EPITHERMAL ENVIRONMENTS AND STYLES OF MINERALIZATION:VARIATIONS AND THEIR CAUSES, AND GUIDELINES FOR EXPLORATION

Noel C. WhiteBHP Minerals International Exploration

Jeffrey W. HedenquistMineral Resources Department, Geological Survey of Japan

This paper is modified slightly from

White, N.C. and Hedenquist, J.W., 1990. Epithermal Environments and Styles ofMineralization: Variations and their Causes, and Guidelines for Exploration. In: J.W.Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal gold mineralisation of theCircum Pacific: Geology, Geochemistry, Origin and Exploration. Journal of GeochemicalExploration, 36: 445-474.

EPITHERMAL ENVIRONMENTS AND STYLES OF MINERALIZATION:VARIATIONS AND THEIR CAUSES, AND GUIDELINES FOR EXPLORATION

Noel C. WhiteBHP Minerals International Exploration

Jeffrey W. HedenquistMineral Resources Department, Geological Survey of Japan

ABSTRACT

Epithermal precious and base metal deposits are diverse, reflecting the different tectonic,igneous and structural settings in which they occur, the complexities of their local setting, andthe many processes involved in their formation. Most epithermal deposits form at shallowcrustal levels where abrupt changes in physical and chemical conditions result in metaldeposition and attendant hydrothermal alteration. The principal factors that influence theconditions prevailing in the epithermal environment, and which ultimately determine the sitesand character of mineralization, include: geology (structure, stratigraphy, intrusions and rocktype, which affect the style and degree of permeability and the reactivity of the host); pressureand temperature (which in the epithermal environment are related on the boiling point withdepth curve); hydrology (the relationship between permeability and topography which governsfluid flow, and discharge/recharge characteristics, as well as access of steam heated waters);chemistry of the mineralizing fluid (which determines the metal-carrying capacity, as well as theassociated vein and alteration assemblage); and syn-hydrothermal development of permeabilityand/or changes in hydraulic gradients.

Many attempts have been made to classify epithermal deposits based on mineralogy andalteration, the host rocks, deposit form, genetic models, and standard deposits. All have theirstrengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry(high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) asreadily inferred from vein and alteration mineralogy and zoning, together with the form of thedeposit, and using comparative examples to clarify the character of the deposit.

Guidelines for exploration vary according to the scale at which work is conducted, and arecommonly constrained by a variety of local conditions. On a regional scale the tectonic, igneousand structural settings can be used, together with assessment of the depth of erosion, to selectareas for project area scale exploration. At project area scale direct (i.e. geochemical) or indirectguidelines may be used. Indirect methods involve locating and interpreting hydrothermalalteration as a guide to ore, with the topographic and hydrologic reconstruction of the systembeing of high priority. These pursuits may involve mineralogic, structural, geophysical orremote sensing methods. On a prospect scale both direct and indirect methods may be used;however they can only be effective in the framework of a sound conceptual understanding of theprocesses that occur in the epithermal environment, and the signatures they leave.

INTRODUCTION

A hydrothermal system undergoes abrupt physical and chemical change at the shallow depththat characterizes most epithermal deposits. This occurs because of the change from lithostaticto hydrodynamic pressure (resulting in boiling), interaction of fluids derived at depth with near-surface water, permeability changes, and reaction between fluid and host rocks. These changesnear the surface are the reason that an 'epithermal' ore environment exists, as they affect thecapacity of the hydrothermal fluid to transport metals in solution. Focusing of fluid flow nearthe surface, in conjunction with changes which decrease the solubility of metals in the fluid, willthen result in metal deposition within a 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.Although the interpretations of his observations have not changed substantially, ourunderstanding of the epithermal environment has now broadened as a result of a greatlyincreased observational base. The discovery and study of a large number of epithermal depositsoutside of the classic western US setting (e.g., 1990) shows the variety of geologicenvironments which are potential hosts to near-surface precious and base metal mineralization.

The purpose of this paper is to demonstrate the usefulness, limitations, and dangers associatedwith 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 thiswith syntheses in the published literature (e.g. White, 1955, 1981; Sillitoe, 1977, 1981, 1988a,b; Buchanan, 1981; Graybeal, 1981; Berger and Eimon, 1983; Henley and Ellis, 1983; Gilesand 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 etal., 1995). Several of these papers have improved our understanding of the epithermalenvironment by stressing its relationship to presently active hydrothermal systems. Geothermalsystems are presently active examples of the systems which produced many epithermal deposits.What is termed the epithermal environment is represented by the upper regions (

hydrothermal system (Giggenbach, 1986), and their possible contribution of metals has beenspeculated on (e.g. Hedenquist, 1987; Berger and Henley, 1989).

The character of volcanic settings which host epithermal deposits is most commonly central toproximal, 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 occurin 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 basicvolcanics host epithermal deposits the volcanics commonly have shoshonitic or alkalineaffinities (e.g. Emperor, Fiji; Anderson and Eaton, 1990). Various authors have attempted toidentify favourable volcanic rocks on the basis of their chemical composition (e.g. Keith andSwan, 1988). In general it seems that more prospective suites are produced by I-type or A-typemagmas (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, andhave been classified into silicic depressions (commonly calderas or graben), andesiticstratovolcanos, cordilleran volcanism, and oceanic islands (Bogie and Lawless, 1987; White etal., 1995). Each of these is characterized by a different hydrological regime, which controls thedischarge and recharge of the hydrothermal system, the distribution of conduits, the types anddistribution of hydrothermal alteration products, and the potential sites of deposition of oreminerals.

Tectonic Settings

Subaerial volcanism may occur in a variety of tectonic settings. The igneous settings describedabove occur mainly as volcanic arcs in the convergent tectonic settings characteristic of oceanic-continental, or oceanic-oceanic plate subduction (le Pichon et al., 1973). Dilationalenvironments may develop in these settings, producing back-arc rifting, and this may evolveinto a marine back-arc basin (Karig, 1971). Back arc basins are characteristically submarine, andif so, are not prospective for epithermal deposits; in contrast, massive sulfide (Kuroko) depositsare formed in this setting (Cathles et al., 1983). The Basin and Range region of the western USis an example of a wide back arc rift (Hamilton, 1985), and it is extensively mineralized withepithermal deposits (White, 1982). Similar examples include the Taupo Volcanic Zone(Hedenquist, 1986a; Cole, 1987) and the Coromandel Peninsula of New Zealand (Christie andBrathwaite, 1986).

Several subaerial volcanic settings do not appear to be prospective for epithermal deposits. Theregions of continental flood basalts, whether tholeiitic or alkaline, do not contain epithermaldeposits. This is probably because their magma chambers are deep and/or small, and theirconduits narrow, resulting in small near-surface heat anomalies which preclude the developmentof a major hydrothermal system. Oceanic ridge settings do not appear prospective, probablybecause they are typically submarine. Iceland offers a modern example of an oceanic ridgewhich is subaerial (le Pichon et al., 1973), and has extensive geothermal activity involvingmeteoric fluids. It is not apparent whether this setting may be prospective for epithermaldeposits; however, as it is a very rare setting in the geological record it may not be significantfor 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 necessaryheat-flow conditions for major hydrothermal activity are not established.

Structural Settings

Strong structural control is almost universally recognized for gold deposits (Henley, 1990), dueto the permeability enhancement caused by fractures in the near surface. Many epithermaldeposits are regionally associated with volcanic-related structures (Rytuba, 1981). A closeassociation with felsic calderas and andesitic vent complexes has been observed in the San JuanMountains of Colorado (Steven et al., 1977), and in some parts of Japan (Kubota, 1986) and thesouthwest 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 sourceand influencing subsequent hydrothermal activity (Hedenquist, 1986a). Although major faultshave a regional control on the localization of deposits, mineralization is commonly not locatedon 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 structuralfeatures such as bedding planes, joints, and joint intersections may have influenced thepermeability 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 epithermalenvironment). Several different factors influence the physical and chemical conditionsprevailing in that environment, which ultimately determine the sites and character ofmineralization. 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 constrainsthe temperature in the system to not significantly more than that corresponding to thevapour pressure at a given depth, i.e. the boiling temperature.

3. Hydrology - in conjunction with permeability characteristics, topography determines thedirection and degree of hydraulic gradients in the shallow hydrothermal system, whichin turn govern fluid flow. The formation of perched, steam-heated waters, with theirpotential to penetrate into the hydrothermal system, is also enhanced as topographicrelief increases. Palaeoclimate, and its variations during hydrothermal activity mayinfluence availability of recharge water, and the condensation of vapours.

4. Chemistry of the mineralizing fluid - the total gas content of a fluid is important indetermining several critical physical and chemical factors, including the solubility ofgold and other metals; to a large extent the composition and concentration of the gasesin 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 magmaticvolatiles contributed to the system, though host rock reactivity is also a factor. Coldmarginal groundwater and/or steam-heated waters (the latter commonly weakly tomoderately acid) may mix with the mineralizing fluid, affecting alteration and ganguedeposition 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 secondarypermeability of rocks through which hydrothermal fluids flow. In any stratigraphic sequence ofhighly contrasting permeabilities, the least permeable units serve as aquitards, and the mostpermeable as aquifers. Fluid flow through the least permeable units is confined to joints,fractures and brecciated zones. Where impermeable units are intruded, fracture permeability isenhanced along intrusive margins (e.g., at Kelian, Kalimantan; van Leeuwan et al., 1990) whichthen act as fluid channels. When permeable rocks become silicified their brittleness andsusceptibility to subsequent fracturing increases (e.g. Round Mountain, Nevada; Tingley andBerger, 1985). In stratigraphic sequences with little lateral continuity (e.g., andesiticstratovolcanos with discontinuous lava flows) primary permeability occurs along (brecciated)formation contacts. Secondary permeability is related to faults and fractures. These featureshave 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 maygenerate permeability depending on the reactivity of the rocks with the fluids. Although therehas been little research to document this, hydrothermal alteration that accompaniesmineralization in most deposits may result in volume changes which could enhance or inhibitchanges in permeability. An extreme example of permeability generated by fluid-rock reactionis found in epithermal deposits such as Summitville, Colorado (Stoffregen, 1987) and those inthe 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) generatedzones of high permeability by leaching everything except silica from the rock; these zones thenserved as conduits for later mineralizing fluids.

Pressure and Temperature

Pressure-temperature profiles in the upper 2 to 3 km of drilled geothermal systems reflecthydrodynamic conditions, i.e. the hydrostatic pressure due to a column of hot water plus thepressure due to natural upflow (Grant et al., 1982; Donaldson et al., 1983). Where permeabilityis high, large volumes of fluid ascend under boiling conditions at

steam-heated water is also common at shallow levels, as well as on the deeper margins ofsystems (Hedenquist and Browne, 1989; Hedenquist, in press), leading to marginal temperaturereversals.

Hydrology

The importance of hydrology governing fluid upflow and outflow has long been recognizedfrom geothermal exploration drilling. Surface hot spring discharges need not occur above thedeep upflow zone, as geothermal fluids may be deflected in the near-surface environment if ahydraulic gradient is encountered (Hanaoka, 1980). This situation is prevalent in active systemsin high relief areas such as andesitic terranes in the Philippines (e.g., Tongonan, Bacon Manitoand 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 occurat a distance of several kilometres, and below areas of fumarolic discharges. By contrast, fluidflow in the Hatchobaru system in Kyushu is mostly vertical, due to strong fracture control in athick sequence of andesitic lava flows (Taguchi et al., 1986), despite relief of 500 m over adistance 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, lateralflow can extend as far as 10 km from the upflow, with hydrothermal alteration attendant overthe 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 ofvarious 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 belowthe epithermal environment. The chemistry of near neutral pH geothermal fluids is determinedby interaction of the convecting meteoric cell with the host rocks, and with an inferredmagmatic fluid component (Giggenbach, 1980, 1981, 1984, 1986, 1988). The total gas contentof the ascending fluids, which is quite variable (Hedenquist and Henley, 1985b), is largelydetermined 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 alterationassemblage 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 someof these minerals (particularly clays, zeolites and calc-silicates) is temperature sensitive, andreflects 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 thehydrology 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 andOhmoto, 1985).

Some epithermal deposits are associated with advanced argillic alteration that was produced athigh temperatures (Rye et al., 1989), and is distinct from the surficial, hybrid acid waterscommonly developed in geothermal systems. These deposits may contain pyrophyllite and/ordiaspore, zunyite, and sulfides including enargite, tennantite and covellite, as well as gold. Thehost rocks may also show evidence of acid leaching, with only residual silica left; atSummitville 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 resultfrom a direct, unreacted contribution of magmatic volatiles such as HCl, HF and SO2 (Bethke,1984; Hedenquist, 1987). These fluids are distinct from most geothermal fluids in that they havenot interacted sufficiently with the host rocks to become neutralized. They are probably similarto 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 geothermaland magmatic hydrothermal environments (Giggenbach, 1981, 1987). Even though theprocesses initially controlling the chemistry of these fluids occur beneath the epithermalenvironment, an understanding of their origin is required to appreciate the difference inalteration and mineralization style which they cause.

In the surficial environment of geothermal systems, steam-heated waters form fromcondensation of steam and H2S separated from underlying boiling fluids. The sulfide oxidizes tosulfate near the surface, thus generating acidity, which results in advanced argillic alterationassemblages (Browne, 1984). The typical mineral assemblage produced by surficial,steam-heated acid waters includes kaolin clays, alunite and cristobalite as well as native sulfurand pyrite. Since most oxidation occurs in the vadose zone, condensation of steam and gasesbelow this level results in formation of a CO2-rich water on the margins of and overlying thedeep fluid (e.g. at Waiotapu and Broadlands; Hedenquist and Browne, 1989; Hedenquist, inpress).

Dilution of ascending fluids (by marginal or shallow water) is the other principal process (apartfrom boiling) occurring in the geothermal environment (Giggenbach and Stewart, 1982). As thedeep fluids rise they tend to entrain marginal fluids. This is evident from the fluid chemistry andalteration mineralogy patterns of geothermal systems (e.g. Henley and Plum, 1985; Hedenquistand Henley, 1985a; Hedenquist and Browne, 1989; Hedenquist, in press) and epithermaldeposits (Roedder, 1972; Barton et al., 1977; Hayba et al., 1985). Commonly the dilutionpattern in geothermal systems is towards a steam-heated fluid located on the margins of thesystem. This steam-heated water typically has a temperature of about 150oC, and causes amarginal alteration halo of interstratified clays (Hedenquist, in press). Like boiling, dilution mayalso 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 significantdecreases in permeability during hydrothermal activity (Fournier, 1985). Leaching by acidfluids, however, may enhance permeability, as may syn-hydrothermal fracturing. Fracturing willmostly occur during catastrophic events such as faulting or hydraulic brecciation, and is quitecommon in extensional terranes associated with volcanic-related geothermal activity. Hydraulicfracturing, and related hydrothermal eruptions, is common in geothermal systems (Hedenquistand 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 thicksequence 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 formdue to the system re-establishing new and higher hydraulic heads. Conversely, erosion duringthe 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 andHayashi, 1983; Taguchi et al., 1985; Reyes, 1990). In Japanese and Philippine geothermalsystems water tables have fallen by as much as 200m over the lives of systems in areas subjectto little erosion (perhaps due to deepening of drainage channels some distance away); there isevidence for up to 450m of erosion since activity began in the Palinpinon system of southernNegros (Reyes, 1990). Such changes in the water table and/or palaeosurface will have strongeffects on the thermal stability, hydrology and patterns of boiling and mixing over the life of asystem. Appreciating that these changes are possible, and seeking evidence for them, such asoverprinting 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 frommesothermal 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. Apparentchanges in the boiling point curve (as indicated by fluid inclusions) for different stages ofveining at Acupan (Cooke and Bloom, 1990) may also relate to erosion/deposition changesduring the life of the system.

Climatic changes over the life of the system can also affect the water table by increasing ordecreasing the availability of groundwater, which is important both as a condenser of steam, as amarginal 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 avariety of ways in which deposits have been grouped for their intercomparison; each has itspositive and negative points. What we wish to gain from this discussion are the practicalbenefits 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 ifand where mineralization will occur during the life of a hydrothermal system. Hydrothermalalteration and ore mineralogy are good indicators of fluid chemistry and temperature, and inepithermal systems provide the only means other than fluid inclusions to estimate these criticalvariables. The hydrothermal alteration and ore mineralogy we use to identify the chemistry ofthe mineralizing fluid must be that related to mineralization, and not a later, overprinting phaseof alteration during the waning of a system. Overprinting of hydrothermal alteration by anincompatible assemblage is rare in active geothermal systems, as the hydraulic head keeps thecondensed hybrid fluids on the margins of the system; though in high relief terrane perchedwaters 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 iscorrectly identified.

Based on the characteristics of many epithermal deposits it is possible to distinguishmineralization produced by two contrasting deep fluids having respectively near-neutral pH, andacid pH (Hayba et al., 1985; Heald et al., 1987; Hedenquist, 1987). In the near-neutral pHsystem, 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 toadvanced argillic alteration, the systems appear to be analogous to hydrothermal systemsadjacent to near-surface magma bodies and volcanic vents (e.g. White Island, New Zealand), inwhich 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 to10 km?) than in the observed volcanic vent-related systems, resulting in the acid volatiles beingneutralized 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 beenvariously 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 ifany adularia; though they may contain kaolinite and/or alunite, usually peripheral to, or as a lateroverprint to mineralization. Epithermal deposits formed from near-neutral pH geothermalsystems 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 metaldeposits are sulfide-rich. Although deposits formed from acid pH fluids commonly containmassive 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 eitherdeposit type is clearly not appropriate, nor is the sulfur content, which varies widely in bothtypes. Despite the variability of mineralogy and mineral abundance, the deposits always reflectthe character of the fluids that produced them: this can be inferred from observations of the veinmineralogy, and the mineralogy and distribution of the hydrothermal alteration products (Table1), 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 theredox 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 neutralpH. The "geothermal" versus "magmatic" association of the fluids can be distinguished quiteclearly on the basis of stable isotope studies (Bethke, 1984; Rye et al., 1989), with one of theprincipal 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. assulfide with an oxidation state of -2. This is termed low sulfidation. By contrast, the sulfur involcanic hydrothermal discharges can approach an oxidation state of +4, with all the sulfurpresent 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 maybe intermediate between the two endmembers (indicated by sulfur isotope chemistry) reflectingthe degree of dilution of the acid high sulfidation fluids by low sulfidation meteoric waters, andneutralization 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 beresponsible for mineralization (Stoffregen, 1987, 1989; Hedenquist et al., 1988; Berger andHenley, 1989; Jannas et al., 1990). The degree of fluid evolution is likely to be variable betweendeposits.

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 arecommonly regarded as 'typical' epithermal deposits; they are usually low-sulfidation veinshosted 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 alsooccur in other host rocks. Some epithermal deposits have a vertical extent exceeding 1000metres (e.g. Cripple Creek, Colorado; Thompson et al., 1985); in many instances the hostvolcanic rocks may not be this thick, so mineralization may extend below the volcanic rocksinto basement. This is illustrated by the Hishikari epithermal vein deposit in Japan, whereapproximately the upper one-third of the deposit occurs in andesitic volcanic rocks and theremaining majority of the deposit is confined to veins within basement sedimentary rocks of theShimanto Group (Izawa et al., 1990). At the Umuna deposit (Misima Island, Papua-NewGuinea; Clarke et al., 1990) the deposit is hosted by metamorphosed basement rocks, butprobably originally extended upwards into the thin Miocene volcanics which are preservedelsewhere on the island.

In some cases the related volcanic formations may have been of limited areal extent, sovolcanics need not have been present at the time of mineralization, though they were probablyrepresented nearby. Sinters and chalcedonic epithermal veins occur at the Pliocene Puhipuhiprospect in New Zealand (Williams, 1974), although the only volcanic rocks present are basaltswhich probably post-date the geothermal activity. Acid volcanic domes, however, arewidespread regionally, and they are possibly related to the mineralizing event.

Classifying a deposit as 'volcanic-hosted' explains little about the deposit apart from the natureof the host-rocks, and as similar deposits may occur in non-volcanic hosts, the term ispotentially misleading.

'Carlin-type' deposits, i.e., disseminated fine-gold deposits hosted by carbonaceous calcareoussediments, are described commonly as 'sediment-hosted', but this term fails to convey that aparticular type of deposit, hosted by very specific sediments, is referred to. Using thisterminology the lower two-thirds of the Hishikari deposit mentioned above would not beclassified as a sediment-hosted deposit (as it is not 'Carlin-type'), despite being hosted bysediments. A similar difficulty arises from the term 'carbonate-hosted' for the same type ofdeposit; although it more adequately describes the host-rocks, it conveys nothing about the veryspecific features that characterize these deposits, except that the host rocks were relativelyreactive.

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 thedeposit. This basis for description conveys nothing about the host rocks, textures or genesis ofthe deposit, but does convey important information on the spatial distribution of themineralization, which in turn has important implications for exploration.

Epithermal deposits are diverse in their forms; however, in most cases they can be regarded ascombining in various proportions the characteristics of three end-members; vein deposits,stockwork deposits, and disseminated deposits. The deposit form is essentially a result of thehost rock permeability during mineralization.

Vein deposits consist of a limited number of discrete veins with well-defined vein walls; theyhave sharp grade cut-offs. Veins may pass laterally into local zones of vein breccia having theform of veins, but consisting of breccia containing clasts of wall-rock and vein material in ahydrothermal vein matrix (e.g. Mt. Muro; Simmons and Browne, 1990; Acupan; Cooke andBloom, 1990). The matrix commonly consists of fine-grained quartz with abundant veryfine-grained pyrite giving it a dark grey colour. In areas of structural complexity or where theveins change orientation, they may locally become stockworks. Veins can be mineralized oververtical 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; LebongDonok, Indonesia; Pajingo, Australia; Karangahake and Waihi, New Zealand; Fresnillo,Mexico.

Stockwork deposits consist of relatively narrow interconnected veins forming complex zones; asufficiently high concentration of veins may allow the zone to be mined in bulk. Althoughindividual veins have sharp cut-offs in grade, the stockwork zone is normally defined by anassay cut-off, determined by the concentration and grade of individual veins. Stockworks aremost likely to develop in areas of structural complexity where faults intersect or changedirection, or intersect competent (i.e. brittle) lithologies. Exclusively stockwork-type epithermaldeposits are not numerous, as they typically occur as part of a vein deposit. Examples which areat 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; GoldRidge, Solomon Islands.

Disseminated lode deposits are characterized by mineralization dispersed through the (typicallyaltered) host rocks, rather than confined to discrete veins. The orebody boundaries are definedby assay cut-offs. Disseminated deposits are less common than veins for low sulfidationdeposits 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 incalcareous sediments.

Examples of disseminated low sulfidation deposits include Round Mountain, Nevada, andKelian, Indonesia.

High sulfidation disseminated deposits include Summitville, Colorado; Akeshi, Iwato, andKasuga, Japan; Chinkuashih, Taiwan; Masbate, and Nalesbitan, Philippines; Temora, Australia.

Examples of disseminated fine gold deposits in calcareous sediments include Carlin, JerrittCanyon, 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 classificationof epithermal deposits. The now classic epithermal vein cross-section by Buchanan (1981)modelled the vertical distribution of alteration and mineralization, and related the mineralogy tothe depth of first boiling. As a subset of this generalized schematic, the 'hot spring' model hasbeen widely used for low sulfidation epithermal vein deposits, on the basis of the interpretationthat they form below thermal springs (Giles and Nelson, 1984). Similarly 'open cell' and 'closedcell' models (Berger and Eimon, 1983) have been proposed to explain differing distributions ofvein and alteration mineralogy.

Apart from the problem of being unduly simplistic, genetic classifications of this type are likelyto be subject to re-interpretation, and consequently re-classification. At best they offer aconceptual genetic model which may be tentatively applied after the deposit has been classifiedin 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 wellestablished 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 epithermaldeposits the terms Carlin-type, and Nansatsu-type are in established use.

To someone unfamiliar with the 'type' deposit, the name conveys no information about thedeposit (however, this is essentially true for most brief deposit classifications). For anyonefamiliar with the 'type' deposit (which to be useful must be well-described and significant), thename 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 iscontained in the name.

The unique ability of this basis of classification to convey large amounts of diverse informationabout a deposit with succinctness means that it can be very useful, and so it will continue to bewidely used. It also has the advantage that it draws attention to the important 'type' deposit, andmay be used to imply that other prospects or deposits of this type might have the potential to beas 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 toorigorously in exploration, then important differences from the 'type' deposit may be overlooked(or ignored). In this situationthe 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 aseries of characteristics, principally mineralogy and texture, and from this he inferred lowtemperature and shallow depth of formation. Some authors subsequently have taken theseinferred characteristics as diagnostic. Fluid inclusion studies have allowed temperatures offormation to be measured, and from this the depth of formation (below the palaeowater table) iscommonly inferred. This inference is based on the observation that in many active geothermalsystems rising fluids have temperatures close to the boiling point for depth curve. This can betransferred to analogous mineralized systems if hydrostatic pressure is assumed, and boiling canbe demonstrated. The assumption of boiling has commonly been made even when there is noevidence to support it, and this has been used to infer depth. In fact unless boiling wasoccurring, only a minimum depth can be inferred (Roedder, 1984), and the estimated depths offormation for some epithermal deposits may be too shallow. Even in cases where boiling didoccur, deep depression of the palaeowater table may result in deposits forming at substantialdepths below the surface, as for example in stratovolcano settings in the Philippines, where thewater table may be 500-800 m below the surface (Reyes, 1990).

The two original characteristics used to classify deposits as epithermal were mineralogy (of bothveins and hydrothermal alteration), and texture. It is proposed that the basis of classificationchosen by Lindgren (1933) be followed, and that the principal bases for characterizing depositsas epithermal remain mineralogy and texture. Both these are readily observed, thereby providingan accessible basis for classification. Both also provide information pertaining to mineralizingconditions, 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 usefulclassification schemes should be brief, simple, descriptive, observationally based, andinformative. These various requirements are mutually in conflict (especially brevity andinformation). Of the various schemes outlined above, none satisfies all requirements. The mostinformative is the one based on type deposits; however, it is necessary to have a goodunderstanding of the type example before this classification can be properly used. The mostinherently informative schemes are those based on mineralogy, alteration, and the form of thedeposit. Classification into high and low sulfidation systems expresses a fundamentalcharacteristic of the deposit, which in most cases can be easily inferred on the basis of simpleobservations, or at most supported by simple laboratory observations of mineralogy. It hasimplications for the mineralogy, alteration, and mineral zoning, as well as the genesis of thedeposit, some aspects of which we are only beginning to understand. Classification by the formof the deposit expresses very simple observable characteristics that have important implicationsfor its exploration, evaluation, and exploitation.

The best way to classify epithermal deposits would seem to be by a combination of the mostuseful of the above schemes. As a fundamental characteristic, the classification into high or lowsulfidation should come first. The descriptive form of the deposit should then be given, and ifdesired, followed by a comparative example. Thus an informative classification of depositscould 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 thesedeposits are characterized by an alteration assemblage similar to the low sulfidation epithermaldeposits, this alteration is normally not readily observable in the field, and the characteristictextures which are features of epithermal deposits are lacking. These deposits also differ in hostlithologies and regional setting. Our understanding of their origin suggests that they aregenetically dissimilar to most epithermal deposits, and form outside the normally recognizedepithermal environment (***Berger and Henley, 1989). Therefore, we prefer to regard them as aseparate class of deposit, which we term disseminated fine-gold lode deposits. It would be bestthat the term epithermal not be applied to them, although it should be understood that they doexhibit SOME of the characteristics of epithermal deposits (notably alteration assemblage andtemperature).

GUIDELINES FOR EXPLORATION

The approach adopted in exploration of any area depends on the available data. In regions wherelittle is known, discrimination between more and less favourable regions may be difficult, butopportunities for new discoveries are great. Conversely, relatively well known regions allowbetter discrimination of favourable areas; however, it is more likely that these opportunitieshave been investigated already. In many parts of the circum Pacific region, particularly thewestern 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 toprospect 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 formthe basis for the first stage of area selection. The frequent occurrence of epithermal deposits inconvergent tectonic settings allows environments favourable for Recent epithermal activity tobe readily discerned. Former convergent plate margins are most readily recognized from thetypes and distribution of igneous rocks present, so in many older environments the igneoussetting is most useful in selecting favourable regions for exploration.

The critical aspect controlling whether epithermal mineralization occurs is not the distributionof volcanic rocks, but rather the distribution of the intrusions deep below the surface thatprovide the heat for meteoric water circulation, and magmatic components to the hydrothermalsystem. Therefore, the extent of the igneous province should first be determined; this includesall the igneous rocks (volcanic and plutonic) related to the igneous phase. Calc-alkaline toalkaline provinces are most prospective.

Epithermal deposits are mostly formed at shallow crustal levels, so regions which have beendeeply eroded are in general less prospective. The depth of erosion may be roughly estimatedfrom the extent of preservation of the volcanic rocks, and the character and size of intrusions. Ingeneral, the more extensive the intrusions are, the more deeply they are likely to have beenemplaced, 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. Majorstructural zones may be recognized on a variety of regional data sets, from geological maps toaeromagnetic surveys and satellite images. Other favourable structures such as those that occuraround calderas may also be located from these data sets. The distribution of known mines andprospects 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 ofmineralization is commonly on a subsidiary structure within the structural zone, rather than on amajor regional fault.

Consideration of these three regional guidelines will, in many cases, indicate areas of enhancedprospectivity that are sufficiently limited in area for project scale exploration methods such asregional geochemistry, or regional airborne geophysical surveys (Irvine and Smith, 1990).

Project Area Scale Guidelines

The characteristics of epithermal environments assume greater significance when exploration isundertaken on a project area scale. The hydrothermal alteration effects that commonly envelopeepithermal mineralization can provide a broad target and assist in locating more favourableareas. 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 ofgeothermal systems, where lateral spread of the deep reservoir fluids, together with the effectsof cooling and fluid mixing, result in widespread blankets of argillic alteration. Although thesemay aid location of favourable areas, a potential ore deposit occupies only a very small part ofthis area, and the widespread alteration may hinder subsequent exploration. Conversely, therelatively narrow margin of alteration that may surround veins at greater depth does not greatlyenhance 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 airborneremote sensing techniques, can all assist in locating areas of hydrothermal alteration. It must beremembered, however, that searching for alteration is an indirect approach to exploring forepithermal deposits. The primary objective of an exploration program is the location ofeconomic accumulations of metals. Studies of alteration mineralogy and zoning may providevaluable 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 mineralexploration, 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). Thegreater the dispersion of the potential ore fluid, the more widely dispersed are the geochemicaleffects of the fluids. Thus the geochemical response detected over an area of epithermalmineralization depends both in extent and chemistry on the hydrology of the system, and itslevel of exposure. This latter point is particularly important in interpreting the anomalous levelof some elements (e.g. Hg, Tl, As), as their concentrations can increase at least two orders ofmagnitude in the upper few hundreds of metres of a system; hence the level of exposure is amajor factor in the magnitude of an anomalous element. Using extremely sensitive analyticalmethods, anomalous gold values have been detected in Devonian-Carboniferous volcanics innorth 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 frominterpretation of airborne geophysical surveys, satellite images, aerial photographs, and eventopographic maps. Typically many structures are not mineralized, so it is necessary todistinguish the more prospective structures by examining the correspondence betweenstructures, geochemistry, mineral occurrences, and hydrothermal alteration. Favourableconjunctions of these features become the focus of prospect-scale exploration. In general theexploration techniques applied at project area scale should not be model specific. Explorationshould be conducted so that ANY type of economically significant mineralization will belocated, not only one model.

Prospect Scale Exploration

Exploring at prospect scale confronts the explorationist with his greatest challenge. A drill holewill very effectively test the small volume of rock it penetrates; the challenge is to put the drillhole 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 fromfield observations, which consequently provide simple practical guides in exploration.

Focusing of the ore fluids occurs in zones of enhanced permeability. In some cases thesepalaeopermeable zones can be recognized directly by an increased density of mineralizedfractures (typically veins), and the occurrence of hydraulic brecciation and hydrothermaleruption breccias. In other cases they may be recognized indirectly from vein mineralogy, andfrom the mineralogy and zoning of hydrothermal alteration products. These may also bedetected using geophysical techniques (Irvine and Smith, 1990).

Deposition of gold in epithermal deposits can have several causes. These include boiling, fluidmixing, cooling, and wall-rock reaction, and evidence for each can be recognized from vein andalteration mineralogy. By far the most important of these is boiling, which is the dominantprocess controlling the temperature of hydrothermal fluids near the surface. Characteristictextural 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 andhydrothermal eruption breccias involve sudden pressure releases which must have causedviolent boiling.

So the presence of these textures provides strong presumptive evidence for boiling, which mayhave resulted in gold deposition. Fluid mixing has the potential to cause gold deposition throughchanges in fluid chemistry, and has been invoked to explain mineralization in some volcanichosted epithermal deposits (e.g. Kwak, 1990; Henley et al., 1984); however, its importance isprobably much less than that of boiling. Because of the hydrodynamic pressure accompanyingthe high fluid flux required to form a substantial ore body, fluid mixing is probably onlyimportant near the margins of hydrothermal systems (except as noted previously in high-reliefareas). The main evidence for fluid mixing is seen in the alteration assemblage (notably mixedlayer clays around the margins, or an acid overprint near the top). It is probably important incarbonate-rich late-stage veins (e.g. at Fresnillo; Simmons et al., 1988; at Acupan; Cooke andBloom, 1990; at Emperor; Kwak, 1990); however, these are typically barren, or poorlymineralized.

Both boiling and fluid mixing are likely to be enhanced in structural zones which are alsoeffective for fluid focusing. Gold mineralization localized along, and at intersections ofstructures 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 epithermaldeposits, as it implies a slow rate of fluid ascent. Deposition as a result of wallrock reactions isapparent at Emperor (Anderson and Eaton, 1990), where gold-rich pyrite has formed bysulfidation of iron-bearing minerals. Although this may be important in some deposits, it isprobably never the dominant ore-forming process in epithermal deposits.

Efficient prospect scale exploration requires the careful integration of all available data, coupledwith a good understanding of the processes that occur, and their likely effects. Possibletopographic effects should also be considered, as these will influence the spatial distribution ofconduits and hybrid fluids, and resultant mineralization and alteration.

CONCLUSION

We favour a simple approach to classification of epithermal deposits. Whether a deposit isclassified 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 fluidchemistry. Readily observable features of mineralogy, texture, and alteration zoning allowclassification into high and low sulfidation deposits. This, combined with a simple descriptionof the form of the deposit (vein, stockwork, disseminated), conveys a large amount ofinformation on mineralogy, alteration, and spatial characteristics of the mineralization, andallows inferences to be drawn regarding likely regional controls, and the characteristics of theore-forming fluids. Comparison with a relatively well-known example (if one exists) is anestablished 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 andchemical processes occur within a complex and dynamic geological environment. Consequentlythe features observed, and their spatial relationships vary widely. The unifying theme thatcharacterizes all epithermal deposits is the processes that occur in their formation. Therefore thediversity of features observed, and their significance in exploration, can only be understood withthe aid of a strong conceptual understanding of the processes which occur in hydrothermalsystems. Reliance on models without a firm understanding of the underlying processes leads totheir inflexible, and consequently, ineffective use. Many different data sets provide the sourcesof information for exploration for epithermal deposits. Only by imaginative integration of alldata within the framework of a strong conceptual understanding can we ensure that we obtainmaximum benefit from the data for our exploration program.

ACKNOWLEDGMENTS

NCW acknowledges the permission of BHP Minerals International Exploration to publish thispaper. M.S. Bloom, P.R.L. Browne, D.R. Cooke, R.W. Henley, S.F. Simmons and D.G. Woodcritically reviewed the manuscript.

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TABLE 1: Characteristics of epithermal gold deposit types

Low sulfidation High sulfidation

Host rocks Acid to intermediate subaerial Acid to intermediate subaerialvolcanics, and underlying volcanics, and underlyingbasement rocks of any type. basement rocks of any type.

Localizing controls Any faults or fracture zones Major regional faults orespecially closely related to subvolcanic intrusions.volcanic centres.

Depth of formation Mostly 0 to 1000m. Mostly ?500 to ?2000m.

Temperature of formation 100 to 320oC 100 to 320oC(mainly 150 to 250oC)

Character of ore fluids Low salinity Mostly low (some high) salinity

Meteoric waters, interaction with Magmatic fluid source mixing withmagmatic fluid possible. meteoric waters.

pH near neutral, may become pH acid from magmatic HCl, and byalkaline from boiling; phase disproportionation of SO2 becomesseparated 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).

Associated alteration Extensive propylitic alteration Extensive propylitic alterationin surrounding regions with low in surrounding regions with lowwater:rock ratios. water:rock ratios.Intensive white mica in regions Deep deposits have intensewith high water:rock ratios. pyrophyllite-white micaClay alteration becomes dominant alteration.with decreasing temperature. Shallow deposits have core ofBoiled off gases may produce massive silica (from acidargillic and advanced argillic leaching and silica mobilization),alteration peripheral to, or with narrow margin of alunite andoverlapping alteration from deep kaolinite, out to white mica andfluids. Interlayered clays. Near-surface

deposits may have pervasive clayalteration.

Character of Ore mineralization characterized Ore mineralization typicallymineralisation 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 notcommonly with multi-stage common. Mineralization usually

brecciation. Near-surface may be associated with advanced argillicstockwork or disseminated, alteration, and pyrite typicallydepending 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-grainedbanded quartz-chalcedony, drusy quartz).cavities, vugs, vein breccia,silica pseudomorphs after bladedcalcite (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, AustraliaEmperor, Fiji Mount Kasi, FijiLebong Donok, Indonesia Motomboto, IndonesiaWapolu, Papua New Guinea Nena, Papua New GuineaAcupan, Philippines Lepanto, PhilippinesGolden 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 Inference

Vein Mineralogy/Texturechalcedony present rapid cooling has occurred; may indicate boiling; deposition

temperature between 190 and 100oC; can infer depth of lessthan 400m below water table, assuming hydrostatic conditions.

adularia present boiling has occurred, causing an increase in pH.

lattice texture (i.e.silica replacement of

boiling has occurred, resulting in CO2 loss, and consequentbladed calcite crystals) calcite saturation.

Wallrock Alteration

sericite (white mica) fluid pH near neutral to slightly acid; temperature aboveabout 220oC.

mixed-layer clays paleotemperatures below about 220oC; can be semi-quantified by XRD analysis of basal spacing.

zeolites and calc-silicates very temperature dependent; also indicate low CO2content of fluid.

kaolin pH of fluid depressed; may result from CO2-rich steam-heated waters marginal to the system, from acid sulfate,steam-heated surficial waters, or from condensationof magmatic volatiles.

pyrophyllite fluid acid; if fluid silica supersaturated with respect toquartz, temperature below 260oC, may be down toambient; if fluid saturated with respect to quartz,temperature about 260oC, and depth greater than 800m.

alunite conditions acid with high sulfate concentration; canform under hydrothermal or weathering conditions;wide temperature stability range.

silicification (quartz) saturation with respect to quartz required; may result fromdevitrification of volcanic glass. If from cooling of silica-saturated fluids, at low pressures (