Chapter l9

CHARACTERTSTICS OF HIGH.SULFIDATION EPITHERMALDEPOSITS, AND THEIR RELATTON TO MAGMATIC FLUID

Antonio Arribas Jr.Mineral Resources Department, Geological Survey of Japan,

l-l-3 Higashi, Tsukuba 305, Japan

[NTRODUCTIoN

A consequence of the increased exploration forgold deposits during the Iate 1970s and early1980s was tlre revision of the classification ofepithermal deposits in order to account for thevariations observed in styles of mineralization andinferred genetic environments. Among thenumerous classifications that followed, one groupof deposits clearly showed a common set offeatures, this deposit type is characterized by thepresence of minerals diagnost ic of high-sulfidation states (e.g., enargite and luzonite) andacidic hydrothermal conditions (e.g., alunite,kaolinite, pyrophyllite). The terms enargite-gold(Ashley 1982), Goldfield-type (Bethke 1984, afterRansome 1909), high-sulfur (Bonham 1984,1986), quartz-alunite Au (Berger 1986), acid-sulfate (Heald et crl. 1987), and alunite-kaolinite(Berger & Henley 1989) were appl ied to thisgroup in reference to some of its mineralogical orinferred geoclremical attributes. The term high-sulf idat ion (HS) (Hedenquist 1987) is now widelyused; the term was proposed originally to refer toa fundamental genetic aspect, the relativelyoxidized state of sulfur contained in thehydrothermal system (i.e., initially SO2-rich). Thisaspect is significant because it l inks HS depositswith one of the two main types of terrestrialmagma-related hydrothermal systems (Henley &El l is 1983), those associated with andesit icvolcanoes whose surface manifestation includeshigh-temperature fumaroles and acid sulfate-chloride hot springs and crater lakes. By contrast,Iow-sulfidation deposits form from neutral-pH,reduced (H2S-rich) hydrothermal fluids similar tothose encountered in geothermal systems (Henley& Ellis 1983), with surface manifestation

including si l ica sinter-deposit ing hot spr ings andsteam-heated acid-sulfate alteration.

The main objective of this review is tosummarize the characteristics of HS minerali-zation formed primarily within the epithermalenvironment, tlrough recognizing the potential forHS conditions to occur at greater depths. Earlierstudies have argued for a magmatic fluidcomponent in HS deposits (e.g., Si l l i toe 1983,1989, 1991a; Hayba et al . 1985; Henley t99tWhite 1991; Rye 1993; Hedenquist et al. 1994a),and the identification and characterization of HSdeposits has contributed to a re-evaluation of therole of magmatic fluids in other types ofhydrothermal systems (Hedenquist & Lowenstern1994; Simmons this volume; de Ronde thisvolume). In this context, particular aftention isgiven to the characteristics that are helpful indetermining the nature of the magmatic contri-bution to the hydrothermal system through timeand space. This review considers features of manyof the deposits listed in Table l, with locationsshown in Figure 1, but is based on a selection offourteen deposits for which the results of detailedgeological and geochemical studies are available(Tables 2, and 3). For simpl i f icat ion, bibl io-graphic references are not given in the text forgeneral deposit features; these references may befound in Table 1. For regional studies of HSdeposits, particularly with respect to other types ofmagmatic-hydrothermal base- and precious-metaldeposits, the reader is referred to reviews byHeald et ul . (1987), Bonham (1989), Si l l i toe(1989, l99la), Berger & Bonham (1990), Camus(1990), White & Hedenquisr (1990), Mitchel l &Leach (1991), Mitchell (1992), and White et al.( I 99s).

7-

A. Arribas../r.

Table l . Pr inc ipal h igh-sul f idat ion deposi ts or documcnted prospects ordered geographical ly

N ' i nt ; ig . I Deposit References

I231561d

9l 0l lt 2l - )

l 4l-5l 6t 1l 8

l a

202 l22L-)

24

26

212829303 l

.1.')

343-536

3839404 l42

4344A <

4641484950

Dobroyde, AustraliaRhyolite Creek, AustraliaTemora, AustraliaPeak Hi l l , Austra l iaMt. Kasi , F i j iWafi River, Papua New GuineaNena, Papua New GuineaMotomboto, IndonesiaNalesbi t iur , Phi l ipp inesLepanto, Phil ippinesChinkuashih, TaiwanZi.j inshan, ChinaSeongsan & Ogmaesan, South KoreaNansatsu (lwato, Akeshi & Kasuga), JapanYoji, Japan-feine,

JapanAkaiwa, JapanMitsumori-Nukeishi, Japan

Northwestem Vancouver Island, CanadaSummitvil le, ColoradoRed Mtn-Lake City, ColoradoRed Mtn-Sil verton, ColoradcrGoldfield, NevadaParadise Peak, NevadaPueblo Viejo, Dominican RepublicMulatos, Mexico

Julcani, PeruCastrovirreyna, PeruCcarhuarso, PeruSan Juan de Lucanas, PeruCerro de Pasco, PeruColquij irca, PeruSucuitambo, PeruLaurani, BoliviaChoquelimpie, ChileGuanaco, ChileEl Hueso, Chi leEsperanza, ChileLa Coipa, ChileNevada & Sancarr6n, ChileEl Indio-Tambo, ChileLa Mejicana-Nevados del Famatina, Argentina

Rodalquilar, SparnFurtei-Serrenti. SardiniaSpahievo, BulgariaChelopech, BulgariaWestem Srednogorie region, BulgariaBor, YugoslaviaLah6ca, HungaryEnisen. Sweden

Asia & AustralasiaWh i tee ta l . ( 199 -5 )Raetz & Panington (1988)Thompson et ul. (1986)Cordery (1986), Harbon (1988), Masterman (1994)Turner ( 1986)I -each & Erceg (1990), Erceg et u l . (1991)Asami& B r i t t en (1980 ) , Ha l l e ra l ( 1990 )Perel l6 (1994)Si l l i toe et u l . (1990)Gonzalez ( l9-59) , Carc ia ( 1991), Arr ibas er a/ . ( 199-5b)Huang (195.5) , Hwang & Meyer (1982), Trn et u l . (1993)Zhang er ul. (1991)Yoon (1994 )Izawa & Cunningham ( 1989), Hedenquist et ul. (l994tt)Yu i&Matsueda (19921 )I t o (1969 )Akamatsu & Yui (1992), Akamarsu (1993)Aoki & Watanabe (1995)

North & Central AmericaPanteleyev & Koyanagi (1994)Steven & Rat t6 (1960), Stof f regen ( 1987), Rye (1993)Bove e/ c1. (1990), Rye (1993)Burbank (1941), Fisher and Leedy (1973)Ransome (1907, 1909), Ashley (1911), Vikre (1989)John er u l . (1991), Si l l i toe & Lorson (1994)Muntean et ul. (1990), Russell & Kesler ( l99l )Staude (1994)

South AmericaPetersen er u l . (1911), Deen (1990), Rye (1993)V ida l& Ced i l l o (1988 )Vidal er a1. ( 1989)Vidal & Cedi l lo (1988)Graton & Bowdi tch (1936), Einaudi (1911)Vidal et ri1. ( 1984)Vidal& Cedi l lo (1988)Muril lo et al. (1993)Gri'ipper et ul. (1991)Puig et a/ . (1988), Cui t i f lo et a i . (1988)S i l l i t oe (1991a )V i l a (1991 ) , Moscoso e t a l . ( 1993 ) , Cu i t i f r o e t u l . ( 1994 )Oviedo et ul. (1991), Cecioni & Dick (1992)Siddeley & Araneda (1990)Siddeley & Araneda (1986), Jannas er ul. (1990)Losada-Calderon & McPhail ( 1994)

EuropeSzinger-von Oepen era1. (1989), Arribas et ul. (l995tr)Ruggieri ( l993a,b)Velinov er al. (1990)Bogdanov (1982, 1986)Bogdanov (1982), Velinov & Kanazirski ( 1990)Jankovic et ul. (1980), Jankovic ( 1982)Baksa ( 1975 , 1986 ) , F i r s t ( 1993 )Hallberg (1994)

420

H igh-sulfidation Epithermal Depos its

Figure l. Worldwide distribution of high-sulfidation deposits and principal documented prospects. The main high-

suifidation metallogenic provinces are indicated. See Table I for deposit names and selected references.

OPSNTNC REMARKS ON GENETICENVIRONMENT

Based on detailed research of the Summiwille

Au-Cu-Ag deposit, Stoffregen (1987) demon-

strated that a nearly ubiquitous feature of HS

deposits, fracture-controlled vuggy silica rock

(intensely leached volcanic rock consisting

dominantly of quartz; Fig. 2) is the product of

very acidic conditions (pH <2 at T : -250 "C) that

occur within a sulfate-rich hydrothermal fluid

formed by absorption of magmatic vapor' In

addition to SOz disproportionation to H2SOa,

significant concentration of HCI from the

magmatic vapor contributes to the acidic

conditions necessary for alumina to be soluble,

leading to vuggy silica alteration (Hedenquist e/

al. 1994a,b). Neutralization of the acidic solution

by reaction with the wallrock results in a sequence

of alteration zones, oufward from the

hydrothermal conduit, which is indicative of

decreasing acidity and is defined by the presence

of alunite, kaolinite, il l ite, and montmorillonite +

chlorite (Steven & Ratte 19601'Fig.2).This same alteration sequence' without the

vuggy silica zone but with enargite-bearing ores,

was documented in the Butte polymetallic deposit

(Meyer et al. 1968) and in the roots of the

advanced argill ic zones that commonly cap

porphyry copper systems (e.g., Sill itoe 1973; Corrt

1975; Gustafson & Hunt 1975; Koukharsky &

Mirre 1976; Wal lace 1979). lndeed, several of the

deposits considered in this review are underlain by

porphyry-type mineralization (Table 2). Tliis

advanced argill ic assemblage is also typical of

that associated with acidic crater lakes atop active

volcanoes (Christenson & Wood 1993; Delmel le

& Bernard 1994; Rowe 1994; Hedenquist this

volume).The implications of a genetic relation between

porphyry and epithermal mineralization, e.g', with

respect to the origin of metals or the nature of the

fluid inclusions in HS deposits, are discussed

below. The observation made here is that an

alunite-enargite assemblage records a similar

geochemical environment, whether forming arl

epithermal deposit or as part of the alteration

zoning of an orebody formed at greater depths.

High-sulfidation deposits forrn in a position

intermediate between intrusions and the surface;

therefore, they may be located close to a porphyry

copper deposit or in a near-surface environment,

such as the roots ofan acid crater lake.

Comprehensive genetic models for HS

deposits have been proposed only recently (e.8.'

Berger & Henley 1989; Si l l i toe 1989; White l99l ;

q9.: Balkans\ ,--<+s-+s

421

A. Arribas, Jr.

Table 2. Main geological characteristics of l4 selected high-sulfidation epithermal deposits

Deposit/disrict. Agelocation {Ma)

Metals.( tonnes) I

Local volcanicsetting

Principal hostroc Ks

Geneticallyrclated rtxks

Timebetwecn host

rock & deposit Dcposit l i)rm

Motomboto .Indonesia

Na lesb i t in .Ph i i ipp incs

Lcpanto.Ph i l i pp incs

Chi n kuash ih .k iwu

Z i i i n s h a n .Ch ina

Nansatsu ,Japiur

Sunrnr in l l l c .Color:rdo

Gold l i c ld .Ncvadir

Central-vcntvolcanir

Small cenral-vent volcano

Diatremecomplex

Dome complex

Domc akrngcaldcra m:trgin'l

Snrall volcanosin a c:rldrra'i

Dome alongpreexrstlnSc:ildera margin

Dac donr, zrnds/dac/rhyl-1ows. pyr and volx

Ands pyr + l lows

Ands/dac vol.Mioccnc + oldervolx + metavol

Dac volcMioccnc sed

Jurassic granite.Cretaceous dacporpyhry +pyr

Ands pyr. l lows +vo l x

Qtz-latite porphyry

Miocene andesitc

Diorit ic. qtz-diorit ic stocks

None observed

Qtz{ioriteporphyry

Dacite domesmd llows

Not reported

Horblende :rrds(Middlc Volcs)

<1 .0 n r . y .

N/A

<analyl. error(10 .1 m .y . )

7 .0 r n . y . ( l )( Poorl Y rJatul )

<0.5 nry

Hbx . vc ins . d is inV S

Hbx. vc in lc ts

Vcrtical brcccilrs,vcins. slralab0urdreplacenrnls

Vcins or "letl{cs

.hhx , d is and s tksunounding veins

Vc ins . hbx . s tk

Dis in stratatrountlVS/MS bodies.vc ins . hbx

''hdgcs" with

vc ins . hbx + d isi n V S

Stratabourrrl btxlicscommonly withhbx

Mushrtxrnrshapcdbodics with stk +d ls

Vc ins

Ve ins + s tk

Ve ins ; a lso hbx a tN. dc l Famt ina

Vc ins . hbx . d is inV S

l t l r A u ( p ) + l 8 rAu reserves

Au, Cu. Ag1 7 t A u

1 .9 Cu . Au . Ag6 0 , 0 0 0 t C u . 4 tAu. t80 r Ag (c)

Pliocene Aul 5 t A u ( c )

I . 5 - I . 2 Cu . Au . Ag900.(100 r Cu.120 r Au ( c )

1 . . 1 -1 .0 Au . Cu . Ag92 t Au. 183 r Ag120.000 r Cu (p)

-94 Cu. Au> t0 r Au ( c )

5-J.-s

22. .5

2 l

Qtz-monzonite <analyl.errorporphyry (10.-5 m.y.)

Andesitc

And/dac vol <analyt. crror( + 1 . 0 n ) . y . )

CA bimodal N/A(Rhy + basalr)volcanic suite

Dac/rhyulacitic <analyr.crrorporphyry (+0. I m.y.)

<analyl. crror "[.cdges"

with( t0 . ,1 m.y . ) ve ins . hhx + d is

i n M S

Au (Ag. Cu) Domes alongl -10 t Au. t 4.1 Ag. preexisting ring-17.000 Cu (p) fracrue

Paratl isc Pcak. lg-lt i Au, Ag. HgNevai:r 47 t Au, 12-55 Ag

457 r Hg (p )

Pue b lo V ie jo . - l - l ( . ) Au . AgDonr in ican Rcp. >600 I Au (p ;

S i l l i toe . 199.1)

Ju lcan i , g . l t Ag . Cu. Pb. Au.Peru W, Bi. Zn

El Ind io , l l - t t Au . Ag. CuC h i l e - 1 4 0 t A u .

- 1 . 1 0 0 r A g ( c )

La Mejicana & Ne- 4.0 -1.6 Cu. Au, Agvados de l Fan ia t ina . > l l l5 t Au (c )Argcntlna

RrxJ : rJ t lu i lu . I l -10 AuSpa in 10 t Au (p )

Within or close Compositc welded tulf.to a central-vcnt volx + ands f ' lowsvolc ano

Mzurdiatreme Mau sed + basalticcomplex vol (spil i tc )

Dome complcx Dac to rhyodaciticaround a cenual domes and tuft.sdlareme

Stratovolcano('?) Dac. rhy pyr;in cirl ier caldera dac + ands vol

Dome complex( l) Paleozoic seds +granitcs. Plioceneil ltrusivc dacite

Caldera margin Ands to rhy pyr flows.collapse bxs + domes

N/A

Dac/rhyrxlactic <1.2 nr.y.porphyrystocks

Ands flows <analyt. error+ dykes (+0 .7 m.y . )

CA vo l

Abbrcviations used: CA : calc-alkaline, MS - massive silica, VS : vuggy silica, ands : andesitic, bre : breccias, dac - dacitic, dis =

disseminations, hbx = hydrothermal vein breccia or breccia pipes, pyr - pyroclastics, qtz = quartz, rhy : rhyolitic, sed: sedimentary rock, stk- stockwork, vol : volcanic rock (unspecifled), volx : volcaniclasticsI

1p; : produced, (c) : estimaled total contained 2

Approximate number, quoted from paper or estimated fiom ligures: 150 nr lbr Paradisc

Peak is fbr indiv idual orebodies

Giggenbach 1992a; Rye 1993; Hedenquist et al.1994a). However, the basic genetic controls, as weunderstand them now, were formulated almostninety years ago by Ransome (1907) following hisclassic study of the Goldfield Au-Ag-Cu deposit.In his own words "the

[ore depositingJ solutionswere essentially emanations from ct solidifuingbody rf dacitic magma " and " . . the initially acidemonqtions would be neutralized and modified intheir ctscent through fissured rock. .by thedistance emd kind o.f rock traversed, the quantiQand characler of admixed surface-derived waters,

422

and the pressure and temperature gradients". Thisconcept formed the basis for Ransome's "direct

volcanic hypothesis", though it was quicklyabandoned in favor of a "simultaneous solfatarismand oxidation" model (Ransome 1909). Thechange in genetic interpretation has more thananecdotal value because it i l lustrates the source ofa not-uncommon misconception on the environ-ment of mineralization of epithermal deposits.

The crucial aspect is identification of theorigin of alunite or acid-sulfate alteration, whichcan be generated by different mechanisms in three

H igh-sulfidalion Epithermal Deposits

Table 2 (continued)

Dcposit/districtlocatlon Control on mineralization

Vertical ext-ent of epiri.

ore (m)2Relation to

porphyry systen) Relerences

Motomhoto .lndurcsia

Nalcshitan.Ph i l ippi nes

Lcpanto.Ph i l i ppi ncs

Chin luash ih ,Taiw;rrr

Z i i inshan.Ch ina

Nansatsu .Japiut

S u n r m r t v i l l e .Colorado

Goldlrcld.Nevalir

Paradisc Pcak.Nev:da

Puch lo V ie . jo .Don) in ican Rcp.

Ju lcan i .Pcru

E l lnd io .Ch i le

La Me.jic:na & Ne-vados dcl FantatinaAJScntlna

Rrxllr it1uiIar.Spa i t r

Contact bctwcen dome andvolcmic Krk. steep lault

Stecp strike-slip lault

Major steep + minor faults.diatrcn)e contact. unc0mlormrty, permeable layers

Stecp normal laults +thcir intcrscctions,bedding plancs

Steep srike-slip faultzones + contact 0fvolcztntc vent

Stecp lractures + permeablepyroclastic layers

Steep rldial fracturcs +dtxnc contact

Modcratcly + shallowdipping faults & fissures

Stccp Iaults antl permeablepyroclastic layers

Diatreme rinq fault +permcable layers

Steep liactures

Stccp normal faults

LOcal I aults

Caldera ring faults +nornral local faults

Porphyry Cu-Auprospects nearby. agewi th in 1 .0 m.y .

hoposcd,none lo)owr)

Above + adjaccntsamc age porynyryCu-Au dcposit

Nonc k-nown

None known

None known

Inrusion-ccncredscricit ic, low gradestk mineralization

Nonc lnown

Sericit ic. stk Auminerahzation (East

Zone)

Ntne l*rown

None klown

Porphyry Cu-Momineralizationnearby

HS ore il Nevado delFamatina is a pirt of aporphyry Cu prospect

Nrne Lrown

Pcrcll6 ( I 994)

Sil l i toe ?r a/. ( I 990)

Garcia ( l99l ),Anihas et a/. ( I 995b)

Huang ( 1955) ,Tan et al. (1993)

Rcn er a / . ( 1992) ,Zhang et ul. (1994)

Izawa & Cunningham ( I 9fl9).Hedentluist et al. \1991a)

Steven & Rat t i (1960) . Menhcr let al. (19'7 3). Stoffregen ( I 987 ).Rye (199, j )Gray & Coo lbaugh( 1994)

Ransomc (1909) , Ash ley (1974) .

Ash lcy & S i lberman (1976) .V ikc (1989. wr i t ten conrmun.I 995)

John ? / a / . ( 1991 ) .S i l l i toe & Lorson (1994)

Russe l l & Kes le r ( 1991 ) .Muntean et a1. ( I990)

Petersen eI al. (19'11\.

Noh le & S i lberman (198,+) .

Dccn ( I 990)

Siddcley & Araneda ( 1986).Jannas el a1. ( 1990)

Losurda-Calder(xr & McPhail( I 994). Losada-Caldcr6n el a/.

{ I 994)

Anibas e/ d/. ( I 995a)

250

150

500

800

60()(')

< 1 5 0

250

400

< 150

4UX t)

600

>l(x)

< 1 5 0

principal geologic environments (Bethke 1984;Rye el al. 1992): (l ) by the disproportionation of

magmatic SOz to H2SO4 and HzS following

absorption by groundwater (magmatic-

hydrothermal), (2) by atmospheric oxidation of

H2S in the vadose zone over the water table,

associated with fumarolic discharge of vaporreleased by deeper boiling fluids (steam-heated),

and (3) by atmospheric oxidation of sulfides

during weathering (supergene). Magmatic-

hydrothermal alunite occurs with mir-rerals such as

d iaspore , pyrophy l l i te , kao l in i te , d ick i te , and

zunyite, which are typical of hypogene (T : 200-350 "C) acidic condit ions (advanced argi l l icassemblage; Meyer & Hemley 1961). This type ofalunite is characteristic of HS deposits, but it mayalso appear in areas of advanced argill ic alterationvoid of ore mineralization (e.g., Iwao 1962; Flall1978). Alunite in steam-heated environmentsforms with kaolinite and interlayered ill ite-smectite at about 100 to 160 'C where fumarolicvapor condenses above the boiling zone ofneutral-pH, H2S-rich fluid, typical of geothermals y s t e m s t h a t f o r m l o w - s u l f i d a t i o n d e p o s i t s .

423

A. Arribcts,,Ir.

Propylitic Argillic + Adv. argillic

rock rock rock

Because of the relatively shallow and dynamicenvironment of mineral izat ion, overpr int ingamong the three types of acid-sulfate alteration( including sLrpergene) is possible; however, thespatial relation of each type of alunite to ore isdifferent, and correct identification is importantfor exploration (Rye et al. 1992:. White &.Hedenqu is t 1995) .

DISTRIBUTION, AGE AND ECONOI\{ICStcNInrcaNcB

In common with other magmatic-lrydrothermal deposits (e.g., porphyry copperdeposits), HS deposits coincide worldwide withplutonic-volcanic arcs. This associat ion is bestobserved in the Cenozoic deposits of the Circurn-Pacific and the Balkan belt of southeastern Europe(F-ig I ) . These deposits occur in two mainsettings: in island arcs and at continental margins.The tectonic regime during formation of the

deposits seems to be dominantly extensional(Si l l i toe 1993). Some deposits (e.g., Goldf ield,Rodalquilar, Summitville) formed in intra-

cont inental regions during periods of extensiot lthat followed regional compression and sub-duct iorr by several m.y.

Tertiary HS deposits predominate, and only afew deposits are Mesozoic (e.g., Pueblo Viejo,Zijinshan), Paleozoic (e.g., Temora and others insoutheastern Australia), or PreCambrian (the early

Quartz alunite

Mineralized vuggyquartz rocl(

Proterozoic EnAsen Au deposit located in theBalt ic shield of central Sweden; Fig. I ) . Theyoungest deposits are Pleistocene (<1.6 Ma) andoccur in the central western Pacific (Kelly,Lepanto, and Chinkuashih). The concentration ofdeposits in young volcanic areas is mainly areflection of the fact that older HS deposits aremore likely to be eroded.

Gold. copper, and variable arnounts of silverare the main products of HS deposits (Table 2).Gold (Nalesbitan, Rodalqui lar) , occasional ly withsilica by-product (Nansatsu), is the only economicmetal in the smal ler deposits. No copper isproduced at Paradise Peak and Pueblo Viejo.Mercury is produced at Paradise Peak, and theJulcani district has been a source of a remarkablepolymetal l ic assemblage consist i rrg of Ag, Cu, Pb,Au, W, Bi, and Zn (Table 2). The six largestdeposits or distr icts (Chinkuashih, El Indio,Goldfield, La Coipa, Lepanto, arrd Pueblo Viejo)each contains more than about 100 tonnes of gold.The economic potential of this type ofmineralization is clear in regions such as theChi lean Andes (S i l l i toe 1991a) .

VoLCANIc SITTTnC AND ASSOCIATEDIGNEOUS ROCKS

The high-sulfidation deposits considered inTable 2 occur within intennediate-compositionvolcanic rock sequences having ages broadly

Leachedsilicic

, I'100 m

1Kaolinitic

rock

Figure 2. Cross-section of alteration zones characteristic of high-sulfidation deposits, as observed at the

Summitvil le Au-Cu deposit, Colorado. Diagram at left (simplif ied from Steven & Ratte 1960) shows schematic

outward zonation from a subvertical mineralized body, shown at right (from Stoffregren 1987).

H ig h-s ulfi dat ion Ep i t he r m a I Dep os i ts

Figure 3. K2O versus SiO, variation diagramfor rocks thought to be genetically related tohigh-sulfidation deposits. The samples from12 deposits or districts (r : 140) define asmall compositional f ield, which contrastssharply with the large field defined byvolcanic rocks associated rvith low-sulfidation or intrusion-related Au deposits(> 100 samples f rom l6 d is t r ic ts ; Si l l i toe1991b, 1993; Mr i l ler & Groves 1993). Thedegree of alteration of the rock samples andprecision of the analytical data are Iargelyunknown; however, according to theindividual data sources, most of the samplesare unaltered or very weakly altered. Circlesindicate average values for each high-sulfidation deposit or district: ChChinkuashih, Cq = Choquel impie, Go -

Goldf ie ld, In : E l Indio. Ju : Ju lcani . LaLaurani , Le : Lepanto, Mo - Motomboto,

Na - Nansatsu, PP : Paradise Peak, Ro : Rodalquilar, Su - Summitvil le. Compositional f ields afrer Keith et al.( 199 l). See Appendix fbr references and information on data plotted.

50 60 70SiO2 (wt"/")

simi lar to that of mineral izat ion. Where abundantradiometric ages are available, the age of the hostrocks and the age of mineral izat ion are withinanalyt ical precision: where a di f ference isindicated, i t is typical ly less than -1.0 m.y. (Table2). A comrnon spatial association exists betweenthe deposits and shal low. typical ly porphyri t icintrusions. These intrusions are interpreted to bethe roots of volcanic domes or the feeders ofcentral-vent volcanoes or maar-diatrerne com-plexes, the three rnain volcanic settings for HSdeposits (1 'able 2). Some deposits are hostedent irely within a single dome (Summitvi l le), orwithin a dorne complex (Julcani) . In most casestfre mineralization extends frorn the subvolcanicintrusion into country rocks, such as the MainVein Cu-ALr-Ag deposit and associated brecciadeposits in the Penshan area of the Chinkuashihdistr ict . Some deposits, however, do not show any(known) spat ial associat ion with subvolcanicintrusions thought to be genet ical ly related tomineral izat ion (e.g., Nalesbitan. Nansatsu). In theRodalqui lar Au deposit , dykes and smal lintrusions of hornblende andesite which areinterpreted to be temporally related to themineral izat ion reprcsent only a fract ion of thealtered and mineralized area exposed at thepresent depth of erosion; a larger intrusive body is

tlrought to exist at depth (Arrrbas et al. 1995a).The main control on locat ion of mineral izat ion atRodalquilar is the structural rnargin of two nested,resurgent calderas. With the exception ofRodalquilar, the role of calderas in the formationof HS deposits seems to be l i rni ted to faci l i tat ingthe emplacement of late intrusive magrna alongpreexisting caldera ring-fractures (Rytuba cl rzl.1990) .

The magmas thought to be genetically relatedto HS deposits have a remarkably limitedcompositional variation. The ranges of wt.% K2Oand SiO2 for twelve deposits overlap greatly andshow a dominance of calc-alkal ine andesit ic anddacit ic composit ions, with subordinate rhyol i te(Fig. 3). Intermediate calcic volcanic rocks arelimited to porphyritic intrusions in the Lepantoand Motomboto Cu-Au-Ag districts, andintermediate-to-felsic alkali-calcic rocks arecharacter ist ic of the Summitvi l le and Lauranidistr icts (Fig. 3). Interest ingly, no deposits havebeen discovered in associat ion with alkal ine ormafic magmas, even though these magmas can begenet ical ly related to low-sulf idat ion andintrusion-related Au deposits (Si l l i toe 1991b,1993; Miiller &. Groves 1993; Richards thisvolume). The data shown in Figure 3 suggest arelation exists between Inagma cornposition and

/ ' \ ca\c.,^t 3\Katt"

*t" '""

A Arribas, Jr.

Table 3. Main alteration and mineralization characteristics of 14 selected high-sutfidationepithermal deposits

Dcposit

Lateral alteration zoning(outward from nrinem-

lizcd txxlies)

Vertical altcrationzonin-9

(shallow t() dccp) Pnncial ore nrinerals

( )remineralization

rn: Ag/Au

Silica corc VcryIo* As

N/A

Si l icu core <2

Silica eorc 2-10

Siliclr core < |

Si l icu cure 10-30

I n A A + 7MS zoncs

Vcins .170

Vcins

Si l icu core 10-10

Silica core < I

Motonlboto

Nalcsbitan

Lcpanto

Chinkuashih

Zi j rnshar

N:ursaLsu

Sunrnr i tv i l le

Goltllicld

Pu'irdisc Peak

Puehlo Vrejo

Julcur i

El Indrr

La Mcjicaua.Nevrdrx drlFiunatina

Rulalquilar

VS ,qr-alu |qt7-kao )kao-smc r i l l - chl

Silicificd Hbx rqtz-kao-alu rill-sme-chl-cal

VS/MS , tz-alu-kao rkao-qtz-ill rchl-ill

VS,MS rtltz-alu-kao ri l l -chl -kao

VSA4S tqtz-dic-alu t9?-dic-\er rqtz-Scr

VSA4S ralu-dic-pyo 'ill-kao-smc tPRO

VS(MS) tqtz-aiu- tqu-kao 'kao-ill Isnrc-chl

MS(VS) rqrz- : r lu-kao ri l l -smc IPRO

Vertical (due to deF)sits t y l c ) : MS(VS) |q?-alu-kao rsme-chl

Conrplex + overprinted

pre<rrc:VS/MS )qtz_alu-kart rqtz-kao: Syn<rre:

qtz-pyo-py rqu- kao-py +q?-siir-py lq(Z-kao-smc

Cu stage veins rkao-alu-scr-qU: Au stagcvcins rser-kao-pyrt-q?

VS,MS rqtz-aiu-kao rqtz-kao-ill r ill-sme-chl

VS,MS rqtz-alu rqtz-kao r i l l -kao lchl

Silicificd Hbx )qL.-kx>alu rill-sme-cbl-cal

MS /VS rAA ISER r( K-silicate in subiacentFSE porphyry copper)

VSA4S rqu-dic-alu r90-dic_Sor )qU_ser

VS/h4S ralu 'dic-ser-pY )ser-chl tPRo

vs(MS) ,qrz_kao_xlu rqtz-kao rSER

MS>VS .qr1__alu-kao |q?-klolpyo

MS(VS) )q lz-alu-k()(SER in laulted. deeper('l)East Zone deposit)

Early: Kao-py-qu rdu-py<ltz

Lare: MS rpyo_dia

Alu-kto rqu-scr ) (K-s i licatc in N. del Farnatinaporphyry copper)

VSMS rqtz-alu-kao rqtz-kao-sel qt !-ser-py

Py. ena-luz, mzu. sph. gal. tcn- Silica corc 35--15lct. ars. cpy. arg. nat.Au. tcll

Py. chalc.qtz, cco. hor. cov.ena. tell

Ena-luz, py. ten-te t. cpy. p_v-.. e lc. Sil ic:r corcsph. gal. nrar. sele. tell. Sn-bearing sull'

py. ena-luz. f :rm. tcn-tet. nal.Au. Sil ica corce lc . hu . na t .Hg. tc l l . sp l r . g r l .cpv. geo. hou

py . d ig . ena. cov . n ro l . na t .Au S i l i ca corccpy. hor. tet-ten. gal. sph

ena- luz . p1 ' . c lc . na t .Au. a rg .plr. cpy. bor. sph. gal. cas. stirn)()1. can

py. cna-luz. c()v. mar. nat.S.nat.Au. sph. gal. bar. cpy. ten

py. lam. ten-tet. bls. gol,nat.Au. cna-luz. bru. tell. sph.cov

bar. stb. his. nat.Au. nrnr. pl.nat.S. cin, sph. gal, cpy. ars.tet. arg, cov. f:ul

py. sph. cna. nal.Au. nal.S. b2rr.tcn-tet. fan. gal. bar. stb. cle.selc, tcl l, Bi- Pb- Ag- sull '

py. wol. c:rs. nat.Au. ena. lur.tel tcn. cpy. gal. sph. biu. sid.Pb- Bi- Ag-bearing sull '

Ena. py. tel, nat.Au. ten. cp-y.gal. sph. hue. hor. dig. cnrp.cco. nlar. Dar.

pV. cna. cpy. Splr, ten-lct. cov.cco. lam, luz. nat.Au. gal. nlole le . te l l , co l . Sn-B i -Ph-Ag-su l l

Py, nat.Au. cna. tell. cas. col.cov, dig. bor, gal. sph. Bi- sult

Abbreviat ions uscd: AA: advanced argi l l ic , Hbx - hydrothermal brcccia, MS: massive s i l ica, PRO - Propyl i t ic . SIJI{ :ser ic i t ic , VS - vuggy s i l ica, VS (MS) = vuggy s i l ica dominant, a lu - a luni te, ars: arsenoyr i te, bar - bar i tc . b is:b ismuthin i te, bor = borni te, bou - bournoni te, cal : calc i te. cco = chalcoci te, chal .qtz : chalcedony or chalcedonic quartz,

chl : chlor i te, c in = c innabar, can: canf ie ld i te, cas: cassi ter i te, col - colusi te, cov: covel l i te, cpy - chalcopyr i te, d ic:d ick i te, d ig : d igeni te, e le : e lectrum, emp : emplect i te, fam - famat in i te (st ib io luzoni te) , gal : galcna, gco - geocroni t r ,

gol : goldf ie ld i te, hue - h i ibner i te , i l l : i l l i te , kao: kaol in i te, luz: luzoni te, mar: marcasi tc, mol : molybdcni tc, nat .Au :

nat ive gold, nat .S : nat ive sul fur , nat .Te : nat ive te l lur ium, oro = orpiment, py - pyr i te. pyo : pyrophyl l i tc , qtz : quartz,

r e a = r e a l g a r , s e l e : s e l e n i d e s , s e r = s e r i c i t e , s i d - s i d e r i t e , s m e : s m e c t i t e , s p h : s p h a l e r i t e , s t a = s t a n n i t e . s t b : s t i b n i t c ,sul f - sul f ides or sul fbsal ts, te l l : te l lur ides, ten: tennant i te, tet : tet rahedr i te, tou: tourmal ine, wol : wol f iamiteI

Based on f lu id- inclusion ( f l inc) or geological (geol) evidence; b lank rvhere not speci f led.'Boi l ing (Hbx) - boi l ing due to abrupt pressurc reduct ion a-ssociated wi th hydrothermal brecciat ion

A. Arribus, Jr.

is difficult, but useful for discussion of the

differences among deposits and design of

exploration strategies. In this context, White

(1991) dist inguished three end-member styles of

HS deposits, named after deposits of the Circum-

Pacific: Temora, El Indio, and Nansatsu. Irregular

bodies of disseminated, si l ic i f ied ores dominate in

the Temora-style. Cavity-fil l ing veins with

sericitic and clay-rich haloes are characteristic of

El Indio-style ALr deposits. A large group of

deposits fal ls into White's (1991) Nansatsu-style,

which is characterized by wallrock-alteratiort

zoning simi lar to that shown in Figure 2, and by

the occurrence of enargite-bearing ores within a

si l ica core consist ing of vuggy or massive si l ica

rock (Table 3). Mineralization in this style of

deposit forms irregular stratabound bodies (e.g.,

Nansatsu, Lepanto) or subvertical vein-like

masses or " ledges" (e.g., Chinkuashih, Goldf ield,

Lepanto, Rodalqui lar, Summitvi l le). These

deposits contain breccia bodies, veins, stockworks

of small veins. and disseminated ores that replace

or irnpregnate intensely altered country rock'

Ericksen & Cunningham (1993) dist inguished two

styles of HS deposits in the Andean province: Ag-

and Au-rich polymetallic base-metal veins' and

low-grade vuggy silica and breccias; the two types

are broadly comparable with El Indio- and

Nansatsu-styles, resPectivelY.l,ocal subvertical faults and fractures are the

dominant control on HS mineralization and they

are present in rnost deposits (Table 2). Other

examples of structural controls observed in some

districts arnong the foufteen selected include:

rnoderately to shallow-dipping faults (Goldfield)'

caldera ring and radial faults (Rodalquilar), the

di lat ional jog of a str ike-sl ip faul t (Nalesbitan),

diatreme ring-faults (Lepanto, Pueblo Viejo), the

contact between a dome or volcanic conduit and

country rock (Motomboto, the Missionary

orebody at Summitvi l le), and a l i thologic

unconfbrmity (Pueblo Viejo, Lepanto). In three of

the fburteen deposits, the principal control is

l i thological (maar sediments at Pueblo Viejo, and

interbedded pyroclastic layers at Paradise Peak

and Nansatsu; Table 2).A unique cornbination of the structural and

lithological controls characteristic of HS deposits

is exhibi ted by the Lepanto Cu-Au-Ag deposit .

The deposit is 3 km long and consists of a tnainzone of breccia and replacement mineralizationalong the Lepanto Fault (Fig. 4A). Mult ip le veins

associated with smaller diagonal faults branchfrom the rnain zone and extend into both the

hanging wall and foot'ivall (Garcia l99l). The

characteristic mushroom-shaped cross-section of

many of the orebodies at Lepanto is related to the

intersection of the steeply dipping Lepanto fault

and branch veins with the unconfonnity at the

base of Imbangui la dacite (Fig. aB). Li thologicvariations in the host rocks also played an

important role in the fonnation of the deposit. as

shown by lenses of stratiform enargite-luzotrite

ore which resulted from replacernent of detrital

layers within volcaniclastic and sedirne ntary

basement units (Garcia l99l ) .

AITEN.ITION MINERALOGY AND ZONING

As mentioned above, the lateral alteration

zoning that is characteristic of HS deposits

reflects the reaction and neutralization of high-

temperature acidic fluids with wallrock. The

innermost zone of vuggy or tnassive si l ica

alteration commonly has sharp boundaries with a

zone that may contaitr quartz, alurrite, kaolinite,

dicki te, pyrophyl l i te, diaspore, and zunvite ' . l 'h is

advanced argill ic assemblage grades into a second

envelope of argi l l ic al terat ion, composed of

minerals such as quartz, kaol ini te, i l l i te, ser ic i te,

and smectite, and an outermost halo of propylitic

al terat ion, with chlor i te. i l l i te, smect i te. and

carbonate (Fig. 2, Table 3). The width o1' eacl.t

zone varies widely; for example, vuggy si l ica and

advanced argill ically altered rock fonn narrow

(<70 cm) vein selvages at Julcatr i (Deen 1990) '

but form wide (>50 m) rock bodies at Sumrnitv i l le

or Lepanto (Figs. 2 and 4). Late-stage', cavity-

f i l l ing planar veins at Julcani and E, l Indio may

extend outside the zone of aluni te-kaol ini te ' ln the

majority of HS deposits, however, most of the ore

is contained within the si l ica core, inside the

advanced argi l l ic envelope ( ' Iable 3).

l ln Russian and eastern IJuropcan tcrrninology lhcse rtlcks are

conrmonly termcd 'metasomatic quartzites" with nrorc spe cilic

names such as porous quartzites, diasporc quartzitcs' alunite

quartz i tes, and dick i te quartz i tes (e.g. . Vcl inov et u l .1990)r '

@NW

High-sulfidation Epithermal Deposits

Figure 4. Longitudinal (A) and transverse (B) cross-sections of the Lepanto-FSE Cu-Au-Ag deposits (phitippines),showing structural and lithologic controls on formation of the high-sulfidation and porphyry-type ores (simplif iedfrom Garcia l99l ). Potassium-argon dating of country rocks and alteration minerals associated with the porphyry andhigh-sulfidation deposits indicates that hydrothermal Cu-Au mineralization took place in the middle of a pliocene toPleistocene event of dacitic-andesitic magmatism (Arribas et al. 1995b). Note the overall spatial overlap of themagmatic and hydrothermal "plumbing" systems (i.e., volcanic vents of Pliocene dacite, quartz diorite intrusions.porphyry deposit, and deeper parts of epithermal mineralization).

The zones of alteration with increasing depthtypically grade from a shallow silicic zonethrough advanced argi l l ic, argi l l ic, argi l l ic/ser ic i t ic, into a ser ic i t ic or phyl l ic zone withquartz, sericite, and pyrite. This alterationsequence occurs over a vertical interval thatranges from a few hundred meters to more than1000 m, and has been best documented by deepdri l lholes in the deposits of smal ler s ize, in whichthe vertical span of rnineralization is less thanabout 300 m (e.9., Rodalqui lar, Summitvi l le; Fig.5B). At Lepanto, sericitic alteration at depths of400 to 500 m below the epithermal deposit givesway, laterally towards the south, to K-silicatealteration of the FSE porphyry Cu-Au deposit.Porphyry-type stockwork mineralization atParadise Peak is contained within the sericitic oresof the East Zone deposit which, according toSillitoe & Lorson (1994), formed underneath themain HS orebodies irr the area. A quartz-sericite-pyrite zone with trace amounts of chalcopyrite andmolybdenite surrounds an intrusion of monzoniteporphyry >300 m below the HS deposit atSummitvi l le (Grav & Coolbaush 1994\.

The lateral and vertical alteration zonesdescribed above correspond to a generalizedmodel. They are useful in exploration becausethey help in understanding the genetic environ-ment of a deposit and provide spatial "markers"

within the extinct hydrothermal system.Experimental data on the relative stability ofrninerals such as alunite, kaolinite, pyropliyllite,and diaspore (Hemley et al. 1969, 1980), coupledwith the temperature ranges noted for these andother related acid minerals in active systems(Reyes 1990; Reyes et al. 1993), also provideinformation that contributes to definition of thepaleoconduits in extinct systems.

If studied in detailed, several superimposedand crosscutting stages of pervasive as well asfracture (conduit)-related mineralization may berecognized in the majority of deposits. These arethe expected result ofvariations, during the courseof mineralization, in temperature, pressure, andcomposition of the hydrothermal fluid and thedegree of wallrock interaction. Detailed field andpetrographic studies at the Monte Negro orebodyin the Pueblo Vieio deposit have resulted in

A. Arribas, .Jr.

Vuggy silicaAdvanced argillicArgillicS€riciticPropyliticInlense supergene acij-sulfate ovsrprint

-100

I K M

Au-(Cu-Te-Sn) htgh-sulfidation deposits

particular features of the deposits listed in Table

3. Pyrite and enargite (and its low-temperature

dimorph luzonite) are the dominant sulfides in HS

deposits; pyrite is abundant but the amount of

enargite and luzonite is variable. Common ore

minerals, listed by decreasing abundance from

variable to very minor, include tennantite-

tetrahedrite, covellite, native gold and argentiangold (electrum), marcasite, chalcopyrite, spha-

lerite, and galena. Famatinite is locally abundant

in some deposits (Goldfield, La Mejicana). Sparse

ore minerals include bornite, cassiterite, ctnnabar,

molybdenite, orpiment, realgar, stibnite, and

wolframite (the last locally important at Julcani).

Other minerals present in minor amounts in

several deposits include Pb-, Ag-Pb, Bi- and Sn-

bearing sulfbsalts (Table 3).Fine-grained quartz is the dominant gangue in

HS deposits. Other comrnon but minor gangue

minerals include bari te, kaol ini te, aluni te,

pyrophyllite, diaspore, and Ca-,Sr-, Pb- and REE-

bearing phosphate-sulfate mineral(s) such as

svanbergite-woodhouseite or crandallite (Stoff-

regen & Alpers 1987). For example, high-grade

Elsvation (m) | 5 0 0 m I

I

ffil ^ ^ af - ' - ^ l

tlm@

Figure 5. Generalized surface alteration map (A) and cross-section (B) of the Rodalquilar

HS deposit in the Rodalquilar and Lomilla calderas, southeastern Spain (fiom Arribas e/

at. 1995a). The boundaries shown between alteration zones are irregular and gradational.

identification of two stages of mineralization,

interpreted to correspond to two distinct magmatic

pulses (Muntean et al. 1990). During the first

stage (responsible for -600/o of the Au in the

deposit), shallow kaolinite-quartz-pyrite and deep

alunite-quartz-pyrite-quartz zones were de-

veloped, with gold mineral izat ion in associat ion

with disseminated pyrite in the wallrock; during

the second stage (responsible for about 40% ofthe

Au), an extensive zone of silicification with pyrite

+ sphalerite + errargite veins formed at shallow

levels. above a zone of pyrophyllite-diaspore

alteration (Muntean et al. 1990).

Ono aNu GANGUE MINERAL0GY' AND

TIMING OF MINERALIZATION

White et ul . (1995) and White & Hedenquist

(1995) presented detai led discussions on var ious

aspects of epithermal gold mineralization on the

basis of observations from a large number of

deposits around the Pacif ic; their conclusions with

respect to ore and gangue mineralogy in HS

deposits are included here, in addition to the

II!

vein specimens from Chinkuashih, Goldfield, andLa Mejicana have spectacular intergrowths of oreminerals with kaolinite, alunite, or pyrophyllite.This observation implies that ore formation

occurred under moderately acidic to acidicconditions, which are inconsistent with transport

of Au as a bisulfide complex (Seward 1973).

Recent studies of Au solubi l i ty in high-

temperature acid sulfide solutions have resulted in

identification of AuHS" as one of the principalgold complexes in HS mineral izat ion (Bening &

Seward 1994), the other possibility being AuCl2(e.g., Hedenquist e/ al. 1994a).

The number and order of mineralizing eventsprovide critical information for reconstruction of

the hydrothermal system that results in HS

mineral izat ion. A minimum of two stages of

alteration/mineralization has been recognized in

most deposits on the basis of crosscuttingrelations (Table 3). The most common evolutionis from an early leaching and alteration stage to a

later ore-forming stage. Vuggy silica rock and the

advanced argill ic assemblage with disseminatedpyrite form typically early-stage acidic alteration,

and are followed by Cu + Au + Ag deposition.

Detai led studies in some distr icts (e.g., El Indio,

Lepanto), however, have resulted in identification

of two metal stages, an early Cu-rich, Au-poor

stage, dominated by enargite-luzonite, and a late

Au-rich, Cu-poor stage, associated with

intermediate-sulfidation-state sulfides such as

tennantite-tetrahedrite and chalcopyrite, and

tellurides. The transition from quartz-alunite-pyrite alteration to enargite-pyrite and finally to

tennantite-tetrahedrite, the last typically without

sulfate (alunite) but with quartz-sericite gangue

and wallrock alteration, indicates a fluid

progressively more reduced and less acid. At

Summitvi l le and Chinkuashih (also Tambo and

Furtei-Serrenti; Table l), a late stage of barite-

gold has been documented.

CsaRactnRISTICS AND SoURCES oF

HvuRorsnRMAL FI-utos

Results of recent detai led f lu id- inclusion and

stable-isotopic studies reveal much about the

composition, temperature and sources of

hydrothermal f lu ids in HS deposits" Combinat ion

H igh-suffidation Epit hermal Depos its

of these data with geological and mineralogicalobservations mentioned above allows the natureof the altering and ore-forming fluids to bedetermined. The framework for the interpretationhas benefited from information on the compo-sition and fluxes of volcanic discharges and activemagmatic-hydrothermal systems (Hedenquist &Lowenstern 1994; Giggenbach this volume;Hedenquist this volume).

F I uid-in c I usio n Ev idenceSuitable hosts for fluid-inclusion studies are

scarce in HS deposits, as the gangue minerals aretypically fine-grained and even millimeter-sizehydrothermal quartz crystals are usually late stageand vug-fil l ing. Satisfactory results are obtainedon secondary fluid-inclusions in igneous quartzphenocrysts from altered wallrocks; althoughlacking temporal information, these inclusionsseem to provide a representative cross-section ofthe fluids involved. The most reliable data on theore-forming fluids are obtained through infraredmicroscopy directly on ore minerals, such asenargite (Deen 1990; Mancano & Campbell1ee5).

The temperatures and salinities estimated forHS deposits define a wide range, from 90o to 480oC and <l to 45 equiv. wt.% NaCl, respect ively(Table 4). There is no systematic difference insalinity among Au-, and Ag- or base-metal-richdeposits, in contrast to that noted for low-sulfidation Au versus Ag deposits (Hedenquist &Henley 1985). Large var iat ions in bothtemperature and salinity also occur within a singledeposit; these reflect the dynamic environment,with high- and low-temperature and high- andlow-salinity fluids interacting during the course ofmineralization. Four broad groups of hydro-thermal fluids are recognized here on the basis ofthe estimated temperatures and interpretationsgiven by most workers. The temperatureboundaries chosen for each group are onlyindicative, as significant variations exist amongand within deposits; each group, however,provides relevant information on various genetic

aspects.Group 1. Higher temperature (e.g., >300 "C)

fluids of variable salinity, which have beendocumented in several deposits and are generally

431

A. Arribas, Jr.

Table 4. Summary of f luid-inclusion microthermometric data for high-sulfidation deposits

DepositHost-mineral

studicdTcmpcrature Salinity Asstriatcd

("C)t (cquivwt.%NaCl) a l tcrat ion

Mrxoniboto, IndoncsiaNalcsbi tan, Phi l ippinesLcpanto, Phi l ippines

Chinkuashih, Taiwan

Zi . l inshan. China

Nansatsu, Japan

Akaiwa, JapanMitsumori-Nukeishi, Japan

Sunimitv i l le , Colorado

Coldlielcl, Nevada

Pradise Peak, Nevada

Julclni, Peru

Ccarhuaraso, PeruColqui.jirca, PeruCan-Can (La Coipa),

Chi lcEl Indio, Chile

La Mejicana (LM) andNcvados Famatina (NF),Argentina

Rrxlalquilar, Spain

Furtei-Serrenti, ltaly

Barite

QuartzEnargitc

Quartz, baritc.a luni te

Qu:rtz (no dctailsrcfx)rtcd)

Quartz

DiasJnre

Quartz. ba-ritc,quanz pnen(x

Quartz phcnoc

Baritc

Quartz- phenoc

Quartz, baritc

Quartz, barite

Quartz

Quartz phenrr

Quartz phenocWol, ena, quartzSidcritc

Quartz phenoc

Qufiz phcnoc

Sphalcritc. quartzhiibnerite

Quartz phenocN/A

Quartz , quartzphcnm

Quartz, barite,quartz phentr

<l Atusi lAA/si l

0.24.5 AA/si l

150-18022(J-260t](\-290

I8{). 330

I 6(f-3(X)220 380l 0(I- 1 60

(300-+20)13(l250

-21025F310I 9(I-2402I ( I 330

l8(}_280

(300-390)- t (x)

230-480+2 I (),280

(37(H10)r80-2I0300,380

(up to 4-50)I 60-280360 45t)230-330220,250330-38023(f,260l7(I 350

I 90-280i4(}-l80(>3(n)

2(XI'+60l6(i-340230-480

17F30022(}.450

I 9(),3209(I 140

(390-5m)

o .2 - t2

a 1 a

3 , t 90-5

(3-2( ))< l

up to 30

0.5- 1.1

2 - 1 8

(up to 9)

5 1 80.2-8

<33[r35

\ ) 438,469 2 06-97 1 8.+- l l< l-40

0 . l 40 . t -2 .1

(up to 27)t 3 l

0 .3 ,1 2341

2 3 0245

Scrs i r

AA/si lAA/s i lScrAAis i l

AA/sil

AAis i lAAis i l

AA/s i lSer

AAis i l

AA/silAA-/silAA/sil

donrinantAA"/Ser

AA + scr

Scr

AA/silSer

0.4-23 AA/sil0.4- I .6(32 45)

Ahbreviations used: AA = advanccd argillic, ena = enargite, phenoc = phenocrysts, ser =sericitic, sil = silicicl wol = wolfiamite: see Tablc J for rraleoderrth estimations

Irem;^*raturc,arcr()undedl() t icncircst l t ) " : hraekcisuscdtoindrcatctugh-tcnrpcraturr .inclusions typically interpreted is having formcd early or being anomalous

interpreted as "anomalous" or unrelated to ore andare associated with early stages of alteration.Two-phase entrapment may explain some of theunusually high homogenization temperatures (4),particularly considering the shallow minerali-zation depth inferred for many of the deposits(Table 3). However, most workers agree that such

432

entrapment cannot account for all the high 17,values. The consistent presence of these fluids inseveral deposits indicates a high temperaturegradient, and implies the presence of a shallow-depth intrusion, and possibly lithostatic confiningpressures. On the basis of fluid-inclusion, as well

as isotopic (634Srrrrut.-rurna") temperatures (see

H igh-sulfdation Epithermal Deposits

Table 4. (continued)

Dcposit Commcnls Rcl'crcnccs

M()tornboto, Indoncsian*alcsbi lan. Phi l ippincsLcpanto, Phi l ipprncs

Chinkuashih. Taiwan

Zi . j inshan, China

Nansatsu, J lpan

Akaiwa, JapanMitsumori -Nuke: ishi , Japan

Su rnmitv i l lc . Colorat io

ColtlliekI, Ncvatla

Piiraclise Pcrk, Ncvatla

Julcani , Pcru

Ccrrhuaraso, PcruColc lu i l i rca, PcruCan-Can (La Coipa).

Ch i l eEl Indiu, Chi lc

La Mc.jicana (LM) andNcvacirs Famatina (NF),Argcnt ina

Rrxlalquilu, Spain

Furtci-Scrrcnti, Italy

Rcconnaisancc srudy in latc-stagc bariteReconnaissancc studyi liquid CO2 observcdSamplcd intcrval 3 knl long by 0.5 kn hieh t ctnling tluitls

awav fionr subjaccnt porphyry Cu-Au degrsit, whcrcTh >.150'C & salinity up to 5.1 eq wt.rl NaCl

Prxrr ly-documented samples along a '15( lnr vert ical intcrval :the highcr Ths in sanples lt -7,50 m dcpth: CO2 ohserved

Asstrciatcd with main stagc CuDorrp altcration zonc (>6(X) nr depth)Associatcd with late. shallow silica-AuAssrriated with carly silica and quartz-dickiteLate, vug-lill ing quirtz

Qtz in brcccia. salrne liquid and krw-salimty vapor cmxistVein quartz -4(X) m helow Kasuga depositCoarsc-grained clilsgrreNot (known) Au or Cu mincralization, but high salinity

l lu idsLic lu i t l - r ich: sal in i ty >6 eq wl .7 NaCl only in vuggy s i l ica

associated with Cu mineralization: CO2 obscrvcdLrquid- and vapor-rich inclusions: also polyphase inclusionsLatc barite-Au assemblagcTruc T5 is interpreted to be 25(1290"CHydrostatic and ncar-lithostatic prcssures suggested

Latc, vug-lill ing crystals in hydrothermal brcccia:Frorn stockwork Au East Zonc dcoosit: COr observed

Quaru-alunitetpyritePro-ore tourmalinc brcccia dykes, lithostatic pressures likely.Main-stagc orc fluicls, also inner veins, liquid-rich inclusionsLatc-stage ore fluids, also in outcr vcinsl P correction appliedQuartz-alunitctpyriteQuartz-al u ni tetpyriteTwo generations idcntillcdl both may be very salinc. Evidcncc

firr P abovc hydrostatic and higher salinities at dcplhCoppcr and gold stagesLate stageInterprctcd as carly, with vapor-rich inclusions, CO2 observetlLM & NF. includes liquid-, vapxrr-rich and potyphasc inclusionsNF: complctc transiLion liom porphyry-type fluids in K-

silicatc stage (30(),6(X)+"C, up to 67 eq wtq, NaCl)through sercitic to epithcrmal f'luids in HS (AA) stage;vapor-rich inclusions typically less saline

Vcrtical temperature and salinity gradient: high-lcmperaturebrines coexist with low -;Llinity vapor inclusions:hydrostatic and near-lithostatic pressures suggested

Includes hi-eh + low-salinity fluids (22-23, <6 eq wt% NaCl)Latc stagc

Percll6 ( I 99:l)Si l l i tm el rr1. (1990)Mancano & Campbell (1995),

G a r c i a ( 1 9 9 1 )

Folinsbee et tr l . ( .1912).Ycn( 1976), Tan et uL. (.1991)

Zhang er al. (1991)

Hcdcnquist et ul. (1991.t)

Akamatsu & Yui (1992)Aoki & Watanabc (199-5)

Bruha & Noblc ( 1983), R.Stoflicgcn (writtencommun. , 1994)

Cunningham (1985)Bruha & Noble (1983)Vikre ( 1989)

John e t a1 . (1991)Sil l i toe & Lorson ( l99tl)

Bruha&Noble(198.1)Shclnutt & Noble (1985)Dccn ( 1 990)Deen ( 1 990)Bruha&Noble( l9 t i3 )Bruha & Nohlc (198.1)Townley ( 199 1)

Jannas er a/. ( I 99(l)

Losada-Calder6n & McPhail( l 994)

Sdnger-von Oepen at a/. ( I 989),Arribis et al. (1995a)

Ruggieri ( I 993b)

below), pressures above hydrostatic have beensuggested for several deposits, including Julcani(Shelnutt & Noble 1985), Goldf ield (Vikre 1989),Summitvi l le (Rye 1993), and Rodalqui lar (Arr ibaset al. 1995a).

Group 2. Intermediate-temperature fluids(e.g.. 180-330 "C), with sal ini t ies var iable from

<1 to -18 equiv. wt.% NaCl. With the possibleexception of deposits for which only the late-stageminerals have been studied, these typically liquid-rich inclusions are found in all deposits. Main-stage ore fluids are contained within this group.The temperatures measured in fluid inclusions inenargite at Lepanto (Mancano & Campbell 1955)

433

A. Arribas, Jr.

and Julcani (Deen 1990) are broadly similar, buttheir sal ini t ies are dist inct ly di f ferent (0.2-4.5equiv. wt.% NaCl versus 8-18 equiv. wt.% NaCl,respectively), providing constraints on the role ofa sal ine magmatic l iquid (versus Iow-sal ini tyvapor) in the generation of HS deposits.

Group 3. Lower temperature (e.g., 90-180"C), dilute (typically <5 equiv. wt.yo NaCl)liquids; these have been documented in a fewdeposits associated with late-stage (e.9., Au-barite) mineralization. The late-stage ore fluids atJulcani are hotter (220-250 oC; Deen 1990) andsl ight ly more sal ine (6-9 equiv. wt.% NaCl), thanthese averages, but no correlation among the latestages in different deposits is attempted here.

Group ./. "Sericitic" fluids. As mentionedabove, sericitic (quartz-sericite-pyrite) is the mostcommon alteration assemblage observed belowthe ore zone in some HS deposits. Althoughdetailed documentation is lacking for manydeposits, higher temperatures and higher salinityfluid-inclusions seem to characterize the sericiticzone with respect to the shallower zones ofalteration (Table 4). For example, at Rodalquilar(Arribas et al. 1995a), documentation of tem-perature and salinity along a >600-m verticalinterval (extending 500 m below the ore zone; Fig.6) shows a gradient which correlates with thechange in dominant alteration, from silicic andadvanced argi l l ic Q : 170-300 oC, sal ini ty :2-15

equiv. wt.% NaCl at the elevation of the orebody)to ser ic i t ic (T: 220-450 oC, sal ini ty :2-45 equiv.wt.% NaCl) assemblages.The transition from advanced argill ic alteration,through quartz-sericite-pyrite, to K-silicatealteration and typical porphyry-type high-temperature (600+ "C) and high-salinity (up to 67equiv. wt.% NaCl) f lu ids of magmatic or igin isdisplayed, among the examples reviewed, at theLepanto-FSE and La Mejicana-Nevados delFamatina epithermal-porphyry copper systems.The cooler and less sal ine inclusion f lu idsdocumented in the ore zone of the HS deposits areinterpreted to reflect mixing of magmatic andmeteoric fluids in an environment shallower thanthat of porphyry mineralization. Furthermore, incommon with porphyry-type deposits, high-temperature, vapor-r ich. low-sal ini ty f lu idinclusions coexist with high-temperature, liquid-

434

Temperature ("C)

200 300 400

Figure 6. Elevation versus temperature diagramshowing the range (horizontal l ine) and average(vertical l ine) of f luid-inclusion homogenizationtemperatures measured in the Rodalquilar Au deposit,Spain. Also shown are the temperatures calculated, on

the basis of 63aS surfide-surrare for four coexisting alunite-pyrite samples (large fi l led circles), reference boil ing-point curves, and vertical spans of the alteration zonesmentioned in the text. Estimated salinit ies of f luidinclusions in the shallow advanced argil l ic/sil icic zoneand deep sericit ic zone range between 2 to 30 equiv.wt.% NaCl and 2 to 45 equiv. wt.% NaCl, respectively(modified from Arribas et al. 1995a).

r ich hypersal ine inclusions ( i .e. , with Groups 1and 4, above). These fluids may be the result ofboiling of a high-temperature liquid, or they mayreflect immiscible vapor and hypersaline liquidderived directly from shallow-emplaced magma(Rye 1993; Hedenquist & Lowenstern 1994Shinohara 1994; Hedenquist this volume).

S ulfur-is otope Ev iden ceThe abundance of coexisting hydrothermal

sulfides and sulfates, in addition to the possibility

o0)

q)

dno 6- ' ( g

o(s3q)

Ann ;- - ' oa,

E

q)o

H 2 O + 5 w f / . N a C l

2-6

4

5

rtIltItFIi:.a

Lepanto

Chinkuashih

Nansatsu

Summitvi l le

Goldf ield

Pueblo Viejo

Julcani

El Indio

Rodalqui lar

of measuring 'oS/"S in host rock and geneticallyrelated igneous rock (Sasaki et al. 1919), allowssulfur-isotope studies to provide information onthe composition, temperature, and sulfur sourcesof the hydrothermal fluids. The results of detailedstudies in nine HS districts show a remarkableconsistency (Fig. 7). In agreement with theobservations in active volcanic-hydrothermalsystems (e.g., Kiyosu & Kurahashi 1983), sul f ideand sulfate minerals are mainly in isotopicequilibrium, and, therefore, their overall 'oS/1'S

depends.on the temperature of mineralization andthe '"S/"S of total sulfur in the hydrothermalsystem. Only the data for alunite from theCampana vein in El Indio (Fig. 7) are different. Ifthe measured El Indio alunites are not steam-heated or supergene (unlikely as they contain fine-grained pyrite; Jannas et al. 1990), the most likelyexplanation is a "magmatic-steam" (Rye et al.

1992) or igin, in which the 63aS of aluni te is closeto the composition of total sulfur in the system(e.g., Alunite Ridge in Marysvale; Cunningham elal. 1984: Rve el al. 1992\ . Combined with the

p0 - 420

20 -270

200 - 240

200 - 390

200 - 350

180 - 260

210 - 270

220 - 330'(minerat pairs)

63aS values of pyrite and enargite from the samevein, these values indicate drastic changes inH2S/SO4 during the course of mineralization(similar to those for the Red Mountain alunitedeposit; Bove et al. 1990; Rye 1993).

The main conclusions of the sulfur-isotopestudies in HS deposits are: ( I ) sulfur in thedeposits is magmatic, but the magmatic sulfur is

overall heavier than mantle values (from 63aS : 2+2o looat Summi fv i l le , to 9 +2o/noat Roda lqu i la r ;Fig. 7). This is not surprising given the mostcommon geological setting of the deposits;isotopically heavy igneous sulfur is common involcanic arc environments (e.g., Ueda & Sakai1984). (2) A simple mass-balance calculat iondone in several deposits using the 3oS/"S valuesof the igneous rocks and the average 'oS/"S

values of sulfides and sulfates indicates thatH2S/SO4 in the hydrothermal fluids was generallyabout 4 * 2 (Fig. 7; Rye et al. 1992; Hedenquist elctl. 1994a; Arribas et al. 1995a). This is aminimum value for ore-forming fluids because itapplies mainly to the early stage of hydrothermal

-Sultides - * Sulfates ̂ V& V= 634515

r V--F

I

'1-

- t vI

! v- l i

r s z--.--*r J

Yl v-I

- ; o

r Y lt - ' lt l

l r f f it lI

@II

I

ry

I

I@"f"I

H igh-sulfidation Epithermal Depos its

aSSHzs-sor

Temp. ("C)' H2S/ SO4

10 206345 (%", CDT)

!-igure 7. Range of 63o5 (per mil) values for sulfides and sulfates from nine high-

sulfidation deposits. Also shown are the values calculated for 5'oS for total sulfur in thehydrothermal system (triangles), H2S/SO4. and the range of temperatures determined

from sulfide-sulfate mineral pairs. Solid triangles indicate deposits in which 6toS* wascalculated on the basis of isotopic analyses of samples of unaltered whole rockgenetically related to mineralization. See Appendix for references and information ondata plotted.

43s

.4. .4rrihns. ./r.

al terat ion. which is character ized bv a sul fate-r ichalunite-pyr i te assemblage (3) lsotopic equi l ib-rium between sulfide and sr-rlfate in thehydrothermal solut ions results, in a nrajor i ty of thedeposits. in rel iable temperatures calculated on the

basis on A3aSrr:s-so+ (Fig 7). Pyr i te-aluniternineral pairs were used most commonly, andrvhere sampling rvith depth is available, thev shorva thermal gradient: e.g.,220 to 330 oC over 200-mclevat ion at Rodalqui lar (Arr ibas et al . 1995a).200 to 390 "C over .--900 m at S'.tmmitville (R1'e

1993)1 220 to 420 'C over 500 m at l -epanto(Hedenquist and Carcia 1990: J \ \ r . Hedenquist .unpr-rb. data). Other mineral pr i rs used withconsistent results include p1'rite-barite (Vikre

1989: Deen 1990), sphaler i te-bari te (Venncmann

et al . 1993), and plr i te-g1'psurn (Vikre 1989). Therange of isotopic temperalures is consistent lv i thtemperatures estimated from fluid inerlusions andalteration mineralogy (e.g., Flemley' et ul. 1980;Reyes 1990; Rey'es et ul. 1993). l-he range is alsoconsistent with formation of altrnite attemperatures belorv -400 "C, rvhen SO2 gas startsto dispropottionate in the h1'drothermal solution(Sakai & Matsubaya 1911;, Bethke 1984).

Oxygen- and Hydrogen-isotope EvidenceIn terms of oxygett and hydrogen isotopic

composition, the fluids that form HS deposits arearguably some of the better documented andunderstood in ore-deposit studies. This si tuat ioncontrasts sharply witli that of a decade ago, atwhich time no data were available to corroboratethe affinity suggested between fluids in activevolcanic-hydrothermal systems and HS deposits(e.g., Heald et al . 1987; Hedenquist 1987). Stable-isotope studies of HS deposits are particularly

i l luminat ing because of: ( l ) the abundance andvariety of oxygen- and hydrogen-bearing minerals(e.g., aluni te, i l l i te, kaol ini te), (2) the developmentof analytical procedures for complete stable-

isotope analysis of aluni te, including 6l8oroo and

6'tOu' that help to dist inguish the var ious typesof alunite and associated acid-sulfate alteration(Rye et al. 1992; Wasserman et ctl. 1992), (3)

fewer limitations on the interpretation of theisotopic data because of the relatively young ageof mineralization of most HS deposits and general

lack of post-deposit ional ef fects that disturb thestable- isotope systematics. and (4) the avai labi l i tyof detai led information on the isotopiccomposit ion of f lu ids in act ive geothermal andvolcanic-hvdrothermal systems. which al lowsfluids estimated in HS deposits to be comparedwith those in their act ive equivalents.

Some l imitat ions st i l l exist . ' fhese

rnay berndependent of obvious factors such as sampl ingor mineral-preparat ion procedures ( fundamentalfor achieving representat ivc and rel iable results).analyt ical imprecision. and natural var iat ions, asobserved in act ive systems (c.g., Aoki 1991, 1992,Rowe 1994). Important l imitat ions that rnust betaken into accourrt for optimum use of the stable-isotope data are related to ( l ) the choice oftemperature of mineral formatiott fbr calculationof the l lu id isotopic composit ion. (2) thc lack ofmineral-water lractionation factors for someminerals (e.g , pyrophyl l i te), and (3) thedisagreement among fractionation constantsproposed lbr even common minerals such as i l l i te(see Di l les er a/. 1992, for a discussion) andkaol ini te. For examole. at 200 oC there is adifference of -20" lno between tlte D/lI fiac-tionation constants for kaolinite - water as givenby Marumo et al. (1980) on the basis of samplesof minerals and rvater from active systems, and byt, iu & Epstein (1984) on the basis of experimentalresults. For these reasons. discussion of thesources of water during acidic alteratiorr in thedeposits considered here is based on the averageof the data collected for alunite, for whichfractionation factors are well-known (Stoffregenet al. 1994). The magmatic-hydrothermal alunitetypical of HS deposits gives good results becauseit is relatively coarse-grained (post-mineral D-Hexchange is not a problem; Stoffregen et al. 1994)and commonly is closely associated with ore, thusrecording equi l ibr iurn condit ions of a f- lu id closerin composit ion to the ascending mirreral iz ingsolution than the kaolinite or il l ite from outeralteration zones.

Oxygen and hydrogen isotopic compositionsof water in HS deposits are clearly consistent withmixing between a high-temperature magmatic

f lu id o f 6180:9 + 1o /no and 6D: -30 + 20" /oo andmeteoric groundwaters (Fig. 8). In part because of

F

H igh-su(idation Epithermal Depos its

-1 00

- t z v

-1 40

6180 (%", sMow)

Figure 8. Summary diagram showing variation in oxygen- and hydrogen-isotope composition of hydrothermalfluids in high-sulfidation deposits. The average isotopic composition for the main stages of acidic alteration(squares) and ore-mineralization (circles) f luids are shown. Where possible, only alunite data were used for the

alteration stage (6D and 6r8O5eo); 6'tOo, is not used because hydroxyl oxygen requil ibrates with the hydrothermalfluid during cooling (Rye et al. 1992), Tie-lines befween data points connect samples from the same deposit. Insetshows the isotopic composition of fields defined by waters from active geothermal systems and high-temperaturefumarole condensates in subduction-related andesitic volcanoes (from Giggenbach 1992b). Go: Goldfield, Ju:Julcani, Le- Lepanto, Nansatsu district: Ka - Kasuga, Iw : Iwato, NF : Nevados del Famatina, PV : PuebloVeijo, Ro : Rodalquilar, RM : Red Mountain, Lake City, Colorado, Su : Summitvil le. The approximatecompositions of groundwaters suggested for several deposits are indicated by the intials parallel to the meteoricwater line. See Appendix for references and information on data plotted.

the very l ight isotopic composition of local relations are identical to those of volcanic-

meteoric water, this meteoric-magmatic water- hydrothermal and geothermal systems associated

mixing trend is displayed particularly well by the with subduction-related volcanism (Giggenbach

three stages of alterationlmineralization at Julcani 1992b; Fig. 8, inset). The similarity is even closer

(Deen 1990; Rye 1993): from a magmatic-water- between the composition of acidic alteration fluids

dominated early stage of (alunite) acid-sulfate (large shaded field, Fig. 8) and the vapor

alteration (Ju, Fig. 8), through main ore-stage condensates from high-temperature fumaroles of

fluid-inclusion waters (Ju1 and Ju2), to meteoric- andesitic volcanoes (dark shaded field, Fig. 8,

water-dominated late ore-stage fluid-inclusion inset), such as Nevado del Ruiz, Satsuma

-40

3> -oua

d

t9 -eoota

waters (Ju3). In addition to Julcani, the ore fluidsat Summitville (Rye et al. 1990:' Rye 1993) andRodalquilar (Arribas et al. 1995a) also have lower

6180 values than those of acidic alteration fluids,indicating greater dilution by groundwater (Fig.

8). The extent of an O-shift in the groundwatercomponent due to water-rock interaction, astypically seen in some neutral-pH geothermalsystems, is not known, but such a shift is notindicated by the Julcani data.

The overall oxygen- and hydrogen-isotope

Iwojima, or White Island, the last documented tohave a geochemical environment similar to that ofHS mineralization (Hedenquist et al. 1993).

The origin of the D-enriched magmatic (end-member) fluid of HS deposits has been interpretedin two ways. Most workers conclude that theacidic fluid in HS deposits is derived fromabsorption of magmatic vapors outgassing fromarc volcanoes or felsic magmas in crustal settings(e.g., Hedenquist & Aoki 1991; Matsuhisa 1992;Giggenbach 1992q' Vennemann et al. 1993;

n Alunite alteration stg.

Q Ore mineralization stg.

O Alteration/ . ^?y,

Subduction-relatedvolcanrc vapor

437

A. Arribas, Jr.

ALTERATION

Figure 9. Model showing the two main stages of evolution of HS deposits. A: Early stage of advanced argil l ic

alteration dominated by magmatic vapor. B, and Bt: Two genetic hypotheses proposed for the stage of oreformation. B, - absorption of high-pressure vapor by entrainment in meteoric water cell at depth to explain low-

salinity, mixed magmatic-meteoric ore fluid (Hedenquist this volume). B, - ascending metal-bearing magmatic

brine with shallow cooler meteoric waters to explain high-salinity, mixed magmatic-meteoric ore fluid (White

I 99 I ; Rye I 993; Hedenquist et al. 1994a).

metals strongly partitioned into the high-densityl iquid (Hemley et al . 1992; Hedenquist thisvo lume) .

At this early intrusive stage, several modes ofmagma degassing may occur which wi l l lead todifferent styles of magmatic-hydrothermalsystems with or without associated mineralization(Giggenbach 1992a). To form the styles ofalteration and the spatial distribution of alterationzones characteristic of HS deposits, degassingmust be very efficient, with oxidized high-temperature magmatic vapor reaching shallowdepths with little reaction with rock or dilution bygroundwaters at greater depths (Fig. 9A). Dilutionwith groundwaters is unlikely because the hightemperatures surrounding the cooling magmacause meteoric water cells to be displaced fromthe magma core (Fig. 9A). In addition to therelat ively low pressure at the depth of intrusion,effective degassing will be favored by thestructural factors characteristic of HS deposits,such as fractured volcanic domes or roots ofdomes, caldera or diatreme faults, volcanic ventcontacts. and active faults with a dilationalcomporrent.

As thc high-temperature Inagmatic vapor

440

reaches shallow depths of less than a kilometer, itmay be absorbed by groundwater if it does notdiscliarge as a fumarole. The acidity of thisgroundwater-absorbed vapor condensate increasesas the liquid cools, first at temperatures below-400 "C by disproportionation of SO2 to formH2SO4 and H2S (Day & Al len 1925; Sakai &Matsubaya 1971), then by progressive disso-ciation of H2SOa and HCI at lower temperatures(<300 oC). React ion of the increasingly acidicliquid with wallrock results in the upwardalteration sequence of sericite-+kaolinite*+alunite+vuggy silica (Fig. 9,A'), the residualvuggy silica rock results frorn complete leachingof the rock components, except silica, by ahydrothermal solution with a pH <2 andtemperatures probably <250'C (Stoffregen 1987).The extremely acidic conditions may even lead toforrnation of dissolution cavities in which the onlyremnant of the host rock is a basal sedimentarylayer of quartz phenocrysts (e.g., Rodalquilar;Arribas et ctl. 1995a).

For the quartz-alunite-pyrite assemblage ofthe advanced argill ic zone, the stable-isotopeevidence is consistent with magmatic vapor beingabsorbed by meteoric waters, with tlre latter

1II

Heatedgroundwatet

Mixing withshallowmeteoric water

\I

Metal-bearing -

hypersal ine /liquid l-

Absorption ofhigh P vapor

Juagmatic-' j

brine l l

Heatedoroundwater

\ Ionvective cel l

r$i

const i tut ing a relat ivelv smal l part of the rnixturc-(gcneral ly <113, l t ig. 8).

- l 'he f lu id- inclusiorr

evidence, by contrast. is inconclusive because ofthe lack of ternporal infonr-ration. Nevertheless,high-temperature, high-sal ini ty inclusion f lu idshave been interpreted to form early in most I{Sdeposits (e.g., Bruha & Noble 1983: Ruggier il993bl Arr ibas et ul . 1995a). These f lLr ids rnal 'berestricted to greater depths. as demonstrated atRodalqui lar and in other deposits wlrere high-sal ini ty inclr-rs ion f lu id is associated rvi th the deepseric i t ic al terat ion (Table 4). This lat ter obser-vat ion suggests an episodic asccnt of high-sal ini tymagmatic l iquid f iom the greater depths of thehydrothermal system, rvhere the hypersal ine l iquidtends to stay because of i ts high density (Fig. 9A).These nragmatic br ines rnay be rnore closelyrelated to the K-si l icate al terat ion (and, in places.porphyry m ineral izat ion) that envelopes thein t rus ion (F ig . 9A; S i l l i toc 1989) .

The condit ions during the rnain stage of orefbrmation are not yet as lve ll-understood, and thisref lects the much rrore var iable geochemicalenvironment in cornparison with that associatedwith acidic al terat ion. During the ore stage, thehydrothennal l iquid may bc less dominated by amagnratic vapor phase arrd its associated "sulfur-

gas buf ibr" (Giggenbach 1987). ' fhe presence ol-

this SO2-H2S buf l t r is the reasorr that the earlystage of alteration is so oxidized, as reflected bythe alunite-pyr i te assernblage (Whitney 1988;Giggenbach 1992a). Instead, condit ions during theore stage f'luctuate within a range of redoxpotential that is reflectcd by enargite-pyrite +

alunite and enargi te-tennant i te-chalcopyri te asso-ciat ions, which are relat ivcly high to intermediatesulfidation-state assemblages, respectively (seeFig. 3 in Hedenquist , this volurne). tn the Lepanto(Claveria & l ' ledenquist 1994) and El Indio(Jannas et al . 1990) deposits, these twoassemblages are related to CLr-r ich and Au-r ichmineral izat ion, respect ively, with the lat ter beingof later stage in both cases. The more reducedcorrdi t ions are a l ikely consequeltce of increasedwater-rock interaction, and, to some extent,increased di lut ion of the oxidized rnagrnat ic f lu idby meteoric water; this trcnd is also consistentwith the isotopic composit ion of waters in t l remain ore stage of var ious deposits (Fig. 8). No

l { i gh-.s u I/idat ion Epit hermctl Depos its

discrimination. hou'ever. can be made between ameteoric-rvater componeut that is incorporated atdeep or shal lorv levels rvi thin the hydrothermalsystem. lmportant ly. sal ini t ies during the main orestage can be low (c.g., Lepanto and El lndio. <4equiv. rvt .oZ NaCl: T'able 4; or moderate to high(Ju lcanr , up to lB equ iv . w t .% NaCl ; Z i j inshan. upto 22 eqLr iv . w t .% NaCl : ' l ab le

4 ) .

Assessment o.f a |lfodelNo single model adequately explains al l oi

tltese various observations. and several hypotheseshave been propcsed. each rel lect ing an emphasison individual deposits or di l - ferent interpretat ionsof the 1' lu id- inclusion and stable- isotope data. Abasic urrderstandirrg o{' this ore-forming eur,,iron-rnent may be gained by considering the pr incipalend-rnember t lu id components and ore-formingprocesses. The spectrum of characteristicsdisplayed by HS deposits may be then analyzed inthe context of such a genetic framer,vork.

Four lluid regimes have been iderrtified in the[{S environrnent; evidence for al l is present in theearly stage of IIS alteration, and three of them arecritical to fbnnation of porphyry systerns (c.g.,Henley & IvlcNabb 1978; Si l l i toe 1989).

' Ihese

end-members are: ( l ) a metal-r ich hypersal incrnagmatic l iquid which tends to remain in thevicini ty of the intrusion, but mav ascend (or bedriven) to shal lorve r depths i f the ambientternperature is low enough (<400 "Cl) for themechanical strength of the rock to increasesLrfficiently to result in brittle fiacturing (F'ournier1992), (2) a lou'-salinity' magmatic vapor whosemetal-transporting capacity decreases sharply withdecreasing pressure ( lJedenquist this volume), (3)heated meteoric or connate water in deepconvecl ion cel ls that col lapse inward anddownward as the intrusive stock progressivelysol idi f ies and cools, and (4) shal low and coolmeteoric groundwater.

Two nrain end-member ore-forminghypotheses are considered. In the "volat i le

transport" hypothesis (Fig. 981), the magmatichypersal ine l iquid may remain at depth throughoutthe evolution of the hydrotherrnal system, and thelow-sal ini ty vapors are responsible for mineral i -zat ion (Si l l i toe 1989; Vennemann et al . 1993);deep meteoric water entrainment of high-pressure

441

A. Arrihas, Jr.

vapor is required for transport of sufficientamounts of metals (Hedenquist this volume;Si l l i toe this volume). These condit ions areconsistent with the low salinity of the Lepanto andEl Indio f lu id- inclusion data. Mineral deposit ionin this case may be caused by mixing with coolergroundwater or by boiling, possibly resulting fromthe abrupt pressure reduction associated withhydrothermal brecciation.

In the "hypersaline liquid transporl"hypothesis (Fig. 9B2), fol lowing waning of thernagmatic vapor plume responsible for earlyalteration, the lithostatic-pressured system frac-tures and the metal-bearing hypersaline liquidascends into the porous leached zone (Deen 1990;White l99l ; Rye 1993; Hedenquist e/ al . 1994a).The dominant ore-forming mechanism in this caseis rnixing of the metal-bearing hypersal ine l iquidwith cooler groundwaters at the site of deposition,not at depth in the meteoric water convect ion cel l .This hypothesis has been proposed to explain thehigh sal ini t ies recorded by inclusion f lu ids inseveral deposits (e.g., Julcani) .

A part of the ore-fbrming components mayoriginate frorn leaching of wallrock, but bothhypotheses agree on a dominantly magmaticsource fbr metals, with an increase in the meteoricwater component with t ime. The pr incipaldifference between the two hypotheses is in thenature of the magmatic phase responsible fortransporting the metals into the epithermalenvironment. and in the site of meteoric waterdilution. A potential contributor to ore fbrmationin HS deposits involves remobi l izat ion of themetals by a meteoric-water-dominated hydro-thermal system fiom a subjacent K-silicateassemblage and porphyry-type protore, such asthat which may have formed close to the intrusion(e.g., Br imhal l 1980). This mechanism, however,has not been suggested as the main ore-fbrmingprocess in any of the deposits reviewed in thisstudy.

The three models for formation of HS ores.assimilated here from the literature, are notmutually exclusive; on the contrary, they mayoccur in the same HS deposit as the magmatic-hydrothermal system evolves, with complexitiesarising from multiple intrusions, variations indepth of emplacement, and changes in the local

442

tectonic and hydrodynamic environment. None ofthe three rnodels satisfies the overall evidence. Forexample, if metals were supplied only by a dense,high-salinity liquid, a relation would be expectedamong estimated salinities, metal associations,and ore grade or metal abundances of the variousdeposits. Such seems rrot to be the case. Similarly,if alteration and mineralization were solely theresult of interaction between groundwater andlow- and high-pressure vapor, respectively. highsalinities should not be as comtnon as theyunless they are explained by local boi l ing of di luteto moderately saline meteoric or seawater-dominated fluids.

SYNTHESIS

Gold, Cu, and Ag (and in a few exceptionalcases also Hg, W, Bi, Pb, and Zn) are producedfrom HS deposits. As a source of Au, and becausetheir mode of occurrence and the potential tooverlie porphyry-type rnineralization have beenwidely recognized only within the past 10 to l5years, HS deposits represent a valuableexploration target that has been overlooked insome regions. Most known HS deposits are youngin age, Tertiary and even Quaternary. High-sulfldation deposits fbrm dominantly insubduction-related plutonic-volcanic arcs,commonly during crustal extension.

' fhe deposits

form at a depth intermediate between the surfaceand shallow (few kilometers depth) intermediate-composit ion intrusions.

The int imate relat ionship among HS deposits,volcanic host rocks, and oxidized magrnatic fluidderived from a degassing intrusion is supported bythe fol lowing observat ions: ( l ) the volcanic rockshosting HS deposits were erupted immediatelyprior to mineralization, (2) the ore-fbrminghydrothermal system commonly follows the sameplumbing as that of the magmatic system ( i .e. ,rnineralization spatially associated with domes orvolcanic conduits), (3) the isotopic composit ion ofhypogene sulfides (e.g., enargite and pyrite) andsulfates (e.g., aluni te gnd.bari te) commonly can bemodel led from the 'oS/"S of sul l 'ur in rgneousrocks thought to be genetically related, byequilibrium fractionation between H2S and SOa insolution at T -200-400 oC, and (4) on the basis of

I

oxygen and hydrogen isotopic ratios, the watersinvolved in formation of HS deposits are identicalto waters in active volcanic-hydrothermal sys-tems, in which the same HS geochemical

environment has been documented.Ore formation in some HS deposits may

accompany acidic alteration, and recent studies ofthe hydrothermal geochemistry of Au providepreliminary evidence that this element may be

transported in HS and low-sulfidation systems as

different hydrosulfide complexes (AuHS" andAu(HS)2, respect ively; Bening & Seward 1994;

Seward 1913). On the other hand, the presence of

moderate to high sal ini t ies in many HS deposits.the intimate association with porphyry copper-type deposits, and the assumptions of the most

recent genetic models (transport of Au and Cu by

either hypersaline liquid or high-pressure vapor)indicate that chloride complexes must also be

considered for metal transport.Most HS deposits evolve from an early period

of acidic wallrock alteration to a late period ofprecious- and base-metal rnineralization. Acidic

alteration is characterized by advanced argill ic

assemblages and porous (leached) rock, and the

hydrothermal fluid responsible for this alteration

is dominated by high-temperature magmatic vapor

containing SO2, H2S, and HCl. Less reactive and

oxidized fluids are typically responsible for ore

mineralization. Factors such as multiple intrusions

and opening or closing of fractures (conduits)

result in variations in the temperature, pressure,

and composit ion of the ascending solut ions.

Combined with the shal low environment of

mineralization, these conditions lead to a variety

of deposit styles (mainly replacements, breccias,

and veins) that usually occupy a limited vertical

span of <300 to 500 m (except for >800 m at thegiant Chinkuashih deposit) . The geological ,

mineralogical , and geochemical evidence,particularly the association between the orebodiesand the lateral and vertical zones of alteration,

illustrates the basic genetic condition of HS

deposits, that a magmatic fluid interacts extensive

ly with country rock and groundwaters on its

relatively short path to the earth's surface.

High-sulfidation Epithermal Depos its

ACKNOWLEDGMENTS

Valuable insight on various aspects related tothis exciting ore-forming environment was gainedthrough discussions and field work with M. Aoki,A. Arr ibas Sr. , C. G. Cunningham, J. Hedenquist .W.C. Kelly, R. O. Rye, J. J. Rytuba,and T. A.Steven. Earlier versions of this manuscriptbenefited from constructive reviews by PhilBethke, Andrew Campbell, Anne Thompson, JohnThompson, Peter Vikre, Noel White, and JeffHedenquist, who also provided abundantdocumentation on HS deposits worldwide.

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H igh-sulJidation Epithermal Deposits

APPENDIX I

Summary of data and references used to compileFigures 3, 7, and 8.

Figure 3K2O versus SiO2 variation diagram. The name

of lithologic units analyzed, number of sarnples(n), and data sources are given: Chinkuashih,dacite n : 18 (Chen & Huh 1982); Choquelimpie,Choquel impie volcanic complex (5 units). n - 20(Gropper et al. 1991 : chemical data fbr thefeldspar porphyries genetically related tomineralization are not available); Goldfield.rhyodacite n : 6 (Ransome 1909; Ashley, unpub.analyses in Si l l i toe 1993); El Indio, Cerro de lasTortolas Formation, n: 15 (Maksaev et al. 1984in Si l l i toe 1993); Julcani, daci te and rhyodacite, n: 10 (Petersen et al. 1917); Laurani, Lauranivofcanic and intrusive rocks, n : 10 (Jimenez etal. 1993); Lepanto, Imbanguila dacite and leastaltered quartz diorite porphyry, n : 4 (A. Arribas.unpub. data); Motomboto, porphyritic intnrsions,n: 10 (Perel l6 1994, and wri t ten comm. 1995):Nansatsu, Upper Formation and hornblendeandesite in Middle Volcanic rock, n :2 (E lzawa,written comm. 1995); Paradise Peak, average ofYounger andesites, n : 3l (John et al . l99l) ;Rodalquilar, hornblende andesite, dacite tuff, andrhyolite domes, n : 7 (Arribas et al. 1995a);Summitville, Fisher quartzlatite, n: 7 (Steven &Rattd i 960).

Figure 7

Range of 63aS 1o/oo) values. Giverr below arethe number of measurements for sulfides (nrirs),sulfates (nso+), sulfide-sulfate mineral pairs

(rA'oS), and references: Lepanto, flr2s: 52, n.no:38 (Hedenquist & Garcia 1990; J. Hedenquist &M. Aoki, unpub. data); Chinkuashih, nvzs : 4,

^ 3 4trsoo : 2, ,L"S : 2 (Folinsbee et al. 1972);Nansatsu, nszs: 6, n soq: 9 (Hedenquist et al.1994a); Summitvi l le, f lLt s

: >11, n ssa : 17,

,A tos :7 (Rye e t a l . leeb ; : co tane ld . n l1 rs : 16 ,

n so+:16, n63ag : 7 (Jensen et al . 1911; Vikre

1989); Pueblo Viejo, ngzs: 19, n s174:7, ny3aS:4 (Vennemann et al. 1993); Julcani, n11rs : 183,

453

A. Arribas, Jr.

r?so+: 55, n6345 :7 (Deen 1990); El lndio, ns2s_11, n5sa : 3 (Jannas et ol . 1990), Rodalqui lar,

r ,gzs :44 , nssa : l l , ,A3aS : 4 (Ar r ibas e t a l .1995a). Temperatures for Chinkuashih werecalculated using the 'oS/"S data from Folinsbee e/al. (1912) and more recent fractionation equations.Sulfide-sulfate mineral temperatures higher than350 oC were documented only at depth atSummitvi l le (T : 390 oc, -900 m below thepresent surface; Rye e/ al.1990) and Lepanto (I:

420 "C at the 700-m level, immediately above theFSE porphyry copper deposit; Hedenquist &Garcia 1990). On the basis of phase equi l ibr ia, thesulfide/sulfate values for the Pueblo Viejo stage Iand stage 2 mineralization were estimated byMuntean et al. (1990) to be about 3 and 35,respectively.

Figure 8

6D versus 6'80 variation diagram.Explanation: Go : Goldfield, hypogene alunite, n: I (Rye et al . 1992); Ju: Julcani, aluni te (n:6),

Jui average of main-stage ore fluids inwolframite, enargite, tetrahedrite, and galena fluidinclusions, Ju2 : average of main-stage ore fluidsin sphalerite and chalcopyrite, Ju3 : late-stage orefluids in barite, siderite, and botroidal pyrite (Deen

1990); Le : Lepanto, alunite, n : 2 (Y. Matsuhisa& J. Hedenquist, unpub. data); Nansatsu district:Ka : Kasuga, aluni te f t : 7, lw : Iwato, aluni te r

2, 6r80 values of residual vuggy si l icaassociated with ore in both depositsfall befweenKa and 1w (Hedenqu is t e / a l . 1994a) ; NF :

Nevados del Famatina, stage V alunite-kaolinite, nI (Losada-Calderon & McPhail 1994); the

average 5D and 5l80 values for La Mejicana (n :

9) are similar to NF; K-silicate and quartz-sericite

at Nevados del Famatina have 6180 between 4 and10o/oo, reflecting a larger magmatic component(Losada-Calderon & McPhail 1994); PV : PuebloViejo: PVI : stage I alunite and kaolinite, PV2 :

stage 2 pyrophyllite (Fig. 9 in Vennemann et al.1993); Ro Rodalqui lar, aluni te, 10,chalcedonic ore, n:6 (Arribas et al. 1995a); RM: Red Mountain, Lake City, Colorado, alunite, n :

12 (Bove et al . 1990; Rye 1993); Su

Summitville, alunite, average of n - l0 (6D) and n: 16 (6'80) (Rye er at. 1992), ore fluids from Rye(1993). The main ore stage at Rodalquilar (stage

2) is based on SltO of chalcedonic quartz; 6D arenot available for this stage but present-daygroundwaters, alunite, kaolinite, and ill ite fluids in

the deposits have a limited range of 6D values,suggesting significant variations are unlikely(Arrlbas et al. 1995a). Stage 2 (pyrophyllite)fluids for Pueblo Viejo involve severalassumptions with respect to the choice offractionation factors for oxygen and hydrogen.The data for stage 2 at Rodalquilar and PuebloViejo should be viewed as approximate. Data for asingle alunite for Goldfield (Rye et al. 1992)

suggest that mixing of a the 6D- and 6180-enriched magmatic fluid with isotop^ically lightwaters may result in D- and 'oO-depleted

hydrothermal acid-sulfate fluids (see also Vikre1989) .

F