Supergene Silver Enrichment Reassessed_Sillitoe

©2009 Society of Economic Geologists Special Publication 14, 2009, pp. 15–32

Chapter 2 Supergene Silver Enrichment Reassessed RICHARD H. SILLITOE† 27 West Hill Park, Highgate Village, London N6 6ND, England

Abstract Supergene silver sulfide enrichment has been widely accepted for the last 100 years, but has warranted little or no mention in descriptions of several silver-rich, bulk-tonnage orebodies defined over the last three decades. This dichotomy is addressed by reassessing the importance of enrichment in 40 of the world’s premier silverdominated and other silver-rich deposits, including several of historical significance. The deposits are of highgrade vein and low-grade, bulk-tonnage styles and varied genetic types, but are dominated by representatives of the intermediate-sulfidation epithermal and carbonate-replacement, chimney-manto classes. The results of this preliminary analysis show that only 12 (30%) of the deposits contain(ed) appreciable amounts of silver ore generated by silver sulfide enrichment, mainly in the form of acanthite and argentian chalcocite-group minerals in the cases where its mineralogic characteristics are recorded. Silver-rich oxidized zones are, however, well developed in 60 percent of the deposits and, locally, display silver enrichment of either residual or chemical origin. Irrespective of whether oxidative weathering takes place under acidic or alkaline conditions, a factor controlled mainly by hypogene iron sulfide and carbonate contents, silver tends to be retained in oxidized zones, with comparatively little remaining available in solution to generate underlying silver sulfide enrichment. The extreme insolubility of the silver halides (chlorargyrite, embolite, bromargyrite, iodargyrite) over broad pH and climatic ranges, besides efficient silver fixation as native silver, argentojarosite, or silver-bearing manganese oxides under the appropriate chemical conditions, explains the metal’s relative supergene immobility. The efficient dissolution and downward transport of copper under acidic supergene conditions, as exemplified by porphyry copper leached cappings and underlying multicyclic enrichment blankets, appears to have no counterpart in either silver-only or other silver-rich deposits. Nor are the silver equivalents of exotic oxide copper deposits, the products of lateral metal transport in the acidic supergene environment, considered likely to exist. Furthermore, the processing benefits accruing from supergene oxidation and enrichment of copper deposits are not as evident in the silver environment, in which the main supergene oxidation products, especially the silver-bearing manganese oxides and argentojarosite, commonly present metallurgical difficulties.

deposits (e.g., Bastin, 1922). In such deposits, some of the widely proposed enrichment zones were reinterpreted as products of paragenetically late hypogene processes, particularly where silver sulfosalts were the putative supergene species. Furthermore, argentite (now known to be predominantly acanthite, its lower temperature dimorph) and native silver are not necessarily indicators of a supergene origin, since they have been long recognized as valid and important hypogene species (e.g., Bastin, 1925; see below). The supergene parts of the deposits in which silver sulfide enrichment was originally defined (e.g., Chañarcillo) were fully exploited as long ago as the early 1900s and, therefore, are no longer accessible to detailed study. However, during the last 30 years or so, a number of low-grade, bulk-tonnage silver deposits have been discovered and explored, and some exploited. Although such deposits, by analogy with porphyry deposits in the case of copper, should provide optimal sites for supergene profile development, little mention has been made of appreciable upgrading due to silver sulfide enrichment. Therefore, in view of the obvious economic implications, it is considered timely to reassess the importance of supergene silver enrichment across a broad spectrum of the world’s major silver-dominated and other silver-rich deposits (Table 1). The deposits selected, markedly concentrated in western North and South America, span most of the world’s climatic

Introduction DURING the early decades of the 20th century, several eminent investigators proposed that silver sulfide enrichment beneath zones of oxidation is a widespread and economically important supergene process in silver-only and other silver-rich deposits (e.g., Weed, 1901; Cooke, 1913; Ravicz, 1915; Emmons, 1917; Lindgren, 1933), a concept that has since become entrenched in the literature (e.g., Bateman, 1942; Boyle, 1968; Guilbert and Park, 1986; Fig. 1). The concept of silver enrichment is unsurprising given the geochemical affinities between silver and copper, the latter a metal that commonly undergoes several-fold sulfide enrichment in the supergene environment (e.g., Emmons, 1917; Titley and Marozas, 1995). A few deposits have been repeatedly cited as type examples of particularly well-developed silver sulfide enrichment, beneath thick oxidized zones, with Chañarcillo in northern Chile arguably being the most famous (Whitehead, 1919, 1942; Lindgren, 1933; Segerstrom, 1962; Ruiz et al., 1965; Guilbert and Park, 1986). Nevertheless, the volumetric and economic importance of supergene silver sulfide enrichment was extensively debated during the early decades of the last century, and some experts downplayed its significance, especially in epithermal vein † E-mail:

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RICHARD H. SILLITOE

and physiographic regimes (Fig. 2) besides representing all the main deposit types with their correspondingly different mineralogic constitutions. In this latter regard, the amount of acid- and Fe3+-generating sulfide minerals and acid-neutralizing gangue minerals are critical to supergene processes, as they also are in copper deposits (e.g., Emmons, 1917; Anderson, 1982; Sillitoe, 2005). This review should be considered as a first attempt at reassessment of the importance of silver sulfide enrichment because of the difficulty of obtaining reliable data for many of the deposits selected. The problem is two-fold: supergene ores were commonly exploited and described, some rather poorly, as much as a century ago (e.g., Chañarcillo, Comstock Lode), since when only their relative importance vis-à-vis the deeper, hypogene parts of the deposits has been addressed; and several of the more recent discoveries await full documentation, including detailed mineralogic study. Although this analysis focuses on silver sulfide enrichment, silver enrichment in the oxidized parts of supergene profiles is also considered. The economic consequences of the resulting supergene mineral assemblages are also the subject of brief comment.

Surface

Oxidized zone

Ag halides

Native Ag

Enrichment zone

Ground water table

Acanthite

Hypogene zone

Pyrargyrite, proustite

Major Silver-rich Deposits The 40 silver-only and other silver-rich deposits selected for consideration (Table 1; Fig. 3) either rank among the world’s largest silver concentrations or are particularly well known, because of either their prominence as major producers during the 19th and early 20th centuries (e.g., Chañarcillo, Comstock Lode, Tintic, Tonopah) or their relatively recent

FIG. 1. Idealized supergene silver profile presented by Boyle (1968), which is at variance with the conclusions reached herein. Note importance assigned previously to silver sulfide enrichment and the supergene origin of silver sulfosalts.

Tintic Park City

Coeur d’Alene Eskay Creek Keno Hill Greens Creek

Dukat

60°N

Cobalt Cove Rochester Comstock Lode

30°N

Leadville

Peñasquito Providencia

Paradise Peak Tonopah Hardshell Santa Eulalia Tayoltita Fresnillo Zacatecas Pachuca

Imiter

Real de Angeles Guanajuato

Fankou

0°

Cerro de Pasco Colquijirca Cerro Rico de Potosí Corani Oruro San Cristóbal El Peñón Chañarcillo Pascua-Lama

30°S

Pulacayo La Coipa Cannington Broken Hill

FIG. 2. Locations of the 40 major silver and other silver-rich deposits selected for consideration. Note the concentrations in western North and South America and distribution across a broad spectrum of physiographic and climatic zones.

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150°E

120°E

90°E

60°E

30°E

0°

30°W

60°W

90°W

120°W

150°W

Navidad

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Carbonate replacement Zn-Pb-Ag

Broken Hill-type Zn-Pb-Ag

Intermediate-sulfidation epithermal vein Ag-Au

Mesothermal (metamorphogenic) vein Ag-Pb-Zn-(Cu-Sb)

Intermediate-sulfidation epithermal vein Au-Ag

Intermediate-sulfidation epithermal vein and carbonate-replacement Ag-Au-Zn-Pb-Cu

Intermediate-sulfidation epithermal disseminated (diatreme brecciarelated) Ag-Au-Zn-Pb Broken Hill-type Ag-Pb-Zn

Cerro de Pasco, Peru

Broken Hill, Australia

Pachuca, Mexico

Coeur d’Alene, Idaho

Guanajuato, Mexico

Fresnillo, Mexico

Peñasquito, Mexico

Intermediate-sulfidation epithermal vein Ag-Pb-Zn-Cu-Au

Intermediate-sulfidation epithermal disseminated and breccia-hosted Ag-Zn-Pb

Zacatecas, Mexico

San Cristóbal, Bolivia

Cannington, Australia

High-sulfidation epithermal stockworkdisseminated Ag-Sn

Deposit type1

Cerro Rico de Potosí, Bolivia

Deposit, location

685

750

758

864

910

1,140

1,208

1,364

1,400

1,600

3,700

Contained Ag, Moz

63

120

538

29

425

270

500-850

350

148

150

NA

Average grade, g/t Ag

9% oxidized (to 300 m at Cerro Proaño); SE veins blind and unoxidized; sulfide enrichment absent 13% oxidized (40-120 m thick); sulfide enrichment absent No oxidation; concealed beneath postmineral cover Minor oxidation (up to 40-60 m); minor sulfide enrichment 10% oxidized (10-35 m thick); sulfide enrichment minor (avg 4 m thick)

Minor oxidation (20-30 m); sulfide enrichment absent

Minor oxidation (up to 30 m); many veins blind; sulfide enrichment absent Shallowly oxidized (0-60 m); sulfide enrichment absent

60-120-m oxidized zone; sulfide enrichment up to 1 m thick

50% oxidized (up to 90 m thick); up to 30 m sulfide enrichment

95% oxidized (to 300 m); sulfide enrichment absent

Supergene contribution

Freibergite, galena, pyrargyrite, allargentum, acanthite, dyscrasite, native Ag Acanthite, polybasite, pyrargyrite, freibergite (miargyrite, stephanite, freieslebenite) Acanthite, galena (polybasite, pearcite, pyrargyrite)

Acanthite (freibergite, polybasite)

Acanthite, aguilarite, polybasite, pyrargyrite, electrum, galena, stephanite, miargyrite Pyrargyrite, polybasite, pearceite, acanthite (stephanite, native Ag)

Freibergite (galena, polybasite, proustite, pyrargyrite)

Tetrahedrite, dyscrasite, pyrargyrite, native Ag, stephanite (acanthite, mckinstryite, miargyrite, polybasite, proustite, allargentum) Acanthite (miargyrite, pyrargyrite, proustite, native Ag)

Tennantite-tetrahedrite, aramoyoite, polybasite, acanthite

Acanthite, andorite, pyrargyrite, tetrahedrite, matildite, miargyrite

Hypogene Ag mineral(s) (minor)

Argentojarosite (native Ag)

Native Ag (bromargyrite, chlorargyrite)

Chlorargyrite(?)

Chlorargyrite, native Ag, acanthite, bromargyrite

Chlorargyrite, embolite, bromargyrite

Native Ag

Embolite, native Ag, Mn oxides, iodargyrite, iodoembolite, chlorargyrite, bromargyrite Chlorargyrite, bromargyrite, Mn oxides,

Chlorargyrite, native Ag, iodargyrite, embolite (argentojarosite, manganese oxides) Plumbojarosite

Supergene oxidized Ag mineral(s) (minor)

TABLE 1. Selected Supergene and Other Characteristics of Major Silver-Only and Other Silver-Rich Deposits

Acanthite, native Ag

Acanthite, native Ag

Native Ag, acanthite

Stromeyerite, acanthite, jalpaite, native Ag

Chalcocite (stromeyerite)

Supergene enrichment Ag mineral(s) (minor)

Buchanan (2003), Lozano (2007), L. Buchanan (writ. commun., 2008)

Bastin (1941), Ponce and Clark (1988)

Bailey (1998)

Bryson et al. (2007), Brown (2008)

Gemmell et al. (1988); Trejo (2001)

Ransome and Calkins (1908), Hobbs and Fryklund (1968) Wandke and Martínez (1928), Querol et al. (1991)

Bastin (1948), Thornburg (1952)

Geological Staff of the Corporation (1950), Amstutz and Ward (1956) Stillwell (1953), van Moort and Swensson (1982), Plimer (1984)

Lindgren and Creveling (1928), Steele (1996), Bartos (2000)

Main data source(s)

SUPERGENE SILVER ENRICHMENT REASSESSED

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High-sulfidation epithermal disseminated Au-Ag-(Cu) Intermediate-sulfidation epithermal vein Ag-Au

Intermediate-sulfidation epithermal breccia, stockwork, and disseminated Ag-Pb-Cu-Zn Carbonate-replacement Ag-Pb-Zn-(Sn)

Bolivian-type vein Ag-Sn-(Pb-Cu-Sb)

Intermediate-sulfidation epithermal vein Ag-Au

Carbonate-replacement Ag-Zn-Pb-Hg Intermediate-sulfidation epithermal vein and stockwork Ag-Pb-Zn-Au VMS Ag-Au-Zn-Pb-Cu

Pascua-Lama, Chile/Argentina

Navidad, Argentina

Santa Eulalia, Mexico

Oruro, Bolivia

Tayoltita, Mexico

Fankou, China Corani, Peru

18

Carbonate-replacement Ag-Au-Pb-Cu-Zn

Carbonate-replacement Ag-Pb-Zn-Cu-Au

Carbonate-replacement Ag-Pb-Zn-Cu-Au

Tintic, Utah

Leadville, Colorado

Park City, Utah

Greens Creek, Alaska

Dukat, Russia

Ag-Co-Ni-As vein: Ag

Deposit type1

Cobalt, Ontario

Deposit, location

254

260

274

485

320

486

540

51

278

278

102

460

>500

367

110

500

66

NA

Average grade, g/t Ag

292

320

349

436

457

569

585

600

Contained Ag, Moz

25% oxidized; sulfide enrichment not reported

Oxidized from 120180 m; minor sulfide enrichment in veins

70% oxidized (300700 m); minor sulfide enrichment

50% oxidized (commonly to >450 m); sulfide enrichment not reported Oxidized from 20-150 m; sulfide enrichment absent Minor oxidation; many veins blind; sulfide enrichment absent Supergene profile absent Oxidation to 30-40 m; sulfide enrichment not reported Supergene profile absent

Minor oxidation and sulfide enrichment2 (glaciated terrain) 20% oxidized3 (up to 350 m thick); sulfide enrichment absent 10% oxidized (average 100 m, up to 400 m); minor sulfide enrichment 100 Ag-Co-Ni-As vein: Ag Chañarcillo, Chile

Hypogene Ag mineral(s) (minor) Deposit, location

Deposit type1

Contained Ag, Moz

Average grade, g/t Ag

Supergene contribution

TABLE 1. (Cont.)

Supergene oxidized Ag mineral(s) (minor)

Supergene enrichment Ag mineral(s) (minor)

Main data source(s)

RICHARD H. SILLITOE

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21

SUPERGENE SILVER ENRICHMENT REASSESSED

0

500

1000

1500

2000

2500

3000

3500

4000

M oz

Cerro Rico de Potosí Cerro de Pasco Broken Hill Pachuca Coeur d’Alene Guanajuato Fresnillo Peñasquito Cannington Zacatecas San Cristóbal Cobalt Pascua-Lama Dukat Navidad Santa Eulalia Oruro Tayoltita Fankou Corani Greens Creek Tintic Leadville Park City Pulacayo Keno Hill Real de Angeles Rochester Comstock Lode La Coipa Imiter Tonopah Colquijirca El Peñón Cove Providencia Eskay Creek Chañarcillo Hardshell Paradise Peak

Intermediate-sulfidation epithermal High-sulfidation epithermal Low-sulfidation epithermal Carbonate replacement Zn-Pb-Ag Ag-Co-Ni-As vein Mesothermal Ag-Pb-Zn Bolivian-type vein Broken Hill type Zn-Pb-Ag VMS Sediment-hosted

FIG. 3. The 40 silver and other silver-rich deposits selected for consideration, showing deposit types and total silver contents (in million oz). Deposits arranged in decreasing order of size, as listed in Table 1, which also indicates deposit locations.

carbonate-replacement, Ag-Co-Ni-As veins, stockworks, and breccias, Coeur d’Alene veins, Broken Hill-type deposits, and Cove sediment-hosted deposit typically contain abundant carbonate gangue, to which is added carbonate wall rocks in the specific case of the carbonate-replacement deposits and a few others (Chañarcillo, Cove). Importantly, the intermediatesulfidation epithermal, Broken Hill-type, and some carbonate-replacement deposits as well as the Keno Hill veins and Cove sediment-hosted bodies contain manganoan carbonate (± rhodonite ± alabandite) gangue.

in most supergene profiles developed in the upper parts of silver-rich deposits (Ravicz, 1915; Emmons, 1917; Boyle, 1968; Shcherbina, 1972; Fig. 5). The solubility of Ag+ increases dramatically with increasing Eh and decreasing pH (e.g., Gammons and Yu, 1997) so dissolution of the native metal and most silver-bearing sulfides and sulfosalts takes place readily in oxygenated water under near-surface, acidic conditions. Molecular O2 and/or Fe3+ ions act as the oxidants. Oxidation of pyrite and other iron-bearing minerals (pyrrhotite, arsenopyrite, siderite) produces Fe2+, which then oxidizes, commonly with the catalytic assistance of acidophilic bacteria (e.g., Nordstrom and Alpers, 1999), to produce the Fe3+. The potential for appreciable mobility of silver in such sulfate-rich solutions is confirmed by the high silver contents of some efflorescent sulfate salts in mine openings (Morris and Lovering, 1952). At the iron redox front, the oxidation of Fe2+ to Fe3+ is accompanied by reduction of any Ag+ to Ag0. Hence, supergene native silver tends to be more abundant on approach to underlying sulfide zones, as correctly shown in Figure 1 (Boyle, 1968).

Supergene Silver Geochemistry Dissolution, migration, and reprecipitation of silver in the supergene environment are less well documented than is the case for copper, in part because of the large number of potentially stable silver complexes under low-temperature, aqueous conditions (e.g., Webster, 1986; Renders and Seward, 1989; Akinfiev and Zotov, 2001). However, a few key solubility- and redox-controlled processes appear to explain the observed distribution of silver and silver-bearing minerals

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RICHARD H. SILLITOE TABLE 2. Chemical Compositions of Main Supergene and Hypogene Silver-Bearing Minerals Referred to in the Text and Table 1

1 2 3 4 5 6 7 8

Intermediate-sulfidationvein

Supergene oxidized zone Chlorargyrite (cerargyrite) Embolite Bromargyrite Iodargyrite Iodembolite Argentojarosite Argentian plumbojarosite Argentian beudantite Manganese oxides and oxyhydrates (e.g., cryptomelane)

Intermediate-sulfidationbulk tonnage Carbonate replacement Zn-Pb-Ag High-sulfidation bulk tonnage Ag-Co-Ni-As vein Broken Hill type Zn-Pb-Ag VMS Mesothermal Ag-Pb-Zn Bolivian-type vein Low-sulfidationvein Sediment-hosted

FIG. 4. Bar graph showing the relative importance of the different silver and other silver-rich deposit types listed in Table 1 and Figure 3. References to less widely known deposit types are provided in the text.

Under neutral to alkaline, oxidizing conditions, consequent upon deficiency of iron sulfides and/or abundance of carbonate gangue (Fig. 5), silver tends to be much less mobile, although limited transport as the slightly soluble hydroxycarbonate is possible because of the abundant bicarbonate ions produced by acid attack of carbonate gangue or wall rocks. Thiosulfate and other metastable sulfur species, generated during the oxidative conversion of sulfide minerals to sulfate, may also solubilize silver under alkaline (i.e., pyrite-deficient) conditions (Webster, 1986), but the resulting Ag(S2O3)3– is likely to be only a transient species. In saline, oxygenated groundwater, silver is also readily soluble as chloride complexes (AgCl0, AgCl2–, AgCl23–; Gammons and Yu, 1997). Nevertheless, the chloride, bromide, and iodide anions are also the most effective precipitants of silver at ambient temperatures over a wide range of redox and pH conditions (Gammons and Yu, 1997; Fig. 6) because of the extreme insolubility of the resultant silver halides, particularly where the supply of descending groundwater is relatively limited. Indeed, under even moderately saline supergene conditions and irrespective of solution pH, silver sulfides and sulfosalts as well as native silver may undergo direct replacement by the halide minerals (Fig. 5). Vertical silver halide zoning, from chloride near surface through bromide to iodide near the base of the oxidized zone, as described at Tonopah (Burgess, 1911) and Chañarcillo (Moesta, 1928), is a result of the initial formation of the least soluble halide, iodargyrite,

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AgCl Ag(Cl,Br) AgBr AgI Ag(Cl,Br,I) AgFe3(SO4)2(OH)6 (Pb,Ag)Fe3-6(SO4)2-4(OH)6-12 (Pb,Ag)Fe3AsO4SO4(OH)6 K1.2(Mn3+Mn4+)8O16 · xH2O

Supergene and hypogene zones Native silver Argentite Acanthite Stromeyerite Mckinstryite Jalpaite

Ag αAg2S βAg2S Ag1-xCuS Ag1.2Cu0.8S Ag3CuS2

Hypogene zone Electrum Amalgam Allargentum Dyscrasite Aguilarite Polybasite Pearceite Canfieldite Stephanite Pyrargyrite Proustite Tetrahedrite-tennantite (freibergite) Freieslebenite Owyheeite Miargyrite Aramoyoite Matildite Andorite Imiterite

(Au,Ag) (Ag,Hg) AgSb Ag3Sb Ag4SeS (Ag,Cu)10Sb2S11 (Ag,Cu)10As2S11 Ag8SnS6 Ag5SbS4 Ag3SbS3 Ag3AsS3 (Cu,Fe,Ag)12(Sb,As)4S13 Pb3Ag5Sb5S12 Pb5Ag2Sb6S15 AgSbS2 Ag(Bi,Sb)S2 AgBiS2 PbAgSb3S6 Ag2HgS2

and its subsequent conversion to bromine- and chlorine-bearing species under higher oxidation states (Gammons and Yu, 1997; Fig. 6). In stark contrast to the case of copper, other naturally occurring silver compounds, including oxides, silicates, hydroxycarbonates, hydroxysulfates, arsenates, and phosphates, are either rare or unknown. Precipitation of native silver is favored by the progressive neutralization of acidic, silver-bearing supergene solutions (Gammons and Yu, 1997; Fig. 6), conditions that also lead to sorption of Ag+ by ferric oxyhydroxide (goethite) or its coprecipitation as argentojarosite or argentian plumbojarosite/beudanite (Table 2) during hydrolysis of ferric sulfate in solution (Fig. 5). Under near-neutral or alkaline conditions, in manganoan carbonate-bearing deposits, either biotic or abiotic oxidation of Mn2+ to Mn4+ (Mills, 1999) causes efficient precipitation of silver, which may be an integral component of several of the Mn4+ oxides that constitute manganese wad. Alternatively, the silver may be directly adsorbed onto these negatively charged minerals, especially at high pH values (Nicholson, 1992), or finely intergrown with them as the native metal. 22

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SUPERGENE SILVER ENRICHMENT REASSESSED

Pyrite-rich deposit Low pH, Fe3+: sulfide oxidation giving Ag + in solution Fe 2(SO 4) 3 hydrolysis: Ag° precipitation as argentojarosite Fe 2+ reprecipitates native Ag

Pyrite-poor deposit Direct AgCl replacement of Ag minerals Ag+ + halide ions giving Ag halides Acanthite preservation because resists oxidation

Surface

Neutral pH: minor hydroxycarbonate + thiosulfate transport of silver

Ag° precipitated by Mn2+ : Mn4+ oxyhydroxides (wad)

S2- ions from FeS or ZnS oxidation or bacterial SO 42- reduction: precipitate acanthite Precipitation of any Ag+ by sulfides: acanthite, argentian chalcocite group

Ag+ and sulfides: native Ag precipitation

Water table

Silver sulfosalts ± acanthite ± native Ag as hypogene minerals

FIG. 5. Schematic representation of supergene processes in pyrite-rich and pyrite-poor silver-only and other silver-rich deposits. Important processes and minerals are highlighted. Compiled from Emmons (1917), Boyle (1968), Shcherbina (1972), and references therein.

Under reducing conditions at and immediately below the water table, Ag+ in solution progressively substitutes for copper, zinc, and iron in less-soluble sulfides to form acanthite (Figs. 5, 6), the process of silver sulfide enrichment. The process has been replicated experimentally at 25°C (Scaini et al., 1995) and its results observed microscopically (e.g., Greffié et al., 2002). If copper accompanies the silver in solution, stromeyerite or argentian chalcocite-group minerals may either coprecipitate with or form instead of the acanthite. Many sulfide and sulfosalt minerals, including chalcocite, enargite, galena, sphalerite, pyrite, chalcopyrite, tetrahedrite, arsenopyrite, and even cobalt-nickel arsenides, whether below the water table or as remnants above it, reportedly also cause precipitation of native silver under neutral to slightly acidic conditions (Palmer and Bastin, 1913), although the quantitative importance of such reactions in naturally formed supergene profiles is difficult to ascertain. Where sulfide ions are present in solution, resulting from either pyrrhotite or sphalerite oxidation or bacterial reduction of aqueous sulfate, silver may be precipitated as acanthite, stromeyerite, or, where arsenic and antimony are present, perhaps even as silver-bearing sulfosalts. Sulfur isotope evidence suggesting bacterial involvement in acanthite formation was

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recently obtained from the Pierina high-sulfidation epithermal gold-silver deposit, Peru (Rainbow et al., 2006). Acanthite, whether of hypogene or supergene origin, is commonly reported from the oxidized zones of silver deposits (Table 1), probably because of its high degree of resistance to oxidation (Shcherbina, 1972); however, it may also form locally in oxidized zones if sulfate-reducing bacterial populations are present (Rainbow et al., 2006). Supergene Silver Profiles Emmons (1917), Lindgren (1933), and Boyle (1968) recognized that supergene profiles developed in silver deposits are far less orderly than those in copper deposits, but proposed that despite the appreciable mineralogic admixture and complexity, a generalized downward progression from silver halides through native silver to acanthite and, finally, silver sulfosalts was typical (Fig. 1). However, essentially all the sulfosalt minerals, an appreciable proportion of the acanthite, and at least some of the native silver seem likely to be hypogene in many cases (e.g., Bastin and Laney, 1918). For example, Table 1 reveals that 75 percent of the deposits contain one or more hypogene silver sulfosalts, 72.5 percent contain acanthite of probable hypogene origin, and 35 percent contain

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RICHARD H. SILLITOE

native silver, acanthite, and silver sulfosalts of necessarily hypogene origin all occur at Cannington, Cobalt, and Greens Creek (Table 1). The remaining 34 major silver-rich deposits have supergene profiles, 70 percent of which may be considered to include reasonably well-developed oxidized zones (Table 1; Fig. 7). These attain maximum subsurface depths in the 300- to 500-m range at Cerro Rico de Potosí (Lindgren and Creveling, 1928; Fig. 8a), Cove (Emmons and Eng, 1995), Dukat (Konstantinov et al., 1995), El Peñón (Robbins, 2000), Pascua-Lama (Chouinard et al., 2005), Providencia (Triplett, 1952; Mapes et al., 1964), and Santa Eulalia (Maldonado, 1991), but an extreme maximum of 700 m in the karsted carbonate terrain at Tintic (Lindgren and Loughlin, 1919). Some 95 percent of the mined and remaining ore at Cerro Rico de Potosí, the world’s largest silver deposit, is oxidized (Table 1; Fig. 8a), with 50 to 100 percent oxidation reported for at least 13 other deposits (Table 1; Fig. 7). Besides being located in arid to semiarid environments, these deeply oxidized deposits also have high intrinsic permeability. This permeability commonly results from pyrite and/or carbonate dissolution in carbonate-replacement and other massive to semimassive sulfide bodies (e.g., Paradise Peak; Fig. 8b) or is provided by steep veins and, in the case of high-sulfidation epithermal deposits, bodies of vuggy quartz (Plumlee, 1999; Sillitoe, 2005). Onethird of the deposits affected by supergene processes have oxidized zones that are rather poorly developed, thin (maximum 60 m), and economically unimportant because of inappropriate geomorphologic and climatic histories, as exemplified by the Navidad, San Cristóbal, and Guanajuato deposits (Wandke and Martínez, 1928; Lhotka et al., 2005; Buchanan, 2003; Table 1; Fig. 7). The oxide silver minerals typically either attain or approach the present surface, without development of thick, silver-deficient leached cappings or gossans although, in this regard, the manganese- and lead-rich gossan at Broken Hill is a notable exception (van Moort and Swensson, 1982). Silver sulfide enrichment is reliably reported from only 14 (35%) of the supergene profiles (Table 1; Fig. 9): Broken Hill (Stillwell, 1953), Cobalt (Boyle and Dass, 1971), Cerro de Pasco (Geological Staff of the Corporation, 1950), Colquijirca (R. Bendezú, writ. commun., 2007), Dukat (Konstantinov et al., 1995), Keno Hill (Boyle, 1965), Leadville (Tweto, 1968), Pachuca (Bastin, 1948), Providencia (Triplett, 1952), Real de Angeles (Pearson et al., 1988), San Cristóbal (L. Buchanan, writ. commun., 2008), Tintic (Morris, 1968), Tonopah (Bastin and Laney, 1918), and Zacatecas (Bastin, 1941). However, as noted above, the silver enrichment at Cobalt and Pachuca is of trivial importance at the deposit scale because most of the ore shoots were unaffected; hence, it is excluded from Figure 9. The enriched horizons are commonly poorly defined, but typically thin (Table 1), ranging from ≤1 m at Broken Hill (Stillwell, 1953; van Moort and Swensson, 1982) and 4 m at San Cristóbal (L. Buchanan, writ. commun., 2008) to 30 m at Cerro de Pasco (Geological Staff of the Corporation, 1950). The enrichment factor is also commonly low, only 1.3 at San Cristóbal (L. Buchanan, writ. commun., 2008). In the high-grade vein deposit at Chañarcillo, however, a major enrichment zone, up to 150 m thick and, hence, comparable to those developed in many copper deposits, was claimed

Ag2O(s)

0

AgCl(s)

log fO2

-20

AgI(s)

- 40

Ag(s) -60

Ag2S(s) -80

0

2

4

6

8

10

12

pH FIG. 6. Eh-pH diagram to show the stability fields of chlorargyrite, iodargyrite, native silver, and acanthite at 25°C. Ligand concentrations approximate those of rainwater. The bromargyrite stability field, between chlorargyrite and iodargyrite, is not shown. Note the broad stability field of the silver halides and confinement of acanthite to reduced conditions. Taken from Gammons and Yu (1997).

native silver that is also judged to be hypogene. In this section, the world’s premier silver-bearing deposits are used to test the broad-scale validity of the idealized supergene profile reproduced as Figure 1. Six of the silver deposits listed in Table 1 lack supergene profiles (Fig. 7). The main Pulacayo vein deposit in Bolivia is largely blind and, hence, protected from supergene processes (Ahlfeld and Schneider-Scherbina, 1964). Immediately postore rocks cover the Eskay Creek VMS deposit (Roth et al., 1999), whereas much younger sedimentary rocks conceal the Broken Hill-type deposit at Cannington, beneath which no supergene effects are reported (Bailey, 1998). Geomorphologic conditions inhibited development of supergene profiles at Cobalt, Fankou, and Greens Creek (Petruk, 1971; C. Allen, writ. commun., 2007; M. Satre, writ. commun., 2007). However, a single productive vein in the Cobalt district partially escaped the widespread effects of glacial erosion and retained a preglacial supergene profile (Boyle and Dass, 1971; Table 1), as indeed did all the veins at Dukat where permafrost conditions prevail today (Konstantinov et al., 1995). Most of the intermediate-sulfidation epithermal ore shoots at Pachuca and Tayoltita as well as those in the Southeast sector at Fresnillo are also blind and, hence, unaffected by supergene processes (Thornburg, 1952; Clark, 1991; Trejo, 1991). It is important to emphasize, though, that the silver mineralogy of the six deposits that lack supergene profiles is not greatly different from that of the sulfide zones in the rest of the deposits in which supergene effects are variably developed. For example,

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SUPERGENE SILVER ENRICHMENT REASSESSED

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Cerro Rico de Potosí Cerro de Pasco Broken Hill Pachuca Coeur d’Alene Guanajuato Fresnillo Peñasquito Cannington Zacatecas San Cristóbal Cobalt Pascua-Lama Dukat Navidad Santa Eulalia Oruro Tayoltita Fankou Corani Greens Creek Tintic Leadville Park City Pulacayo Keno Hill Real de Angeles Rochester Comstock Lode La Coipa Imiter Tonopah Colquijirca El Peñón Cove Providencia Eskay Creek Chañarcillo Hardshell Paradise Peak

Dominant Ag mineral Halides Mn oxides Argentojarosite/plumbojarosite/ beudantite

a

Native Ag

Pre- or postmineral cover

FIG. 7. Approximate proportion and dominant silver mineralogy of oxidized ore in the 40 silver-only and other silver-rich deposits selected for consideration. Deposits lacking supergene profiles because of presence of pre- or postmineral cover are indicated. Layout as in Figure 3.

a

b

b

FIG. 8. Views of classic silver-rich oxidized zones. a. Cerro Rico at Potosí where the 300-m-thick oxidized zone coincides with an advanced argillic lithocap composed almost entirely of vuggy quartz (darkest brown). Arrows indicate the base of oxidation within the mountain. Photograph taken in 1973. b. Paradise Peak, where masses of semimassive sulfide were transformed to gossanous oxidized ore that was porous, broken, and rubbly as a result of compaction and possible collapse before breakage by blasting. Photograph of the basal part of the orebody taken in 1992.

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Cerro Rico de Potosí Cerro de Pasco Broken Hill Pachuca Coeur d’Alene Guanajuato Fresnillo Peñasquito Cannington Zacatecas San Cristóbal Cobalt Pascua-Lama Dukat Navidad Santa Eulalia Oruro Tayoltita Fankou Corani Greens Creek Tintic Leadville Park City Pulacayo Keno Hill Real de Angeles Rochester Comstock Lode La Coipa Imiter Tonopah Colquijirca El Peñón Cove Providencia Eskay Creek Chañarcillo Hardshell Paradise Peak

Silver sulfide enrichment Pre- or postmineral cover

FIG. 9. Approximate proportion of enriched ore in the 40 silver-only and other silver-rich deposits selected for consideration. Deposits lacking supergene profiles because of presence of pre- or postmineral cover are indicated. Layout as in Figure 3.

by Whitehead (1919, 1942). Notwithstanding its location in the southern Atacama Desert, a region that was especially conducive to the generation and preservation of major copper sulfide enrichment blankets during the last ~40 m.y. (Sillitoe, 2005, and references therein), doubt is believed to surround Whitehead’s (1919, 1942) interpretation of the vertical mineralogic zoning at Chañarcillo. Although a detailed reappraisal is now impossible because the mine is depleted and its waste dumps repeatedly reprocessed, Sillitoe (2007) proposed that much of the putative enrichment, beneath a thick (50–190 m) oxidized zone, reflects hypogene zoning in a native Ag-Co-Ni-As-type deposit (Table 1). Four main lines of evidence combine to argue strongly against appreciable supergene sulfide enrichment (Sillitoe, 2007): (1) the low acidand Fe3+-generation and high acid-neutralization potentials of the pyrite-deficient and carbonate-rich vein material and enclosing wall rocks; (2) the crystalline nature of some of the supposedly supergene sulfide minerals, in particular acanthite pseudomorphs after argentite; (3) the several-times higher silver grades in the oxidized ore than in the underlying sulfide

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zone (Whitehead, 1919), an unusual situation for supergene profiles in either silver or copper deposits; and (4) the physical separation of the oxidized and sulfidic parts of the veins by thick (up to 165 m), relatively impermeable, tuffaceous horizons in which the veins are represented only by tight, sulfidefree fractures (Whitehead, 1919; Fig. 10). Supergene Silver Mineralogy The supergene mineralogy of the 34 oxidized zones considered herein is characterized by a relatively restricted number of silver-bearing species (Tables 1, 2), although oxidized minerals containing lead, zinc, copper, manganese, and other metals as well as many textural varieties of limonite (mainly composed of jarosite, goethite, and/or hematite) are commonly also abundant. The silver halides, of which chlorargyrite is typically the most common (Table 1), are dominant in just over half of the oxidized zones and present in at least minor amounts in 68 percent of them. Embolite, bromargyrite, and iodargyrite, besides chlorargyrite, are widely reported. Native silver of assumed supergene origin dominates 26

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100m

NEGRO

Oxidized

AHUESADO

Barren

DELIRIO

High-grade hypogene

CONSTANCIA

Barren

AZUL

Low-grade hypogene

Silver Enrichment in Oxidized Zones In many of the oxidized zones considered herein, silver contents seem likely to broadly reflect the former hypogene distribution patterns. Hence, vertical changes, like the upward increase in Ag/Au ratios in the completely oxidized La Coipa deposit (Oviedo et al., 1991), most likely reflect hypogene zoning on approach to the base of the partially preserved steam-heated horizon (i.e., the paleowater table). Where grade distribution patterns for silver and gold in oxidized ore are closely similar, as at Paradise Peak (Fig. 11), appreciable supergene silver mobilization is essentially precluded (Sillitoe and Lorson, 1994). If upward increases in silver content commence in the hypogene sulfide zone and continue upward uninterruptedly into the oxidized zone, as at San Cristóbal (L. Buchanan, writ. commun., 2008), then hypogene zoning is also the most likely explanation. Nevertheless, in some oxidized zones, silver enrichment consequent upon oxidative sulfide destruction is clearly discernable, and may be the result of physical and/or chemical processes. The main physical process seems to be residual enrichment, whereby the specific gravity of the oxidized ore is lowered

10m Limestone

Tuff

PARADISE PEAK FIG. 10. Schematic section of a typical silver vein at Chañarcillo, constructed using data reported by Whitehead (1919) for the southern part of the district. Note the marked expansion of the vein within bituminous limestone horizons and its contraction to a tight, sulfide-free fracture in intervening tuffaceous horizons. The high-grade hypogene vein interval was previously considered to be a supergene enrichment zone. Names down the left side are those used locally during mining for some of the limestone and tuff units. Note the horizontal scale is five times the vertical. Taken from Sillitoe (2007).

DRILL HOLES

20 percent of the oxidized zones as well as occurring as a subsidiary silver mineral in another 43 percent, whereas argentojarosite, argentian plumbojarosite, and argentian beudantite or silver-bearing manganese oxides (wad) are the main minerals in only a few percent each. The wad may be completely amorphous or contain minerals such as cryptomelane, chalcophanite, coronadite, and hetaerolite (e.g., Koutz, 1984). Acanthite, of unspecified hypogene or supergene origin, is reported from 23 percent of the oxidized zones (Table 1), where it persists because of its resistance to oxidation (see above). The silver sulfide enrichment zones (Table 1) are typified by the presence of powdery, black sulfide aggregates, which, where studied in any detail, prove to contain acanthite and, where copper is also present, argentian chalcocite-group minerals and stromeyerite, in some cases accompanied by native silver. Jalpaite and mckinstryite, sulfides of silver and copper like stromeyerite (Table 2), are reported from single deposits (Table 1). The unambiguous presence of supergene silver sulfosalts, such as pyrargyrite-proustite, pearcite-polybasite, and stephanite (Table 2), as enrichment products (e.g., Emmons, 1917; Lindgren, 1933; Bateman, 1942; Boyle, 1996) remains to be authenticated. Even Boyle’s (1965) detailed description of putative supergene pyrargyrite crystals at Keno Hill is more in keeping with an end-stage hypogene origin (Lynch, 1989).

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a

PARADISE PEAK

b

DRILL HOLES

FIG. 11. Gold (a) and silver (b) distributions in the completely oxidized Paradise Peak deposit, constructed on the basis of life-of-mine blast-hole assay data. Note the near coincidence of the two distribution patterns, a feature strongly suggesting that the silver underwent no significant supergene mobilization. Taken from Sillitoe and Lorson (1994).

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Oxidized zone

with respect to that of the former sulfidic material because of removal of components, most notably sulfur and carbonate, and, in some cases, also zinc (Fig. 12). Element subtraction during oxidative weathering leads to volume loss and consequent compaction, subsidence, and even collapse, as documented at Broken Hill (van Moort and Swensson, 1982; Plimer, 1984), Cerro de Pasco (Bowditch, 1935), Oruro (Chace, 1948), Paradise Peak (Sillitoe and Lorson, 1994; Fig. 8b), Providencia (Triplett, 1952), Santa Eulalia (Prescott, 1916), and Tintic (Morris, 1968), although unfilled cavities and even caverns commonly remain. Residual enrichment is most prevalent in sulfide- and manganese carbonate and/or silicate-rich ores, such as those typical of carbonate-replacement, VMS, Broken Hill-type, and Bolivian-type vein deposits. Graybeal et al. (1986) observed that the silver contents of oxidized, carbonate-replacement, chimney-manto deposits are typically four times those of the underlying sulfide zones, a situation spectacularly exemplified by production data from the Providencia deposit (Fig. 12). Even greater degrees of silver enrichment may have occurred in some oxidized carbonate-replacement deposits (e.g., Leadville; Cappa and Bartos,

2007), although it is unclear if this was entirely residual in origin or also had a chemical contribution. At Cerro de Pasco, Bowditch (1935) recorded residual silver enrichment accompanying a 44 percent decrease in specific gravity consequent upon the oxidative transformation of massive silica plus pyrite, a limestone-replacement product, to friable quartz and limonite: the pacos of colonial Spanish miners. Lead, because of the extreme insolubility of the carbonate (cerussite) and sulfate (anglesite), is also enriched with the silver in such oxidized zones, but zinc is severely depleted (Sangameshwan and Barnes, 1983; Fig. 12). The most common forms of chemical enrichment seem to take place as a result of the preferential precipitation of either argentojarosite-argentian plumbojarosite (e.g., Cerro de Pasco; Geological Staff of the Corporation, 1950) or silver-bearing manganese oxides (e.g., Hardshell; Koutz, 1984). The contained silver appears to have been coprecipitated during the hydrolysis of ferric sulfate or oxidation of Mn2+ in solution, respectively. The basal parts of the gossan above the Rio Tinto massive sulfide deposit in Spain display extreme silver enrichment, much of it also in the form of argentojarosite and argentian beudantite (García Palomero et al., 1986). Silver halides may also become enriched in the lower parts of a few oxidized zones, presumably because of progressive downward flushing resulting from protracted groundwater flux. Examples include the greater than two-fold increase in silver content in the lower compared to the upper parts of the deeply developed oxidized zone at El Peñón (S. Kasaneva, pers. commun., 2008) and the exceptionally high-grade silver ore (up to 9,500 g/t) near the base of the Broken Hill gossan (average 900 g/t Ag; Plimer, 1984).

2500

Elevation, m

Transitional

Ag Zn

Supergene Profile Interpretation This review of the world’s major silver-bearing deposits concludes that supergene sulfide enrichment is an economically unimportant process, and that in the majority of deposits it is largely absent or, at best, only incipiently developed. Based on the best estimates used to construct Figure 9,