Depositos epitermales

r

•

Geo

l

ogical Charactelistics of Epi

th

e

rmal Precio

us

a

nd

Base

M

etal

D

eposits

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 DAvlI) A.JOHN

U.S. (~I!.iall SUI~. 345 AHddlefJeld lid., Mellio Park, Cali/omla 9.1025

Abstract

Epithermal deposits are impol'tallt sources of gold and siiver that foml at <l.S-km depth Bnd <3O()OC in

higb-tciliperahlre, mainly subaerial hydrollu:::nnal systems. Such hydrutlJcnnlll systems oommonly develop in llS5OC'iatiOIl with calc-alkaline to alkaline Illllgmatism. in \'Olcanic arcs at convcrgent plate margins, 35 well as in

intra-arc, back-arc, and ~lcolllsional

rin

settings. Many important deposits are Tertiary and younger in age and are conccntntt(.>d llfOunrl thl" Pacific Rim and ill the Mediterranean IUld Carpathian region.~ of Europe.

()1cler deposits oc'(:ur in the Tethyan arc from Europe to Asia and others are scattered UI vokanie IlJ'CS of all ages

with rare examples os old as Archean.

Precious m~al mineralization ili:velops in zones of high pakoupcnneability, hosted within sequences of (:0-cval volcanic and underlying basement rocks. Veins with steep dips are common and these tend to host h.igh -t.'St gnule ores. PreciOuS metal mineralization also occurs in breccias, coarse clliStic rocks, lind intensely leacht.'<i rocks: such disseminated Or1! is much lower in gnade but greater in total tonnage and may he amenable to bulk

mining Ulcll ... Kb. DeposiLS and districts. comprising one or more orebodies. cover areOilS from < 10 to -200 kml. Epithemlal depoSits have been classified on the basis of a1teratinn and gangue mincnUassemblages, metal contents, sulfide contents, 1lI1U sulfide mineral assemblages, IUld eliCh cla.~siAcation scheme has it~ merits. Sli -cause ores are oxidized by weathering, we preflir a classification that utili7.e5 gangue mineralllSSCmblages. Wli

describe

""'0

types of minemlil'..llotiOIl anoc!ated with quartz .:t: calcite :I: adularia :I: lillie and quartz .. afunitc :I:

pyrophyllite :I:-dickite.:t: kaolinite 8SSCmblag<.'S, which reflect the pH orhydrulhennal solutions.

Epilllcrmal deposits associated with qua.u.:t: <."aldte :I: adularia :to illite contain Au-Ag, Ag-Au, or Ag-Pb·Zn ores. Electrum. ac.:aJlllLite, silver 51llfosalts. silver SciCllidt.'S, and AII.Ag tellurides are lILe main gold-and silver-hearing minerals. with generally minor sphalerite. galena. and chalcopyrite; In some deposits base metals dom·

inate the metal assemblage_ Quartz is the principal" gangue mineral accompaniM by variable amounts of chal -cedony. adularia, illite. pyrite, cllkite, antVor rhodochrosite. the latter ill more Ag-and bllse metal-rich deposits. Distilll.'tively banded crustiform-collofonn textures, and lattice textures comprising aggregates of ploty calcite

and their quartz pseudomorphs, are common. Hydrothennal alteration is wned and comprises deep regional

propylitic alteration. which gives way upward to iLK'f"Casing amounts of clay, carbonate. and zeolite minerals,

whereas quartz. adularia, illite, and pyrite fonn proximal alteration zones enveloping orebodies. Ore-grade mlnernHL-1tion commonly tcnninates upward. and where there has been minima.! erosion. it call be (''Oncealed beneath regionally extensive blllllkebi or c1ay-carbonate-pyrite or kaolinite-alunite-opal :r pyrite alteration.

Fluid inclusion data Indicate salinities arc (''OLmnoniy <5 wt percent NaGI equiv for AIL.Ag deposits and -dO to >20 wt percent NaGl equiv for Ag-Pb-Zn deposits. Stable isotope data indkate that hydrothennal solutions

were composed mostly of deeply circulated meteoric water, with a nil to small and variable component of

mag-matic wnter.

Epithennal deposits associalM with quartz + alunite :I: pyrophyllite :r dickite :r kaolinite assemblages con

-tain Au :to Ag:t Gu 0(<.'$. Native gold and elect:mm are lIlC mail! ore-bearing minerals, with \'Wiablc IlInounls of pyrite, eu-bearing sulfides 1lI1U sulfosalts such as enargite. luwllite. (,'O\'eIUte, tetrahedrite. and tcnnantite, plus sphalerite and telluride minerals, emugite dominates the Gu )"ulfides and Indicates a high-sulfidatiun mte.

Quartz (both massive and \'uggy) and alunite are the main gangue minerals with kandite minerals (dickite

alldfor kaolinite) andlor pyrophyllite_ CoTl(:entric paNems ofhydrothemlal alteration envelop the zone of vuggy

ulld massive quartz alteration. which hosts Uri:. Outward. these comprise WILes uf quartz and alunite. dickite .t

kaolinite ur pyrophyllite. and illite or smectitc a1tenltion. surrounded by regional propyUUc alteration. Zones of illite or pyruphyUite alteration occur in the roots beneath some deposits. Fluid inclusion data indicatli that salin

-ities are Iypkally <5 to 10 wi percent Nael equiv but may be as hid! as >30wt percent NaGI equiv. Stable iso-tope data indicate that the altering nuids llIe (''OlIlpost.>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 1I1<.-tt.'OriC compo-nents in solution, and lIle amount of subsequent water-rock interaction during ascent to lIle epillLcrmal envi· I Corresponding author tl-mail. ,f.simmorulihuc1c4nd.ac.nz

486

SIMMONS £T

A1-mnment; (2) at epithermal depths. the dev!:lopm~nl of boiling andl"or mixing conditions which create sharp physical and chemical gradients conducive to precious and base metal precipitation; and (3) at shallO\\l' level. the position of the WIIter bl.ble, which controls the hydrostatic pressure-temperature gradients lit depth where epilhennal mUlcnd.i:tatioli fonlls.

Epithenm:al mineralization can occur in large areas, with orebodies that mnge in shape, size, and gnuJc. lind

lie easily conCt.'aled beneath blankets uf clay alteration or unaltered volcanic deposits. Efficient explOrllliOIJ re-quires integration of all geological, 8,,""OChcHlical, and geoph)'5lcaJ data, from regional to deposit scale. Vein min -eralogy and texture. patterns of hydrothcnnal alteration, patterns of geochemical dispersion. and three-di -mensional interpretation of related geophysical siglla.tures are Important guides. Willingness to drill is crucial, as surface feat\lre5 may not reliably indicate what is pn .. "SC1l1 al depth.

Introduction

EpmlERMAL deposits fonn in the shallow parts of high-tem

-perature hydrothennal systems that commonly develop in volcanic arcs (Fig. 1). The deposits are host to both precious and base metals, but in the

pas

t

three decades, Uley have l.Jeen mined mainly for their gold and silver contents. The

totn! metal contents of some orebodics are substantial, and lo-cally the precious metal concentrations of some achieve

b0-nanza grades (>1 MaL Au at >30

gIt;

Sillitoe. 1993a). Some

deposits have been amenable to mining by simple methods dating back many cenhuies (e.g .. Abbot and Wolfe. 20(3).

The Spanish empire reached prominem:e during the colonial period (ca. 1500-1800 AD) through exploitation of ule ep-ithermal ores of Mexico, Peru. and Bolivia. rich in either gold or silver. [n the mid lSOOs to early l000s, epilhcnnal diSl:UV~ crics fueled gold-silver rushes to Nevada and New Zealand. During the past few decades, improved ret:overy methods

and favomble gold and silver prices (since the late 19705)

have enabled many low-grade orebodies to be mined. [n total,

A

Magmatic-

Hydroth

ermal

B

Geothermal

~ N

~

~

_{, , ,i '}

_,

_,

" v " " v

,

~

•

L:

2km volcanIc rocks basement intrusion ~

.

,

.""

,

v "

y.

~-t

R

water table

t

magmatic fluid ... " meteoric water , epithermal deposit

Flc. 1. Simplified COnceptllal models of high-temp ern til III hydruthcmlal5)lnems. showing the reladonlhlp be~n

ep-ithermal envimnmenu, nmgmnli(!' Intrusions, !luiu (,1rculllUon paths, and volcaniC and basement hOllI mc:kli, A, TIll, "pither.

mal environment forms in 1I111agl11l1tic-hydrothennai system dominated by acid hydrothemlaJ fluids, where tllere is II strung OWl of magmatic IIqulU and vapor. containing lit<), COl. IICI, IItS. and SOt. witll variable inpllt frum lot'al meteoric water. Ibis type of environment Is analogous tn those existing in modem vok'VnOC!i (e,g .• Hedenquisl et al.. 1993: Chrl!!tenson and Wood, 1993). B, The epithennal environr1l("nt fornu in a geothermal system dominated by near .. ne11lral pi I chloride waters,

where there i5 a strong

n'LA

of lk>L-ply dn:ulated water (mcmIy of meteoric origin), containing COs. NBC], and HiS.

nlis type

of system is analogous to those exploited for generation of electricity (e.g., Simmons wid 8""""". 2(l()(#. b). The inferred lo-cation of the underlying ~chamben; in hoIh (A) 811d (8) lllC purtnlyed to show the different path IengthJ thai deep nIl' Ids tra\'erse before encountering the ore-forming enviromTlCnt. The relatively short path 10 the epithermal environment in

(Al meall5 there Is minimal _tef'>ltx:k interaction during ascent. whereas the relatively long path to the epithennal en'; .. ronmcnt in (8) ml"llJ\S tllcre Is considerable water .. rock Interaction during ascent. TI,,, m.wnmlll prcssun. .. -temperature gra. dient under hydrostatic conditions is represented by boiling pooul for Ot.'Ptll (8PD) temperatures. which are also Ihown for reference.

EPITHERMAL PRF.CIOU$ AND BASE METAL DEPOSITS

48

7

about 6 percent of

all

gold and about 16 percent of all silver

mined have C(Jme frulll epithermal deposits (Singer. 1995). and their wide range of tonnage-grade chanl.{:teristics (Hedenquist et ai., 2()(x)) make them an attractive target for both large and small exploration and mining companies.

The term epithermal derives from the genetic classification scheme for hydruthermal are deposits proposed by Lindgren (1933). On the basis of stratigrophic relationships in volcanic sequences, and by analogy with minerdl and metal occur -rences and mineral textures in active hydrotherm31 systems, Lindgren inferred that epithermal deposits fomled at <200"C and <100 atmosphere.~ (-100 ban). Aside from the seminal paper by D.E. White (1955) that strengthened the link to all active hydrothermal environment, there W.iS little advance in the understanding of epithemlal deposits until the late 1970s when exploration interest rose due to the increaosing value of gold and silver. New research techniques applied to these de -pusH's included fluid inclusion studies that extended the range of formation temperature to lIhout 3(X)"C (e.g., Nash, 1972; Ca.~atlevall and Ohmoto, 1977; Kamilli and Ohmoto. 19TI; Sawkins et 31.. 1979; Buchanan, 1981), and stable isotope studies that indicated the prevalence of meteoric waters in the formation of gangue minerals from some epithermal d e-posits (e.g .• O'Neil and Silbennlln, 19i4; Casadevall and Ohmoto, 1977; Kamilli and Ohmoto. 19TI; Sawkins et aI.,

1979). Experimental and theomti<.w tl.'dmiqucs were used to t1etcnniuc metal solubilities and mineral stabilities tlTlder hy-drothermal conditions (e.g., Sewanl, 1973; Barton et al.,

1977; Barnes, 1979), which led 10 numerical simulations of reaction paths and ore formation (Reed, 1982; Drummond and 01.111010, 1985; Reed and Spycher, 1985; Spyeher and Reed, 1989).

Meanwhile, in Nt!W Zealand, Japan, Philippines. United Stales, and other countries, the demand for altenlative sources of electricity em:ouragcd geothermal exploration drilling and development. Temperatures and pre.~nres s imi-lar to those in the epithemlal environment were encountered at depths of less than 1 km (e.g .. White. 1981; Henley lUlti

Ellis, 1983), and precious amI ha.~e metals were found de· posited in springs. wells, and surface pipes (e.g .. Weissberg, 1969, Hedenquist and Henley, 1985a; Brown, 1986; Krupp and SeWHro, 1987). The rapid increase in understanding at the time

was

such that the

first

two volumes uf Reviews ill Economic

Geology

focused on the nature of epithermal e nvi-ronments (Henley et al .. 1984; Berger and Bethke, 1985). Thus,

by

the mid 1980s, genclic models were formulated to explain the occurrence and zonation of metals and minerdls, to define the physical-chemical col1ditiuns of ore deposilion in several epithermal depoSits, and to prOvide a basis (or

spec-ulnuon on the sources of fluids and metals (e.g., Barton et

a.I.,

1977; Kamilli and Ohmntu, 1977; Sawkins et 31., 1979; Buchanan, 19SI; Berger and Eimon, 1983; Henley and Ellis, 1983; Hayba et

aI

.,

1985; Heald et aI., 198i; Stoffregen, 1987). In these models, hydrology \VQS seen to

be

an e5sential fador in producing Ofe deposits, with boiling and fluid mixing being recognized as causative agent.s for metal deposition. Because they o\'erlap in temperature and metal suite,

Carlin-type

depoSits were initially included in the epithermal realm by several workers (e.g .• Radtke et al., 1980; Berger and Bethke. 1985; Radtke. 1985; Berger and Henley, 1989), but

they were later defined as a distinct class of sedimentary rock -hosted hydrothermal ore deposits (Kuehn and Rose, 1992, 1995; Horstra and Cline, 2000; Cline et

aI

..

2005). The mod-em use of the term epithennal thus retains much of Lind -gren's intent and inSight.

Since 1990, numerous articles have reviewed the nature and genesis of epithennal gold-silver deposits (e.g., \Vhile and Hedenqwst. 1990, Sillitoc, 1993a, h; Arribas, 1995, Hichards, 1995; Simmons. 1995; Cooke and Simmons, 2000; Jensen and Barton, 2000; Sillitoc and Hedenquist. 2003). in this paper ... 'C draw heavily on these references together \vith data, mostly published since 1975, tabulated for more than 70 epitllCrmal depoSits (sec App. Table AI) that were selected to represent the range of typical characteristics and geographiC distribution. The ileposits cited below as examples nre listed in the Appendix along with their relevant references. and their locations are shown in Figure 2. Plan maps of some d e-posits (Fig. 3) show the main geologiC features and the di-mensions of ore zones. Our aim in this paper is to provide an overview of the tliversity of features that characterizes ep -ithennal deposits and to relate the..>;e to C(Jmmon ore-forming processes and to suggest stnltegie5 for exploration.

Definition and Classification

The term epitherm31 refers to a range of temperature ver-sus depth (pressure) ore-forming contlitions that develop within much larger, mainly subaerial, hydrothermal systems (Fig. 1). Depth relates directly to pres.sure in the shallow en -virunment where near-hydrostatic conditions prevail. with maximum temperature largely controlled by the boiling -point-for.depth curve (e.g., Haas, 1971; Fig. 4). Ore minerals precipitate at temperatures ranging from _150° to _300°C and at depths ranging frolll

-so

to as much as 1.500 m below the water table, caused by chemical changes that result from sharp pressure and temperature gradients in this environ -ment. These physical controls define the epithermal environ-ment. although ore genesis also depends on the composition of the hydrothcnnal solutions, whiell controls metal transport and deposition (e.g., Henley, 1985). Such metal-transporting solutions vary in composition and differ in origin (e.g., Ar-ribas, 1995; Simmons, 1995) and thus vary in their metal en-dowment (e.g., A1binson e1

aI.,

2(01).

Lindgren (1933) showed that despite sharing common gangue minerdl assemblages, metal inventorie5 of epithermal deposits range Widely, with varying proportions of gold. silver, ana base metals. including mercury, a.lllimony. tellurium, and seleuium. Lindgren recognized their diverse characteristics when he distinguished nine deposit type.~ based on metal oo

n-tents (Le., dnnabar, stibnite, base metal, gold, argentite gold, argentite, gold telluride, gold tellUride with alunite, gold se

-lenide). Nevertheless, epithennal depoSits mined todny are prindpallya source of precious metals.

Since the late 19705. over a dozen classification schemes have been proposed (Table 1). All or them consider some as -pect of ore or gangue mineralogy and most reflect some as

-pect

of the fluid chemistry (pH, oxidation state, or sulfidation state) associated with proximal hydrotherm31 31teration andlor ore mineralization (Table 2). That so many schemes have been proposed reflects the wide rd,nge of chlltl:l.cteristic features displayed by orebodies, as well as the evolution in

488 SIMMONS ET A.L

Ftc. 2.. Lxation of t-pithcnllal deposi.u)me<! in Appeodb: Table AI. labels include 1nosl okpos.l'I mcntiunl.-U in the ,"" but some Ilre leA: out for darity. Abbreviations: Sa ,. Baguil'l cI~rict (ACUpotn); 8M • 8m Marc; Bo ... Bolide"; CC .. Crip-ple Crce\c.: Ch .. Chinkuashlh. ehe ... Chelopecll: CP ... Cerro de Pasco:> lind Culquijiru.-San Gregorio; Cr ... Cracow; en ...

Cerro RIc:o; CV ... CeITO Vangllardia: EI-P ... EllndiO-PIlSt:UIl: Em ... Emperor. EP ... 1::1 Pel'lon; Es .. Esqnel; Fr .. Fres.nillo: ru .. Fllrtel: Cto .. Guanajll.lll0; H8 ... Hope BrooI<; HI .. Hlshikarl;

I

n

...

Jukanl: Kc .. Kelian: La .. LadOlolll; Lto-Vi .. t..ep. anto-Victoria; I.

e ..

Ltt Coipa: Ma .. Murtha Hill-favona: Me .. Mclaughlin: Mi .. Mi~;llIa; 0.. ... Ovacik: PII. PilChm:a-Real del Monte: Pi .. furina; Pj .. Pajingo: Po .. Porgera; rv .. Puehlo Viejo; R1o.'1 .. Round Mountaiu; Ro .. RodaIqullar; Ta • TM)\'ltihl: Tc .. Temora; Y . .. Yanacocha.

thinking. Acid-base and redllction-oxidation fluid-mineral

equilibria underpin the par.unefers that distinguished acid from alkaline types (Sillitoe. 1977), aCid-sulfate or

a1unite-kaolinite from adularia-sericite types (Hayba et al.. 1985;

Heald et aI .• 1987; Berger and Henley. 1989), and high -from low-sulfidatioll types (Hedenquist. 1987: White and Heclenqllist. 1990, 1995; Sillitoe, 19933.; White and POi7..at.

1995).

TABU: I. Evolution of Claulr~!ion Sehemi!! Applied 10 Erid...,n",.! Olpos;t' C .. >Odlfll"ti from SllIItoe ilnd Iledenquist. 20(3)

Silliloe (1977) BuehalWl (1981)

Ashley (1982)

Gik'S all,] Nelson (1!l!:I2) Bonham (1986. 1988)

lIayba I!t aI. (1985) i1e-ald et al. (I98i)

Hl-dcnquist (1987). White and

Hcdcnqui$t (1990, 1995)

BlI'rgll'rand IllI'nley(l9AA) Alhino alld Margolis (1991) Sillituc (iu.s9. 1993a)

\\o1l1le and Poiw! (1995)

Hedenquist ct ai, (2000). Elnaudi et aI_ (2003), Sillitoe and Hedenquisl (2000)

Cooke &lid Dc)~U (!!003)

Acid En2r'gitll' gold High $ulfur Acid sulfate High sulfidation Alunite-kaolinite IlighSulr;cbtion High sulf1dation An-Ag-CII deposits Wllh vujlg)' qUIl.l1:t iIlteration AU-Ag-Cu w.~ltswith pyrophylUte-sericlte alleration High sulf.datiull Alkalii'll! Epithl!':nnaI Hot-spring type Low sulfur Aduram.-seridl!: Low ,ulfldaUon AtI.,L~nQ-S<!rilile

T)'pII' I Il<tulllrill-scrilile l)-pe 2 adularla-SI!ricite

Low sulfldation

High sulfide + h.:ue metal Low mlftdc • base metal

Low sulfldatlon Sn-Ag-base metal deposits Ag-Au-base

m,'"

depos,tJ; Inlennedi:lle sulfidation Au-Agc!epos,15

Wilh calc-ultu.liuc \\~th alkaline ,,,lclllile rocks yolcanic rocks Low suilldallon

Descripth"fl nomenclature based on ore metals. dcpoJlt form, diagnostic h)pogene gangup. and alt~l"lIhu" ",iIlCOOs,

EPITHE.RMAL PRE.CIOUS AND BASE ME.TII.L DE.POSITS

489

l'..\I;ILE 2. DiIIgnl)slic Minerals and Textures of Variow; Stutes of pH, Sulfldation and OxIdation Stille U~ 10 DIstinguish Epithcrmal Ore-F"orming En\1ronmentf (Ciggl."bad" 199i; Einaudi et aI., 2(00)

(the.~ of h)Jlhens between minerals md/Ulles an equilibrium assem~e for wltid. all

phases

need to be present) kid IIII

Alunite. kaolinite (clicklte), pyrophylUte. reskluab-uggy q...rt:"l

Hig/, .ulfodlltiofl

Ntlltrol pH

Quartz-adularia J. illik, •.• ;<J...~le

t.l/ernwiiat, ,ulftdotkm

Pyritc~nargite.% IUZ(lnite. co\,eUite-digenite, famatinitc, orpimenl

Te.lIlantite, tetrahedrite, hematite-pyrite -magnetite. pyrite, chalcopyrite, Fe-poor 'Pfl&lcrile-pynte

Low tu!fidDtion

A~nopyrite.loellingite-pyrrl,otitt', pyrrhOlite, Fe-rich sphalerite.pyritc

Oxidlzccl

Alunite, hematitc-lIl11j1;llChte

Application of the term -sulfidation,~ which in respect to

cpithermal deposits was initially

used

to describe the oxida.

tion state of aqueous sulfur species of deep ore.forming solu-tions (Hedenquist, 1987; Hetlcnquisl and Lowenstem, 1994). was merged to agree with its other wide.~pread usc in ore petrology to describe the stabilities of sulfur-bearing minerals in terms of sulfur fugacity (e.g., Barton and Skinner, 1967, 1979; Hedenquist et al., 1994; Einaudi et

al.,

2(03). This

re-sulted from recognition that epithermal ore mineml

assem-blages (:ould be distinguished in terms of lhcir high-, inter

-mediate-, or low-sulfidation state (John et a1., 1999) and iJlat fluids forming these assemblages could change sulfidation

stutes in response to chemical evolution both in space and

time (Ein3udi el

aI.,

20(3). The variability in ore minerology (cspcciruly Fe-, Cu-, and As-bearing sulfides) and in the s ulfl-dation states (.~.i11

be

correlated to pnxJe!i.Ses wiiJlin the ep

-ithermnl environment, as well as to igneous rock compositions and tectonic setting, the latter reflecting fundamental con -trols beneath iJle ore·fonning environmcnt (John et al.. 1999

:

John, 2001; Sillitoe and Hedenquist, 2003; see Table 3), Al

-though there is much remaining to learn about these rela· tionships, the slllfidation-stnte tenninology refleds the cvolu· tion of ascending hydrothemlal fluids and assists in understanding the genesis of epithermal deposits (Einaudi ct

al .• 2003).

111e classification schemes in Table 1 that are based on al-teration and gangue minerals associated wiiJl gold and silver

",",,ad

MlIgnetile'pyrite'p)'IThotlle, chlorite-pyrite

ore are useful, cspccinlly at the early stages of prospect eval· uation, because their respet.1:ive rock textures and alteration

zonation patterns can

be

distinguished in the field or with

lIlinimni petrographiC study. By comparison, the sulfide min-enlis in epithennal deposits that occur near the surface and in tht! vadose zone are susceptible to rapid oxidation and con·

version to supergene minerals. Thus, the potential inSights

re-sulting from determining the sulAdation states of precious metal mineralization may be elusive or difficuJt to determine from field examination of

rocks

and may not be precisely es

-tablished until exploratiun uf a prospect or deposit is well un-derway. The hvo cnd·member types descril>ed below are

based on the hypogene gangue millenll assemblages that con· tain quam. :t (.-alcite ;z; adularia :t illite and quart~ + alunitc :t pyrophyllite :t dickite :t kaolinite. These minerol o.ssemblages form from solutions of near·neutml and acid pH, respectively, but as discussed later in this paper, iJ\C fluid compositions in· ferred from these mineral assemblages may differ from iJIC (.'ompositions of ore-forming nuids transporting metals.

Cooke and Deyell (2003) suggested another means of

classi-fying deposits that is based on the metal contents, deposit form. diagnostic hypogene gangue and alteration minerals, and iJle dominant Cu·bearing mineral (Table I), similar to the deSCriptive proposal made earlier by White and Hedenquist (1990). This classification has merit but is dependent on de -posit familiarity. the lellbrth of the names may limit its future use.

TABLE 3. Summll1)' of RcUtl.iUnships ~ Sulfldadon Stille of Ore-Forming Environment, Related Igneous RocIc ComjXl>&itioN,

and Tectonic Setting

Proposed

by Sillitoc antfHedeoquist (20(3)

Sulfldat10n nate Igneous mek composition'

<..:ak-a1lailinc. andesillHiacite

il)Iermcdiale Cak:-alkalil1c, audesite-myolile

I~ Caic-allcatine, alkaline, tholeiitic biTnudul hasalt-rhyolite

I Genetic relaliollslul' ... ferred hy temporal-spatial condation

Tectonic St.-thllg

Magmatic an: In a neutral to milc1ly eJ.tensional stress state: COll1pres5i't'l!; stress state UTKOmmon but se .... -es 10 suppress \"O!clI.llic acti>ity

Magmatic arc In I neutral to mildly Cl<Iell$ionaJ stress state; compressive Slreu stale I"Ilre

Magmatic arc undergoing exteTUion leading to T1fting;

490 SIMMONS ET Al_

Cerro Vanguardia 131 tAu

1,605 tAg Guanajuato

...

175 t Au 34,850 t Ag

,

Cracow

)

_.-f

(

,

,

'

,

,

Pueblo Viejo

--26

tAu

30

tAg

1,

2

4

2

t Au 7,062 t Ag

-·-

t

·

K-\

"

•

-

""-iii . . . .

•

--•

\.

_...

'

\

'

.

" ~" '.~ ".

.

"-

... ...

,-,

....

.

"'"'--. o·--Fd Rodalquilar

/'

,:

,

,

,

" "

-

_

--

AdIIWlCed IrgIIIic

....

_ Intenntodiatl arviIIiC

•

.

~

....

•

•

•

..

_..

..

_..

..

_..

..

_..

••

..

-'-\ \, \, . '

..

-" 10 t Au

_

....

0 _ I r~ .lbl.ndanI s~ 2 ...

u.

'"

115

t

393

tAg ...

1"'1(;, 3. Sketch maps of f!pitM:rmal depos,ts, SllO"o1ng the outlines of orebodles grouped according to K:aJe. 11leSe

.nus-half! the great vlIriahility in lilt: sizes llml shapes of orcbodles. Note that total production conelates poorly ,,;ti, the IIn·a] Cl[

-I .. nl of orcs. ",hidl is further n:l1ectcd In the data set reported In APpP.ndit Tahle Al MUI)S IIfC n.-dn<v. .. , U pn.'scntoo in pub-lications 10 there Is some Inconsistency In, for example, locahng the uct'Um:TI~.,"" uf veins. References for each dqx>sil a~

given In the Table Al.

Fresnillo Acupan

,/---,.-'

..-

..

-.,<

/ '

•

Ellndio

r.PITHE.RMA1~ PRECIOUS AND BASE METAL DE.POStTS

-200 t Au

200

t

Ag

•

26 t Au

16

,

050 I Ag

Emperor _{136 t Au}

Martha Hill-Favona

263

I

Au

1

,

253 lAg

•

Hishikari

260

I

Au

_~:;;~

140lAg

-~-

~~~-}ole. 3. (WI'.) Summitville 17 t Au

231Ag

-:;

.

:.:..'--.

... .

'

.

....

-...,

...

,

....

_

u.--.. _____ ...

~"'O' ~.'-

_•

,-4

9

1

-492

SIMMONS ET AL..

A

B

C

~

Vertical distribution Alteration zonation at

,,~ _{of minerals the water table}

i

'

.

.

•

I~:S ~~~

~ "e~

=:

~ in boiling upflow zone

i06~ ~

...

_{... 1>.}

it

iT~'~ (J~

• =

~{

H~"

TemprC) E-·~.Q _{.... 1OciiiiD _3'}ij'" _{OII Depth (m) ~}

t

i

acid alteration massive opal

".

0

_"'

_'iil

_I

~

I

.

ii

,

_,

,

.

,

_,

,

_,

_1;

,

,

_,

i!~

,

D

,

"'

,

!!!

QI

,

,

-

\

_•

!

i

_•

Depth (m) crlstobalite + sulfur

..

_\

o

-J

massive opal

,

_,

1

,

,

,

_•

,

\

a

"

,

..

\

~

11

..

I

,

, ,

, , ,

~

".

_Temperature

.. ..

_rCJ

FtG. 4. Kfl)' indieator minenls in "''Pitilol"rmal er,\;mnme:n15. A.. Stability range uf temperature-sensitive cbys. phy!IosiH·

elliei'. IIntl ".oolite'! (Jll'mley and Ellis, lU83: Keyes. 1990). B. Vertocal distrihulion of some of the l'IIJrle lIlirlera~~ lot'led

ac-cordill 10 tlepth. 1I~lng the hydmstatk: holling curve.., th", reference Temperature gr.«I.iCIlt. C. Diagnostie h roIhermai

mine~J

fanning at the waleI' table, comprising silica sinter where near.neutral 1'1-1 waters dbcharge arotJlI ooiliug hOlt

springs and vmicall)' "(:on~1 acid alterntion (modified from $illilue, 199.%). D. MagnifICation of ~rtically ZUll(.oU steam.

ho!:ued acid alteration lit th.., wate~ table (Schoen ~t aI., 1974, SlmlTlOlU and Browne. 2000a); cristoballte and sulfur form lit and alxwe the water table; tabular rn~ive opal forms at and below the water table; lIIunite and kaolinite form III and below the WlIter table and the 'WIle of manl\'e opilI.

General ChlUllcteristics of Epithennnl Deposits Epithermal deposits comprise epigeneHc ores that are

g(:n-erally hosted by coeval and older volcanic rocks and/or

un-derlying basement rocks and rarely by subvolcanie intrusions.

They cover areas that range from dO to >l(X) km! (Fig. 3).

The orebodies occur in a diversity of shapes that reflel.1 the

influence of structural and litholOgical controls, and they

rep-resent zones~f leopenneability within the shallow parts of

once adive h rothermal systems (Figs. 3, 5). Most com

-monly, ore 'es occur in veins with steep dips that formed

through dilation and extension. Some are hosted by major

faults but more commonly they are hosted by minor faults

(seoond-or third-ortler structures) with small displacements

(dO m). Optimum structural development generally de-pends on rock rheology and briUlc failure. Uthology is also important, especially where contrasts in porosity and perrTle-ability focus the fluid flow through spedfie unilS, along rock contacts, or through pemleable masses of breeciated rock. These lithologic features may

be

an inhinsic chamcterislie of the original rock; alternatively, th!:!y may

be

a by·product of

hydrothermal altemtion and chemical dissolution or

hy-drothermal brecciation (Sillitoe, 1993b). Thus, faults and fracture neI:Y.'Orks. as welt as breccias, coarse clastic rocks. and

intensely leached rocks account for the spectrum of vein-

re-lated to disseminated ore.'i (Table 4), which can extend for l00s to 1.000s of meters laterally and lOs to 1005 of meters

vertically. The dominant gangue mineral is quart-z. making

ores hard and generally resistant to weathering. and the

dom-inant sulfide mineral is pyrite, with sulflde contents that can range from <,1 to >20 vol percent.

Melal endowments

Most orcs arc mined for gold and silver, and there is a

spec

-trum of gold-rich (AWAu ratio dO, locally <1; e.g .• Cracow ;lnd Pajingo, Australia; Hishikari, Jap.m; ACl1pan lmd Antamok

in Baguio, Philippines: Ladolam, Papua New Guinea: Hound

Mountain, United St'.ites) to silver-rich deposits (Ag/Au ratins

-20-200; e.g., Pascua-L:\ma and La COipa, Chile; Tayoltita, Guannjuato, and Paehuea-Real del Monte. Mexico; Comstock and Tonopah, United States). Some of these are copper bear-ing, with high- to intermediafe-suUid,lfion-state mineroJ as

-semblages containing As and Sb (e.g., Yanacocha, Peru: EI

Indio, Chile; Lepanto, Philippines; Goldfield. United States). There are also Ag-Pb-Zn deposits (with subordinate Cu, As,

and Sb) that are poor ill Au (AWAu ratio >4(0). and their

N

--.

m

-.

.

'"

.

,.,

EI'lTIIF.RMAL I'RECfOUS ANU BASE METAL DEPOSITS

Martha

H

ill

s

s

Lado

l

am

- - - -

----\ _{+ + + + +}

•

\+ + + + + + \ + + + + + I..+-~+\++ +/ +- ~ + ~+j+ ...

+

'"

-¥y'l

.IJ

'I

I'

""'-

['""']

,

-

3

...

,

.

_ 3

,

-

_

7

15

,

,

_=m

_ >\5

-I

,

___

~_~''''m

,"'IG. 5. Eumples of strul'l.ural and litliological oontrols on orebody ~ry. At Mllrtha l!ill, preo:.Wus metal mint'niliza·

tIon is l!lItill'ly host<!rl in steeply dippIng ,'('irl! e~lencl"'g O'o'er j\ 500'111 vertic.;a.l inlervW (red""wu (rom Morg.~n, 19~); not ..

thallh., tops of the \"irlll are cut Ioy .. n erosional ""t.'tmfonnil)' ... ·riain by 1111:Ilt ... n.'<I Ignlmh",c, AI ~m, Jissemlnal/!rl £Okllllusdyoccurs '" subhorV.nnh.] tabular 7.DntlS J,usted by n culllpiCJ( 5ef1uell(.'O;: of volcanic IIml hydrothermal breec\a.$ (re-dl1lWJl from C.armllll, 20(3).

493

N

".

+ + +

.,

,,,

+

•

. =

...

N

''''

-.

,,,,

...

...

exemplified by those CM.:c.:urring in northern Mexico (e.g., F

res-nillo and Zocah."cas). Even more isolated in occurrcm.:e are ex

-amples like Cerro Rico de Potmi, Bolivia, which, albeit ep

-ithcnnal in style and the largest silver deposit in thl! world, is a vmiant of mineralization found in the Ag-Sn belt of Bolivia,

wl,ere deposits fom,ed III <.'Ondltions generally deeper and hot

-ter than eplthem)al (e.g., Sill.itoe et

at.,

1998).

Geologic panllneters afTet.1ing produd:ion of epithennal orcs

include mineralization !>tyle (e.g., structurally t.'Ontrolled versus

disseminated), grade distribution, and supergene oxidation, all

of which can afTe<:t the cost of mining and metallurgical

pro-<'"eS.o;ing. Open-pit metl,od~ are used for large lonnage l ow-grade (1-2

g

Au/I; 90

g

AWt) oreborues (e.g., Round Mountain,

United State .. ; Real de Angeles, Mcxk'O), with gold in oxidized

ores being amenable to low-cost heap-leach treatment. Under

-ground mctllods are generally used to exploit small to mode.'iI: tonnage but higll-brrnde (10->100

g

Au/I, >500

g

AWl) orebod

-ies (e.g., Hishikari, Japan; Emperor, Fiji: Fresnillo, Mexico),

ex-cepl where they inter.;ect the surface and can be mined from

open cuts (e.g., Cerro Vanguardia, Argentina). At finer scale,

within individual orebodies, the

highl

y

variable nature of gold

and silver H.'isays over distances of [c.'iS than a few meters, makes

gmde

co

n

tro

l

a critical part of successful mining, especially

where ores are hosted in structum17.Qnes.

llclation.sli'p to igneous rocks

Most epithennal deposits are affiliated with coeval VOIC-.lOic

rocks alld subvolC'.mic intrusive eCJuivalents of predominantly

T.ul.f; 4. Ceometric-ltl Controls on Penueabllit;.' and Epithennal Ordxxlleli (modifit'() rrom SiUitoe, 1993b)

Structural,

Infiuef\<.'t.'<i by raulu and rrBCtu~

~Iydruthemul: ",fiuenced

loy the prfl~.sure and

fellcthity of fluids Lithological: mf\ucnced by the physi\."! dwact",rUticl

or

rocks Orebodies

~:r~~~

;

veIU

fault Inlel'S(!(;tions

Hydrothennal breccia;

d'al~llles; lind resid'~11

V\'ggy (Iuam

Str.lt:a·oo.mu dinemil\:lliollS

Permeability CQ!ltml

Exl<!ll5ion·tTllllstension $t.'t.'Ond· and thin.l-order

struelu~; dllational jOgS and ~Iea.~mg bends:

hriltk rl1tCturing

o.."rpressuring follw.'Cd by hydruuUc rr.letlLnng

or hydmthemml eruption: III"-'Ic;e II(:Id leaching

Coa~.gralned Ignimbrite :md/or r imel.tal)'

unit that is \.IJl<.vnsolidalf)(\ or ,hat has easily

dissolYed l-crncnt: rook ~ .. U\tacts S<!Jlo,,,,ung

p!!mlt."I\bIc and impt'rJlocable ~Imtn

Examples Martha HiU Mclaughlin Tayo!titll Hi5hikari

"'.-

Comstock t.ndolam Cripple Creek Summitville Nansa'$\1 district Acupwi GtJldflcld ¥lInacocha Pascua·Lam~ Round Mool\lain Chlnkuru;hih y.".",.;,.

494 SIMMONS £T A1~ calc-alkaline affinity that form in magmatic arcs resulting

from convergent plate movement and plate .subduction

(Sawkins, ]990; Sillitoe and Hedcnquist. 2003). Cold-silver,

Au :i: Ag ::I: Cu, and Ag-P!rZn deposits are all found in vol-canic sequences containing andesite, dacite, and rhyolite. Th~se calc-alkaline magmas are relatively oxidi7-ed (magmatic oxygen fugacity ;j?: nickel-nickel oxide bulTer; e.g., Hifdreth, 1981; John, 2001; Einaudi ct

aI.

,

20(3) and generated

by

par-tial

melting of the mantle wedge above suLc:luding oceanic

lithosphere (e.g., Gill, 1981; Lulu, 1992). Epithermal Au-Ag deposits of relatively low A'J/Au ratio are

also

found with

vol-canic rocks that erupted in 1ack-arc and continental-rift

envi-mnments, producing reduced tholeiitic magmas with bimodal basalt-rhyolite t.'OmIHlSitions. The

best

documented examples are in the Creat Basin of the western United States (Hildreth, 1981; Jolm, 2001).

There are some important exceptions to these gcneml

trends, including the few, but very large, Au-Ag:t Te deposits that

are

closely related to alkaline volcanic

rocks

that were

de-rived from oxidized and hydrous mafic magmas (Hiehards,

1995; Jensen and Barton, 2000). Such magmas form outside conventional volcanic arcs in zones of crust where deeply penetrating tensional structures developed through rifting (e.g., Cripple Creek, United States; Ladolam, Papua New Cuinea; Emperor, Fiji) or postsubduction tectonism (e.g., Porgera, Papua New Guinea; Sillitoe, 1993a; Richards, 1995; Jensen and Barton, 2000). The corrclation of magma compo-sition and metru assemblage is also seen at Cerro Rico de Po-tos!, where host volcanic rocks for the Ag-Sn ores consist of relatively rooue<..>d ilmenite-bearing rhyodacite (Sillitoe et

aI

.,

1998). The late Pliocene-Pleistocene age MCLaughlin de-posit, Cruifomia, formed during activity of tite Clear Lake vol. canic Aeld that erupted in response to upwelling of mantJe through a slab window in a largely tmnspressional environ-ment east of the San Andreas tnmsform fault (Sherlock et

aI.,

1995; Dickinson, 1997). These exceptions highlight the \vide range of tectonic settings that can host mineralization noted by SiUitoe and Hedenqwst (2003).

PreseflXlt/on In the geologiC record

Civen the relatively shallow depth of formation,

epither-mal deposits may have poor preservation potential in the

ge-ologiC record, because they commonly form in high-renef volcanic arc settings and because (.'Onvergent plate

bound-aries

are espet.'ially prone to phases of rapid uplift and ero-sion. Thus, 3 majority of deposit' are Tertiary or YOUllger

(Table A I), and there are major deposits that have formed since 2 Ma (e.g., Lepanto. Philippines; Hishikari, Japan; Ladolam, Papua New Cuinea; McLaughlin, United States). However, older deposits have been preserved where their host volcanic belts are well preserved, such as tJlC Mesozoic deposits (e.g., Cerro Vanguardia, Argentina) of the Desendo massif in Patagonia and the Paleo7.0ic deposits (e.g., Temora, Pajingo, and Cracow) of the Tasman fold belt in eastern AIlS-lralia, as well as simiJar examples in Mongolia and Hussia (Yakubchuk et

aI.,

2005). Precambrian examples are also re-ported for Canada, Scandinavia, and Australia but, to date, the known very ancient epithennal deposits are small (Dube et

at.,

1998; Hallberg. 1994; Turner et

aI

.,

2001; Huston et

at.

,

2002).

Active Epithermal Environments

Active epithermal environments in geothermal and mag-matic hydrothermal systems (Fig. 1) were important to the conception and classification of epithermru deposits ( Ran-SOll1e, 1907; Undgren, 1933). Such high-temperature hy-drothennal systems are located in geologic settings analogous to epithermal deposits (Henley and Ellis, 1983; Henley, 1985), and they provide a context in which the mineral

prod-ucts ofhydrotJlennai act.ivity can be compared with (.'uexisting fluids at

known

tempemtures,

pressures

,

mass flows, and chemical compoSitions. For example, the occurrence of spec-tacular sulfide scales, containing 6 wt percent Au and 30 wt percent Ag, 011 back-pressure plates (downstream of the

throttle point) within surface pipe work at the Broadlands

-Ohaaki

geothermal field

was

shown to be the direct conse-quenceofboiling (flashing) of a fluid at 260° 10 lSOoC initially (''Olltaining about J to 2 ppb Au (Brown, 1986). Although the low-saliltily

«0.

5

wt % NaC!) and near-neutrAl pH solution is

initially undersaturated in brold and silver, the flashing e nvi-ronment results in quantitative precipitatiun of precious met -als, highlighting the efficiency of metal precipitation induced by boiling in tile epithermal regime. With geothennal wells

drilled to >2.5-km depth (>300°C), such active systems pro-vide an overview of hydrothermal

processes

occurring witltin, above, below, Imd on the periphery of tlle epithermal envi-ronment (e.g., Henley and Ellis, 1983; lIedenquist, 1990; Heres, 1990; Simmons and Browne, 2000a, b). Here we briefly examine the main fluid types and corresponding h y-drothcmlal miner& assemblages of active environments (Henley and Ellis, 1983; Ciggenbach, 1992a, 1997) as a framework for understanding hydrothermal minernls in ep -ithennal depoSits (Table 5), described in greatcr detail below.

Ceothennal systems

Ceothermal systems in volcanic arcs and rifts involve deep convective circulation of meteoric water driven by shallow in -trusion of magma at >4-km (?) depth. At the deepest level ex-plored by geothcnnal wells, these chloride waten;-sn-called due to the dominant anion-are reduced and have near-neu-tral pH and contain from 0.1 to > 1 wt percent CI, up to 3 wt

percent CO2, and lOs to l00s of ppm H2S; the latter is an im-portant 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, chl o-rite, 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 chlo-ride waler may rise to the surface to discharge and deposit

£fITHERMAL PRECIOUS AND BASE METAl. DEPOSITS

495

TAIlL£.5. Summaz or lIyc!mrhennaJ A1t~rulion Asscmbla~es Fonnlng in Erithermal Environmcnts

Altention

Adv. AzglIlic: (lteam·heated)

Adv. AzglIlic: (magmatic:

'"""""nnol)

Mine~

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

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

Opal, alunite ( .. ttit". powdery. fine-gralned. l)Seudocubic-), bolinltc, pyrite, marcasite

Qllart7~ alulI;t" (lIIbular). dickite. pyrophyllite. (duaspore. 'mfIytte)

Occurrence IlIld orl~n

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

Dt.:vc!ops at <18O"C on the pericb!!ry and In tlle sh..tlow epithermal environment through altemtion by sh.'aI1l-heated COrrieh waters Develops at 0( 12O"C 1M.'IlC the water tab/f! IlI\d In tIN! $hal~

q)ltlocnnal CJl~itonment through altemlioo by steflm-I .... 'Il\(.-d lI(.;d-sulrate waters; locally associated with silica sinter but only In geotlwmnall)'Stems

Develops at ,2O)"C .. ithin the epithermal environment through altellition by magrnatk«rived acidic w.llers

Adv. A<gilllc (rupergene) A1unit!!, kaolinite. halloysite. jttrorltc, Fe oxioo Dllveklps at 0(40"C tI.lUugh wt:athcring IlI\d oxidation

or

sullld6-b.:arillg rock,

silica sinter where topography intersects the geothermal water tablc; wternativcly, this liqUid may disperse laterally through an outflow zone.

By contrast, dissol\lt!d gases (mainly COl and H2S) separate from the liqUid into vapor due to boiling and rise to the sur

-face wong Ii path distinct from the residunlliquid. The rising

gases, COl and H2S, may be partially absorbed into cool

ground waters at sh3.lIow levels, along with (:ondensed water

vapor, to form two types of steam-heated waters, COt-rich and acid-sulfate. C02-rich steam-heated waters (."Uutain high concentrations of dissolved CO2 (> 1 wt %) and tend to

accu-mulate at shallow levels. They drape the stagnant margins of the upflow zolle to depths &, much as J ,000 m below the

water table. Their distribution is best known at Bmadlands

-Ohaaki, where weaklr. addic steam-heated waters alter vo l-canic rocks to an argillic assemblage dominated

hy

clay min

-erals (illite, ilIite-smet..tite, smectite, and kaolinite), calcite, and siderite at temperatures up to about 150°C (Hedenquist,

1990; Simmons ana Browne, 2000b).

Acid-sulfate steam-heated waters are close to 100°C and form in the vadose 7..one where H2S comes into atmospheriC

contact and oxidizes to H2S04. Their pH is -2, and they con -tain relatively high concentrations of sulfate (-1,000 mJikg). These waters alter rocks to an advanced argillic assemblage of opal (cristobalite), alunite, kaolinite, and pyrite as the so-lution is neutrAlized near the water table (Sehocn et al,

1974). The distribution of these three WAter types largely de

-pends upon topographically controlled hydraulic gradients. In low-relief volcanic settings (e.g., calderas, flow-dome

(.'Omplexes, rifts), the steam-heated waters occur above and on the pt!riphery of the chlOride-water plume, whereas in high-relief settings (e.g., andesitic composite cones), the

steam-heated waters may extend from the snmmit to the

lower flanks of the volcanic edifice; under the influetl(:e of such a steep hydraulic gradient, chloride waters may flow lat -erally long distances (>5 km) to form subsurface outflow zones (Henley and Ellis, 1983). Hybrid compositions form where the waters mix.

Magmfltic "yd,·othenrw[ system.r

Magmatic hydrothem,al systems, unlike geothermal sys -tems, are rarely drilled because of their acidic conditions

and high lcmperahlTes. What we

know

of subsurface condi

-tions is from gases discharged from fumaroles at 100" to

>8oo°C, acidic hot springs, and hydrothennally altered rocks ejected by explOSive eruptions <e.g., Hedenqulst et w., 1993), An exception is in the Philippines, where several magmatic hydrothennal systems with zones of very reactive fluids have been explored for their gcothennal energy po-tential (Heyes, 1990; Delfin et aI., 1992; Reyes et

al.,

1993, 2003). Existing data on the metal contents of high-temper a-ture volcanic discharges indicate the potential for subs tan-tial flux of both precious and

base

metals (Hedenquist,

1995). Within the ccntral upflow column overlying shallow intnlsions, the fluids in these systems are dominated by magmatic componeuts, including BCI, S02, and HF. When these gases condense into the hydrothermal system, S02 disproportionates, forming H2S and lhS04 (Sakai and Mat· subaya, 1977; Rye et al., 1992) and a very acidic (pH -1) so-lution, (.'Ontaining subequal amounts of HCI and H2S04, up to -1 wt percent each (Giggenbaeh, 1997). HydrolysiS reac

-tions with igneous country rocks progressively neutralizes the acidity while forming hydrothermal minerals that in-clude alunite, pyrophyllite, dickite, quam.., anhydrite,

dias

-pore, and topaz, as well as kaolinite and illite, eharaderistic

of ~fluid-dominated~ alterdtion conditions (Reyes, 1990;

Giggellhach, 1992a, 1997). Surficial steam-heated acid-sul -fate waters also form in magmatic hydrothermal systems,

just as they do in thc wdose zone over geothermal systems, due to the presence of HtS in the vapor. Silica sillters, how -ever, are absent due to the acidic conditions that inhibit sil-ica polymerization and deposition of vitreous amorphous sil -ica (Fournier. 1985). Tn thiS setting. two styles of advanced argillic alteration, magmatic hydrothennal and stc am-heated, develop with different origins hoth containing alu

496

SIMMONS £T

AI-Advanced argillic alteration

The origin of advanced argillic alteration can be

deter-mined from its morphology, as well as mineralogy and

zona-tion (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). Ma

g-matic hydrothemlaJ or hypogene. advanced argillic alteration

includes minerals that fonn at >200a

C, such as pyrophyWte.

dickite, diaspore, zlinyite, and topaz..

with

alunite that is gen -erally tabular and sometimes coarse grainoo. This a1terdtioll is

epigenetic in nature, so it generally cuts across stratigraphy

and follows high-angle structures, although it can be

strati-fonn in permeable host rocks.

Steam-heated advauced argillic alteration forms above the water 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 il1l.hickness. Tabular but diseoTltinuolls

bodies of massive

opal

mimic and mark the water table, un -derlain by

a

discontinuous zone L'Omprising alunite. 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). These 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 su-pergene 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, kaol i-nite, halloysite, jarosite, and iron oxides and hydroxides. Su-pergene advanced

argillic

a1temtion also has a blanket like

geometry that mimics topography, but it may line sub-vertical

fractures that were patJlways for deS<.'t!nding lIurfare W3ter. A combination of Coveful geologiC mapping and mincral

identification (with a hand lens, infrared spectrometer,

petro-graphic microscope, X-ray diffraction, or scanning electron

microscope) are generally sufficient for distinguishing the

ori-gins of advanced argillic alteration. Rye et al. (1992) and Rye

(2005) further describe how the alunite and kaolinite forming

in these three environments can be distinbruishcd all tJle basis of sulfur, oxygen, and hydrogen isotope analysis.

Minerali7.ation Associated \vith Quartz :t Calcite :t

AduJaria :t Illitc Assemblages

Dllc type of epithcnnal minerAlization is distinguished by its intimate association with quart'Z J: calcite :i; adularia :i; illite

that fonns from the near·neutral pH chloride waters in ex-tinct geothermal systems. This gangue minerAI assemblage

hosts a spectrum of Au-to Ag-rich ores, as weU as the Au-Ag

:t Te ores ass<K."iated with albline rocks, and the Ag-Pb-Zn ores of northern Mexico. Quartz ancVor chalcedony domi-nate. accompanied by lesser and variable amounts of adularia, calcite, pyrite, illite, chlorite and rhlKlochmsite; with the ex-ception of quart:2;, there are many examples where one or

more of these phase.~ is missing or is trace in amount. Ores

occur in veins and stockworks, making up subvertica1 frac-hires, or. more rarely. in pore space of breccias and

pcnne-able rocks, forming disscminated minerali7..ation. In Au-Ag depoSits, gold typically occurs as microsL'Opic tu suhmiem· scopic brrAins

or

electrum and rare tellurides, whereas silver

occurs as ele<:t:mm. acanthite, sulfosalts (e.g., pyrargyrite

-proustite, Ag-rich tctmhedrite) ancVor silver sclcllide miner

-als. 80th precious metals are found with highly variable

amounts of base metal sulfides (sphalerite. galena, and lesser

chalcopyrite) and pyrite, marcasite, ancVor pyrrhotite. Sul-fides constihlte from < 1 to > 10 vol percent of the ore, and the

sulfide abundance, particuJarly tJle base metal sulfides, in

some deposits increase with depth or with changes in

host-rock

composition. Sulfidatiol1 states inferred from ore-related

sulfide minerals range from intermediate to low (Heald et al., 1987; John, 2001; Einaudi et

aI

.,

2003~ Sillitoe and Heden

-quist, 20(3).

Epithennal deposits are also distinguished by the gangue

mineral tcxtures (Fig. G). Cmstifonn banded quartz is com-mon, typically with interbanded, discontinuous layers of

sul-fide minerals (mainly pyrite) amL'or selenide minerals, adu -laria. and/or illite. At relatively shallow depths, tJIC bauds are

L'OJlolunn in texture and millimeter-scale. whereas at greater depths, the quartz becomes more coarsely crystalline. Lattice textures, comprised of platy calcite and its quam pseudo-morphs, occur

as

opcn-spaL't! filling in veins, and along

with

vein adularia indicate boiling fluids of near-neutral to alkaline

pH (Simmons and Chri~tenson, 1994; Simmons and Browne

2000b).

Bre<.-cias in veins and subverticaI pipes commonly show

ev-idence of multiple episodes of (onnation. They comprise

jum-hied anb'ular clasts of altered host rock and earlier vein fill, supported by a matrix of mainly qulll1Z, calcite, and/or adu-laria and sulfide minerals (Fig. 6), suggesting rapid pressure

release and violent fonnation that can be ascribed to seismic· ity (e.g., Sibson, 1981) and hydrothennal eruptions (e.g., Hedenquisl and Henley, 1985a).

8road-scale patterns of alteration zoning surround

orcbod-ies and reflect the level of exposure (Fig. 7). At regional scale,

deep level (>400 m below the water table) alteration is prop y-litic (e.g., Aeupan, Philippines; Comstock Lode and Round

Mountain, United States; Tayoltita, Mexico: Martha Hill,

Ncw Zealand; cf. Hu(l~on, 2003). At intermediate levels

(400-150 m below the Wilter table), clay ami c-arbonate min·

erals incrclL~e at the expense of aluminosilicate minemls, whereas zonation of clays (illite to smectite), and zeolites

(waimkite tu heulandile to mordenite) reflect decreasing temperature (Fig. 4). Intense quartz, adularia, illite, and pyrite alterAtion commonly surrounds ores and reflects the sharp increase in permeability associated with fluid conduits: accordingly, In host rocks with low penneability, alteration may be closely restricted In tJle selVAges of veins and veinlets. At shallow levels (150-0 m below the water table), blankets of argillic aitenttion, illite and other clays (with or without

dis-seminated pyrite, carbonate, minor barite, and minor anhy-drite) are generally well developed, especially in host volcanic

rocks, and may conl.'eai underlying orebodies (e.g., Creede, United States; Pachuca-Heal del Monte, Mcx.iI.'O; Bartun el al., 1977; Dreier, .1982). At the shallowest depths in the ep -ithennal environment, steam-heated advanced argillic alter-ation occurs with or without silica sinters that foml near tJle paiL'Owater taille and the paleosurface (Figs. 4, 6). Silica

sin-ter, which deposits as amorphous silica and then converts to '1uartz (Herdianita et al .. 2(00), shows rhythmiC banding,

F.I'IHlflUIAL PRfX /O(JS AND HA';E M£TAI. DEPOSITS

Flc.. 6. l'hot~raphs of minerals and textures II'al rommonly occ·.,r in e\,ithermal deposits associated "ith quart!;:o eal. dte., a.1"bri" ., illite: /\. Ci"nabar~bcarillg silic-.t sinter (I>uhipuhi, ~ew 7.ea and: scale bar. 2 em), 11. eolloform cnlstifornl handing in)!old_sil\l·._l>caring ore (~Iarthn Hill. New Zcabml; ,,,,;ale bar" 2 ern). C. Adnbria encnlsted on op"n fractnn; (~Iarth:t Ilill. ~t·w Zeabnd; seale har • I em), I), LatliCC texture_ in which pbty e-.ileite j. replact'd b)' qll,lrt? in gold-sil -\'Cr-beo,uingorc (~l.artha Ilill. 1\cw7.caiand; ~c 11.1 •• 3 em), E. Vein containing coarsely crystalline quartz. ~l'ha1crite, and galena (I'aehuca-Ih'al del Monic, ~!cxioo; scale bar. 1,25 CIII). E llJ1.'C('iated \'ein material in goJd,~il"er-h"aring or,' (Gold,:u Cross, New :t"":tland: sc;,Je bar. '1 em),

498

SIMMONS E:T AL..

Quart

z

±

Calcite

±

Adular

ia

±

I

ll

i

t

e

clsy catb0n8te -~.:::

pyrite

--5O-100m

qUlJrtz, chB/c&dony. edulBJia. carbonates pyrite, Au-Ag, Ag-Pb-Zn lattice textures, crustiform-colloform banding

1-10 m

Quartz

+

Alunite

±

Pyroph

y

l

li

t

e

±

D

icki

t

e

±

K

ao

lin

i

t

e

quartz, sJunite • • • dlck1te (klIoIlnft8) - •• pyrophyilite. pyrite •

!If

propylific

•

~L

vuggy to massive quartz

native Au, sulfosalts, pyrite

5O-100m

'-10m

f',c,7. Sketch diagrams showing the mineralogic v::maUon at two different _Ie!! 11Il)",KI el'ithermal nrebollies M.'iOclalecl with quartz

*

calcite .. adularia", Illite and quartz + alunite i. pyrophyllite .. ~kite .t: kQOlinite S"ngue mineral o.s:semblages. Thll diagrnms on the len show the large-scale pattern, and the rectangle arca outlh\(:d 1.$ 1n1l!.;nllk.J on the right tu show al· \l'lratKm 1.onation patterru: in thll vicinity of ore (after SlIHtoe, 1993b),

be preserved in rock sequences containing epithennal

de-posits (White et al., L989).

Fluid inclusion data

Fluid inclusion studies, mostly on transparent gangue

phases

(

quartz,

calcite

)

nnd sp

halerite (the

main

ore-related

sulfide mineral suitable for fluid inclusion study), indicate ore

deposition from dilute to moderately saline solutions at tem

-peratures between 150° and 300°C. Gold-silver deposits

gen-erally have dilute solutions of <5 wt percent NaCI equiv, whereas Ag-Pb--Zn deposits commonly have brines of dO to >20 wt percent NaC\ equiv (Fig. 8). Coexisting liquid-lind vapor-rich fluid indusions are common and indicate boiling

conditions at the time of trapping (Bodnar et al., 1985). This

allows

temperatures of boiling to be used to calculate

pres-sures and estimate depths of formation (Hoedder

and

Bodnar, 1980). Therefore. assuming

a

hydrostatic boi ling-point-for-depth grndient (Haas. 1911), consistent wilh estimates of

vertical temperature gradients (Vikre. 1985; Simmons et aI., 1988; Cooke and Bloom, 1990; Sherlock et al., 1995) and

ge-othennal system analogues, ore deposition occurs over

a

depth r.mge of about 50 to 1,100 In below the water taLle.

These are minimum values, however, because the presence of

small amounts of dissolved COl, the main

gas

in geothennal

fluids (Hedenquist and Henley, 1985b), inereases the total

fluid pressure by

as

much

as

several tens of

bars

and

in

creases

the depth r.mge of ooiliug up to hundreds of meters (e.g.,

Simmons, 1991; Sherlock et aI., 1995).

Stable

isotope data

Stable isotope studies, comprising measurements of bD

and 6111_{0 , have been made on several gangue minerals}

(quartz, adularia,

days

,

wld carbonates) and on fluid i

nclu-sions to determine the provenance of the fluid responSible for alteration and mineralization; few of the studies have de-termined the isotopiC composition of the ore solutions

EPlT1IERMJ\L PRECIOUS AND BASE METAL DEPOSITS

499

o

Au-Ag

- _ _ - - Au (Te) (alkaline rocks)

Ag-Pb-Zn

Au (Cu)

10

20

30

wt

% NaCI equivalent

F'lc.8. F'lwd Indusian Mlinities vs. metal contf'nts m L-piiliermal depn5.its.

CoId·)ih.,:r, ROkI (Tfl), and Ag-!'b-Zn dflpOOu are ~ted with quartz .t.

calcite.t. adularia S Illite gllngue, whereas the Au (eu) deposits lire lWQ(.'iated

with qlWtt ~ alunite to p)TOphyililfl s dickitc gangufl.

themselves (Fig. 9). The interpretation of such data is not

straightforward, because the data typicaHy are seattered,

water compositions generally have to be construL1ed from

aualyses of differentlllinerais (hydroxyl-bearing clays) or nuid

inclusion waters. and equilibrdtion (or fractionation) temper

-ahlres have to

be

estimated. In addition, doubt has been cast

on the validity of

6D

analyses of quartz-hosted Iluid inclusion

waters, as they may yield unreliaLle values that arc too low if

the quam. crystallized from originally Jlrecipitated amor-phous silica or if the .. vaters

arc

extracted by thermal decrepl

-tatioll (Faure et al., 2002; Faure. 2003). Deposits younger

than

a

few million years gemmilly allow more act,''Urate

con-straint'> on the comlx>sition of local meteoric water, with

pre-sent-day values serving as a reliable proxy. Notwithstanding

0 -20

~

-40

_>'

.'

0

_if'

..

::;:

_-60

~

• .!/'

(()

~

-SO

0 K> _{-1 DO} ~Calc%Ad:tlHite -120 epithermal deposits -140 -20 -15 -10 -5

these problems, the results generally plot between the m

ete-oric water line and compositions associated with magmatic

water (Fig. 9), suggesting that mixing of water.> from both

sourt:es accounts for the compositions measured (e.g., O'Neil

and Silbennan, 1974; Faure et al., 20(2). Commonly,

inter-pretations are inconclusive. bt..'Cause water-rock illter..tction of

deeply circulated meteoric water results in an evolution of

isotopic compositions-tlle "IHO_shift" (Cmig, 1963; Taylor,

1979). This overlap in isotopic compositions has caused

con-sidcmule debate on the origins of waters in subaerial geot

-hennal systems (e,g., Giggenbach, 1992b, 1993). Two points

are clear about epithennal deposils: a Significant portion of

near-neutral pH chloride wu.ters is derived from deeply

circu-lated meteoric water, and there is evidence in some deposits

for n component of magmatic water. thus a potential source of

some components, even metals (e.g., Simmons, 1995).

Mil1cmli:.:ation affiliated with alkaline rocks

Cripple

Creek,

Ladolam, Emperor, and Porgera

arc grouped

as

a subtype of the deposits a.~socIated with quartz ::t calcltc :t

adularia ::t illitc assemblages but are distinguished Lecause

they show a number of distinctive features, including

associa-tion with alkaline igneous rocks, and the common ()(,'currence

of telluride minerals in their ores (Bonham, 1986; Hichards,

1995: Jensen and Barton, 2000; Sillitoe. 20(2). AltllOugh they

are relatively few, the.~e alkaline rock-related deposits have Significant gold contents ilnd grAdes, and they display features suggesting genetic aspects that differ from most other

ep-ithermal deposits formed from near-neulral pH solutions

(Table O. Cold occurs In native form, 1.11 electrum, in tel-lurides, and in refractory pyrite, the latter of which can

be

a

Significant component of ores (Cannan, 2003; Pals et al..

vapor.i

magmas

0 5 10 15 20

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