Archean lode-gold deposits: fluid flow and chemical evolution in vertically extensive hydrothermal systems

ORE GEOLO(,Y RE\qEWS ELSEVIER

Ore Geology Reviews 10 (1996) 279-293

Archean lode-gold deposits" fluid flow and chemical evolution in vertically extensive hydrothermal systems J. Ridley l, E.J. Mikucki, D.I. Groves Key Centre for Strategic" Mineral Deposits, Department of Geology and Geophysics, The Unit,ersity of Western Australia, Nedlands, WA 6907, Australia

Received 11 May 1995; accepted 8 August 1995

Abstract The range of conditions of formation of lode-gold deposits from the sub-greenschist to the lower-granulite facies in Archean greenstone belts, and the generally steeply plunging, vertically continuous pipe-like or tabular geometries of individual deposits, indicate long-distance hydrothermal fluid advection along well-defined channelways in the upper and middle crust. From presently available gold solubility data, destabilisation of gold-bisulphide complexes through H2S loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole. This inference is supported by the S:Au ratios of ores. Sulphur and Au precipitation in the hydrothermal system is estimated to be relatively inefficient, with only 10-50% of S or Au contained in the fluid precipitated over any kilometre length of fluid channelway. The relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile.

1. Introduction

Lode-gold deposits that formed essentially synmetamorphically and syn-tectonically, late in the tectonic history of an individual craton, occur in all rock types in and adjacent to Archean greenstone belts. The pressure-temperature conditions of mineralisation at individual deposits in the Yilgarn Block, Western Australia, are estimated to range from subgreenschist-facies conditions at 200-300°C at about 1 kbar, through mesothermal, or greenschist-facies conditions, to upper-amphibolite facies and lowergranulite facies conditions at 700°C at 5 kbar (Groves

i Present address: Institut •r Isotopengeologie und Mineralische Rohstoffe, ETH-Zentrum, CH-8092, Ziirich, Switzerland.

et al., 1992), indicating, therefore, deposit formation at crustal depths of 3 to 18 km. The deposits that formed at different depths and temperatures are considered genetically related through a common hydrothermal process because they share a hydrothermal fluid of a similar composition with respect to major molecular components (Hagemann and Ridley, 1993), major salts (Mikucki and Ridley, 1993) and isotope ratios (e.g., McNaughton et al., 1993), a similar ore geochemistry (Perring et al., 1991), and, at least for the majority of deposits, a similar timing of formation late within the late-Archean deformation history of the Yilgarn Block (Groves et al., 1992). Given the range of mineralising conditions and depths of formation, trace-element and isotope evidence that the ore fluids interacted with granitic

0169-1368/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0 169- 1368(95)00027-5

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J. Ridley et al. / Ore Geology Reuiews 10 (1996) 279-293

rocks at some stage in their evolution (e.g., Kerrich, 1989; McNaughton et al., 1992), and evidence for single-pass, upward advection of fluid (e.g., Ridley, 1993; and cf. Peters, 1993b), many recent workers have concluded that the deposits tbrmed by advection of ore fluids from a deep source, below the greenstone belts in which they are typically hosted. There are two implications of a deep fluid source: firstly, that the deposits are generally distal to the source, with fluid travel lengths from source to deposit of potentially greater than 10 km, and secondly, that ore deposition may have occurred over several kilometres vertical distance within any single hydrothermal system (e.g., Colvine et al., 1988; Foster, 1989). The "crustal continuum" model of Groves et al. (1992) implies further that gold deposition may take place along the extent of a fluid flow path through a greenstone belt. Whether gold can both be efficiently transported and be precipitated along the length of vertically extensive channelways are unexplored constraints on ore-genetic models that involve long distance transport of ore solutions and gold deposition over a range of depths. This paper, therefore, firstly considers the likely characteristics of gold transport and deposition at different levels in the crust, and, secondly, the efficiency of gold precipitation from the fluid at different depths in the crust. The exposure of deposits at different depths of erosion, and within individual deposits over significant vertical extents, allows the compositional evolution of the fluid to be traced with respect to parameters which may control gold transport and deposition. The discussion is not based on a specific deposit, but considers the general mineralogical and geometrical characteristics of deposits formed at different depths in the crust in the Yilgarn Block and elsewhere. The features of deposit geometry which constrain the geometry of fluid flow, and the geochemical, mineralogical and fluid-inclusion constraints on ore-fluid composition are reviewed first.

2. Geometry of lode-gold deposits: implications for patterns of hydrothermal fluid flow Recent publications have considered the structural controls on the geometry of lode-gold deposits, of

individual veins within deposits (Robert and Brown, 1986; Hodgson, 1989), and on the siting and orientation of ore shoots within deposits (Peters, 1993a, b). In this section, a review is given of the gross, three-dimensional geometries of deposits, and of the overall patterns of hydrothermal alteration at lodegold deposits, in order to define typical patterns of fluid flow within major gold-depositing hydrotherreal systems.

2.1. Gross deposit geometry In almost all deposits in which the extent of mineralisation has been well-defined by exploration and mapping, the ore bodies are vertically continuous, steeply plunging, and are tabular to pipe-like in shape. The known vertical depth extent of almost all larger deposits exceeds 1 km, and extends in the case of the Kolar deposits in India to more than 3 km (Hamilton and Hodgson, 1986). There is rarely an indication that mineralisation dies out at depth, though grade may fall, or lodes lose strike length. At many large deposits, the major part of the ore occurs within one or more well-defined tabular, or gradually tapering, ore shoots within a hosting structure. The lodes are typically moderately to steeply dipping, and ore shoots generally pitch steeply on the lode surface. At the Sons of Gwalia deposit, for example, the total strike length of the major ore shoots remains relatively constant at between 300 and 400 metres, down to at least 1 km depth (Kalnejais, 1988). At other deposits, e.g., Lancefield (Hronsky, 1993), and Big Bell (Chown et al., 1984), the strike length of the major ore shoot is mapped as shortening with depth, or splaying upwards, although it is difficult to determine whether this interpretation is influenced by restricted data coverage at greater depth, by economic considerations, or by subtle and gradual changes in the average grade of the shoot. At deposits in which individual ore shoots have a shallow plunge, there are multiple shoots on a single ore surface, or multiple, en-echelon overlapping shoots on parallel surfaces, and it is generally possible to define an ore-body envelope which plunges steeper than the individual shoots (e.g., Mararoa-Crown, Norseman in Campbell, 1990). Deposits formed of strata-bound stockworks or vein arrays, of which Mt Charlotte is the best de-

J. Ridley et al. / Ore Geology Reviews 10 (1996) 279-293

scribed example (Clout et al., 1990), have similar gross shapes. At Mt Charlotte, stockworks of relatively thin quartz veins of two orientations are developed within approximately 200 m strike lengths of the 40-50 m wide granophyric unit 8 of the Golden Mile Dolerite. Ore bodies occur where the spacing of quartz veins is close enough so that an effectively continuous zone of mineralisation is developed and form well-defined pipes with steep (70 °) plunges. In the giant Golden Mile deposit at Kalgoorlie, multiple lodes of various orientations form a network of mineralised structures with intervening weakly or unmineralised rock (e.g., Clout et al., 1990). Despite the geometric complexity of the deposit, the enveloping surface of the ore body is a pipe or upward-opening funnel: in plane view, the ore body extends over about 4 km 2 at the present surface, and is mapped as covering a slightly smaller area at the deepest wellexplored levels at 1-1.5 km depth. The giant Hollinger-Mclntyre deposit at Timmins in the Superior Province, Canada, has a similarly complex internal geometry, but pipe-like to funnel-shaped overall deposit geometry (Wood et al., 1986). The overall shapes of lode-gold deposits indicate that fluid movement was dominantly along steeplyplunging, well-defined conduits. Vertical continuity

281

indicates that fluids kept within coherent conduits over a minimum of two to three kilometres vertical distance, potentially over several kilometres, and is consistent with the concepts of large-scale vertical movement of ore fluid and ore deposition over significant distances of fluid transport. The upwardopening funnel shape of some ore bodies suggests a component of fluid dispersion during advection, but not to an extent that the overall channelisation was destroyed. 2.2. Geometry of alteration zonation Within gold deposits, lateral alteration zonation is on a scale of a few centimetres to metres around lodes and veins. This zonation reflects the metasomatic interaction of the hydrothermal fluid and wallrock. Gradients in the values of intensive chemical variables across wallrock alteration zones (e.g., for Xco ~ in the fluid, Clark et al., 1989; Guha et al., 1991; for aH~s in the fluid, Neall and Phillips, 1987; Mikucki and Heinrich, 1993), show that the lateral zonation can be viewed as the result of increasing degrees of metasomatic modification of wallrock chemistry as the lode is approached, or, alternatively, of progressive modification of hydrothermal fluid increasing intensity

of alteration

dlffuslonal _.....-.--------1 exchange "~:"~',, ,"~¥~/.. /I!] [.coo( c,','~',~, " , t , , o O ,,~,,

l /

iiiiiiiii!iii/i/iiiiiiiiiiiiiiii! II

~

~

Ill . . ~ .v , ~ . . .. Ill

I[ rr~

~ .

.

.

~.

.

.

.

~

\

.

.

[ ] l l k ] I ~ o~,',',',', ",

I

~',',',%'~

I

.,,,~,,,,,~.

I I t

olsperslon " " ' ~ ~ ' , " , ' ~~l::loo'.'.'.'.~. '~into '.'.'.%>.oo11~ ,..Lu ,, ~ _ . _ o , . ' ~ , ~ ' " wallrock ,,,,~ a I liar, o's,,',.',.',',,"

(schematic)

Fig. 1. Schematic representation of fluid flow in the hydrothermal system of a lode-gold deposit. Fluid flow is dominantly channelised: interaction with the wallrock is through diffusion and dispersion of flow, that is, excursions of channelised flow into and out of the wallrock.

282

J. Ridley et al. / Ore Geology Reviews 10 (1996) 279-293

chemistry as it infiltrates and interacts with wallrock (Dub6 et al., 1987). In contrast to the lateral zonation around lodes, zonation with depth within the lode of ore and alteration assemblages is generally subtle, and is not detectable in some deposits, even over vertical distances of a few hundred metres or kilometres. The limited vertical zonation within lodes indicates both limited temperature gradients during mineralisation and little change in fluid composition along the flow path. The distances of fluid advection along the channelways are thus orders of magnitude larger than the distances of fluid infiltration into the wallrock around a lode. Interaction with the wallrock involves movement of components at fight angles to the major direction of fluid flow, and is presumably due to

"C" 05

diffusional transport between the channelway and wallrock and to dispersion of fluid flow, such that there is a component of flow both into and out of the wallrock - - a fluid flow geometry here termed "imperfectly channelised flow" (Fig. 1, and cf. Cox et al., 1991; Ridley et al., 1991; Cartwright, 1994; Heinrich et al., this volume). The limited fluid compositional evolution along the flow path suggests that, despite the apparently strong interaction with the wallrock, only a proportion of the fluid in the lodes equilibrates with the wallrock.

3. Ore fluid chemistry From fluid-inclusion data, and ore and alteration mineralogy, the gold-carrying hydrothermal fluid at

350 °C, 1 kbar

-7

-5

550 °C, 3 kbar

CO

~5 o

o o Lo o3

-6

Hem

o

-4 sulfate sulfide

sulfate 04

-r-

-5

sulfide

Py

O

Mag

\(~

co2 (g)

/

o

/IN \

\~f.L\

/

04 "I"

t ° -3

-1-~

~

~.

-4 -~

CH 4 (g)

I I -3

I -2

I

I

I

-1

0

~

I

[

1

log all2 s Fig. 2. Estimated compositions of gold-bearing hydrothermal fluids with respect to fluid oxidation state (auz) and sulphide content (aH2s), based on the common vein and proximal alteration assemblages of pyrite 4- arsenopyrite, pyrite-pyrrhotite or pyrite-chalcopyrite-anhydrite at mesothermal conditions (light stipple) and pyrite-pyrrhotite, pyrrhotite, or pyrrhotite-loellingite _+ ilmenite at amphibolite-facies conditions (darker stipple). For further discussion of the assemblages, see Mikucki and Ridley (1993): thermodynamic data from Johnson et al. (199¢2).

J. Ridley et al. / Ore Geology Reviews 10 (1996) 279-293

almost all Archean lode-gold deposits is determined to be a water-dominated, mixed carbonic-aqueous fluid of near neutral or slightly alkaline pH and low to moderate salinity with NaCI as the dominant salt (e.g., Robert and Kelly, 1987; Spooner et al., 1987; Ho et al., 1990). Fluid oxidation potential is calculated to range from weakly reducing to moderately oxidising, and does not change significantly with the temperature and pressure of ore formation relative to important aqueous oxidation:reduction equilibria and buffers, including the C O 2 : C H 4 buffer (Fig. 2 and Hagemann and Ridley, 1993; Mikucki and Ridley, 1993). At some sub-greenschist facies deposits, fluid inclusion evidence suggests that a low- to moderatesalinity aqueous fluid was the dominant fluid in the ore system (Gebre-Mariam et al., 1993; Hagemann et al., 1994), but the significance of this fluid to gold mineralisation has not been determined, and alteration assemblages in these deposits are not significantly different from those at slightly higher temperature deposits. The stability of carbonate minerals in the ore zone shows further that a fluid of Xco" similar to that recorded in higher-temperature deposits was present during mineralisation, even though such a fluid is not strongly represented in the fluidinclusion record of these particular deposits. One parameter that is determined to vary significantly with temperature is the sulphur content of the fluid. The sulphide assemblages in veins and proximal alteration zones indicate that the fluid had a relatively high sulphur content at all temperatures, and that sulphur contents were up to two orders of magnitude lower at mesothermal conditions than at amphibolite-facies conditions (Fig. 2). In all but the most strongly oxidised fluids, sulphur would be present in solution dominantly as reduced sulphide species.

4. Gold transport At mesothermal conditions (i.e., 300-350°C), the experimental database shows that gold can be transported in solution at several orders of magnitude greater abundance as a sulphide complex than as a chloride complex in a low- to moderate-salinity,

283

high-sulphur fluid of near neutral pH and relatively reduced nature (Seward, 1984; Hayashi and Ohmoto, 1991). Other ligands have been suggested as being potentially important, for example, ditetluride, thioarsenite or thioantimonite complexes (e.g., Seward, 1973; Seward, 1984), but there has been no experimental determination of their stability, nor any clear evidence given to date that any of these was the dominant gold-complex at any deposit. The exact stoichiometry of the major gold-carrying bisulphide complex is uncertain. For near neutral pH conditions, Hayashi and Ohmoto (1991) suggested that the dominant complex is either Au(HS)~_or HAu(HS) °, depending on fluid pH, and that gold solubility can be expressed by: Au + 2H2S(aq) ~ Au(HS)2 + H + + 1/2H~(aq ) (1) or

Au + 2H2S(aq) ~ HAu(HS) ° + 1/2H2(~,q,

(2)

Subsequent work by Benning and Seward (1994), Benning and Seward (in press) has shown that the Au(HS) ° complex, rather than HAu(HS) °, dominates at relatively low pH, and hence gold solubility under these conditions is a result of the reaction: A u + H2S(aq) ~ Au(HS) ° + l/2H2~aq ~

(3)

For simplicity of argument, it is assumed in this paper that the two potentially significant complexes are Au(HS) ° and Au(HS)~-. There have been almost no gold solubility experiments at amphibolite facies conditions. Gold-chloride complexes are indicated to become more stable relative to bisulphide complexes with increasing temperature (Romberger, 1991). However, higher sulphur contents of the fluids at higher temperatures will favour the stability of bisulphide complexes, and extrapolation of the available data indicates that these are dominant throughout the range of conditions of gold deposit formation (Fig. 3). Empirical evidence in support of this conclusion is discussed below. By analogy with the behaviour of other metal complexes, a neutral bisulphide complex (Au(HS) ° or HAu(HS) °) is likely to be favoured over charged complexes at higher temperatures (cf. Benning and Seward, 1994).

284

J. Ridley et a l . / Ore Geology Re~,iews 10 (1996) 279 293

amphibolite facies deposits~ .

6 wt% NaCI Iogf02 = C02:CH4 +1 pH= 5.5 ~3 ¢M

3:0

bisulphide

133

-1

complexes dominant

mesothermal deposits

-2

I

I

/ d "

,s

~,

chloride complexes dominant

-3 -4

~sub-gre~ facies deposits

/

250 '

36 0

'

46 0

.

.

500 .

.

600

T (°C)

Fig. 3. Predominance fields for gold-chloride and gold-bisulphide complexes with respect to temperature and sulphide content of the fluid. The predominance boundary is constrained below 400°C by data of Hayashi and Ohmoto (1991) and Romberger (1991), and is extrapolated to higher temperatures. Plotted are conditions of mineralisation at Racetrack (a subgreenschist facies deposit; Gebre-Mariam et al., 1993), Hunt (a mesothermal deposit; Neall and Phillips, 1987) and Mt York (an amphibolite-facies deposit; Neumayr et al., 1993).

5. Gold precipitation For gold transport as a sulphide complex, Reactions 1-3 indicate that gold precipitation would result from destabilisation of gold-sulphide complexes through fluid desulphidation or reduction, and, if the dominant complex is a charged complex (Reaction 1), also by decreasing fluid pH. Changes in dominant sulphur species in the fluid with changing pH and oxidation state mean that oxidation or increasing pH of the fluid can cause gold precipitation under specific conditions (e.g., Hayashi and Ohmoto, 1991), but the conditions required are well outside those determined for the gold-bearing fluids in most Archean deposits. Mechanisms of gold precipitation from bisulphide complexes that have been suggested are: (1) H2S loss from the fluid due to wallrock sulphidation reactions (Neall and Phillips, 1987); (2) reduction of the fluid due to reaction with reducing wallrocks, particularly graphite-beating shales (Springer, 1985); (3) decreasing pH of the fluid as a result of K- and CO2-fixing reactions in wallrock alteration (Kishida

and Kerrich, 1987); (4) H2S loss from the fluid due to phase immiscibility (Spooner et al., 1987; Naden and Shepherd, 1989); or (5) fluid cooling (Phillips and Powell, 1993). The depositional mechanism that is most likely to be effective over the full range of the pressure and temperature conditions determined for deposits formation, and within the full range of hostrocks to mineralisation, is H 2S loss due to wallrock sulphidation. Although other gold-precipitation mechanisms may be important at individual deposits or groups of deposits, none can explain mineralisation over the large range of environments in which it is observed. Both reduction-induced precipitation and pH effects will be hostrock specific. Mikucki and Heinrich (1993) show, for example, that the ore fluid became more oxidising on interaction with wallrock at Mt Charlotte, hence acting against the decrease in gold solubility induced by desulphidation. High-temperature deposits are generally not associated with significant wallrock carbonation (e.g., Mikucki and Ridley, 1993; Neumayr et al., 1993), and the pH of the fluid is thus less likely to have been significantly reduced through CO2-fixing wallrock rections at these deposits. A number of authors have indicated a correlation in mesothermal deposits between highgrade shoots in quartz veins, and fluid inclusion evidence from the veins for fluid immiscibility (e.g., Walsh et al., 1988; Guha et al., 1991). However, the evidence for fluid immiscibility is generally restricted to certain areas within a deposit. In addition, for typical gold-bearing fluids, fluid immiscibility will generally occur only at the lower end of the spectrum of temperatures of mineralisation. The change in the stability of the gold-bisulphide complexes with temperature indicates that gold is unlikely to be precipitated in significant amounts at temperatures above about 250°C as a result of fluid cooling without simultaneous fluid-wallrock reaction (Shenberger and Barnes, 1989). The empirical correlation of gold grade with strong wallrock sulphidation or sulphide-bearing veins in these deposits supports H2S loss from the fluid as the dominant cause of gold precipitation. Neall and Phillips (1987) showed that wallrock alteration haloes record progressive desulphidation of the fluid due to fluid-wallrock reactions to degree that would cause significant gold precipitation. For the Mt Charlotte

J. Ridley et al. / Ore Geology Reviews 10 (1996) 279-293 deposit, aH,s~,q ) was reduced by up to two orders of magnitude "on fluid infiltration into the wallrock (Mikucki and Heinrich, 1993). Mineral zonation in alteration haloes of amphibolite-facies deposits at Transvaal (Dalstra et al., in press) and Mt York (Neumayr et al., in press) indicate similar gradients of a.~s.,,,~ in these deposits.

285

28

6 -30

ol

8

O3 0 5

5.1. Constraints.from S.'Au ratios

O

"3"

-32

Element ratios of ores can further constrain the relative importance of different gold precipitation mechanisms. If gold precipitation is due to wallrock reactions, there should be relationships between changes of fluid fo2, pH and an2 s, and the S:Au ratio of the ore (cf. McCuaig and Kerrich, 1994) which can be quantified if the equilibrium constants ( K ' s ) for the relevant reactions are known. For gold transport as Au(HS) ° complexes, gold precipitation will be through: Au(HS) ° + 1/2H2c~,ql--* Au + H2S(aq)

-34

-36

-38

(4)

-4

-2

-1

log aH2s(aq )

for which: aH~S

K(4 ) - aAu(Hs)oal~2 Assuming initial saturation with respect to both gold and a sulphide phase, in an increment of reaction at constant P and T, equilibrium changes in activities of the species ((~a i) are related through:

(, )

~aAu(HS)0 = ~aH~ S K(4)a{t/2

6aH,S

~an~

aH2S

aH~

Fig. 4. Hydrogen activity-an2 s conditions of lormation of mesothermal deposits and molal S:Au ratios that would be precipitated from a fluid due to fluid H2S loss through wallrock sulphidation reactions at 300°C and 500 bars. Thick solid lines are for precipitation from the HAu(HS)2° complex (K(5) = 10.1, Benning and Seward, submitted); thick dashed lines precipitation from the Au(HS)° complex (K(4)= 7.5, Benning and Seward, in press). The narrow dashed line shows the predominance field boundary at these conditions for the two complexes calculated from the data of Benning and Seward.

2K(4)a~f/2 )

Taking 6aAu(HS)0 as a measure of the gold precipitated from the fluid, the first term on the right-hand side of the above equation reflects precipitation due to H2S loss from the fluid, the second due to fluid reduction. The relative importance of H2S loss and fluid reduction on gold precipitation is given by a ratio of the fractional changes in an,s and all2: 2

-3

If H2S loss from the fluid is the major cause of gold precipitation, assuming activity coefficients of H2S(aq) and Au(HS)gaq) are similar, the ratio of S:Au

precipitated from the fluid is thus given approximately by

K(4)a~/2 and hence, through K(4 ) and all2, is a function of temperature, but is independent of a n 2s" Using similar arguments, if gold is carried as the Au(HS) 2 complex, and precipitated through: A u ( H S ) [ + H + + 1/2U2(aq ) ~ a u + 2H2Su,q) the expression for the S:Au ratio becomes

g(s)an+ 2an,s

a 1/2

H2

(5)

J. Ridley et al. / Ore Geology Ret'iews 10 (1996) 279 293

286

Fig. 4 shows S:Au ratios that would be precipitated

tions,

as

Benning

functions

of

an2 s

fluid-desulphidation

and

an2

in

increments

r e a c t i o n s at m e s o t h e r m a l

of

condi-

given

KI4 ) a n d

and Seward

K¢5 ) f r o m (1994)

Seward

and Benning

(1984), and Se-

ward (in press).

Table 1 Sulphur and gold contents of bulk ore from deposits in the Yilgarn Block (from Perring et al., 1991; Knight, 1994). Distinction of deposits by condition of formation is based on the mineral assemblages of ore and proximal alteration zones. Roman numerals refer to analyses of samples from different parts of a single deposit. Two contrasting stages of mineralisation are distinguished at Race Track (see Gebre-Mariam et al., 1993) Deposit

Hostrock

Style

S (ppm)

Au (ppm)

log (S:Au)

conditions qtz vein in shear zone, Au in vein vein set, Au in veins shear zone vein set strata-bound fracture set vein set shear zones qtz vein in shear zone vein set strata-bound fracture set shear zone vein breccia, Au in veins vein breccia, Au in veins shear/fault zones

7400 6500 44000 86000 34000 39000 82000 43000 3900 91000 55000 21000 5400 20000

51 46 3.4 7.6 3.6 12 14 5.9 2.35 6.55 14.5 31 105 8.1

2.16 2.15 4.11 4.05 3.98 3.51 3.77 3.86 3.22 4. l 4 3.58 2.83 1.71 3.39

Deposits formed at upper greenschist-facies conditions Junction mafic shear zone Lawlers felsic shear zone Paddington mafic vein set Reedys (I) ultramafic shear zone Reedys (II) mafic shear zone Sons of Gwalia mafic shear zone Victory mafic shear zones

8900 10000 16000 22000 43000 15000 21000

6.85 2 3.5 2.55 15.5 2.4 12.5

3.11 3.70 3.66 3.93 3.44 3.80 3.22

Deposits formed at lower amphibolite-facies conditions Bayleys felsic-pelite qtz vein in shear zone Corinthia ultramafic-mafic shear zone Kings Cross mafic qtz vein in shear zone Lindsays mafic vein set Mystery Mint mafic qtz vein in shear zone Patricia Jean mafic qtz vein in shear zone Rothsay ultramafic vein set, Au in veins Three Mile Hill mafic vein set Tindals ultramafic-felsic shear zones

8600 34000 7000 5000 14000 4900 1300 8600 3600

1.3 6.9 2 0.56 I1 6 12.5 0.33 1.18

3.82 3.69 3.54 3.95 3.10 2.91 2.02 4.42 3.48

Deposits formed at middle amphibolite- to lower granulite-facies conditions Big Bell mafic shear zone Bounty Fe-rich sediment shear zone Edwards Find mafic qtz vein, Au in vein Griffins Find (I) mafic-pelite shear zone Griffins Find (II) mafic-pelite vein set in shear zone Hopes Hill ultramafic-mafic shear zone Marvel Loch ultramafic shear zone Nevoria Fe-rich sediment strata-bound vein set Westonia felsic vein set

10000 48000 530 55000 47000 10000 2100 20000 6000

7.4 38 34 12 4.2 0.57 8.4 4.9 3.2

3.13 3.10 1.19 3.61 4.05 4.25 2.40 3.61 3.27

Deposits formed at lower to middle greenschist-facies Golden Crown mafic Lady Bountiful felsic Lancefield Fe-rich sediment Mt Charlotte mafic Mt Morgans Fe-rich sediment Mt Pleasant mafic North Kalgurli mafic Ora Banda mafic Phar Lap ultramafic Prohibition Fe-rich sediment Race Track (Stage I) mafic Race Track (Stage I + If) mafic Race Track (Stage II) mafic Wiluna mafic

J. Ridley et al. / Ore Geology Reviews 10 (1996) 279-293

Analyses of bulk ore compositions are available for over thirty lode-gold deposits from the Yilgarn Block (Table 1, and Perring et al., 1991; Knight, 1994). As there is ore-textural evidence for disequilibrium gold precipitation processes at many deposits, e.g., surface sorption on sulphide phases (Jean and Bancroft, 1985), or an association with chalcedonic quartz (Herrington and Wilkinson, 1993), the bulk ore S:Au ratio is considered to be a better measure of the dominant gold precipitation process in a deposit than the composition of individual ore samples. The bulk ore S:Au ratio will reflect the integrated changes in gold solubility of all aliquots of hydrothermal fluid that passed through that volume of rock. Note, that as most gold and sulphur is precipitated in the initial stages of progressive fluid desulphidation, the exact fluid desulphidation path with respect to all2 s and an~ will not have a major effect on the bulk ore S:Au ratio. Sulphur:gold ratios of most lower-greenschist facies deposits fit the values predicted for fluid desulphidation induced gold precipitation well (Fig. 5). A number of quartz-vein deposits, in which gold is present predominantly as coarse free gold in the quartz veins, have significantly lower ratios. The mean S:Au decreases gradually with temperature (Fig. 5), as would be consistent with gold

precipitation dominantly through the same or similar reactions throughout the temperature spectrum of mineralisation and gradual changes in the values of the equilibrium constants of the precipitation reactions. The equilibrium constants of the gold dissolution reactions are not known for the higher temperatures and pressures, but an estimate of the equilibrium constant of the gold dissolution reactions at amphibolite-facies conditions may be made using the bulk ore S:Au ratios, and estimates from phase equilibria of fluid pH, aH~ s and a l l . Sulphur:gold ratios in amphibolite-facies deposits average 10 3s (Fig. 5). If precipitation is through reaction 5, substituting into the relation, K(5)aH+ a / / 2

S :Au = 2an2s for pH = 5.5, log all2 s = 0, and log aH~ = - 2 (Fig. 2), gives log K(5) = 10.3 for amphibolite-facies conditions (550°C at 3 kbar). This is within one order of magnitude of K calculated for mesothermal conditions. The association of gold with wallrock sulphidation in deposits formed at all temperatures and depths in the proposed vertically extensive hydrothermal

106

10 5

S/Au (molar)

S/Au (weight)

<>

105'

10 4

1 0 4.

103

287

[]

•

13

[]

.103

•

Gold dominantly hosted in wallrock alteration haloes

.102

<> Low-grade ores (<1 ppm)

[]

102

10 ¸

If01 low mid greenschist

upper greenschist

lower amph.

[] Gold dominantly hosted in quartz veins

midupper amph.

Fig. 5. Sulphur:gold ratios of deposits of different metamorphic grade in the Yilgarn Block, as detailed in Table 1. Distinction is made of deposits in which gold occurs dominantly as free gold in quartz veins, and of low-grade ( < l ppm) ores. Horizontal lines are logarithmic means of the S:Au ratios of deposits formed over each temperature range in which gold is predominantly present in wallrock alteration haloes. Each data point represents the mean of a minimum of two composite 30-50 kg samples collected from pulp, drill core, grab sampling from high-grade stockpiles, or channel sampling of ore faces (see Perring et al., 1991).

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systems, the relatively high all: s of the fluids, and the gradual change in S:Au ratios with temperature, are consistent with gold transport in the hydrothermal fluids as bisulphide complexes over the whole range of pressure and temperature conditions of gold deposit formation. The S:Au ratios of bulk ores, together with the limited changes of fluid an: and pH with depth, suggest that, in most deposits, gold was precipitated from solution dominantly as a result of destabilisation of gold bisulphide complexes brought about by loss of aqueous sulphide from the fluid through fluid-wallrock reactions.

6. Efficiency of gold precipitation from the hydrothermal fluid Gradients of sulphide content and sulphidation state in zoned alteration haloes around lodes reflect sulphur loss from the fluid due to sulphidation reactions during progressive infiltration of the wallrock. In Types 2 and 3 alteration at the Mt Charlotte deposit, for example, pyrite is stable in and adjacent to the veins, whereas pyrrhotile is stable in distal alteration zones, and magnetite in relatively unaltered wallrock (Clark, 1980; Mikucki and Heinrich, 1993). The all2 s of the fluid in the veins is estimated to have been two orders of magnitude higher than that in equilibrium with the wallrock (Mikucki and Heinrich, 1993): fluid infiltrating and equilibrating with the wallrock at the outer cage of the pyrrhotite zone would have lost 99% of its dissolved sulphur and 99-99.99% of its dissolved gold, depending on which gold-bisulphide complex was dominant. It is thus predicted that gold will be efficiently precipitated from fluid that infiltrates and equilibrates with the wallrock. How much of the gold in the system as a whole is precipitated from the fluid, and whether significant gold can be transported over long distances, depends on what proportion of fluid in the system equilibrates with the wallrocks over a given length of fluid transport path. An estimate of this is obtained here by considering the vertical ore zonation in lode deposits and its implication for the behaviour of sulphur. In a number of deposits in the Yilgarn Block, pyrrhotite is the sulphide phase in proximal alteration zones at depth, whereas pyrite occurs at shallow levels (e.g., Mt Charlotte, Clark, 1980; Mikucki

and Heinrich, 1993; Hill 50, Kelly, 1994). The apparent increase in fluid sulphidation state along the flow path can be modelled as the result of cooling of the fluid, essentially along a paleogeotherm, together with minimal sulphur loss from the fluid (Mikucki and Heinrich, 1993). Vertical temperature gradients of the order of 50°C/kin are predicted for Archean granite-greenstone terrains (e.g., Grambling, 1981), and a reduction of 50°C over the 1 km of vertical exposure at Mt Charlotte can explain the vertical zonation of sulphide species at this deposit (Mikucki and Heinrich, 1993). Given the uncertainties in the determination of the temperature gradient and of the S content of the fluid, the loss of S from the fluid within the exposed section at Mt Charlotte is too small to be accurately constrained from the vertical mineral zonation. However, typical sulphide contents of the fluid in mesothennal deposits are one to two orders of magnitude lower than in amphibolite-facies deposits (Fig. 2). Assuming a difference of depth of formation of the two types of deposit of about 5-8 km, a loss of 10-30% of contained S per kilometre is thus inferred. As fluid all2 and pH do not vary significantly with depth (Fig. 2 and Mikucki and Ridley, 1993), and the stabilities of the gold-bisulphide complexes do not vary strongly with temperature above about 300°C, this rate of S loss would give precipitation of 10-50% of contained gold per vertical kilometre. Mass-balance calculations give independent estimates of the efficiency of gold precipitation from the fluid. Hronsky (1993) estimated ore fluid volumes in Main Lode, Lancefield, using total volumes of quartz precipitated, and, from the all2 s and all: of the fluid, gold concentration in the fluid as bisulphide complexes. Given the total gold resource at Lancefield, 5-20% of the gold in the fluid is calculated to have been precipitated in the exposed 1 km vertical section of the lode, in approximate agreement with the estimate above.

7. Discussion

7.1. Significance of the relative inefficiency of gold precipitation in lode-gold systems The contrast between strong sulphide activity gradients across alteration zones around a lode and

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subtle gradients along a fluid flow path implies that fluid-wallrock interaction in lode-gold systems is relatively inefficient, in the sense that only a small proportion of the fluid equilibrates with the wallrock. Two consequences of this conclusion are: the fluid composition remains relatively constant along the channelway, and the concentrations in the fluid of many solute species will not be buffered by fluidrock equilibria. The low rate of sulphur loss from the fluid, together with the likelihood that equilibrium constants for the gold dissolution reactions do not vary rapidly along flow paths, imply likewise that only a small proportion of contained gold is precipitated within any segment of the system. Lode-gold hydrothermal systems therefore have suitable physical and chemical characteristics to allow both long distance gold transport and gold precipitation at all levels along a fluid conduit. 7.2. Efficiencr o f gold precipitation at different crustal let~els

Comparison of the efficiency of gold deposition at different crustal depths requires an estimation of gold contents of the fluid at different depths. The stabilities of pyrite in veins and ore zones in most greenschist-facies deposits and pyrrhotite in most amphibolite-facies deposits imply fluid sulphide molalities, respectively, of 10-25-10 -j at 300-350°C, and 1 - l 0 at about 550°C. Gold contents of mineralising fluids are estimated to be 10-8-10 6 m at mesothermal conditions (e.g., Hayashi and Ohmoto, 1991). In high-temperature deposits, gold concentrations in solution would be 10-6-10 -3 molal, or 0.2-200 ppm, given the estimate above of the equilibrium constant for gold dissolution at amphibolitefacies conditions. These estimates for sulphide and gold contents of the fluid imply that, if the same volumes of fluid pass through amphibolite facies terrains and greenschist facies terrains, and if ftuid-wallrock interaction, measured as the percentage of fluid in the system that equilibrates with the wallrock, is similarly efficient at the two crustal levels, then about an order of magnitude more gold should be precipitated per cubic kilometre of greenstone belt in an amphibolite-facies terrain. In the Yilgarn Block, however, gold production per square kilometre is marginally

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lower from amphibolite-facies terrains than from greenschist-facies terrains (Groves et al., 1990). Assuming that there is an approximate correlation between gold production and gold content of a terrain, the relatively low gold production of the amphibolite-facies terrains implies either that fluid-wallrock interaction at higher temperatures was less efficient, in the sense that there was a lower proportional reduction in aH,s per kilometre of fluid transport, or that there were regional variations in fluid flux such that significantly smaller volumes of gold-bearing fluid passed through the terrains in which the greenstone belts are metamorphosed to the amphibolite facies at the present level of exposure than through those terrains in which the present level of exposure is of greenschist-facies rocks. The possibility of less efficient fluid-wallrock interaction at higher temperatures is tested here by considering silica addition as an independent measure of hydrothermal fluid volume. Quartz precipitation in veins and pervasive silicification in alteration haloes show that gold-bearing hydrothermal fluids are almost invariably quartz saturated. If silica is carried in solution as a neutral hydroxyl complex (Walther and Helgeson, 1977), then silica addition in a lode should be effectively independent of fluidwallrock interaction, and be dominantly a function of changing SiO 2 solubility with respect to pressure and temperature (see also Helgeson and Garrels, 1968). For a fluid rising and cooling at 50°C/km, silica precipitation per unit volume of hydrothermal fluid at middle amphibolite-facies conditions should be about three times greater than at lower-greenschistfacies conditions. Silica addition has not been rigorously determined for an amphibolite-facies deposit, however, there is no evidence for significantly higher added-SiO2:Au ratios in these deposits. In general, alteration haloes are not strongly silicified, and quartz veins are usually a relatively minor component of these deposits (e.g., Neumayr et al., 1993; Bloem et al., 1994), and are certainly not significantly more abundant than in mesothermal deposits of a comparable gold grade. It is therefore considered unlikely that fluid-wallrock interaction was significantly less efficient at these deposits. Regional variations in fluid flux would not be inconsistent with the concepts of the "crustal continuum" of gold deposition, or of derivation of the

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ore-bearing fluids from beneath the greenstone belts. Groves et al. (1990) showed that the different provinces in the Yilgarn Block have different gold prospectivities, as measured by kg gold produced per km 2. These provinces are distinguished by differences in their Archean geological histories, including their thermal histories (Gee et al., 1981). The Southern Cross Province has a relatively high proportion of amphibolite-facies greenstone belts and a relatively low prospectivity, and the province as a whole may thus have had a relatively low flux of goldbearing fluids. There may also have been variations in fluid flow within a single greenstone belt. Phillips et al. (1987) noted in this respect that many larger deposits occur in the centres of greenstone belts of large areal extent, which are typically greenschistfacies terrains. Amphibolite-facies terrains occur dominantly in relatively narrow greenstone belts, or along the margins of the aerially extensive belts.

and in them the majority of gold is present as free gold in quartz veins. As such, they have similarities to deposits discussed by Walsh et al. (1988) and Guha et al. (1991), for which it was argued that gold precipitation, at least in high-grade ore shoots, was a result of fluid immiscibility. Fluid immiscibility may have been a significant gold precipitation mechanism at the low S:Au deposits listed in Table 1, though evidence for immiscibility has not been recorded at all the deposits concerned (e.g., Racetrack, GebreMariam et al., 1993). If this is the case, the relative abundance of low S:Au deposits potentially gives an estimate of the relative importance of fluid immiscibility as a gold precipitating mechanism in the hydrothermal systems as a whole.

7.3. Additional controls on the efficiency of gold precipitation

It has been argued in this paper that Archean lode-gold deposits are the result of hydrothermal systems involving long distance, upward directed fluid flow in the crust along well defined tabular or pipe-like channelways. Gold may have precipitated continuously over vertical intervals of 10-15 km in the channelways, The following inferences are made about the chemical evolution of the systems based on the ore and fluid chemistry of deposits formed at different temperatures, and hence different depths. (1) Gold transport was predominantly as bisulphide complexes throughout the exposed extent of the system. (2) Gold precipitation at all levels in the exposed system was dominantly due to loss of H2S from the fluid to the wallrock as a result of wallrock sulphidation reactions. (3) Gold and sulphur deposition from the fluid was relatively inefficient, at most only 10-50% of contained gold was precipitated over a 1 km vertical interval of a hydrothermal system. This relative inefficiency was critical in allowing both the formation of vertically extensive mineralised systems, and the formation of deposits distal to the fluid source. (4) Exposed terrains in Archean cratons that were metamorphosed under amphibolite-facies conditions generally experienced a lower flux of gold-bearing fluids than greenschist-facies terrains.

One implication of the above calculation of the rate of S and Au loss from the hydrothermal fluid is that gold grade within a conduit may be strongly influenced by the efficiency of fluid-wallrock interaction. The importance of wallrock chemical composition has been argued by Phillips et al. (1984) and BiShlke (1989), who showed that iron-rich wallrocks and wallrocks with high F e / ( F e + Mg) ratios favour the stabilisation of sulphides, and hence the precipitation from the fluid of sulphur and gold at mesothermal deposits. The physical nature of the conduit may control the efficiency of fluid-wallrock interaction through affecting distances of fluid ingress into the wallrock, or the proportion of fluid in the system that interacts with the wallrock. A minority of deposits in the Yilgarn Block have S:Au ratios significantly lower than those predicted for wallrock sulphidation. The S contents of these deposits may have been underestimated as they are quartz-vein hosted deposits and the analysed ore was dominated by the vein and excluded much of the wallrock alteration halo, but it is considered unlikely that S content was underestimated by an order of magnitude or more. The deposits in question mainly formed at low to mid greenschist-facies conditions,

8. Summary

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Acknowledgements This paper develops results of research work of staff and students at the Key Centre for Strategic Mineral Deposits, The University of Western Australia, and a number of co-workers. The input of Jon Hronsky, Joe Knight, Peter Neumayr and Chris Heinrich in this respect are particularly acknowledged. Careful and helpful reviews of the paper by Rob Kerrich and Chris Heinrich are acknowledged. JR and DIG thank MERIWA for financial support towards research on high-temperature gold deposits.

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