Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits

Ore Geology Reviews, 6 ( 1991 ) 333-364 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits Gregg W. Morrison, William J. Rose* and Subhash Jaireth Geology Department, James Cook University of North Queensland, Townsville, Qld 4811, Australia (Received August 1, 1990; accepted after revision December 12, 1990)

ABSTRACT Morrison, G.W., Rose, W.J. and Jaireth, S., 1991. Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits. Ore Geol. Rev., 6: 333-364. A compilation of microprobe fineness data and a re-evaluation of bullion production fineness data suggest that the gold fineness model of Fisher (1945) can be updated and applied to a wide range of gold-mineralised environments. The major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows: Archaean 940 (780-1000) Slate Belt 920 ( 800-1000 ) Plutonic 825 (650-970) Porphyry 700-1000 (650-1000) Volcanogenic 650-850 (520-870) Epithermal 440-1000 (0-1000) The Archaean (including Witwatersrand), Slate Belt and Plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range. There is no clear subdivision within these classes according to deposit style or inferred genesis. The Porphyry, Volcanogenic and Epithermal classes have a variable average deposit fineness and a wide total range of fineness values. Within these three classes, higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings; deposits with Cu-Au rather than Cu-Mo or Pb-Zn element associations; and deposits with teUuride or selenide minerals. Solubility calculations indicate that in all these environments, gold and silver form the same aqueous species with AgCI~- dominating for silver and Au (HS)~- dominating for gold. In all the environments, the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes. Therefore, the observed gold-fineness values can not be explained by the transporting mechanism only. Low silver in the ores and higher gold-fineness values observed in the Archaean, Plutonic and Slate Belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition. Sulphidation of wall rocks destabilises only the bisulphide-complexes of gold and silver. In contrast Epithermal, Volcanogenic and Porphyry environments are characterised by a diversity of ore-depositing mechanisms (cooling, boiling, mixing) which act alone or in combination and destabilise chloro- and bisulphide-complexes of gold and silver. The high silver concentration in the ores, low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore-depositing mechanisms.

Introduction

Natural gold grains vary widely in composition, especially with respect to silver content, reflecting the complete range of solid-solution substitution in gold-silver alloys. Analyses of *Now Western Mining Corporation, 5 5 MacDonald Street, Kalgoorlie, Western Australia.

0169-1368/91/$3.50

gold grains demonstrate that Cu, Fe, Hg and platinoids are common constituents and that a range of other elements may also be present (Antweiler and Campbell, 1977; Knight and McTaggart, 1986). As the proportion of components other than Au and Ag does not commonly exceed one percent, the composition of gold is normally expressed as fineness [ 100×Au/(Au+Ag) ]. Gold-silver alloys

© 1991 Elsevier Science Publishers B.V.

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G.W. M O R R I S O N

with fineness greater than 800 are referred to as gold, those with fineness 200-800 as electrum and less than 200 as silver. Historically, fineness has been determined from mint returns of bullion recovered by amalgamation and cyanidation. Amalgamation recovers silver from the native state and cyanidation recovers silver from silver-bearing phases as well. Thus, in some areas bullion fineness is significantly lower than true fineness. Nowadays, microprobe analysis of individual gold grains

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gives an accurate record of silver and trace-element contents, but a large number of grains may need to be analysed to give true representation of deposits or fields. Since the pivotal studies of Fisher ( 1945 ), the fineness of gold grains and their trace-element contents have been used to classify hypogene gold deposits and to interpret the source of placer gold (Badalova and Badalov, 1967; Desborough et al., 1971; Berman et al., 1973; Fayzullin and Turhinova, 1974; Guindon and

Gregg Morrison graduated from the University of Otago, New Zealand in 1973. He obtained his Ph.D. from the University of Western Ontario in 1981, studying the skarn deposits of the Whitehouse Cooper Belt, Yuhon. Since 1983 he has led a major industryfunded research group studying the gold deposits of north Queensland, Australia. His research interests are the genesis and exploration of epithermal and porphyry-type ore deposits, and regional tectonics and metallogeny.

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Bill Rose graduated from the University of Queensland in 1970 and joined Broken Hill South as an exploration geologist. In 1978 he joined Western Mining Corp. as a senior exploration geologist. In 1988 he completed and M.Sc. at James Cook University. His interests are in economic geology, particularly exploration for gold, base metals and diamonds.

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Subhash Jaireth graduated from the Patrice Lumumba Peoples Friendship University, Moscow in 1974 and obtained Ph.D. from the same University in 1978. He has taught Economic Geology for more than eight years. His principal research interests are in economic geology, particularly the genesis of tin-tungsten, base-metal and gold deposits. He has a special interest in fluid inclusion research and thermodynamic modelling of ore forming systems.

SI LVER C O N T E N T ( FINENESS ) O F G O L D IN G O L D - S I L V E R DEPOSITS

NichoU, 1982; Oberthur and Saager, 1986). Fisher (1945) established that gold fineness calculated from bullion production records of a wide range of deposits could be used to assign deposits to the hypogene depth classes of Graton ( 1933 ): epithermal deposits 500-800, mesothermal deposits 750-900 and hypothermal deposits 800-1000. An essential feature of both Graton's and Fisher's classifications is the interference that intrusive rocks are the principal source of hydrothermal fluid and ore-forming elements. Modern ore-deposit classifications are generally based on the distinction of broad ore forming environments (Bache, 1981; Meyer, 1981; Morrison, 1988). In such classifications, it is acknowledged that ore-forming fluids may be (a) derived directly from magma; (b) from connate water, seawater, or groundwater heated and circulated by magma; (c) from metamorphic dewatering; or (d) some combination of these. The six major classes of gold-silver-forming environments discussed in this paper are based on geologic features, alteration and ore mineral zoning and ore fluid characteristics. Gold deposits in the Archaean environment are characterised by quartz veins and replacement bodies in sheared and metamorphosed rocks. Mineralisation is hosted by a variety of rocks, although at a single-deposit scale an association with greenstone-type supracrustal sequence with tholeiitic and komatiitic mafic volcanics is very common (Hodgson et al., 1982, referred to in Keays and Skinner, 1989 ). The mineralising environment is mesothermal with carbonate-sulphide alteration dominant in mafic rocks and sericitic alteration in felsic rocks. The ore fluids are metamorphic in origin (Groves et al., 1989 ), but locally can have significant contribution from meteoric (Nesbitt and Muehlenbachs, 1989) and magmatic (Burrows et al., 1986) fluids. For the purpose of this review, all styles and genetic types of Archaean gold mineralisation have been grouped together. While this is not necessarily

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consistent with the definition of environment used for the other deposit classes, it does highlight the consistent fineness from the range of environments (including the Witwatersrand) represented in the Archaean. The Slate Belt environment is characterised by podiform quartz veins and replacement bodies in fissures and reactivated brittle-ductile shear zones hosted in metasedimentary rocks. Veins have narrow sericitic alteration selvages and are sulphide-poor with pyrite and arsenopyrite dominant over base-metal sulphides. The ore-depositing conditions are mesothermal and the ore fluid is a mixture of magmatic, metamorphic and meteoric components with a dominant contribution of the metamorphic component. The Plutonic environment is characterised by simple veins that occupy fissure and shear zones in pluton level granitoids and metamorphosed wall rocks. Veins have narrow selvages of sericitic and argillic alteration and consist largely of quartz with a simple base-metal sulphide assemblage. There is no evidence for a direct genetic link with the host pluton. The ore fluid is considered to originate at deeper crustal levels during orogenesis, to deposit dominantly under mesothermal conditions, and to be a mixture of magmatic, metamorphic and meteoric components (Brhlke and Kistler, 1986; Nesbitt and Muehlenbachs, 1989 ). The Porphyry environment is represented by numerous skarn, vein, stockwork, and breccia deposits for which a genetic link to subvolcanic intrusions can be demonstrated. The veins occupy brittle structures, have narrow envelopes of potassic and sericitic alteration, a base-metal-bismuth association and evidence for origin from magmatic fluids under conditions typical of porphyry copper-molybdenum systems. In the Volcanogenic environment, gold-silver mineralisation is associated with galenasphalerite ores concentrating in the upper parts of zinc-rich massive sulphide lenses and with chalcopyrite-pyrite ores occupying lower por-

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tions of copper-rich lenses (Huston and Large, 1989). The ores are characterised by chloritic and argillic alteration and the ore fluids are dominated by seawater with a possible magmatic component. The Epithermal environment is represented by single veins, vein networks, breccias, and replacement bodies hosted in unmetamorphosed volcanic and sedimentary rocks. Extensive zones of propylitic, sericitic, argillic, advanced argillic and silicic alteration, together with fluid inclusion and stable isotope data, suggest formation under near-surface conditions at temperatures less than 300°C with a predominant contribution from the meteoric fluids. By assigning many of the deposits referred to by Fisher ( 1945 ) into a classification based on environments, suggestions can be made about the fluids responsible for the reported fineness values. In the classification adopted here, additional subdivisions by deposit style and element association can be used to explain variations within classes. In the general classification, most of Fisher's epithermal deposits remain as Epithermal with a few deposits, particularly his deep epithermal group, assigned to the Porphyry class. Fisher's mesothermal class is subdivided into Slate Belt, Plutonic, Porphyry and Archean classes and the hypothermal class mainly represents Archean deposits with some Slate Belt, Plutonic, Porphyry and Volcanogenic deposits. A second implication of Fisher's (1945) subdivision is that gold deposits should increase in bullion fineness with depth. This appears to be the case in the Timmins-OchaliBerlin mine, Columbia (Fisher, 1950) and at the O'Brien mine, Quebec (Mills, 1954). However, Fitzgerald et al. (1967) demonstrated that increasing bullion fineness with depth at Lamaque and Sigma, Quebec corresponded with increasing silver content of pyrite associated with gold grains of consistent fineness. Elsewhere, it has been demonstrated that variations in bullion fineness relate to po-

G.W. MORRISON ET AL.

sition in the paragenetic sequence (Eales, 1961 ) and specific mineral associations in complex deposits (Badalova and Badalov, 1967 ). Variations of several hundred fineness units have been documented in some deposits, but paragenetically late gold and gold in telluride associations tend to be of higher fineness (Badalova and Badalov, 1967 ). It was clearly recognised by Fisher (1945) that epithermal deposits had much greater internal variation than hypothermal deposits and this was also reflected in the overall average and range for the deposit class. That there is a limited range of average fineness values within each of the major deposit classes suggests there are consistent physicochemical conditions for transport and deposition of Au and Ag within ore-forming hydrothermal systems. Internal variations within deposit classes and within individual deposits may reflect differing conditions of Au/Ag deposition within hydrothermal systems. In earlier studies, the Au/Ag ratios in deposits have either been explained by the specific nature of the geological setting of these deposits (Titley, 1987) or by the conditions of transportation and deposition of gold and silver (Cole and Drummond, 1986; Shimazaki and Shimizu, 1986, 1987). In recent years, new thermodynamic data on the bisulphide-complexesof gold and silver have appeared (Sugaki et al., 1987; Gammons and Barnes, 1989; Shenberger and Barnes, 1989). In this study we have attempted to re-evaluate Fisher's ( 1945 ) model in the light of microprobe fineness data and modern deposit classifications. The most recent thermodynamic data on the chloro- and bisulphide-complexes of gold and silver have been used to calculate the solubilities of gold and silver in the six different environments and to evaluate the role of chloro- and bisulphidecomplexes in each environment. Based on the calculated solubilities, the effect of different ore-depositing mechanisms (sulphidation, mixing boiling, reduction and cooling) on the precipitation of gold and silver has been inves-

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SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

tigated to explain the observed gold-silver ratios. Source of thermodynamic data

Basic thermodynamic data on the stability of various aqueous species used in these calculations has been obtained from the datafile SOLTHERM (Reed, 1982 ), which was modified by Heinrich ( 1987 ) to be compatible with the data file CPDMRL of the CSIRO-SGTE-THERMOCHEMISTRY system (Turnbull and Wadsley, 1988) and with the data on aqueous ions calculated by Cobble et al. ( 1982 ). Data for chloride-complexes for gold and silver has been taken from Helgeson (1969) and Seward (1976), respectively. For sulphide-complexes of gold and silver, data were obtained from Shenberger and Barnes (1989) and Gammons and Barnes ( 1989 ), respectively. In acidic solutions Seward ( 1973 ) suggested the presence of Au(HS) °, but later (Seward, 1982) opted for a neutral complex HAu (HS)2. Renders and Seward (1989) and Shenberger and Barnes (1989) have confirmed the presence of Au(HS) ° in acidic fluids, but due to the lack of reliable thermodynamic data, this complex has not been included in the calculations. Activity coefficients for the charged aqueous species are estimated using the extended DebyeHuckel equation of Helgeson (1969). The "distance of closest approach" (h in angstroms) for AgCI~-, AuCI~-, Ag(HS)~-, Au (HS) ~-, and Au2 S (HS) ~-2, was taken equal to 4.0, 4.0, 5.0, 4.0 and 4.0, respectively (Seward, 1976). The h for other charged aqueous species were obtained from Truesdell and Jones (1974). An activity coefficient of one was adopted for all neutral species. Fineness data for gold-silver deposits

A literature search reveals probe data for 115 deposits world wide. In some papers, individual data points are listed, but more commonly ranges and averages for the deposit are noted.

Consequently, statistical analysis of the data is generally not possible and so only an empirical evaluation has been attempted. Bullion fineness data, largely derived from Fisher ( 1945 ), have been reassigned in the new classification to facilitate comparison of bullion and probe data. In silver-poor deposits (Archaean and Slate Belt classes) there is good correlation of bullion and probe data, but in silver-rich deposits, particularly the Epithermal class, there is only a broad correlation. This is partly because probe analysis highlights the complete internal variation within deposits rather than averages, whereas bullion figures tend to skew the data to the lower fineness end by incorporating other silver phases.

Archaean deposits Data for Archaean, mainly greenstonehosted deposits show a very consistent pattern of high average fineness and limited fineness range (Fig. 1 ). For example, of the 550 probe analyses from 19 major deposits in the Abitibi Belt of Canada reported by Guindon (1982), 44% plot in the fineness range 940-950. Only 15% of the data from three deposits plot outside the range 890-960. Similar results appear typical of deposits in the Archean cratons of Western Australia, South Africa, Brazil and India. Limited ranges of fineness are also typical within many deposits. For example, the Strydom reef, Barberton has a mean fineness of 889.2 with a standard deviation of 28 (Von Gehlen, 1983). Within the Archean data set there is no clear subdivision between deposits of inferred different origins and no real comparison with Phanerozoic counterparts. For example, average fineness is 945 for Bousquet (volcanogenic - Valliant and Hutchinson, 1982), 942 for Fairview (BIF hosted - Liebenberg, 1972), 930 for Matachewan (intrusive-related - Sinclair, 1982), 924 for Dome (metamorphic Kerrich and Fryer, 1979) and 900 for Hollinger - Mclntyre (magmatic/mantle sourced -

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Fig. 1. Gold fineness data for Archean and Witwatersrand deposits. Typical Archean deposits average 950 and have a very narrow fineness range. Witwatersrand deposits average 900 and also have very narrow fineness ranges. Source of data: Archean: 1-18, 20, 38: Guindon (1982), 19: Fitzgerald et al. (1967); 21-26: Liebenberg (1972); 27: Chisholm ( 1978 ); 28a: Golding ( 1978); 28b-34, 35b: Fisher ( 1945 ); 35a: Desborough et al. ( 1971 ); 36: Harris ( 1986); 37: Nickel (1983). Witwatersrand: 1-5: Von Gehlen ( 1983); 6: Hirdes ( 1984); 7: Oberthur and Saager (1986). Data for Proterozoic Homestake BIF-hosted deposit is included here for comparison with Archaean BIF-hosted deposits.

Burrows et al., 1986). Of the deposits with fineness less than 900, some are associated with zones of strong carbonate alteration (Barder Lander, Mclntyre, Hollinger), some are deposits relatively rich in base metal sulphides (Lily, Kolar) and others (Homestake, Morro

Velho) are banded iron formation (BIF)hosted deposits. In each case there are other deposits of the same type with high fineness. A small number of greenstone-hosted deposits have a divergent fineness character. Gold and silver grains from the Ross Mine, Ontario,

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average 740 in fineness and range from 0 to 770. Native electrum grains (fineness 0.5 ) are found in contact with some of the gold grains. The fineness character is not unlike some silver-rich epithermal systems. Akande ( 1985 ) reported argentite and silver sulphosalts in the vein assemblages suggesting a further similarity with low fineness epithermal systems. The association of copper-rich stringer veins in pipe-like zones, base-metal veins and possible potassic alteration in intrusive rocks (Troop, 1986) suggest the Ross Mine may have a different origin to typical Abitibi Belt deposits. A further exceptional deposit of some interest is Hemlo, Ontario. Average fineness recorded is )41 but locally ranges down to 709 (Harris, 1986 ). Hg values range up to 22.1 wt% within Au grains compared with values of less than 0.65% recorded for Abitibi deposits (Guindon, 1982). Cinnabar and aktashite are common minerals in the deposit and Hg occurs as a significant solid solution component in tetrahedrite, tennantite, and sphalerite (Harris, 1986). Homestake, South Dakota, although of Proterozoic age (Wolfgram, 1979 ) is included here for comparison with Archaean deposits hosted by banded iron formation (BIF). It has been argued as a remobilised syngenetic exhalative deposit (Rye and Rye, 1974), but could be considered as gold-sulphide mineralisation formed by replacement of carbonate-facies banded iron formations using the model of Phillips et al. (1984). Homestake has gold fineness (average 848, range 800-875) lower than the other BIF-hosted deposits reported here and lower than Phanerozoic carbonate replacement deposits, such as Cariboo, Salsigne (Fig. 2 ) or Carlin (Fig. 7 ). A number of genetic studies of Witwatersrand reefs involve the statistical analysis of large numbers of probe determinations of gold fineness (e.g., Saager, 1969; Viljoen, 1971; Von Gehlen, 1983; Hirdes, 1984). Recent data presented by Von Gehlen (1983) and Hirdes (1984) indicate several distinct populations of

gold grains with mean values in the range 832910. Individual populations cluster with standard deviations of less than 30 fineness units. They used these data to support the modified placer theory which suggests that individual populations were inherited from particular mineralised provenances within greenstone belts. In contrast, Oberthur and Saager (1986), from a detailed study of fineness variation in gold grains within the Carbon Leader reef of the Carletonville goldfield, concluded that most grains were authigenic, annealed, and homogenised over distances of a few metres. The Ag and Hg contents of gold grains were the product of post-depositional overprinting processes, including metamorphism. Viljoen ( 1971 ) noted a distinct Ag enrichment in the range for the Witwatersrand compared with those of the Barberton greenstone belt. However, the ranges and means of individual Witwatersrand reefs are only slightly lower than those for Archaean greenstone belt deposits in general. Probe results essentially range from 775 to 970. Average gold fineness values for various reefs fall in the range 880-920. The data are consistent with both an Archaean source terrane for placer gold (Von Gehlen, 1983) and an epigenetic metamorphic origin for Witwatersrand gold (Phillips and Meyers, 1989).

Slate Belt deposits The Slate Belt class of deposits includes a small but distinctive group of goldfields characterised by regionally extensive swarms of sulphide-poor quartz veins hosted in greenschist grade turbidite sequences in Palaeozoic and Mesozoic fold belts. The classic fields of Victoria, (Australia); Otago, (New Zealand ) and Meguma, (Nova Scotia) are as much known for their alluvial as for their lode production. Two other deposits, Cariboo, (British Columbia) and Salsigne, (France) have carbonate replacement as well as quartz vein ore bodies, but their overall setting is similar to the classic

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hosts is prominent, but the exact genetic link is uncertain. Possibilities include favourable host rock structure and chemistry for postgranitoid ore solutions (Bohlke and Kistler, 1986); a plutonic-magmatic heat source for circulating connate or meteoric water (Criss and Taylor, 1983); and/or a plutonic magmatic source for the hydrothermal ore-forming fluid (Wilton and Strong, 1986 ). In the deposits considered here there is a close spatial and temporal association between pluton emplacement and mineralisation, but no necessary implication of a granitoid host or a magmatic ore fluid. This class is comparable to the idealised mesothermal deposits of Lindgren (1933) but we make a clear distinction between the plutonic and subvolcanic (porphyry) environments. Probe data for most Plutonic deposits show a limited range 650970 and average values close to 825 (Fig. 3). Bullion data reflect very similar values. These fineness values are distinct from the Slate Belt class in average fineness and from the Prophyry class in the more limited range of values (Figs. 2, 3 ). An interesting exception to the overall pattern is the Croydon Goldfield, Queensland, Australia. In this field, gold-base-metal-quartz veins are hosted in fissures and shear zones in Proterozoic rhyolitic volcanics and cogenetic

deposits. There are very limited probe fineness data for Slate Belt deposits (Fig. 2 ); however, all available probe data are consistent with average bullion fineness values for the major Slate Belt fields in the range 900-1000. Bullion fineness data for the Victorian fields are mostly high (over 950), with exceptions in the Woods Points-Walhalla and Omeo districts where values in the range 700-800 are prominent, possibly reflecting the presence of native silver (Fisher, 1945). A probe analysis from the AI Mine at Woods Point, for example, has a fineness of 941 (Fig. 2). The reported narrow range of fineness values and deposit averages for the Slate Belt class compares closely with values for the Archaean group (Fig. 2). Metamorphic dewatering models initially proposed by Henley et al. ( 1976 ) for deposits in the Otago Schist have been broadly applied to other Slate Belt deposits, such as Victoria (Wall et al., 1983) and Nova Scotia (Graves and Zentilli, 1982), to Archean BIF-hosted replacement deposits (Phillips et al., 1984 ) and Archaean vein/shear zone deposits in Western Australia (Phillips and Groves, 1983).

Plutonic deposits In some metallogenic provinces, an association of auriferous quartz veins with granitoid 0

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Fig. 2. Gold fineness data for Slate Belt deposits. Typical deposits average 940 and range 900-1000. Sources of data: 1-3, 6, 9, 10, 12: Fisher ( 1945); 4, 5: Hinman ( 1981 ); 7: Barr ( 1980); 8: Andrew ( 1910); 11: Smith ( 1986); 13: Bonnemaison et al. ( 1986); 14: Desborough et al. ( 1971 ).

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Fig. 3. Gold fineness data for Plutonic deposits. Typical averagefineness is 825 and range less than 150 finenessunits. Sources of data: 1: Spencer (1906); 2, 3, 5, 6, 8, 9, 10a, 11, 12a, 12c: Fisher (1945); 4: Coveney (1981); 7: Knight and McTaggart (1986); 10b: Rose (1987); 12b: Shelton (1987); 13: Wilton and Strong (1986). (N.b. 9, 11 figuresare for derived alluvials.) granite. A notable feature is an abundance of graphitic metasedimentary inclusions in both granite and rhyolite, particularly near the margin of the granite and hosting the major gold producers of the field (Shelton, 1987 ). Bullion fineness at Croydon averaged 554 for the granite-hosted and 744 for the volcanic-hosted deposits with extremely low fineness, and native silver being reported from some mines• Wilton and Strong (1986) found a range in fineness of 500-915 in the Cape Ray fault zone of southwestern Newfoundland. They present geochemical evidence for a genetic relationship to an alaskite plug, which is adjacent to the fault zone. The range of fineness values is more comparable to that of Porphyry deposits suggesting there may be some gradation between the classes. Investigation of Korean granite-hosted vein mineralisation (Shelton, 1986) suggests consistent relationships between precious metal compositions and levels of emplacement of the genetically related plutons. Deep level (2-3 km) granite-hosted mineralisation was of high fineness ( > 900). Shallow-level mineralisation was of lower fineness ( < 600), but characterised by epithermal isotope signatures. Possibly plutonic hydrothermal systems are transitional to epithermal

systems with increased meteoric groundwater involvement at shallow depths as suggested by Shelton (1986).

Porphyry deposits Gold fineness data for Porphyry deposits range widely ( 500-1000) and average values show no apparent consistency in the range 7001000 (Fig. 4). Subdivision by deposit style crudely separates vein and stockwork deposits with values mostly 700-900 from skarn, breccia and diatreme deposits with values mostly 850-1000 (Fig. 4 ). Another apparent feature is that the Cu-Mo porphyry systems typical of the western Americas have lower values than Cu-Au porphyry systems in island arcs and elsewhere. For example, the porphyry Cu-Mo deposits referred to by Antweiler and Campbell ( 1977 ) have fineness around 700, whereas the Guinaoang Cu-Au deposit in the Philippines has a fineness range 870-988. For the Cu-Mo systems, electrum is commonly associated with Pb, Zn and Ag phases. At Guinaoang, lower fineness gold is associated with chalcopyrite, whereas higher fineness gold is associated with high sulphur phases such as digenite (Ramsden et al., 1985). High fineness

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lOOO

FINENESS

Fig. 4. Gold fineness data for Porphyry deposits. Typical average fineness is 700-1000 and range 200 fineness units. Sources of data: 1, 11, 16, 17: Antweiler and Campbell (1977); 2: Morrison (1981); 3: Korobyeinikov (1976); 4,5: Katchan ( 1982); 6-10, 12-15, 19-21, 24, 27, 29: Fisher ( 1945 ); 18: Ramsden et al. ( 1985 ); 22: O'Dea ( 1980); 23: Rose ( 1987); 25: Porter and Ripley ( 1985 ); 26: Rose ( 1987); 28: Sillitoe et al. (1985) (N.b. 7 figure is for alluvials. )

in some skarn deposits may be related to the presence of tellurides (Morrison, 1981). Bowles (1984) suggested that distinctive, high fineness, Cu-rich, alluvial gold grains recovered along a major Mesozoic structural trend through Sumatra, Kalimantan, and East Java, originated in zones of skarn and porphyry Cu-Au mineralisation. High Cu contents of high fineness Au grains may be a more general feature of porphyry environments. Porphyry-related subvolcanic breccias and diatremes typically have a fineness range 8001000 and average fineness around 900 (Fig. 4). The lower average fineness (570) of the Montana Tunnels and Wau phreatomagmatic diatreme deposit may be related to the development of a late-stage epithermal system by encroachment of heated meteoric or seawater (Sillitoe et al., 1984, 1985 ).

Volcanogenic deposits Small amounts of electrum are commonly present in volcanogenic massive sulphide deposits (Shimazaki, 1974). Whereas the totalfineness range (510-875; Fig. 5) and withindeposit ranges are large, there is an apparent distinction between high fineness in Cu-Au deposits such as Mt. Morgan (850) and low fineness (down to 510) in the Pb-Zn-Ag-Au deposits of Tasmania and Japan. Probe data for individual deposits are also wide ranging. A consistent feature is strong zonation of individual gold grains from silver-rich rims to higher fineness cores with a range of probe values over one or two hundred fineness units (Shimazaki, 1974 ). Zoning is also common in gold grains in Epithermal deposits, but more complex reverse zoning is typical. This differ-

SILVERCONTENT(FINENESS)OF GOLDIN GOLD-SILVERDEPOSITS 0 h

100 I

200 I

300 I

400 I

500 i

600 I

343 700 L

800 i

900 i

1000 I

VOLCANOGENIC

1 Shakanal 2 Uchlnot ai- Nlahi

KE_Y

3 Mount Morgan 4 Q u e River

average" probe

range

&

range

& average i

bullion i

~ L

I

500

5 I

i

i

Rosebety

L

1000

FINENESS

Fig. 5. Gold fineness data for Volcanogenic deposits. There is insufficient data for a reliable estimate but the average is approximately 700 and the range 520-870. Sources of data: l: Sato (1974); 2: Shimazaki (1974); 3: Fisher (1945); 4: Ramsden and Creelman ( 1985); 5: D. Huston (Univ. Tasmania, pers. commun., 1987).

ence in zoning character in gold grains may help to distinguish these two types.

Epithermal deposits Fineness data for Epithermal deposits range widely from 0-1000 with deposit averages mostly 450-900 (Figs. 6 and 7 ). The most distinctive feature is the wide range of fineness, commonly several hundred fineness units within individual deposits. Variations of up to 600 fineness units are characteristic of epithermal deposits in the USSR (Berman et al., 1973 ). Even individual gold grains have wide fineness ranges and adjacent grains may differ significantly (Berman et al., 1973 ). The paragenetic position of gold is also complex. It may occur in altered wall rocks or within any of the several stages of veining (Berman et al., 1973; Domingo and Sato, 1981 ). An early gold bearing assemblage dominated by base-metal sulphides and a late assemblage characterised by a variety of silver sulphides, sulphosalts, or tellurides is a common paragenesis. Epithermal deposits have been classified on the basis of (a) Au/Ag ratios of ore, (b) ore, alteration and gangue mineralogy, (c) host rock lithology, and (d) geologic setting (Lindgren, 1933; Bonham, 1986; Heald et al., 1987). In compiling data for the present model, subdivisions based on all of these features were useful for evaluating fineness data (Figs. 6 and 7). The Epithermal Au-Ag group represents most of the typical volcanic-hosted adulariasericite-type epithermal deposits reported in

the literature (Heald et al., 1987 ). Many of the deposits are veins with silica or quartz, silverbearing phases and base-metal sulphides. Fineness values for the group range 0-880 with most in the range 300-800, deposit averages well clustered at 440-700 and an overall average of 600 (Fig. 6). The deposit averages, in particular, are distinctly lower than any of the other classes of deposits and suggest a distinctive hydrothermal fluid regime for the adularia-sericite type. The Epithermal sediment-hosted, or Carlinstyle, have distinctly higher fineness than the adularia-sericite types (Fig. 7). The range plotted for Carlin is based on Au/Ag ratios (930) for GetcheU, Cortez, Carlin and Gold Acres (Boyle, 1979) and as such represents minimum figures only. Although the fineness range for Cinola is large, the typical range for the group as a whole is 750-970. These values are higher than for the other Epithermal deposits and more comparable to the skarns and breccias of the Porphyry class and to the Slate Belt and Archaean classes. Epithermal Au-Te-Se deposits are characterised by the dominance of silver telluride and selenide minerals (hessite, naumanite and aguilarite) over argenite and Au/Ag production ratios close to 1 (Shikazono, 1985). Included in this group are gold telluride deposits such as Emperor, Fiji and Ogancha, USSR, gold selenide deposits such as Wolumla, Australia, and the selenide-bearing Ginguro ores of Japan (Fig. 7). For the whole group, fineness ranges 520-960, with most average values

344

G.W. MORRISONET AL. 100

200

300

400 l

500 I

600 J

700 ~

800

900

I

l

1000 1 Agatovakoye

EPITHERMAL AU-AG

2

Vesonneye

3

Karamken

4 Belaya Gora 5 Bukhtyanka 7

Pepenveyam S o p k a Rudnaya

8

Utesnoye

6

Valuniatyy Fvenskoye 1 1 Ohmidarfl-Fusei g

10

12 m

-

B - -

Taio-No 9

14

Todoroki-Shuetsu

15

Nawaji

16

~;eigoshi No. 3

17

Tel

18 20

Yatani Tengu Yatani-Kanizawa Ohe-Senzai

21

T a i o - N o 13

22

Sado

23

Nebazawa-Manzai

24

Chitose Daikoku

25 26

Yatani Honpi National Vein Bell Vein Tonapah

19

27 28 29

30

33 34

Guanajuato

35

Martha Hill

36

Thames Karangahake

32

range & average

• ¢,

L 0

37

:

range

±____

& average

I

38

probe

I

40

Waihi Beach Simau Manganl

41 42

Redjang Lebong Upper Ridges (Wau)

43

G ualilan

39

bullion

L

J 500

NO. 3

Manhattan Jay Gould Delamar Bodie Round Mtn

31

KEY

Todoroki-Chuetsu

13

1 0100

FINENESS

Fig. 6. Gold fineness data for Epithermal Au-Ag deposits. Typical average fineness is 450-700 with a range of 500 fineness units. Sources of data: 1-10: Berman et al. ( 1973); 11-25: Shikazono ( 1985 ); 26, 27: Vikre ( 1985); 28-32, 36-41, 43: Fisher ( 1945 ); 33: Mills ( 1984); 34: Petruk and Owens (1974); 35: Brathwaite et al. ( 1986); 42: Webster and Mann (1984).

700-900. These values are higher than those for the Au-Ag epithermal deposits and more comparable to the Porphyry and Plutonic classes. One suggestion, consistent with empirical observations both in the Epithermal and Porphyry classes, is that gold fineness is influenced by the presence of alloying phases in the system and in particular increased in the presence of Te or Se. Acid-sulphate type epithermal deposits, most of which are within the Epithermal Au-Te-Se group are also characterised by higher fineness

than adularia-sericite type deposits (Fig. 7). The acid-sulphate type (Heald et al., 1987) are characterised by the ore assemblage enargitepyrite-covellite, by advanced argillic (kaolinite-alunite) alteration, and by a Cu-Au +_Bi element assemblage. Their fineness values (900-1000) and other geological characteristics are comparable to those of acid-sulphate or enargite-type massive sulphide deposits in porphyry systems (Sillitoe, 1983 ). Epithermal deposits formed in the andesitic environment are also characterised by high

345

SILVER CONTENT (FINENESS) OF GOLD 1N GOLD-SILVER DEPOSITS

0L•

100 1

200 I

EPITHERMAL

300 I

400 I

500 I

600 I

700 I

800 L

gO0 I

1000 I

AU

SEDIMENT HOSTEO

•

44

• • ANDESITE HOSTED --

• •

Carlin Type

45

Cinola

46

Antamok

47

Acupan

48

Paracale Gumos

49

Masara

50 Cracow

EPITHERMAL

AU-TE-SE

ACID SULFATE

•

TELLURIDE • • - -

•

•

Q g•

l = ¢

• -I1--

°

•

range & average p r o b e : range & average bullion '

~

'

~

.8~o

~

'

Goldfield Nv

52

Summit vil(e

•

53

Mt. Kasi

54

Emperor

55 56

Choukpazat Mnogover- S h i n n o y e

57

Kochbulak

58

Zod

l

,

Fiji

59

Ogancha

60

Vostochnoye Tevinskoye

61

SELENIDE

GINGURO

81

•

62

Wolumla

63

Yalwal

64

Grassy

Gully

65

Pambula

66

Takeno

67

Ohkuchi Uahio A

68

Sanru

69

Fuke

70

Kushikino

Honpi

71

Hishikari

lOJOO

FINENESS

Fig. 7. Gold fineness data for Epithermal Au and Au-Te-Se deposits. Typical average fineness for Epithermal Au deposits is 800-900 and the range approximately 200 fineness units and for the Epithermal Au-Te-Se deposits 600-1000 and approximately 250 fineness units. Sources of data: 44: Boyle ( 1979); 45: Champigny and Sinclair ( 1982); 46-49: Doraingo and Sato ( 1981 ); 50, 51, 53-55: Fisher ( 1945); 52: Perkins and Neiman ( 1982); 56, 59-61: Berman et al. ( 1973); 57, 58: Smirnov ( 1977); 62-65: Glaser ( 1986); 66-69: Shikazono ( 1985); 70, 7 l: Izawa and Urashima (1983).

fineness (720-980; Fig. 7 ). Typical deposits in the Philippines formed in plutonic-subvolcanic regimes, commonly hosted by quartz diorite to granodiorite intrusives, and older sedimentary or volcanic units (Domingo and Sato, 1981 ). The metallogenic environment is often shared with porphyry Cu-Au deposits. High Au/Ag ratios reflect a paucity of silver phases. The most productive stage is typically replacive or paragenetically early sulphide-poor quartz vein assemblages with associated minor base metal sulphides, magnetite, pyrrhotite and arsenopyrite and very minor Au-Ag tellurides. Thus, a compilation of microprobe fineness data and re-evaluation of the bullion production data suggests that the six major classes of gold-silver forming environments are charac-

terised by distinct overall averages and ranges of fineness values (Fig. 8 ). The Archaean (including Witwatersrand), Slate Belt and Plutonic classes are characterised by high and consistent average fineness and narrow fineness range. There is no clear subdivision within these classes according to deposit style or inferred genesis. The Porphyry, Volcanogenic and Epithermal classes have variable average deposit fineness and a wide total range of fineness values. Within these three classes, higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings; deposits with Cu-Au rather than Cu-Mo or Pb-Zn element associations; deposits with acid-sulphate rather than K-feldspar-sericite assemblages and deposits with telluride or selenide minerals. The

346

G.W. MORRISON ET AL.

100 J

200 I

300 I

400 I

500 I

600

700

800

900

ARCHEAN

KEY ~ i"

- -

Overall

Average

Range

of A v e r a g e s

1000

WITWATERSRAND

Range SLATE

BELT

"k PLUTONIC

PORPHYRY

"~

VOLCANOGENIC

"k EPITHERMAL

AU-AG

EPITHERMAL

AU

"A"A-

EPITHERMAL

AU-TE-SE

HYPOTHERMAL

MESOTHERMAL

EPITHERMAL I 0

1

J

i

I

i

I

500

I

L

[

L 1000

FINENESS

Fig. 8. Overall average,range of averagesand range of fineness for major deposit classes. Compiledand simplifiedfrom Figs 2-8. Hypothermal,Mesothermaland Epithermalrange adapted from Fisher ( 1945) for comparison. limited range of average fineness values within each of the major classes suggests some possible consistency in the geochemical conditions of transport and deposition of gold and silver from fluids.

Conditions of gold and silver transport and deposition The distribution of gold and silver in precious metal deposits depends on speciation of gold and silver in the fluids, and the mechanisms of precipitation. The speciation of gold and silver in hydrothermal fluids depends upon temperature, pressure, redox state, pH and the activity of ligands.

In fluids with the same activity of ligands, speciation is controlled by the nature of metalligand interaction in the fluids. In recent years, the concept of hard and soft acid and bases has been applied to predict metal-ligand interaction (Crerar et al., 1985; Seward, 1984). The metals in a complex ion bound by ligands serve as electron donors to the molecule. Therefore, interaction between specific metal ions and ligands (co-ordinating species, such as C1- or H S - ) can be regarded as acid-base reactions, in which the metal acts as an electron acceptor and the ligand as a donor. The metal-ligand interaction can be predicted by using the hardsoft classification summarised by Huheey ( 1978 ). Metals and ligands classed as hard are

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

347

prefer sulphide ligands over C1-. As Ag ÷ is harder than Au ÷, it will preferentially bind with C1- whereas Au ÷ will prefer sulphide ligands. In addition to the chloro and sulphide complexes, in some specific environments gold can possibly form ammonium (Skibsted and Bjerrum, 1981 ), carbonyl (Kerrick and Fyfe, 1981), thioarsenite (Grigor'yeva and Sukneva, 1981 ), thioantimonite (Mironova and Zotov, 1973), and ditelluride (Seward, 1973 ) complexes. In the case of silver, carbonate complexes have been thought to be of some importance, but experimental studies (Kozlov, 1984 ) indicate very low solubility of silver in the form of carbonate complexes. At fixed temperature and pressure, speciation is a function of the redox state, pH and the

generally small in size, highly charged and show slight polarisability. The soft species (metals and ligands) in contrast are large, low in charge and highly polarisable. Hard species behave ionically, whereas soft species behave covalently. In an aqueous fluid with different ions, metal ions and ligands compete with each other to bind with a suitable ion or ligand. The soft metals bind preferentially to soft ligands and hard metals prefer hard ligands. Brimhall and Crerar (1987) classified geologically important metals and ligands. Both Ag ÷ and Au ÷, with filled d-orbital electrons are typical soft acids. Therefore, both Au + and Ag ÷ should readily form stable complexes with soft bases like C N - , CO, H2S and HS-. CI- is a borderline ligand and can form stable complexes with Au ÷ and Ag +, but these two metal ions will TABLE 1

Fluid compositions used in the calculations for ore deposits of major gold-silver forming environments Archaean

T(°C)

250-400

Fluid composition (m/kg) CO2 10 NaCI 0.155 KCI 0.005 CaCI2 0.015 MgCl2 0.005 FeCI2 H2S 0.005

Slate Belt

Plutonic

Porphyry

Volcanogenic

Epithermal

Galenasphalerite associated

Pyritechalcopyrite associated

adulariasericite associated

acid-sulphate associated

200-400

200-400

300-400

<250

>250

200-275

275-300

2.4 0.531 0.290 0.029

0.5 1.34 0.06 0.005

3.0 0.99 0.01

0.2 0.6 0.08 0.06 0.013 0.2X 10 -4 -

0.2 0.6 0.08 0.06 0.013 0.2X 10 -4 0.01

0.56 0.081 0.019

0.56 0.91 0.09

0.005

0.005

so~-

_

.

BaCI:

-

-

.

-

.

0 . 1 X I 0 -4

.

0.1X10 -4

pH buffer* redox buffer* Source

dol

cal

mus-kao

kf-mus-qz

mus-qz-mgchl

mus-kao

bio-chl-pyqz Nealland Phillips ( 1987 ); Ho (1986)

chl-kfmus-py-qz Patterson (1987)

chl-kaopy-mus-qz Peters (1987)

chl-kfmus-py-qz Baker (1987)

py-anh Huston and Large (1989); Pisutha-Arnond and Ohmoto ( 1983); Ohmoto etal. (1983)

-

-

0.5 X 10 - 4

0.005 0.005

kf-mus-qz

kao-kf-aluqz chl-kao-pychl-kf-mt- py-kao-alumus-qz mus -qz qz Huston and Large Hayba et al. Hayba et al. ( 1989); Pisutha(1985) (1985) Arnond and Ohmoto ( 1983); Ohmoto et al. (1983)

Alu - alunite; bio - biotite (annite), chl - chlorite, cal - calcite, dol - dolomite, kao - kaolinite, k f - potash feldspar, mgchl - magnesian chlorite, mus - muscovite, m t - magnetite, py - pyrite, qz - quartz. *Only mineral species are shown.

G.W.MORRISONET AU

348 TABLE 2 Equilibrium constants for some important reactions Reactions

~g~ 200°C

250°C

CaMg(CO3)2 + 4 H + = Ca 2+ (aq) + Mg 2+ (aq) + 2 CO2(aq) + H 2 0

11.67

10.23

2 KAIFe3Si3OIo(OH)2+2 H + + 2 H 2 S ( a q ) + ½O2(g)=FesAI2Si3Olo(OH)s + F e S 2 + 3 SIO2+2 K + (aq)

33.48

30.39

CaCO3 + 2H + = Ca :+ (aq) +CO2 (aq) +H20 4 KAIaSi3OIo(OH)2 + 20 FeS2+ 12 SiO2+ 52 H 2 0 = 4 FesA12Si3OIo(OH)8+ 10 O2(g) + 4 KA1Si308+40 H2S(aq) 2 KA13Si3OIo(OH)2+ 2 H + + 3 H20-3 AI2 (Si205) (OH)4 31- 2 K + (aq) 4 KA13Si3OIo(OH)2 + 10 FeS2+2 SiO2+ 4H + + 30 H 2 0 = 2 FesAl2Si3Olo(OH)8+ 4A12Si205 ( O H ) 4 + 4 K + (aq) + 3 H2S(aq) + 5 O2(g ) 3 KAISi3Os+ 2 H+=

6.632

- 580.6

7.480

-274.2

9.159

5.919

-498.6

6.592

-234.7

8.766

300°C 7.729

27.79

5.026

-429.8

5.807

-201.6

8.417

350°C 3.480

25.54

4.011

- 370.7

5.131

-173.1

8.103

400°C -3.256

23.57

2.915

--318.9

4.557

-148.1

7.818

KAI3Si3Olo(OH)2+6 SIO2+2 K + (aq) 2 KAI3Si3OIo(OH)2 + 15 Mg 2+ (aq) + 3 SiO2 + 24 H20 = 3 MgsA12Si3OIo(OH )8 +28 H ++2 K +(aq)

-73.23

-52.91

-20.06

36.61

FeS2+ 2 Ca 2+ ( a q ) + H 2 0 + 7/2 O2(g) = 2 CASO4+2 F e + 2 ( a q ) + 2 H +

124.8

111.1

100.5

92.25

85.74

128.9

3 FesAI2Si301 o ( O H ) 8 + 3 KAISi3Os + 5 / 2 O2(g) = 5 Fe304+ 3 KA13Si3OIo (OH)2 + 9 SIO2+9 H20

99.21

87.82

78.34

70.26

63.23

3 Al2Si205 (OH)4 + 6 H + + 2 K + (aq) + 4 SO2- (aq) = 2 KA13(SO4)2 ( O H ) 6 + 6 SiO2 + 3 H20

43.92

46.33

49.47

53.05

56.89

3 A12Si205 ( O H ) 4 + 2 H + + 2 K + ( a q ) + 2

262.0

229.4

202.6

180.2

161.2

FeS2+ 7 O 2 ( g ) = 2 KA13(SO4)2(OH)6+6 S I O 2 + H 2 0 + 2 Fe 2+ (aq)

CO2 (g) +4 H2(g) =CH4(g) + 2 H20 A u + 2 H2S(aq) =Au(HS)~- (aq) + 1/2 H2(g) + H + 2 A u + 3 H2S(aq)=Au2S(HS)22 - (aq) +H2(g)+2H + A u + 2 C1- (aq) + H + = AuClz(aq) + 1/2 H2(g)

6.686

4.101

1.955

0.126

-9.057

- 9.202

-9.529

-9.978

-16.72

-9.102

-17.83

- 7.090

-18.75

- 5.431

- 19.54

-3.978

- 1.465

-10.51

-20.21

-2.688

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

349

TABLE 2 ( c o n t i n u e d ) .

Reactions

Log Kr 200oc

Au+4 CI- (aq) +3 H ÷ = AuCI~- ( a q ) + 3 / 2 H2(g)

-28.98

250°C

-24.84

300°C

-20.88

350°C

- 16.70

Ag+2 H/S(aq) = Ag(HS)~- ( a q ) + 1/2 H2(g) + H +

-8.679

-8.925

-9.245

-9.624

A g + 2 C1- ( a q ) + H 2 =AgC12(aq) + 1/2 H2(g)

- 2.012

--0.437

1.242

3.054

400°C

- 11.88

- 10.04

5.013

TABLE 3 Calculated solubilities of gold and silver and characteristic features of ore deposits in major gold-silver forming environments Archaean

T(°C)

250-400

Solubility (ppb) at T (°C) 350 Au(HS)~0.2 Au(CI)~ 0.004 Ag(HS)~0.2 Ag(CI)~24,000

Slate Belt

Plutonic

Porphyry

Volcanogenic

Epithermal

galenasphalerite associated

pyritebachalcopyrite associated

adularia sericite associated

acid sulphate associated

200-400

200-400

300-400

<250

>250

200-300

275-300

350 0 0.016 10 96.000

300 1 0.007 1 19,000

400 17 0.17 24 4,800,000

250 0.05 0.015 0.04 36,000

350 1.4 0.1 1.6 560,000

300 2 3×10 -5 2 74

300 16 0.03 14 76,000

py,po cp,sp,gn Au~,Ag2S

py,mt cp,gn,sp sulfosalts bar,anh Ag2S, Agsulphosalts

py,mt cp,gn,sp

py,po cp,gn,sp asp,sulfosalts Ag2S, Agsulphosalts

py,po,hm,mt cp,gn,sp orp,real Ag2S, Agsulphosalts

py,po,hm,mt cp,gn,sp enar bar,anh,gyp,alu Ag2S, Ag-sulphosalts

Au, Ag, As, Cu, Zn, Pb, Sb, Bi, "re, Mo cooling, mixing

Au, Ag, Cu, Pb, Zn, As, Sb, Bi, Te, Ba

Au, Ag, As, Sb, Hg, Te, Pb, Zn, Cu, T1, U

cooling, mixing, dilution

boiling, cooling, mixing

Minerals: Fe-S-O base-metal As-S Sulphates Ag phases

py,po gn,cp asp Au~

py cp,gn,sp asp

Elemental association

Au, Sb, As, W, B, Te, Hg

Au, As, Sb, Zn, Pb, Cu, W

Au, As, Pb, Zn, Cu

Deposition mechanism

sulphidation

sulphidation, reduction

sulphidation

Au~

bar,anh,gyp Ag2S, Agsulphosalts

alu - alunite, anh - anhydrite, asp - arsenopyrite, bar - barite, cp - chalcopyrite, gn - galena, hm - hematite, py - pyrite, mt - magnetite, orp orpiment, real - realgar. Auss- gold-silver solid solutions.

350

G.W. MORRISON ET AL.

TABLE 4 Calculated solubilities of gold and silver and relative importance of chloro and bisulphide complexes of Au and Ag in major gold-silver forming environments Archaean

T ( °C )

250-400

Slate Belt

200-400

Plutonic

Porphyry

Volcanogenic

Epithermal

galena-sphalerite associated

pyrite-chalcopyrite associated

adularia-sericite associated

acid-sulphate associated

200-400

300-400

< 250

> 250

200-300

275-300

300 1 0.007 0,007

400 17 0.17 0.01

250 0.05 0.015 0.300

350 1.4 0.1 0.07

300 2 3X 10 -5 1.5× 10 -5

300 16 0.03 0.002

Concentration of gold based on 3 ppb Au in the source as: Au(HS)~2.99 2.94 2.98 2.97 AuCI~0.01 0.06 0.02 0.03

2.18 0.82

2.98 0.02

3 6X 10 -4

2.99 0.01

Maximum solubility o f A g ( p p b ) a s Ag(HS)f 0.2 10 Ag(C1)f 24000 96000 Agr 12000 9600

24 4800000 200000

0.04 36000 900000

1.6 560000 350000

2 74 37

14 76000 5400

Concentration of silver based on 70 ppb Ag in the source as: Ag(HS)~0.0006 0,007 0.004 0.0004 Ag(C1)f 70 70 70 70

0.0002 70

0.0002 70

1,8 68.2

0.013 69.9990

Maximum solubility of Au (ppb) at T (°C) 350 350 Au(HS)f 0.2 9 Au(Cl)f 0.004 0.016 Aur 0.02 0.002

1 19000 19000

A U r = A U ( C l f / A u ( H S ) f , Agr=Ag(Clf/Ag(HS) f .

activity ofligands. The redox state, pH and the activity of ligands depend upon the geochemical environment which buffers the fluids. In general, gold and silver transporting fluid can undergo two different types of evolution. In rock buffered evolution pH, redox state and the activities of the ligands are controlled by the mineral assemblage of the surrounding rocks through which the fluid is channeled. The existence of pervasive and vein-controlled alteration zones (propylitic, phyllic, argillic, etc.) suggests that pH and redox state were at least initially controlled by the altered wall rocks. In fluid-buffered evolution, species of the fluid (e.g., HCI, carbonate ions, C02/CH4) along with the minerals precipitated from the fluids control buffering. In those gold-silverbearing quartz veins, where vein quartz effectively isolates the wall rocks, the fluids are buffered by the fluid-species. Fluid-buffered conditions can also be generated by high fluid/ rock ratios or high fluid flow rates. During the

formation of a deposit the fluid can fluctuate from rock-buffered to fluid-buffered conditions. As the redox state, pH and the activities of ligands change differently on cooling under rock- and fluid-buffered conditions, such changes of conditions are significant in controlling transportation and deposition of oreforming elements. In Carlin-type deposits, where the mineralisation is hosted by silicified calcareous rocks, pH of the fluid is buffered by the calcareous rocks. Similarly in the Archaean greenstonetype deposits a significant part of the mineralisation is localised in the altered wall rocks indicating rock-buffered conditions. In brecciahosted deposits, clasts of altered or unaltered rock within the veins create rock-buffered conditions. In such deposits, changes in the fluid/ rock ratios, and the fluid flow rates can generate alternating rock and fluid-buffered conditions.

SILVERCONTENT(FINENESS)OFGOLDINGOLD-SILVERDEPOSITS

Solubility of gold and silver

In order to evaluate the relative contribution of chloro- and bisulphide complexes of gold and silver, the solubility of gold and silver in major gold-silver forming environments have been calculated using the CSIRO-SGTE THERMOCHEMISTRY system of Turnbull and Wadsley (1988). Geochemical constraints and the fluid composition used for these calculations have been summarised in Table 1. The fluid composition used in the calculations is based on the available fluid inclusion data. The mineral assemblages controlling pH and the redox state of the fluid have been derived based on the composition of the alteration zones and mineralised veins. Equilibrium constants of the buffering reactions have been summarised in Table 2. Figures 9 and 10 show the solubilities of gold and silver (as log molality of chloroand bisulphide complexes) in ore-forming fluids typical of each class as a function of temperature. In each case, pH and the redox state of the fluid are assumed to be buffered by the altered rocks. Gold and silver solubilities (in ppb) at the reference temperature for each class have also been shown in Tables 3 and 4. The Witwatersrand-type of gold deposits are characterised by a marked diversity in genesis (placer to metamorphic-hydrothermal ) due to which it was difficult to estimate composition of the typical ore-forming fluid. Therefore, gold and silver solubilities for this class of deposits have not been calculated. The solubilities of gold and silver in the fluids for all the classes of gold-silver deposits decreases with fall in temperature except for the fluids responsible for the acid-sulphate type of epithermal gold-silver deposits. For these fluids, solubility of gold and silver as chlorocomplexes decrease with decrease in temperature, but the amount of gold and silver associated with their bisulphide complexes increases slightly with fall in temperature (Fig. 10d).

35l

The calculated solubilities thus suggest that fluids saturated in gold and silver when cooled under rock-buffered conditions will start depositing gold and silver. In the epithermal acid sulphate type of fluids, cooling under rockbuffered conditions will only deposit silver, as more than 99% of the silver is associated with AgCI~-. With respect to gold for which Au (HS) ~- is the dominant complex, cooling will keep the fluids undersaturated in gold and the amount of gold deposited from AuCI~- will be geologically insignificant. The calculated solubilities also show the relative contribution of chloro and bisulphide complexes of gold and silver in each environment. Table 4 gives the ratio of chloro- and bisulphide associated gold (Aur) and silver (Agr) at the reference temperature for each environment. For silver, the ratio of chloro and bisulphide associated silver (Agr) is very high, indicating predominance of AgCI~- over Ag (HS)~-. The only exception is the fluids associated with the adularia-sericite type of epithermal deposits for which the ratio is around 40. For gold, in contrast, the ratio (Au~) is low indicating predominance of Au (HS) £ over AuCI~-. In fluids responsible for the formation of gold-silver mineralisation associated with the galena-spahlerite ores (volcanogenic environment), the ratio is relatively high suggesting some contribution of the chloro-complexes of gold. In all environments, silver is 30 to 20,000 times more soluble than gold, though amounts of gold and silver associated with their bisulphide complexes is broadly similar. The relative contribution of chloro and bisulphide complexes does not show any change with temperature (Figs. 9 and 10 ). At all temperatures, AgCI~- remains the dominant complex of silver, accounting for more than 99% of the total dissolved silver. In contrast, for gold, Au (HS) ~- dominates at all temperatures and the contribution of AuCI~- is geologically insignificant though it is possible that in some hypersaline fluids responsible for some goldsilver mineralisation associated with Cu-Mo

G.W.MORRISONET AL.

352 SOLUBILITY e)

Archaean

OF Au A N D Ag IN R O C K B U F F E R E D c)

Environment

FLUIDS

PIutonic Environment -4

~.~.~------~--"---L-'-- 1 0 0 p p b A g

.

.

.

.

.

• 10 ppb Au . . . . . . . . . . . . . . . . . . . . . . .

:~ - 8 e E -10

~

~°_10

. ' ~ - ' - - - - ~~2"i ~ -

o=

-

. . . . . . . . . . . . A;Cl; -t4

I 250

.

. . 270

.

. 290

.

.

-12

. 310

r 330

~350

................. ~.c~

-14 200

220

Temperature ( ° C )

b)

240

250

280

300

Temperature(°C)

Slate Belt Environment

Porphyry Environment

d)

-3

-1

-5

-3 ¸

.=

E

o E

-7-

to

b Au

Ag(HS)~,

~

--

-5.

--

-100

ppb Ag .

.

.

.

.

.

.

.

.

.

.

.

-9

-9

.................... ~,ci;

-11

o=

........... ~,c~;

-lt -13-

-250

279

290

310

330

. ...

350 -13

Temperature ( ° C )

260

280

300

320

340

360

380

400

Temperature ( ° C )

Fig. 9. Solubility of gold and silver in fluids buffered by altered rocks. The composition of fluids used in the calculations and the buffering assemblages for each gold-forming environment are shown in Table 2: (a) Archaean environment, (b) Slate Belt environment, (c) Plutonic environment and (d) Porphyry environment. The solubility is shown in the form of log molality of chloro- and sulphide-complexes of gold and silver. Dashed horizontal lines with caption indicate solubilities in ppb of gold and silver (e.g., for silver log m-- - 6 . 0 3 is equivalent to 100 ppb, - 7 . 0 3 equivalent to 10 ppb, for gold - 6 . 2 9 is equivalent to 100 ppb, - 7 . 2 9 equivalent to 10 ppb).

porphyries, geologically significant amount of gold might be linked with AuCl~-. The precipitation of gold and silver depends on the degree of saturation of these fluids in gold and silver. Saturated fluids on cooling under rock-buffered conditions will start precipitating gold and silver immediately (see Figs. 9 and 10). Slight undersaturation will cause precipitation to commence at relatively lower temperatures. Highly undersaturated fluids might require geologically unrealistic degrees of cooling, vapour loss, changes in pH and redox levels to cause precipitation. The calculated solubilities can be used to evaluate the degree of undersaturation of fluids in gold and

silver for different environments. Table 4 shows the partitioning of 3 ppb of gold (crustal abundance of gold ) and 70 ppb of silver (crustal abundance of silver) into their chloro and bisulphide complexes calculated as based on the ratios of chloro and bisulphide-associated gold and silver (Aur and Agr). A comparison of these numbers with the maximum solubility of gold and silver reveals that in all the environments the fluids will be saturated or slightly undersaturated ( < 10 times) in gold, whereas with respect to silver, except for the epithermal fluids, all other fluids will be highly undersaturated.

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

353

S O L U B I L I T Y OF Au AND Ag IN R O C K B U F F E R E D FLUIDS Volcanogenic Environment : GI - Spl Type

c) Adularia - Sericite Type Epithermal Environment

~"'-'------'--

-7--lOppb

-51

Ag . . . . . . . . . . .

~

~

~=.....~.

v

-100

_= -7 t

-lO

ppb Ag pp~

................................

>,

A~(.._._s~:: "

A. . . . . . . . . . . . . . . . . .

-11-13-

E

...........

-15-

.....

-17250

270

290

Temperature

310

330

,

200

350

q

220

"" " ""

, --

, 240

280

300

280

Tempereture(*C)

(*C)

Volcanogenic Environment : Cpy

b)

~,Cl;

• .........

Aue,2 ....

-

Py Type

Acid - Sulphate Type Epithermal Environment -2

-2

-4

-100

=

a 0 E

-8

ppb AO .

-,oppb,

c=

x:

. . . . . . . . . . . . . . . . . . . . . .

..........

-6

-100

10

...... _T--Lzt

ppb ag. ppb Au .

. . . . . .

Ao(HS);

~

. . . . . . . . . Au(HB);

.

.

.

.

.

.

.

.

E

o=-10

............. L,;e~; -12

-12 250

270

290

310

330

350

Tempereture (*C)

, 200

220

240

Temperature

260

280

300

(*C)

Fig. 10. Solubility of gold and silver in fluids buffered by altered rocks. The composition of fluids used in the calculations and the buffering assemblages for each gold-forming environment are shown in Table 2: (a) gold-silver mineralisation associated with galena-sphalerite ores in the Volcanogenic environment, (b) gold-silver mineralisation associated with pyrite-chalcopyrite ores in the Volcanogenic environment; (c) adularia-sericite type of deposits in the Epithermal environment, and (d) acid-sulphate type of deposits in the Epithermal environment. The solubility is shown in the form of log molality of chloro- and sulphide-complexes of gold and silver. Dashed horizontal lines with captions indicate solubilities in ppb of gold and silver (e.g., for silver log rn= -6.03 is equivalent to 100 ppb, -7.03 equivalent to 10 ppb, for gold -6.29is equivalent to 100 ppb, -7.29 equivalent to 10 ppb).

Mechanism of deposition and gold/silver ratio The solubility calculations have indicated that in major gold-silver forming environments transportation of gold and silver takes place in the form of the same aqueous species with AgCI~- dominating for silver and Au(HS)~- for gold. Therefore, the observed gold/silver ratios cannot be explained only by the transport mechanism. Hence, the mechanisms of deposition and the degree of undersaturation of fluids in gold and silver become significant in explaining the observed gold/silver ratios.

In addition to cooling, which according to the solubility calculations can be an effective mode of ore deposition, some gold-forming environments are characterised by the predominance of one or more processes of ore deposition. In Archean greenstone-hosted deposits, dilute CO2-rich fluids precipitate gold as a result of reaction with the iron-rich wall rocks (Neall and Phillips, 1987). This reaction results in sulphidation of the wall rocks with associated fall in the activity of reduced sulphur in the fluid. In the Slate Belt environment, ore-forming fluids generated by metamorphic dewatering

354

reactions are dilute chloride fluids with 2 to 3 m/kg of CO2. Gold deposition is thought to be the result of reaction between wall rocks and gold transporting fluids during which the fluids are reduced by the carbonaceous material of the rocks (Goldfarb et al., 1986 ). For some deposits, a more complex model has been proposed (Cox et al., 1983 ) with gold deposition attributed to mixing of a gold-transporting fluid with methane-bearing fluid derived from the wall rocks. In the Otago schist belt, gold deposition is thought to result from cooling and an increase in the fluid pH due to wall rock reaction (Paterson, 1986). Ore localisation in the vicinity of pyritic shales in some deposits (Cox et al., 1983 ) indicates that sulphidation of the wall rocks could also have played an important role in gold deposition. FO't gold-silver deposits in the Plutonic environment, the model proposed by Bohlke and Kistler (1986) may be more generally applicable. The noted similarity of the Mother Lode deposits to Archaean deposits (Taylor, 1986) suggests the broad model of mantle-lower crust interaction, metamorphic dewatering (Coveney et al., 1984) and melting may also be applicable here. Ore precipitation in some of these deposits results from cooling, reaction with the wall rocks (sulphidation) and reduction by mixing with reduced fluids derived from the wall rocks (Coveney, 1981; Bohlke, 1989). In gold-silver deposits of the Porphyry environment, the early mineralisation phase of pyrite-chalcopyrite +_ molybdenite, bornite forms from high-salinity magmatic vapor condensate at temperatures of 350-550°C (Eastoe, 1982). This mineralisation type contains free gold of moderate to high fineness intimately associated with copper sulphides and is typical low-grade ore in many porphyry CuAu deposits. The major phase of gold mineralisation in gold-rich porphyry systems, such as the Kidston breccia pipe, is related to the post-breccia stages of mineralisation associated with K-silicate and phyllic alteration

G.W. MORRISON ET AL.

(Baker, 1987). Fluid inclusion studies indicate that the post-breccia vein forming fluid, was relatively dilute (5-10 instead of 30-50 wt% eq. NaC1) and gold-silver precipitation occurred at temperatures between 250 ° and 400°C (Baker, 1987). Gold precipitation in this class of gold deposit is largely controlled by cooling, boiling or by mixing of the fluids with cooler meteoric fluids. Detailed studies on the distribution of goldsilver mineralisation in the Volcanogenic environment have revealed two distinct associations: (1) gold associated with galena-sphalerite ores concentrating in the upper part of the zinc-rich massive sulphide lenses, and (2) gold associated with chalcopyrite-pyrite ores localised as stringers and in the lower portions of the copper-rich lenses (Huston and Large, 1989). Fluid inclusion studies in similar Kuroko-type volcanogenic deposits indicate that the ore-forming fluids for the two types of ores are essentially similar except for the temperatures of mineralisation; 150°-275 °C for galena-sphalerite ores and 275 °-350°C for chalcopyrite-pyrite ores (Pisutha-Arnold and Ohmoto, 1983). Mixing of ore transporting fluids with more oxidised marine water and associated cooling are thought to be the dominant mechanisms of ore formation (Ohmoto et al., 1983). The conditions of gold-silver mineralisation in the Epithermal environment have been clearly defined from compilation of data on numerous occurrences (Buchanan, 1981 ) and by analogy with active geothermal systems (Henley, 1985). In volcanic-hosted epithermal systems, a dilute ( 1-5 wt% eq. NaCl) fluid undergoes boiling and loss of acidic volatile components, which condense and mix with more oxidised meteoric fluids in the shallower parts of the system. Ore formation in many adularia-sericite type deposits is controlled by boiling of the fluid and the associated loss of H2S (Hayba et al., 1985). In some deposits, mixing of ore-forming fluid with meteoric fluids has been suggested as the main mecha-

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

nism of ore formation (Hayba et al., 1985 ). In contrast with the adularia-sericite type, there is limited information on the geology and geochemistry of acid-sulphate type deposits. Most of the information on the geology and fluid composition for this environment has been obtained from the studies on Summitville, Colorado (Hayba et al., 1985 ). The goldsilver mineralisation is late compared to the alteration and is localised in vuggy quartz surrounded by pyrite-bearing quartz-alunite, quartz-alunite-kaolinite and illite-kaolinite zones. Limited fluid inclusion studies conducted at Summitville indicate that the ore fluids were dilute (4-6 wt% eq. NaC1) chloride solutions and that mineralisation occurred at temperatures between 230 ° and 270°C (Stoffregen, 1985, in Hayba et al., 1985) due to mixing with oxidised meteoric fluids. Thus in Archaean, Plutonic and Slate Belt environments, sulphidation of the wall rocks and the associated loss of reduced sulphur is the dominant mechanism of ore formation. In contrast, precious metal deposits in the Porphyry, Volcanogenic and Epithermal environments are characterised by more diversity in ore-forming mechanisms. In all these environments cooling, boiling and mixing of the ore fluids with meteoric fluids can cause ore formation. Whereas cooling affects chloro and bisulphide complexes of gold and silver in the same manner, decreasing their solubility, the effect of changes in pH, fo2, salinity and activity of reduced sulphur on the stability of chloro and bisulphide complexes is different. At fixed temperature, an increase in the pH and a drop in the oxidation level induces a drop in the solubility of gold and silver in the form of chlorocomplexes (Fig. 11 ). In the case of bisulphide complexes, the effect of changes in pH, oxidation potentials and the activity of reduced sulphur depends on the initial pH and oxidation state of the fluid. For a fluid with pH of 7.0 at 250 oC and buffered by the pyrite-pyrrhotite

355

assemblage (point A on Fig. 1 la and b), an increase in pH and oxidation state will increase the solubility of gold and silver as bisulphide complexes. Similar fluid, when buffered close to the magnetite-hematite buffer (point B in Fig. 11 a and b) will deposit gold and silver with increase in pH and oxidation state. Boiling of this fluid from point B will also be an effective mode of precipitating gold and silver, because boiling will be accompanied by an increase in the pH and drop in the activity of reduced sulphur. Adiabatic boiling in contrast to the isothermal vapor loss causes cooling of the fluid, which also facilitates deposition of gold and silver. The effect of mixing of the ore-forming fluid with dilute, oxidised and cooler fluid on the stability of chloro and bisulphide complexes is more complicated. An increase in the redox state of the fluid due to mixing will destabilise the bisulphide complexes, but will make the chloro-complexes more stable. Dilution of the fluids due to mixing will affect the activity of both the ligands (chloride and sulphide) and hence will destabilise chloro and bisulphide complexes. Cooling due to mixing likewise will destabilise chloro and bisulphide complexes of gold and silver. A fluid typical of the Archaean greenstone belt deposits will have almost all the gold as Au (HS)~-, whereas all the silver will be present as AgCI~- (Table 4). Sulphidation of the wall rocks due to reaction with the fluids will destabilise the bisulphide complexes and precipitate gold. As sulphidation and the associated drop in the activity of reduced sulphur will not affect the chloro-complexes, no silver will be precipitated. Hence, high fineness of gold grains and very low values of total silver in these deposits can be explained by the specific nature of the precipitation and concentration mechanism, which influences only the bisulphide complexes. In the Slate Belt and Plutonic environments, the fluids will similarly have almost all of the silver as AgCI~-, while all of the gold will be present as Au(HS)~-

356

G.W. MORRISON ET AL.

250 (*C) Total chloride = 6 0 0 0 p p m , total iron = 10ppm, total sulphur = 100ppm Gold Solubility (ppb)

a)

/o"" \

\\

b)

Silver Solubility (ppm)

Hso; so:'

'\

Io I"/'/A

- " -

|

-,o

-.o/ ,,

4

i

HzS : HS"

;

6

8

/ .....

4

'\

10

pH

12

_,oY//I 2

4

6

\:,

8

10

1

pH

Fig. 11. log ao2-pH diagram at 250°C showingthe solubilityof gold and silver as chloro- and bisulphide-complexes:(a) solubility of gold in ppb, and (b) solubility of silver in ppm. Thick full lines represent solubility contours of gold and silver. Dotted lines denote the stability fields of aqueous speciesof sulphur and thin full lines represent the stability fields of solid phases in the Fe-S-O system.For the source of thermodynamicdata see text. (Table 4 ). If a fluid, typical of the Slate Belt environment is able to dissolve all of the silver from the source rock (70 ppb), at 350°C it will be at least 1000 times undersaturated with silver. A cooling of more than 100°C would be required to start precipitating silver (Fig. 9b). Similarly, a very high degree of reduction would be essential to precipitate silver from such an undersaturated fluid at 300 ° or 350°C. With respect to gold, this fluid will be only three times undersaturated at 350 ° C. Therefore, the effect of cooling and reduction will be more pronounced in the case of gold. In some Slate Belt deposits, it is possible that cooling and sulphidation of the wall rocks are equally important in ore formation. In such deposits, high fineness of gold can be attributed to sulphidation and cooling. Sulphidation will destabilise only bisulphide complexes, precipitating gold. Cooling likewise will only precipitate gold because high degree of undersaturation in silver will obstruct precipitation of silver.

In the Plutonic environment, sulphidation is the main mechanism of ore formation which as discussed above, will only destabilise bisulphide complexes and precipitate gold. The gold-silver deposits of the Croydon field are an exception. In this field, gold-silver bearing veins hosted by the altered granites have low average fineness values compared to the veins hosted by the volcanics. The altered granites and the veins within it are marked by an abundance of graphitic metasedimentary inclusions. It has been argued that graphitic carbon played a major role in gold precipitation (Shelton, 1987). Reduction of ore-forming fluids, as discussed above affects chloro-complexes much more intensively than the bisulphide complexes, precipitating more silver and decreasing the fineness. Therefore, low fineness in the granite-hosted deposits in the Croydon field can be attributed to reduction as the dominant mechanisms of ore formation. Epithermal, Volcanogenic and Porphyry environments are characterised by complex

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

mechanisms of ore formation acting in isolation or in combination with each other, due to which precipitation of gold and silver can take place from chloro- as well as bisulphide-complexes. As discussed above, boiling, cooling and mixing of the fluids are all capable of precipitating gold and silver. Thus, higher silver concentration of ores, low fineness and a large variation in the fineness of gold in the Epithermal, Volcanogenic and Porphyry environments can be attributed to the complexities in the mechanism of ore formation. Variation in the fineness values within these three classes are difficult to rationalise, though relatively low silver concentration of silver in the ores from Porphyry-related deposits might be the result of a higher degree of undersaturation of the ore forming fluids in silver. Similarly, higher fineness of gold associated with the chalcopyritepyrite ores in the Volcanogenic environment can be attributed to higher level of undersaturation in silver of fluids responsible for the formation of these ores. The fluids responsible for gold mineralisation associated with the galena-sphalerite ores are more saturated in silver. In the case of Epithermal environment, higher silver concentrations in the ores are probably related with lower Agr value (Table 4 ), indicating that these fluids have significant amount of silver as Ag(HS)~-. Fluids associated with the adularia-sericite type of deposits are saturated in gold and close to the saturation level with respect to silver. In contrast, fluids associated with the acid-sulphate type of deposits are highly undersaturated in silver and slightly undersaturated in gold. Higher level of undersaturation with respect to silver in the fluids associated with acid-sulphate type of deposits can explain their higher fineness. Discussion

A compilation of microprobe fineness data and re-evaluation of the bullion production

3 57

suggests consistency in the range of deposit average and the total range of fineness values for each mineralising environment (Fig. 8 ). This may be sufficient to characterise each environment. However, there is enough overlap in the fineness ranges due to which fineness values alone will not always be able to discriminate between different environments. Therefore, fineness data should be used in conjunction with other characteristic features of the deposits, such as geological setting, ore and gangue mineralogy and geochemistry, and quartz textures (Dowling and Morrison, 1989). Interpretation of bullion and probe fineness data together with quartz textures and ore mineralogy have been used to discriminate Plutonic deposits with limited bulk tonnage potential from Porphyry deposits with significant bulk tonnage potential in North Queensland (Rose, 1987; Morrison, 1988 ). The fineness data are also useful as a crosscheck on the proposed origin of some deposits. For example, the average bullion fineness of 850 for Mt. Morgan, Queensland, is more typical of the Porphyry or Plutonic rather than the volcanogenic environment (Fig. 5 ). The generally accepted volcanogenic model for Mt. Morgan (Taube, 1987 ) has been challenged by Arnold and Sillitoe ( 1989 ), who favour a Porphyry-related origin for the deposit. Similarly, the fineness data for the Rosebery deposit, Tasmania (Fig. 5 ), is more consistent with the generally accepted volcanogenic model (Green et al., 1981 ) than the recently proposed metamorphic (Slate Belt) model (Aerden, 1990). The most remarkable feature of the fineness values is the consistency of values for many styles of Archaean deposits (Fig. l ) and differences with Phanerozoic deposits of similar style. There are three possible explanations for this consistency of fineness values: ( 1 ) a common origin for all the styles of mineralisation; (2) a common source for gold and silver-bearing fluids without modification during formation; and (3) a mechanism of homogenising the fineness values through post-mineralisa-

35 8

tion deformation and metamorphism. Given the variable degrees of deformation and metamorphism of the deposits and the overall similarity in fineness values with the Phanerozoic Slate Belt deposits, the most reasonable explanation is that the deposits have a common origin. The mineralising fluids may have had diverse sources, but mineralisation was only possible under a limited range of conditions (Colvine et al., 1988). There is a good comparison of average fineness values for the environment classes used here and the hypogene depth classes used by Fisher (1945). The Archaean and Slate Belt classes have average fineness values comparable to Fisher's hypothermal class, the Plutonic and Porphyry classes are comparable to the mesothermal class and the Epithermal Au-Ag and Volcanogenic classes are comparable to the Epithermal class (Fig. 8 ). This suggests a possible depth control on the origin of these deposits. In earlier studies, fineness values were either explained by the specific nature of geological setting (Titley, 1987 ) or by specific conditions of transport of gold and silver (Shimazaki and Shimizu, 1986, 1987). In these studies, the contribution of bisulphide complexes of silver was not evaluated because of the lack of basic thermodynamic data. In our study, both chloro- and bisulphide-complexes of gold and silver have been considered. As basic thermodynamic data on other aqueous species of gold and silver (carbonyl, carbonate, hydroxy, etc. ) are either absent or are not very reliable, contribution of these species in controlling the fineness values has not been evaluated. Thermodynamic data on various species allows calculations of solubilities of gold and silver at temperatures below 350°C and pressures along the liquid-vapor curve of water. For all but the Porphyry environment, the estimated temperatures of formation are below 350°C. The fluids responsible for the formation of porphyry-related gold deposits have salinities of at least 1 molal total chloride,

G.W. MORRISON ET AL.

which will extend the liquid behaviour of the fluids to about 430 ° C for above (Bodnar et al., 1985 ) and will reduce the effect of pressure on the fluid-mineral equilibria in the 350-400°C range. Hence, extrapolation of free-energy data without explicit consideration of pressure should not contribute significant errors even in the case of porphyry deposits. In this study, solubilities of gold and silver have been calculated for fluids which are typical for a specific gold-silver forming environment. In addition, pH and redox buffering assemblages are based on typical wallrock alteration described for these deposits. It has been assumed that the ore-forming fluids at least initially are buffered by the wallrocks. Because of this simplification in fluid composition and wallrock buffering assemblages, the calculated solubilities are at best the first approximation for the whole class. Therefore, the calculated solubilities of gold and silver have been used only to explain the general trends in the fineness values. These trends in the fineness values have been explained based on the assumption that gold and silver precipitating from a fluid readily form solid solution. In those deposits, where gold and silver are associated with other gold and silver phases (acanthite/argentite, teUurides and selenides) and base metal sulphides and sulphosalts (tetrahedrite-tennantite, enargite) partitioning of silver between coexisting gold and other host phase should be important in controlling the gold fineness. Preliminary thermodynamic calculations in the Au-Ag-Te-CI-S-O-H system (Jaireth, 1989) indicate that in the epithermal environment hessite (AgaTe) is much more stable than calaverite (AuTe2), and cooling of a fluid saturated in gold, silver and tellurium at 300°C precipitates hessite, decreasing significantly the activities of silver and tellurium in the fluid. As a result, the fluid only precipitates gold with hessite, which due to the unavailability o f silver should be of high fineness.

359

SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

Conclusions

( 1 ) A compilation of microprobe fineness data and re-evaluation of the bullion production data has suggested that the six major classes of gold-silver forming environments are characterised by distinct overall averages and ranges of fineness values. The Archaean (including Witwatersrand), Slate Belt and Plutonic classes are characterised by high and consistent average fineness and narrow fineness range. The Porphyry, Volcanogenic and Epithermal classes have variable average deposit-fineness and a wide total range of fineness values. Within these three classes, higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings; deposits with Cu-Au rather than Cu-Mo or Pb-Zn element associations; deposits with acid-sulphate rather than K-feldspar-sericite assemblages and deposits containing telluride or selenide minerals. (2) The calculated solubilities of gold and silver in fluids buffered by the altered rocks in major gold-forming environments indicate that silver is 30 to 200,000 times more soluble than gold. In each environment, most of the silver is associated with AgCl~-, whereas for gold, Au (HS)~- remains the dominant complex. The contribution of Au (C1) ~- in the solubility of gold is insignificant though it is possible that hypersaline fluids ( > 30 wt% eq. NaC1) associated with some gold mineralisation in the Porphyry environment might dissolve significant amounts of gold as Au (C1)~-. In each environment, an equal amount of gold and silver can be transported in the form of their bisulphide complexes. (3) As transportation of gold and silver in all the discussed environments takes place in the form of the same aqueous species the observed gold/silver ratios cannot be explained by the transport mechanism alone. Hence, gold/silver ratios in the source rock and mechanisms of deposition should be of vital signifi-

cance in explaining the observed gold/silver ratios. (4) Sulphidation of wall rocks by fluids and the associated loss of H2S destabilises the bisulphide-complexes of gold and silver without influencing the chloro-complexes. Hence, low silver concentration in the ores and high fineness values observed in deposits of the Archaean, Plutonic and Slate Belt environments can be attributed to this mechanism. In those Slate Belt deposits where reduction is the main cause of ore formation, the high fineness values can be explained by the high degree of undersaturation of ore-forming fluids in silver. (5) Precious metal deposits in Porphyry, Volcanogenic and Epithermal environments are characterised by more diversity in oreforming mechanisms. In all these environments, cooling, boiling and mixing of the fluids with meteoric water can cause ore formation. Boiling and cooling destabilise chloro and bisulphide complexes of gold and silver, while the effect of mixing of ore-forming fluids with a dilute, oxidised and cooler fluid is much more complicated. An increase in the oxidation state due to mixing destabilises only the bisulphide complexes. The chloro-complexes in contrast become more stable. Dilution and cooling due to mixing, destabilise chloro- as well as bisulphide-complexes. Higher concentrations of silver in the ores, the low fineness values and a large variation in the fineness values can be explained by the complexities in the mechanism of ore formation in these deposits.

Acknowledgements This work was supported by the AMIRA projects "Gold Deposits of Northeast Queensland" and "Epithermal Gold Deposits of Queensland". The authors wish to acknowledge the financial support of the project sponsors. W.J. Rose had additional support from Western Mining Corporation to undertake an M.Sc. on this project. S. Jaireth wishes to acknowledge the benefit of discussions with Chris

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