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Geological Charactelistics of Epithermal Precious and Base Metal Deposits STUART F. SIMMONS,'

Cmlngy lJcpartmclI/ , U" i~it!j of Auckland. Prioow Bug 92019, AIlCkulIId, Nell) Zeo/wul NOEL C. WHITE, PO. Box 5181, Kemllore EMt , Queells/fIIld, Australia 40ti9 AND

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Abstract Epithermal deposits are impol'tallt sources of gold and siiver that foml at d mostly of magmatic nuids with a minor to moderate component of mctt.'Oric water. Critical genetic factors include; ()) at several-kilometers depth. the delo-elopment of oxidU.ed and acidic \~r­ sus reduced and ncar-neutral pH solutions, controlled by lite proportions of magmatic and 1I11 M aL Au at >30 gIt; Sillitoe. 1993a). Some

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Flc. 1. Simplified COnceptllal models of high-tempern til III hydruthcmlal5)lnems. showing the reladonlhlp be~n epithermal envimnmenu, nmgmnli(!' Intrusions, !luiu (,1rculllUon paths, and volcaniC and basement hOllI mc:kli, A, TIll, "pither. mal environment forms in 1I 111agl11l1tic-hydrothennai system dominated by acid hydrothemlaJ fluids, where tllere is II strung OWl of magmatic IIqulU and vapor. containing li tity

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Magmatic arc In I neutral to mildly Cl4-km (?) depth. At the deepest level explored by geothcnnal wells, these chloride waten;-sn-called due to the dominant anion-are reduced and have near-neutral pH and contain from 0.1 to > 1 wt percent CI, up to 3 wt percent CO 2 , and l Os to l00s of ppm H 2 S; the latter is an important ligand for aqlleous transport of gold and silver as bisulfide complexes (Seward, 1973; Seward and Barnes, 1997). The concentrations of the main aqueous constituents represent equilibrium with quartz, albite, adularia, illite, chlorite, pyrite, calcite, and epidote, which form as secondary minerals during alterAtion of igneous rocks (Barton et aI ., 1977; Giggenbach, 1997). The fluid reaches equilibrium with the rock alld its constituent minerals where flow is slow, through a Hrock-domillated·' or rock-buffered environment, to form a propylitic alteratioll assemblage (Ciggenbach, 1997). BOiling occurs in the central upflowiug column of fluid down to 1- to 2-km depth helow the water table, controlled by near-hydrostatic pressure-temperature conditions (Fig. 4). In this environmcnt, quam., adularia, and calcite (usualry platy) deposit in open spaces and subvertical channels from the boiling and cooling liqUid (e.g., Simmons and Browne, 2OOOb). Depending 011 the permeability structure, the chloride waler may rise to the surface to discharge and deposit

495

£fITHERMAL PRECIOUS AND BA SE METAl. DEPOSITS

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Quartz. K.feldqm (adularia), albi te, Illite. chlorite, calcite, epidote, pyrite

Develop! at >24O"C deep ill the "pithcnnal environment through alt!!mtion by near.neutnil pH "''Ilters

Illite. SIIll-'CtitC. chlorlte,lnter-layered d:ty.t, pyrite, calcite (slderlt!!), chaJoedooy

Dt.:vc!ops at 1 wt %) and tend to accu- fluids have been explored for their gcothennal energy pomulate at shallow levels. They drape the stagnant margi ns of tential (Heyes, 1990; Delfin et aI., 1992; Reyes et al., 1993, the upflow zolle to depths &, much as J ,000 m below the 2003). Existing data on the metal contents of high-temperawater table. Their distribution is best known at Bmadlands- ture volcanic discharges indicate the potential for substanOhaaki, where weaklr. addic steam-heated waters alter vol- tial flux of both precious and base metals (Hedenquist, canic rocks to an argillic assemblage dominated hy clay min- 1995). Within the ccntral upflow colum n overlying shallow erals (illite, ilIite-smet..tite, smectite, and kaolinite), calcite, intnlsions, the fluids in these syste ms are dominated by and siderite at temperatures up to about 150°C (Hedenquist, magmatic componeuts, including BCI, S02, and HF. When 1990; Simmons ana Browne, 2000b). these gases condense into the hydrothermal system, S02 Acid-sulfate steam-heated waters are close to 100°C and disproportionates, forming H 2S and l hS04 (Sakai and Mat· form in the vadose 7..one where H2S comes into atmospheriC subaya, 1977; Rye et al., 1992) and a very acidic (pH -1) socontact and oxidizes to H2S04. Their pH is -2, and they con- lution, (.'O ntaining subequal amounts of HCI and H2S04, up tain relatively high concentrations of sulfate (-1,000 mJikg). to -1 wt percent each (Giggenbaeh , 1997). HydrolysiS reacThese waters alter rocks to an advanced argillic assemblage tions with igneous country rocks progressively neutralizes of opal (cristobalite), alunite, kaolinite, and pyrite as the so- the acidity while forming hydrothermal minerals that inlution is neutrAlized near the water table (Sehocn et al , clude alunite, pyrophyllite, dickite, quam.., anhydrite, dias1974). The distribution of these three WAter types largely de- pore, and topaz, as well as kaolinite and illite, eharaderistic pends upon topographically controlled hydraulic gradients. of ~ fluid -dominated ~ alterdtion conditions (Reyes, 1990; In low-relief volcanic settings (e.g., calderas, flow-dome Giggellhach , 1992a, 1997). Surficial steam-heated acid-sul(.'Omplexes, rifts), the steam-heated waters occur above and fate waters also form in magmatic hydrothe rmal systems, on the pt!riphery of the chlOride-water plume, whereas in just as they do in thc wdose zone over geothermal systems, high-relief settings (e.g., andesitic composite cones), the due to the presence of HtS in the vapor. Silica sillters, howsteam-heated waters may extend from the snmmit to the ever, are absent due to the acidic conditions that inhibit sillower flan ks of the volcanic edifice; under the influetl(:e of ica polymerization and deposition of vitreous amorphous silsuch a steep hydraulic gradient, chloride waters may flow lat- ica (Fournier. 1985). Tn thiS setting. two styles of advanced erally long distances (>5 km) to form subsurface outflow argillic alteration, magmatic hydrothennal and stcamzones (Henley and Ellis, 1983). Hybrid compositions form heated, develop with different origins hoth containing aluwhere the waters mix. nite and kaolinite (Rye et aI., 1992).

496

SIMMONS £T AI-

Advanced argillic alteration

The origin of advanced argillic alteration can be determined from its morphology, as well as mineralogy and zonation (Table 5), and this information can be used to interpret the level of exposure and proximity to potential epithennal mineralization (Sillitoe. 19933; I-ledenquist et aI., 20(0). Mag-

matic hydrothemlaJ or hypogene. advanced argillic alteration includes minerals that fonn at >200 a C, such as pyrophyWte. dickite, diaspore, zlinyite, and topaz.. with alunite that is generally tabular and sometimes coarse grainoo. This a1terdtioll is epigenetic in natu re, so it generally cuts across stratigraphy and follows high-angle structures, although it can be stratifonn in permeable host rocks. Steam-heated ad vauced argillic alteration forms above the wate r table at - lOO"C in horizons with pronounced vertical miner.u wnation. Tn general, this blanket of alteration does not exceed 10 to 20 m il1 l.hickness. Tabular but diseoTltinuolls bodies of massive opal mimic and mark the water table, u nderlain by a discontinuous zone L'Omprising alu nite. kaolinite, opal, and variable amounts of pyrite and marcasite that gives way with depth to a kaulinite zone comprising kaolinite plus opal (Schoen et al, 1974; Simmons and Browne, 2000a; Fig. 4). T hese alterAtion minerals are typically very fine grained, and the alunite generally occurs as pscudocubic crystals. A third type oT advanced argillic alteration is formed by supergene weathering and oxidation of sulfide-rich rock... that postdate hydrothennal activity. This alterotion forms at c:40"C, within the vadose zone, and comprises alunite, kaolinite, halloysite, jarosite, and iron oxides and hydroxides. Supergene advanced argillic a1temtion also has a blanket like geometry that mimics topography, but it may line sub-vertical fractures that were patJlways for d eS

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0" 0(%0. SMOw) Flc.9. Stahle isotope (c) O YS. c)1"O) patterns foc flpithenmll tlt..-posits (compiled from Arribu. 1995; SiIllTllQIU. 1995; Cookfl and Sioulllons. 2000: and A~n$Ofl ct al .. 20(1). The trend fl,lr LepanlD 15 based I,In hydmthflnnal u1unile that Is a hall,l 11,1 Ihfl euaTlc.bearlng ore; Ihe trend lodicalflS condensatil,ln Df magmatic vapor by Ioca1 meteoric water (Hedenqui5t et aL 1998). 11le trend for r..IoIam represents Ill(I(\em Kt'CJIhennal walflrs and s1l(1\\13 m~ng bet"""",n magmatic ancIlo;x,.I meteoric WlIttr (Cannan. 20(3). 11lfl 0 shift due 11,1 wtllel'roc:k inlflmctit;m is bufld on Tllyktr (1919).

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2003). Adularia is a dominant gangue mineral, probably at the expense of quartz, which is generally subordinate (Je nse n and

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Barton, 2000), perhaps due to the higher quartz solubility Ilnde r alkaline conditions (Sillitoe, 2002). Fluolite, ros(;oelite (vanadium-bearing mica), and telluride min erals are common, although not essential accessOlv mine rais, und the occurrences of magne tite ± hematit e, "'Fe-rich sphalerite, and tetrahedlite-te nmllltite indicate low- to lTI(xlerate-suHklauon states (jensen and Bmtoll, 2000 ); lattice and (;ollofonn, handed vein textures are rare. Ores extcild over unusually large vertical inlen'als (500-1 ,000 m) and can be ilSsociated \\ith telescoping of epithermal lind porphyry environments (Je nsen anJ Barton, 2000; Sillitoe, 2002). Hydrothermal alte ration is re stricted to areas immediately ad.jacent to ore, where there is extensive deve lopment of propylitic and argillic asse mblagcs. The re is also a lack of zoning among temperature-sensitive alt e ration minerals , such as clays. Fluid inclusion studies indi cat e that ore fluids had saliniti es of vJasterman et aI. , 2004). III somc cases, the adjacellt oecurrences of mineralization have nearl), the same age, implying a close genetic link between hydrothermal activity, igneous iutn lsious, and orc min emli7';ltion, sHeh gro&pfIY is ~red by ~nse ~atioll and the rocb are~. _ therd. (ncUed by stlTamJ and m-eTS. Alilhe ep1lltennal depostts are assoa.ated witli quam tilanomagnetite), whereas oogenetic rhyolites locally contain quartz and Fe·rich olivine. Hydrol1.~ mineral phenocrysts are absent in most rocks of all compositions. The phenocryst mineralogy associated with the two igneous suites indicates that the western ande.'Iite magmas were more oxidized and water rich than the bimodal basalt-rhyolite magmas (Johll, 20(H ). The re are also distinctio ns in mineralogy and ~emical signatures of the associated epitJle nnal deposits tJlat c::orrelate wilh magma composition. In the western an· desite assemblage, most Au-Ag (C u) deposits with quartz + alunite :t: pyrophyllite :I: dickite % kaolinite gangue contai n yrite and/or marcasite, native gold, e nargite-Iuwnite. sphaerite, covellite, % chalcopyrite, galena, tetrahedrilc-lennalltite, bismuthinite, stibnite, and gold te llurides, indicating an Intennediate- 10 high-sulftdation state. They gcnerally have silver/gold ratios of 0.2 to 2 a nd chemical Signatures of Au, Ag, As, Sb, Pb. Cu :t Bi, Hg. Mo, Sn, Te, and Zn. In tJlis sallie magmatic assemblage, there also are Au-Ag (Cu·Pb-Zn) deposits with quartz % cnIcite ~ adularia :I: illite gangue. These deposits l.'Ontain pyrite, e lectrum , acanthite, silver sulfosalts, tetrahedrite-tennantite, gale na, Fe-poor sphale rite, and chalcopyrite ofintermediate-sulfi.datton state affinity. They generally have silver/gold ratios of 10 to 100 and chemical signa. tures ur Au, Ag, Ba, Mn, ± Cu, Pb, Se, and Zn. By contrast, the bimodal basalt-rhyolite-related deposits are relative ly sulfide IXlOr whe re hosted by rhyolites but relatively sulfide rich whe re hosted by basalts. They contain pyrite, marcasite. arsenopyrite. elcctrum . acanthite, siJver selenides, and mUlor chalcopyrite, stibnite, and Fe-rich sphalerite of low·sulfldation-state affinity and chemical signatures of Au, Ag. As, Sb, Sc, Hg, :t: Mu, TI, and w. Silver/gold ratios are generally S:1O (commonly near 1). These minenalogical and geochemical chanl(!teristics correlate closely with magma composition; water-poor and reduced lII afiC lllag:IIla.~ correspond to the development of do minantJy low·sulfidation- state ore mine rdl assemblages, \Y h e rea.~ w.lter-rich and oxidized intermediate magmas correspond to the development of intermed iate- (to lligh-) sulfidation-state assemblages. The work on the Great Basin deposits described above has no known counterpart in northern Mexico or anywhert: else in tJle world. I evertheless, Clark e t aI. ( 1982) and Staude and Barto n (2001 ) show, in a morc gellcr.u way, that metallogeny throughout the Mexican region correlates in space and time to tJ1C magmatic evolution a..~sociated with plate convergence during the mid to late Tertiary. Isotopic Jatillg and fluid

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5 wt % NaCI equiv) have no modem counterpart in analogous active hydrothennal syste ms, and the stable isotope data indicate that salinities renect fluid pulses having sibrnificant magmatic input (Simmons, 1991. 1995; A1hinson e t al., 2(01). The occurrence uf evaporites within the host-rock stratigraphy may in some cases account for high flUid salinities, as seen in tJle Salton Sea geothermal system (e.g., McKibben and Hardie, 1997) and in some Mexican Ag-ll b-Zn deposits, discussed above, but these appear to be the exception rather than the rule. Thus, the range of salinities observed in fluids that form deposits dominated by intennediate- versus low-sulfldation- slate sulfides hosted by quartz :1 calcite ::I: adularia .:!: illite appears to renect different sources. For fluids that Conn high- to intermediate-sulfidation-state sulfides hosted by quartz + alunite :I: p)'TOrhyllite ::I: dickite.:!: kaolinite, mode rate- to high-salinity flui( inclusions reflect the history of fluid exsolution during c rystallization of underlying magmas (e.g., HcJenquist et aI. , 1998; Heinrich et aI., 2004; H einrich, 2(05). aItJlOugh most salin.ities are low, rtance of boiling (Cooke and Sim mons, 2000; Deycn et aI., 20(4), as does the occurrence of ores in hydrothennal breccias of explOSive origin. The development of steam-heated watcrs and corresponding hydrolytic alteratioll in shaJlow and near-surface environme nts corroborates that boiling is (.'Ommon in epithennal e nvironments hosted by both magmatic hydrothennal and geothennal systems. Mixing between fluids of Jiffe rent t;olllpusitiollS is anuthe r viable mechanism of precious metal precipitation, as supported by some fluid inclusion data (e.g., Robinson and Norman , 1984; Mancano and Campbell, 1995; Hayba, 1997), extensive stable isotope data (Fig. 9), and numerical simulations (Reed and Spycher, 1985; Plumlee, 1994). There are also cases where mixing has not produced metal depoSition (e.g., Simmons and Browne, 2000b); however, in cases where metal depoSition oc'Currcd, it probably resulted mainly from dilution and cooling and, to a lesser extent, from changes in oxidation state and pH. For e ither mixing or boiUng to occur and cause ore deposition, a favorable hydrological setting must exist. Boiling requires sharp tempemture-pressuJ'e gmdie nts and a free lluiJ path to the surface (e.g., Simmons and Browne, 2OOOa, b),

whe reas mixing requires sustained interaction between Iluids of different compositions andlor temperatures (e.g., Hayba, 1997), preferAbly in a turbulent envimnment. Bnt not all epithermal deposits can be easily cxplained as simple products o f boiling andlor mixing. Among these are the large gold d eposits that l.'Ontaill te lluride-bearing ores over long vertical intervals (>500 m) in association with alkaline igneous rocks (Emperor, Fiji; Porgera, Papua New Cuinea; Cripple Creek, United States). For these, mechanisms of metal deposition seem complex, involving a combination of factors including boiling, mixing, and water-rock interaction (Ronacher et al. , 2(04), and posSibly reactions involving condensation o f Te bearing magmatic ga.~ into metal-be aring solutions (Cooke and McPhail, 2(01 ).

The role of the water table The regional water table controls the hydrostatic pressure gradie nt in subae rial hydrothe nnal syste ms (He illey, ] 985). Thus, the e levation of the water table and its shift with time relative to the land surface play an important role in dictati ng the vertical position of the epithermal environment (e.g., Steven and Eaton, 1975; He nley and Ellis, 1983; H enley, 1985; Sillitoe, 1993b). The position of tbe water table is inlluenced by topography and climate, so that in steep or arid te rrains there may be as much as several hu ndred meters between the water table and the land surface, in contrast to fl at and wet terrdins, where the two closely l.'OinciJ e. Steep terrains also inllue nce the regional hydraulic gradient, which induce.~ lateral flow and mixing in the epithe nnal environment, potentially conducive to metal deposition (e.g., He nley and Ellis, 1983; Hayba, 1997). Despite the dynamic nature of su rface changes in volcanic terrains, the effects of the water table on mine rali7.ation are not commonly documented. There are relatively few examples where the wate r table and its evolution have been interpreted and substantiated a.~ integral to ore genesis (Sim mons, 1991; Sillitoe, 1994; E bert and Rye, 1997; Bissig et al., 2002; John et al., 2003; WallaL'C, 20(3). The wate r table rises and falls in volcanic arcs under the influence of uplift, subsidence, erosion , volcanic e mption, and lake drainage. These effects range from local ( 10,000 km 2 ) in extent. Whe reas the minimum amount of time required 1.0 form an cpil.hennal orebody might be a thousand years o r more (He nley, 1985; Brown, 1986; Hedenquist et al., 1993), large shifts in the water table (> 100 m) can occur on the scaJe or hours to months, for example, due to voieanic e ruption ({;(Jne bUilding or calderA formation), sector collapse, or breakout ll00ding causing catastrophiC drainage of a lake-Ailed depression (e .g., Coff et aI. , 1989; Simmons et ai. , 1993; L6pcz and Williams, 1993; Manville et al., 1999). In such examples where the water table falls mpidly, the accompanying pressure drop may trigger hydrothermal eruption, brecciation, and p recious metal depoSition , whereas in examples of progressive e rosion , a fall in the water t.able may teles~l~ altemtion styles and ure l}(~lies (e.g., Simmons, 1991; SLlhtoe, 1994, 1999). By companson, water table changes induced by steady regional uplift of a few mmlyear requirc several thousand years or more to have a comparable effect, although this can be well within the lifes pan (dOO,OOO yr) of a Single hydrotherlllal system (e.g., Heyes. 1990; Bignall and Browne, 1994).

EPrTHERMAL PRECIOUS AND 8ASE METAL DEPOSITS

The overall effect of shifting the water table up or down during hydrothermal activity is to extend or contrnct the distance separating rocks altered and mineralized in deep and shallow environments and. if precious metal mineralization is forming at the same time, to change the vertical distribution of ores. Evidence of the position of the water table is deduced from fluid inclusion data and shallow alterntion patterns (blankets of steam-heated advanced argillic alteration, silica sinter). In regions where the water table is rising (e.g., due to regional subsidence or damming of a drainage), 110tter alteration assemblages prograde on to cooler altemtion assemblages and silica sinters may become stacked in the stratigrapbic record (e.g., Hasbrouck Mountain, United States, Graney, 1981; Drummond basin, Australia, Cuneen and Sillitoe, 1989). In regions where the water table drops, cooler altemtion assemblages retrograde on to hotter assemblages and mineraliZll.tion is telescoped. This latter situation may explain occurrences of epithermal- over porphyry-style mineralization (e.g.. Sillitoe, 1999) or. in the very rare case, in the vicinity ofbatholit.h intrusions (Kesler et al., 20(4). J mplications for Exploration

Epithennal deposits are variable in size. shape, and grade, and eommonly the ore zones are not exposed. These characteristics make them elusive to find and a cballenge to explore (e.g., Sillitoe, 1995,2(00). Although some deposits are large and continuous, many are not (Fig. 3). Efficient explonllion thus requires integration of all available geological, geochemical, and geophysical data with a good understanding of deposit characteristics and ore-fonning processes. plus a \.Villingness to drill targets generated from these data. Epithermal deposits have the benefit that there are many features , as discussed above, that provide valuable information on erosion level and mineral zonation. There bave been many exploration successes since the revival of interest in epithennal depoSits in the late 1910s. Some have resulted from reevaluatioll of the potential for low-grnde bulk mining of deposits Originally exploited as high-grnde deposits (e.g., Martha Hill, New Zealalld; Rouml Mountain , United States), and others have resulted from di.scoveries of new deposits in known mining districts (e.g., McLaughlin and Sleeper, United States; Colden Cross, New Zealand; Hishikari , Japan; San Cristobal, Bolivia). Some have been the result of expanding exploration into previously unexplored districts (e.g., Ladolam, Papua New Guinea; Mount Muro and Kelian, IndoneSia; Ellndio, Chile; Cerro Vanguardia, Argentina). The discoveries of epithermal deposits in areas originally being explored for other types or styles of minerdliZll.tion (e.g., Midas, United States; El Penon, Chile; Victoria, Philippines; Nevada. Chile; Veladero, Argentina; Sillitoe, 1995, 2(00) highlight tbc critical importance of explorers being familiar with the key characteristics of the different styles of epithennai minemlizatiol1 so they can re{.'(}!,l"Jli:r.tl its Significant traces, even if exposed in only small areas. The first objective in exploration for epithermal deposits is to choose the favorable regions to explore and then to narrow this extensive prospective region to a manageable area for detailed exploration. and finally to define targets for drill testing. Once a potentially economic intersection has been achieved, further work is most1y resource definition and evaluation,

515

rather than true exploration, although it is commonly COIlducted by the same staff and many of the same skills are required. Tile distinetion ilere is between explOring for a deposit and assessment of a deposit. There are few technical papers that address exploration issues directly (Sillitoc, 19950., b, 2(00). White and Hedenquist (1990, 1995) and Hedenquist et al. (2000) described and discussed attributes of epithermal gold deposits that are useful for exploration. More specific geochemical and geophysical aspects were addressed by Allis (1990), Clarke and Covett (1990), IlVine alld Smith (1990), Ellis and Robbins (2000), and Wright and Lide (2000). Table 6 shows the main goals alii..! methods used at different scales of the explordtion process. Some exploration programs cover all scales, whereas olhel1l begin \vith a relatively well-defined target. In all cases, the aim is to reduce the area, and then the volume of rock being explored, with the goal of ultimately defining an economic deposit. Exploration techniques can be conveniently clasSified as geological, geochemical, and geophysical in emphasis. In general, the geologie techniques are tbose that geolOgists do themselves and are dependent on recognition and mapping of geologic features. Ceochemical techniques are those that involve determining the concentration of various chemical elements. Geophysical techniques involve measurement of geophysical parameters that are subsequently inverted to produce a geologie interpretation. Geologic techniques The aim is to use geolOgiC characteristics to identify prospe-.... ClaVl!ria, R.J.R .• 2:001. MinerW p!U"ageneJls the l.eranto .....,ppcr and gold and the Vidorill gold deposits. Mankayan minerW district. PhiUpplneJ: Re· source GeoJosty. Y. SI. p. 97-106. Cline. j.S .• 1I0fst... A.H.. Muntcltn. J.L.. TosdaI. n.M., ancI Hit-ky. K.A .. 200:5. Carlin.type gold dt.-pmitlln Nevada: Critical geologic dwacteristlcs

or

EPITHERMAL PREC10US/oND BASE AlETAt DEPOSITS

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