Acid Sulfate Type Research Articles

The geology of hydrothermal gold deposits in Chile

A detailed geologic study has been undertaken of the characteristics of 20 of the best known hydrothermal gold deposits in Chile. Characteristics include production and reserve data, metal grades, geotectonic setting, morphology, mineralogy, zonation, alteration assemblages and fluid-inclusion studies. Based on this geologic data base, two broad categories of gold deposits have been recognized: volcanic-hosted epithermal deposits of the adularia-sericite and acid-sulphate types and porphyry-related deposits. Volcanic-hosted epithermal deposits of the adularia-sericite type appear to be more abundant than the acid-sulphate type. Very few examples of porphyry-related deposits have been recognized. The majority of the acid-sulphate type deposits were formed during the Oligocene-Quaternary time-span and no adularia-sericite type examples are yet known from this period. This latter epithermal type was formed exclusively during the Mesozoic and Early Tertiary. Presently accepted ore-deposit models have been applied to the Chilean epithermal gold deposits, and an empirical model for adularia-sericite epithermal system is presented.

An acid-sulphate-type lode gold deposit at Nalesbitan, Luzon, Philippines

The Nalesbitan lode Au deposit in the Bicol peninsula of southeastern Luzon, Philippines, is of the acid-sulphate type, and originally contained a minimum of about 15 tonnes of Au. Mineralization is hosted by Pliocene andesitic volcanic rocks transected by a northwest-striking, steeply west-dipping, sinistral strike-slip fault zone, which is considered to be subsidiary to the major Philippine fault zone. The principal lode comprises two discrete structures which flared and coalesced upwards to produce a 1300-m-long body of Au-bearing rock ranging from 12 to 145 m wide. Flanking subsidiary lodes represent mainly parts of the principal lode that were detached and transported several hundred metres left-laterally by post-mineralization motion on the fault zone. The two component structures of the principal lode are marked by steeply west-dipping, linear bodies of hydrothermal breccia, which are transitional outwards to irregular patchy breccias and swarms of subparallel veins and veinlets. Breccias, veins and veinlets are composed of chalcedonic quartz and up to 50 vol.% pyrite of replacement origin, and are enveloped by advanced argillic alteration composed of quartz and kaolinite, subsidiary alunite and lesser amounts of sericite and diaspore. Illite increases in amount as the margins of the lode are approached, and is transitional outwards to a halo of intermediate argillic alteration. Gold in the Nalesbitan lodes is associated closely with high-sulphidation Cu and Cu-Fe sulphide assemblages containing, in order of decreasing abundance, chalcocite, bornite, covellite and chalcopyrite. Enargite accompanies these sulphides, especially in some of the subsidiary flanking lodes. At least 95% of the principal lode, to depths of at least 130 m, underwent complete supergene oxidation, a process which resulted in increased rock permeability and the overprinting of supergene kaolinite on intermediat argillic assemblages. The highest Au grades are centred on the hydrothermal breccias, which die out in the lower, bifurcated part of the principal lode. Reconnaissance fluid-inclusion measurements indicate that mineralization at Nalesbitan took place some 300–500 m beneath the paleosurface from intermittently boiling fluids at temperatures ranging from 223 to 255°C. The Nalesbitan deposit is believed to be centred on a minor dilational jog on the fault zone, where two closely spaced en-echelon fault strands may be represented by the two component structures of the principal lode. Repeated hydraulic fracturing caused by pressure reductions across the jog could have caused the brecciation and fluid ascent responsible for lode construction. Fluid boiling induced by the pressure reductions may have facilitated Au precipitation. The magmatic volatiles and metals required to form the Nalesbitan deposit are inferred to have been supplied from a subjacent porphyry Cu-bearing stock. Advanced argillic alteration is believed to have been more widespread and alunite-rich in shallower, now-eroded parts of the system, which also may have been Au-bearing. The age, geological setting, morphology, alteration characteristics and zoning pattern, mineralogy and conditions of formation of the Nalesbitan deposit are closely similar to those described for the Nansatsu-type Au deposits of southern Kyushu, Japan.

The distribution of arsenic (III) and arsenic (V) in geothermal waters: Examples from the Massif Central of France, the Island of Dominica in the Leeward Islands of the Caribbean, the Valles Caldera of New Mexico, U.S.A., and southwest Bulgaria

Data for As (III), As (V) and total As for various types of geothermal spring water are given. The different species of As were separated at the time of sampling after filtration, acidification and dilution where necessary, using an anion-exchange method. Total As, As (III) and As (V) were determined by flameless atomic absorption spectrometry. The total As content ranges from 5 to 1800 μg l −1. Samples from Dominica show evidence of different sulfur-arsenic oxidation kinetics, As (III) being the major component (85–99%) in fluids from deep levels of the geothermal reservoir and also (99%) in oxidized waters of the acid sulfate type. The bicarbonate spring water of the Valles Caldera, New Mexico, contains more As (III) than As (V), but there As (V) is the major component of the acid sulfate springs. As (III) proves to be the predominant form (60–100%) in the warm alkaline waters from southwest Bulgaria. A selection of CO 2-rich spring waters from the Central Massif, France (Cézallier) is analysed. Two groups are distinguished on the basis of major-element chemistry and total As content. Oxidized forms are mainly encountered in the first group, including a well sample containing 0–5% As (III). The As (III) content of the second group ranges from 10% to 70%. These preliminary results indicate that the fluids directly related to hydrothermal systems contain mainly As (III), whereas acid sulfate springs have a variable As (III) content. Bicarbonate waters are generally enriched in oxidized As. In the absence of springs or sinks within a geothermal region, the total As concentration may display conservative properties, but both As (III) and As (V) may be reactive during mixing and cooling.

Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo volcanic zone, New Zealand

Erosion and excavations at Ohakuri in the Taupo Volcanic zone have exposed the upper portion (100–150 m) of a hydrothermal system that was active sometime between 700,000 and 160,000 years ago. Extensive hydrothermal alteration occurred within a host sequence of young, relatively undeformed, chemically and lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs. Mapping and petrologic work have identified six distinct alteration types. An early event formed a concentrically zoned suite of alteration through the pervasive movement of alkaline chloride type water. In the innermost zone, primary rock components were almost entirely converted to quartz + adularia ± illite ± hematite ± leucoxene. Mineralized veins and breccias of quartz ± pyrite ± adularia ± chlorite formed here in response to episodic hydraulic fracturing. This zone grades outward and upward into a zone of less intense, lower rank alteration with a mordenite + clinoptilolite + smectite + opal ± hematite assemblage, then a zone of weak clay alteration and into fresh rock. Calcite is conspicuously absent from the entire suite. Acid-sulphate type water, formed from steam-condensate, dominated the shallow activity in a second stage of alteration that followed local erosion. Widespread but discontinuous alteration converted the ignimbrite to kaolinite + opal ± hematite, with alunite occurring in the more intense zones. This alteration locally overprints the early alkali-chloride produced suite, but the focus of the second-stage activity was north of the focus of the older event. Scattered opaline sinters and silicified surficial deposits are products of either still later activity or the waning part of the second stage. Chemical analysis shows that the various alteration types have characteristic patterns of major element addition and removal; these reflect the key hydrothermal mineral reactions that formed the new assemblages. Quartz-adularia alteration involved mainly silicification, dehydration and cation exchange (K + for Na 2+, H +, Ca 2+, Mg 2+), whereas alteration in the mordenite zone was mostly a moderate hydration process. Kaolinite alteration involved strong hydration, hydrolysis and redistribution of silica. Trace elements show varying degrees of mobility and correlation with major elements. Alteration features identify the important upflow zones, zones of mixing between hydrothermal and shallow groundwater, and changes in alkali chloride water level. They also reflect a transition from diffuse to channel flow as sealing eliminated original rock porosity, and led to hydraulic fracturing which maintained fracture permeability in the system. Mineralogy and fluid inclusion studies indicate that the primary fluid at now-exposed levels was a high-pH (7–8), low-CO 2 and low-H 2S water cooler than 200°C, probably modified by boiling at depth.

Comparative anatomy of volcanic-hosted epithermal deposits; acid-sulfate and adularia-sericite types

Two types of epithermal precious and base metal deposits hosted primarily by Tertiary volcanic rocks have been distinguished through a detailed examination of the characteristics of 16 well-documented epithermal districts. These characteristics include the ore, gangue, and alteration mineral assemblages, the spatial and temporal distributions of these assemblages, host-rock composition, age relations between ore deposition and emplacement of the host rock, size of the district, temperatures of mineral deposition, chemical composition of the fluids, paleodepth estimates, origin(s) of the fluids, and regional geologic setting. The relative value of each of these features for distinguishing epithermal precious and base metal deposits is estimated. The vein and alteration mineral assemblages are the most important factors, and age relations between ore deposition and emplacement of host rock also appear to be significant. The two major types are designated the acid-sulfate type (Goldfield, Nevada) and the adularia-sericite type (Creede, Colorado; Round Mountain, Nevada). Both types have gold-rich and silver-rich subsets. Base metal contents in the acid-sulfate-type deposit are relatively high and relatively copper rich; there is a wide range in the base metal contents in the adularia-sericite-type deposit, although most are relatively copper poor. Adularia-sericite-type deposits are considerably more abundant than acid-sulfate-type deposits.The two types of deposits appear to form under similar pressure-temperature conditions but in different geological and geochemical environments in ancient geothermal systems. The acid-sulfate-type deposit forms in the root zones of volcanic domes from acid waters that contain residual magmatic volatiles. The adularia-sericite-type deposit forms in a geothermal system where surficial waters mix with deeper, heated saline waters in a lateral flow regime, high above and probably offset from a heat source at depth; neutral to weakly acidic, alkali chloride waters are dominant.

Isotopic geochemistry of acid thermal waters and volcanic gases from Zaō volcano in Japan

The chemical composition and D/H, 18O 16O and 34S 32S ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO 2, SO 2 and N 2, exclusive of H 2O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ 34S values of around +15‰, are enriched in 34S compared to Zaō H 2S, while the acid sulfate waters at Kamoshika contain supergene light sulfate ( δ 34 S = ∼ + 4‰ ) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ 34S than those of other volcanic areas, reflecting the difference in total pressure.