Volcanic-hosted Epithermal Deposits Research Articles

Fluid inclusion and O–H–C isotopic constraints on the origin and evolution of ore-forming fluids of the Cenozoic volcanic-hosted Kuh-Pang copper deposit, Central Iran

The Kuh-Pang copper deposit (2.8 Mt @ 1.65% Cu, 0.52 g/t Au, 34 g/t Ag) is a volcanic-hosted epithermal deposit in the central part of the Late Eocene-Oligocene Urmia-Dokhtar Magmatic Arc of Iran. Three stages of fluid evolution are identified at the Kuh-Pang deposit: (i) early pre-ore stage: with homogenization temperature of 205–372 °C (298 ± 45 °C), salinity of 11.3 ± 2.9 wt% NaCl equiv., fluid trapped at ∼200 bars, (ii) main-ore stage: with homogenization temperature of 175–310 °C (253 ± 43 °C), salinity of 2.1–12.5 wt% NaCl equiv. at ∼150 bars, and (iii) late post-ore stage: with homogenization temperatures of 148 to 231 °C (199 ± 24 °C), and salinity of 1.1–9.8 wt% NaCl equiv. These data record an evolution of mineral precipitation from deeper (>2 km) to shallower environments (<500 m). Fluids trapped in the early (quartz I) and main-ore stages (quartz II) yielded δ18Ofluid values of +5.7 to +5.9‰ and +3.2 to +3.8‰, and δDfluid values of −77 to −41‰ and −84 to −60‰, respectively. These data indicate a major magmatic fluid source, with slightly increasing mixing of meteoric waters. Calcite in the late post-ore stage has δ13CV–PDB values of −5.8 to −5.6‰ and δ18OV–SMOW values of 8.2–8.6‰, with calculated δ18Ofluid and δ13Cfluid values of −1.0 to −1.4‰ and −5.4 to −5.2‰ respectively, that are also consistent with a predominantly magmatic carbon source and a significant fluid mixing by meteoric waters.

Unconventional non-magmatic sulfur source for the Mazarrón Zn–Pb–Cu–Ag–Fe epithermal deposit (SE Spain)

The present study shows the existence of textural, mineralogical, microthermometric and isotopic data that evidences processes such as thermochemical sulfate reduction (TSR) that are not typical features of volcanic-hosted epithermal deposits formed at shallow submarine and/or subaerial environments. Geological evidence and mineralogical, fluid inclusion temperature and geochemistry data suggest that the Mazarrón Zn–Pb–Cu–Ag–Fe deposit is a volcanic-hosted epithermal deposit. It is hosted by dacites with different alteration halos: K–Ar dating of illite from the argillic alteration gives an age of 9.7±1.2Ma, which suggests that mineralization took place very close to the volcanism. The fluid inclusion study in the ore veins shows epithermal temperatures (between 190 and 260°C), but moderate salinities, between 12 and 18wt.% NaCl eq., compatible with mixing between surface waters and polygenic deep hydrothermal fluids. Evolution of the hydrothermal activity shows the existence of three stage of mineralization: in the first and last stages, with Fe-sulfides as main phases, mineralization was mainly formed by interaction between basinal fluids that leached Triassic sulfates (+ surface waters), and C-bearing reduced deep waters. During interaction of both fluids, the sulfur was subjected to extreme and geologically unusual fractionation under TSR conditions, with ranging from −2.9 to +28.4‰ δ34S, reaching exceptional values up to +53‰ δ34S. The intermediate stage is mainly formed by sphalerite–galena–chalcopyrite–fahlore bearing sulfides; this event is compatible with the influx of a magmatic fluid discharge. Finally, this study suggests that Mazarrón could be classified as an intermediate sulfidation deposit.

Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand

Golden Cross is a late Miocene volcanic-hosted epithermal Au-Ag deposit associated with quartz ± calcite ± adularia ± illite assemblages located in the Coromandel peninsula, New Zealand. Gold- and Ag-bearing quartz veins occur in intensely altered flows and lesser pyroclastic volcanic rocks of andesitic and dacitic composition. The primary control of whole-rock geochemistry at Golden Cross, as at other volcanic-hosted epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages, is the destruction of igneous plagioclase by hydrothermal alteration and K metasomatism from hydrothermal fluids. Quantitative mass change calculations indicate that, irrespective of the rock type (andesite versus dacite), hydrothermally altered rocks are enriched in K, Rb, As, and Sb around the Empire vein system and stockwork. In contrast, Na and Sr are strongly depleted around the veins, with Fe locally depleted. Silicon, Ca, Mg, and Ba are variably enriched and depleted, with Si enriched immediately adjacent to the veins and Ca only locally enriched. The enrichment and depletion trends for these elements are directly related to the degree of alteration and the alteration mineralogy. Intensely altered rocks show addition of K and Rb, coupled with loss of Na and Sr, which reflect the replacement of plagioclase by adularia and/or illite. Calcium depletion also reflects the destruction of plagioclase, although Ca loss is locally offset by the formation of hydrothermal calcite. Progressively higher Na and Sr away from the veins reflect greater preservation of plagioclase and the transition into less altered rocks on the margins. Potassium, Rb, Na, and Sr all define anomalies that extend 200 to >350 m east of the veins and are more widespread than As and Sb, which are anomalous within 150 to 200 m of the veins. The geochemical zonation patterns identified at Golden Cross are very similar to those at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns provide insight into the intensity and type of alteration, thereby complementing traditional analyses of Au, Ag, and other pathfinder elements, such as As and Sb, that are used to explore for epithermal Au-Ag deposits. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns at Golden Cross are larger than halos defined by precious metals and other trace element pathfinders, thereby providing a larger target area for exploration. Potassium metasomatism is particularly important because the Empire vein system occurs in altered rocks that are significantly enriched in K. Potassium metasomatism can be evaluated by raw K 2 O data or, more effectively, by K mass changes, molar element ratios of (2Ca + Na + K)/Al versus K/Al (wt % K 2 O * 100)/ppm Sr, Rb/Sr ratios, and K numbers [K/(2Ca + K + Na) molar]. Whole-rock oxygen isotope patterns at Golden Cross are similar to those described at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. However, the area defined by low δ 18 O values is smaller than the area that can be defined by field mapping of alteration minerals or whole-rock major and trace element geochemistry.

The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada

The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada

Porphyry-copper mineralisation in the central Srednogorie zone, Bulgaria

The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district. The hydrothermal systems are related to Late Cretaceous calc-alkaline igneous complexes. Ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks. Several of the porphyry systems are spatially associated with shallow-level intermediate and high-sulphidation volcanic-hosted epithermal deposits of economic importance, such as the major gold-copper mine at Chelopech located 10 km from the Elatsite porphyry-copper deposit. Mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks. K-silicate alteration is characteristic for pre-ore hydrothermal activity in all of them, and it is located mostly in their central parts. Propylitic alteration is prominent in the Medet and Assarel deposits. The Assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre-ore alterations, including propylitic, sericitic, and advanced argillic assemblages. The initial stages of the hydrothermal systems are characterised by high temperatures (>550–500 °C) and highly saline (50–20 wt% NaCl equiv.) and vapour-rich fluids of likely magmatic origin. The composition of the fluids gradually changes from H2O–NaCl±FeCl2 to H2O–NaCl–KCl and H2O–NaCl-dominated as the fluids cool, react with wall rocks, and may become diluted with meteoric water. Fe-Ti-oxide mineral associations were formed early in all deposits, later followed, in the Elatsite deposit, by an assemblage of bornite, chalcopyrite, platinum group element (PGE) phases, Co-Ni thiospinels, Ag- and Bi-tellurides, and selenides. The main ore stage in all deposits is dominated by chalcopyrite ± pyrite, associated with propylitic alteration at Medet, K-silcate-sericite alteration at Elatsite, and chlorite-sericite alteration at Assarel. Specific geochemical features of the ore systems are marked by precipitation of numerous fine-grained mineral inclusions in chalcopyrite and pyrite (palladoarsenide, Pd-rammelsbergite, Cu-Ni sulpharsenides in Elatsite; Co-and Ni-pyrite, carrolite, vaesite, sulvanite and colusite in Medet; enargite, As-sulvanite, colusite, goldfieldite, wittichenite, calaverite and aikinite in Assarel). Native gold and electrum are observed as small grains, except in Elatsite where larger aggregates up to several millimetres are found. Molybdenite is present as small flakes within chalcopyrite, or forms felted aggregates within thin quartz veinlets in the Elatsite and Medet deposits, and very rarely at Assarel. Thin veinlets of quartz-galena-sphalerite are found at the upper parts of the deposits. Supergene alteration of the primary sulphide minerals is developed in the Assarel deposit where a thick blanket of secondary copper mineralisation (chalcocite, covellite) has formed. In the other two deposits, secondary mineral associations (Cu and Fe oxides and hydroxides) are limited to near the surface. Differences in the deposits studied suggest that Elatsite and Medet are formed probably at deeper levels of the porphyry systems, whereas Assarel is developed at rather shallower and lower-temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure.

Epithermal precious metal deposits of Italy-an overview

Epithermal precious metal deposits have only quite recently been added to the metallogenic view of Italy. It was only in the late 1980s that two districts were recognized, in Sardinia and southern Tuscany, previously long known for a large variety of other commodities. The Sardinian epithermal precious metal district is associated with the Oligocene-Miocene Sardinian calc-alkaline magmatic cycle. The most relevant areas include the Au deposit at Furtei, already in production, and the Osilo prospect, where extensive exploration is under way. The deposit at Furtei contains at least six tonnes of gold metal. The mineralogy of the deposit and of the alteration assemblages is typical of the acid-sulfate (high-sulfidation) class of volcanic-hosted epithermal deposits. Fluids associated with alteration and mineralization have moderate temperatures (200–300 °C) and low salinities (less than 6% wt. NaCl equivalent); high-temperature, high-salinity fluids similar to porphyry-style systems also circulated at Furtei. At Osilo, a number of quartz veins containing up to several ppm Au have been identified. The alteration assemblage includes adularia and illite, i.e. is typical of low-sulfidation deposits. Fluid inclusion homogenization temperatures range from 198 to 270 °C, with salinities of less than 4% wt. NaCl equivalent. In southern Tuscany, a number of carbonate-hosted (“Carlin type”) gold showings occur at the edges of the geothermal fields of Larderello, Amiata and Latera, an area previously known for Sb mineralization. In fact, many showings coincide with former Sb mines, and stibnite, along with pyrite, is the most common sulfide mineral. Gold is typically invisible. Fluids hosted in a variety of minerals from these occurrences span a relatively large temperature range (132 to 245 °C), with constantly low salinities (less than 7% wt. NaCl equivalent). The onset of (presumably meteoric) hydrothermal fluid circulation can be ascribed to the emplacement of Neogene Tuscan magmatic rocks, but the ultimate source of gold remains speculative.

Gold-rich porphyry systems in the Maricunga Belt, northern Chile

The Maricunga belt is a linear metallogenic unit defined by at least 14 zones of gold and/or silver mineralization between latitudes 26 degrees and 28 degrees S in the Andean Cordillera of northern Chile. After 10 years of exploration, three precious metal deposits are in production, two more have proven reserves, and others are being drilled. Total geologic reserves are 420 metric tons of gold and about 14,000 metric tons of silver divided between intrusion-hosted porphyry-type deposits and volcanic-hosted epithermal deposits of high sulfidation, acid sulfate type.Precious metal mineralization is related to a belt of Miocene volcanic rocks, most of which constitute a series of large compound stratovolcanoes of calc-alkaline composition. The volcanic rocks overlie unconformably and largely conceal an igneous and sedimentary basement of Paleozoic and Mesozoic age. Volcanic rocks and contained alteration and mineralization are subdivided on the basis of K-Ar dating into partly overlapping, western early Miocene (24-20 Ma) and eastern middle Miocene (14-13 Ma) subbelts. High-angle reverse faulting occurred between these two epochs in response to regional compression induced by subduction zone flattening. A northwest alignment is also prominent in the belt as reflected by the strike of several components of the alteration and mineralization zones.Nine zones of porphyry-type mineralization have been discovered in the Maricunga belt, where they are present in both western and eastern subbelts. The porphyry-type mineralization was generated beneath andesitic-(dacitic) stratovolcanoes, which were dissected more extensively in the western than in the eastern subbelt. Volcanic rocks were intruded by isolated, composite porphyry stocks which are dioritic in the eastern subbelt but include quartz dioritic phases in the western subbelt. Weakly porphyritic microdiorite and associated intrusion breccia are prominent stock components. Minor inter- to late mineralization hydrothermal breccias are commonly present.Gold-copper mineralization is believed to have been introduced with K silicate alteration, which is well preserved only at the Amalia, Refugio, and Casale Hill (Aldebaran) prospects. K silicate alteration is overprinted and commonly obliterated by sericite-clay-chlorite assemblages of intermediate argillic type. Much of the gold is present in quartz stockworks. Iron oxides, both early magnetite and late hematite, constitute 5 to 10 vol percent of mineralized zones. Sulfides are dominated completely by pyrite but include minor chalcopyrite and trace bornite and molybdenite. Supergene leaching of copper is developed to various degrees, but enrichment is developed only incipiently.Several porphyry-type stockworks are overlain by pyrite- and alunite-rich advanced argillic alteration, which carries barite, native sulfur, enargite, and at La Pepa, high-grade, vein-type gold mineralization of high sulfidation, epithermal type. The quartz stockworks and advanced argillic caps are telescoped at Marte, Valy, Santa Cecilia, and La Pepa but are separated by a chloritized zone transected by a swarm of gold-poor, polymetallic veins with quartz-alunite selvages at Aldebaran.Marte and Lobo are rich in gold (1.43 and 1.6 ppm) and poor in copper (0.05 and 0.12%) and molydenum (46 and approximately 10 ppm), and may be designated as porphyry gold deposits. However, gold contents are lower (0.6-1 ppm) and hypogene copper contents probably higher at Refugio and Casale Hill. Zinc tends to occur as a geochemical halo to the Au-Cu-Mo mineralization, which is always deficient in silver (<0.1 ppm). Arsenic and mercury are enriched in the advanced argillic caps, and along with lead, are also weakly anomalous at Marte in the underlying, shallow-level quartz stockwork.The depth of erosion of Maricunga porphyry-type systems is believed to decrease from the K silicate zones exposed at Refugio and in the Casale Hill sector at Aldebaran, through Marte, Valy, Santa Cecilia, and La Pepa where remnants of advanced argillic caps are present, to the highest, mercury-rich part of the Cathedral Peak sector at Aldebaran and zones higher than and west of Marte which comprise advanced argillic alteration rich in native sulfur. Reconstruction of the porphyry systems suggests that gold-bearing quartz stockworks were generated 600 to 1,000 m beneath paleosurfaces.Gold-rich porphyry mineralization in the Maricunga belt is closely comparable to that in the Philippines because of the association with dioritic to quartz dioritic porphyry stocks intruded into andesitic-(dacitic) stratovolcanoes, the widespread distribution of intermediate argillic assemblages as partial or complete overprints to K silicate alteration, the abundance of hypogene iron oxides with ore, and the common preservation of advanced argillic caps. The Maricunga porphyry-type mineralization stands in contrast to the porphyry deposits else-where in northern Chile, which are associated with more felsic intrusions lacking coeval volcanic rocks, contain large volumes of sericitized rocks, lack abundant ore-related iron oxides, are molybdenum rich, gold poor, and more than ten times larger, and include major supergene enrichment blankets. However, both the gold-rich Maricunga belt and the molybdenum-rich deposits in northern Chile are underlain by thick continental crust, whereas the Philippine copper-gold province is in an island-arc setting.

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.

Geology of the DeLamar silver mine, Idaho; a volcanic dome complex and genetically associated hydrothermal system

The DeLamar silver mine, located in southwest Idaho, produced over 17 million ounces of silver and 230 thousand ounces of gold by bulk mining methods between 1977 and late 1987. The mine occurs within the Silver City rhyolite--an extensive and complex sequence of middle Miocene silicic volcanic flows and domes that unconformably overlie a thick pile of Miocene alkali olivine basalt. The basalt overlies Late Cretaceous granite which is correlative with the Idaho batholith. All three major rock types contain epithermal silver-gold veins, although rhyolites such as those at DeLamar, are the only hosts for low-grade, large tonnage mineralization.The DeLamar mine occurs within a sequence of six pervasively altered silicic volcanic units. From oldest to youngest these units have the following informal designations: (1) porphyritic latite, (2) quartz latite, (3) tuff breccia, (4) porphyritic rhyolite, (5) banded rhyolite, and (6) millsite rhyolite. The porphyritic rhyolite unit is spatially, temporally, and genetically related to silver-gold mineralization and is the principal host rock at DeLamar. This unit was emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings. The emplacement of tuff rings and lava domes was controlled by structures related to a shallow level magma body and regional basin-and-range faulting. Onlap of banded rhyolite preceded hydrothermal activity and served as an important control on localizing ore deposition. The mineralogic character of ore and alteration at DeLamar is similar to adularia-sericite-type volcanic-hosted epithermal deposits, but the geologic setting is more like that of acid-sulfate deposits.

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.