NRCAN-Synthesis_ Taylor_EpithermalAu

8/14/2019 NRCAN-Synthesis_ Taylor_EpithermalAu

1/27


2/27


3/27


4/27

B.E. Taylor

116

Archean

Golddeposittypes:

Cenozoic

Mesozoic

Paleozoic

Phanerozoic

Precambrian

Proterozoic

Proterozoic-P

hanerozoic

Le

gend

High

Sulphidation

Low

Sulphidation

HotSpring

Transitional/Intrusion

Related

GoldenBear

SprinpoleLake

Phoenix5

)Caribooetal.

Blackdome

6)NickelPlate

etal.

3)Silbak-Premieretal.

EquitySilver

Warman

Sunbeam-Kirkland

Elk

SurfInlet

Cinola

Tillicum

Sulphuret

GrewCreek

2)MountSkukumetal.

Zeballos

NicholasLake

Arcadia

Troilus

Newfield

Hemlo

LaurelLake

8)MatachewanConsolidatedetal.

Kiena

4)SilverButteetal.

7)CentreStarGroupetal.

Hope

Brook

KetzaRiver

MountNansen

Specogna

Kemess

M

alleryLake

CampbellMine

1)DublinGulchetal.

AllMineslistedarelargestdepositsforarea;inc

aseswheremorethanonedepositislocated,etal.hasbeenindicatedasfollows:

1)

2)

3)

4)

5)

6)

7)

8)

DublinGulchetal.,includes:DublinGulch,EagleZone,BreweryCreek;

MountSkukumetal.,includes:MountSkukum,SkukumCreek,MountReid,Berney;

Silbak-Premie

retal.,includes:Silbak-Premier,Spectrum,Banks,Banker,Tel,YellowGiant,JohnnyMountain,

Stonehouse,Snip,TwinZone,Scottie,SalmonGold,

Premier,Bush,Silbak,PremierGold;

SilverButteetal.,includes:SilverButte,SIB,Goldwedge,;

Caribooetal.,includes:Cariboo,Aurum,QR,Dome,QuesnelRiver;

NickelPlateetal.,includes:NickelPlate,Hedley;

CentreStarGroupetal.,includes:CentreStarGroup,Josie,LeRoiNo.2;

MatachewanConsolidatedetal.,includes:MatachewanConsolidated,Young-Davidson,RyanLake

.

HislopEast

Ho

lt-McDermott

EastM

alartic

Douay

BachelorLake

PoplarMountain

BotwoodBasin

HickeysPond

SteepNapProspect

HolyroodHorst

Laforma

MountClisbako

NiziProperty

QueenCharlotte

Islands

SixtymileRiverArea

WatsonBarProperty

HowellCreek

References:

Dub

BrownandCameron,1999;

GosselinandDub,2005b,d;Panteleyev,1996a,b,c,2005a,b;Poulsen,1996;Poulsonetal.,2000;Taylor,1996,thispa

per;Turneretal.,2003.

etal.,1998;

Lawyers

EPITHERMALANDSELE

CTED

INTRUSION-RELATEDGOLD

DEPOSITS

INCANADA

GSC

FIGURE

1.LocationofselectedCanadianepithermalAudepositsandprominentexampleselsewhereintheworld,classifiedbysubtypeasreferredtointhetext.Namesandlocationsofdepositsfromsources

arelisted.


5/27

Epithermal Gold Deposits

1

WAIHI

Temora

EMPEROR

LADOLAM

Wafi

PORGERA

KELIAN

BAGUIO

L

epanto

CHINKUASHIH

HISHIKARI

Nansatsu

PAGUNA

GRASBERG

FA

R

SO

UTHEAST

Ensen

Rodalqu

ilar

M

ahdadhDhahab

McLaughlin

COMSTOCKLODE

GOLDFIELD

SLEEPER

ROUNDMO

UNTAIN

CRIPPLE

CREEK

Creede

PUEBLOVIEJO

Fresnillo

PACHUCA

CerroRico

JulcaniChoquelimpie

YANACOCHA L

aCoipa

ELINDIO

BajodelaAlumbrera

THAMES

REPUBL

IC

OATMAN

HopeBrook

Hemlo

Blackdom

e

Zeballos

EquitySilver

Cinola

MountSkuk

um

PilotMountain

Milos

DonlinCreek

Summitville

Pascua

/Veladero

Pierina

MalleryLake

Tambo

Mulatos

ParadisePeak

Midas

Boliden

Lahca

Chelopech

MallinaBasin

References:

Arancibiaetal.,2006;Bethkeetal.,2005;Carman,2003;Deyelletal.,2005;Dub

etal.,1998;Fifarekand

Rye,2005;Goldfarbetal.,2004;GosselinandDub,200

5a,c;Hedenquistetal.,2000;Hustonetal.,2002;Kleinan

dCriss,1988;Nadenetal.,2005;

Panteleyev,1996a,b,c,2005a,b;Poulsen,1996,2000;Sillitoe,1992,1997;Taylor,1996,thispaper;Turneretal.,2003.

N.B.:GiantandBonanzaGolddepositsindicatedbycapitalizationofdepositname,e.g.,ELINDIO.

CERROVANGUARDIA

CerroLaMinaProspect

SELECTEDEPITHERMALANDINTRUSIO

N-RELATED

GO

LDDEPOSITSOFTHEWOR

LD

ElPeon

Archean

Golddeposittypes:

Cenozoic

Mesozoic

Paleozoic

Phanerozoic

Precambrian

Proterozoic

Proterozoic-Phanerozoic

Legend

High

Sulphidation

Low

Sulphidation

HotSpring

Transitional/Intrusion

Related

GSC

FIGURE

2.GlobaldistributionofselectedCanadianandnon-Canadianepithermaland

intrusion-relatedAudepositsoftheworld.The

associationofmany(young)depositswiththecircum-PacificBeltempha-

sizestheirgeneticlinktomagmaticcentres.GiantorBonanzadeposits(Sillitoe,1992)a

relabeledinuppercasefont.Depositsshownincludethosenotedinthetextorrepresentedinaccompanyingfigures.Names

andlocationsofdepositsfromsourcesarelisted.


6/27


7/27


1

Belt is host to Au deposits containing pyrophyllite,andalusite, topaz, and traces of diaspore (metamorphosedadvanced argillic assemblage) marked by oxygen isotopedepletion from meteoric-recharged hydrothermal systems(Klein and Criss, 1988), confirming an epithermal origin(see also Feiss et al., 1993).

Among Canadian examples, oxygen and hydrogen iso-tope measurements of vein quartz and of fluid inclusions,

respectively, from the unmetamorphosed Au-Ag mineral-ized, low-sulphidation altered stockwork associated withMiddle Proterozoic rhyolite dykes in the Mallery Lake area,

Nunavut, confirm a meteoric origin and epithermal setting(Turner et al., 2001). Oxygen isotope data on rocks from theHope Brook zone (B. Taylor and P. Stewart, unpub. data,1990) suggest a magmatically dominated (high-temperature)origin rather than a shallow, meteoric (e.g. steam-heated)alteration system. Detailed geologic and mineralogical stud-ies (Dub et al., 1998) have corroborated this conclusion.Similarly, the association of Hg-Sb-As-Tl (e.g. Harris, 1989)and range in 34S of pyrite (Cameron and Hattori, 1985) atthe controversial Hemlo mine (Ontario), a disseminated Audeposit in the Precambrian Shield, might suggest a meta-morphosed epithermal deposit. However, mineralized hostrocks are not depleted in 18O (Kuhns, 1988), suggestingdeeper level, magmatically dominant fluids. This is also sup-

ported by S isotope fractionations between sulphate-sulphidemineral pairs (Hattori and Cameron, 1986).

Associated Mineral Deposit Types

Other deposit types that may be found broadly associatedwith epithermal deposits (i.e. within epithermal districts orcamps) are those that share a common genetic link to mag-matic centres (e.g. veins, skarns, and mantos; Sillitoe, 1993;Sillitoe and Thompson, 1998; Lang and Baker, 2001). Somevein and/or replacement deposits, typically of the intermedi-

ate sulphidation subtype (e.g. Silbak Premier, BritishColumbia, Table 1; Hedenquist et al., 2000), might be gen-erally referred to as deep epithermal or transitionalaccording to Panteleyev (1986, 1991). Because of the possi-

bility of association, the locations of selected intrusion-related or transitional Au deposits in Canada have beenincluded in Figure 1.

Often a barren gap intervenes between the epithermaland deep epithermal portions of magmatically heated geot-hermal systems. Close juxtaposition of epithermal vein and

porphyry- or intrusion-related deposits may imply inducedchanges in the relative positions of meteoric geothermal sys-tems and magmatic heat sources. Superposition of Au-bear-ing epithermal veins in the Coromandel Peninsula, New

Zealand on slightly older Cu-Mo-Au porphyry deposits (e.g.Waihi deposit: Brathwaite and Faure, 2002), and superposi-tion of a low-sulphidation subtype deposit on a porphyry Cu-type deposit in the Philippines (Acupan: Cooke and Bloom,1990) are two examples. The extent of such superposition, ortelescoping, may be tectonically and/or climactically con-trolled by rapid rates of uplift and high erosion, and volcanicsector collapse (Sillitoe, 1993; Mller et al., 2002a,b).

Deep epithermal veins (transitional deposits ofPanteleyev, 1986) or replacement deposits associated withconventional epithermal deposits may comprise a compo-

nent of the genetic link between a degassing high-levelmagma and an overlying mineralized epithermal systems.The quartz-monzonite porphyry intrusion known beneath theSummitville, Colorado high-sulphidation magmatic-hydrothermal system is essentially coeval with Au mineral-ization and hydrothermal alteration (Bethke et al., 2005) and

provides an example of the porphyry-epithermal linkage forhigh-sulphidation deposits. Intrusion-related vein deposits in

the Sulphurets, Mt. Washington, and Zeballos camps, BritishColumbai, are possible Canadian examples (BritishColumbia Ministry of Energy, Mines, and PetroleumResources, 1992; Margolis, 1993). Other, related hydrother-mal deposits that may be associated with epithermal veindeposits, and represent the mesothermal counter part,include Au-bearing skarns (high-temperature, silica-replace-ment deposits; e.g., Hedley, British Columbia) and mantodeposits (sulphide rich replacement; e.g., Ketza River,British Columbia) in carbonate rocks, and intrusion-adjacentdeposits sometimes referred to collectively as such intru-sion-related deposits (e.g. Thompson et al., 1999; Lang andBaker, 2001).

Disseminated and vein Au deposits associated with alka-lic intrusions (e.g. Howell Creek, Fernie, British Columbia:Brown and Cameron, 1999) have gained attention as a dis-tinct type of intrusion-related Au deposit (e.g. Richards,1995; Jensen and Barton, 2000; Robert, 2001). The Au-Teepithermal deposit(s) at Cripple Creek, Colorado, represent aclassic association with alkalic (diatreme) magmatic rocks;no extrusive rocks are present at this level of erosion (e.g.Kelley et al., 1998).

Hot-spring deposits, including siliceous sinters, steam-heated alteration zones, and brecciated root zones (e.g.Cinola, British Columbia: Christie, 1989; see also Izawa etal., 1993), cap many modern geothermal systems, and may

be associated with either low- or high-sulphidation epither-

mal deposits. Their formation at the Earths surface makesthem very susceptible to erosion, however. Modern andrecent deposits are most common, and examples are foundworldwide, including Japan, Indonesia, New Zealand (e.g.Champagne Pool, Wairakai), Nevada (Round Mountain:Sander and Einaudi, 1990), and California (McLaughlin:Sherlock, 2005). Older, Jurassic, sinters, and related Au-Agepithermal deposits are also known from Patagonia,Argentina (Schalamuk et al., 1997). Gold grades are typi-cally variable and subeconomic, but the presence of Au,along with Hg, Sb, As, S, and Tl, may suggest a mineralizedroot zone, or deeper epithermal deposit.

Volcanogenic massive sulphide (VMS) deposits that format or near the seafloor from submarine hot springs and sub-

seafloor geothermal systems are epithermal deposits in thebroad sense (e.g. Sillitoe et al., 1996). Gold-bearing VMSdeposits (e.g. Horne mine, Noranda: Dub et al., 2007;Eskay Creek 21B, British Columbia: Roth et al., 1999) arerecognized as a type of VMS deposit (Dub et al., 2007), and

both high-sulphidation and low-sulphidation variants havebeen recognized (Sillitoe et al., 1996). Similarly, island arcsettings may also host submarine calderas and epithermaldeposits (e.g. Pueblo Viejo high-sulphidation subtype: cf.Kesler et al., 2005; Sillitoe et al., 2006; Milos, Greece:

Naden et al., 2005). Indeed, many of the same processes


8/27

regarding origins and causes of metal deposition and enrich-ment associated with continental epithermal deposits applyto these shallow marine settings. Comparison of S isotopegeochemistry between adjacent subaerial and submarinemineralized systems in Papua New Guinea (Gemmell et al.,2004) offers further supportive evidence. Thus, the subma-rine environment should not be ignored with regard toepithermal deposits, despite the fact that some features

(steam-heated alteration caps, dominance of meteoricwaters, etc) familiar among subaerial epithermal and hot-spring deposits are necessarily absent.

Economic Characteristics of Epithermal Gold Deposits

Summary of Economic Characteristics

Gold (Ag) is the principal commodity of epithermal Audeposits, occurring usually as native Au, or in electrumalloyed with Ag. It may also occur in tellurides, or as inclu-sions in sulphides. Copper and the other base metals, Pb andZn, may also occur with Au, especially in transitionalepithermal deposits with high Ag grades. Indeed, the com-mon presence of enargite has led to the term enargite-Au

deposits for some high-sulphidation subtype deposits(Ashley, 1982).

Epithermal (vein-) deposits, compared to the low-grade, bulk-tonnage porphyry deposits or the Carlin-typedeposits, are typically small in size (e.g. 106 to 108 tonnes ofore; high-sulphidation deposits tend to be smaller than low-sulphidation deposits) and, consequently, have a short min-ing life. However, epithermal Au deposits can reach highgrades, a few to several tens of g/t, or more in exceptionalcases (e.g. 70 g/t, Hishikari, Japan; to ~200 g/t, El Indio,Chile; Table 1).

Epithermal Au deposits represent a minor proportion, typ-ically a few percent, of the total Au (reserves + production)in Canada. For example, epithermal deposits yielded an

average annual production of 2725 kg/year, or about 2.7% ofthe total annual Au produced in Canada from 1985 to 1987.Owing largely to occurrence tending to favour geologicallyyounger terrane, epithermal Au contributed relatively more(as much as 24%) of the total Au produced in BritishColumbia and the Yukon during the same period.

Grade and Tonnage Characteristics

The sizes (in 106 tonnes) of the principal Canadianepithermal Au vein deposits and selected type depositselsewhere are listed in Table 1, and plotted versus Au grade(g/t) by class of deposit in Figure 3. The mean grade and ton-nage of several classic examples of non-Canadian deposits(low-sulphidation: Creede, Colorado, and Hishikari, Japan;

high-sulphidation: Summitville, Colorado; average Au-bear-ing porphyry deposit; and average Carlin-type deposit,

Nevada) are plotted for comparison. The estimated sizes (ca.0.05 to 42 Mt of ore) give an order of magnitude basis forcomparison; definition of size depends on cut-off grades andeconomics. Ore comprises disseminated Au in silicifiedand/or finely veined rocks in the Cinola deposit, BritishColumbia, and in areas of the Sulphurets district, BritishColumbia. Here, grades are typically lower, but tonnageslarger, than in other, vein-type, epithermal deposits (Table 1).Based on a reported grade of 2.45 g/t and 23.80 Mt of ore,

the Cinola deposit is potentially the second largest epither-

mal Au deposit in Canada. For vein-style epithermaldeposits, the Au grades of Canadian examples (most ~2.5-25g/t) are similar to those of a majority of mesothermalquartz-carbonate deposits (see Gosselin and Dub, 2005a-d),

but of generally smaller size, and are distinguished from thelatter by higher Ag:Au ratios (>1:1).

Canadian epithermal Au deposits are comparable in sizeand grade to many deposits found in the major epithermalterranes of the world, as illustrated in Figure 3. The largestepithermal deposits (in tonnes of ore) and the richestdeposits (in g/t) are found outside of Canada. Fields for

prominent low-sulphidation subtype epithermal Au deposits

B.E. Taylor

120

1000tAu

10tAu

1t

Au

H

I

C

+

100tAu

+

EPITHERMAL

INTRUSION RELATED

SEDIMENT HOSTED

VMS - Au

P

Prominent Global EpithermalHigh-Sulphidation Subtype

Prominent Global EpithermalLow-Sulphidation Subtype

Y

GSC

S

BSK

BD

LAF

N

AL

DM

V

L

SP

EQ

S

HB

C

100

10

1GOLD

GRADE

(g/t)

109108107106104 105

TONNES OF ORE

FIGURE 3. Plot of Au grade (g/t) versus tonnage (economic, orreserves+production) for selected Canadian epithermal Au deposits and

prominent examples elsewhere in the world, classified by subtype asreferred to in the text. Canadian epithermal deposits (filled circles; seeTable 1) include AL = Al; B = Baker; BD = Blackdome; C = Cinola; DM =Dusty Mac; EQ = Equity Silver; HB = Hope Brook; L = Lawyers; LAF =Laforma; N = Mt. Nansen; SK = Mt. Skukum; SP = Silbak Premier; S =Sulphurets camp (Brucejack Lake, Sulphuret, West Zone deposits); and V= Venus. Hydrothermal vein deposits of a possible transitional or deepepithermal deposits are represented by open circles, sediment-hosteddeposits by a green square with cross, and Au-bearing VMS deposits(marine epithermal) by open red squares (see Appendix 1 in Dub et al.,2007). The median grades and tonnages for several comparable types ofdeposits (yellow-filled circles) from Cox and Singer (1986) include por-

phyry Cu-Au [P]; low-sulphidation Creede-type [C]; intermediate sulphi-dation: polymetallic vein deposits associated with felsic intrusions [M]; andhigh-sulphidation: Summitville deposit [S]; and Lawyers deposit,Toodoggone River district, British Columbia [L is similar to theComstock-type, Nevada (not plotted) of Cox and Singer, 1986]. Medianvalues for the low-sulphidation Hishikari, Japan vein deposit [H], and forthe high-sulphidation El Indio, Chile, deposit [I] are from Hedenquist et al.(2000). Fields for prominent low-sulphidation (blue shading) and high-sul-

phidation (dashed line) epithermal Au deposits worldwide (global) arebased on data in Hedenquist et al. (1996; 2000).


9/27


12

and for high-sulphidation subtype epithermal Au deposits(from data in Hedenquist et al., 1996, 2000) overlap, but sug-gest that high-sulphidation subtype deposits tend to be com-

parable in size and grade to the smaller of the low-sulphida-tion subtype deposits. Several non-Canadian epithermaldeposits selected by subtype to reflect the global range ofgrade and tonnage are plotted together with Canadiandeposits in Figure 3.

Exploration Properties of Epithermal Gold Deposits

Physical Properties

The mineralogy, textural features, host rocks, morphol-ogy, and selected chemical properties found typically inepithermal Au deposits are summarized in Table 2. Key fea-tures are emphasized below.

Mineralogy

Quartz is the predominant gangue mineral in all epither-mal Au deposits, whereas distinctive ore and gangue miner-als characterize high-sulphidation and low- sulphidationdeposit subtypes. Mineralogical zoning around veins or

replacement zones may be present in both subtypes, record-ing chemical and/or thermal gradients. Both subtypes ofdeposits can contain very fine-grained Au and gangue min-eral assemblages, especially in hot-spring and steam-heatedenvironments that form above boiling hydrothermal systems(Henley and Ellis, 1983).

In high-sulphidation subtype deposits, native Au and elec-trum are typically associated with pyrite+enargitecovel-litebornitechalcocite. In addition to sulphosalts and basemetal sulphides, tellurides and bismuthinite are present insome deposits. Total sulphide contents are generally higher inhigh-sulphidation than low-sulphidation subtype deposits,

but high sulphide contents may also characterize transitionalpolymetallic low- sulphidation deposits (e.g. Silbak Premier,

British Columbia). Where base metals are present in high-sulphidation deposits, the Cu abundance can vary signifi-cantly (0.1-5%: Sillitoe, 1993), and typically dominate thatof Zn. Principal gangue minerals include quartz (vuggy sil-ica), alunite, barite (especially associated with Au), and, insome deposits, S; manganese minerals and fluorite are rare.Calcite is not characteristic of high-sulphidation subtypedeposits due to the high acidity of the hydrothermal fluids.

Native Au and electrum occur in low-sulphidation sub-type vein deposits that often contain only a few percent orless of sulphides (usually pyrite; e.g., Blackdome, BritishColumbia). In deposits in which sulphide minerals are abun-dant (e.g. Venus; Silbak-Premier: sulphide-rich stage), sul-

phides commonly include chalcopyrite, tetrahedrite, galena,

sphalerite, and arsenopyrite in addition to pyrite. The princi-pal gangue minerals include calcite, chlorite, adularia, barite,rhodochrosite, fluorite, and sericite.

In sediment-hosted low-sulphidation deposits, the charac-teristic assemblage of gangue minerals commonly includescinnabar, orpiment-realgar, and stibnite, in addition to jas-

peroid, quartz, dolomite, and calcite. Chalcedonic quartzveins and jasperoid are typically associated with ore,whereas calcite veins are often more common further fromore, or are paragenetically late. In siliceous sinter associatedwith hot-spring deposits, sulphate minerals, clays, and minor

pyrite constitute the typical gangue assemblage; verticalzoning of alteration mineral assemblages is characteristic.

In some deposits hosted by volcaniclastic rocks (e.g.McDermitt, Nevada), micrometre-size Au grains are typical,although visible (recrystallized) native Au may occur in oxi-dized portions of some deposits. Gold can occur coating sul-

phides and/or encapsulated in quartz in silicified rocks,accompanied by Hg-, Sb-, and As-bearing sulphides. At

Cinola, British Columbia, a rare example in Canada(Christie, 1989; see also Poulsen, 1996), Au is most abun-dant in the subsurface silicified sediments and hydrothermal

breccias. Inclusion of sediment-derived hydrocarbons mayoccur during vein formation in sedimentary rocks, ordeposits within hydrothermal systems encompassing sedi-mentary rocks (e.g. Owen Lake, British Columbia: Thomsonet al., 1992).

Textures

Vuggy silica has a porous texture formed by removal ofminerals, particularly feldspars during reaction with veryacidic fluids and concentration of residual silica (e.g.Summitville, Colorado: Stoffregen, 1987). Massive, quartz-rich zones may result from further silicification (i.e. by addi-tion of silica). Examples include alteration zones at Mt.Skukum, Yukon (alunite cap zone: Love, 1989) and at the Aldeposit (Toodoggone River, British Columbia: Diakow et al.,1993). In high-sulphidation subtype deposits, coarse-grainedalunite is characteristic, whereas alunite from steam-heatedzones (high-sulphidation subtype caps to epithermal sys-tems), and from supergene weathering of sulphide deposits,is typically very fine grained to microcrystalline.

Lamellar or platy (angel wing) calcite, in some cases pseudomorphically replaced by silica (e.g. Mt. Skukum,Yukon), is of particular significance because it forms in boil-ing zones in low-sulphidation subtype systems (e.g. de

Ronde and Blattner, 1988; Simmons and Christenson, 1994).Rhombic adularia has been similarly associated with boiling(Keith and Muffler, 1978; Dong and Morrison, 1995).

Unique to (unmetamorphosed) hot-spring deposits, arenon-horizontal laminated or bedded lenses that may con-tain textures formed by silica fossilization of plant matter(root casts, etc.), and vertical crystallization textures andstructures.

Dimensions

High-sulphidation deposits of magmatic hydrothermalorigin (e.g. Rye et al., 1992) are typically of smaller dimen-sion than low-sulphidation subtype deposits, and are foundin close proximity to, and often topographically above, a

related source of magmatic heat and volatiles. Altered rocksof the Summitville, Colorado, deposit, for example, crop outover an area of 1.5 by 1.0 km (Heald et al., 1987). Shallow,steam-heated environments, in contrast, may producewidespread altered areas, typically (but not always) barren;

bulk-tonnage mining of these zones may be possible if theyare mineralized. For example, mineralized areas altered toquartz+clay+alunite (+barite+dickite) at the Al deposit,Toodoggone River area, British Columbia, measure about250 m by as much as 1.5 km (Diakow et al., 1993). Fault-controlled, quartz-(kaolinite)-alunite alteration zones meas-


10/27

uring roughly 200 by 250 m occur topographically above theMt. Skukum deposit, in an area partially removed by erosion(McDonald, 1987). Similarly altered prospective areas occurin folded Neoproterozoic (Avalonian) rocks in the BurinPeninsula, Newfoundland affected by advanced argillicalteration (pyrophyllite-alunite-specularite; e.g., OBrien etal., 1999). Two of these measure approximately 125 by 225m (Hickeys Pond prospect) and 4700 m x 4 km (Stewart

prospect), although evidence of similar alteration is presentover a length of approximately 100 km. The AvalonianCarolina slate belt hosts high-sulphidation subtype gold

deposits (abandon mines) of similar age, alteration style, anddimension (c.f. Klein and Criss, 1988; OBrien et al., 1999).

Low-sulphidation subtype deposits in some cases coverlarger areas than typical of high-sulphidation deposits, eventhough alteration mineral assemblages are restricted to gen-erally narrow zones enclosing veins and breccias. At theBlackdome mine, British Columbia (Fig. 4), quartz veins asmuch as 0.7 m thick and 2200 m long, are contained withinan area approximately 2 by 5 km. Veins comprising theLawyers deposit and the Baker mine in the Toodoggone dis-trict, British Columbia, are commonly 2 to 7 m wide and asmuch as several hundred metres in length. Veins and breccia

zones as wide as 40 m and as long as 1200 m comprise theMain zone of the Silbak-Premier deposit in British Columbia(McDonald, 1990). Elsewhere, mineralized veins in low-sul-

phidation subtype epithermal deposits have been mined for astrike length of more than 5 km at Creede, Colorado (Healdet al., 1987), and occur for a distance of about 2 km at thehigh-grade (e.g. 70 g/t) Hishikari mine, Japan (Izawa et al.,1990). Alteration zones around the veins of the Hishikarideposit have been mapped in an area measuring as much as2 km wide by more than 3 km long (Izawa et al., 1990).

Hot-spring deposits comprising surface lenses or aprons

of silica (siliceous sinter), may be several hundred metres indiameter, but only metres to tens of metres in thickness.Discordant hydrothermal conduits beneath these depositsmay extend over a hundred or more metres in the verticaldimension, and resemble funnel-like forms in section,decreasing from perhaps many tens of metres to a few metreswith depth (e.g. Christie, 1989).

Morphology

The morphology of epithermal vein-style deposits can bequite variable. Deposits may consist of roughly tabular lodescontrolled by the geometry of the principal faults they

B.E. Taylor

122

0 km 300

Blackdomemine

ALBERTA

BRITISHCO

LUMBIA

Yalakom

Fault

Fra

se

rRiv

er

Stra

igh

tCre

ekF

au

lt

Mid-Eocenevolcanic rocks

Basalt

Contact .....................

Vein ..........................

Adit ...........................

Upper andesite

Rhyolite and sedimentaryrocks

Dacite

Lower andesite

Metamorphicrocks

Chilliwackbatholith

CANADA

U.S.A,

Z

km0 1

GIANT

VEIN

SOUTH LEVEL(1960 LEVEL)

NORTH LEVEL

(1960 LEVEL)

1920 LEVEL

VEIN

NO

.2

VEIN

NO

.1

GSC

BLACKDOMEMOUNTAIN

RED

BIRD

VEIN

FIGURE 4. Geological map illustrating setting of the epithermal Au vein deposit at the Blackdome mine, British Columbia (from Taylor, 1996; data from

D. Rennie, unpub. rep., 1987). Inset map illustrates the regional tectonic setting of the Blackdome mine area (red square), between the Yalakom (55-45 Ma)and younger Fraser River Straight Creek (40-35 Ma) strike-slip fault (Coleman and Parrish, 1990; R.R. Parrish, pers. comm., 1991).


11/27


12

occupy (e.g. Cirque vein, Mt.Skukum; Fig. 5; Table 1), or com-

prise a host of interrelated fracturefillings in stockwork, breccia, lesserfractures, or, when formed byreplacement of rock or void space,they may take on the morphologyof the lithologic unit or body of

porous rock (e.g. irregular brecciapipes and lenses) replaced. Volumesof rock mineralized by replacementmay be discordant and irregular, orconcordant and tabular, dependingon the nature of porosity, perme-ability, and water-rock interaction.In deposits of very near-surface ori-gin (e.g. Cinola), an upwardenlargement of the volume ofaltered and mineralized rocks may

be found centred about thehydrothermal conduits. Hot-springdeposits tend to comprise subhorizontal aprons or lenses of

sinter about their upflow zones and subhorizontal replace-ment zones in the shallow subsurface. Phreatic eruptions pro-duce discordant zones of breccia-like deposits; clasts may be

partially rounded (e.g. Izawa et al., 1990).

Brecciation of previously emplaced veins (e.g. Mt.Skukum, Yukon) can form permeable zones along irregular-ities in fault planes: vertically plunging ore zones in faultswith strike-slip motion and horizontal ore zones in dip-slipfaults. Topographic (i.e. paleosurface) control of boiling byhydrostatic pressure can also result in horizontal or subhori-zontal mineralized zones, limiting the vertical distribution ofore (as suggested in Fig. 5; Cirque vein, Mt. Skukum,Yukon). The distribution of high-sulphidation alteration insteam-heated settings (possibly in the Toodoggone Rivercamp, British Columbia) may also reflect a topographic con-trol of the paleo-water table.

Host Rocks

Nearly any rock type, even metamorphic rocks, may hostepithermal Au deposits, although volcanic, volcaniclastic,and sedimentary rocks tend to be more common. Typically,epithermal deposits are younger than their enclosing rocks,except in the cases where deposits form in active volcanicsettings and hot springs. Here, the host rocks and epithermaldeposits can be essentially synchronous with spatially asso-ciated intrusive or extrusive rocks, within the uncertainty ofthe determined ages in some cases (e.g. high-sulphidation

Summitville deposit, Colorado: Bethke et al., 2005; low-sul-phidation El Peon deposit, Northern Chile: Arancibia et al.,2006).

Chemical Properties

Ore Chemistry

Gold:silver ratios of epithermal Au deposits may varywidely both between and within deposits from 0.5 for thehigh-sulphidation type Kasuga deposit, Japan (Hedenquist etal., 1994), for example, to >>500 in the Cerro Rico de Potosideposit, Peru (Erickson and Cunningham, 1993). Differing

magmatic metal budgets (Sillitoe, 1993) and depths of for-

mation (Hayba et al., 1985) have been suggested to influencethis ratio. At the Lawyers deposit, Toodoggone River district,British Columbia, the Ag:Au ratio varies northward in thedeposit, from less than 20 to more than 80 (average = 46.7;Table 1), and higher ratios are also found at deeper levels ofthe deposit (Vulimiri et al., 1986). Typically, Ag:Au ratiosfor epithermal deposits, though variable, tend to be higher inlow-sulphidation subtype deposits than in high-sulphidationsubtype deposits (Table 1). The deep epithermal (mesother-mal) Equity Silver deposit, British Columbia (e.g. Cyr et al.,1984; Wojdak and Sinclair, 1984) has the highest Ag:Auratio (approximately 128; Table 1) among Canadian epither-mal deposits.

High precious metal/base metal ratios in hot-springdeposits (and steam-heated zones in general) are thought to

be characteristic. Buchanan (1981) suggested that base met-als precipitate in deeper, more saline, liquid-dominated por-tions of the system, whereas deposition of Au occurs in anupper, gas-rich, or boiling portion of the geothermal system,resulting in the observed metal separation.

Whereas base metals may accompany Au (Ag) in vari-able amounts in intrusion-related or transitional deposits,more volatile elements commonly occur with Au (Ag)in shallower epithermal and hot-spring environments.These elements characteristically include Hg, Sb, Tl, As, andnative S.

Alteration Mineralogy and Chemistry

Examples of alteration mineral zoning and its relationshipto lithology are illustrated for portions of three Canadiandeposits in Figure 6A-C (sediment-hosted low-sulphidationsubtype: Cinola, British Columbia; volcanic-hosted low- tointermediate-sulphidation subtype: Silbak-Premier, BritishColumbia; Fig. 5C), and rhyolite/andesite-hosted high-sul-

phidation alteration topographically above the low-sulphida-tion subtype (Mt. Skukum deposit, Yukon). Alteration was,in each case, structurally controlled, cross-cutting the hostrocks. Symmetrical zoning developed about some veins (e.g.

< 0.4 0.4 - 0.8 0.8 - 4.0 4.0 - 8.0 8.0 - 15.0 > 15.0

GSCmetres x grams/tonne

N S1730

1710

1690

1670

1650

1630

Limit of data

0 m 20

Surface

FIGURE 5. Longitudinal section of the Cirque vein, Mt. Skukum (from Taylor, 1996, modified afterMcDonald, 1987) illustrating distribution of Au (thickness x grade).


12/27

Mt. Skukum, Yukon), but was markedly asymmetrical inother cases (e.g. Cinola, British Columbia).

In both high-sulphidation and low-sulphidation depositsubtypes, hydrothermal alteration mineral assemblages arecommonly regularly zoned about vein- or breccia-filled fluidconduits, but may be less regularly zoned in near-surfaceenvironments, or where permeable rocks have been replaced.Characteristic alteration mineral assemblages in both deposit

subtypes can give way to propylitically altered rocks con-taining quartz+chlorite+albite+carbonatesericite, epidote,and pyrite. The distribution and formation of the earlierformed propylitic mineral assemblages generally bears noobvious direct relationship to ore-related alteration mineralassemblages.

Altered rocks in low-sulphidation deposits generally com-prise two mineralogical zones: (1) inner zone of silicification(replacement of wall rocks by quartz or chalcedonic silica);and (2) outer zone of potassic-sericitic (phyllic) alteration(quartz+K-feldspar and/or sericite, or sericite and illite-smectite). Adularia is the typical K-feldspar, but its promi-nence varies greatly; it may be absent altogether. Chlorite

and carbonate are present in many deposits, especially inwall rocks of intermediate composition, and in some cases(e.g. Shasta deposit, Toodoggone River, British Columbia:Thiersch and Williams-Jones, 1990; Silbak-Premier:McDonald, 1990) chloritic alteration accompanied the potas-sic alteration and silicification. Argillic alteration (kaoliniteand smectite) occurs still farther from the vein. In somedeposits (e.g. Cinola: Christie, 1989), argillic alteration pre-dates silicification, giving evidence of the waxing and wan-ing of hydrothermal systems. Argillic mineral assemblagesare commonly superposed on the above, or form in higherlevel alteration zones (e.g. Toodoggone River area, BritishColumbia: Diakow et al., 1993), where adularia is replaced

by kaolinite; smectite may occur furthest from the veins.

Silicified rocks are common in epithermal deposits, as isquartz gangue in veins. For example, in both the volcanic-hosted Blackdome deposit, British Columbia, and sedimen-tary hosted Cinola deposit, irregularly silicified and mineral-ized wall rocks are common adjacent to faults and fractures.Silicified and decarbonated host rocks characterize Carlin-type Au deposits in Nevada (e.g. Bagby and Berger, 1986).The silicification of wall rocks (and the distribution of ore)was apparently controlled by available primary permeabilityof bedding planes or rock fabric. Secondary permeability canalso be produced by physical and chemical processes involv-ing the hydrothermal fluids themselves. The sudden releaseof pressure on hydrothermal fluid (e.g. by faulting) can cause

brecciation, creating pore space permeability (e.g. Cinola,

British Columbia, breccia zone in Fig. 6A). This can occur ingeothermal systems within several hundred metres of theEarths surface (e.g. Hedenquist and Henley, 1985).Dissolution of carbonate upon reaction between hydrother-mal fluids and wall rocks also can produce secondary per-meability.

Based on the nature of silicification, Bagby and Berger(1986) distinguished two types of sediment-hosted deposits:

jasperoidal- and Carlin-type; the latter is no longer consid-ered a subtype of epithermal Au deposits. Jasperoidaldeposits may occur in clastic sedimentary rocks, where sili-

B.E. Taylor

124

SW NE

Breccia

Rhyolite

Mudflow breccia

Pebble conglomerate

Boulder conglomerate

Shale (Haida Fm.)

Silicic alteration

Speconafault

Argillic alteration

Surface

Argillic

Silicic

3

3

3

4

4

4

4

4

3

2

1

3

3

3

2

1

1

0 100m

200

100

0

N

Argillic alterationAlunite + breccia

Rhyolite ignimbritePyrophyllite + kaolinite

Flow-banded rhyolitePyrophyllite

Andesite brecciaSericite

Andesite flows and tuffs

'Silica cap'

3

4

4

5

2

5

4

3

2

1

?

2

1

0 100m

2100

2000

1900

1800

1700

S

SW

NE

3

3

3

3

33

2

2

2 1

1

1

650

600

550

500

GSC

Stockwork veins and breccia

K-feldspar porphyritic dacite

Dacite lapilli tuff

Sericite

Chlorite

Calcite

Fault zone

Hornblende-plagioclase porphyry

3

2

1

0 5

0

m

FIGURE 6. Geological cross-sections of representative Canadian epithermaldeposits illustrating alteration mineral zoning and selected features (from

Taylor, 1996). (A) Cinola sedimentary-hosted Au deposit (after Christie,1989; low-sulphidation subtype), also illustrating localization of bothmagma (interpreted from faulted dyke) and hydrothermal fluids by theSpecona fault, and the control exerted by primary lithological permeabilityon the distribution of zones of silicification and argillic alteration. (B) Cross-section through a portion of a principal normal fault in the Mt. Skukum area(after Love, 1989; low-sulphidation subtype, volcanic host) illustrating thedistri bution of associated alunite+silica, and advanced argillic alterationmineral assemblages. Supergene alteration has been superposed on quartz-(kaolinite)-alunite alteration zones. (C) Cross-section through a portion ofthe Silbak-Premier deposit (intermediate sulphidation; after McDonald,1990) illustrates hydrothermal propylitic, sericitic, and potassic alterationmineral assem blages in relation to fault-controlled vein stockwork and

breccia, and to porphyritic dacite.

B

A

C


13/27


12

cification is characterized by quartz veins (commonly)and/or replacement (e.g. Cinola, British Columbia). Othereffects of alteration are otherwise similar, and include decar-

bonation (where rocks originally contained carbonate) andargillization. Alteration minerals include alunite, quartz, cal-cite, illite, cinnabar, orpiment, realgar, stilbite, pyrite,

pyrrhotite, marcasite, and arsenopyrite.

Advanced argillic alteration mineral assemblages that

characterize high-sulphidation deposits includequartz+kaolinite+alunite+dickite+pyrite in and adjacent toveins or zones of replacement in the magmatic-hydrothermalenvironment. Pyrophyllite occurs in place of kaolinite at thehigher temperatures and pressures of deeper deposits. Insome outer zones (e.g. alunite cap; Mt. Skukum), argillic(smectite)sericite mineral assemblages may occur (Fig.6B). These alteration minerals indicate a very low pHhydrothermal environment (possibly below even that for alu-nite stability; Stoffregen, 1987) of high oxidation state(hematite and sulphate are stable). Zones of silica replace-ment and vuggy silica are characteristic, and carbonates areabsent. Topaz and tourmaline in high-temperature zonesindicate the presence of F and B in the acidic hydrothermalfluids.

Acid-sulphate (high-sulphidation) type alteration fluidsform by the dissolution of large amounts of magmatic SO2 inhigh-temperature hydrothermal systems, and also by reac-tion of host rocks with steam-heated meteoric waters acidi-fied by oxidation of H2S (probably of magmatic origin: e.g.,Rye et al., 1992; Bethke et al., 2005), or by dissolution ofCO2. Two contemporaneous fluids are typically found tohave been significant in epithermal Au deposits, and partic-ularly in the high-sulphidation subtype (e.g. Summitville,Colorado: Bethke et al., 2005; Pierina, Peru: Fifarek andRye, 2005). These are a saline fluid (typically ~10-40 wt.%

NaCleq) found often in the deeper portion of the hydrother-

mal system, associated with mineralized zones, and a low-density (


14/27

deposit). The mixing of meteoric waters (PDMW) witheither modified fluids or magmatic fluids is indicated bystraight lines (e.g. such indicated for the Nansatsu deposit,N). Variations of D or 18O of the fluids may be accom-

panied by variations in wt.% NaCl, such as by mixing of a(deeper) saline fluid and a (shallower) dilute fluid (e.g.Taylor, 1987). Evidence of the mixing of distinct fluids with

distinct isotopic ratios and salinities has been reported forsome vein deposits of the deep epithermal or transitionalcategory (e.g. Finlandia vein, Peru: Kamilli and Ohmoto,1977).

Altered sedimentary wall rocks are generally less depletedin 18O and the hydrothermal fluids are more enriched in 18Othan in volcanic terranes. For example, the markedly higher18O/16O ratios of hydrothermal fluids at Cinola, compared tothose at Mt. Skukum and Blackdome (see Fig. 7), can beattributed to higher initial 18O/16O ratios of the local watersand to their reaction with sedimentary wall rocks.

Involvement of seawater or low-latitude meteoric water isindicated for the Sulphurets area (Margolis, 1993). Meteoricwaters formed the major component of the ore-forming flu-ids at the Blackdome (Vivian et al., 1987), Dusty Mac(Zhang et al., 1989), and Mt. Skukum (McDonald, 1987)deposits. Data reported in Diakow et al. (1991, 1993) indi-cate a broadly similar scenario for low-sulphidation depositsof the Toodoggone River area, British Columbia An unusual

range in D (-151 to -54 per mil) and 18O (recalculated: -7.6 to -2.6 per mil) for vein-depositing fluids in the Laformavein (Table 1; McInnes et al., 1990) are consistent, alongwith S isotope data, with a mixing scenario involving mag-matic volatiles and meteoric waters. Margolis (1993)inferred progressive mixing of magmatic water and seawaterduring potassic, sericitic, and advanced argillic alteration atSulphurets, British Columbia, on the basis of isotopic dataand water-rock reaction modeling. In some cases (e.g.Creede, Colorado), incorporation of dilute (fresh meteoric)fluids occurred abruptly, and late, in the paragenesis (e.g.Foley et al., 1989).

Recognition of the source of S by means of its isotopiccomposition depends on the relative mass balance for thecontributing sources that are isotopically distinct. Host rockS, or biogenically precipitated S (e.g. Eskay Creek: Roth etal., 1999; Roth and Taylor, 2000, submitted) may comprise asignificant component in some low-sulphidation deposits,whereas in high-sulphidation deposits, magmatic S (as S02;34S about 0-4 per mil for felsic magmas: Taylor, 1987)dominates. Where magmatic sources of wall rock S (e.g. sul-

phide minerals) dominate, magmatic S isotope values maycharacterize low-sulphidation deposits also.

Carbon isotope data for calcite or fluid inclusion CO2 typ-ically reveals its magmatic (ultimately mantle) origin, evenin systems dominated by meteoric water (e.g. Laforma,Yukon: McInnes et al., 1990; Mt. Skukum, Yukon:

McDonald, 1987). Admixture with terrestrial C sources mayalso occur (e.g. organic carbon in the Owen Lake deposit:Thomson et al., 1992).

Fluid inclusions typically have been shown to contain pre-dominantly fluids of low salinity (less than approximately 5wt.% NaCleq) and have filling temperatures of 150 to 300C,with maxima in the range of approximately 260 to 280C(e.g. Equity Silver: Shen and Sinclair, 1982; Blackdome:Vivian et al., 1987). Vapour-dominated systems at or near a

boiling water table tend to evolve toward a rather uniformtemperature of about 240C due to the limitation imposed bya maximum in the enthalpy of steam+liquid (e.g. White etal., 1971). Some deep epithermal (transitional) environmentsclose to genetically related intrusions are characterized by

higher temperatures, salinities, and CO2 contents (e.g.Baker, 2002). The occurrence of fluid inclusions formed atdifferent times in a dynamic system complicates interpreta-tion of the evolution of the system. Temporal changes in theCreede hydrothermal system, identified by abrupt changes inthe chemical and isotopic compositions of fluid inclusions

between different growth zones, or in fracture planesthrough crystals, demonstrate that the identity of ore-trans-

porting fluids can be obscured by inappropriate samplingand analysis (Foley et al., 1989).

B.E. Taylor

126

SW

W

0

-40

-80

-120

-160

S

Au

F

U.S.A Sed. Au

U.S.AVol. Au

JapanAu-Ag

Sulphurets

Magmatic water

-20 -10 0 10 20

Mt. Skukum

Blackdome

Cinola

Dusty Mac

PDMW

18 O ( )V-SMOW

V-S

MOW

- Arc magmas

- Continentalmagmas

Eskay Crk - VMS

C H

PL-EI

MalleryLake

Summitville

McLaughlin

N

D

(

)

FIGURE 7. Plot of 18O versus D for present day meteoric waters and forwaters in equilibrium with gangue minerals in selected epithermal Au

deposits; Canadian examples are shown in red (modified from Taylor, 1996,with additions). This diagram illustrates the origin and oxygen isotopeenrichment of meteoric waters in many epithermal vein systems.Abbreviations are for the Finlandia vein [F], Colqui district, Peru (S = sul-

phide stage; Au = precious metal stage); PDMW = present day meteoricwater; SW = seawater (note similar composition for fluids in the EskayCreek Au deposit: Sherlock et al., 1999); U.S.A. SED. Au = sedimentaryrock-hosted Au in the United States; U.S.A. Vol. Au = volcanic rock-hostedAu in the United States. Magmatic water composition defined by Taylor(1987, 1992). Sources of data for Canadian deposits: Taylor (1987);Blackdome: Vivian et al. (1987); Cinola: B. Taylor and A. Christie (unpub.data, 1991); Dusty Mac: Zhang et al. (1989); Mallery Lake: Turner et al.(2001); Sulphurets: Margolis (1993), and Mt. Skukum: McDonald (1987)and B. Taylor (unpub. data, 1991);. Sources for non-Canadian deposits: [H]Hishikari, Japan: Faure et al. (2002); McLaughlin, California: Sherlock etal. (1995); [N] Nansatsu, Japan (two fluid compositions identified on plau-sible mixing line): Hedenquist et al. (1994); [PL-EI] Pascua-Lama - El

Indio, Chile (data for Pascua-Lama): Deyell et al. (2005); Summitville,Colorado: Bethke et al. (2005); [W] Waihi, New Zealand: Braithwaite andFaure (2002); others, including Creede, Colorado [C], Finlandi vein Colqui,Peru [F: Au- and S-rich stages] as cited in Taylor (1987). Mixing withmeteoric waters that evolved by water/rock reaction is indicated by theSummitville field of waters. Mixing of magmatic fluids and local meteoricwater in the Nansatsu deposit, Japan [N] is shown by the straight dashedmixing line.


15/27


12

Geological Properties

Continental Scale

Exploitation of deposits in active volcanic systems (e.g.Ladolam, Lihir Island, Papua New Guinea: Mller et al.,2002a,b; Carman, 2003), the lure of shallow, easily extractedhigh-grade deposits, and even the sensational exposure ofassay sample adulteration at the Busang epithermal Au

prospect, Borneo (e.g. Hutchinson, 1997), have resulted inincreased general public awareness of epithermal depositsand mining activities. Recent increase in scientific/technicaldocumentation of deposits, review papers, exploitation ofgeothermal systems, and laboratory/theoretical studies havehelped to clarify geological settings, epithermal Au depositcharacteristics worldwide, and clarified processes of trans-

port and precipitation of Au. Large deposits appear to requirea sustained (magmatic) heat source, and efficient, localized

processes (e.g. cooling, degassing/boiling, fluid mixing, andwall-rock reaction) leading to supersaturation and precipita-tion of ore minerals. Whether an Au-rich source, especiallyefficient Au precipitation, or a particular setting or climaticinfluence is necessary to produce very large deposits remains

unanswered (cf. Sillitoe, 1992).Epithermal Au deposits may be found in association with

volcanic activity in numerous tectonic settings, includingisland-arc volcanoes (e.g. Papua New Guinea: Sillitoe,1989), and continental-based arcs and volcanic centres (e.g.Silverton caldera, Colorado). The shallow formation ofepithermal Au deposits suggests a higher probability of ero-sion, especially the high-sulphidation deposits that fre-quently occur in active arc environments. Accordingly,epithermal Au deposits, especially in volcanic terranes, arecommonly Tertiary in age, although numerous examples arealso known of pre-Tertiary deposits, including the LowerPaleozoic high-sulphidation subtype Gidginbung Au deposit,Lachlan fold belt, New South Wales, Australia (Lindhorst

and Cook, 1990). Early Devonian hot spring sinter depositsin Scotland (Nicholson, 1989), and other examples ofPaleozoic age are known in Australia, from Queensland(Wood et al., 1990) and the Pilabara Craton (Huston et al.,2002). Even much older, Late Proterozoic epithermal Audeposits are also known: the Hope Brook mine,

Newfoundland, (Dub et al., 1998) and the Mahd adhDhahab deposit, Arabian Shield (Huckerby et al., 1983).Some deposits of even greater antiquity have survived ero-sion, deformation, and metamorphism (e.g. ProterozoicMallery Lake deposit, Nunavut: Turner et al., 2001, 2003),whereas many others were subsequently metamorphosedand deformed (e.g. Paleoproterozic Ensen deposit, Sweden:Hallberg, 1994), which inhibits recognition of their epither-

mal (especially low-sulphidation) origins.

The tectonic setting of epithermal Au deposits is charac-terized by extension, at least at the district scale or larger,localizing and facilitating emplacement of magma and, athigher levels, hydrothermal fluids. Regional strike-slip faultsystems may bind rhomb-shaped extensional zones or pull-apart basins. Fault jogs or transitions from one fault toanother create local environments of extension (see alsoGoldfarb et al., 2004, for similar control on location of intru-sion-related Donlin Creek deposit, Alaska). Regionallyextensive rift zones can also provide the extensional frame-

work (e.g. Northern Nevada rift: John et al., 2003). Thedeposits of the Toodoggone River area, British Columbia, forexample, are thought to have formed in an elongate, tectoni-cally controlled graben in the medial portion of an island arc(Diakow et al., 1991, 1993). The preservation of these EarlyJurassic epithermal deposits may have to do with the fact thatthe Toodoggone River area was one of active deposition ofyounger rocks, rather than one of constructional volcanism

and uplift, in a climate providing high erosion rates such asfound today in Melanesia (e.g. Chivas et al., 1984).

Small volcanic- and volcaniclastic-hosted deposits inCanada are also found in other structural-tectonic settings ofa more local nature. These include the Dusty Mac deposit,British Columbia (e.g. Church, 1973; Zhang et al., 1989,Table 15.1-1), located in breccia and stockwork zones alongreverse faults at the margin of the White Lake basin. Gold-mineralized zones of silicification and argillic alterationalong faults in the Tintina Trench with Eocene rhyoliticdyke, are characterized by superimposed steam-heated orsupergene high-sulphidation alteration mineral assemblages(cf. Duke and Godwin, 1986). Sediment-hosted Au (Ag)deposits occur in a variety of settings in which sedimentarysequences have been intruded by magmas and also in sedi-mentary rocks not obviously closely associated with intru-sions. In some cases, the deposits are located in the outerzones of paleo-hydrothermal systems associated with intru-sions (e.g. Cinola; Equity Silver, British Columbia).

District Scale

Epithermal Au deposits are, in many cases, structurallycontrolled; the same features that served as the conduits forhydrothermal fluids may have facilitated processes leadingto Au deposition (e.g. rapid cooling, boiling, fluid mixing,water-rock reaction, decompression, to name a few). Thedeposits may be of similar age to their host rocks where

these are volcanic, or they may be much younger. A mag-matic heat source is commonly inferred. The deposits com-prise veins and/or related mineralized breccia and wall rock(e.g. Mt. Skukum), or replacement bodies associated withzones of silicification (e.g. Cinola). Principal geological andother characteristics of each subtype of epithermal Au (Ag)deposit are listed in Table 2 (see Table 1 for data on indivi-dual examples). Both high-sulphidation and low-sulphida-tion deposit subtypes (distinguished by alteration character-istics) share many features in common. Modern geothermalsystems have many features in common with epithermaldeposits.

Caldera ring fractures (e.g. Summitville, Colorado:Lipman, 1975), radial fractures (e.g. Lake City, Colorado:

Slack, 1980), extensional faulting due to tension aboveresurgent domes (e.g. Creede, Colorado) may createfavourable vein-hosting environments in volcanic terranes.Extensional, pull-apart basins formed between regionalstrike-slip faults, or at transitions between these faults, pro-vide favourable sites for intrusions and epithermal deposits.

Northeast-trending, regional Eocene strike-slip faulting wasrelated to extensional synvolcanic faults at Blackdome,British Columbia, for example, that controlled the emplace-ment of dykes and Au- bearing quartz veins (Fig. 4).Synchronous tectonic and hydrothermal activity is indicatedin some deposits by the fact that many of the vein-bearing


16/27

faults were active during and after vein filling (e.g.Blackdome, Mt. Skukum, and Toodoggone River deposits);tectonic vein breccias and displaced mineralized and alteredrocks resulted. Similar orientations of normal faults south-east of the Blackdome area and east of the Fraser River-Straight Creek fault have been attributed to northwest exten-sion along the Yalakom fault (e.g. Ewing, 1980; Colemanand Parrish, 1990).

Knowledge Gaps

Tertiary terrane was once thought to be virtually the onlyfertile ground for the occurrence of epithermal Au deposits.And, certainly, a greater number of important epithermaldeposits are known in association with young centres ofmagmatic activity. Thus, the focus of much of the explo-ration in Canada for epithermal deposits has been in theCordillera. Within the last twenty-odd years, however, anincreasing number of epithermal Au deposits have been rec-ognized in pre-Tertiary terranes. The apparent metamorphicstability of alunite, recognition of abundant aluminosilicateminerals as potential indicators of pre-metamorphic argillicalteration, and the association of zones of replacement by

massive quartz have led to the recognition of high-sulphida-tion subtype deposits in older terrane, even when the depositshave been extensively deformed (e.g. Hope Brook: Dub etal., 1998). These discoveries emphasize the need to recog-nize the preservation of near-surface crust in ancient ter-ranes, and to better understand the tectonic environments andconditions that hold higher potential for such preservation.

Low-sulphidation subtype epithermal Au deposits areharder to recognize in ancient terranes, owing to the factsthat their commonly found alteration mineral assemblagesare not unique, especially in regional metamorphic terranes,or may no longer be present, depending on the grade of sub-sequent metamorphism, and that these deposits are often not

as intimately associated with igneous rocks as is the ten-dency for the high-sulphidation subtype deposits. In thiscase, oxygen isotope techniques can be used to support geo-logical evidence for an epithermal environment by providinga measurable, unique, and robust criterion of near-surfaceorigin of the paleo-geothermal system. Moreover, the distri-

bution of the geothermal system can potentially be mappedusing oxygen isotope data, even in deformed rocks. A widerapplication of this approach could enhance recognition of

potentially fertile terrane.

Modern geothermal systems, both subaerial and subma-rine, commonly associated with centres of active volcanism

provide excellent analogs to mineralized epithermal systems(e.g. Cooke and Simmons, 2000). These systems, as well as

epithermal districts themselves, should be examined toestablish to what extent high-sulphidation and low-sulphida-tion subtype deposits represent a spectrum of characteristics.The essential contribution of magmatic volatiles to formhigh-temperature, high-sulphidation alteration, and simplythe need of a viable heat source to sustain low-sulphidationsystems may explain the apparent lack of a common overlapin space and time.

Where do intermediate sulphidation systems fit in timeand space relative to low- and high-sulphidation with respectto the development and evolution of high-level magmatism

and their related geothermal systems? The spatial, geochem-ical, and chronological links need to be strengthened

between deposit subtypes in the classic epithermal Au envi-ronment. Notwithstanding the utility of general relationshipsdepicted by schematic cross sections in common usage (e.g.Fig. 15-2 in Poulsen, 1996), a stronger understanding of anysuch connections between these environments could facili-tate the task of the explorationist.

Comparative studies of both poorly and highly mineral-ized hydrothermal systems need to be undertaken in order tounderstand and better define characteristics or specific geo-logic features of a regional nature (e.g. magmatic-hydrothermal evolution with respect to tectonic setting andclimate) as possible predictors of better mineralized terrane.

Deposit Scale

The geological settings of low-, intermediate- and high-sulphidation subtype epithermal deposits are summarized forcomparison in Table 2. With respect to a high-level mag-matic intrusive centre, the geological properties of thesedeposit subtypes are broadly those of a distal versus prox-imal settings, but in both time (relative to magmaticemplacement and active versus passive degassing; e.g.,Taylor 1987, 1992). These environments and selected geo-logical properties are illustrated schematically in Figure 8.

The locations of epithermal Au deposits are typicallydetermined by those features that define the hydrothermalsystem plumbing, i.e., provide the hydrological control andcontrol on magmatic emplacement (e.g. structural controlson fluid flow and magmatic emplacement; topographical/

paleosurface control of hydrology, boiling elevation,hydrothermal eruption). Extensional faults are especiallyimportant, whether due to local, volcanic-related features(e.g. resurgent doming: Creede, Colorado), or to regionaltectonism (e.g. rifting zones, or pull-apart basins associated

with strike-slip faults: Mt. Skukum, Yukon: Love, 1989;Love et al., 1998; Blackdome, British Columbia: Colemanand Parrish, 1990; R.R. Parrish, pers. comm., 1991; ElPeon, Chile: Arancibia et al., 2006). Fault intersections andfault plane inflections provide zones for vein thickening andzones of brecciation during synchronous movement and veingrowth.

High-sulphidation deposits are typically associated withandesitic to rhyolitic rocks and with geologic features asso-ciated with sites of active volcanic venting and doming,including among others ring fractures, caldera fill breccias,hot springs, and acidic crater lakes. It is the dominance ofdirectly derived or evolved magmatic fluids that buffer thehydrothermal fluids to low pH and result in the distinct char-

acter of the high-sulphidation subtype. Orebodies primarilyconsist of zones of silica-rich replacement. Bodies of mas-sive vuggy silica and marked advanced argillic alterationmineral assemblages are typical.

Low-sulphidation deposits that occur further removedfrom active magmatic vents may be more apparently con-trolled by structural components, zones of fluid mixing, andemplacement of smaller magmatic bodies (e.g. dykes).Meteoric waters dominate the hydrothermal systems, whichare more nearly pH neutral in character. Low-sulphidationrelated geothermal systems are more closely linked to pas-

B.E. Taylor

128


17/27


12

sive rather than to active magmatic degassing (if at all), andsustained by the energy provided by cooling, subvolcanicintrusions or deeper subvolcanic magma chambers.

Some deposits with mostly low-sulphidation characteris-tics with respect to their alteration mineral assemblages havesulphide ore mineral assemblages that represent a sulphida-tion state between that of high-sulphidation and low-sulphi-dation deposits. Such deposits tend to be more closely spa-tially associated with intrusions, and Hedenquist et al. (2000)suggest the term intermediate sulphidation for thesedeposits.

Zones of boiling, as indicated by mineral textures (bladedcarbonate, rhombohedral adularia), are potential sites of Audeposition, especially in low-sulphidation subtype deposits,

and that may be related to (and predicted from) paleo-topog-raphy. A stationary zone of boiling increases the potential fora high-grade deposit. Similarly, structural control may influ-ence sites of fluid mixing, which can also lead to metal pre-cipitation.

Distribution of Canadian Epithermal Districts

Epithermal, deep epithermal/transitional, or intrusion-related deposits in Canada, as illustrated in Figure 1, are pri-marily found in the Cordillera in close association with cen-tres of magmatism. Although dominantly young (Tertiary) as

a deposit class, older epithermal Au deposits also occur inCanada (Fig. 1, Table 1), or may be suspected in older (evenmetamorphosed) terranes based on evidence of epithermalalteration. Examples include the low- and high-sulphidationtype, Jurassic epithermal deposits of the Toodoggone Riverarea, British Columbia, the low-sulphidation ProterozoicMallery Lake deposit, Nunavut, and the high-sulphidationHope Brook deposit, Newfoundland. The Avalonian terraneis notably prospective for both high- and low-sulphidationdeposits from the abundant evidence of advanced argillicalteration (e.g. Mills et al., 1999; OBrien et al., 1999, 2001),and this type of evidence suggests Ordovician terrane in

Newfoundland may also be prospective (e.g. ODriscoll andWilton, 2005). Whereas advanced argillic alteration can be

recognized in metamorphosed terranes, recognition of theorigins of sericitic and argillic alteration that formed in high-and low-sulphidation hydrothermal systems may be prob-lematic in metamorphosed terrane on the basis of texturaland mineralogical grounds alone. Yet, the existence ofancient unmetamorphosed examples (e.g. Mallery Lake,

Nunavut), plus occurrences of deeper, transitional or intru-sion-related deposits (Fig. 1), suggest that older depositsremain to be discovered. Exploration of preserved continen-tal volcanic centres and associated epithermal and transi-tional or intrusion-related deposits in rocks at least as old asthe Late Proterozoic should be considered.

GSC

CONTINENTALISLAND ARC

Meteoric water

Hot SpringSteaming

ground

B

EH

G

D

A

Water table Seawater

Acid crater lake

Dilutegroundwaters

Dilutegroundwaters

Boiling duringupflow

200o

200o

300o200o

250o

300o

250o

1

0km

HCO /SO- --

3 4

3

2

2

increasing

2

Liquid +CO -richvapour

Liquid +vapour

Nonvolcanicdegassing viacracking front

Reducedneutral chloride

waters T > 200C

Nonvolcanic degassingof rhyolitic magma via

cracking front

3

2

2

C

SO >H S,

CO , HCl

Vuggy Silicazone

F

2 2

2

Volcanicdegassing

FIGURE 8. Schematic cross-section illustrating the general geological and hydrological settings of quartz-(kaolinite)-alunite and adularia-sericite deposits(from Taylor, 1996; partially adapted from Henley and Ellis, 1983, and Rye et al., 1992). Characteristics shown evolve with time; all features illustrated arenot implied to be synchronous. Interpreted settings are indicated for several Canadian deposits discussed in the text; see also Table 1. Local environmentsand examples of low-sulphidation deposits include: (A) basin margin faults: Dusty Mac; (B) disseminated ore in sedimentary rocks: Cinola; (C) veins indegassing, CO2-rich, low sulphide content, low-sulphidation systems: Blackdome, Mt. Skukum; (E) porphyry-associated vein-stockwork, sulphide-rich(intermediate sulphidation) and sulphide-poor stages: Silbak-Premier; and (H) disseminated replacement associated with porphyry-type and stockworkdeposits, involving seawater: Sulphurets. Examples of high-sulphidation environments include: (D and G) steam-heated advanced argillic alteration (quartz-kaolinite-alunite) zone: Toodoggone River district , British Columbia; (F) magmatic-hydrothermal, high-sulphidation vuggy quartz zone ( aluminosilicates,corundum, alunite) Summitville, Colorado, or Nansatsu district, Japan. Fluid flow parallels isotherms. Upflow zones are shown schematically by arrowhead-shaped isotherms. Volcanic degassing refers to magmatic degassing driven by depressurization during emplacement (first boiling). Nonvolcanic degassingrefers to vapour exsolution during crystallization (second boiling). The SO2 disproportionates to H2S and H2S04 during ascent beneath environment (F).

Note that free circulation occurs only in crust above about 400C. All shown temperatures are in Celsius degrees.


18/27

Genetic and Exploration Models

Stable isotope and fluid inclusion studies have contributedsignificantly to our knowledge of the origins, pressures, tem-

peratures, and chemical compositions of hydrothermal fluidsresponsible for epithermal Au deposits. Studies of moderngeothermal systems, hot springs, and volcanic gases havegreatly increased our understanding of epithermal deposits

because active geothermal systems offer modern-dayanalogs for physical and chemical parameters that can bedirectly measured, and compared to inferences made fromancient water-rock interaction and alteration zones(Hedenquist et al., 2000). For example, comparisons

between active, steam-dominated geothermal systems likethe Geysers-Clear Lake system, California, and hot-springdeposits such as Cinola, British Columbia or theMcLaughlin deposit, California (Sherlock, 2005) aid theinterpretation of features of hot-spring deposits in the geo-logic record, and add quantitative constraints on parametersof formation, metal segregation, and concentration.

Determination of mineral solubility, metal volatility andtransport, and phase relations, as well as numerical water-

rock reaction simulations, have contributed to our quantiza-tion of the chemical and physical nature of mineralizinghydrothermal fluids, and also to our understanding of the

processes that lead to the transport and deposition of Au, Ag,and base metals.

Lindgren (1922, 1933) suggested that degassing magmasare sources of many ore-forming constituents in epithermalAu deposits, and this supposition appears to be essentiallycorrect for magmatic-hydrothermal high-sulphidation depos-its (Stoffregen, 1987; Rye et al., 1992). However, for manydeposits (e.g. the majority of low-sulphidation subtypes) O-and H-isotope data permit only a very small fraction (i.e.


19/27


13

alization: low to very low pH, oxidized fluids (high-sulphi-dation subtype) and near neutral, more reduced fluids (low-and intermediate-sulphidation subtypes). These two environ-ments are contrasted in Figure 9, which also represents sta-

bility fields for selected mineral and isopleths of Au solubil-ity (after Giggenbach, 1992). Field (G) in Figure 9 illustratesthe temperature and oxidation state of the meteoric geother-mal fluids discharged from the Geysers geothermal field,

California (Lowenstern and Janik, 2003) as an example oftypical low-sulphidation systems. Such fluids may furtherevolve by water/rock reaction or by mixing with fluidschemically influenced by water/rock interaction (e.g. Mt.Skukum, field SK, in Fig. 9). Magmatic fluids of higher tem-

perature and a relatively more oxidized (i.e. more negativeRH) nature dominate hydrothermal systems hosting high-sul-

phidation subtype deposits. These fluids may mix with sur-face waters and/or with geothermal waters similar to thosefrom the Geysers, as shown by the field for Summitville flu-ids (field S in Fig. 9).

The upwardly welling, highly acidic, magmatic-hydrothermal plume may produce a high-sulphidation min-eralization event that is likely to be short-lived, limited bythe shallow degassing of the magma in response to depres-surization during its ascent (so-called first boiling), and bythe eventual neutralization of the fluids due to reaction withwall rocks and/or dilution by meteoric fluids. In contrast,meteoric fluids heated by cooling magmatic rocks can pro-vide potential fluids for mineralization and alteration oversomewhat longer periods of time, and at sites furtherremoved from the magmatic heat source. With time, themeteoric water dominated environment may encroach uponthe earlier, hotter, hydrothermal-magmatic environment.

Active geothermal systems provide instructive analoguesto low-sulphidation hydrothermal systems. Geochemicalstudies of dominantly volcanic-hosted geothermal systems

in the Taupo Volcanic Zone, New Zealand (see Henley andHedenquist, 1986) have demonstrated the existence of twoprincipal types of fluids: (1) a deep chloride water, generally200 to about 300C, and (2) a shallower, less than 100 to200C steam-heated, low-chlorinity, acidic water. The inter-face between waters of markedly different salinity has beendescribed in the Salton Sea geothermal system by Williamsand McKibben (1989). These deep chloride waters producelow-sulphidation subtype alteration (e.g. Henley, 1985), andwhere they are rapidly depressurized, degas CO2 and H2S,cool, and precipitate precious and base metals (Clark andWilliams-Jones, 1990). The well scales studied by Clark andWilliams-Jones (1990) revealed a vertical separation of pre-cious metals (higher) and base metals (lower) analogous to

that described by Ewers and Keays (1977) for theBroadlands geothermal field (New Zealand), and byBuchanan (1981) for a number of deposits.

Sub-millimetre-scale variations in 18O, as much as 6 permil in vein quartz from Hishikari at times of Au precipita-tion, indicate that intermittent vein opening permitted intro-duction of deep, metal-bearing fluids to the veins. The deep-sourced fluids mixed with meteoric water, boiled (indicated

by bladed quartz), cooled, and precipitated Au (Hayashi, etal., 2001). The bladed quartz analyzed by Hayashi, et al.(2001) formed subsequent to initial boiling by replacement

of calcite. Any inheritance of18O from the isotopically heav-ier calcite would indicate a lower estimate for the 18O of thedeeper, evolved fluid. Nevertheless, the mixing of metallif-

ROCK BUFFER

GASBUF

FER

-3

-4

-5

-6

-7

100 200 300 400

GSC

STM

Temperature (C)

RH

HSO4

-

H SO32

H S2

al

SP

SK

ES

po

popy

py

(FeO)

1.5(FeO )

pH= 6

a = 1S

pH= 6

a= 1

S

INTERMEDIATE

SULFIDATION

HIGHSULPHID

ATION

anhy

0.0001

Au(

g/kg)

0.01

11

00

tnen

cc

mt

hm

py

anhy

pH = 3

py

al

HS

2

H

SO

2

3

HSO

4-

HSO

4 -

0.0001

0.01

1

100

100

0.01

0.0001

1

cpy

bnAuCl 2

2

HAu

(SH)

S

CL

B

4

G

LOW

SULFIDATION

FIGURE 9. Diagram of redox potential (RH = log fH2/fH2O) versus temper-ature (from Taylor, 1996; modified after Giggenbach, 1992; Hedenquist etal., 1994). Calculated isopleths of Au in g/kg solubility (as the dissolvedspecies HAu(SHh; (Giggenbach, 1992) are shown in red. An equimolar iso-

pleth for HAu(SH)2 and AuCl2 (after Hedenquist et al., 1994) is shown forpH=3 and 1.0 wt.% Cl (or pH=5 and 10 wt.% Cl). The thiogold complexHAu(SH)2probably dominates as the Au-transporting agent in much of the

epithermal environment at pH


20/27

erous and dilute fluids and consequent metal precipitation(via cooling, dilution, and oxidation) are clearly demon-strated.

Simple, conductive cooling of Au-bearing fluids is suffi-cient to cause Au precipitation (see Fig. 9). Boiling can alsocause cooling, chemical fractionation, and an increase in pHassociated with acid vapour loss that leads to saturation and

precipitation of chloride-complexed metals (e.g. Cu, Pb, Zn:

Drummond and Ohmoto, 1985; Spycher and Reed, 1989;Williams-Jones and Heinrich, 2005). Also, degassing of ini-tially CO2-rich fluids in gas-rich systems depletes the liquidin H2S that is carried off in a CO2-rich vapour. The loss ofH2S eventually leads to precipitation of sulphide-complexedmetals (e.g. Au; Drummond and Ohmoto, 1985; Henley,1985; Hayashi and Ohmoto, 1991). Carbon dioxide andhydrogen sulphide are well correlated in some geothermalfluids (Fig. 11 in Taylor, 1987). Boiling and chemical frac-tionation of the hydrothermal fluid provides an explanationfor the separation of precious and base metals. This separa-tion results in a vertical zoning where fluids are upwardlyflowing (Clark and Williams-Jones, 1990), or in relativetemporal stages, such as at Silbak-Premier, BritishColumbia, and EI Indio, Chile. As a corollary, larger veindeposits require the movement of larger amounts of fluidthrough localized zones of boiling, and thus the importanceof structural analysis in exploration is obvious.

Neutralization and cooling of ore fluids may also occur (1)by mixing with dilute groundwaters, and (2) by water-rockreaction (e.g. sulphidation of ferrous iron-bearing minerals),especially during formation of disseminated and replace-ment-type orebodies.

Steam-heated acid waters formed by the oxidation andcondensation of H2S (boiled off deeper geothermal reser-voirs) in groundwater produce high-sulphidation subtypealteration of the volcanic rocks (Henley and Hedenquist,

1986). The Champagne Pool, in the CO2-rich Waiotapugeothermal field (steam-heated, high-sulphidation subtypealteration), New Zealand, is a hydrothermal eruption feature

below which Au and Ag are being deposited in response toboiling and loss of H2S over the approximate temperatureinterval 250 to 175C (Hedenquist, 1986). Ore-grade, Au-

bearing amorphous sulphides precipitate in the pool at 75C,and base metal sulphides occur below the zone of boiling.Acidic waters produce advanced argillic alteration and, withvariation in PCO2, evolve to cause the replacement of adu-laria and albite by sericite. Thus, by chemical evolution, ageothermal field, initially boiling and producing high-sul-

phidation subtype alteration, may eventually produce miner-als characteristic of low-sulphidation subtype alteration.

The precious metal content of steam-heated alterationzones may also be related to the rate of fluid ascent versusthe extent of boiling and H2S loss: faster moving fluidsand/or those less depleted in H2S may produce higher gradesof precious metals in steam-heated alteration zones. Thismight apply to the ascension of boiling magmatic hydrother-mal plumes as well as to boiling meteoric and marine geot-hermal fluids.

Exploration for epithermal Au deposits entails, for a com-prehensive approach, judicial application of methodologiesto assess the geological characteristics (e.g. tectonic/struc-

tural setting, petrological association, mode of occurrence,geochronology), mineralogical and geochemical characteris-tics (mineral assemblage, mineral/rock chemical composi-tions, isotopic composition, exploration geochemical tech-niques), and geophysical characteristics (e.g. electrical andmagnetic properties). The results of the application of thesetechniques are compared to one or more models that repre-sent empirically determined associations of characteristics.

Hedenquist et al. (2000) is a useful and comprehensive ref-erence in this regard.

Assessment of Geological Characteristics

Volcanic arcs and belts with abundant intermediate to fel-sic rocks and associated rift systems host epithermal Audeposits of many ages. Evidence of high-level magmatism inmore deeply eroded terrane may still offer possibilities fortransitional or intrusions-related deposits. Geologic map-

ping, including alteration of mineral assemblages, and atten-tion to structural control(s) provides a fundamental means ofassessment.

Assessment of Mineralogical and Geochemical

CharacteristicsMapping and recognition of alteration mineral assem-

blages are reasonably straightforward in unmetamorphosedterrane. New instrumental technologies, such as Short-WaveInfrared Spectroscopy (SWIR; e.g., PIMA, PortableInfrared Mineral Analyzer; Ducart et al., 2006), in additionto portable (to field offices) XRF analyzers and X-Ray dif-fractometers, are finding increased application for miner-alogical and elemental identification in the field. Indeformed and metamorphosed terranes, however, interpre-tive mineralogical or alteration mapping may be problem-atic. In particular, distinction of high-sulphidation alterationformed in steam-heated zones (which may form above eitherhigh- or low-sulphidation systems), from high-temperaturealteration may affect interpretation of the deposit subtypeand exploration strategy. The nature and origin of highly sili-cic zones should also be determined, particularly indeformed terranes where the usual textural criteria may nolonger be applied. Oxygen isotope mapping, using whole-rock analysis, can be used to map paleo-hydrothermal sys-tems, even in highly metamorphosed and deformed terranes.In particular, oxygen isotope techniques can be especiallyuseful to decipher the origins of chlorite-sericite-bearingmineral assemblages, and assist in interpreting the origins(e.g. residual, vuggy silica zone from silicic zones from near-surface, lower temperature silicification).

Two applications of oxygen isotope techniques for the

exploration of epithermal Au deposits are shown in Figures10 and 11 representing examples of high-sulphidation andlow-sulphidation systems, respectively. The Pilot Mountainarea in the Carolina Slate Belt contains a number of previ-ously mined high-sulphidation epithermal Au deposits. Thegreenschist-metamorphosed terrane bears mineralogical evi-dence of argillic and advanced argillic alteration shown onthe basis of oxygen isotope characteristics (Klein and Criss,1988) to have formed in a meteoric-water recharged, high-sulphidation system. Isotopic zoning above the associatedhigh-level stock documents upflow of magmatic-hydrother-mal fluid that was most intense in the area of vuggy silica

B.E. Taylor

132


21/27


13

alteration and Au mineralization (Fig. 10). The isotopically

mappable affects of related meteoric hydrothermal alterationcover at least 30 km2, well beyond definitive mineralogicalzones, and distinguish this area from other, non-mineralizedsites of plutonism in the slate belt (Klein and Criss, 1988).The application of this technique to mapping meteoric high-sulphidation systems in undeformed terrane is straightfor-ward (e.g. Tonopah, Nevada: Taylor, 1973), whereas thestudy at Pilot Mountain demonstrates the potential of thistechnique for exploration in older terranes.

Oxygen isotope zoning about veins in the low-sulphida-tion epithermal Au deposit at Hishikari, Japan (Fig. 11)shows definitive effects of hydrothermal water/rock interac-tion in a surface zone as much as 200 m or more in lengthabove blind vein deposits. Whereas clay mineral alteration

can also be recognized and mapped at the surface of thisyoung unmetamorphosed deposit (Izawa et al., 1990),whole-rock oxygen isotope anomalies as telltale indicatorsof epithermal fluid flow can survive even high-graderegional metamorphism.

Soil and rock geochemical analyses may prove fruitful.The Pilot Mountain, North Carolina, district (Keith andCriss, 1988; see also caption to Fig. 10) provides a particu-larly good example, in a greenschist-facies metamorphic ter-rane, of the correspondence between mineralogical alter-ation, oxygen isotope zoning, and geochemical soil anom-

alies. In Canada, geochemical mapping of the typical

epithermal pathfinder elements (Hg, Sb, As, Tl, in additionto Au and Ag), plus intrusion-related elements (e.g. Mo,Cu, Sn, B) may also be tested in both soil and till, as well asrocks. Aluminosilicates, corundum, sulphides, specularhematite, and alunite may, among other minerals, also proveuseful in till analysis.

Assessment of Geophysical Parameters

In contrast to their application in the exploration of othertypes of ore deposits, geophysical techniques have been lessuseful in the discovery of epithermal deposits (Sillitoe, 1995;Hedenquist et al., 2000). Except for the use of aeromagneticsurveys as a very powerful aid in regional geologic mapping,the application of other geophysical techniques for epither-

mal Au deposits in Canada appears less fruitful.

Knowledge Gaps

Upon comparison of many features, both regional andlocal, of 16 bonanza (>30 tonnes Au) and giant (>200 tonnesAu) epithermal Au deposits, Sillitoe (1992) concluded that,although complex arc environments and unusual igneousrock types seemed more prospective, no single feature could

be isolated as an apparent cause or explanation. Either anunusually rich source of Au or an unusually effective depo-sitional process was necessary to effect such concentrations

N

3

32

4

4

5

6

4

5

Volcanic and VolcaniclasticRocks

Intrusive Rocks

Alteration

Felsic rocks(Uwharrie Fm.)

Arenite and argillite

Andesite

Dacite porphyry

Quartz granofels

Chlorite-sericite

Quartz-sericite

Au prospect

Abandoned Au mine

Quartz-monzonite

Quartz-pyrophyllite-andalusite

km 10

North Carolina

Pilot MountainKlein and Criss (1988)

35

40

79 42 30

FIGURE 10. A 18OWHOLE-ROCKisopleth map of the high-sulphidation epithermal Au district of Pilot Mountain in the Carolina-Avalon slate belt, RandolphCounty, North Carolina (after Klein and Criss, 1988). Klein and Criss (1988) infer the greenschist metamorphosed terrane to be westward tilted, exposing avertical section through part of a high-level quartz monzonite stock, dacite porphyry, and argillic and advanced argillic alteration zones at the apex of thestock. Areas mapped as quartz granofels are interpreted to represent a metamorphosed vuggy silica zone associated with Au (note greater number of Aumines). Alteration and mineralization at Pilot Mountain are analogous to that at the Hope Brook mine, Newfoundland (Dub et al., 1998), and similar to areasof high-sulphidation alteration of broadly similar age in the Burin and Avalon Peninsulas, Newfoundland (see discussion in text). Fifty-three samples, virtu-ally all of which yielded low 18OWHOLE-ROCK values, record a near-surface, meteoric water-recharged geothermal system over an area of more than30 km2 (larger than the 4 x 6 km area shown). The relative increase in 18OWHOLE-ROCK in altered rocks above the stock probably reflects the upflow ofisotopically heavier, magmatic-hydrothermal fluids. Mapped anomalies of Au, Mo, Sn, Cu, and B in soil samples agree well with isotopically mapped zonesof magmatic-hydrothermal influence (Klein and Criss, 1988). Continued hydrothermal upflow is indicated by the northwest-trending zone of chlorite-sericite;a higher density of samples would permit a more detailed isotopic definition of hydrothermal flow patterns.


22/27

of Au. This chicken or egg conclusion remains as a princi-pal enigma, a key question in the knowledge gap. A completespectrum in Au endowments in epithermal deposits world-wide might be expected if Au endowment depended only onthe Au content of the source materials. Sillitoe (1992)

emphasized the necessity for the geologically unexpectedin the environments of these rich deposits and the likelihoodthat that this factor resulted in usually effective Au precipi-tation. Despite many detailed studies since Sillitoes paper(1992), universal agreement on this central question appearsto elude.

A firmer understanding of links between porphyries andepithermal systems is evolving, and an understanding of the

temporal differences in magmatic and hydrothermal evolu-tion that explains the lack of direct linkages (e.g. low-sul-

phidation and porphyry Cu-Au deposits). Efficient trappingof hydrothermally transported Au is certainly required for aneconomic deposit, and processes such as mixing, boiling,cooling, and oxidation are known to have occurred at thetime of gold precipitation (e.g. Hayashi et al., 2001). As acorollary, further studies of the processes of magmaticdegassing (both active and passive), associated metal migra-tion, and the influence of the oxidation state of the magmaon metal availability and migration would seem to be help-ful.

A sufficient number of ancient epithermal Au deposits,

both low- and high-sulphidation subtypes, are now known toraise the level of understanding needed regarding the likeli-hood of preservation and rates of destruction of the epither-mal regime of the crust. Clearly very old examples have sur-vived.

Acknowledgements

This chapter was, in part, extracted and updated fromTaylor (1996). Lillian Munro assisted greatly by data, map,and literature compilation. Benoit Dub, Wayne Goodfel

NRCAN-Synthesis_ Taylor_EpithermalAu

Documents

Transcript of NRCAN-Synthesis_ Taylor_EpithermalAu