24th Au Workshop CODES - David R. Cooke

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CODES Special Publication 5 Edited by David R. Cooke, Cari Deyell and June Pongratz

r CODES SRC

Centre for Ore Deposit Research University of Tasmania Australia 2004

UTAS Centre for Ore Deposit Research University of Tasmania Private Bag 79 Hobart Tasmania Australia 7001

www.codes.utas.edu.au June 2004 Second edition August 2004

ISBN 1 86295 175 6

Cover: Jurassic intrusion-related auriferous quartz pyrite vein, Johnny Mountain deposit, BC, Canadá. Sample provided by J.B. Gemmell.

another Pongatz Production 2004 Printed by the Printing Authority ofTasmania.

Contents Francois Robert: Characteristics of lode gold deposits in greenstone belts Andrew Tunks: Contrasting styles of Proterozoic gold mineralisation in Ghana, West África

1 13

David R. Cooke, Alan J. Wilson and Andrew G. S. Davies: Characteristics and génesis of porphyry copper-gold deposits

17

Doug Kirwin: The Oyu Tolgoi copper and gold porphyry deposits, South Gobi, Mongolia

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Richard M. Tosdal: Tectonics of porphyry copper and epithermal deposits as constrained by vein geometry

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Alan J. Wilson, David R. Cooke and Tully Richards: Veins, pegmatites and breccias: examples from the alkalic Cadia Quarry gold-copper porphyry deposit, NSW, Australia

45

J. Bruce Gemmell: Low- and intermediate-sulfidation epithermal deposits

57

Andrew G. S. Davies, Theo M. van Leeuwen, David R. Cooke, J. Bruce Gemmell: The Kelian gold deposit — exploration history, critical factors and deposit summary Joey S. Garcia Jr: Geology and mineralisation characteristics of the Victoria and Teresa gold deposits,

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Mankayan mineral district, north Luzon, Philippines

77

M. D. Hannington: Spectrum of gold-rich VMS deposits from the Archaean to the present

79

Guy Gosselin: Agnico-Eagle's LaRonde mine — a world-class gold-rich VMS deposit

87

Peter Pring: Recent gold-rich VHMS discoveries at Gossan Hill Jeffrey W. Hedenquist, Richard H. Sillitoe and Antonio Arribas: Characteristics of and exploration for

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high-sulfidation epithermal gold-copper deposits

99

Stephen E. Kesler: Geology and exploration at Pueblo Viejo

111

Brian Levet: The Martabe high-sulfidation epithermal gold deposits of North Sumatra, Indonesia

121

Lawrence D. Meinert: Characteristics of gold skarn deposits

125

Jean S. Cline, Albert H. Hofstra, John Mintean, Richard M. Tosdal and Kenneth A. Hickey: Characteristics and génesis of Carlin-type gold deposits, Nevada, USA

133

APPENDIX Stephen E. Kesler, Norman Russell and Karr McCurdy — Trace-metal content of the Pueblo Viejo precious-metal deposits and their relation to other high-sulfidation epithermal systems. Reprinted from Mineralium Deposita, with permisssion of the publishers Springer-Verlag GmbH.

Sponsors

ANGLOGOLD ASHANTI

BARRICK AUSTRALIA

Preface DAVID COOKE, ANDREW TUNKS AND CARI DEYELL This volume contains proceedings of the 24 Carat Gold Workshop, which was held in Hobart, Tasmania, on 14-16 June 2004. At the time of writing, the workshop had attracted more than 100 delegates from Canadá, the United States of America, the Philippines, Papua New Guinea, Ghana and Australia. The workshop consisted of 13 one-hour keynote presentations from selected industry and academic specialists who, where possible, have addressed the following issues: • Characteristics of a hydrothermal gold deposit type • Anatomy of a giant gold deposit discovery Seven of the papers contained within this volume summarise the characteristics and origins of gold deposits: lode gold (Robert), porphyry copper-gold (Cooke), low and intermedíate sulfidation epithermal gold (Gemmell), volcanic-hosted massive sulfide gold (Hannington), high sulfidation epithermal gold (Hedenquist), gold skarns (Meinert) and Carlin-type gold (Cline). There is also one overview paper on the structural architecture of porphyry and epithermal deposits (Tosdal). Five papers deal with the anatomy of giant ore deposit discoveries, and characteristics of those deposits (Tarkwa/Damang - Tunks; Oyu Tolgoi — Ivanhoe Mines; Kelian — Davies; La Ronde - Gosselin; Pueblo Viejo - Kesler). In addition to the keynote presentations, each halfday session concluded with a one-hour forum that dealt with exploration strategies for future discoveries of giant ore deposits. These forums included presentations by industry geologists on recent discoveries of gold deposits, which were followed by group discussions on the requirements for finding new gold deposits. This volume contains papers summarizing the characteristics of four of those deposits discussed in the exploration forums: Cadia Quarry (Wilson), Teresa and Victoria (Garcia), Golden Grove (Pring) and Martabe (Levet). Presentations were also given on Siberia lode gold (Byass) and relationships between different types of carbonate-hosted gold deposits (White), but no accompanying papers were provided.

The aim of the 24 Carat Gold Workshop is to help maintain the high level of interest in gold deposits that currently exists amongst industry and academic geologists. It is hoped that some of the insights gained from reading this volume will help geologists in their quests for new gold deposits, and in furthering our understanding of the origins of the deposits that have already been discovered.

Acknowledgements We thank the speakers for their written and oral contributions which have helped to make this workshop a success: Francois Robert, Andrew Tunks, Adrián Byass, David Cooke, Doug Kirwin, Dick Tosdal, Alan Wilson, Bruce Gemmell, Andrew Davies, Joey Garcia, Mark Hannington, Guy Gosselin, Peter Pring, Jeff Hedenquist, Steve Kesler, Brian Levet, Larry Meinert, Jean Cline and Noel White. The editors of the workshop volume subjected each paper to peer reviews. The authors are thanked for their patience in making corrections and for providing high quality versions of their illustrations, wherever possible. The publisher of Mineralium Deposita is thanked for providing permisssion to reprint the paper by Kesler. Special thanks are owed to three individuáis who have helped to make this workshop a success. June Pongratz is thanked for her tireless efforts in editing and typesetting the workshop volume. Karin Orth and Kylie Kapeller are thanked for taking on so many of the logistical tasks associated with running the workshop, including registrations.organisationof social functions, arranging the conference venues and dealing with the accommodation and travel bookings for the guest speakers. David Cooke and Cari Deyell Editors, Workshop Volume June 2004

Andrew Tunks Workshop Convener

Characteristics of lode gold deposits in greenstone belts FRANQOIS ROBERT

There is general agreement that: • Many deposits share a number of recurring characteristics • Deposits also display significant diversity in other important attributes which defines a number of recurring styles of deposits • Some of this diversity reflects variations in host rocks, metamorphic grade and depth of emplacement, • Some of the variations also reflect the existence of different types of gold deposits. However, there is no consensus on: • The criteria that distinguish 'orogenic' deposits from other types • Which deposits belong to the orogenic group and which ones represent other deposit types • The interpretation of timing of mineralisation relative to deformation and metamorphism. Such ambiguities make it difficult to review the characteristics of lode gold deposits in greenstone belts in a unifying way. The approach taken here is to review the geologic setting and characteristics of deposits by highlighting commonalities and by adequately capturing the variations observed. This provides a base for discussing recurring 'styles' of deposits and for touching on the question of permissible models. The focus is on gold deposits greenstone belts and their associated clastic sedimentary rocks. Deposits in clastic sedimentary belts lacking any significant volumes of volcanic rocks, such as those in Central Victoria, are excluded, as this introduces even more ambiguities. All types of gold deposits present in greenstone belts are considered except those of clear VHMS origin (for example Horne, Bousquet and La Ronde in Abitibi). These are discussed in a separate session.

Introduction Greenstone belts are an important source of gold, especially those of late Archaean and Paleoproterozoic age, and continué to be the focus of significant gold research and exploration efforts. Their importance is further illustrated by the fact that they are well represented among the global population of giant gold deposits. Of the 28 giant gold deposits (>20 Moz) considered by Sillitoe (2000), eight are from greenstone belts (Golden Mile, Homestake, Hollinger—Mclntyre, Ashanti, Kolar, Kirkland Lake, Hemlo, and Boddington). Much has been written about greenstone gold deposits, and their general deposit characteristics and genetic models have been summarised in several reviews arricies (Colvine, 1989; Hodgson, 1993; Goldfarb et al., 2001; Groveset al., 2003). Despite all the work in the last two decades, there still remains ambiguity with respect to the classification, nomenclature, and origin of these deposits, and as to how many (and which) deposit types are present in greenstone belts. The main unresolved issues relate to existence and importance of different types and ages of deposits in greenstone belts. Two main school of thoughts are that the majority of deposits belong to a single 'orogenic' class, with few 'anomalous deposits' of other types (Groves et al, 2003; Fig. 1), and that there several deposit types and ages represented in the population of large deposits, including epithermal and porphyry-style deposits (Robert and Poulsen, 1997; Penczak and Masón, 1995; Robert, 2001).

Barrick Gold of Australia Ltd., Locked Bag 12, Cloisters Square, Perth, WA 6850, Australia

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Frangois Robert

Figure 1 Schematic representación of orogenic gold deposits (A), gold deposits with clear anomalous ore geochemistry (B) and intrusion-related deposit (C), all of which may be present in greenstone belts. From Groves et al. (2003).

Table 1 Definitions of terms used to desígnate gold deposits in deformed terranes. Terms/type Original definition and comments Lode gold

Originally used to distinguish bedrock from placer gold deposits. Also later used by Lindgren (1933) to refer to relatively wide zones of irregularly interconnected veinlets and intervening mineralized wallrocks. Lode gold applies to several types of deposits, including epithermal and 'intrusion-related'. It is appropriate to refer in general to gold deposits in greenstone belts, but is not synonymous with mesothermal or orogenic.

Orogenic gold

Gold-only deposits formed during the main period of crustal shortening of their host belt (Groves et al., 1998). The term applies to such deposits in both greenstone belts and clastic sedimentary basins From a strict point of view, only deposits that can be demonstrated to have formed during crustal shortening (e.g. syntectonic veins) should be included in this category.

Gold-only

Refers to deposits in which Au is the only significant metal enriched in the deposit (including relative to Ag), to distinguish them from other types of deposits with low to anomalous concentrations of base or other metáis. Commonly equated with orogenic but the issue is that other deposit types, argued to be present in greenstone belts, can also share this charactetistic. Examples include alkalic and other epithermal deposits.

Mesothermal gold Hydrothermal deposits formed at temperatures of 15O-300°C (Lindgren, 1933). The term was widely applied to gold-only deposits in greenstone belts and slate belts in the 1980's. However, some of the deposits used as classic mesothermal examples in that period were classified by Lindgren (1933) as hypothermal. Greenstone gold

General term that encompasses all gold deposits in greenstone belts, irrespective of their origin and characteristics. I would view it as equivalent to iode gold' deposits.

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Characteristics oflode gold deposits in greenstone belts

Definition and nomenclature

Size distribution

Numerous terms have been used to refer to all or to specific subsets of gold deposits in metamorphic belts in general and in greenstone belts. This situation, compounded by different classification schemes and diverging interpretations of many deposits, leads to significant confusión. Table 1 summarises the original definition of the more commonly used terms, some of which have been used appropriately, but others less so.

• As for most deposit types, most of the gold endowment in greenstone belts is contained in a few large deposits. This is shown in Figure 2 for gold deposits with >3 t Au in the Superior, Slave, Churchill, Yilgarn and Zimbabwe cratons: 28 of the 270+ deposits, i.e., 10% of the population, contain 67% of the gold (Hodgson, 1993). • Within cratons or granite greenstone terranes, the bulk of the gold endowment is commonly contained in specific greenstone belts or even specific areas within them. For example, the southern Abitibi belt, Eastern Goldfields Province, and Ashanti belt host the bulk of the gold in the more extensive Superior, Yilgarn and West African cratons, respectively. • These differences in endowment between the different áreas, very important in exploration, are due in a large part to differences in the proportion of large deposits (Hodgson, 1993). These differences are apparent in Figure 2 for deposits > 100 t of contained gold.

Time distribution • Lode gold deposits occur in greenstone terranes that span the nearly all the geologic timescale, from Paleoarchean (Pilbara) to Mesozoic (Mother Lode). • The most prolific greenstone belts have formed at specific times in the evolution of the Earth as follows (Goldfarb et al., 2001): Neoarchean (2.6-2.7 Ga, in the Yilgarn and Superior cratons), Paleoproterozoic (2.1 to 1.8 Ga, in the Black Hills and West African craton) and Mesozoic (0.14 Ga, in the Sierra Foothills province). • However, not all belts or cratons of these prolific ages are equally endowed (see below).

Figure 2 Probability plot of size of gold deposits with >3 t Au in greenstone belts of five Precambrian provinces. From Hodgson (1993).

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Fran^ois Robert

rocks present in a greenstone belt and, consequently over a wide range of stratigraphic positions, from lower manc-ultramafic volcanic to upper clastic sedimentary stratigraphic levéis. However some belts display a preferred association of deposits with mafic-ultramafic volcanic rocks (e.g., Wiluna—Norseman belt) and others with sedimentary rocks (e.g., Barberton belt) • In some belts, gold deposits may show a spatial association with specific rock types whose distribution is also controlled by crustal scale structures. A good example is the association with felsic porphyries and Timiskamine conglomerates in southern Abitibi, as

Belt-scale distribution of deposits

Relative to structures • Gold deposits and occurrences are not uniformly distributed within greenstone belts. While occurrences display significant scatter, the deposits themselves tend define corridors that are commonly spatially associated with regional (crustal-scale), belt-parallel structures. • The larger deposits, representing the bulk of the gold endowment, tend to be restricted to specific crustal-scale structures such as such as the Larder Lake—Cadillac and Porcupine Destor 'Breaks' in Abitibi (Fig. 3). Other examples include the Boulder—Lefroy fault in the Wiluna-Norseman belt, and the Obuasi Fissure between the Ashanti Belt and Kumasi basin in West África. • Despite their distribution along crustal-scale structures, most large deposits are actually associated with lowerorder faults and shear zones, away from the transcrustal structure. For example, the >5 Moz deposits in the Abitibi belt occur up to 6 km from these crustal breaks (Fig. 3; Robert, 2003). Only a few large deposits actually occur directly within transcrustal structures themselves, such as Kerr Addison in Abitibi and Jubilee-New Celebration in the Eastern Goldfields.

emphasised by Hodgson (1993). • Gold deposits globally show only a weak association with large granitic intrusions within the belts, mainly occurring along their edges rather than in their cores. A notable exception is the Grass Valley deposit in California.

Relative to metamorphic grade • Although gold deposits and occurrences are present in rocks of all metamorphic grades, from granulite to subgreenschist, the majority occurs in greenschist grade rocks, as represented in Figure 4. • The large deposits and camps occur dominantly in áreas of greenschist grade metamorphism, although a few large deposits also occur in áreas of mid-amphibolite or higher grade (e.g., Kolar, Hemlo). • The question of whether deposits have formed at ambient metamorphic conditions or have been overprinted by metamorphism is a challenging one, especially at high metamorphic grades. Examples of both situations have been clearly been documented.

• This spatial association of large gold deposits with crustal-scale structures is not universal, and it appears to be lacking in the Uchi greenstone belt of Superior Province (Card and Poulsen, 1998) or in the Black Hills. • Within these corridors and along crustal-scale structures, deposits display a strong clustering into camps (Fig. 3), typically defined by at least one large deposit, a few small ones, and numerous occurrences (Hodgson, 1993). • These camps are localised at bends or major splay intersections along the crustal-scale structures. Along some faults, there appears to be a regular spacing of camps, for example along the Larger Lake—Cadillac in Abitibi (Fig. 3) and Boulder-Lefroy fault system in the Eastern Goldfields Province, where gold camps occur at intervals of 30—50 km.

Local settings of deposits Gold deposits in greenstone belts occur in a whole range of lithologic and structural settings, as represented in Figure 4. Within this range, there are a few recurring patterns and associations that appear to be favorable for the development of large deposits. The recognition of these favourable settings is important from an exploration point of view. As a general rule, areas of high geometric, structural and lithologic complexities appear to be more favorable.

Relative to rock types • By virtue of their occurrence along crustal breaks, many gold camps and large deposits are situated near boundaries between major lithologic units. The Obuasi deposit, for example is located near the structural contact between the mafic volcanic rocks of the Ashanti Belt and clastic sedimentary rocks of the Kumasi basin. • Gold deposits can occur in any type of supracrustal

Structural setting Nearly all gold deposits show a close spatial association with shear zones, faults, or folds, as shown in Figure 4. This attests to the importance of structure as a control of mineralisation, as would be expected given the hydrothermal origin of the deposits.

4

Characteristics of lode gold deposits in greenstone belts

Figure 3 Simplified geologic map of parts of the Abitibi greenstone belt showing the distribution of transcrustal structures and gold deposits. The different types of deposits are represented by different symbols, and the larger symbols mark deposits with >5 Moz gold. The main gold districts are also identified.

Figure 4 Schematic representation of the diverse lithologic, structural and metamorphic settings of gold deposits in the Yilgarn craton. From Groves etal. (1990).

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Francois Robert

• The character of structures with which the gold deposits are associated generally reflects the local metamorphic grade. It ranges from relatively brittle in areas of lower to sub greenschist grade, to brittle-ductile (depending on rock type) in mid to upper greenschist grade, to clearly ductile at amphibolite or higher grade (Fig. 1). • Favorable structural settings in the vicinity of transcrustal structures are linked to the distribution and geometry of lithologic units (Fig. 4): -Shear zones and faults developed along lithologic contacts between units of contrasting competencies and along thin incompetent lithologic units. Common examples include mafic-ultramafic volcanic contacts (Kerr—Addison), volcanic-sediment contacts (Obuasi), the edge of granitic intrusions (Tarmoola, Granny Smith), and interflow sedimentary rocks in volcanic piles. Along these contacts and incompetent units, deposits will preferentially develop at sites of bends, splays or other structural intersections. -Competent rock units enclosed in less competent ones, a setting that favors the development of fracturing and veining. Examples include felsic dykes and stocks in clastic sedimentary (Wallaby) or volcanic rocks (SigmaLamaque), granophyric layers in differentiated dolerite sills (Mt Charlotte and Fimiston), and magnetite-rich sedimentary rocks units in mafic volcanic or clastic sedimentary sequences. -Folds hinges and anticlines, especially in layered rocks units such as BIFs and sedimentary rocks (Homestake, Musselwhite).

the Quadrilatero Ferrifero district of Brasil for its association with iron-formation, and the southern Abitibi for its association with conglomerates and high-level porphyry intrusions.

Characteristics of deposits As indicated in the introduction, lode gold deposits in greenstone belts display a wide range of geometric, structural and hydrothermal characteristics, reflecting the existence of a spectrum of mineralisation styles (Colvine, 1989; Groves et al., 1993; Hodgson, 1993, Robert and Poulsen, 1997). Despite such diversity, there are recurring characteristic among the deposits, which have prompted the development of unified models. In the sections below, these recurring characteristics are clearly identified, while the most commonly noted variations are also covered.

Dimensions and overall geometry • The deposits range in overall shape from tabular to pipe-like bodies that, in most cases, conform to the regional structural and lithologic trends. • In a majority of cases, tabular deposits have steep dips (Kolar, Obuasi) and elongated to pipe-like deposits have moderate to steep plunges (Kanowna Belle, Wallaby). Ore shoots internal to tabular deposits have moderate (Kirkland Lake) to steep plunges (Kolar; Fig. 5), and some deposits also have well-defined funnel shapes, either in cross-section (Fimiston) or in longitudinal section (Kerr Addison, Kolar; Fig. 5). • In several cases, however, the deposits and their main structures have only moderate dips (Lancefield, Norseman, Granny Smith) and plunges (Homestake, Morro Vehlo). • Greenstone gold deposits tend to be vertically extensive. Large deposits commonly have a vertical extent in excess of 1 km, reaching -3 km in the case of Kolar (Fig. 5). As an illustration, all of the nine >5 Moz deposits in southern Abitibi exceed 1 km of vertical extent, and five of them exceed 2 km (Robert, 2003). The downplunge extent of the moderately plunging Homestake deposit reaches 5 km at Homestake (Caddey et al., 1991). • Large deposits (>5 Moz) commonly have kilometrescale strike length, and in several cases in excess of 5 km (Kolar, Fig 5). The footprints of the large deposits (>5 Moz) are substantial and commonly exceeds 1 km2. They can reach 10 km2 for large deposits such as Hollinger-Mclntyre deposit (31.4 Moz).

Lithologic setting • As indicated above, gold deposits can occur in all rock types present in the belt. However there are particular host rocks associations that appear to be particularly favorable for mineralisation (Fig. 4): -Fe-rich mafic igneous rocks such as tholeiitic basalt and differentiated dolerite sills -BIFs (of oxide and silicate facies) and iron-rich clastic sedimentary rocks Porphyry stocks and dykes of dioritic to felsic compositions, whether they intrude mafic-ultramafic volcanic or clastic sedimentary rocks. • Fe-rich lithologies are regarded as favorable chemical hosts, in light of their high Fe content, whereas intermediate to felsic porphyry intrusions are viewed a favorable structural host, in light of their high competencies and their brittle response to regional deformation (fratcturing and veining). • Specific lithologic associations also appear to dominate in some greenstone belts or districts: the Norseman-Wiluna belt is well know for its association of gold deposits with differentiated dolerite sills,

6

Characteristics of lode gold deposits in greenstone belts

Figure 5 Longitudinal projection of the Kolar gold deposit, illustrating the dimensions of large gold systems. From Hamilton and Hodgson (1986).

mass to concentrated in bands parallel to foliation or to bedding. • Veinlet systems: arrays of either sulfidic fractures or of thin (25% carbonate). • Replacements zones: wallrock replacements without any significant veins or veinlets. They range from zones of disseminated sulfides (25% sulfides). Sulfide distribution ranges from evenly distributed in the rock

7

Francois Robert

Figure 6 Schematic representation of the main constituents of mineralised zones and how they combine to form the different types of mineralisation discussed in the text.

Table 2 Preliminary compilation of characteristics of common styles of deposits in greenstone belts Style of deposit Quartz-carbonate veins and vein arrays ( 5 As, W, ±Te, Mo, B

Alteration assemblages

Structural or lithologic associations

Carb-ser (biot)py (apy) ± alb, at greenschist grade;

Veins in brittle-ductile shear zones and faults, commonly along lithologic contacts or within incompetent rock units

Biot-act-py ± carb, at amphibolite grade

Extensional vein arrays of variable complexity in competent host rocks

Examples (from Superior and Yilgarn cratons) Dome, Pamour, HollingerMclntyre, Sigma-Lamaque; St. Ives; Norseman; New Holland; Mt. Charlotte, San Antonio

Commonly centered on clusters of felsic dykes or stocks Campbell-Red Lake (in Disseminated sulphide Au/Ag > 5 1- alb-carb-ser-py; Mafic volcanic and plutonic hosts; zones ± stockworks of mafic As, W, 2-biotite-pyrrhotite commonly stratabound part); Kerr Addison (flow ore), Sons of Gwalia, association Plutonic Disseminated sulphide Hemlo; Malartic; Ross; Au>Ag, 1- Kfsp-ser-silica; Tonalitic to syenitic zones ± stockworks of felsic Wallaby; Kanowna Belle; As, Te +Hg, 2- albite-carbonatestocks and dykes, commonly Binduli; Mo, Sb, V, sericite porphyritic; clastic and epiclastic hosts association Ba common Iron-formation-hosted veins, Au>Ag; As Fe-amphibole; chlorite; Folded iron-formation; commonly cut Musselwhite; Geraldton; stockwork and stratabound common garnet; +/- carbonate by intermediate to felsic intrusions Cleo-Sunrise (in part) sulfides Disseminated sulfides of calc- Au>Ag Stratabound zones in iron-formation Madsen; Akasaba; Epidote-actinolitedioside-garnet Daveyhurst, Nevoria silicate association ("skarn") Sulphide-rich veins (> 20% sulfides) and veinlet systems Colloform-crustiform carbonate-quartz veins, breccias and sulfidic replacements

Au/Ag is 1-sericite-chloritoidvariable; alumino-silicate Cu, Zn 2- Sericite-chlorite Au/Ag > 5 Ser (biot)-carb at As, Te, V, greenschist grade Hg, Sb, W, Zn

Sequences with felsic volcanic rocks and Doyon; Bellevue, Copper synvolcanic intrusions Rand Felsic high-level porphyry intrusions and dykes;

In brittle structures

Campbell-Red Lake; Dome (ankerite veins); Fimiston, Kanowna Belle (in part), Jundee, Wiluna

Abbreviations: act - actinolite; alb - albite; asp - arsenopyrite; biot - biotite; carb - carbonate; Kfsp - K-feldspar; py - pyrite; ser - sericite.

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Characteristics of lode gold deposits in greenstone belts

Crustiform-colloform carbonate-quartz veins and associated wallrock sulfides. This type of mineralisation is distinguished on the basis of the presence of epithermalstyle veins. It consists of narrow high-grade zones of carbonate-quartz veins, veinlets and breccias, with a variable but complex combination of sulfidic fractures, wallrock sulfide replacements, and silica—arsenopyrite replacements. This type of mineralisation is relatively uncommon but is present in several large deposits. It is dominant at Fimiston, Jundee, Wiluna, and Campbell Red Lake, and is also present at Kanowna Belle, Racetrack, and Dome (ankerite veins).

laminated veins or sigmoidal vein arrays in moderately to steeply dipping reverse shear zones with arrays of shallow-dipping extensional veins in adjacent competent and lower strain rocks. The reverse character of the shear-zone-hosted veins and shallowdips of extensional veins attest to the formation of these quartz—carbonate vein deposits during crustal shortening (Robert and Poulsen, 2001). • Iron-formation-hosted deposits are stratabound and their geometry is largely dictated by the shape of the iron-formation unit, which is commonly folded. In some deposits, the mineralised zones are localised in fold hinges and plunge parallel to the fold axis, as Homestake, while on others mineralisation is controlled by intersecting structures and plunges parallel to the line of intersection. • Colloform-crustiform carbonate—quartz vein and sulfidic replacements also form complex arrays of mineralised zones in brittle structure in several deposits (Jundee, Wiluna), consistent with the high crustal levels suggested by their internal textures and common breccia character. In a few deposits (Campbell—Red Lake, Fimiston), the mineralised zones are clearly overprinted by penetrative structural fabrics. • Deposits dominated by disseminated ± veinlet systems show significant structural variability. They range from low-strain deposits intimately associated with felsic porphyry intrusions with or without clear fault controls (Kanowna Belle, Wallaby) to high strained and folded deposits (Hemlo, Plutonic)

• Iron-formation-hosted sulfidic replacements ± quartz veins and veinlets. Combines varying proportions of discordant quartz veins and veinlets and concordant replacement of iron-rich beds. It is relatively common, and is the BIF equivalent of quartz—carbonate veins described above. Examples include Homestake, Geita and Sunrise (in part). • Sulfide-rich veins, veinlets ± sulfide disseminations. Characterises a small number of deposits, but including significant ones, such as Doyon (see Robert, 2003). Deposits of this type range from individual sulfide-rich veins (Copper Rand, Bellevue) to vein arrays (e.g., Doyon Zone 3, Sleeping Giant), to zones of sulfide veinlets and associated disseminated sulfides (Doyon Main Zone, Mount Gibson). Many deposits consist of a single type of mineralisation, while others combine more than one type. Each of the above type of mineralisation is represented by large deposit examples.

Ore composition Structure of deposits Recurring compositional characteristics of deposits are as follows: • The ores of a majority of deposits are consistently enriched in silver, arsenic and W, and have Au/Ag > 5. Other commonly but not systematically enriched elements include B, Te, Bi, Mo. • Although the deposits can be vertically and laterally very extensive, there is only cryptic mineralogical zoning • The dominant sulfide mineral is pyrite at greenschist grade, pyrrhotite at amphibolite grade. Arsenopyrite is the dominant sulfide in many clastic-sediment-hosted ores at greenschist grade, and loellingite is also present at amphibolite grade. Significant variations include: • Ores range form quartz-rich, in the case of quartzcarbonate veins to quartz-poor in disseminated sulfide ± veinlet systems and iron-formation hosted deposits. • Disseminated ± veinlet systems and sulfidic vein ores may have Au/Ag 20 g/t/) goldmineralised pegmatite at Dinkidi in the Philippines demonstrates that they can be very important to the economics of an alkalic porphyry deposit. At Cadia Quarry, the pegmatite-cemented breccias are mineralised and contribute to the overall resource of the system. Exploration for deeper extensions of these breccias is continuing. • The 'G-faults' and quartz-fragment breccias at Cadia Quarry are late stage magmatic-hydrothermal features akin to deep level low sulfidation style epithermal (carbonate-base metal) veins. These structures dilute or truncate grade at Cadia Quarry. It is possible that the fluids responsible for the G-faults and quartz fragment breccias actually stripped early-formed gold from the

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AlanJ. Wilson, David R. Cooke andTully Richards Landtwing, M.R, Dillenbeck, E. D., Leake, M.H. and Heinrich, C.A., 2002, Evolution of the breccia-hosted porphyry Cu-Mo-Au deposit at Agua Rica, Argentina; progressive unroofing of a magmatic hydrothermal system: Economic Geology, v.97, p. 1273—1292. Lickfold, V, Cooke, D.R., Smith, S.G. and Ullrich, T.D., 2003a, Endeavour copper-gold porphyry deposits, Northparkes, New South Wales: Intrusive history and fluid evolution: Economic Geology, v.98, p. 16071636. Lickfold, V, Wilson, A.J., Harris, A., and Cooke, D.R., 2003b. The alkalic Au-Cu porphyry deposits of NSW, Australia: evidence for coexisting melt and hydrothermal fluids in comb quartz layers: In Eliopoulos, D. et al.,: Mineral Exploration and Sustainable Development1 - Proceedings of the Seventh Biennial SGA Meeting: Millpress, Rotterdam, v.l, p. 315-318. Lowenstern, J.B., and Sinclair, W.D., 1996, Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada: Transactions of the Royal Society of Edinburgh, Earth Sciences, v. 87, p. 291-303. McCallum, M.E., 1985, Experimental evidence for fluidization processes in breccia pipe formation: Economic Geology, v. 80, p. 1523-1543. Newcrest Mining Staff, 1996, The Cadia 'wallrock-porphyry'style gold-copper deposit, NSW: Porphyry-related copper and gold deposits of the Asia-Pacific region, Cairns, Australian Mineral Foundation, p. 16.116.10. Newcrest Mining Staff, 1997, The Cadia gold-copper deposit, I NSW, New developments in research for ore deposit exploration, Third national conference of the Specialist Group in Economic Geology, Geological Society of Australia v. 44, Sydney, Geological Society of Australia, p. 54. Newcrest Mining Staff, 1998, Cadia gold-copper deposit, in Berkman, D. A., and Mackenzie, D. H., eds., Geology of Australian and Papua New Guinean mineral deposits,, Australasian Institute of Mining and Metallurgy, Monograph Series v. 22, Melbourne, Australasian Institute of Mining and Metallurgy, p. 641—646. Packham, G., Percival, I., and Bischoff, G., 1999, Age constraints on strata enclosing the Cadia and Junction Reefs ore deposits of central New South Wales, and tectonic implications: Geological Survey of New South Wales, Quarterly Notes, v. 110, p. 1-12. Pogson, D.J., and Watkins, J.J., 1998, Bathurst geological sheet 1:250 000, Sydney, Australia: Geological Survey of New South Wales, Dept. of Mineral Resources, 430 p. Pollard, P.J., and Taylor, R.G., 2002, Paragenesis of the Grasberg Cu-Au deposit, Irian Jaya, Indonesia: results from logging section 13: Mineralium Deposita, v.37, p.117-136. Seedorf, E., 1988, Cyclic development of hydrothermal mineral assemblages related to multiple intrusions at the Henderson porphyry molybdenum deposit, Colorado, in Taylor, R. P., and Strong, D. F., eds., Recent advances in the geology of granite-related mineral deposits, CIM Special Volume 31, p. 367-393.

Giggenbach, W.F., 1992, Origin and composition of volatiles associated with andesitic magmatism: Eos, Transactions, American Geophysical Union, v.73, no. 14, p.371. Gilluly, J., 1946, Ajo Mining District, Arizona: USGS Professional Paper 0209, 112p. Glen, R.A., and Walshe, J.L., 1999, Cross-structures in the Lachlan Orogen; the Lachlan transverse zone example: Australian Journal of Earth Sciences, v. 46, p. 641— 658. Grant, N. J., Halls, C, Sheppard, S. M. E, and Avila, W., 1980, Evolution of the porphyry tin deposits of Bolivia, in Ishihara, S., and Takenouchi, S., eds., Granitic magmatism and related mineralization, Tokyo, Society of Resource Geologists of Japan, p. 151—173. Green, D., 1999, Geology, geochemistry and genesis of the Big Cadia deposit, N.S.W., Unpublished B.Sc. Honours Thesis, University of Tasmania, 154p. Gustafson, L. B., and Quiroga, J., 1995, Patterns of mineralization and alteration below the porphyry copper orebody at El Salvador, Chile: Economic Geology, v. 90, p. 2-16. Harris, A.C., and Golding, S.D., 2002, New evidence of magmatic-fluid-related phyllic alteration: implications for the genesis of porphyry Cu deposits: Geology, v. 30, p. 335-338. Hedenquist, J.W., and Richards, J.P., 1998, The influence of geochemical techniques on the development of genetic models for porphyry copper deposits, in Richards, J. P., and Larson, P. B., eds., Reviews in Economic Geology Volume 10. Techniques in Hydrothermal Ore Deposits Geology, Littleton, Society of Economic Geologists, p. 235-256. Heithersay, P.S., and Walshe, J.L., 1995, Endeavour 26 North; a porphyry copper-gold deposit in the Late Ordovician, shoshonitic Goonumbla volcanic complex, New South Wales, Australia: Economic Geology, v. 90, p. 1506— 1532. Holliday, J., McMillan, C, and Tedder, I., 1999, Discovery of the Cadia Ridgeway gold-copper deposit: New Generation Gold Mines - case histories of discovery, Perth, Australian Mineral Foundation, p. 101—107. Holliday, J. R., Wilson, A. J., Blevin, P. L, Tedder, I. J., Dunham, P. D., and Pfitzner, M., 2002, Porphyry gold-copper mineralisation in the Cadia district, eastern Lachlan Fold Belt, New South Wales, and its relationship to shoshonitic magmatism: Mineralium Deposita, v. 37, p. 100-116. Horita, J., Cole, D. R., and Weslowski, D. J., 1995, The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: III. Vaporliquid water equilibrium of NaCl solutions at 35O°C: Geochimica et Cosmochimica Acta, v. 59, p. 1139— 1151. Kirkham, R.V., and Sinclair, W.D., 1988, Comb layer quartz in felsic intrusions and their relationship to porphyry deposits, in Taylor, R. P., and Strong, D. F., eds., Recent Advances in the Geology of Granite-Related Mineral Deposits, Canadian Institute of Mining and Metallurgy Special Volume 31, Montreal, Canadian Institute of Mining and Metallurgy, p. 50-71.

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Veins, pegmatites and breccias: examples from the alkalic Cadia Quarry gold-copper porphyry deposit, NSW, Australia Shannon, J.R., Walker, B.M., Carten, R.B., and Geraghty, E.P., 1982, Unidirectional solidification textures and their significance in determining relative ages of intrusions at the Henderson Mine, Colorado: Geology, v. 10, p. 293-297. Sillitoe, R.H., 1976, A reconnaissance of the Mexican porphyry copper belt: Transactions of the Institute of Mining and Metallurgy, v. 85, pB170-B189. Stanley, C. R., Holbek, P. M., Huyck, H. L. O., Lang, J. R., Preto, V. A. G., Blower, S. J., and Bottaro, J. C, 1995, Geology of the Copper Mountain alkalic coppergold porphyry deposits, Princeton, British Columbia, in Schroeter, T. G., ed., Porphyry Deposits of the Northwestern Cordillera of North America. CIM Special Volume 46, Quebec, Canadian Institute of Mining, Metallurgy and Petroleum, p. 537-564. Tedder, I. J., Holliday, J., and Hayward, S., 2001, Discovery and evaluation drilling of the Cadia Far East goldcopper deposit: NewGen Gold 2001 - Case Histories of Discovery, Perth, Australian Mineral Foundation, p. 171-184. Vargas R., Gustafson, L.B., Vukasovic, M., Tidy E. and Skewes, A., 1999, Ore breccias in the Rio Bianco-Los Bronces porphyry copper deposit, Chile, in Skinner, B.J., ed., Geology and ore deposits of the Central Andes: Society of Economic Geologists, Special Publication No..7, p. 281-297. Wallace, S.R., 1991, Model development: Porphyry molybdenum deposits, in Hutchinson, R. W, and Grauch, R. I., eds., Historical Perspectives of genetic Concepts and Case Histories of Famous Discoveries, New Haven, The Economic Geology Publishing Company, p. 207-224. Wellman, P., and McDoueall, I., 1974, Potassium-argon ages on Cainozoic volcanic rocks of New South Wales: Geological Society of Australia Journal, v. 21, p. 247— 272. Wilson, A.J., 2003, The geology, genesis and exploration context of the Cadia gold-copper porphyry deposits, New South Wales, unpublished Ph.D. Thesis, Centre for Ore Deposit Research, University of Tasmania, 335p. Wilson, A.J., Cooke, D.R. and Harper, B.L., 2003, The Ridgeway gold-copper deposit: A high-grade alkalic porphyry deposit in the Lachlan Fold Belt, New South Wales, Australia, Economic Geology, v. 98, p. 1637— 1666. Wolfe, R. C, Cooke, D. R., and Joyce, P., 1999, Geology, mineralisation and genesis of the alkaline Dinkidi CuAu porphyry, north Luzon, Philippines: PACRIM '99 Congress, Bali, Indonesia, The Australian Institute of Mining and Metallurgy, p. 509-516. Wood, D., and Holliday, J., 1995, Discovery of the Cadia gold/copper deposit in New South Wales: New Generation Gold Mines - case histories of discovery, Perth, Australian Mineral Foundation, p. 11.1-11.10.

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Low- and intermediate-sulfidation epithermal deposits J. BRUCE GEMMELL

types (Hedenquist et al., 2000). This classification was initially based on the oxidation state of sulfur in the fluid (Hedenquist, 1987) but this proved to be an impractical way of interpreting deposits in the field. Currently the classification is based on the sulfidation states of observed hypogene sulfude assemblages (Hedenquist et al., 2000; Sillitoe and Hendequist, 2003). Low-sulfidation deposits contain pyrite-pyrrhotite-arsenopyrite and high Fe sphalerite while intermediate-sulfidation epithermal deposits contain an assemblage of pyrite-tetrahedrite/ tennantite-chalcopynte and low Fe sphalerite (Hedenquist et al., 2000). A further subdivision of low-sulfidation deposits is based on the associated magmatic rocks (e.g., alkalic-type low sulfidation-deposits). However to many economic geologists working in the epithermal realm, intermediate-sulfidation deposits are considered a subset of low-sulfidation deposits and not a separate deposit

Introduction Low- and intermediate-sulphidation epithermal deposits have produced significant quantities of gold and silver, along with minor amounts of base metals, and continue to be a major target for precious metal explorers. As well as research on ancient deposits, investigations of their modern analogues, active geothermal systems, have led to many new concepts in the understanding of epithermal deposits which aid in the development of genetic and exploration models. The salient geologic and genetic features of low- and intermediate-sulfidation gold-silver and gold-silver-leadzinc deposits (Table 1) are outlined here, as Hedenquist et al. (2004) cover the characteristics of high-sulfidation deposits. Information presented in this abstract is based on the author's own observations and selected papers that summarise the geologic and genetic characteristics of epithermal deposits (e.g., Buchanan, 1981; Heald et al., 1987; Reyes, 1990; Sillitoe, 1993, 1997; Hedenquist and Lowenstern, 1994; White et al., 1995; White and Hedenquist, 1990, 1995; White and Poizat 1995; Hedenquist et al., 2000; Cooke and Simmons, 2000; Sillitoe and Hedenquist, 2003; Einaudi et al., 2003; Simmons and White, in review).

typeClassic examples of low-sulfidation epithermal deposits are Hishikari and Kushikino (Japan), Waihi and Golden Cross (New Zealand), Pajingo-Vera Nancy (Australia), Round Mountain, McLaughlin, Midas, Bullfrog and Sleeper (USA), El Penon (Chile), and Cerro Vanguardia and Esquel (Argentina). Ladolam and Porgera (Papua New Guinea), Cripple Creek (USA), and Emperor (Fiji) are alkalic-type, low-sulfidation epithermal deposits. Examples of intermediate-sulfidation epithermal deposits are Fresnillo, Guanajuato and Pachuca-Real de Monte (Mexico), Rosia Montana (Romania), Baguio and Victoria (Philippines), Creede and Comstock Lode (USA) and Kelian and Mt Muro (Indonesia).

Classification Since the early 1900s epithermal Gold and Ag deposits of both vein and bulk-tonnage styles have been characterised by a myriad of terms, but recently they have been are classified into high-, intermediate-, and low-sulfidation

Tectonic setting and distribution Most low-sulfidation deposits are associated with subaenal bimodal (basalt-ryholite) volcanic suites in a broad spectrum of extenstional tectonic settings, including

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Australia 7001

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/. Bruce Gemmell Table 1 Geologic and genetic characteristics of low- and intermediate-sulfidation deposits. Characteristics

low-sulfidation deposits

alkalic-type low-sulfidation deposits intermediate-sulfidation deposits

Tectonic setting

extensional continental and island arcs

extensional island and continental arcs

intra-, near-, back-arc, postcollisional

extensional continental and compressive island arcs

Genetically related

calc-alkaline,

alakaline,

calc-alkaline,

igneous rocks

rhyolite—basalt (bi modal)

basalt-trachyte

andesite—rhyodacite, locally rhyolite

Typical host rocks

domes, volcaniclastic & sedimentary domes, volcaniclastic & sedimentary lava flows, pyroclastic, basement, units diatremes units

Metal signature

Au-Ag,

Au-Ag,

Ag-Au, Zn, Pb, Cu

(Pb, Zn, Cu, Mo, As, Sb, Hg)

(Te, Zn, Pb, Cu, Mo, As, Sb, Hg)

(Sb, As, Mo, Te, Se, Hg)

Au content

1000 (t)

70 to > 1000 (t)

20 vol. % in intermediatesulfidation deposits (Sillitoe and Hedenquist, 2003). In the vertically attenuated deposits, mineralogical and textural zonation is generally observed, with deep-level vein mineralisation characterised by coarse euhedral textures, and by abundant carbonates and base metal sulfides. Shallow-level systems tend to be more siliceous, have finer-grained vein material and are associated with clay-bearing assemblages (illite and smectite). Phreatic breccias are common in some systems; in others, vein mineralisation is intimately associated with dilational structures.

mineralogical, chemical or isotopic zonation within areas of alteration may provide a basis for developing vectors to the ore deposit.

Genesis The distinct differences between epithermal deposit types, specifically the ore mineralogy and metal complement, are largely controlled by the composition of the ore fluid (Sillitoe and Hedenquist, 2003). In general, low- and intermediate-sulfidation mineralisation is precipitated in the shallow crustal environment ( 0 - 1 km) by near-neutral (pH « 6), reduced (H2S > SC>4~2), variable salinity, moderate temperature (» 150°—300°C), boiling, gassy (CO 2 - and H2S-rich) fluids. Essentially the main difference between the fluids responsible for lowor intermediate epithermal mineralisation is salinity. Low-sulfidation deposits generally have salinities 80 deposits and >500 Mt). The origin of this remarkable concentration of gold remains one of the outstanding questions in economic geology in Canada. One argument is that the high gold contents reflect the proximity of the deposits to the Cadillac Break, a major controlling structure for later Archaean lode gold mineralisation, and that the gold was introduced during a late syntectonic event. However, a number of compelling arguments in each of the deposits, including metal zonation, alteration characteristics, and the unique gold geochemistry and mineralogy, suggest that the majority of the gold belongs to the VMS systems (Poulsen and Hannington 1996, Poulsen et al., 2000).

Classification and distribution Gold-rich VMS deposits can be divided into two groups: deposits from which gold is recovered as a by-product of base metal mining and deposits in which gold is a primary commodity. Deposits of the first group typically have modest gold grades but large tonnages (e.g., 180t Au at Flin Flon, Manitoba). Deposits of the second group are true gold deposits in a strict economic sense (Poulsen et al., 2000). Several have been 'world-class' gold producers (>100t Au at grades of more than 1 g/t). Four subtypes are recognised: (1) large tonnage copper-gold deposits,

Geological Survey of Canada, Ottawa, Canada

Other examples include the Early Proterozoic Boliden copper—gold—arsenic deposit in the Skellefte district

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M. D. Hannington of Sweden (128 t Au), the gold-rich VMS deposits of the Cambrian Mount Read volcanic belt (c. 1001 Au, collectively, at Rosebery, Hercules, Que River and Hellyer), the nearby Mount Lyell copper—gold deposits (40 t Au), and possibly the Paleozoic Mount Morgan deposit of Queensland (240 t Au). Significant gold is also contained in several large-tonnage, low-grade (c. 1 g/t) deposits in the Iberian Pyrite Belt (e.g., Aljustrel, Tharsis, and the giant Cerro Colorado gossan overlying the Rio Tinto deposit) and in several deposits of the southern Urals (e.g., Uchaly).

the possibility of a direct genetic link with high-level magmatic-hydrothermal systems commonly associated with andesitic arc volcanoes. However, the uncertain preservation potential of deposits in this environment is an important consideration.

Deposit characteristics Most gold-rich VMS, like other VMS deposits, are stratabound, occurring within or along-strike of welldefined volcanic and sedimentary packages. Individual deposits may comprise stratiform lenses (e.g., Eskay Creek, LaRonde), however, many pyritic gold and copper-gold deposits formed as subseafloor replacements underlain by substantial stockwork mineralisation and, strictly speaking, may not have been exhalative in origin (e.g., Boliden). Some stockwork-type deposits lack massive sulfide mineralisation and are capped only by cherty exhalite or bedded barite. Other deposits may have formed in transitional shallow submarine-to-subaerial settings and have a number of important attributes in common with epithermal gold deposits. The large size and relatively high Cu/Cu+Zn ratios of some gold-rich VMS, such as Home and Mount Lyell, distinguish these deposits from other massive sulfide deposits in the same district and likely reflect processes that were i also responsible for the primary enrichment of gold. It is noteworthy that Home and LaRonde, in addition to having the highest gold grades, are among the largest of : any VMS in the Abitibi greenstone belt (only Kidd Creek I is larger).

Most of the gold production from VMS has come from deposits that have a close spatial and temporal association with anomalous concentrations of rhyolite and associated felsic subvolcanic intrusions. In the classification scheme of Barrie and Hannington (1999), felsic-dominated, bimodal volcanic successions, which account for only about 20% of the total tonnage of VMS worldwide, contain more than 40% of the VMS-hosted gold. Few gold-rich VMS occur in mafic-dominated volcanic successions. A similar association of gold-rich VMS with felsic volcanic rocks has been noted on the modern seafloor (Herzig and Hannington, 1995). Such deposits may derive their gold via direct contributions from felsic magmas (see below). Tonalitic to granitic subvolcanic intrusions are spatially associated with the deposits at Home (Flavrian—Powell), Bousquet (Mooshla intrusion), Boliden (Jarn granitoids), and Mount Lyell (Cambrian granites). In some cases, the intrusions are hydrothermally altered and contain vein- and stockworktype mineralisation that locally constitutes significant gold ore in itself (e.g., Doyon deposit near Bousquet: Poulsen, 1995; subeconomic porphyry copper—gold mineralisation in the Boliden area: Allen et al., 1996). However, the temporal relationships of different phases of the intrusions to the overlying volcanic packages are generally not well known.

A common feature of the older, metamorphosed deposits is a strong foliation subparallel to the regional lithologic trend and complete transposition of bedding, with the possibility of dislocation of the ore lenses from their altered host rocks. The development of tectonic fabrics that overprint the mineralisation in some cases has caused significant remobilisation of gold into faults and shear zones adjacent to the original massive sulfide lenses. This has led to considerable debate about possible | syntectonic versus synvolcanic origins for the deposits and their contained gold. A definitive conclusion about the timing of mineralisation with respect to deformation and metamorphism is often impossible. However, the presence of obvious exhalative mineralisation is an important distinguishing feature.

In nearly all cases the tectonic setting is inferred to be that of island arcs, rifted arcs, or nascent back-arc rifts. An association with older continental crust also appears to have been important for a number of deposits, both on the modern seafloor and in ancient volcanic belts (e.g., Okinawa Trough, Boliden, Eskay Creek). On the modern seafloor, deposits in island arc settings or those associated with the early stages of arc rifting have higher gold contents than deposits on the mid-ocean ridges or in mature back-arc basins in the advanced stages of opening. This may reflect a fundamental petrogenetic control on gold enrichment or greater contributions of magmatic volatiles associated with near-arc magmas. A number of gold-rich VMS have been found along the active volcanic fronts of modern arcs (e.g., Izu Bonin arc and Southern Kermadec arc). This setting contrasts with the majority of ancient VMS, which formed in back-arc rifts, and raises

Two main geochemical associations are evident in gold-rich VMS: a copper-gold association, typical of pyritic gold and copper—gold stockwork deposits, and a zinc-gold association, typical of auriferous polymetallic sulfides. A strong gold-barite association is also present in many Phanerozoic auriferous polymetallic sulfide deposits. The mineralogy of the gold-bearing ores is typically more complex than in gold-poor massive sulfides

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Spectrum of gold-rich VMS deposits from the Archaean to the present

(e.g., the Boliden deposit contained nearly 50 different ore minerals). The gold-rich ores commonly include a suite of complex sulfosalts, high-sulfidation Cu-minerals, abundant gold-silver—bismuth-tellurides or selenides, or an unusual abundance of arsenopyrite (e.g., 6.8 wt% As at Boliden). A number of deposits are distinguished from ordinary VMS by strong enrichments in the epithermal suite of elements (e.g., Ag, As, Sb, and Hg at Eskay Creek), and some auriferous polymetallic deposits contain rare minerals such as orpiment, realgar, stibnite, and cinnabar that are not normally found in VMS. However, not all deposits with distinctive epithermal characteristics are gold-rich (e.g., Selbaie, northwestern Quebec). Pyritic gold and copper-gold deposits typically have much higher Au/Ag ratios than auriferous polymetallic sulfides and may also possess notable copper—gold-bismuth— tellurium (±Se±In±Sn) associations similar to those of many intrusion-related gold deposits. In copper-gold deposits, the gold is present mainly as the native metal and as Au-tellurides; auriferous polymetallic sulfides typically contain electrum, which may be silver-rich or mercurian. In some deposits, arsenic-rich pyrite and arsenopyrite are the principal gold hosts. This gold may be refractory (present as submicroscopic inclusions or structurally bound in the crystal lattice), but in older, metamorphosed deposits the recrystallisation that accompanies deformation may result in reconcentration of this gold in a recoverable form at grain boundaries (e.g., brecciated arsenopyrite ore at Boliden).

some highly deformed and metamorphosed regions, it may be difficult to distinguish between aluminosilicate assemblages derived from acid-leached volcanic rocks and those produced by metamorphism of unalteredpelitic rocks (e.g., Carolina Slate Belt). A lack of biotite (±cordierite ±garnet±anthophyllite) in the aluminosilicate-rich zones usually indicates that the metamorphic assemblage did not form from pre-existing sericite—chlorite alteration. Recognition of this type of alteration and mineralisation in the VMS environment has important implications for exploration, as the volatiles from a high-level degassing magma may contribute substantially to the gold content of the mineralising system. However, aluminosilicaterich alteration is also found in association with goldpoor massive sulfides (e.g., Sturgeon Lake), and some aluminosilicate-rich alteration zones are found with little or no sulfide mineralisation at all. This situation may be analogous to subaerial epithermal systems, in which volatile-rich fluids readily escape to the surface, causing widespread argillic and advanced argillic alteration, while metal-rich fluids reside at greater depths. Other gold-rich VMS of the auriferous polymetallic type (Eskay Creek, Que River) have alteration that more closely resembles the quartz—adularia—sericite assemblages found in lowsulfidation epithermal gold deposits. And other goldrich VMS have discordant alteration pipes that are not noticeably different from those of ordinary massive sulfide deposits (e.g., Home). The result is a wide range of alteration types that must be considered during exploration.

Bornite is especially common in some deposits, including both copper—gold deposits (e.g., Bousquet No. 2) and auriferous polymetallic sulfides (e.g., DumagamiLaRonde). Similar bornite-rich ores in ordinary VMS are also sometimes gold-rich, and may reflect similar conditions of mineralisation (e.g., HW-Myra Falls and some Kuroko deposits). The presence of distinctive highsulfidation sulfide minerals, including bornite-pyrite, enargite-tennantite, luzonite, chalcocite, and digenite in some deposits may be analogous to that of highsulfidation copper—gold deposits. The ore zones of these deposits are commonly hosted by distinctive aluminous alteration, including kaolinite, diaspore, pyrophyllite, and alunite or their metamorphosed equivalents (quartz-pyrite—andalusite—muscovite schists at Boliden and Bousquet—LaRonde). The aluminous nature of this alteration is interpreted to reflect strong acid leaching of a type normally associated with high-sulfidation epithermal systems. Zones of advanced argillic alteration are found in closest proximity to the ore, whereas chlorite— sericite alteration more typical of seawater-dominated hydrothermal systems may be present at the margins. The outward zonation from advanced argillic to chloritic alteration most likely reflects mixing of highly acidic ore fluids with seawater adjacent to the upflow zones. In

Controls on gold enrichment The range of different deposit types and the variability in gold grades implies that there are a number of different factors that influence gold enrichment. Tectonic setting and host-rock geochemistry appear to be important first order controls. Second order controls include aspects of the ore fluid evolution and precipitation mechanisms, which, irrespective of the source of gold, dictate whether gold can be transported in the hydrothermal fluids and whether it is likely to be concentrated with other metals at the seafloor. Different P—T paths for the fluids, redox controls, and fluid—rock interaction can affect the ability of the fluids to transport gold to the seafloor, and different processes such as conductive cooling, mixing, oxidation, and boiling can affect the efficiency of gold deposition. Third order controls include post-depositional processes such as hydrothermal reworking and zone refining (including possible over refining), seafloor weathering, supergene enrichment, overprinting by later hydrothermal fluids, and metamorphism. Although processes by which gold may become geochemically enriched in ordinary VMS are generally known, it seems unlikely

81

Spectrum of gold-rich VMS deposits from the Archaean to the present

(e.g., the Boliden deposit contained nearly 50 different ore minerals). The gold-rich ores commonly include a suite of complex sulfosalts, high-sulfidation Cu-minerals, abundant gold—silver—bismuth—tellurides or selenides, or an unusual abundance of arsenopyrite (e.g., 6.8 wt% As at Boliden). A number of deposits are distinguished from ordinary VMS by strong enrichments in the epithermal suite of elements (e.g., Ag, As, Sb, and Hg at Eskay Creek), and some auriferous polymetallic deposits contain rare minerals such as orpiment, realgar, stibnite, and cinnabar that are not normally found in VMS. However, not all deposits with distinctive epithermal characteristics are gold-rich (e.g., Selbaie, northwestern Quebec). Pyritic gold and copper—gold deposits typically have much higher Au/Ag ratios than auriferous polymetallic sulfides and may also possess notable copper-gold—bismuthtellurium (±Se±In±Sn) associations similar to those of many intrusion-related gold deposits. In copper-gold deposits, the gold is present mainly as the native metal and as Au-tellurides; auriferous polymetallic sulfides typically contain electrum, which may be silver-rich or mercurian. In some deposits, arsenic-rich pyrite and arsenopyrite are the principal gold hosts. This gold may be refractory (present as submicroscopic inclusions or structurally bound in the crystal lattice), but in older, metamorphosed deposits the recrystallisation that accompanies deformation may result in reconcentration of this gold in a recoverable form at grain boundaries (e.g., brecciated arsenopyrite ore at Boliden).

some highly deformed and metamorphosed regions, it may be difficult to distinguish between aluminosilicate assemblages derived from acid-leached volcanic rocks and thoseproducedbymetamorphism of unalteredpelitic rocks (e.g., Carolina Slate Belt). A lack of biotite (±cordierite +garnet±anthophyllite) in the aluminosilicate-rich zones usually indicates that the metamorphic assemblage did not form from pre-existing sericite-chlorite alteration. Recognition of this type of alteration and mineralisation in the VMS environment has important implications for exploration, as the volatiles from a high-level degassing magma may contribute substantially to the gold content of the mineralising system. However, aluminosilicaterich alteration is also found in association with goldpoor massive sulfides (e.g., Sturgeon Lake), and some aluminosilicate-rich alteration zones are found with little or no sulfide mineralisation at all. This situation may be analogous to subaerial epithermal systems, in which volatile-rich fluids readily escape to the surface, causing widespread argillic and advanced argillic alteration, while metal-rich fluids reside at greater depths. Other gold-rich VMS of the auriferous polymetallic type (Eskay Creek, Que River) have alteration that more closely resembles the quartz—adularia-sericite assemblages found in lowsulfidation epithermal gold deposits. And other goldrich VMS have discordant alteration pipes that are not noticeably different from those of ordinary massive sulfide deposits (e.g., Home). The result is a wide range of alteration types that must be considered during exploration.

Bornite is especially common in some deposits, including both copper—gold deposits (e.g., Bousquet No. 2) and auriferous polymetallic sulfides (e.g., DumagamiLaRonde). Similar bornite-rich ores in ordinary VMS are also sometimes gold-rich, and may reflect similar conditions of mineralisation (e.g., HW—Myra Falls and some Kuroko deposits). The presence of distinctive highsulfidation sulfide minerals, including bornite—pyrite, enargite-tennantite, luzonite, chalcocite, and digenite in some deposits may be analogous to that of highsulfidation copper-gold deposits. The ore zones of these deposits are commonly hosted by distinctive aluminous alteration, including kaolinite, diaspore, pyrophyllite, and alunite or their metamorphosed equivalents (quartz-pyrite—andalusite—muscovite schists at Boliden and Bousquet—LaRonde). The aluminous nature of this alteration is interpreted to reflect strong acid leaching of a type normally associated with high-sulfidation epithermal systems. Zones of advanced argillic alteration are found in closest proximity to the ore, whereas chlorite— sericite alteration more typical of seawater-dominated hydrothermal systems may be present at the margins. The outward zonation from advanced argillic to chloritic alteration most likely reflects mixing of highly acidic ore fluids with seawater adjacent to the upflow zones. In

Controls on gold enrichment The range of different deposit types and the variability in gold grades implies that there are a number of different factors that influence gold enrichment. Tectonic setting and host-rock geochemistry appear to be important first order controls. Second order controls include aspects of the ore fluid evolution and precipitation mechanisms, which, irrespective of the source of gold, dictate whether gold can be transported in the hydrothermal fluids and whether it is likely to be concentrated with other metals at the seafloor. Different P—T paths for the fluids, redox controls, and fluid-rock interaction can affect the ability of the fluids to transport gold to the seafloor, and different processes such as conductive cooling, mixing, oxidation, and boiling can affect the efficiency of gold deposition. Third order controls include post-depositional processes such as hydrothermal reworking and zone refining (including possible over refining), seafloor weathering, supergene enrichment, overprinting by later hydrothermal fluids, and metamorphism. Although processes by which gold may become geochemically enriched in ordinary VMS are generally known, it seems unlikely

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M. D. Hannington that the spectacular gold contents of some deposits can be adequately explained by precipitation from highly undersaturated ore fluids. The very high gold-to-base metal ratios in these deposits require an inherently goldrich fluid and an efficient means of concentrating gold at the seafloor. Clues to the origin of this enrichment lie in the fact that the most gold-rich deposits have a number of attributes, apart from their high gold contents, that clearly distinguish them from ordinary VMS (e.g., atypical ore mineral assemblages and alteration).

volatile species (As, Sb, Hg) above the boiling zone. The extent of boiling is therefore a major control on the bulk composition of the deposits that are likely to form at the seafloor. A simple model, which equates fluid temperatures with pressures on the boiling curve for seawater, illustrates this point (Fig. 1). Although water depth is an important consideration in the VMSepithermal transition, deposits with similarities to high sulfidation-type epithermal systems may form at any water depth (Sillitoe et al., 1996).

Temperature-dependent solubility controls, in large part, account for the two main geochemical associations of gold found in VMS (i.e., high-temperature Cu—Au associations and lower temperature Zn—Pb-Ag—Au associations: Hannington et al., 1999; Huston 2000). However, the wide range of co-enrichments in other trace elements (e.g., Cu—Co—Bi—Se—Te±Sn±In in some Cu-Au assemblages and Ag—As—Sb—Hg±Tl in some Z n Au assemblages) suggests that other factors may also be important. The distinctive epithermal-like characteristics of some deposits may be related to formation in shallow water and subseafloor boiling, whereas the presence of high-sulfidation copper-minerals and advanced argillic alteration may indicate contributions from a magmatic source deeper in the system (e.g., magmatic brine). The recent findings of copper-rich magmatic fluid trapped in melt inclusions from andesite in the Eastern Manus Basin provide important clues to additional sources of metal, including gold, that might be present in the deep parts of some seafloor hydrothermal systems (Yang and Scott, 1996). Thus many of the factors considered to be important for the formation of volcanogenic gold deposits in the porphyry-epithermal environment may also be important for enrichment of gold in VMS (e.g., Sillitoe 1995, Hedenquist et al., 2000).

Sources and sinks for gold in VMS systems Evidence from modern seafloor vents indicates that simple hydrothermal convection of seawater and leaching of the underlying volcanic rocks can deliver significant amounts of gold to the seafloor. Gold concentrations in the quenched products of high-temperature (35O°C) black smoker fluids at mid-ocean ridges indicate end-member fluid concentrations of about 0.1 |xg/kg or 0.1 ppb Au (Hannington et al., 1991). At these concentrations, a black smoker vent field with a total mass flux of 100500 kg/s of high-temperature fluid can transport more than 1 million oz. Au (>30 t Au) to the seafloor in less than 10 years. In the open oceans, most of this gold is lost to a diffuse hydrothermal plume. As with other metals, construction of a sulfide mound on the seafloor or precipitation of the sulfides below the seafloor is necessary to capture this gold. However, the efficiency of this process is very low. An important exception is the precipitation of metals from metalliferous brines of the Atlantis II Deep in the Red Sea, which have deposited 90 Mt (dry weight) of metalliferous mud with an average gold grade of 0.5 g/t Au. The amount of gold contained in the Atlantis II Deep (45 t Au) is close to that which would be expected from 100% efficiency of deposition from a typical mid-ocean ridge black smoker system (see Hannington et al., 1991, and references therein).

A number of the deposits forming in modern arc environments are transitional in character between deep-sea hydrothermal vents and subaerial hot springs and exhibit characteristics of both VMS and epithermal gold-depositing systems. This comparison is supported by the fact that many active gold-depositing systems on emerging arc volcanoes of the western Pacific are close to sea level and that similar volcanic and hydrothermal activity commonly extends offshore into nearby shallow submarine environments. Here, boiling is an important mechanism for sulfide deposition because of cooling of the fluids during decompression and the large increase in pH andyO that accompanies the loss of dissolved gases. As a boiling fluid rises to the seafloor and cools, the base metals carried as chloride complexes at high temperatures may be precipitated within a vertically-extensive stockwork zone, whereas gold, which is transported as an aqueous sulfur complex, may be effectively separated from the base metals and concentrated along with other

Cu-rich sulfides in black smoker chimneys on the mid-ocean ridges contain about 200 ppb Au (Hannington et al., 1995). This is close to the measured Au concentrations in the quenched products of hightemperature fluids emanating from the same chimneys, Similar high-temperature, copper-rich sulfides in some black smoker chimneys from volcanic arcs in the western Pacific contain orders of magnitude more gold than in samples from the mid-ocean ridges (e.g., >10 ppm Au in Cu-rich chimneys from Suiyo Seamount on the IzuBonin arc and in the Pacmanus deposits in the Eastern Manus basin: Watanabe and Kajimura 1994, Moss and Scott 2001). If these copper-rich samples similarly reflect the composition of the highest temperature endmember fluids, then gold concentrations in vent fluids from arc volcanoes may be on the order of 10 [Ag/kg or

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Spectrum of gold-rich VMS deposits from the Archaean to the present

Figure 1 Schematic model of the VMS-epithermal transition. In subaerial hot spring, boiling of the hydrothermal fluids results in a highly telescoped system. Base metals are deposited as polymetallic veins at depth, with high-grade gold and silver concentrated above the boiling zone (after Buchanan 1981). In submarine hot springs, fluids will rise to the seafloot along the boiling curve for seawater and fractionate base and precious metals duting cooling, depositing Cu and Zn at depth while transporting Au, As, Sb and Hg to seafloor. Submerging the hydrothermal system in deeper water will compress the boiling zone, limit the separation of base and precious metals, and reduce the vertical extent of mineralization. Polymetallic ore is deposited, together with gold, as auriferous massive sulfide on the seafloor (cf. Eskay Creek). If the water is too deep, boiling does not occur, and the high-temperature fluids arrive at the seafloor to form black smoker vents. In this case, much of the gold may be lost to a diffuse hydrothermal plume.

level magmatic-hydrothermal systems in andesitic arc volcanoes such as White Island, New Zealand, which has been shown to be emitting significant quantities of Cu, Au and Bi (e.g., 110,000 kg/yr Cu, 2190 kg/yr Bi and >36 kg/yr Au: Le Cloarec et al., 1992; Hedenquist et al., 1993). Similar quantities of metal can be expected to be available to seafloor hydrothermal systems in submarine arc volcanoes. In just 10,000 years, the amount of Cu and Au that might be delivered to the seafloor via a magmatic vapor plume, similar to that at White Island, would be on the order of 1 Mt Cu and 360 t Au (Hedenquist and Lowenstern, 1994) — an amount of metal roughly equivalent to that contained in the Home deposit or the entire Bousquet district.

10 ppb Au. At these high concentrations, the fluids may deposit gold at much higher temperatures, producing the observed Cu-Au association in some deposits. The origin of the high gold concentrations remains uncertain but probably reflects contributions to the high-temperature end-member fluids from magmatic sources. Certain trace elements, such as Bi and Te, that are closely associated with gold in ancient copper-gold deposits have not been detected in mid-ocean ridge black smoker fluids. However, high concentrations of Bi (up to 200 ppm) and Te (up to 150 ppm) have been found in some Cu—Au-rich chimneys in western Pacific arc settings (e.g., Watanabe and Kajimura 1994, Moss and Scott 2001), possibly indicating a link to a felsic magmatic source similar to that suggested for Cu-Co—Bi-Se—Te±Sn±In-Au associations in subaerial magmatic-hydrothermal systems. Isotopically distinct fluids at several modern seafloor vents in the western Pacific provide positive evidence for the presence of magmatic volatiles in the hydrothermal fluids (see review in Hannington et al., 1999). Such volatiles are considered to be the primary source of metals in a number of high-

Significance for exploration As a group, gold-rich VMS deposits possess a number of characteristics that are important for exploration: (1) Gold-rich deposits may occur in any base metal district, including established mining camps where no such deposits are presently known. In many cases, they

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M, D. Hannington suggests that this setting may be particularly important for gold-rich VMS. Such shallow-water environments are underexplored and may be targeted for a wide range of epithermal gold-base metal deposits. (10) Uniquely gold-rich systems may result from superposition of multiple mineralising events (e.g., epithermal-style mineralisation superimposed on preexisting massive sulfides), making these hybrid deposits particularly attractive for exploration. Overlapping styles of alteration and mineralisation within a single volcanic complex may reflect multiple intrusive events or changing environmental conditions (e.g., gradual emergence or submergence of an arc volcano). The result is a wide range of potential targets that cannot be readily assigned to a single genetic model.

co-exist with other massive sulfide deposits that do not contain anomalous gold. (2) Gold-rich VMS occur in the same volcanic sequences that host other massive sulfide deposits but are commonly associated with large volumes of rhyolite and high-level felsic intrusions. (3) Stratiform massive sulfides typically comprise at least some of the ore, although many pyritic gold and copper-gold deposits consist mainly of disseminated or pipe-like stockwork zones (e.g., subvolcanic replacement deposits). Because of their large dimensions and consistent ore grades, these stockworks may be amenable to open pit mining. (4) Barren pyrite lenses and siliceous or barite-rich caps are commonly present. In some cases, gold-bearing ores are found stratigraphically below large deposits of relatively gold-poor massive sulfides (e.g., No. 5 Zone at Home). (5) The ores may contain complex assemblages of minerals, including abundant sulfosalts, bornite-pyrite, enargite-tennantite, arsenopyrite, or tellurides. They are commonly enriched in the epithermal suite of elements (e.g., Ag, As, Sb, Hg) or other elements such as cobalt, bismuth, tellurium, selenium, indium and tin. By analogy with high-sulfidation copper-gold gold deposits, some gold-rich VMS may have formed by direct contributions of metals from magmatic volatiles or a related brine phase derived from a feslic subvolcanic intrusion.

References Allen, R.L., Weihed, P., and Svensson, S-A., 1996, Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and fades architecture of a 1.9 Ga marine volcanic arc, Skellefte district, Sweden. Economic Geology, 91: 10221053. Barrie, C.T. and Hannington, M.D., 1999. Chapter 1. Classification of volcanic-associated massive sulfide deposits based on host-rock composition. Reviews in Economic Geology, 8: 1—11.

Buchanan, L.J., 1981. Precious metal deposits associated with volcanic environments in the Southwest. Arizona Geological Society Digest, 14: 237—262.

(6) At the mine scale, the presence of advanced argillic alteration or aluminosilicate-rich rocks may be a particularly useful guide to exploration, as the volatiles from a high-level degassing magma that cause this alteration may also contribute substantially to the gold content of the mineralising system. However, it is an important observation that similar alteration may occur in areas where productive mineralisation is absent, and not all gold-rich deposits possess aluminous alteration assemblages (e.g., Home, Eskay Creek).

Hannington, M.D., Herzig, P.M. and Scott, S.D., 1991. Chapter 8. Auriferous hydrothermal precipitates o the modern seafloor. In R.P. Foster (ed.), GoldMetallogenj and Exploration, Glasgow, Blackie and Son: 249-282. Hannington, M.D., Jonasson, I.R., Herzig, P.M. and Petersen, S., 1995. Physical and chemical processes of seafloor mineralization at mid-ocean ridges. AGU Monograph 5>7: 115-157. Hannington, M.D., Poulsen, K.H., Thompson, J.F.H. and Sillitoe, R.H., 1999. Chapter 14. Volcanogenic gold in the massive sulfide environment. Reviews in Economic Geology, 8: 325-356. Hedenquist, J.W., Arribas, A.R. and Gonzalez-Urien, E.., 2000. Chapter 7. Exploration for epithermal gold deposits. Reviews in Economic Geology, 13: 245-277. Hedenquist, J.W., Simmons, S.E, Giggenbach, W.E and Eldridge, C.S., 1993. White Island, New Zealand, volcanic hydrothermal system represents the geochemical environment of high-sulfidation Cu and Au ore deposition. Geology, 21: 731-734. Herzig, P.M. and Hannington, M.D., 1995. Polymetallic massive sulfides at the seafloor: A review. Ore Geology Reviews, 10: 95-115. Huston, D.L., 2000. Chapter 12. Gold in volcanic-hosted massive sulfide deposits: Distribution, genesis, and exploration. Reviews in Economic Geology, 13: 401426. Le Cloarec, M.F., Allard, P., Ardouin, B., Giggenbach, W.E

(7) Deposits in older volcanic terranes may be significantly upgraded as a result of structural and metamorphic redistribution and reconcentration of gold. Although a spatial association with late structural breaks or other intense zones of deformation is often found, this should not be regarded as essential for exploration. (8) In modern volcanic settings, gold-rich VMS are most common in rifted-arc and incipient back-arc environments rather than mature back-arc spreading centers. An association with rifted continental crust and continental margin arcs may be particularly important in some cases (Okinawa Trough, Boliden, Eskay Creek). In ancient volcanic belts, these settings can be readily identified by volcanic geochemistry. (9) The occurrence of some high-grade gold deposits in areas of transitional subaerial-to-submarine volcanism

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Spectrum of gold-rich VMS deposits from the Archaean to the present

and Sheppard, D.S., 1992. Radioactive isotopes and trace elements in gaseous emissions from White Island, New Zealand. Earth and Planetary Science Letters, 108: 19-28. Moss, R. and Scott, S.D., 2001. Geochemistry and mineralogy of gold-rich hydrothermal precipitates from the Eastern Manus Basin, Papua New Guinea. Canadian Mineralogist, 39: 957-978. Poulsen, K.H. and Hannington, M.D., 1995. Volcanicassociated massive sulfide gold. In Eckstrand, R.O., Sinclair, W.D. and Thorpe, R.I. (eds.), Geology of Canadian Mineral Deposit Types. DNAG Vol. P-l, Geology of Canada, No. 8: 183-196. Poulsen, K.H., Robert, F. and Dube, B., 2000. Geological classification of Canadian gold deposits. Geological Survey of Canada Bulletin 540: 106 pp. Sillitoe, R.H., 1995. The influence of magmatic-hydrothermal models on exploration strategies in volcanoplutomc arcs. Mineralogical Association of Canada Short Course, 23:511-525. Sillitoe, R.H., Hannington, M.D. and Thompson, J.F.H., 1996. High-sulfidation deposits in the volcanogenic massive sulfide environment. Economic Geology, 91: 204-212. Watanabe, K. and Kajimura, T, 1994. The hydrothermal mineralization at Suiyo Seamount in the central part of the Izu-Ogasawara arc. Resource Geology, 44: 133-140. Yang, K. and Scott, S.D., 1996. Possible contribution of a metal-rich magmatic fluid to a seafloor hydrothermal system. Nature, 383: 420-423.

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Agnico-Eagle's LaRonde mine — a world-class gold-rich VMS deposit GUYGOSSELIN

Since the beginning of the 20th century, exploration in the Abitibi Greenstone Belt has lead to the discovery of significant gold and base metal deposits. In the Quebec portion of the belt, the world-class Au-rich Home VMS deposit (54 Mt @ 2.2% Cu and 6.1 g/t of Au; >10 Moz Au) was discovered in the Rouyn-Noranda camp in 1920. In the Val d'Or camp, the structurally controlled vein system of the Sigma-Lamaque deposit (>46 Mt @ 6 g/t of Au; 9 Moz Au) was discovered in 1933 along the Cadillac-Larder Lake Break.

The LaRonde mine lies between these two major mining camps, within the prolific 25 Moz Au Bousquet mining camp (Fig. 1). Prior to the 1970s, small deposits were put into production along the Cadillac—Larder Lake Break in the Cadillac area. However, the emergence of the Bousquet camp took place in the late 1970s and now it is the most important gold camp in Quebec and one of the most important in Canada. Since the beginning of mining at LaRonde in 1988, Agnico-Eagle Mines Ltd has produced more than 2.4 Moz of gold, 13.5 Moz of silver, 0.051 Mt of copper and 0.180 Mt of zinc from 14.2 Mt of ore. Proven and probable reserves stand at 37.8 Mt @ 4.1 g/t Au, 56.2 g/t Ag, 0.33% Cu and 2.6% Zn for 5.0 Moz of Au and

Cheif Geologist, Mine Office-LaRonde Division P.O. Box 400, Cadillac, Quebec JOY ICO

Figure 1 Location map showing the Abitibi SubProvince of the Canadian Shield. Modified from Daigneault et al. (1990).

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resources at 17.2 Mt grading 5.8 g/t Au, 36.5 g/t Ag 0.34% Cu and 1.0 % Zn for 3.2 Moz Au (Fig. 2). Total production-reserves and resources are now estimated at 69 Mt hosting 10.6 Moz Au, 102 Moz Ag, 0.23 Mt Cu and 1.34 Mt Zn on the LaRonde property. Exploration work on the LaRonde property goes back to the 1930s when outcrop stripping and trenching led to the discovery of quartz—sulfide veins and massive sulfide mineralisation in 1937 near the western limit of the property. In 1960s and 1970s sporadic exploration campaigns took place on the property. In 1985, a 435 m exploration shaft was sunk to allow underground drifting, sampling and diamond drilling as surface exploration continue to progress.

of the property on surface and from underground along an exploration drift located at a depth of 860 m. The surface program resulted in the discovery of three new surface to sub-surface lenses: zone 4 (1991), and zones 6 and 7 (1992). Underground exploration led to the major discovery of new sulfide lenses at depth (zones 6, 7, 20 North Gold, 20 North Zinc and 20 South) in 19921993. In 1994, development of zones 6 and 7 was initiated at Shaft #2, while major underground exploration and development of the deep resources started with the sinking of the Penna Shaft. In 2000, the mine and mill facilities were expanded to reach the daily production rate of 4500 tons per day and the Penna shaft was commissioned. At 2250 m depth, this is the deepest single lift shaft of the western hemisphere (Fig. 3). A subsequent expansion project in 2002 brought the LaRonde mine and mill complex to the actual production rate of 7000 tons per day.

The turning point in the exploration program on the LaRonde property occurred in 1986 when the West zone was discovered at Shaft #1 intersecting 7.76 g/t Au over 9.1m at 854 m depth. The exploration campaign that followed that discovery hole led to a positive feasibility study. Shaft #1 was deepened to 975 m, a mill facility was constructed and the mine was put into production in October 1988 at a rate of 1360 tons per day. In 1990, an aggressive exploration program was initiated. This covered the unexplored eastern portion

Since 1999, exploration work has moved to new depths and is now taking place along an exploration drift situated on level 215 (2150 m depth). A major exploration program is presently ongoing below the bottom of the Penna Shaft between 2 and 3 km below

Figure 2 Longitudinal view of the LaRonde mine property with reserve and resources outline and past production.

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Agnico-Eagle's LaRonde mine — a world-class gold-rich VMS deposit

the vicinity of shaft no. 1 to over 550 m in the Penna shaft area. This stratigraphic interval is characterised by a dominance of quartz and feldspar phyric rhyodacitic to rhyolitic flows and coarse to fine-grained flow breccia deposits. Andesitic to dacitic flows and flow breccia deposits are common in the northern part of the unit, whereas blue and grey-quartz phyric horizons occur in the southern portion of the unit. Minor andesite flows or sills horizons have also been observed (Mercier-Langevin etal., 2004). Regional scale alteration zones hosts steeply dipping, east-trending, anastomosing shear zone structures (Marquis et ah, 1992;Tourigny et ah, 1988). These highstrain zones comprise a larger structure, the DoyonDumagami Structural Zone, which has been traced over 10 km within the Blake River Group from the LaRonde property westward to the Doyon—Mouska properties. Among the most important deposits of the camp, the LaRonde deposit (10.6 Moz Au), the Doyon deposit (8 Moz Au) and the Bousquet deposit (5 Moz Au) host most of the known reserves and resources (Fig. 5). Several other small occurrences are also known along the belt. At the LaRonde mine, more than a dozen economic massive to disseminated polymetallic sulfide lenses are known. These vary in size from 50,000 t to >20 Mt and are hosted within four different mineralised horizons. The mineralised zones are generally east-trending and dip steeply to the south (parallel to the geological fabric). The most important lens, the 20 North zone, has thicknesses that can reach up to 40 m along the north-trending axis, 600 m along the east-trending axis and can be traced over 2.3 km along its steeply dipping southwest-trending long axis.

Figure 3 Surface facilities at LaRonde Pcnna Shaft installation. surface. The program is focused on the deep extension of the deposit where reserves stand at 18.3 Mt (3.4 Moz Au) and resources at 16.3 Mt (3.1 Moz Au). Geologically, the LaRonde property is located within the Archaean (2.7 Ga) Abitibi Sub-Province (near it's southern boundary with the Pontiac Sub-Province) of the Superior Province of the Canadian Shield (Goodwin and Ridley, 1970). The most important regional structure is the Cadillac—Larder Lake break, which marks the contact between the Abitibi and the Pontiac sub-provinces, located approximately 2 km to the south of the LaRonde property. The geology that underlies the LaRonde mining property consists of three east-trending, steeply southdipping and southward-facing regional lithological units. These units are, from north to south: (1) the Kewagama Group which is made up of thick bands of interbedded wacke; (2) the Blake River Group, a volcanic assemblage which hosts all the known economic mineralisation on the LaRonde property and the massive sulfide deposits of the Noranda mining camp; and (3) the Cadillac Group, made up of thick bands of wacke interbedded with pelitic schist and minor iron formations (Dimroth et al., 1982). At LaRonde, the Blake River Group is composed of the Hebecourt and Bousquet Formations (Fig. 4). The regional sequence shows a basement of basalt flows overlain by andesitic to rhyolitic flows, domes and fragmental rocks associated with local volcanic centres (Lafrance et al., 2003). Three members present on the property have been identified regionally, from north to south: (1) the Northern Tholeiitic Basalt member within the Hebecourt Formation, (2) the Lower Transitional member, and (3) the Upper Felsic member within the Bousquet Formation. The Upper Felsic member, which hosts all the significant gold and base metal mineralisation on the LaRonde property, varies in thickness from 150 m in

Strong alteration zones at LaRonde are typically associated with the different mineralised zones (Fig. 6). Rhyodacite occurs in the footwall of the most important lens, the 20 North zone. It is characterised by a banded quartz-garnet—biotite-sericite major alteration zone. The second most important lens, the 20 South zone, is most commonly hosted within andesitic flows or sills characterised by a quartz—sericite-green micas-titanite alteration zone (Dube et al., 2004). The 20 North lens is a south facing sequence that contains two types of mineralisation: a gold-copperrich portion (the 20 North Gold Zone) at the bottom and a zinc—silver—lead-rich upper portion (the 20 North Zinc Zone; Fig. 7). In the higher portion of the deposit (closer to surface), the gold—copper-rich mineralisation is mostly restricted to the highly transposed sericitised pyrite-chalcopyrite stinger zone at the bottom of the lens whereas the zinc—silver—lead-rich mineralisation is located within the massive sulfide lens in a 'low sulfidation' volcanogenic massive sulfide environment (Dube et al., 2004).

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Agnico—Eagle's LaRonde mine — a world-class gold-rich VMS deposit

Figure 5 Aerial view looking west of the Bousquet mining camp.

Figure 6 Alteration zonation surrounding the sulfide lenses of the LaRonde deposit. From Dube et al. (2004).

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Dimroth, E., Imreh, L., Rocheleau, M., and Goulet, N., 1982, Evolution of the south-central part of the Archean Abitibi belt, Quebec. Part I: stratigraphy and paleogeographic model: Canadian Journal of Earth Sciences, 19, p. 1729-1758. Dube, B., Mercier-Langevin, P., Hannington, M. D., Davis. D. W., Lafrance, B., 2004, Le gisement de sulfures massifs volcanogenes auriferes LaRonde, Abitibi, Quebec: alteration, mineralisation genese et implications pour l'exploration : Ministere des Ressources Naturelles, Quebec, MB 2004-03, 112 p. Goodwin, A.M., and Ridley, R.H., 1970, The Abitibi orogenic belt. In: A.J. Baer, (Ed.), Symposium on basins and geosynclines of the Canadian Shield: Geological Survey of Canada, Paper 70-40, p. 1-30. Lafrance, B., Moorhead, ]., Davis, D.W., 2003, Cadre geologique du camp minier de Doyon-BousquetLaRonde : Ministere des Ressources Naturelles, Quebec; ET 2002-07, 45 p. et cartes ET 2002-07C001 au 1 :20 000. Marquis, P., Hubert, C, Brown, A.C., Scherkus, E., Trudel, E and Hoy, L.D., 1992, Geologie de la mine Donald J. LaRonde (Dumagami), Cadillac Quebec : Ministere de l'Energie et des Ressources du Quebec, ET 89-06. Mercier-Langevin, P., Dube, B., Hannington, M. D., Davis, D.W., Lafrance, B., 2004, Contexte geologique et structural des sulfures massifs volcanogenes auriferes du gisement LaRonde, Abitibi : Ministere des Ressources Naturelles, Quebec; ET 2003-03, 47 p. Tourigny, G., Hubert, C, Brown, A.C., and Crepeau, R.. 1988, Structural geology of the Blake River at the Bousquet mine, Abitibi, Quebec: Canadian Journal of Earth Sciences, 25, p. 581-592.

With increasing depth, the sericitised pyrite— chalcopyrite stringer zone and a progressively increasing portion of the massive sulfide type mineralisation become more gold—copper rich. Below 2 km from surface, an increasing abundance of kyanite-andalusite porphyroblasts associated with a more strongly developed silica alteration zone suggest a transition toward a 'highsulfidation' volcanogenic massive sulfide environment of mineralisation, similar to that encountered in LaRonde zone 5—Bousquet 2 ore lens (Dube et al., 2004). Since the late 1970s, the Bousquet mining camp has become one of the most important mining camps in Canada, with production reserves and resources estimated to near 25 Moz Au and a significant amount of base metal within a wide range of sulfide associated synvolcanic mineralisation. The LaRonde mine presents diverse and distinct alteration and mineralisation characteristics that are of great interest for the understanding of gold-rich type of VMS deposits.

References Daigneault, R., Archambault, G., 1990, Les grands couloirs de deformation de la Sous-Province de l'Abitibi. In: Rive, M., Verpaelst P., Gagnon, Y., Lulin, J.M., Riverin, G. and Simard, A., Eds., The Northwestern Quebec Polymetallic Belt: A summary of 60 years of mining exploration: The Canadian Institute of Mining and Metallurgy, Special Volume 43, p. 43-64.

Figure 7 Proposed model for the formation of the Bousquet and LaRonde deposits. From Dube et al. (2004).

92

Recent gold-rich VHMS discoveries at Gossan Hill PETER PRING

The Golden Grove Project is located approximately 225 km east of the port of Geraldton and 375 km northnortheast of Perth, Western Australia (Fig. 1). The 100% Newmont Australia owned mine is located in the southern Murchison Province of the Archaean Yilgarn Craton. Mineralisation was first identified at Gossan Hill in 1971 and 4 km to the north at Scuddles in 1979. Copper and zinc production commenced from Scuddles in 1990 and from Gossan Hill in 1998. The Golden Grove mill has the capacity to treat 1.4 Mt of ore per annum with the zinc and copper concentrates transported by road train to Geraldton. From there the concentrates are shipped to overseas smelters primarily in Asia. At June 2001, the total resource was 10 Mt grading 14.9% Zn, 1.9% Pb, 121g/t Ag 2g/t Au and 14.6 Mt grading 3.7% Cu 0.5g/t Au. Because it is principally a base-metals mining operation, few studies have been undertaken on gold at Golden Grove. Mineralisation is hosted by rhyolitic to dacitic volcaniclastic rocks of the Golden Grove Formation (members GG1-GG6; Fig. 2). The footwall Gossan Valley Formation (GVL-GV4) consists of rhyolitic and andesitic flows and volcaniclastic rocks. The hanging wall Scuddles Formation (SCI—SC4) is dominated by coherent felsic lavas, together with volcanic breccias and minor syn volcanic sediments. The sequence has a subvertical to steep westerly dip and forms part of the eastern limb of a regional syncline in the northwest-trending greenstones of the Yalgoo—Singleton Greenstone Belt. The volcanic hosted massive sulfide (VHMS) mineralisation has been shown to be syngenetic with deposition of the host Golden Grove Formation (Sharpe, 1999).

Units GG1 and GG2 have undergone pervasive silica and chlorite alteration (Fig. 2). The magnetite and sulfide mineralisation in GG4 is thought to have formed by replacement and is surrounded by localized silica and chlorite alteration. Banded silica, chlorite and carbonate alteration of GG6 is associated with massive sulfides. The banded style of alteration is less developed in other well banded horizons (Sharpe, 1999). The hanging wall (SC2) adjacent to the GG6 mineralisation at Gossan Hill has been strongly sericite altered. Zinc mineralisation in GG6 at Gossan Hill is stratabound and contained primarily within the same horizon of bedded siltstone, sandstone and polymict breccia. The main sulfide types are pyrite, sphalerite, galena and chalcopyrite with minor tetrahedrite and trace arsenopyrite, cassiterite and bismuthinite. Copper mineralisation at Gossan Hill is contained within the bedded siltstone, sandstone and volcanic quartz rich pebble breccia of GG4 in a footwall position to GG6. Sulfides in this zone are associated with massive fine grained magnetite and include pyrite, chalcopyrite, lesser sphalerite and pyrrhotite towards the hanging wall. Stringer chalcopyrite beneath the zinc provides some copper resources within GG6 at Gossan Hill. Gold at Golden Grove typically occurs as electrum with varying amounts of silver. Most commonly electrum occurs within the zinc and lead mineralisation. In hand specimen electrum forms 1-2 mm clots in the massive zinc and lead sulfides, or remobilised into fractures within larger quartz veins. Petrographic observations show that electrum occurs together with galena and chalcopyrite within microfractures through the sphalerite and pyrite or associated with 'chalcopyrite disease' through the iron-poor sphalerite (Everett, 1990). Minor electrum in the copper zone is associated with bismuthinite and has a notably higher fineness than electrum seen in the zinc zone (Everett, 1990). Electrum in the zinc zone

Newmont Golden Grove Operations Pty Ltd [email protected] nt.com

93

Peter Pring

Figure 1 Locality map.

has a lower more variable fineness than that seen in the copper zone. This is thought consistent with the metal zonation at Golden Grove, with high temperature sulfide mineralisation at depth (Cu—Fe—Au) passing upwards to lower temperature sulfide mineralisation (Zn-Pb-Ag— Au) near the palaeo-seafloor position (Sharpe, 1999). An oxide gold resource occurs in gossanous remnants of the massive sulfldes in the weathered zone above the GG6 primary zinc mineralisation of the Gossan Hill. In 1998 a program of systematic deep diamond drilling was initiated beneath and to the north of Gossan Hill. Drilling before 1998 was primarily in the top 400 m (vertical) with scattered holes down to 800 m. The main focus of this work was to identify extensions to the known zinc mineralisation in areas of thicker GG6 and possible copper mineralisation in the footwall. Prior to the 1998 drilling, mineralisation was presumed to be confined to the GG6 horizon. With the new drilling major zinc and gold intersections were made in previously poorly tested hanging wall and footwall positions (Figs 3, 4). Recognition of the 'poddy' nature of the GG6 mineralisation also gave encouragement to follow up small uneconomic drill intersections or zones of intense alteration as they may potentially represent a 'near miss' to an economic zone. Some of the larger crosscutting dacites may have intruded along the mineralising fluid conduits (Sharpe, 1999). Consequently, holes that intersect dykes in the target horizon contain no mineralisation yet may be within metres of massive

sulfide. Surface and downhole electrical geophysics have been used to help define massive sulfides, however these methods do not highlight massive sphalerite. Historical gold grades to 2000 were in the order of 1 g/t for both Scuddles and Gossan Hill. Recent discoveries beneath and to the north of Gossan Hill indicate a substantial increase in GG6 gold grades with depth. The discoveries also show two new positions of higher grade gold mineralisation: 1. Pods of higher grade gold associated with pyrite in the GG6 footwall position of some of the zinc deposits such as Amity and Catalpa (Figs 3, 4). This mineralisation is associated with trace Co-Bi—In, characteristic of higher temperature magmatically derived copperrich conduits seen in contemporary seafloor systems (McConachy et al., 2002). These deposits grade around 5.5 g/t Au with individual assays up to 350 g/t. 2. High gold grades associated with lead in zinc mineralisation in siltstones and sandstones of the Scuddles Formation (SC3) above GG6 at the Hougoumont deposit (Fig. 3, 4). This mineralisation is associated with trace Cd-Sb—As—Ba suggesting a low temperature association similar to that seen in modern seafloor systems (McConachy et al 2002).This mineralisation grades 6.1 g/t Au and 347 g/t Ag with individual assays upto lOOg/t Au and 570g/t Ag. At June 2001, the total zinc sulfide resource was 9.97 Mt grading 14.9% Zn, 1.9% Pb, 121 g/t Ag 2g/t Au and

94

Recent gold-rich VHMS discoveries at Gossan Hill

Figure 2 Gossan Hill stratigraphic column (Martyn ,2001). plag = plagioclase, comp. = composition.

95

Peter Pring

Figure 3 Gossan Hill geological model showing the relative position of gold mineralisation (Sharpe, 1999). cp = chalcopyrite. GG1-6, SC2-3 and DAC are stratigraphic units — see Figure 2 for detailed descriptions.

Figure 4 Gossan Hill long section showing massive sulphide deposits and their average gold grades.

96

Recent gold-rich VHMS discoveries at Gossan Hill

the copper sulfide resource wasl4.6 Mt grading 3.7% Cu 0.5g/t Au. Due in part to the new discoveries, the total contained gold at Golden Grove is now approximately 1.4 Moz (including past production). The identification of significant gold resources at Golden Grove greatly aid the economics of the project. That mineralisation can occur outside the traditional GG6 horizon has implications for future exploration along the Golden Grove belt.

References EverettjC.E., 1990, The Siting and Timing of Gold Mineralisation and its Relation to Base Metals in the Gossan Hill Volcanogenic Massive Sulfide Deposit of the Murchison Province, University of Western Australia Honours Thesis (unpublished), llOp. McConachy, T.F., Yeats, C.J., Parr, J.M., Binns, R.A., Fraser, S.J., 2002, Characterisation of Gold Mineralisation at Hougoumont and Amity, Golden Grove, Western Australia., CSIRO Exploration and Mining Report 971C (unpublished), 72p. Martyn, J., 2001, Gossan Hill to Scuddles Geology, Outcrop and Interpretation. Unpublished map. Sharpe, R., 1999, The Archaean Cu-Zn Magnetite Rich Gossan Hill VHMS Deposit, Western Australia, University of Tasmania PhD Thesis (unpublished) 371 p.

97

Characteristics of and exploration for highsulfidation epithermal gold-copper deposits JEFFREY W. HEDENQUIST, RICHARD H. SILLITOE2 AND ANTONIO ARRIBAS JR3

deposits. These are referred to by several synonyms, and with subtypes noted (Table 1). More recent study of the variations in deposit style, from tectonic setting to mineralogy, has led to three environments being identified (Sillitoe, 1989, 1993a; Albino and Margolis, 1991; John etal., 1999; John, 2001; Table 1). Herein, we use the most widely employed nomenclature: high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS). These terms were introduced by Hedenquist (1987), with the IS term added more recently as our understanding grew (Hedenquist et al., 2000). HS deposits contain sulfide-rich assemblages of high sulfidation state, typically pyrite—enargite, pyrite-luzonite, pyrite-famatinite, and pyrite-covellite (Einaudi et al., 2003; Fig. 2), hosted by leached silicic rock with a halo of advanced argillic minerals. In contrast, LS deposits contain the lowsulfidation pair, pyrite-arsenopyrite. The latter sulfide mineral is typically present in only relatively minor quantities, within banded veins of quartz, chalcedony, and adularia plus subordinate calcite. Very minor amounts of copper (typically < 100—200 ppm) are largely present as chalcopyrite or, less commonly, tetrahedritetennantite (Einaudi et al., 2003). Pyrrhotite is present in trace amounts in only some LS deposits (e.g., northern Nevada rift; John and Wallace, 2000; John, 2001). As the name implies, IS deposits possess sulfidation states between those of the HS and LS types, typically with stability of chalcopyrite, tetrahedrite—tennantite, and FeS-poor sphalerite, but lacking appreciable arsenopyrite and pyrrhotite (Einaudi et al., 2003).

Introduction Epithermal gold and silver deposits of both vein and bulk-tonnage styles may be broadly grouped into highsulfidation (HS), intermediate-sulfidation (IS), and lowsulfidation (LS) types based on the sulfidation states of their hypogene sulfide assemblages (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Einaudi et al., 2003). This presentation examines the processes that lead to the formation of HS ore deposits, and reviews their volcanic setting (Fig. 1) and characteristics (Sillitoe, 1993a, 1999; Arribas, 1995; Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003), including the nature and zonation of alteration. Diagnostic criteria that are useful during exploration for and assessment of such deposits are discussed.

Terminology Early classification schemes for volcanic-hosted epithermal deposits, almost all of vein type, were based on their dominant elements and minerals (Emmons, 1918; Lindgren, 1933). Even earlier, however, several distinctive epithermal vein varieties had been noted (Lindgren, 1901), with documentation of type examples of gold— alunite (Goldfield, Nevada; Ransome, 1907) and goldtelluride deposits (Cripple Creek, Colorado; Lindgren and Ransome, 1906) being especially important. During the last 25 years or so, two principal epithermal deposit types (Sillitoe, 1977; Haybaetal., 1985; Bonham, 1986; Heald et al., 1987; Hedenquist, 1987) have been recognised widely in both vein and bulk-tonnage style

Like the early classification schemes for epithermal deposits, the current one is also based on mineralogic criteria although we use hypogene sulfide mineral assemblages to classify the deposits, rather than gangue minerals. We interpret the equilibrium sulfide mineral assemblages in terms of sulfidation state (Barton and Skinner, 1967; Barton, 1970) which is defined in terms

1

99 Fifth Avenue, Suite 420, Ottawa, Ontario K1S 5P5, Canada, [email protected] 2 27 West Hill Park, Highgate Village, London N6 6ND, England 3 Placer-Dome Exploration, Reno, Nevada, USA

99

Jeffrey W. Hedenquist, Richard H. Sillitoe and Antonio Arribas Jr

Figure 1 Schematic section of end-member voicanotectonic setting and associated HS epithermal and related mineralisation types (Sillitoe, 1973; Sillitoe and Hedenquist, 2003). Calc-alkaline volcanic arc with neutral to mildly extensional stress state showing relations between HS and IS epithermal and porphyry deposits (note that the complete spectrum need not be present everywhere). Early magmatic volariles are absorbed into groundwater within the volcanic edifice (shown here as a stratovolcano, but it may also be a dome setting) to produce acidic fluid for lithocap generation, over and/or supra-adjacent to the causative intrusion. Later, less-acidic IS fluid gives rise to IS mineralisation, both adjacent to and distal from the advanced argillic lithocap. Where the IS fluid flows through the leached lithocap environment, it evolves to an HS fluid (Einaudi et al., 2003) to produce HS veins or disseminated mineralisation, depending on the nature of the structural and lithologic permeability. The HS fluid may evolve back to IS stability during late stages, supported by paragenetic relationships and lateral transitions of HS to IS mineralogy.

of the sulfur fugacity and temperature of the mineralising fluid. Barton and Skinner (1967) and Barton (1970), along with subsequent workers, subdivided sulfidation state of mineral assemblages into very low, low, intermediate, high and very high. (Fig. 2). Very low and very high sulfidation states are not common in the epithermal environment, whereas low, intermediate, and high sulfidation state sulfide assemblages characterise three styles of epithermal deposits that are also distinguished on the basis of other criteria such as tectonic setting, magmatic affiliation, and alteration assemblage (Table 2; Sillitoe and Hedenquist, 2003, and references therein). Here we use sulfidation state to classify epithermal deposits, and it is fairly straightforward to reinterpret the early classifications in the context of the current scheme (Table 1). Any scheme is acceptable, as long as there is a consistency and proper division of terminology, to allow a reader to understand what the writer is referring to. In this respect, there is much to be said for using 'type' examples to group deposits, as long as the observer is familiar with the characteristics of the type deposit, as

these characteristics will not change even if a genetic interpretation does (White and Hedenquist, 1995).

Setting and origin Most HS deposits are generated in calc-alkaline andesiticdacitic arcs characterised by near-neutral stress states or mild extension. A few major deposits also occur in compressive arcs characterised by the suppression of volcanic activity (Sillitoe and Hedenquist, 2003). Rhyolitic rocks generally lack appreciable HS deposits. Due to this volcanic affiliation, the origins of HS deposits have an intimate relationship with the syn-magmatic intrusions that lie at depth (Hedenquist and Lowenstern, 1994). Highly acidic fluids (i.e., condensates of magmatic vapor; Ransome, 1907) that contain HC1 and SO, produce a leached core of residual silica with a halo of advanced argillic alteration (Steven and Ratte, I960). Where this residual silicic and advanced argillic alteration is hosted by a lithologic unit that overlies the causative

100

Characteristics of and exploration for high-sulfidation epithermal gold—copper deposits

Figure 2 Log sulfur fugacity versus t e m p e r a t u r e diagram s h o w i n g the variety of sulfide assemblages in epithermal deposits t h a t reflect sulfidation state, from very low a n d low t h r o u g h intermediate to high and very high. C o m p o s i t i o n a l fields of arc volcanic rocks, hightemperature volcanic fumaroles, m a g m a t i c - h y d r o t h e r m a l fluids, and geothermal fluids shown, as discussed by E i n a u d i et al. (2003). Simplified by Sillitoe a n d H e d e n q u i s t (2003) from E i n a u d i et al. ( 2 0 0 3 ; see their figure 4 for mineral abbreviations).

Table 1 N o m e n c l a t u r e for epithermal deposit types Goldfield type Alunitic kaolinic gold

Sericitic zinc-silver veins

veins Gold-alunite deposits

Ransome (1907) Gold-silver-adularia veins Emmons (1918) Fluoritic tellurium-adularia gold veins Gold-quartz veins in andesite Lindgren (1933)

Argentite-gold quartz veins Argentite veins Base metal veins Gold-quartz veins in rhyolite Gold telluride veins Gold selenide veins Sillitoe (1977)

Acid

Alkaline Epithermal Enargite-gold

Hot-spring type High sulfur Acid sulfate High sulfidation Alunite-kaolinite

High sulfidation

HIGH SULFIDATION

Low sulfur Adularia-sericite Low sulfidation Adularia-sericite Type 1 adularia-sericite High sulfide + base metal, low sulfidation INTERMEDIATE SULFIDATION

Note: CAPITALISED names used in this paper Modified from Sillitoe and Hedenquist (2003), with references therein.

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Type 2 adularia-sericite Low sulfide + base metal, low sulfidation LOW SULFIDATION

Buchanan (1981) Ashley (1982) Giles and Nelson (1982) Bonham(1986) Haybaetal. (1985), Healdetal. (1987) Hedenquist (1987) Berger and Henley (1989) Albino and Margolis (1991) Sillitoe (1989, 1993a) Hedenquist et al. (2000)

Jeffrey W. Hedenquist, Richard H. Sillitoe and Antonio Arribas Jr

intrusion, hypogene leaching can create a lithocap (Sillitoe, 1995). The early-formed lithocap itself is essentially barren of metals (Hedenquist et al., 1998, 2000), although in places the lithocap appears to have up to 100 ppb or more gold added during the leaching stage. However, such advanced argillic lithocaps may form prior to HS mineralisation, which itself is due to higher-pH, relatively low-salinity fluids. Early lithocap-forming fluids display clear evidence for a close genetic relationship to magmatism (Rye et al., 1992; Arribas et al., 1995; Hedenquist et al., 1998). Although the linkage is less well-defined, later HS fluids that introduce copper and arsenic, as well as the late fluids responsible for much of the gold (and Bi, Sn, Mo, Te, etc.) introduction, also seem to owe much to their magmatic parentage (Sillitoe and Hedenquist, 2003). Where ascending IS fluids enter lithocaps, they can evolve to HS fluids as the result of cooling in a quartzrich environment that lacks buffering capacity (Einaudi et al., 2003). Eventual neutralisation and lowering of sulfidation state by wallrock interaction can convert HS back to IS fluids (Einaudi et al., 2003), as confirmed by both spatial and paragenetic transitions from HS to IS mineralisation (Jannas et al., 1990, 1999; Claveria, 2001; Fig. 1).

Features of HS deposits, with examples Reviews by Arribas (1995), White et al. (1995), and Sillitoe (1999) of 43 HS deposits highlighted their typical affiliations and host rocks (Tables 2, 3). Of 43 deposits in the circum-Pacific region, half are affiliated spatially with volcanic domes, although the domes themselves typically are not hosts, as they generally have a syn-mineral timing. A quarter of the deposits are affiliated with central vent volcanoes (marginal IS veins are common in this setting), whereas about 10% each occur in caldera and diatreme settings. There is insufficient information to deduce the setting of about a quarter of the deposits. In contrast to their affiliation, the actual host rock to lithocaps and subsequent HS mineralisation is most typically andesitic to dacitic flows, breccias, and pyroclastic rocks, the latter variably welded. Intrusions and sedimentary sequences locally are hosts, particularly where deeper portions of the deposit are exposed. The form of HS deposits varies from disseminated or replacement ore to veins, stockworks and hydrothermal breccia bodies (Sillitoe, 1993a; Table 2). Lithologic and structural controls determine the individual deposit form (Sillitoe, 1999) from porphyry to deep and shallow epithermal levels. In particular, there is a diversity in styles of HS ore that is controlled largely by the changing nature of the permeability from the surface to > 1 km depth. The largest, though lowest grade, deposits formed

mainly at shallow depths, where the system mushroom; into permeable lithologies such as volcaniclastic rocks, lacustrine sediments and, in particular, pyroclastic units, The pyroclastic host rocks exhibit varying degrees of welding, but where welded, they are brittle and fracture easily, and may host disseminated mineralisation. Highgrade vein deposits typically have massive accumulations of pyrite and sulfosalt minerals, and are structurally controlled. HS ore deposits commonly show a large degree of structural control, even within the massive zones of vuggy quartz and disseminated sulfides, as the result of their fracture-related roots. These fractures reflect regional-scale features in some cases, whereas in other cases, the fractures appear to be caused by emplacement of the shallow intrusions to which HS deposits are related (Sillitoe, 1999; Sillitoe and Hedenquist, 2003). Gold mineralisation in HS ore deposits is associated most commonly with enargite or its lower temperature dimorph, luzonite. Such high sulfidation-state coppet sulfides (Fig. 2) typically form early in the paragenesis, with relatively low contents of gold, and are cut by sulfides associated with gold ore (e.g., El Indio, Chile and Lepanto, Philippines; Jannas et al., 1990, 1999; Claveria, 2001). The post-enargite gold ore is associated with pyrite, tennantite—tetrahedrite, chalcopyrite and tellurides. These sulfides have an intermediate sulfidation state (Fig. 2), in contrast to the high sulfidation state of the precursor enargite. By contrast, in some HS deposits, e.g., Summitville, there is a transition from tetrahedrite to enargite with decreasing depth (Stoffregen, 1987), Thus, it appears that the roots of HS deposits contain sulfides of intermediate sulfidation state which evolve upward to high sulfidation state due to cooling (Fig. 2), whereas in the late stages, the high sulfidation state of the fluid evolves to an intermediate sulfidation state (Fig. 2), perhaps due to the influence of the rock buffer (Einaudi et al., 2003). Intermediate sulfidation state sulfides are typical in most epithermal veins hosted by volcanic arcs, They evolve to high sulfidation states only when cooling in rock with no buffering capacity, e.g., in lithocaps. Sides of HS deposits: One of the most common characteristics of HS deposits is the alteration zoning outward from the ore body, as first characterised in an alteration section for the Summitville deposit, Colorado (Steven and Ratte, 1960). Alunite is commonly an early alteration and gangue mineral, whereas anhydrite and barite are relatively late. Ore is hosted by rock consisting of quartz recrystallised from residual silica, with grades decreasing sharply at the edge of the silicic core. This silicic core is locally vuggy in texture, depending on the texture of the original rock. Outwards from the core alteration zone is a zone of advanced argillic alteration, consisting of quartz-alunite and the kaolin minerals (Fig. 3), including kaolinite, nacrite, or dickite. Pyrophyllite or

102

Characteristics of and exploration for high-sulfidation epithermal gold—copper deposits able 2 Principal fie d-oriented characteristics of epithermal types and subtypes

Typical examples

High sulfidation Oxidised magma (Reduced magma) ' Lepanto, Potosi, Bolivia Philippines; Chinkuashih, Taiwan; El lndio, Chile; Goldfield, Nevada (vein); Yanacocha, Peru; Pascua, Chile (disseminated) Mainly andesite to Rhyodacite rhyodacite

Genetically related volcanic rocks Typical host rocks Lava flows and pyroclastic units, diatremes, porphyries Deposit form Replacement, dissemination, and breccia bodies, massive veins Notable features Common steam-heated blanket, vuggy quartz host with steep outcrops; structural controls to feeder and high-grade ore; typically underlain by porphyry system, locally overprinted on porphyry features Key proximal Quartz-alunite/ Quartzalteration minerals APS; quartzalunite/APS; pyrophyllite/ quartz-dickite at depth dickite at depth Silica gangue Massive fine-grainec silicification and vuggy residual quartz

Carbonate gangue Absent

Other gangue

Barite common, typically late

Sulfide abundance 10-90 vol %, early pyrite dominant

Ore textures

Massive replacement, ore-cemented breccias, late veins (some bonanza) Key sulfide species Enargite, luzonite, Acanthite, famatinite, covellite stibnite

Main metals Ag/Au ratio

Au-Ag, Cu, As-Sb 1-10 (some >20)

Minor metals

Zn, Pb, Bi, W, Mo, B i , W Sn, Hg Tellurides None known, common; but few data selenides present locally

Tc and Se species

Ag, Sb, Sn 1000

Intermediate sulfidation Oxidised magma

Low sulfidation Subalkaline magma Alkaline magma

Baguio, Victoria, Philippines; Kelian, Indonesia; Rosia Montana, Romania (Au-rich); Comstock, Nevada (Ag-rich); Casapalca, Peru; Creede, Colorado (Ag-base metal rich)

Midas, Sleeper, Ivanhoe, Mule Canyon, Nevada; Hishikari, Japan; El Penon, Chile; Cerro Vanguardia, Esquel, Argentina (vein); Round Mountain (disseminated)

Emperor, Fiji; Porgera Zone VII,

Principally andesite to rhyodacite, but locally rhyolite

Basalt to thyolite

Alkali basalt to trachyte

PNG

Lava flows and pyroclastic units, Domes, volcaniclastic and sedimentary units basement, diatremes Vein, breccia body (disseminated) Vein, vein swarm, stockwork, disseminated in lithologic host Up to 800 m vertical extent, Sinter if paleosurface Limited variable top and bottom of preserved, steam-heated development of ore zones in district; locally blanket, basal chaldedony early quartz veins associated with lithocap ± HS ore horizon; flat top and base of ore horizon, 100 to 300 m vertical extent Sericite; adularia generally uncommon; variable chlorite, hematite

Illite/smectite-adularia

Roscoelite-illiteadularia

Vein-filling crustiform and comb Vein-filling crustiform quartz and colloform chalcedony and quartz; carbonatereplacement texture

Vein-filling crustiform and colloform chalcedony and quartz; quartz deficiency common in early stages Common, typically including Present, but typically Abundant, but not minor and late, commonly manganiferous manganiferous varieties bladed Barite and manganiferous Barite uncommon; Barite, celestite, and/or fluorite silicates present locally fluorite present locally common locally 5->20 vol. %, base-metal sulfides Typically
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