EPITHERMAL ENVIRONMENTS AND STYLES OF MINERALIZATION: VARIATIONS AND THEIR CAUSES, AND GUIDELINES FOR EXPLORATION Noel C. White
BHP Minerals International Exploration Jeffr ey W. H edenqui denqui st
Mineral Resources Department, Geological Survey of Japan
This paper is modified slightly from Whit White, e, N.C. N.C. and and Hed Hedenqu enquis ist, t, J.W. J.W.,, 1990. 990. Epit Epithe herm rmal al Envi Enviro ronm nmen ents ts and and Sty Styles les of Miner Minerali alizat zatio ion: n: Varia Variatio tions ns and and thei theirr Caus Causes es,, and and Guid Guidel elin ines es for for Expl Explor orati ation on.. In: J.W. J.W. Hedenq Hedenquis uist, t, N.C. N.C. White White and G. Siddel Siddeley ey (Editor (Editors), s), Epither Epithermal mal gold gold minera mineralisa lisation tion of the Circum Circum Pacific Pacific:: Geolog Geology, y, Geoche Geochemist mistry ry,, Origin Origin and Explor Exploratio ation. n. Journa Journall of Geoche Geochemica micall Exploration, 36: 445-474.
EPITHERMAL ENVIRONMENTS AND STYLES OF MINERALIZATION: VARIATIONS AND THEIR CAUSES, AND GUIDELINES FOR EXPLORATION Noel C. White
BHP Minerals International Exploration Jeffr ey W. H edenqui denqui st
Mineral Resources Department, Geological Survey of Japan
ABSTRACT
Epithe Epitherma rmall precio precious us and base metal metal deposit depositss are diverse diverse,, reflect reflecting ing the differe different nt tectoni tectonic, c, igneous and structural settings in which they occur, the complexities of their local setting, and the many many process processes es involv involved ed in their their formati formation. on. Most epither epithermal mal deposi deposits ts form form at shallow shallow crust crustal al leve levels ls where where abru abrupt pt chan change gess in phy physical sical and and chemi chemica call cond conditi ition onss resu result lt in metal metal deposit deposition ion and attenda attendant nt hydro hydrothe therma rmall alterati alteration. on. The princi principal pal factor factorss that that influe influence nce the conditions prevailing in the epithermal environment, and which ultimately determine the sites and character of mineralization, include: geology (structure, stratigraphy, intrusions and rock type, which affect the style and degree of permeability and the reactivity of the host); pressure and temperature (which in the epithermal environment are related on the boiling point with depth curve); hydrology (the relationship between permeability and topography which governs fluid flow, and discharge/recharge characteristics, as well as access of steam heated waters); chemistry chemistry of the mineralizing fluid (which determines determines the metal-carrying metal-carrying capacity, capacity, as well as the associated vein and alteration assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic gradients. Many Many attem attempts pts have have been been made made to class classif ify y epith epither erma mall depo deposit sitss based based on miner mineral alog ogy y and and alteration, the host rocks, deposit form, genetic models, and standard deposits. All have their strengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred from vein and alteration mineralogy and zoning, together with the form of the deposit, deposit, and using comparative comparative examples to clarify the character of the deposit. Guidelines for exploration vary according to the scale at which work is conducted, and are commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous and structural settings can be used, together with assessment of the depth of erosion, to select areas for project area scale exploration. At project area scale direct (i.e. geochemical) or indirect guidel guidelines ines may be used. used. Indire Indirect ct methods methods involv involvee locatin locating g and interpr interpretin eting g hydro hydrothe thermal rmal alteration as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. priority. These pursuits pursuits may involve involve mineralogic, mineralogic, structural, structural, geophy geophysical sical or remote sensing methods. On a prospect scale both direct and indirect methods may be used; however however they can only be effective in the framework of a sound conceptual conceptual understanding understanding of the processes processes that occur in the epithermal epithermal environment, environment, and the signatures signatures they leave.
INTRODUCTION
A hydrothermal system undergoes abrupt physical and chemical change at the shallow depth that characterizes most epithermal deposits. This occurs because of the change from lithostatic to hydrodynamic pressure (resulting in boiling), interaction of fluids derived at depth with nearsurface water, permeability changes, and reaction between fluid and host rocks. These changes near the surface are the reason that an 'epithermal' ore environment exists, as they affect the capacity of the hydrothermal fluid to transport metals in solution. Focusing of fluid flow near the surface, in conjunction with changes changes which decrease the solubility of metals in the fluid, will then result in metal deposition deposition within a restricted restricted space. Lindgren (1933) defined the term 'epithermal' from his observations of mineralogy and texture, and he deduced the temperature and pressure (depth) conditions for this style of mineralization. Alth Althou oug gh the the inte interp rpre reta tati tion onss of his his obse observ rvat atio ions ns have have not not chan change ged d subs substa tant ntia iall lly y, our our unde underst rstan andi ding ng of the the epith epither erma mall envi enviro ronm nmen entt has has now now broa broade dene ned d as a resu result lt of a grea greatly tly increased observational base. The discovery and study of a large number of epithermal deposits outs outsid idee of the the clas classi sicc west wester ern n US sett settin ing g (e.g (e.g., ., 1990 1990)) show showss the the vari variet ety y of geol geolog ogic ic environments environments which are potential potential hosts to near-surface preciou preciouss and base metal mineralization. mineralization. The purpose of this paper is to demonstrate the usefulness, limitations, and dangers associated with how we classify the variety of mineralization styles which may be grouped as epithermal. In it we draw upon our own experience in exploration in the western Pacific and combine this with syntheses in the published literature (e.g. White, 1955, 1981; Sillitoe, 1977, 1981, 1988 a,b; Buchanan, 1981; Graybeal, 1981; Berger and Eimon, 1983; Henley and Ellis, 1983; Giles and Nelson, 1984; Berger and Bethke, 1985; Hayba et al., 1985; Henley, 1985; Henley et al., 1986; Bonham, 1986; Heald et al., 1987; Hedenquist, 1986a, 1987; Hedenquist and Houghton, 1987; Giggenbach et al., 1989; Berger and Bonham, 1990; Berger and Henley, 1989; White et al., al., 1995 1995). ). Seve Severa rall of these these pape papers rs have have impro improve ved d our our unde unders rstan tandin ding g of the the epith epither ermal mal environment by stressing its relationship to presently active hydrothermal systems. Geothermal systems systems are presently presently active examples of the systems which produced produced many epithermal epithermal deposits. What is termed the epithermal environment is represented by the upper regions (<2 km.) of geothermal systems, and study of these regions can give insight into the processes that have produced produced epithermal epithermal deposits.
SETTINGS OF EPITHERMAL MINERALIZATION
Epithermal deposits are found in a variety of geological environments which reflect various combinations of igneous, tectonic, and structural settings.
Igneous Settings
In most cases epithermal epithermal deposits are spatially and temporally temporally associated associated with subaerial volcanic volcanic rocks, and their related subvolcanic intrusions (Sillitoe and Bonham, 1984). Even where it can be shown that deposits deposits formed prior to the volcanism volcanism (e.g. the disseminated disseminated fine gold, or 'Carlin-type' deposits of the western US; White, 1982), an igneous heat source is commonly inferred. Clearly igneous activity has an important role in the formation of most epithermal deposits, if only in providing the heat necessary to generate a hydrothermal convection cell. The magm magmas as may may also also cont contri ribu bute te at leas leastt a comp compon onen entt of the the tota totall gases ases to an over overly lyin ing g
hydrothermal system (Giggenbach, 1986), and their possible contribution of metals has been speculated on (e.g. Hedenquist, 1987; Berger and Henley, 1989). The character of volcanic settings which host epithermal deposits is most commonly central to proximal, with volcanic-hosted deposits typically occurring with effusive or pyroclastic rocks (Sillitoe and Bonham, 1984). Some deposits appear to have formed in distal volcanic settings (e.g. Wirralie and Yandan, Australia; Wood et al., 1990), however these seem to be exceptions. Epithermal deposits are abundant in intermediate to acid volcanic settings; they may also occur in bimodal volcanic suites (Mitchell and Garson, 1981), but are rarely found in basic volcanics. Calc-alkaline to alkaline suites can contain significant deposits. In the rare cases where basic volcanics host epithermal deposits the volcanics commonly have shoshonitic or alkaline affinities (e.g. Emperor, Fiji; Anderson and Eaton, 1990). Various authors have attempted to identify favourable volcanic rocks on the basis of their chemical composition (e.g. Keith and Swan, 1988). In general it seems that more prospective suites are produced by I-type or A-type magmas (Ishihara, 1981; Pitcher, 1982), and show some degree of alkali enrichment (Nielsen, 1984; Mutschler et al., 1985). Modern volcanic environments in which hydrothermal activity is occurring vary widely, and have been classified into silicic depressions (commonly calderas or graben), andesitic stratovolcanos, cordilleran volcanism, and oceanic islands (Bogie and Lawless, 1987; White et al., 1995). Each of these is characterized by a different hydrological regime, which controls the discharge and recharge of the hydrothermal system, the distribution of conduits, the types and distribution of hydrothermal alteration products, and the potential sites of deposition of ore minerals.
Tectonic Settings
Subaerial volcanism may occur in a variety of tectonic settings. The igneous settings described above occur mainly as volcanic arcs in the convergent tectonic settings characteristic of oceaniccontinental, or oceanic-oceanic plate subduction (le Pichon et al., 1973). Dilational environments may develop in these settings, producing back-arc rifting, and this may evolve into a marine back-arc basin (Karig, 1971). Back arc basins are characteristically submarine, and if so, are not prospective for epithermal deposits; in contrast, massive sulfide (Kuroko) deposits are formed in this setting (Cathles et al., 1983). The Basin and Range region of the western US is an example of a wide back arc rift (Hamilton, 1985), and it is extensively mineralized with epithermal deposits (White, 1982). Similar examples include the Taupo Volcanic Zone (Hedenquist, 1986a; Cole, 1987) and the Coromandel Peninsula of New Zealand (Christie and Brathwaite, 1986). Several subaerial volcanic settings do not appear to be prospective for epithermal deposits. The regions of continental flood basalts, whether tholeiitic or alkaline, do not contain epithermal deposits. This is probably because their magma chambers are deep and/or small, and their conduits narrow, resulting in small near-surface heat anomalies which preclude the development of a major hydrothermal system. Oceanic ridge settings do not appear prospective, probably because they are typically submarine. Iceland offers a modern example of an oceanic ridge which is subaerial (le Pichon et al., 1973), and has extensive geothermal activity involving meteoric fluids. It is not apparent whether this setting may be prospective for epithermal deposits; however, as it is a very rare setting in the geological record it may not be significant for exploration. Primitive island arc settings (e.g. the Tonga-Kermadec chain) also appear
unprospective, probably as large magma chambers have not yet developed, so the necessary heat-flow conditions for major hydrothermal activity are not established.
Structural Settings
Strong structural control is almost universally recognized for gold deposits (Henley, 1990), due to the permeability enhancement caused by fractures in the near surface. Many epithermal deposits are regionally associated with volcanic-related structures (Rytuba, 1981). A close association with felsic calderas and andesitic vent complexes has been observed in the San Juan Mountains of Colorado (Steven et al., 1977), and in some parts of Japan (Kubota, 1986) and the southwest Pacific. In addition, regional faults commonly exercise important controls on epithermal deposits (Mitchell and Balce, 1990), perhaps in guiding the emplacement of the magmatic heat source and influencing subsequent hydrothermal activity (Hedenquist, 1986a). Although major faults have a regional control on the localization of deposits, mineralization is commonly not located on the major regional structure, but is situated on a subsidiary fault or splay (e.g. Baguio district, Philippines; Fernandez and Damasco, 1979). Within a prospect area even minor structural features such as bedding planes, joints, and joint intersections may have influenced the permeability and hence the distribution of mineralization.
CAUSES OF VARIATION IN THE EPITHERMAL ENVIRONMENT
Many epithermal deposits form in the shallow levels of geothermal systems (the epithermal environment). Several different factors influence the physical and chemical conditions prevailing in that environment, which ultimately determine the sites and character of mineralization. These factors include: 1. Geology - structure, stratigraphy, intrusions and rock type, all of which affect the style and degree of permeability. Rock type determines the reactivity of the host. 2. Pressure and Temperature - in the epithermal environment pressure is generally hydrostatic, or where convection occurs, hydrodynamic. Importantly, it also constrains the temperature in the system to not significantly more than that corresponding to the vapour pressure at a given depth, i.e. the boiling temperature. 3. Hydrology - in conjunction with permeability characteristics, topography determines the direction and degree of hydraulic gradients in the shallow hydrothermal system, which in turn govern fluid flow. The formation of perched, steam-heated waters, with their potential to penetrate into the hydrothermal system, is also enhanced as topographic relief increases. Palaeoclimate, and its variations during hydrothermal activity may influence availability of recharge water, and the condensation of vapours. 4. Chemistry of the mineralizing fluid - the total gas content of a fluid is important in determining several critical physical and chemical factors, including the solubility of gold and other metals; to a large extent the composition and concentration of the gases in the hydrothermal system is determined at depths below the epithermal environment. The reactivity of the fluid relates in part to the degree of neutralization of any magmatic volatiles contributed to the system, though host rock reactivity is also a factor. Cold marginal groundwater and/or steam-heated waters (the latter commonly weakly to moderately acid) may mix with the mineralizing fluid, affecting alteration and gangue deposition as well as mineralization.
5.
Syn-hydrothermal development of permeability and/or changes in hydraulic gradients related to tectonism and faulting, hydrothermal fracturing, and rock deposition/erosion.
Geology
The geological and structural characteristics of a district determine the primary and secondary permeability of rocks through which hydrothermal fluids flow. In any stratigraphic sequence of highly contrasting permeabilities, the least permeable units serve as aquitards, and the most permeable as aquifers. Fluid flow through the least permeable units is confined to joints, fractures and brecciated zones. Where impermeable units are intruded, fracture permeability is enhanced along intrusive margins (e.g., at Kelian, Kalimantan; van Leeuwan et al., 1990) which then act as fluid channels. When permeable rocks become silicified their brittleness and susceptibility to subsequent fracturing increases (e.g. Round Mountain, Nevada; Tingley and Berger, 1985). In stratigraphic sequences with little lateral continuity (e.g., andesitic stratovolcanos with discontinuous lava flows) primary permeability occurs along (brecciated) formation contacts. Secondary permeability is related to faults and fractures. These features have been well documented in the andesite-hosted geothermal systems of the Philippines (Reyes, 1990). In addition to primary and fracture-related permeability, the hydrothermal fluids themselves may generate permeability depending on the reactivity of the rocks with the fluids. Although there has been little research to document this, hydrothermal alteration that accompanies mineralization in most deposits may result in volume changes which could enhance or inhibit changes in permeability. An extreme example of permeability generated by fluid-rock reaction is found in epithermal deposits such as Summitville, Colorado (Stoffregen, 1987) and those in the Nansatsu district of Japan (Hedenquist et al., 1988). In the case of Summitville, Stoffregen (1987) concluded that strongly reactive fluids (related to magmatic acid volatiles) generated zones of high permeability by leaching everything except silica from the rock; these zones then served as conduits for later mineralizing fluids.
Pressure and Temperature
Pressure-temperature profiles in the upper 2 to 3 km of drilled geothermal systems reflect hydrodynamic conditions, i.e. the hydrostatic pressure due to a column of hot water plus the pressure due to natural upflow (Grant et al., 1982; Donaldson et al., 1983). Where permeability is high, large volumes of fluid ascend under boiling conditions at <350 o C (Grant et al., 1982). The resistance to flow is greater in systems of low permeability, resulting in a larger hydrodynamic gradient; the average for geothermal systems is about 10% above hydrostatic (Donaldson et al., 1983), though locally this can reach 40% at shallow levels (e.g. Yellowstone; White et al., 1975). The effect of dissolved salts is to increase the density and hence the hydrostatic gradient (Haas, 1971); dissolved gases have the opposite effect (Sutton and McNabb, 1977). The temperature of two phase formation (the boiling point) is constrained by the fluid vapour pressure existing at a given depth. Not all parts of a geothermal system have pressure/temperature profiles on or near to the boiling point curve. Strong groundwater dilution (common in regions of high relief with a steep hydraulic gradient, e.g. the Philippines; Reyes, 1990) shifts the ascending fluid from its boiling curve at shallow levels. Entrainment of surrounding groundwater (e.g. Mokai, New Zealand; Henley and Plum, 1985) and/or
steam-heated water is also common at shallow levels, as well as on the deeper margins of systems (Hedenquist and Browne, 1989; Hedenquist, in press), leading to marginal temperature reversals.
Hydrology
The importance of hydrology governing fluid upflow and outflow has long been recognized from geothermal exploration drilling. Surface hot spring discharges need not occur above the deep upflow zone, as geothermal fluids may be deflected in the near-surface environment if a hydraulic gradient is encountered (Hanaoka, 1980). This situation is prevalent in active systems in high relief areas such as andesitic terranes in the Philippines (e.g., Tongonan, Bacon Manito and Palinpinon; Allis, 1990; Reyes, 1990), Indonesia, the Andes and elsewhere. In these cases, hot springs commonly discharge in valleys away from their deep upflow zone, which may occur at a distance of several kilometres, and below areas of fumarolic discharges. By contrast, fluid flow in the Hatchobaru system in Kyushu is mostly vertical, due to strong fracture control in a thick sequence of andesitic lava flows (Taguchi et al., 1986), despite relief of 500 m over a distance of 2 km. Only steam-heated features occur at the surface directly over the upflow zone, which has a water table some 200 m below the surface. In high relief (>1000 m) terrane, lateral flow can extend as far as 10 km from the upflow, with hydrothermal alteration attendant over the whole path length. Alteration, and in some cases fluid inclusion studies (Izawa et al., 1981) can help to identify the degree and direction of lateral flow.
Chemistry of the Fluid
The chemistry of the hydrothermal fluid determines metal solubility through the stability of various complexes at the prevailing conditions (Seward, 1981; Henley et al., 1984). Hydrothermal alteration is also affected by the fluid chemistry, together with the temperature, volume of fluid flow, and rock type (Browne, 1978; Giggenbach, 1984). The chemistry of the ascending deep reservoir fluid is mostly determined deep within or below the epithermal environment. The chemistry of near neutral pH geothermal fluids is determined by interaction of the convecting meteoric cell with the host rocks, and with an inferred magmatic fluid component (Giggenbach, 1980, 1981, 1984, 1986, 1988). The total gas content of the ascending fluids, which is quite variable (Hedenquist and Henley, 1985b), is largely determined by the magmatic input (Giggenbach, 1986). Fluids of near neutral pH are present in the upflow zones of most geothermal systems (Browne, 1978; Henley and Ellis, 1983). In volcanic rocks these fluids result in a stable alteration assemblage including minerals such as quartz, albite, adularia, illite and/or smectites, chlorite, zeolites and other calc-silicates, calcite, pyrite and base metal sulfides. The distribution of some of these minerals (particularly clays, zeolites and calc-silicates) is temperature sensitive, and reflects the isotherms in an active system (e.g. Browne, 1978; Cole and Ravinsky, 1984; Cathelineau et al., 1985). In a fossil system this information can be used to reconstruct the hydrology of the geothermal setting (Horton, 1985; reviewed by Hedenquist and Houghton, 1987). Boiling of this near neutral pH fluid results in a change in fluid chemistry. In particular, the loss of gases during boiling can result in saturation with respect to gold bisulfide complexes, leading to gold deposition (Henley et al., 1984; Hedenquist and Henley, 1985a; Drummond and Ohmoto, 1985).
Some epithermal deposits are associated with advanced argillic alteration that was produced at high temperatures (Rye et al., 1989), and is distinct from the surficial, hybrid acid waters commonly developed in geothermal systems. These deposits may contain pyrophyllite and/or diaspore, zunyite, and sulfides including enargite, tennantite and covellite, as well as gold. The host rocks may also show evidence of acid leaching, with only residual silica left; at Summitville this has been attributed to indicate high temperature fluids with a pH less than 2 (Stoffregen, 1987). Extensive leaching of rocks by strongly acid hot ascending fluids may result from a direct, unreacted contribution of magmatic volatiles such as HCl, HF and SO 2 (Bethke, 1984; Hedenquist, 1987). These fluids are distinct from most geothermal fluids in that they have not interacted sufficiently with the host rocks to become neutralized. They are probably similar to fluids now discharging from White Island volcano, New Zealand (Giggenbach et al., 1989). These two contrasting deep fluids are extremes of a possible continuum between the geothermal and magmatic hydrothermal environments (Giggenbach, 1981, 1987). Even though the processes initially controlling the chemistry of these fluids occur beneath the epithermal environment, an understanding of their origin is required to appreciate the difference in alteration and mineralization style which they cause. In the surficial environment of geothermal systems, steam-heated waters form from condensation of steam and H 2S separated from underlying boiling fluids. The sulfide oxidizes to sulfate near the surface, thus generating acidity, which results in advanced argillic alteration assemblages (Browne, 1984). The typical mineral assemblage produced by surficial, steam-heated acid waters includes kaolin clays, alunite and cristobalite as well as native sulfur and pyrite. Since most oxidation occurs in the vadose zone, condensation of steam and gases below this level results in formation of a CO2-rich water on the margins of and overlying the deep fluid (e.g. at Waiotapu and Broadlands; Hedenquist and Browne, 1989; Hedenquist, in press). Dilution of ascending fluids (by marginal or shallow water) is the other principal process (apart from boiling) occurring in the geothermal environment (Giggenbach and Stewart, 1982). As the deep fluids rise they tend to entrain marginal fluids. This is evident from the fluid chemistry and alteration mineralogy patterns of geothermal systems (e.g. Henley and Plum, 1985; Hedenquist and Henley, 1985a; Hedenquist and Browne, 1989; Hedenquist, in press) and epithermal deposits (Roedder, 1972; Barton et al., 1977; Hayba et al., 1985). Commonly the dilution pattern in geothermal systems is towards a steam-heated fluid located on the margins of the system. This steam-heated water typically has a temperature of about 150oC, and causes a marginal alteration halo of interstratified clays (Hedenquist, in press). Like boiling, dilution may also result in metal deposition (Henley et al., 1984).
Changes in Permeability and Hydraulic Gradients During Hydrothermal Activity
The deposition of silica at shallow levels, as well as other minerals, is a cause of significant decreases in permeability during hydrothermal activity (Fournier, 1985). Leaching by acid fluids, however, may enhance permeability, as may syn-hydrothermal fracturing. Fracturing will mostly occur during catastrophic events such as faulting or hydraulic brecciation, and is quite common in extensional terranes associated with volcanic-related geothermal activity. Hydraulic fracturing, and related hydrothermal eruptions, is common in geothermal systems (Hedenquist and Henley, 1985a); textural evidence for hydraulic fracturing is also common in the epithermal
environment (e.g. Tingley and Berger, 1985; Nelson and Giles, 1985; Cooke and Bloom, 1990; Simmons and Browne, 1990). Modification of the shallow hydrology is also possible if a thick sequence of volcanics (or lacustrine sediments, etc.) were deposited during the life of a system (e.g. Berger and Bonham, 1990). In this situation, a prograde mineralogic sequence would form due to the system re-establishing new and higher hydraulic heads. Conversely, erosion during the life of the geothermal system would result in a retrograde mineralogic sequence. Evidence for changes in the level of hydraulic head in active systems is abundant (Taguchi and Hayashi, 1983; Taguchi et al., 1985; Reyes, 1990). In Japanese and Philippine geothermal systems water tables have fallen by as much as 200m over the lives of systems in areas subject to little erosion (perhaps due to deepening of drainage channels some distance away); there is evidence for up to 450m of erosion since activity began in the Palinpinon system of southern Negros (Reyes, 1990). Such changes in the water table and/or palaeosurface will have strong effects on the thermal stability, hydrology and patterns of boiling and mixing over the life of a system. Appreciating that these changes are possible, and seeking evidence for them, such as overprinting of thermal regimes (e.g. in the Palinpinon geothermal system; Leach and Bogie, 1982; Reyes, 1990), will assist in the interpretation of the fossil environment. The change from mesothermal to epithermal style of mineralization at Porgera (summarized by Richards, 1990) has been suggested to be due to the unroofing of the hydrothermal system by erosion. Apparent changes in the boiling point curve (as indicated by fluid inclusions) for different stages of veining at Acupan (Cooke and Bloom, 1990) may also relate to erosion/deposition changes during the life of the system. Climatic changes over the life of the system can also affect the water table by increasing or decreasing the availability of groundwater, which is important both as a condenser of steam, as a marginal diluent and in controlling the hydraulic head of the system.
CLASSIFICATION OF EPITHERMAL MINERALIZATION
There are various classification schemes for epithermal mineralization. We now present a variety of ways in which deposits have been grouped for their intercomparison; each has its positive and negative points. What we wish to gain from this discussion are the practical benefits each has to offer to exploration.
Divided on the Basis of Mineralogy and Alteration
The chemistry of the mineralizing fluid is one of the most important factors in determining if and where mineralization will occur during the life of a hydrothermal system. Hydrothermal alteration and ore mineralogy are good indicators of fluid chemistry and temperature, and in epithermal systems provide the only means other than fluid inclusions to estimate these critical variables. The hydrothermal alteration and ore mineralogy we use to identify the chemistry of the mineralizing fluid must be that related to mineralization, and not a later, overprinting phase of alteration during the waning of a system. Overprinting of hydrothermal alteration by an incompatible assemblage is rare in active geothermal systems, as the hydraulic head keeps the condensed hybrid fluids on the margins of the system; though in high relief terrane perched waters commonly drain back into the system along fractures sealed at depth (Reyes, 1990). Overprinting by acid condensates is commonly observed, however, in epithermal deposits (Hedenquist, 1986b; van Leeuwan et al., 1990; Simmons and Browne, 1990). Overprinting
situations must be recognized so that the alteration assemblage associated with mineralization is correctly identified. Based on the characteristics of many epithermal deposits it is possible to distinguish mineralization produced by two contrasting deep fluids having respectively near-neutral pH, and acid pH (Hayba et al., 1985; Heald et al., 1987; Hedenquist, 1987). In the near-neutral pH system, the fluids are analogous to those in active geothermal systems (Henley and Ellis, 1983). By contrast, in the less well studied deposits where mineralization is more intimately related to advanced argillic alteration, the systems appear to be analogous to hydrothermal systems adjacent to near-surface magma bodies and volcanic vents (e.g. White Island, New Zealand), in which magmatic volatiles generate acidity (Hedenquist, 1987; Giggenbach et al., 1989). Although a near-neutral pH geothermal system may have a magmatic volatile content (Giggenbach, 1986), the magma source (for heat as well as components) is much deeper (5 to 10 km?) than in the observed volcanic vent-related systems, resulting in the acid volatiles being neutralized by interaction with the host rock along their relatively long path of ascent (Giggenbach, 1981). Hydrothermal alteration related to the near-neutral pH and acid pH deep fluids has been variously described. Heald et al. (1987) used the terms 'adularia-sericite' and 'acid sulfate', respectively; Bonham (1986) used the terms 'low sulfur' and 'high sulfur'. Berger and Henley (1989) have suggested replacing the fluid term 'acid sulfate' with the mineralogic term 'kaolinite-alunite', to achieve a consistent basis with the 'adularia-sericite' grouping. Problems with these specific terms are that some adularia-sericite deposits contain very little if any adularia; though they may contain kaolinite and/or alunite, usually peripheral to, or as a later overprint to mineralization. Epithermal deposits formed from near-neutral pH geothermal systems generally have low average sulfide contents (usually less than 1 wt. %; Buchanan, 1981), however, some parts may contain much higher levels, and some epithermal base metal deposits are sulfide-rich. Although deposits formed from acid pH fluids commonly contain massive sulfide veins (e.g., El Indio; Jannas et al., 1990), others such as the Nansatsu deposits (Hedenquist et al., 1988) contain less than 1 wt. % sulfides on average. Classification based on mineral species, some of which are not specific to or diagnostic of either deposit type is clearly not appropriate, nor is the sulfur content, which varies widely in both types. Despite the variability of mineralogy and mineral abundance, the deposits always reflect the character of the fluids that produced them: this can be inferred from observations of the vein mineralogy, and the mineralogy and distribution of the hydrothermal alteration products (Table 1), so the most useful basis for classification is the fundamental contrast in fluid chemistry. Hedenquist (1987) proposed the terms 'low sulfidation' and 'high sulfidation', referring to the redox state of the sulfur present in the mineralizing fluid. This scheme was based on Bethke's (1984) distinction between systems with a high temperature acid fluid and those with a neutral pH. The "geothermal" versus "magmatic" association of the fluids can be distinguished quite clearly on the basis of stable isotope studies (Bethke, 1984; Rye et al., 1989), with one of the principal factors being whether or not there is a component of oxidized sulfur. The sulfur in near-neutral geothermal systems is generally in its lowest redox state, i.e. as sulfide with an oxidation state of -2. This is termed low sulfidation. By contrast, the sulfur in volcanic hydrothermal discharges can approach an oxidation state of +4, with all the sulfur present as SO2 (e.g. at White Island and other volcanoes; Giggenbach, 1987; Giggenbach et al.,
1986). This is termed high sulfidation. There is a possibility that some fluid compositions may be intermediate between the two endmembers (indicated by sulfur isotope chemistry) reflecting the degree of dilution of the acid high sulfidation fluids by low sulfidation meteoric waters, and neutralization by wall-rock reaction. Some high sulfidation systems may evolve from an early, very reactive fluid (responsible for rock leaching) to a later, more reduced fluid, which may be responsible for mineralization (Stoffregen, 1987, 1989; Hedenquist et al., 1988; Berger and Henley, 1989; Jannas et al., 1990). The degree of fluid evolution is likely to be variable between deposits.
Divided on the Basis of Host Rocks
Epithermal deposits are commonly classified on the basis of their host rocks, e.g., volcanic-hosted, sediment-hosted, carbonate-hosted. The volcanic-hosted deposits are commonly regarded as 'typical' epithermal deposits; they are usually low-sulfidation veins hosted by volcanic rocks. This style of deposit is very common in many parts of the world; however, deposits with characteristics similar to those referred to as volcanic-hosted may also occur in other host rocks. Some epithermal deposits have a vertical extent exceeding 1000 metres (e.g. Cripple Creek, Colorado; Thompson et al., 1985); in many instances the host volcanic rocks may not be this thick, so mineralization may extend below the volcanic rocks into basement. This is illustrated by the Hishikari epithermal vein deposit in Japan, where approximately the upper one-third of the deposit occurs in andesitic volcanic rocks and the remaining majority of the deposit is confined to veins within basement sedimentary rocks of the Shimanto Group (Izawa et al., 1990). At the Umuna deposit (Misima Island, Papua-New Guinea; Clarke et al., 1990) the deposit is hosted by metamorphosed basement rocks, but probably originally extended upwards into the thin Miocene volcanics which are preserved elsewhere on the island. In some cases the related volcanic formations may have been of limited areal extent, so volcanics need not have been present at the time of mineralization, though they were probably represented nearby. Sinters and chalcedonic epithermal veins occur at the Pliocene Puhipuhi prospect in New Zealand (Williams, 1974), although the only volcanic rocks present are basalts which probably post-date the geothermal activity. Acid volcanic domes, however, are widespread regionally, and they are possibly related to the mineralizing event. Classifying a deposit as 'volcanic-hosted' explains little about the deposit apart from the nature of the host-rocks, and as similar deposits may occur in non-volcanic hosts, the term is potentially misleading. 'Carlin-type' deposits, i.e., disseminated fine-gold deposits hosted by carbonaceous calcareous sediments, are described commonly as 'sediment-hosted', but this term fails to convey that a particular type of deposit, hosted by very specific sediments, is referred to. Using this terminology the lower two-thirds of the Hishikari deposit mentioned above would not be classified as a sediment-hosted deposit (as it is not 'Carlin-type'), despite being hosted by sediments. A similar difficulty arises from the term 'carbonate-hosted' for the same type of deposit; although it more adequately describes the host-rocks, it conveys nothing about the very specific features that characterize these deposits, except that the host rocks were relatively reactive.
Divided on the Basis of Deposit Form
One of the simplest ways of describing any type of gold deposit is in terms of the form of the deposit. This basis for description conveys nothing about the host rocks, textures or genesis of the deposit, but does convey important information on the spatial distribution of the mineralization, which in turn has important implications for exploration. Epithermal deposits are diverse in their forms; however, in most cases they can be regarded as combining in various proportions the characteristics of three end-members; vein deposits, stockwork deposits, and disseminated deposits. The deposit form is essentially a result of the host rock permeability during mineralization. Vein deposits consist of a limited number of discrete veins with well-defined vein walls; they have sharp grade cut-offs. Veins may pass laterally into local zones of vein breccia having the form of veins, but consisting of breccia containing clasts of wall-rock and vein material in a hydrothermal vein matrix (e.g. Mt. Muro; Simmons and Browne, 1990; Acupan; Cooke and Bloom, 1990). The matrix commonly consists of fine-grained quartz with abundant very fine-grained pyrite giving it a dark grey colour. In areas of structural complexity or where the veins change orientation, they may locally become stockworks. Veins can be mineralized over vertical intervals of 150 to 1000 m, and have a highly variable ratio of vertical to lateral extent (Buchanan, 1981). There are numerous examples of epithermal vein deposits, including Creede, Colorado; Hishikari and Kushikino, Japan; El Indio, Chile; Acupan, Philippines; Lebong Donok, Indonesia; Pajingo, Australia; Karangahake and Waihi, New Zealand; Fresnillo, Mexico. Stockwork deposits consist of relatively narrow interconnected veins forming complex zones; a sufficiently high concentration of veins may allow the zone to be mined in bulk. Although individual veins have sharp cut-offs in grade, the stockwork zone is normally defined by an assay cut-off, determined by the concentration and grade of individual veins. Stockworks are most likely to develop in areas of structural complexity where faults intersect or change direction, or intersect competent (i.e. brittle) lithologies. Exclusively stockwork-type epithermal deposits are not numerous, as they typically occur as part of a vein deposit. Examples which are at least in part stockworks include McLaughlin, California; Las Torres at Guanajuato, Mexico; Golden Sunlight, Montana; Taio, Japan; Gunung Pani, Indonesia; Golden Cross, New Zealand; Hidden Valley and Misima, Papua New Guinea; Motherlode and Placer, Philippines; Gold Ridge, Solomon Islands. Disseminated lode deposits are characterized by mineralization dispersed through the (typically altered) host rocks, rather than confined to discrete veins. The orebody boundaries are defined by assay cut-offs. Disseminated deposits are less common than veins for low sulfidation deposits in volcanic host-rocks. They are, however, common for high sulfidation deposits, where permeability is generated by acid dissolution of silicates, and for fine-gold deposits in calcareous sediments. Examples of disseminated low sulfidation deposits include Round Mountain, Nevada, and Kelian, Indonesia. High sulfidation disseminated deposits include Summitville, Colorado; Akeshi, Iwato, and Kasuga, Japan; Chinkuashih, Taiwan; Masbate, and Nalesbitan, Philippines; Temora, Australia.
Examples of disseminated fine gold deposits in calcareous sediments include Carlin, Jerritt Canyon, and Pinson, Nevada; Cinola, Canada; Siana, Philippines.
Divided on the Basis of Genetic Models
Various genetic models have been constructed, and have been used as a basis for classification of epithermal deposits. The now classic epithermal vein cross-section by Buchanan (1981) modelled the vertical distribution of alteration and mineralization, and related the mineralogy to the depth of first boiling. As a subset of this generalized schematic, the 'hot spring' model has been widely used for low sulfidation epithermal vein deposits, on the basis of the interpretation that they form below thermal springs (Giles and Nelson, 1984). Similarly 'open cell' and 'closed cell' models (Berger and Eimon, 1983) have been proposed to explain differing distributions of vein and alteration mineralogy. Apart from the problem of being unduly simplistic, genetic classifications of this type are likely to be subject to re-interpretation, and consequently re-classification. At best they offer a conceptual genetic model which may be tentatively applied after the deposit has been classified in a more fundamental way.
Divided on the Basis of Standard Deposits
The use of 'standard' deposits as a basis of subdividing and comparing deposits is well established in geology. Broken Hill-type, Kuroko-type, Witwatersrand-type, Sudbury-type, Bushveldt-type, and many others, are all terms which have been widely used. In epithermal deposits the terms Carlin-type, and Nansatsu-type are in established use. To someone unfamiliar with the 'type' deposit, the name conveys no information about the deposit (however, this is essentially true for most brief deposit classifications). For anyone familiar with the 'type' deposit (which to be useful must be well-described and significant), the name briefly encapsulates a large amount of information, including the metal assemblage, mineralogy, host rocks, alteration, geological environment, tectonic setting and inferred origin. With time the genetic interpretation may change; however, even this change in perception is contained in the name. The unique ability of this basis of classification to convey large amounts of diverse information about a deposit with succinctness means that it can be very useful, and so it will continue to be widely used. It also has the advantage that it draws attention to the important 'type' deposit, and may be used to imply that other prospects or deposits of this type might have the potential to be as economically attractive as the 'type' deposit. One major weakness in exploring for a 'type' deposit is that no two deposits are the same, particularly in the highly diverse epithermal environment. If the type example is used too rigorously in exploration, then important differences from the 'type' deposit may be overlooked (or ignored). In this situation the uncritical use of type examples can narrow the imagination of the geologist.
DISCUSSION
What characterizes an epithermal deposit? The description by Lindgren (1933) was based on a series of characteristics, principally mineralogy and texture, and from this he inferred low temperature and shallow depth of formation. Some authors subsequently have taken these inferred characteristics as diagnostic. Fluid inclusion studies have allowed temperatures of formation to be measured, and from this the depth of formation (below the palaeowater table) is commonly inferred. This inference is based on the observation that in many active geothermal systems rising fluids have temperatures close to the boiling point for depth curve. This can be transferred to analogous mineralized systems if hydrostatic pressure is assumed, and boiling can be demonstrated. The assumption of boiling has commonly been made even when there is no evidence to support it, and this has been used to infer depth. In fact unless boiling was occurring, only a minimum depth can be inferred (Roedder, 1984), and the estimated depths of formation for some epithermal deposits may be too shallow. Even in cases where boiling did occur, deep depression of the palaeowater table may result in deposits forming at substantial depths below the surface, as for example in stratovolcano settings in the Philippines, where the water table may be 500-800 m below the surface (Reyes, 1990). The two original characteristics used to classify deposits as epithermal were mineralogy (of both veins and hydrothermal alteration), and texture. It is proposed that the basis of classification chosen by Lindgren (1933) be followed, and that the principal bases for characterizing deposits as epithermal remain mineralogy and texture. Both these are readily observed, thereby providing an accessible basis for classification. Both also provide information pertaining to mineralizing conditions, and from which temperature and, in some cases, depth may be inferred (Table 2). Various classification schemes have been devised for epithermal deposits. As discussed above, all have their particular strengths and weaknesses. For the explorationist, the most useful classification schemes should be brief, simple, descriptive, observationally based, and informative. These various requirements are mutually in conflict (especially brevity and information). Of the various schemes outlined above, none satisfies all requirements. The most informative is the one based on type deposits; however, it is necessary to have a good understanding of the type example before this classification can be properly used. The most inherently informative schemes are those based on mineralogy, alteration, and the form of the deposit. Classification into high and low sulfidation systems expresses a fundamental characteristic of the deposit, which in most cases can be easily inferred on the basis of simple observations, or at most supported by simple laboratory observations of mineralogy. It has implications for the mineralogy, alteration, and mineral zoning, as well as the genesis of the deposit, some aspects of which we are only beginning to understand. Classification by the form of the deposit expresses very simple observable characteristics that have important implications for its exploration, evaluation, and exploitation. The best way to classify epithermal deposits would seem to be by a combination of the most useful of the above schemes. As a fundamental characteristic, the classification into high or low sulfidation should come first. The descriptive form of the deposit should then be given, and if desired, followed by a comparative example. Thus an informative classification of deposits could be expressed thus: low sulfidation vein deposit comparable to Hishikari, high sulfidation disseminated lode deposit, cf. Chinkuashih, high sulfidation vein deposit, cf. El Indio, low sulfidation stockwork deposit, similar to McLaughlin.
This discussion has not included the sediment-hosted (typically carbonate replacement) disseminated fine-gold lode deposits such as Carlin, Jerritt Canyon, Pinson, etc. Although these deposits are characterized by an alteration assemblage similar to the low sulfidation epithermal deposits, this alteration is normally not readily observable in the field, and the characteristic textures which are features of epithermal deposits are lacking. These deposits also differ in host lithologies and regional setting. Our understanding of their origin suggests that they are genetically dissimilar to most epithermal deposits, and form outside the normally recognized epithermal environment (***Berger and Henley, 1989). Therefore, we prefer to regard them as a separate class of deposit, which we term disseminated fine-gold lode deposits. It would be best that the term epithermal not be applied to them, although it should be understood that they do exhibit SOME of the characteristics of epithermal deposits (notably alteration assemblage and temperature).
GUIDELINES FOR EXPLORATION
The approach adopted in exploration of any area depends on the available data. In regions where little is known, discrimination between more and less favourable regions may be difficult, but opportunities for new discoveries are great. Conversely, relatively well known regions allow better discrimination of favourable areas; however, it is more likely that these opportunities have been investigated already. In many parts of the circum Pacific region, particularly the western and southwestern parts, there has been relatively little systematic exploration for gold, so there is the opportunity to approach exploration from the regional scale, and to focus in to prospect scale.
Regional Scale Guidelines
In a previous section the environments of epithermal mineralization were divided into igneous, tectonic, and structural settings. Of these the first two are the most extensive, and logically form the basis for the first stage of area selection. The frequent occurrence of epithermal deposits in convergent tectonic settings allows environments favourable for Recent epithermal activity to be readily discerned. Former convergent plate margins are most readily recognized from the types and distribution of igneous rocks present, so in many older environments the igneous setting is most useful in selecting favourable regions for exploration. The critical aspect controlling whether epithermal mineralization occurs is not the distribution of volcanic rocks, but rather the distribution of the intrusions deep below the surface that provide the heat for meteoric water circulation, and magmatic components to the hydrothermal system. Therefore, the extent of the igneous province should first be determined; this includes all the igneous rocks (volcanic and plutonic) related to the igneous phase. Calc-alkaline to alkaline provinces are most prospective. Epithermal deposits are mostly formed at shallow crustal levels, so regions which have been deeply eroded are in general less prospective. The depth of erosion may be roughly estimated from the extent of preservation of the volcanic rocks, and the character and size of intrusions. In general, the more extensive the intrusions are, the more deeply they are likely to have been emplaced, and the greater the depth of erosion required to expose them. Having defined regions likely to have had favourable heat-flow characteristics, and which have
not been too deeply eroded, structure should be the next regional guideline considered. Major structural zones may be recognized on a variety of regional data sets, from geological maps to aeromagnetic surveys and satellite images. Other favourable structures such as those that occur around calderas may also be located from these data sets. The distribution of known mines and prospects may directly indicate structures, as well as indicating favourability of other structures. On the regional scale only structural zones should be distinguished, as the actual site of mineralization is commonly on a subsidiary structure within the structural zone, rather than on a major regional fault. Consideration of these three regional guidelines will, in many cases, indicate areas of enhanced prospectivity that are sufficiently limited in area for project scale exploration methods such as regional geochemistry, or regional airborne geophysical surveys (Irvine and Smith, 1990).
Project Area Scale Guidelines
The characteristics of epithermal environments assume greater significance when exploration is undertaken on a project area scale. The hydrothermal alteration effects that commonly envelope epithermal mineralization can provide a broad target and assist in locating more favourable areas. The expression of alteration depends markedly on the level of exposure of the system. Extensive areas of intense alteration are commonly associated with the upper levels of geothermal systems, where lateral spread of the deep reservoir fluids, together with the effects of cooling and fluid mixing, result in widespread blankets of argillic alteration. Although these may aid location of favourable areas, a potential ore deposit occupies only a very small part of this area, and the widespread alteration may hinder subsequent exploration. Conversely, the relatively narrow margin of alteration that may surround veins at greater depth does not greatly enhance the size of the target sought, but neither does it inhibit discovery of the mineralization. Aerial photographs, satellite images, airborne magnetic and radiometric surveys, and airborne remote sensing techniques, can all assist in locating areas of hydrothermal alteration. It must be remembered, however, that searching for alteration is an indirect approach to exploring for epithermal deposits. The primary objective of an exploration program is the location of economic accumulations of metals. Studies of alteration mineralogy and zoning may provide valuable insights into the hydrology of the system, and indicate possible sites of deposition; however, only geochemistry offers a direct approach to locating mineralization. Whatever geochemical medium (rock, soil, stream sediments) is sampled during mineral exploration, the source of the detected anomalies is the primary geochemical dispersion halo, which may or may not indicate the presence of an orebody (Clarke and Govett, 1990). The greater the dispersion of the potential ore fluid, the more widely dispersed are the geochemical effects of the fluids. Thus the geochemical response detected over an area of epithermal mineralization depends both in extent and chemistry on the hydrology of the system, and its level of exposure. This latter point is particularly important in interpreting the anomalous level of some elements (e.g. Hg, Tl, As), as their concentrations can increase at least two orders of magnitude in the upper few hundreds of metres of a system; hence the level of exposure is a major factor in the magnitude of an anomalous element. Using extremely sensitive analytical methods, anomalous gold values have been detected in Devonian-Carboniferous volcanics in north Queensland over areas comparable in extent to modern geothermal systems (Wood et al., 1990). At the project area scale structural studies are useful. These may prove difficult in
poorly-exposed areas, but even in these cases useful information is commonly available from interpretation of airborne geophysical surveys, satellite images, aerial photographs, and even topographic maps. Typically many structures are not mineralized, so it is necessary to distinguish the more prospective structures by examining the correspondence between structures, geochemistry, mineral occurrences, and hydrothermal alteration. Favourable conjunctions of these features become the focus of prospect-scale exploration. In general the exploration techniques applied at project area scale should not be model specific. Exploration should be conducted so that ANY type of economically significant mineralization will be located, not only one model.
Prospect Scale Exploration
Exploring at prospect scale confronts the explorationist with his greatest challenge. A drill hole will very effectively test the small volume of rock it penetrates; the challenge is to put the drill hole in the correct place. Whether a potential ore fluid actually forms an ore deposit depends principally on two factors: focusing, and deposition. The conditions necessary for these can commonly be inferred from field observations, which consequently provide simple practical guides in exploration. Focusing of the ore fluids occurs in zones of enhanced permeability. In some cases these palaeopermeable zones can be recognized directly by an increased density of mineralized fractures (typically veins), and the occurrence of hydraulic brecciation and hydrothermal eruption breccias. In other cases they may be recognized indirectly from vein mineralogy, and from the mineralogy and zoning of hydrothermal alteration products. These may also be detected using geophysical techniques (Irvine and Smith, 1990). Deposition of gold in epithermal deposits can have several causes. These include boiling, fluid mixing, cooling, and wall-rock reaction, and evidence for each can be recognized from vein and alteration mineralogy. By far the most important of these is boiling, which is the dominant process controlling the temperature of hydrothermal fluids near the surface. Characteristic textural and mineralogic evidence that suggests boiling can commonly be observed in the field (Table 2). In addition, the conditions that lead to the development of hydraulic breccias and hydrothermal eruption breccias involve sudden pressure releases which must have caused violent boiling. So the presence of these textures provides strong presumptive evidence for boiling, which may have resulted in gold deposition. Fluid mixing has the potential to cause gold deposition through changes in fluid chemistry, and has been invoked to explain mineralization in some volcanic hosted epithermal deposits (e.g. Kwak, 1990; Henley et al., 1984); however, its importance is probably much less than that of boiling. Because of the hydrodynamic pressure accompanying the high fluid flux required to form a substantial ore body, fluid mixing is probably only important near the margins of hydrothermal systems (except as noted previously in high-relief areas). The main evidence for fluid mixing is seen in the alteration assemblage (notably mixed layer clays around the margins, or an acid overprint near the top). It is probably important in carbonate-rich late-stage veins (e.g. at Fresnillo; Simmons et al., 1988; at Acupan; Cooke and Bloom, 1990; at Emperor; Kwak, 1990); however, these are typically barren, or poorly mineralized.
Both boiling and fluid mixing are likely to be enhanced in structural zones which are also effective for fluid focusing. Gold mineralization localized along, and at intersections of structures is described for Acupan, Philippines (Cooke and Bloom, 1990), and Emperor, Fiji (Anderson and Eaton, 1990; Kwak, 1990). Cooling (in the absence of boiling or fluid mixing) is probably of little importance in epithermal deposits, as it implies a slow rate of fluid ascent. Deposition as a result of wallrock reactions is apparent at Emperor (Anderson and Eaton, 1990), where gold-rich pyrite has formed by sulfidation of iron-bearing minerals. Although this may be important in some deposits, it is probably never the dominant ore-forming process in epithermal deposits. Efficient prospect scale exploration requires the careful integration of all available data, coupled with a good understanding of the processes that occur, and their likely effects. Possible topographic effects should also be considered, as these will influence the spatial distribution of conduits and hybrid fluids, and resultant mineralization and alteration.
CONCLUSION
We favour a simple approach to classification of epithermal deposits. Whether a deposit is classified as epithermal should be based on the mineralogic and textural features of the deposit, and not on such inferred features as depth of formation, fluid inclusion temperatures, or fluid chemistry. Readily observable features of mineralogy, texture, and alteration zoning allow classification into high and low sulfidation deposits. This, combined with a simple description of the form of the deposit (vein, stockwork, disseminated), conveys a large amount of information on mineralogy, alteration, and spatial characteristics of the mineralization, and allows inferences to be drawn regarding likely regional controls, and the characteristics of the ore-forming fluids. Comparison with a relatively well-known example (if one exists) is an established and valuable way to convey a large amount of information by analogy. The epithermal environment is extremely diverse in character, as a variety of physical and chemical processes occur within a complex and dynamic geological environment. Consequently the features observed, and their spatial relationships vary widely. The unifying theme that characterizes all epithermal deposits is the processes that occur in their formation. Therefore the diversity of features observed, and their significance in exploration, can only be understood with the aid of a strong conceptual understanding of the processes which occur in hydrothermal systems. Reliance on models without a firm understanding of the underlying processes leads to their inflexible, and consequently, ineffective use. Many different data sets provide the sources of information for exploration for epithermal deposits. Only by imaginative integration of all data within the framework of a strong conceptual understanding can we ensure that we obtain maximum benefit from the data for our exploration program.
ACKNOWLEDGMENTS
NCW acknowledges the permission of BHP Minerals International Exploration to publish this paper. M.S. Bloom, P.R.L. Browne, D.R. Cooke, R.W. Henley, S.F. Simmons and D.G. Wood critically reviewed the manuscript.
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TABLE 1: Characteristics of epithermal gold deposit types
Host rocks
Localizing controls
Low sulfidation
High sulfidation
Acid to intermediate subaerial
Acid to intermediate subaerial
volcanics, and underlying
volcanics, and underlying
basement rocks of any type.
basement rocks of any type.
Any faults or fracture zones
Major regional faults or
especially closely related to
subvolcanic intrusions.
volcanic centres.
Depth of formation
Mostly 0 to 1000m.
o
Mostly ?500 to ?2000m.
o
Temperature of formation 100 to 320 C
100 to 320 C o
(mainly 150 to 250 C)
Character of ore fluids
Associated alteration
Low salinity
Mostly low (some high) salinity
Meteoric waters, interaction with
Magmatic fluid source mixing with
magmatic fluid possible.
meteoric waters.
pH near neutral, may become
pH acid from magmatic HCl, and by
alkaline from boiling; phase
disproportionation of SO2 becomes
separated gases may be oxidised
neutralized by wallrock reaction,
to produce an acid fluid.
and dilution.
Reduced.
Oxidised.
Total S content typically low.
Total S content typically high.
Base metal content low (Pb, Zn).
Base metal content may be high (Cu).
Extensive propylitic alteration
Extensive propylitic alteration
in surrounding regions with low
in surrounding regions with low
water:rock ratios.
water:rock ratios.
Intensive white mica in regions
Deep deposits have intense
with high water:rock ratios.
pyrophyllite-white mica
Clay alteration becomes dominant
alteration.
with decreasing temperature.
Shallow deposits have core of
Boiled off gases may produce
massive silica (from acid
argillic and advanced argillic
leaching and silica mobilization),
alteration peripheral to, or
with narrow margin of alunite and
overlapping alteration from deep
kaolinite, out to white mica and
fluids.
Interlayered clays. Near-surface deposits may have pervasive clay alteration.
Character of mineralisation
Ore mineralization characterized
Ore mineralization typically
by open space and cavity filling,
disseminated, either in white mica-
typically with sharp-walled veins.
pyrophyllite, or in massive silica.
Layered vein fillings typical,
Open space and cavity filling not
commonly with multi-stage
common. Mineralization usually
brecciation. Near-surface may be
associated with advanced argillic
stockwork or disseminated,
alteration, and pyrite typically
depending on nature of local
very abundant.
primary and secondary permeability.
Characteristic textures
Crustification banding, fine comb
Vuggy silica (fine-grained quartz).
texture, colloform banding,
Massive silica (fine-grained
banded quartz-chalcedony, drusy
quartz).
cavities, vugs, vein breccia, silica pseudomorphs after bladed calcite (lattice texture).
Characteristic minerals
Chalcedony veins common.
Chalcedony mostly absent.
Adularia in veins and disseminated.
Adularia absent.
Alunite minor.
Alunite may be abundant.
Pyrophyllite minor.
Pyrophyllite may be abundant.
Enargite-luzonite absent.
Enargite-luzonite typically Present.
Examples
Pajingo, Australia
Temora, Australia
Emperor, Fiji
Mount Kasi, Fiji
Lebong Donok, Indonesia
Motomboto, Indonesia
Wapolu, Papua New Guinea
Nena, Papua New Guinea
Acupan, Philippines
Lepanto, Philippines
Golden Cross, New Zealand
Nansatsu district, Japan
Table summarised, with additions, from Berger and Eimon, 1983; Bonham, 1986; Hayba et al., 1985; Heald et al., 1987; Hedenquist, 1987; Stoffregen, 1987.
TABLE 2: Interpreting observations
Observation
Vein Mineralogy/Texture chalcedony present
Inference
rapid cooling has occurred; may indicate boiling; deposition o
temperature between 190 and 100 C; can infer depth of less than 400m below water table, assuming hydrostatic conditions.
adularia present
boiling has occurred, causing an increase in pH.
lattice texture (i.e. silica replacement of bladed calcite crystals)
boiling has occurred, resulting in CO2 loss, and consequent calcite saturation.
Wallrock Alteration sericite (white mica)
fluid pH near neutral to slightly acid; temperature above o
about 220 C.
mixed-layer clays
paleotemperatures below about 220oC; can be semiquantified by XRD analysis of basal spacing.
