Epithermal Precious Metal Mineralization Research Articles

Fumarolic Activity, Acid-Sulfate Alteration, and High Sulfidation Epithermal Precious Metal Mineralization in the Crater of Kawah Ijen Volcano, Java, Indonesia

Abstract Opinion is divided over whether the fluid responsible for the formation of high sulfidation epithermal deposits is a vapor or a liquid, and whether it is entirely volcanic or of mixed volcanic-meteoric origin. Observations made at Kawah Ijen, an active stratovolcano (mainly andesitic in composition) located in the Ijen Caldera Complex in Java, Indonesia, are used to address these issues. The Kawah Ijen crater is approximately 1 km in diameter, and hosts one of the world’s largest hyperacidic lakes (pH ~0). On the lake edge is a small and actively degassing solfatara field, which is surrounded by a much larger area of acid-sulfate alteration. This area was exposed during a phreatomagmatic eruption in 1817, which excavated the crater to a depth of 250 m, and comprises zones of residual silica, alunite-pyrite, and dickite/kaolinite. Based on the fractionation of 34S and 32S between alunite and pyrite, the acid-sulfate alteration occurred at a temperature between 200° and 300°C. High sulfidation epithermal mineralization accompanied the alteration in the form of massive and vein-hosted pyrite that contains up to 192 ppb Au, 9.2 ppm Ag, 6,800 ppm Cu, and 3,430 ppm As; these elements are invisible at the highest resolution of scanning electron microscopy, and thus either occur in the form of nanoparticles or are in solid solution in the pyrite. Condensed fumarolic gases released from the solfatara field and sampled at temperatures between 330° and 495°C contain up to 3 ppm Cu and 3.8 ppm As; the concentrations of Au and Ag are below detection. The pH of the condensed gas (water vapor) is ~−0.5. The above observations support a model in which highly acidic gases condensed ~250 m beneath the floor of the crater. Depending on the fluid/rock ratio, the condensed liquids altered the andesitic host rocks by leaching them to leave behind a residue of “vuggy silica” (high fluid/rock ratio), by replacing the primary minerals with alunite and pyrite (intermediate fluid/rock ratio), or by converting them to dickite/kaolinite (lower fluid/rock ratio). Gold-, silver-, and copper-bearing phases were undersaturated in the condensed liquids. However, they were able to concentrate by adsorbing on the surfaces of the growing pyrite crystals, which developed p-type conductive properties as a result of the uptake of arsenic. The metals were incorporated in the pyrite either by their electrochemical reduction to form native metal nanoparticles or through coupled substitutions with arsenic for iron and sulfur. The results of this study provide compelling evidence that high sulfidation epithermal precious metal mineralization can form directly from condensed magmatic gases.

Cretaceous epithermal gold–silver mineralization and geodynamic environment, Korea

Epithermal precious-metal mineralization in the Korean Peninsula mainly occurred along NNE-trending major strike-slip fault systems that are associated commonly with formation of pull-apart basins and major volcanic activity during the Cretaceous. Sedimentation in the basins was initiated in the Hauterivian and continued into the Albian, whereas much of the volcanism occurred sporadically from ca. 110 to 50 Ma, with a major episode between ca. 90 and 70 Ma. Epithermal Au–Ag mineralization in Korea took place between approximately 100 and 70 Ma, overlapping with the shallow magmatic activity. Styles of epithermal Au–Ag deposits in Korea include those of the Mugeug-type found in sediment-dominant basins in the central portion, and the Haenam-type in volcanic-dominant basins in the southwest. Epithermal Au–Ag deposits associated with the volcanic-dominant basins in the southern Korea generally formed at very shallow crustal levels (<0.5 kbar) and relatively low temperatures (<300 °C) from fluids containing large components of less-evolved meteoric waters than those associated with sediment-dominant basins. Orthogonal subduction following oblique subduction of the Izanagi Plate along the Pacific continental margin during the Cretaceous probably represents a major control of magmatism and associated Au–Ag mineralization in the Korean Peninsula. In the Early Cretaceous, the left-lateral strike-slip movements due to the northward (oblique) subduction of the plate resulted in the Gongju–Eumseong and Yeongdong–Gwangju fault systems. Late Cretaceous calc-alkaline volcanic activity and associated caldera-related fractures related to an orthogonal convergence that postdates the NNE-trending strike-slip movements may play an important role in the formation of epithermal Au–Ag deposits. Simultaneously with, or soon after heating related to magmatism, continued movement of strike-slip faults may also have been critical to the ore-forming process, leading to relaxation of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.

TEMPORAL EVOLUTION OF ARC MAGMATISM AND HYDROTHERMAL ACTIVITY, INCLUDING EPITHERMAL GOLD VEINS, BOROVITSA CALDERA, SOUTHERN BULGARIA

Plutonic, volcanic, and hydrothermal activity in magmatic arcs is commonly focused spatially, leaving behind complex records of multiple ore-forming processes that can be appreciated in detail only if the timing of igneous and hydrothermal systems is ascertained. The Spahievo ore district, Bulgaria, is adjacent to the Oligocene Borovitsa caldera, one of the largest in Europe. Juxtaposition of porphyry mineralization, barren advanced argillic alteration, and low-sulfidation epithermal alteration and mineralization zones along the caldera margin provides an uncommon opportunity to examine the temporal development of an evolved magmatic-hydrothermal system. 40 Ar/ 39 Ar ages determined using a CO 2 laser to fuse or incrementally heat minerals from fresh igneous and altered rocks indicate a rapid succession of (1) pre-caldera intermediate magmatism including monzonitic intrusion and coeval porphyry-style mineralization and advanced argillic alteration between 32.9 and 32.3 Ma; (2) rhyolitic ash-flow tuffs, dikes, and domes, with adularia-sericite alteration accompanied by epithermal precious metal mineralization at 32.1 Ma; and (3) nearly synchronous collapse of a nested caldera and intrusion of a ring-fault rhyolite dome and intracaldera dikes at 31.8 Ma. These ages link monzonitic intrusion, Cu-Mo mineralization, and advanced argillic alteration, and indicate that rhyolitic magmatism and adularia-sericite alteration with associated base-metal plus Au mineralization is slightly younger, about 100 to 500 k.y. Deposition of Au ore ca. 300 k.y. prior to caldera collapse distinguishes this ore district from many well-known deposits in which ore formation accompanied andesitic or rhyolitic intrusions along ring fractures that postdate the collapse.

Epithermal precious metal mineralization in a strike-slip corridor; the San Dimas District, Durango, Mexico

The San Dimas district, Durango, Mexico, was investigated to define the structural control of epithermal Ag-Au mineralization and to unravel the tectonic history of the area. Three distinct deformational events have been identified. Deformational event D 1 is characterized by the formation of subvertical east-west- to east-north-east-west-southwest-trending tension gashes and hybrid extensional shear fractures that were caused by the ongoing but weakened compression due to subduction west of Mexico during the Early Oligocene. Changing stress conditions during deformational event D 2 developed north-south-trending right-lateral strike-slip faults with accompanying secondary structures, forming a complex pattern of a transtensional corridor. The structures of both D 1 and D 2 are hosted by the Lower Volcanic Group and the underlying Piaxtla intrusive rocks, and they carry low-sulfidation epithermal silver-gold mineralization. Subsequent extension on reactivated north-south-trending strike-slip faults and northwest-southeast-trending normal faults established Basin and Range-type tectonics through both the Oligocene and Miocene. The transition from compression to extension is documented by this strike-slip corridor which was established during the Late Oligocene.

Geochemistry and alteration facies associated with epithermal precious metal mineralization in an active geothermal system, northern Lesbos, Greece

Hydrothermal quartz veins associated with gold and silver mineralization and variable amounts of base metal sulfides have been discovered within an active geothermal system in the Megala Therma area of northern Lesbos. This geothermal system is proba- bly a late evolutionary stage in the formation of this mineralization. The veins are hosted in Upper Miocene volcanic rocks of andesitic composition and consist of quartz, adularia, chlorite, sericite, illite, kaolinite, baryte, small amounts ofjarosite and alunite, and native gold, pyrite, galena, sphalerite, chalcopyrite, bornite, chalcocite, covellite and goethite. The principal types of alteration which occur in the studied area are: silicification, propylitization, argillic alter- ation and potassic, phyllic alteration.

Strike-slip fault reactivation as a control on epithermal vein-style gold mineralization

Epithermal precious metal mineralization develops within contemporaneously active tectonic and volcanic terranes in which co-active faults focus fluids from deep high-temperature reservoirs and magmas into shallower environments. Recognition of such structural controls through analysis of the architecture of prospective volcanic belts and basins is therefore important in the exploration for epithermal gold deposits. Field and high-resolution aeromagnetic data suggest that gold mineralization at Bimurra and Wirralie in the late Paleozoic Drummond Basin (northeast Queensland, Australia) is primarily controlled by reactivation of a northeast-striking strike-slip fault array that may have developed as a transfer fault in the early history of the basin. Gold mineralization is hosted by volcaniclastic and sedimentary rocks whose distribution was also controlled by this reactivated structure.