r

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

Cmlngy lJcpartmclI/ , U"i~it!j of Auckland. Prioow Bug 92019, AIlCkulIId, Nell) Zeo/wul

NOEL C. WHITE,

PO. Box 5181, Kemllore EMt , Queells/fIIld, Australia 40ti9

AND DAvlI) A . JOHN

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

Abstract

Epithermal deposi ts are impol'tallt sources of gold and siiver that foml at d mostly of magmatic nuids with a minor to moderate component of mctt.'Oric water.

Critical genetic factors include; ()) at several-kilometers depth. the delo-elopment of oxidU.ed and acidic \~rsus reduced and ncar-neutral pH solutions, controlled by lite proportions of magmatic and 1I1

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 epilhen nal 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 past three decades, Uley have l.Jeen mined mainly for thei r 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 M aL 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 si lver. [n the mid l SOOs to early l 000s, 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-Hydrothermal 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 1I 111agl11l1tic-hydrothennai system dominated by acid hydrothemlaJ fluids, where tllere is II strung OWl of magmatic IIqulU and vapor. containing li t

EPITHERMAL PRF.CIOU$ AND BASE METAL DEPOSITS 487

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

488 SIMMONS ET A.L

Ftc. 2.. Lxation of t-pithcnllal deposi.u)me

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

Il ighSulr;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

T)'pII' I Il

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

Hig/, . ulfodlltiofl

Ntlltrol pH Quartz-adularia J. illik, . ;

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

1,242 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 IIna] Cl[-I .. nl of orcs. ",hidl is further n:l1ectcd In the data set reported In APpP.ndit Tahle Al MUI)S IIfC n.-dnsil a~ given In the Table Al.

"

Fresnillo

Acupan

,/---,.-' ..- .. -.,<

/'

Ellndio

r.PITHE.RMA1~ PRECIOUS AND BASE METAL D E.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' ~.' - ,-

491

-

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~" Temp rC) E-~.Q ij'" Depth (m) ~ t i .... 1OciiiiD _3' OII 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-cordi ll 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

Epi thermal 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 di lation 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 byproduct 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 d isseminated ore.'i (Table 4), which can extend for l 00s 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 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

N

--.m --.. '" .,.,

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

Martha Hill s s Ladolam

---- ----\ + + + + + \+ + + + + + \+++++ 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

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! wo rld, is a vmiant of mineralization found in the Ag-Sn belt of Bolivia, wl,ere deposits fom,ed III

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 tho leiitic magmas with bimodal basalt-rhyoli te t.'OmIHlSi tions. 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 rooued 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 YOUl lger (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 si miJar 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 of hydrotJlennai 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 envi-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 (>300C), 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 hy-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 l Os 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, chlo-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: (lteamheated)

Adv. AzglIlic: (magmatic: '"""""nnol)

Mine~

Quartz. K.feldqm (adularia), albi te , 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 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 >8ooC, acidic hot springs, and hydrothennally altered rocks ejected by explOSive eruptions

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). Mag-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 natu re, 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 wate r table at - lOO"C in horizons with pronounced vertical miner.u wnation. Tn general, this blanket of alteration does not exceed 10 to 20 m il1 l.hickness. Tabular but diseoTl tinuolls 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, kaoli-nite, halloysite, jarosite, and iron oxides and hydroxides. Su-pergene advanced argillic a1 temtion also has a blanket like geometry that mimics topography, but it may line sub-vertical fractures that were patJlways for deS

F.I'IHlflUIAL PRfX /O(JS AND HA';E MTAI. 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 ( ~Iarth n Hill . New Zcabml; ,,,,;ale bar " 2 ern). C. Adnbria encnlsted on op" n fract nn; ( ~ I arth:t Ilill. ~tw Zeabnd; seale har I em), I), LatliCC texture_ in which pbty e-.ileite j. replact'd b)' qll,lrt? in gold-si l-\'Cr- beo,uingorc ( ~l.artha Il ill. 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),

497

498 SIMMONS E:T AL..

Quartz Calcite Adularia Illite

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 Pyrophyllite Dickite Kaolinite

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 sphalerite (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 300C. Gold-silver deposits gen-erally have dilute solutions of 20 wt percent NaC\ equiv (Fig. 8). Coexisting liquid- lind vapor- rich fl uid 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 boiling-point-for-depth grndient (Haas. 1911), consistent wilh estimates of

vertical temperatu re 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 increases the depth r.mge of ooiliug up to hundreds of meters (e.g., Simmons, 1991; Sherlock e t aI., 1995).

Stable isotope data

Stable isotope studies, comprising measurements of bD and 6 1110 , have been made on several gangue minerals (quartz, adularia, days, wld carbonates) and on fluid inclu-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 different lllinerais (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 e t al., 2002; Faure. 2003). Deposits younger than a few million years gemmilly allow more act,''Urate con-straint'> on the comlx>sition of local me teoric water, with pre-sent-day values serving as a re liable 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 mete-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 thei r o res (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 othe r 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 mete-oric WlIttr (Cannan. 20(3). 11lfl 0 shift due 11,1 wtllel'roc:k inlflmctit;m is bufld on Tllyktr (1919).

500 SIM.IJO.VS ETAL

2003). Adularia is a dominant gangue mineral, probably at the A expense of quartz, which is generally subordinate (Je nse n and Barton, 2000), perhaps due to the higher quartz solubility Ilnde r alkaline conditions (Sillitoe, 2002). Fluolite, ros(;oelite (vanadium-bearing mica), and telluride min erals are com-mon, although not essential accessOlv mine rais, und the oc-currences of magne tite hematit e, "'Fe-rich sphalerite, and tetrahedlite-te nmllltite indicate low- to lTI(xlerate-suHklauon states (jensen and Bmtoll, 2000 ); lattice and (;ollofonn, handed vein textures are rare. Ores extcild over unusually large vertical inlen'als (500-1 ,000 m) and can be ilSsociated \\ith telescoping of epithermal lind porphyry environments (Je nsen anJ Barton, 2000; Sillitoe, 2002). Hydrothermal al-te ration is restricted to areas immediately ad.jacent to ore, B where there is extensive deve lopment of propylitic and argillic asse mblagcs. The re is also a lack of zoning among temperature-sensitive alt e ration minerals , such as clays. Fluid inclusion studies indi cat e that ore fluids had saliniti es of

ErIT/lF.RM."t PRECIOUS AND BASE Mt7AL DEPOSITS 501

sulfides, making minerali7 . .1tion that might otherwise he un-economic (e.g .. Pueblo Vieju. Dominic..1n Republic; Nelson. 2000) amenable to low-cost processing (e.g. , Yanacocha. Peru; Harvcyet al. , 1999).

Vuggy quartz is a residual prooUl .. 1 uf intense acid alterntion, and it is a distinctive feature that reflects the original nx::k tex-tu re and differential lellching of phenocrysts and/or lithic fr;lgments . Its fonnation predates deposition of copper and gold. which life introduced by a fluid of different composi-tion . ilIustr.lting the importwu:e of paleopenneability in prepamtiull fOf metal deposition (e.g., White. 1991 ; Arribas. J995). The vuggy qUHITL. texture in combi nation with dickite 100 m but commonly is confined to zones dO m wide, and the boundary between the centrAl SiliCiC alteration and the outer zones is typically knife sharp (e.g., Summitville, United States; Nansatsu deposits of lwato. Akeshi, and Kasug:a, Japan; Chinkuashih, Taiwan). Propylitic alteration is wide-spread and surrounds the acid-altered core, but zones of illite and pyrophyllite can extend well below some deposits (e.g., EI Indio, Chile; Rodalquilar, Spain; Lepanto, Philippines; Yanacocha, Peru). T hese dlanges in alterntion assemblages reflect outward and upward neutralization of acid fluids through water-nx:k inleraction and cooling (Steven and natt~. 19(0). In some districts and deposits (e.g., Ellndio-Pascna helt , Chile-Argentina; Bissig et aI., 2002, Deye ll et aI., 2004; Pure n prospect near 1 ... 1 Coipa, Chile; Sillitoc. 20(4), the shallow revel is presclVcd and represented by steam-heated advdnced argillic alteration that marks the position of the paleowuter table (Sillitue, 1993b. 1999).

Flui{l inclusion data

Because of the corrosive Illlhire of acid solutions and the character of hydrothennal minerals pnxlllced during the

early leaching (i.e., ruunitc, kaolinite, pyrophyllite), and the lack of

502 Sf.\fMOSS I:.T AI~

eentc red ore minerali:mtion slieh "'.~ pOlllhyry CII (Mo-Au), earbonate replacement deposits, and shlrns (e.g., Meinert et al., 2005; SeedoaiIo.Oac>t ..

D e.-.IIAAdeS