Epithermal gold deposits in volcanic terranes

5 Epithermal gold deposits in volcanic terranes R.W. HENLEY 5.1 Introduction Epithermal mineral deposits are the products of large-scale hydrothermal convective systems driven by magmatic heat in the upper 5-10 km of the Earth's crust. The choice of such a broad definition reflects (a) the wide use of the term 'epithermal' in contemporary mineral exploration and (b) the recognition that active geothermal systems in volcanic terranes are the present day equivalent of the epithermal systems responsible for ore formation in the geologic past (e.g. White, 1981; Henley and Ellis, 1983). The term 'epithermal' was coined by Lindgren (see Lindgren, 1922, 1933, p. 210) in an attempt to systematize hydrothermal mineral deposits on a basis similar to that beginning to be applied to metamorphic rocks. Pressure (or depth) and temperature were recognized as the controlling factors for metamorphic assemblages and both appeared able to be estimated for ore assemblages based on available data for mineral stabilities and textures. Some authors (e.g. Schmitt, 1950) have attempted to use the term 'epithermal' to refer to or to infer a specified depth range; since in most cases depths are derived from temperature estimates, the restriction of the term to a temperature range commensurate with that of explored geothermal fields (50 to -3500C) is preferred here. Such a definition of 'epithermal systems' lacks the accustomed (but generally unwieldy) specificity of many ore-deposit classifications but has the advantage of focusing on crustal environment and crustal process. Deposit types may be subdivided for convenience (e.g. when deriving exploration models) using other criteria depending on the objective of the classification; gold deposits in sedimentary rock overthrust terranes (Carlin-type deposits), for example, are reviewed in this volume by Berger and Bagby. Similarly, volcanogenic massive sulphide deposits are a product of such a hydrothermal environment but are characterized by parageneses and form which reflect seafloor depositional environments. In this chapter, epithermal deposits closely associated in time and space with terrestrial volcanic rocks are considered. Common factors in the origin of the various epithermal deposit-settings are discussed by Berger and Henley (1989). Lindgren (1922) based his classification of hydrothermal deposits on the concept of a fluid continuum from high, magmatic temperatures to lower temperatures at the surface. This approach and his focus on high-grade vein deposits reflects the mineral prices and mining technology of the time whereas today there is a greater, although not exclusive, emphasis on bulk-tonnage, low-grade deposits. As a result, present-day classification focuses more on the geological and hydrothermal 'environment' of mineral deposit formation and far less on an ad hoc use of formation temperatures. In R. P. Foster (ed.), Gold Metallogeny and Exploration © Chapman & Hall 1993 134 GOLD METALLOGENY AND EXPLORATION recognizing the broader scale environment of formation of epithermal deposits, I wish to re-emphasize the concept of Lindgren (see also Sillitoe, 1989, and this volume) that a continuum does indeed exist in ore-forming hydrothermal systems in volcanic terranes from magmatic fluid to almost entirely meteoric-groundwater environments. Thus, porphyry gold and related deposits described elsewhere in this volume relate to a higher temperature hydrothermal-magmatic environment where depositional processes are in part controlled by the depressurization of magmatic vapour and its interaction with surrounding groundwater (Henley and McNabb, 1978). Epithermal gold deposits have had a major impact on world economics past and present. The epithermal districts of northern Greece and Romania (7-10 million oz), for example, have respectively underpinned the economies of Ancient Greece and Rome (P. Eimon, personal communication). More recently, consequent on the increased value of the metal, epithermal deposits have become the focus of major exploration booms in the south-western Pacific and western United States. New discoveries such as Lihir Island, Papua New Guinea, and Hishikari, Japan, with published reserves of 6.0 x 108 g Au and 9.8 x 107 g Au respectively, rank with world-class gold deposits of other types; Kalgoorlie's Golden Mile (Archaean shear zone type), for example, has yielded some 109 g gold. Together with open-pit mining methods, the use of modem cyanide extraction and heap leach techniques has lowered recovery costs for low-grade deposits so that bulk tonnage epithermal deposits may provide a high rate of return on invested capital. Thus, both high-grade vein and largeto medium-tonnage, low-grade disseminated or breccia-hosted deposits are attractive exploration targets. Figure 5.1 provides a summary of ore reserves and grades for 'epithermal' deposits and comparison with porphyry-related Cu-Au deposits in volcanic terranes. 1 million ou nees ,/ Epithermal Hot Spring and Vein Styles 100 Breccia-hosted and Disseminated Styles C ..oJ 0 CJ .... ~"-.[/ 10 C, W C oct IX: CJ Sub economic 10tonnes 10 100 1000 TONNAGE million tonnes Figure 5.1 Summary of grade and tonnage data for epithermal- and porphyry-style gold deposits. The subdivisions reflect more the economics of gold recovery than any intrinsic geological discontinuity between deposit styles. H=Hishikari, B=Bougainville, L=Ladolam, K=Kelian and P=Paradise Peak. EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 135 This chapter will first review some exploration case histories and then proceed to discuss the geological and geochemical environment of ore formation, the distribution of 'epithermal' deposits in time and space, and exploration techniques. A comprehensive review of epithermal deposits is not possible in the space available but a number of excellent reviews and special volumes are available elsewhere (e.g. Berger and Bethke, 1985; Hedenquist, White and Siddeley, 1990). 5.2 Exploration case studies A number of epithermal deposits have been intensively studied and reported in the literature. Here we review the discovery and characteristics of three world-class deposits (_108 g contained gold, '" 30 million oz) illustrative of the bonanza vein and disseminated bulk-tonnage styles of epithermal gold deposits. The discovery histories of a number of other epithermal deposits have been summarized by Hollister (1985). Specific features of other new discoveries or of well-established epithermal deposits are considered elsewhere in this chapter. 5.2.1 Hishikari, Japan The Hishikari deposit, 45 km north-west of Kagoshima in Kyushu, rates as one of the greatest bonanza discoveries of recent times. Drilling commenced in 1980 following a five-year exploration programme by the Metal Mining Agency of Japan and the first hole encountered 0.15 m at 290 glt gold plus 167 glt silver. A series of 18 drillholes further defined the extent of high-grade mineralization and the deposit was then explored from a 2 km decline. Hot water at 6YC was encountered in the ore zone and has required a major pumping programme to lower the water table below the ore zone. Concurrent with production, exploration is continuing on this deposit the published reserves of which currently stand at 1.4 million tonnes with an average grade of 70 glt gold. Through 1988, Hishikari (at US$50/0z) was the lowest cost gold-producer in the world. Figure 5.2 shows the geology of the Hishikari deposit and is based largely on Urashima and Izawa (1982) and Urashima et ai. (1987). Gold (18-30 glt gold in clay-quartz-calcite veins) was discovered in the Neogene andesite volcanic sequence in the eighteenth century and was worked until 1940. The main host to the deeper bonanza mineralization are shales of the underlying Shimanto group. The vein mineralization consists of an anastomosing sequence of veins over a strike length of at least 1100 m and may be interpreted as a dilational jog in a transcurrent fault system. Syn-mineralization faults (Ishihara et ai., 1986) and the presence of large angular clasts of wallrock in some parts of the vein provide evidence of an active tectonic environment during mineralization. The bonanza veins are up to a few metres wide and consist of symmetrically banded (syn. crustiform) chalcedonic silica and up to 30% adularia sometimes interbanded with bladed calcite. The latter has been subsequently leached out. Gold occurs as electrum (70%Au) grains up to 70 11m in diameter. Other vein minerals consist of silver sulpho-selenides and antimonides, pyrite, minor sphalerite and chalcopyrite, and a late-stage, coarse stibnite. Kaolinite bands also occur as well as bands of fine 136 GOLD METALLOGENY AND EXPLORATION (a) 20----- / / o I 500m I Declin~ AGE (b) 0.66 j 1.1 Ma 0.95 I 1.78 Ma Vein 0.84---':".01 Ma Cretaceous Figure 5.2 Hishikari Deposit, Kyushu, Japan. (a) Plan view of veins projected to the + 70 metre level in relation to the upper surface of the Shimanto Supergroup sediments (shown as metres a.s.l.) and the access decline. Ho=Hosen vein, Da=Daisen vein, Ry=Ryosen vein and Zu=Zuisen vein. (b) General NW-SE cross-section of the Hishikari deposit. (Redrawn with permission, R. Suzuki, Sumitomo Mining Company.) EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 137 disseminated hematite. Massive reyerite ([Na, K]z Ca l4 Al2 Si 24 0 58 (OH)8, 6H20) is relatively common in the vein system and zeolites occur in veins in the andesite. Propylitic alteration is most marked in the andesite sequence and consists of K-feldspar, chlorite and mixed layer clays. Shales are only visibly altered within a few centimetres of vein contacts. Fluid inclusion temperatures from vein quartz and calcite generally are 200T with a range from 150 to 300T and some high-grade bands at 100°C. K-Ar dating of adularia provides a mineralization age in the range 0.78-1.05 Ma. The geological setting of the deposit has been discussed by Ishihara et al. (1986). Based on sulphur isotope data and a regional study of the gold content of basement rocks, these authors suggested that the deposit is associated with an unexposed porphyry intrusive related to the magnetite-series magmatic arc of southern Japan. 5.2.2 Kelian, Kalimantan, Indonesia The Kelian district was first recognized through local workings of alluvial gold in the Kelian River in the 1950s and early 1960s. The discovery of the Kelian deposit came after a programme of mapping, soil sampling, deep au gering and trenching between 1976 and 1979 (Van Leeuwen et al., 1990) followed up by a six-hole drilling programme on the most promising anomalies. This programme realized low-grade mineralization at about 2 g/t. Encouraged by the escalation of the price of gold in the early 1980s, further drilling (a total of 300 holes), together with more surface exploration and a successful I.P. geophysics survey, has presently outlined a bulk-tonnage deposit of 40 Mt at an average gold grade of 2 glt (Van Leeuwen et al., op. cit.). Mineralization has been proven well below the 200 m reserve drilling. Figure 5.3 shows the geology of the Kelian deposit; the data are from Van Leeuwen et al. (op. cit.). The deposit is associated with Late Oligocene-Early Miocene andesites, Upper Eocene(?) pyroclastics and minor rhyolites, and some PlioPleistocene basalts. It lies on a north-easterly regional trend which also contains significant epithermal mineralization at Mt. Muro and Masuparia. At the deposit scale, mineralization occurs in the margins of a set of andesite bodies intrusive into the Eocene pyroclastic unit. Mineralization is hosted by fractured andesite and tuff and by a variety of breccias ranging from primary pyroclastic and intrusion breccias to clast-supported hydrothermal breccias. Alteration is intense. A chlorite-carbonate-sericite assemblage is preserved in the cores of the larger andesite bodies and, in the East and West Prampus ore zones (Figure 5.3), alteration occurred in the following stages: sericite-pyrite, quartz-sericite-adularia-pyrite, carbonatepyrite-base metal sulphide. This alteration is overprinted by a widespread kaolin +Fe-Mn carbonate assemblage. The adularia has been dated at 20.2 ± 0.3 Ma. Mineralization is closely associated with pyrite, which makes up a few per cent of the ore, and is disseminated or is present as a fine stockwork throughout the orebodies. Other sulphides include sphalerite, galena and minor chalcopyrite, tennantitetetrahedrite, cinnabar and arsenopyrite. Gold has been observed commonly in polished sections and is associated with the carbonate-base metal sulphide assemblage as inclusions or on grain boundaries. However, much of the gold is submicroscopic, possibly associated with pyrite. Fluid inclusions from quartz, sphalerite and carbonate give temperatures in the range 270-31 WC and generally low salinities, from 0.5 to 4.2 Wt. % NaCI equivalent. 138 GOLD METALLOGENY AND EXPLORATION (a) J, LEGE NO , OUI ~ U\CJry ~ Plio .. Pleistocene CrtlOelOUI un.rODI, floo(l bOtolrs shelt sed imt-ntt []]] Ne04;lene morine ,'!dlm!'nls . SubduCTion ElJ PoltOOlne confinen'a 1marine se<llmenrs , P,e - Ter1lory bolem,n' ond (b) +- Hdimenl1i Cre locltOUI .. Eocene eoml).~ CretoclOUS Qfon ll ... /volC<lnicl KM E -W SE CTION 10,000 N LEGE ND, [;;-::t;d Super'l""" ClOy LEGEND Umllo'mOln ore lOr\!'S ~'\:.i; -.00 H'g" 9'0<1< 'O<>e () 4g/ , Au) 1---, ICoorse gold ,~ I zone )e '- - - - - - - Figure 5.3 Kelian deposit, Kalimantan. (a) Geology and location of ore zones. Kalimantan. (b) cross-sections showing geology, mineralization and alteration in the Kelian deposit. (Published with permission, T.van Leeuwan, Kelian Equatorial Mining, Jakarta.) EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 5.2.3 139 Ladolam deposit, Lihir Island, Papua New Guinea The Ladolam deposit was discovered in 1982 as part of a regional gold reconnaissance programme of the Tabar-Feni island chain to the north-east of New Ireland, Papua New Guinea (Moyle et al., 1990). Earlier geological reconnaissance had recognized hydrothermal activity and alteration but there was no history of gold workings on Lihir or adjacent islands. The discovery outcrops consist of a set of silicified and pyritized rocks adjacent to hot-spring discharges on the beach of Luise Harbour (Figure 5.4). Mapping and trenching plus soil and rock geochemistry followed in 1983 and identified the Coastal, Kapit and Lienetz mineralized zones. Surface work and diamond drilling proceeded through late 1983 to 1984 and outlined a large zone of mineralization. Drilling on vegetation anomalies in 1985 further defined the extent of the orebodies and identified a fourth ore zone, the Minifie area. By 1987, 153 diamond holes (41 000 m) and 420 reverse circulation holes (14 000 m) had been completed, identifying 167 million tonnes of ore at 3.43 glt gold. About 5% of this is oxide ore. Lihir Island lies north-east of New Ireland, within the Tabar-Feni volcanic chain which extends to the south towards Bougainville Island, location of the Panguna porphyry Cu-Au deposit (Fountain, 1972; Eastoe, 1978). Lihir itself lies at the intersection of this chain with a north-north-east-trending zone of seismic activity. The Tabar-Feni Islands are alkaline in composition and may, as suggested by Johnson (1987), be the result of partial melting of mantle lithosphere modified by earlier subduction. The Ladolam deposit lies within the Luise collapse caldera (Figure 5.4) inside the former Luise volcano. The deposit itself underlies an area of about 3 km 2 • The Lienetz, Minifie and Coastal Zone orebodies are subhorizontal, breccia-hosted deposits in volcanics and monzonitic intrusives. As at Kelian, the ore deposit is hosted by very extensive breccias derived from volcanic, intrusive and hydrothermal events. One of these is a unique open-space hydrothermal breccia, consisting of subrounded, potassic-altered intrusive and volcanic clasts, which occurs at 50-200 m below surface in the Lienetz and Coastal areas. This breccia, which typically contains gold grades in excess of 5 glt, grades downward into a more weakly mineralized breccia in which the interstices are filled with anhydrite and calcite (G. Ballantyne, personal communication). Alteration consists of an early porphyry-style event with potassic and propylitic assemblages. This is overprinted by advanced argillic, argillic and phyllic alteration associated with gold mineralization. The former is very widespread and occurs to about 100 m and locally much deeper. Alteration ages range from 0.9 Ma for biotite-altered volcanics, through 0.33 Ma for biotite-anhydrite veins, to 0.15 Ma for alunite (Davies and Ballantyne, 1987). Mineralization consists of pyrite-marcasite with minor base metal sulphides, arsenopyrite, sulphosalts and gold-silver tellurides. Free gold (50-100 /lm, 990 fineness) occurs in the oxide zone but is rare in the sulphide zone where gold occurs as < 20 /lm particles in pyrite (Moyle et al., 1990). Fluid inclusion homogenization temperatures show two maxima of 140-150T and 200-22D"C. They have salinities averaging 3.8 wt.% NaCI equivalent and have relatively high gas contents (Moyle et al., 1990). In addition to its setting on an oceanic island, the Ladolam deposit has a number of important features. The Ladolam ore zones lie within an active geothermal system with boiling hot springs, sol fa tara, and submarine discharge in Luise Harbour 140 GOLD METALLOGENY AND EXPLORATION HARBOUR _ _-.,--r. ::;: NORTH SOUTH Ladolam Creek A MINIFIE AREA "OO ______________L -________ 1.. ,1 Active thermal area ~ -- ~ Caldera -rim scree ~ ." • Kapi/ Creek ~ -L Silica cap D Advanced argillic al/ered volcanics Breccia pipes D Argillic altered Propylilic _........... Em Phyllic volcanics - .. EJ 0 0..: D ~ W9ak/y altered volcanics Argillic altered monzonite Undiff9r9nlia/ed porphyries Potassic Figure 5.4 Ladolam deposit, Lihir Island, Papua New Guinea. Geology, alteration and locations of the principal ore zones (Coastal Lienitz, Minifie and Kapit). From Moyle et al., (1990); redrawn with permission, A.l. Moyle and Kennecott Explorations (Australia) Ltd. and the Australasian Institute of Mining and Metallurgy. EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 141 emphasizing the relation between ore deposition and present-day active processes. The current hydrothermal activity represents a late evolutionary stage in the formation of this unique deposit which commenced with subvolcanic porphyry-style alteration and mineralization and evolved to the present style following caldera collapse. This has resulted in the telescoped alteration zoning and overprinting of assemblages and emphasizes the continuum from magmatic-hydrothermal to meteoric water dominated hydrothermal environments. 5.3 5.3.1 Environment of alteration and mineralization Volcanic association In contrast to the 'Carlin-type' gold deposits described by Berger and Bagby in Chapter 7 (this volume), the deposits described here all show a close temporal and spatial association with volcanism. Detailed descriptions of the geology and geological environment of individual deposits abound in the recent literature (e.g. Hedenquist, White and Siddeley, 1990, proceedings of conferences such as Pac rim [Aus!. I.M.M., Gold Coast, Queensland, 1987], Gold '88 [Geol. Soc. Australia, Melbourne, 1988], Bulk Mineable Precious Metal Deposits of the Western United States (Schafer et al., 1988) and many field excursion guides). Reviews are given by Tooker (1985) and Berger and Bonham (1990). Cenozoic epithermal gold deposits occur in convergent plate margin settings and are a consequence of alkalic, andesitic, or felsic volcanism (Berger and Henley, 1989; Berger and Bonham, 1990; Sillitoe, 1989, and this volume). The latter relates to anomalous heat flow due to intrusions from the lithosphere in back arc environments such as the Taupo Volcanic Zone of New Zealand (Henley and Hoffman, 1987) or analogous extensional environments such as the Northern Great Basin, Nevada (Noble et al., 1988). Mineralization in the San Juan Volcanic Field (Colorado) exemplifies the common time and space association of epithermal deposits with caldera development (Steven and Lipman, 1976; Lipman et al., 1976; Lipman, 1987) and Sillitoe and Bonham (1984) have discussed deposit-style as a function of volcanic landforms such as calderas, stratovolcanoes and domes. Such styles are the consequence of the hydrology induced by the volcanic structure and the relative level of magmatism (cf. Berger and Henley, 1989). Sillitoe et al. (1984) and Sillitoe and Bonham (1984) have also recognized an association of gold mineralization with large maar-diatremes in volcanic fields and cite examples such as Wau (New Guinea), Cripple Creek (Colorado) and Baguio (Phillippines). This association is not restricted to any particular volcanic composition. While most epithermal deposits are associated with calc-alkaline volcanics and intrusives, some show a close association with alkalic volcanics. The Ladolam deposit and the Cripple Creek district, Colorado, are examples and it may be argued that the potential for such rocks to act as sources of gold is dependent on magma evolution, especially with respect to the saturation state of sulphides (Keays and Scott, 1976; Wyborn, 1988) 142 5.3.2 GOLD METALLOGENY AND EXPLORATION Structural controls The structural environment of formation of epithermal deposits has received relatively little attention in the recent literature. Most information derives from earlier mining which focused on high-grade vein deposits such as at Tonopah, Nevada (see for example Wisser, 1960, and McKinstry, 1941, 1948). Both vein and bulk-tonnage deposits are often closely associated with major regional transcurrent fault structures such as the Walker Lane in Nevada. In many cases, high-grade ore shoots formed within dilational transcurrent fault jogs as discussed by Sibson (1987). Examples include Hishikari (Japan), Camp Bird (Colorado) and Pajingo (Queensland) (see Porter, 1988). At Hishikari (and at Golden Cross, New Zealand) low-grade vein sets, formed within a few hundred metres of the palaeosurface in poorly consolidated volcanics, give way at depth to major, highgrade, repetitively mineralized shear zones. This may be controlled by effective rock-stress and hydro-fracturing as discussed by Sibson (1981). The regional scale structural control of ore-forming systems may relate to a variety of structures associated with reactivation of deep crustal shears in a manner analogous to the flower structures associated with fault-reactivation beneath sedimentary basins (Harding, 1985). The Kelian deposit is also related to a major regional trend as are the Porgera and Ladolam deposits, Papua New Guinea. The latter are hosted by extensive zones of phreato-magmatic and hydrothermal breccias the formation of which may well be related to interaction of active faults with cooling magma bodies. 5.3.3 Wallrock alteration From a review of published data, Hayba et al. (1985) and Heald et al. (1987) have subdivided volcanic-hosted epithermal deposits into two classes according to their predominant rock-alteration assemblage: the 'adularia-sericite' style and the 'acidsulphate' style. In this paper (and see Berger and Henley, 1989) the mineralogical term 'alunite-kaolinite' style is preferred for the latter. The characteristics of the two styles are shown in Table 5.1. Their distinctive mineral assemblages reflect formation under quite different chemical conditions relating to the near-neutral and highly acidic pH of the ambient fluid at certain stages in their formation. Adularia-sericite-type deposits are exemplified by the three case histories outlined above. Figure 5.5 shows the general distribution of mineralization and alteration in relation to the hydrological structure of the parent hydrothermal system. Their alteration corresponds to the intermediate argillic and propylitic assemblages defined by Meyer and Hemley (1967), although, where erosion has not occurred for more than a few tens of metres below the palaeosurface, a surficial advanced argillic alteration assemblage (due to oxidized steam condensate) may be preserved. In the field, the mappable zonation of alteration is largely dependent on alteration intensity and primary rock composition, and involves minerals which in the field may not be readily identified. However, in volcanic rocks, selective alteration of phenocrysts (e.g. pyroxene to chlorite) may aid field recognition of alteration zonation. Perhaps more important in exploration is the recognition of alteration-overprinting which may relate to specific phases of ore deposition. The temperature-dependence of common alteration minerals in the propyliticargillic alteration assemblage may be empirically calibrated from geothermal drilling EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 143 data (Figure 5.6) so that the careful determination of clay mineralogy can provide a useful exploration guide to temperature and, used carefully in conjunction with an appropriate boiling point vs depth curve (Figure 5.7), to depth of formation. For example, at Golden Cross, zonation of clay minerals is closely related to ore zones (De Ronde, 1986; De Ronde and Blattner, 1988). and kaoline-alunite-style epithermal deposits Table 5.1 Characteristics of adulri~sect (modified with permission from Hayba et al., 1985) Characteristic Adulari~sect Alunite-kaolinite Structural setting Structurally complex volcanic environments, commonly in calderas Intrusive centers, 4 out of the 5 studied related to the margins of calderas Size length: width ratio Variable; some very large usually 3 : 1 or greater Relatively small equidimensional Host rocks Silicic to intermediate and alkalic volcanics Rhyodacite typical Timing of ore and host Similar ages of host and ore Similar ages of host and ore « 0.5 m.y.) Mineralogy Argentite, tetrahedrite, tennantite, native silver and gold, and base-metal sulphides. Chlorite common, selenides present, Mn gangue present, no bismuthinite Enargite, pyrite, native gold, electrum, and base-metal sulphides. Chlorite rare, no selenides, Mn minerals rare, sometimes bismuthinite Production data Both gold- and silver-rich deposits variable base-metal production Both gold- and silver-rich deposits, noteworthy Cu production Alteration Propylitic to argillic Advanced argillic to argillic (± sericitic) Extensive hypogene alunite, major hypogene kaolinite, no adularia Supergene alunite, occasional kaolinite, abundant adularia 20~3'CI 10~3°C Temperature Salinity 0~13 Source of fluids Dominantly meteoric Dominantly meteoric, possibly significant magmatic component Source of sulphide sulphur Deep-seated magmatic or derived by leaching wallrocks deep in system Deep-seated, probably magmatic Source of lead Precambrian or Phanerozoic rocks under volcanics Volcanic rocks or magmatic fluids wt.% Nacl equiv. 3 1~24 wt.% NaCI equiv. 2 I. Limited data, possibly unrelated to ore. 2. Salinities of 5-24 wt.% NaCI equiv. are probably related to the intense acid-sulphate alteration which preceded ore deposition. 3. Note: a lower salinity range applies more to Au-Ag deposits than to base-metal Ag ± Au deposits (see text). 144 GOLD METALLOGENY AND EXPLORATION boiling and vapour phase separation vein adulariacalcite-quartz Au-Ag Zn. Pb. pyrite vein and disseminated deposits related illite (C0 2 T o .o '" & & i~f ~ • <Ii illite + mixed layer II. and non-Bwe Ing dllorite l;\t( vein deposits related to boiling or mixing 2km Figure 5.5 Schema for the environments of formation of epithermal gold deposits in relation to hydrology. thermal structure, and rock alteration. Note that this 'mushrooming' commonly drawn in such schema reflects dispersion in the plane of major fracture systems and a much narrower thermal anomaly would be seen transverse to such structures. While drawn with reference to a silicic volcanic terrane with deep magmatism, similar patterns apply in other types of volcanic terrane (cf. Henley and Ellis, 1983). Certain minerals act as indicators of original subsurface conditions. Adularia and bladed (or pseudomorphed) calcite, for example (see below), are reliable indicators of original permeability and the occurrence of boiling conditions (Browne, 1978). A number of adularia-sericite-style epithermal precious metal deposits are closely associated with well-preserved silica-sinter deposits containing casts of plants, mud cracks and other diagnostic features. Berger and Eimon (1983) reviewed such deposits, which may be exemplified by McLaughlin, California (Lehrman, 1986), and by the National District (Vikre, 1985), Buckhorn (Plahuta, 1986) and Sulphur (Wallace, 1987) deposits in Nevada. Silica sinters containing plant fossils and other features common to modem sinters are well preserved in the newly discovered Palaeozoic epithermal deposits of North Queensland (Cunneen and Sillitoe, 1989; White, Wood and Lee, 1989). Such 'hot springs-style' deposits are often associated with hydrothermal eruption breccias and lacustrine sediments (Figure 5.8; cf. Figure 5.5) and are a subset of the wider adularia-sericite-style deposits. The most comprehensively studied adularia-sericite-style epithermal system is Creede, Colorado. Originally a major silver district (Steven and Eaton, 1975) with production from an extensive vein set, recent exploration has located high-grade gold mineralization 4-5 km north along strike of the base metal-silver veins (Bethke, 1988). Isotope and inclusion studies now show that the gold mineralization relates to 145 EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES I Amorphous SIlica I I - Chalcedony Quartz K-Feldspar Albite CalcIte -- -- Heulandlte Stllblte Ptllo11te LaumontIte - - I WalraKlte Montmorlllonlte(Smectlte) Mont. - IllIte (MIxed Layer> Illite . - -- .. - ---- ChlorIte (SwellIng) - ChlorIte (MIxed Layer) .- Chlorite - BIotite EpIdote ------ Prehnlte ActInolite Tremollte Dlopslde Garnet I 100 I I 200 Temperature 300 - (oC) Figure 5.6 Temperature ranges for the occurrence of common alteration minerals in active geothermal systems based on data from drill-core and downhole measurements; note that the occurrences of calcic minerals are dependent both on temperature and the CO 2 content of the ambient fluid. (From Henley and Ellis, 1983.) the original, low-salinity, convective up flow of the system controlled by magmatism on the margin of the San Luis caldera. The silver mineralization relates to a satellite convection system sourced by higher salinity fluid derived from the moat lake of the Creede caldera to the south (Rye et ai., 1988). Similarly at Guanajato, Mexico, base 146 GOLD METALLOGENY AND EXPLORATION metal-gold mineralization occurs at depth in the Veta Madre silver veins and in a newly discovered vein set cross-cutting the subparallel Villapando vein. TEMPERATURE 100 200 300'C 400 ....U) CV 800 Q.l E :r: I0- W Cl 1200 1600 Figure 5.7 Temperature-depth relations for 'boiling fluids containing NaCI and CO 2 in relation to boiling point vs. depth relations for pure water. (From Henley, 1985.) Alunite-kaolinite-style deposits are exemplified by Goldfield, Nevada (Ransome, 1909), Summitville, Colorado, Rodalquilar, Spain, and Paradise Peak, Nevada (Thomason, 1986), and their general characteristics are shown in Figure 5.9a. Somewhat smaller but otherwise similar deposits occur in Japan and these are the Nansatsu-type deposits (Hedenquist et al., 1988) such as Iwata and Kasuga (Kyushu). Sillitoe et al. (1990) have described a similar deposit from Nalesbitan, near Luzon, Phillippines, which shows a close association with a major regional transcurrent fault system. In some deposits, the occurrence of pyrophyllite indicates alteration temperatures above about > 280°C (Hemley et al., 1980) and a close genetic association of alunite-kaolinite-style mineralization with magmas and porphyry mineralization is highlighted in a number of deposits, e.g. Guinaoang deposit, Luzon, Phillippinnes (Sillitoe and Angeles, 1985). At Summitville, an assemblage of covellite-Iuzonite ± gold passes downward to chalcopyrite-tennantite; gold also occurs with barite and supergene goethite-jarosite EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES Bedded faiioul malerial 147 Silica sinter ;-:~=FfSb. A •• Au. Ag. Hg in .eam. Opalized rock or porous. vuggy silica Nallve S. cinnabar silcf~ton Disper.ed As. Sb. Au. Ag. TI Pervasive ~ =-.s;:~ I scale 250 m approx. HYdrothemal _ brecciation (low-angle veins) Au. Ag. As. Sb. TI sulfide. and qlZ atz-sulfide veins Au. Ag. As. (Cu. Pb. Zn) in sulfides with adularia _atz-sulfide veins Cu. Pb. Zn. (Au. All) in sulfides with chlorite Figure 5.8 Schematic cross-section showing the main features of a hot-springs sub-type epithermal deposit. (Redrawn with permission from Berger and Eimon, 1983.) near the surface (Stoffregen, 1987). This mineralization is mostly hosted by a zone of vuggy silica developed as pipe-like bodies within a host quartz latite. The quartz-bearing zones (up to 70 m wide) are enclosed by about 30 m of intense alunite-quartz alteration and decrease in thickness downward where they are mantled by quartz-kaolinite with much less alunite (Figure 5.9(a)). At Rodalquilar, where mineralization is associated with the rhyolitic tuffs and domes of a nested caldera in the Miocene Cabo de Gata volcanic field, deep drilling has defined mineralization to more than 860 m. Deep sericitic alteration grades upward into argillic, advanced argillic, and silicic zones and the gold mineralization (averaging 7.8 g/t) is associated primarily with the latter (Arribas et al., 1988; Cunningham et al., 1989). In this example, stable isotope data suggest that seawater was the dominant source of fluid to the deep system. In many alunite-kaolinite-style deposits, mineralization is capricious but the EI Indio deposit in Chile is a spectacular very high-grade example with ore in a more coherent vein system. Bonanza ore grades, averaging 225 glt Au, 104 glt Ag and 2.4% 148 GOLD METALLOGENY AND EXPLORATION Hydrothermal eruption breccia (a) Base of pervasive acid leaching and silicification Ore body Enargite.luzonite,tetrahedrite.tennantite, coveltite,bismuthinite,gold,tellundes : --Advanced argillic alteration Quartz,alunite.pyrophyllite. diaspore,kaolinlte.pyrite Propylitic alteration 1.'00 met'es Propylitic alteration [~OX Scale _ - Quartz-pyrite veins with quartzsericite-pyrite selvages Propylitic alteration _ Quartz-sulfide veins with K -feldspar alteration Fe,Cu,Mo sulfides (b) E W m """ 1000 500 a 0 [ E a: E OJ ""0 .S: co E .I;; '" <{ ~ ~ 0 co .~ co E c co N C ::l - 500 -1000 Basement rocks Fumaroles and acid sulfate steam-heated waters Chloride and mixed waters o I 200 km I 18/05/3 Figure 5.9 Alunite-kaolinite-style epithermal deposits_ (a) Schematic cross-section of an alunite-kaolinite-style epithermal deposit showing alteration, mineralogy and general location of ore zones. (Redrawn with permission from Silberman and Berger, 1985.) (b) Hydrology of the active hydrothermal system within the Hakone caldera and in relation to the occurrence of rhyolite domes. The regional hydraulic gradient controls groundater-flow from west to east across the system. In such an environment, high-level magma degassing at an early stage may produce extensive alunite-kaolinite alteration which subsequently becomes the focus for ore deposition when invaded by the groundwater-based adularia-sericite system. Hakone is used here as an analogy of the hydrodynamic setting of some alunite-Icaolinite systems without speculation on the possibility of gold mineralization in the system_ (Redrawn from Oki, 1983_) EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 149 Cu, occur in quartz-gold veins cross-cutting enargite-pyrite veins several metres in width (Siddeley and Araneda, 1986; Jannas et al., 1989). Barite-alunite-quartz vein and breccia deposits occur nearby at EI Tambo and perhaps represent a nearer-surface part of the system. Alunite-kaolinite-style deposits appear to be more closely related to high-level magma degassing environments than do adularia-sericite-style deposits but Berger and Henley (1989) have stressed some common characteristics which suggest that two stages of alteration and mineralization are involved in their formation. Stage I involves the intense acid alteration of host volcanics and stage 2 is the invasion of these alteration zones by a more-normal near-neutral pH fluid from which gold and associated minerals are precipitated by reaction with the alunite-kaolinite assemblage. Deen et al. (1988), using stable isotope data, have demonstrated this dynamic sequence in the Julcani district, Peru. The type of hydrodynamic environment for such mineralization is illustrated in Figure 5.9(b) through analogy with the Hakone geothermal field in Japan. 5.3.4 Fluid inclusions and light stable isotopes The systematics of fluid inclusions and stable isotopes in epithermal deposits are reviewed by Bodnar et al. (1985) and by Field and Fifarek (1985) respectively. Fluid inclusion data are sparse for alunite-kaolinite systems; Matsuhisa (personal communication) recorded only low salinities for the Nansatsu alunite-kaolinite-style deposits in Japan despite the close association with a magmatic environment. A consistent pattern, however, emerges for adularia-sericite systems (Hedenquist and Henley, 1985a), showing low salinities for epithermal gold-silver deposits and higher (~ seawater) salinities for epithermal base metal-silver ± gold deposits. In addition to geothermometry using mineral pairs, stable isotope data provide information on the dominant source or sources of water, sulphur and carbon in hydrothermal fluids. In adularia-sericite-style epithermal systems in subaerial volcanic terranes, meteoric water generally dominates throughout the evolution of the system but there is evidence in some deposits for transient input of magmatic fluid or of highly exchanged fluids from some deep crustal source. Ocean water may be shown to contribute to or to substitute for meteoric water in the convective systems developed in epithermal deposits in oceanic island settings (e.g. Drake silvergoldfield, New South Wales, Andrew et al., 1985). Carbon and sulphur isotopes commonly show magmatic signatures. In alunite-kaolinite-style deposits, as noted above, oxygen isotopes show an evolution from magmatic to meteoric sources while sulphur isotopes record the breakdown of primary magmatic sulphur dioxide to sulphate and hydrogen sulphide (Rye et al., 1989). Gold in a number of epithermal deposits is often associated with tellurium- and selenium-bearing minerals such as calaverite and naumannite. The specific controls on this element association are quite unknown, although it is often assumed that it relates to input from magmatic fluid; examples include Vatukoula, Fiji (Ahmad et at., 1987), Thames, New Zealand (Merchant, 1986), and Bessie G, Colorado (Saunders and May, 1986). The chemistry of fluids responsible for mineralization and alteration lies within the same self-consistent chemical framework as is observed in active geothermal systems. In the next section, the fluid chemistry, alteration and 150 GOLD METALLOGENY AND EXPLORATION mineralization in the active systems are briefly reviewed. For more comprehensive treatments see, for example, Henley et al. (1984) and Henley and Hedenquist (1986). 5.4 Active geothermal systems It has long been recognized that active geothermal systems in volcanic terranes are the present-day equivalents of those ancient systems responsible for gold- and silver-bearing base-metal mineralization in epithermal mining districts (e.g. Lindgren, 1933; White 1955, 1981; Henley and Ellis, 1983). Weissberg (1969), Weissberg et al. (1979) and Hedenquist and Henley (1985b) have discussed the occurrence and chemistry of hot spring precipitates in the Ohaaki Pool at Broadlands and Champagne Pool at Waiotapu, New Zealand, both of which contain around 3 oz/t gold, 6-16 oz/t silver, about 2% arsenic and antimony and, at Ohaaki, 2000 mg!kg mercury. Similar hot spring precipitates have also been described from Steamboat Springs, Nevada (White, 1967). Hot springs such as Champagne Pool represent major discharge points from extensive subsurface geothermal systems and may be depositing gold at rates of the order 0.13 million oz per 1000 years at depth (Hedenquist and Henley, 1985b). Integrated studies of mineralization and alteration in drill core from active systems and the geochemistry of geothermal fluids have provided a basis for the understanding of the hydrodynamics and the geochemistry of epithermal ore-forming systems. Studies include Browne (1969, 1971) and Browne and Ellis (1970) on Broadlands, Hedenquist and Henley (1985b) on Waiotapu, Christenson (1987) on Kawerau, and Krupp and Seward (1987) on Rotokawa. Although reliable geochemical data are now available for fumarolic gas in active volcanoes (e.g. Giggenbach, 1982,) for selfevident reasons few studies have been published on alteration and mineralization in such extreme environments - the equivalent of the early stage of evolution of alunite-kaolinite-style deposits. Thus the geochemical data reviewed below provide insight to the geochemistry of the adularia-sericite-style epithermal systems and the late stages of the evolution of alunite-kaolinite-style deposits. Exploitation of geothermal energy involves the large-scale discharge of hot water and steam from geothermal fields. The consequent rapid pressure changes and longer term changes in temperature profiles and discharge chemistry of production wells provide critical information on the hydrodynamics of geothermal systems (Elder, 1981) and the recognition of free convection as the control on the gross heat and mass transfer in such large-scale hydrothermal systems. Geological data and simulation studies have provided contraints of 104 to 10 5 years on the duration of such hydrothermal systems. Taken together with geological and geochemical data, these observations provide the basis of an integrated geochemical and hydrodynamic framework for mineralization in epithermal systems (Henley and Ellis, 1983; Henley and Hedenquist, 1986). The well-studied systems of the Taupo Volcanic Zone in New Zealand are hosted largely by porous ashflows and by well-fractured rhyolites. As with epithermal deposits, structural features control the major flows of hydrothermal fluid and these are marked by both intensity of alteration (particularly the abundance of adularia) and often the presence of hydrothermal breccias (Browne, 1978; Grindley and Browne, 1976). Temperatures in the upper few hundred metres of these systems are controlled EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 151 by mixing with cool ground waters or by phase separation (,boiling'). Boiling point vs. depth curves for fluids of differing salinity and gas content have been provided by Haas (1971) and Hedenquist and Henley (1985a) (Figure 5.7). These curves are usually drawn assuming no transfer of groundwater pressure from outside the system, although this certainly occurs in some systems (Fournier, 1987) and leads to higher temperatures at a given depth. While the bulk of the fluid in a typical system is a near-neutral pH, alkaline chloride water containing CO 2 and H 2S (see below), in the near-surface regime phase separation leads to two types of fluid. Steam-heated (syn. acid sulphate) waters result from the condensation of steam and adsorption of H 2S in regions open to atmospheric oxygen (Henley and Stewart, 1983). The resultant oxidation of sulphide to sulphate leads to low-pH conditions and a consequent distinctive, intense alteration of surficial rocks to alunite-kaolinite-silica assemblages often associated with pyrite and native sulphur. At deeper levels, the adsorption of COrrich vapour into cooler groundwater results in COrrich waters of moderate acidity (Hedenquist and Stewart, 1985). It is these waters which are responsible for many of the illite-kaolinite 'clay caps' seen in many epithermal deposits and which, through incursions consequent on declining pressure in the mineralizing systems, probably account for the late-stage occurrence of kaolinite in banded veins (e.g. Hishikari, Golden Cross) and breccia deposits (e.g. Kelian) which otherwise have mineral assemblages (adularia, calcite, quartz) indicative of near-neutral pH, boiling conditions. The second type of fluid evolved in the near-surface discharge regime, consequent on phase separation at greater depths, is an alkaline chloride water supersaturated with silica. During ascent to and discharge at the surface, the alkaline chloride water precipitates amorphous silica to form the familiar sinter terraces seen in geothermal fields and the silicified zones, containing banded chalcedonic silica seen in hot-spring-type epithermal deposits. Remixing of these water types and mixing with shallow ground waters results in the wide range of compositions and temperatures of hot springs. Geothermal systems have been explored by drilling to around 3 km but in the majority of cases only indirect evidence (for example the comparison of trace and major gas contents of geothermal and volcanic gases [Giggenbach, 1986]) provides evidence of deep magmatic fluid-input to these systems. Such an input may be indirectly argued by analogy with porphyry systems and in some cases this is supported by stable isotope (15D, (5 180) data from epithermal deposits. l.W. Hedenquist, (personal communication) has noted that, in the Philippines, one geothermal well is reported to have discharged S02 which may have been of volcanic origin. Table 5.2 provides a summary of the major element composition of a representative suite of geothermal fluids. Apart from fluids in the Salton Trough and a few other cases (e.g. Cessano, Italy), salinities lie in the range 1000-10 000 mg/kg chloride and fluid pH is close to that of neutral for water at the ambient temperature. The major element compositions are controlled by alteration mineral-fluid equilibria (Browne and Ellis, 1970; Giggenbach, 1981, 1984) and, through charge-balance, are indirectly dependent on salinity (Ellis, 1970; Henley et al., 1984). Carbon dioxide concentrations are largely independent of salinity and reflect input from deep magmatic and/or metamorphic sources (Table 5.2). Isotope data tend to favour the former so that analyses of gases from active volcanic environments (Table 5.3) may be representative of such input - both to the rocks of adularia-sericite 152 GOLD METALLOGENY AND EXPLORATION systems and, at a higher crustal level, to the early formative stages of alunitekaolinite-style systems. The concentrations of H 2S in Table 5.2 correlate with CO 2 and Giggenbach (1980) has suggested that the ratio CO 2 : H 2S is strongly controlled by alteration temperature through equilibria involving CaCO r (FeO)-FeS2 where (FeO) represents ferrous iron in aluminosilicates such as chlorite, epidote or prehnite. Table 5.2 Concentrations (mg/kg) of major constituents in fluids discharged from geothermal wells in some representative geothermal fields. Compositions have been recalculated by recombination of liquid- and vapour-phase analyses of samples taken at the wellhead. (For discussion, see Henley and Hedenquist, 1986.) Geothermal fields tOC Wajrakei, New Zealand Broadlands, New Zealand Rotokawa, New Zealand Matilo, Tongonan Philippines Na K 250 926 154 261 705 150 265 87 320 324 Mg Ca 5018 1379 11.8 0.04 5 0.6 0.5 0.01 122 1.4 Cl S04 Sh B H2S CO2 1543 22 484 20 9 348 1238 7 556 34 72 4104 520 4 710 15 245 4495 18 771 194 85 2945 9124 Table 5.3 Chemical composition of gas discharges from three calc-alkaline volcanoes; note that only the most abundant gases are shown; xgis the anhydrous gas content of the fumarole sample (mmole per mole of H20) and n' is the average oxidation state of sulphur (e.g. S = +4 in S02 and -2 in H2S). From Heinrich et al.(1989) based on data from Giggenbach (1982) and Gerlack and Casadevall (1986). Note the abundance of acidic gases and the relative abundance of S02 over H2S relative to the gases in near-neutral pH geothermal fluids - in the latter the acidity of any deep magmatic gas in neutralized by rapid alteration of primary minerals to pyrophyllite, alunite, etc. White Island, New Zealand Ngauruhoe, New Zealand Mt. St. Helens, Washington tOC Xg CO2 S, n* HCI HF NH3 620 165 617 297 +3.2 59 0.66 0.13 520 101 696 179 +1.6 31 2.6 0.31 860 84 824 67 +0.22 H2 24.1 N2 3.3 28.4 142.4 101.4 5.5 Metal transport in epithermal systems The solution chemistry of gold in hydrothermal solutions is now well understood through both experimental studies and direct observation in geothermal systems. These studies demonstrate quite clearly that in the epithermal environment the principal gold-transport species is Au(HS)z (Seward, 1973). Confirmation comes from the observation by Brown (1986) of bonanza-grade (6 wt.% gold) precipitates resulting from boiling of geothermal fluid near the control plates of discharging geothermal wells. These data confirm that the Broadlands fluid is, at depths of a few hundred to a thousand metres, close to saturation with gold as a bisulphide complex. Equivalent gold arsenide species may contribute in a small way to the solubility, but EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 153 the systematics of gold transport and deposition may be discussed satisfactorily in terms of the dominant bisulphide complex. Based on solubility reactions of the form, Au + 2H2S = Au(HS); + H+ + 0.5H2 ZnS + 2H+ + 2Cl- = ZnCl 2 + H 2S for Au, Ag, Cu, Zn, Pb, Henley (1985, 1990) has shown that the geologic solubility of gold and base metals (Ag, Pb, Zn) may be calculated for a wide range of salinities and H 2S contents. These relations are developed from empirical and thermodynamic data relating to pH, fH 2 , aK+/aH+' aH,s/fH2 and alteration assemblages in active geothermal systems. The resulting solubility isopleths are shown in Figure 5.10 for a temperature of 250T. It is important to note that, in constructing these solubility maps, reliable data are not available for the important copper and silver bisulphide complexes, nor for copper-chloride complexes. For gold, the solubility due to AuCl; has been estimated from the thermodynamic extrapolation of Helgeson and Garrels (1986) and experimental data by Bloom and Seward (in prep.) and Henley (unpublished data) and becomes important only under extreme salinity conditions near halite saturation. The chloride complex may become more important for ore transport at higher temperatures ( 40WC or so) in the porphyry environment. The important feature to note in these solubility maps is that the solubility of gold in epithermal environments is inversely proportional to salinity and directly proportional to mH,S' The solubility of chloride-complexed metals is the inverse of this behaviour and this provides the essential chemical distinction between epithermal precious metal and epithermal base metal-silver deposits. This is borne out by fluid inclusion salinity data (Hedenquist and Henley, 1985a). The solubility-salinity data underpin the observation that polymetallic volcanogenic massive sulphide deposits, such as the Kuroko deposits of Japan, are essentially seafloor equivalents of terrestrial epithermal systems. The basic ingredients of their formation are the same but different or additional depositional controls apply due to the overlying seawater. As with epithermal systems, some are gold-enriched and others not, reflecting some fundamental geochemical control in the deep source regime of the systems. Gold transport is maximized in solutions containing relatively high concentrations of H 2S which, as noted above, is inversely related, through fluid-mineral equilibria, to the concentration of CO 2, Using the thermodynamic and empirical relations of Giggenbach (1980, 1981), the H2 : H 2 S : CO 2 ratios may be related to temperature so that the solubility of gold may be expressed in fX0 2-mCI- -temperature space as illustrated in Figure 5 .1O(b). The implication of these data is clear - the solubility of gold in crustal fluids in volcanic terranes is strongly linked to CO 2 concentration and to temperature in the alteration-buffered roots of hydrothermal systems. In addition to its role as a source of metals (Au, Cu, etc.), this highlights the importance of magma degassing and magma evolution in developing epithermal ore-forming systems (note that CO 2 IS02 /H2S IH2 relations in the porphyry environment are controlled both by equilibria in the magma and by progressive high-temperature acid-alteration of country rock). 154 GOLD METALLOGENY AND EXPLORATION pH (a) 6 250·C GoldJ19/kg 4 Sliver Jig/kg Zinc J,l.g/kg Lead Ilg/kg 3 2 5 4 6 log CI (mg/kg) (b) ~ i -;. -1 o(,,) l 100 100 -2 1000 _3L-__________L -__________ ~._ 2 log CI (mg/kg) - - 250·C - 300·C - Ocean ~- - 5 Gold solubility (J,l.g/kgJ Figure 5.10 (a) Solubility controls of gold, silver, and base metals. (a) Relative solubilities of precious and base metals at 250· C in chloride-sulphide solutions the pH and redox state of which are controlled by common alteration mineral assemblages (see text). The composition range for fluids observed in active geothermal systems is shown by the stipple . H2S is the dominant sulphide species across most of the diagram but sulphate occurs under the low-salinity, low-sulphur conditions in the bottom left of the figure. Halite saturation is at about 5.54 log Cl. (b) Solubility of gold at 250 and 300·C in chloride-sulphide-C0 2 solutions buffered by common mineral equilibria (see text). The stippled area shows the general region within which two-phase conditions occur, dependent on pressure. (From Berger and Henley, 1989.) EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 5.6 155 Physico-chemical conditions in the depositional regime In discussing the transport chemistry of gold and base metals, it was shown that the principal controlling variables in rock-buffered systems are pH, mH 2S' fHz and temperature. In the high-level discharge/depositional regime of epithermal hydrothermal systems these factors change rapidly due to phase separation - boiling - in response to decreasing pressure and to dilution by near-surface cold waters (Figure 5.5). Pressures and temperatures in the depositional regime are constrained by regional hydrology and by the phase relations of the COz-H 20-X system; X represents total dissolved solids and is a subordinate factor relative to CO 2 in epithermal gold systems. Conductive heat loss is not significant in mineralized systems since the formation of an epithermal deposit itself implies high mass-flow rates and consequent high rates of convective heat transfer. During phase separation, dissolved gases partition strongly to the vapour phase (Giggenbach, 1980; Henley et al., 1984; Drummond and Ohmoto, 1985). Transfer of CO 2 out of solution in this way leads to a pH increase of 1-2 pH units relative to the original mineral-buffered solution through the reaction, CO 2(g) + H 20 = HCO; + H+ Loss of H 2S and H2 also occurs and these combined effects increase the saturation of gold from initial values close to one to high values. The loss of H2 may be such as to oxidize reduced sulphur species to sulphate, further increasing gold supersaturation. Thus boiling is one of the most important and most efficient processes whereby gold is deposited as ore but there is no preferred temperature range. Calcite and adularia act as mineralogical indicators of this process (see below) and some textures may give circumstantial evidence of episodic boiling (e.g. banded chalcedonic or opaline silica). Dilution can also be an important process leading to gold deposition in more saline systems. In such systems, dilution of chloride ion and accompanying temperature drop lead to precipitation of co-transported base metals as sulphides. The consequent removal of H 2S lowers the solubility of gold and leads to the deposition of discontinuous gold-rich shoots in some polymetallic veins (e.g., El Bronces, Chile Skewes and Camus, 1988). Deposition due to chemical changes associated with wallrock reactions are less important than in other styles of mineral deposit. Exceptions are the Carlin-type deposits (cf. Berger and Bagby, this volume) which involve dissolution of carbonate and the alunite-kaolinite-style deposits where, as discussed above, gold deposition results from the reaction of later stage, near-neutral fluids with acidic, alunite-rich alteration assemblages developed during earlier, magmatic-stage alteration. The gangue mineralogies of epithermal deposits record chemical processes in the depositional regime. Loss of CO 2 due to boiling leads to supersaturation with respect to calcite according to, CaC0:J + CO2(g) + H 2 0 = Ca++ + 2HCOj Due to the retrograde solubility of calcite (Arnorrsson, 1978; Fournier, 1985a), the mass of calcite deposited in this way is (in addition to the original CO 2 or Ca content of the fluid) dependent on the initial and final temperatures. Calcite deposited by 156 GOLD METALLOGENY AND EXPLORATION boiling has a characteristically massive, bladed texture which is often found replaced by silica or leached following silica precipitation in the matrix. The change in pH attendant on boiling commonly leads to the precipitation of adularia in vugs (e.g. Kelian and Ladolam) or interbanded with silica (e.g. Hishikari, Golden Cross, and McLaughlin). In exploration, it must be remembered that the occurrence of adularia and calcite are indicative of phase separation and are not themselves indicative of the gold potential of a deposit or prospect. Increasing concentration of silica due to vapour loss leads to the deposition of quartz or chalcedonic silica from fluids initially at about 250-300°C (Fournier, 1985b). The maximum mass of silica deposition occurs where fluids boil adiabatically to atmospheric pressure and the high supersaturations attained lead to amorphous and opaline-chalcedonic silica deposition as observed in silica sinters and shallow veins. Banding records transient pressure changes in the vein environment due to mineral deposition, permeability changes due to phase separation or, near surface, to changes in atmospheric pressure. The solubility of quartz passes through a maximum at about 320T, so that boiling of fluids at initially higher temperatures leads first to undersaturation with respect to quartz. Thus at Kelian, only small, well-terminated quartz crystals occur sporadically in vugs and there are no major zones of silicification as occur in many other epithermal deposits. Silica deposition from fluids may occur during dilution by cool ground waters (Fournier, 1985b) but the saturation levels attained are low; this favours slow deposition and the formation of well-crystallized quartz above about 200°C. Chalcedony may form at lower temperatures but generally lacks the banding characteristic of boiling environments. Massive chalcedonic silica often occurs with alunite-kaolinite deposits; in this case, the high silica saturations relate to the rapid destruction of primary aluminosilicates by acidic fluids. Over time the various polymorphs of silica recrystallize to quartz but primary deposition textures in banded veins and silica sinters are generally preserved; for example, in the 324 ± 5 Ma Pajingo deposit, Queensland, recrystallization has occurred and destroyed any primary fluid inclusions but vein textures are well preserved (Etminan et al., 1988). 5.7 Epithermal deposits through geologic time Until recently the majority of epithermal deposits were known from Cenozoic to Recent volcanic terranes. However, the ingredients for their formation - high-level magmatism, brittle fracture systems and groundwater flow - are timeless. For this reason it is likely that epithermal deposits have been formed throughout crustal evolution and the only restriction upon their present distribution relates to preservation from erosion or severe metamorphic overprinting. Newly discovered deposits in north-eastern Queensland (e.g. Pajingo-Etminan et al., 1988; WirralieFellows and Hammond, 1988; Yandan-Wood et al., 1989) are Permo- Carboniferous in age and associated with acid to intermediate volcanic suites. Their preservation has been due to younger cover rocks which are only recently being stripped in response to uplift of the Eastern Highlands. Epithermal deposits of similar age also occur at Drake, New South Wales (Bottomer, 1986) and Cracow, Queensland. Palaeozoic epithermal deposits are recognized in the Lachlan fold belt of south- eastern Australia EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 157 and in the Appalachians. In both regions, alunite-kaolinite-type deposits are preserved (e.g. Temora, New South Wales-Thompson et al., 1986; Pilot Mountain - Klein and Criss, 1988; Haile, Carolina - Kiff and Spence, 1988). Epithermal deposits have also tentatively been recognized in Archaean rocks in Canada and Australia (e.g. Adams, 1976; Groves, 1988). 5.8 Exploration Modern exploration for epithermal deposits follows the methodology developed in the 1960s and 1970s for porphyry deposits and volcanogenic massive sulphide deposits. Thus, regional geochemistry and the mapping of alteration in bedrock or float material are of prime importance. Geochemical stream sediment and soil surveys are proving increasingly useful but their utility as a cost-effective exploration method has been dependent on the development of sub-ppm, carbon-rod, atomic absorption techniques. The bulk-cyanide leach technique has also developed as a qualitative and inexpensive tool for regional gold surveys. Arsenic, antimony and mercury are covariant with gold and may be used as additional pathfinders (Silberman and Berger, 1985) and base metals and copper may be useful for some deposit styles. Thallium and tellurium may be useful as discriminants but are generally precluded by the difficulties and cost of analysis. The majority of large epithermal systems are characterized by extensive intense alteration haloes consequent on widespread feldspar and ferromagnesian mineral destructive reactions, silicification, and pyritization. Other characteristics of epithermal systems are the transfer of potassium into the upper few hundred metres of the system and, due to the presence of reduced sulphur, the widespread demagnetization of host rocks. These features ensure that airborne magnetic, radiometric and resistivity surveys are very useful in locating epithermal systems (Allis, 1990). The former have been very successful in epithermal exploration throughout eastern Australia (Tennison Woods and Webster, 1985; Irvine and Smith, 1990). Such surveys also provide basic geological information and, depending on correct flight line orientation, can locate major regional and related structures which may provide a fundamental control on the location of ore districts. The recognition of possible regional structures is also aided by satellite thematic mapper and SLAR radar imagery. Zones of extensive rock alteration may, in some cases, be detected through gravity survey (Locke and De Ronde, 1987). At the prospect scale, geochemical surveys based primarily on gold are obviously of great importance in locating ore zones. Where outcrop is sufficient or pilot drilling has been completed, alteration mapping (backed by X-ray diffraction and petrography) are both important, although the distinction of hypogene or supergene origin for kaolinite and alunite may often be impossible. Recognition of multiple alteration stages may be very important. Similarly, mapping of breccia types provides an important guide to mineralization style and possible targets. Fluid inclusion thermometry is useful in characterizing deposits but only if great care is taken in selecting suitable samples and in interpreting the homogenization data. Strong variations in electrical properties within hydrothermally altered rocks allow conductive clay-pyrite zones and resistive silicified zones to be mapped in detail by standard resistivity methods (Irvine and Smith, 1990). Controlled Source Audio- 158 GOLD METALLOGENY AND EXPLORATION Magnetic-Telluric (CSAMT) surveys have been used with varying success in targeting capricious vein systems at depth and discriminating resistive quartz-rich zones or conductive clay-pyrite zones (Austpac, 1988). In many epithermal districts, recognition of the original geomorphology provides clues to the palaeohydrology which may then aid in developing exploration targets. At Creede, Colorado, for example, the silver-base metal vein district lies on the outflow portion of a much larger epithermal system which up flowed several kilometres to the north near the ring fracture of the San Luis caldera and developed some high-grade gold mineralization. No single technique provides the key to epithermal exploration. Instead, the use of careful mapping in conjunction with geophysics and geochemistry may lead to the recognition of good prospects. Drilling as always is the ultimate test. The discovery of the Hishikari bonanza deposit, for example, was the result of a five-year combined geological and geophysical programme. Exploration drilling tested a series of helicopter-detected electromagnetic and ground resistivity anomalies coincident with a basement high recognized from gravity survey (Johnson and Fujita, 1985; Urashima et ai., 1987). Drilling also tested the complementary concept of depth-extension to the exposed andesite-hosted veins. 5.9 Summary This chapter has focused on the geochemical character of epithermal gold deposits in volcanic terranes rather than the geology of their occurrence. This is deliberate in that epithermal deposits owe more to the chemistry imposed on their host-rock package by hydrothermal fluids at high water/rock ratios than they do on their intrinsic geochemistry. Epithermal gold deposits are the product of large-scale hydrothermal systems in volcanic terranes. Their essential ingredients are a magmatic heat source in the upper few kilometres of the crust, a source of groundwater, metal and reduced sulphur, and zones of brittle fracture. These ingredients have been available throughout crustal history so that there is no restriction on age, only on preservation. Mixing of these ingredients leads to the formation of deposits and deposit- settings which share more common features than differences - one of the primary controls on deposit style is the relative level in the crust where magma degassing occurs. In the past there has been much controversy on the role of magmas as metal sources. To the author, the growing body of field evidence and geochemical data is unequivocal; high-level magmas are the source of gold in epithermal gold systems and in most cases are also the source of the sulphur required for gold transport. The ability of degassing magmas to supply metals is becoming more firmly established; Meeker (1988), for example, has shown that in December, 1986, Mt. Erebus, Antarctica, discharged daily about 0.1 kg Au together with 0.2 kg Cu, 200t HCl and 56t S02 equivalent to 360t Au per 10 000 years. What differs between deposit styles, as discussed above, may relate largely to the relative depth of intrusion. Thus the alunite-kaolinite-style deposits relate to degassing of high level magmas (e.g. rhyolite domes), with later hydrothermal flow driven by the larger, deeper magma system, whereas adularia-sericite-style systems relate to deeper magma bodies degassing into an overlying groundwater system. Convecting groundwaters serve to disperse the magma fluid. In high-permeability systems, dispersion may be so strong as to prevent EPITHERMAL GOLD DEPOSITS IN VOLCANIC TERRANES 159 the formation of an ore deposit even though a billion grams of gold may be disseminated at low grade in the upper few hundred metres of the system (e.g. Broadlands, New Zealand). In lower permeability host rocks, major structures control groundwater flow and focus fluids to a high-level deposition site (e.g. Hishikari, Japan). Extensive brecciation, due to magmatic processes in some way associated with major regional structures, appears to provide an optimum environment (Kelian, Indonesia, and Ladolam, Papua New Guinea, are examples). Epithermal gold deposits are attractive exploration targets. Although many questions remain open, the quantitative understanding of epithermal deposits based on geothermal physics and chemistry provides a much stronger, less speculative geological framework than for many other types of deposit. It is this framework, used wisely, that provides the basis for their systematic and successful exploration. Acknowledgements In this chapter I have attempted to distil some of the essential essence of epithermal systems and their gold mineralization into a few pages. This in no way does justice to the huge and growing body of literature on this style of mineral deposit. Theo van Leeuwan, Roy Suzuki and Geoff Ballantyne very kindly provided review comments on the Kelian, Hishikari and Lihir case histories respectively and permission to utilize published diagrams on these deposits, and Noel White (BHP) kindly provided some of the grade-tonnage data used in Figure 5.1. Jeff Hedenquist, Julian Hemley, Dick Sillitoe, Barney Berger, Pat Browne, Paul Eimon and Mike Smith kindly provided helpful comments on the manuscript. I thank them and the body of explorationists and researchers through whose efforts and enthusiasm our knowledge of epithermal gold deposits has flourished and will continue to flourish into the future. References Adams, G.W. (1976) Precious metal veins of the Berens River mine, Northwest Ontario. Unpublished M.Sc. thesis, Univ. of Western Ontario, London, Ontario. Ahmad, M., Solomon, M.J. and Walshe. J.L. (1987) Mineralogical and geochemical studies of the Emperor gold telluride deposit, Fiji. Econ. Geol. 82, 345-370. Allis, R.G. 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