Low-sulfidation Epithermal Deposit Research Articles

Microtextural evidence for the recrystallization of opal-A to quartz in epithermal veins: A case study from the McLaughlin deposit, California

High-grade ores in low-sulfidation epithermal deposits commonly consist of banded veins containing quartz as the most abundant gangue mineral. Previous studies suggested that at least some of the quartz has formed as a product of recrystallization from a noncrystalline silica precursor. Detailed petrographic studies confirm that high-grade veins from the <2.2 Ma McLaughlin deposit in California were originally entirely composed of opal-AG. The noncrystalline silica is isotropic in crossed-polarized light and consists of compacted microspheres that are up to several micrometers in size. In many bands of the high-grade veins, the thermodynamically unstable opal-AG has matured to quartz. The recrystallization of the noncrystalline silica resulted in the development of quartz and ore textures that mask the original conditions of vein formation. Incipient recrystallization of the opal-AG caused the formation of lepispheres consisting of opal-CT as well as the development of concentrically banded silica spheres. Continued maturation led to the growth of elongated quartz crystals or complexly shaped quartz aggregates in the cores of the concentrically banded silica spheres. Amalgamation of quartz crystals caused the development of mosaic quartz or flamboyant quartz. Ripening produced large prismatic quartz crystals, which are characterized by zones of feathery appearance. Fluid inclusions within the quartz formed through recrystallization are typically highly irregular in shape and show inconsistent liquid-to-vapor volumetric proportions, but assemblages with consistent ratios are also present. Bands rich in fluid inclusions occurring along former recrystallization fronts in prismatic quartz crystals resemble growth zones in zonal quartz. Recrystallization of the silica matrix resulted in grain coarsening of the ore minerals and encapsulation of ore minerals by quartz. In areas where recrystallization of the noncrystalline silica proceeded to completeness, the quartz textures could be easily misinterpreted to indicate that quartz growth occurred in open space with the ore minerals infilling vug spaces. Correct recognition of recrystallization textures has significant implications for paragenetic investigations on vein material from epithermal deposits as well as the design of fluid inclusion studies.

Epithermal Ag–Au mineralization at Galim-Legalgorou, Cameroon Volcanic Line: insights from alteration mineralogy and mineral chemistry of electrum and sphalerite

The Galim-Legalgorou area, located along the north-western branch of the Cameroon Volcanic Line (CVL), is known for artisanal gold mining and primary gold exploration, but the style and origin of the mineralization has not been fully described in the scientific literature. The close spatial association of the mineralization with Tertiary volcanic rocks raises the question of whether or not this is an epithermal system in a region where all previously studied gold occurrences have been classified as orogenic. In this contribution, we use ore mineral and alteration assemblage and mineral microchemistry to constrain the origin and evolution of the mineralization. Gold is alloyed mainly with Ag (max. 53.1 wt%) and show fineness values that vary 379 and 721 with an average of 455. A cumulative percentile plot of Ag concentrations in the cores of all electrum particles, and the observed step-change from Au-rich cores to more Ag-rich mantles in zoned electrum particles, suggest that the mineralization was deposited by two distinct hydrothermal fluid influxes. The observed ore mineral association-electrum, Se/Tl-acanthite, pyrite, Fe-rich sphalerite, ± galena; the measured FeS contents of sphalerite (average 23.1 mol%), and the presence of colloform banded chalcedony and hydrothermal breccia are all consistent with features of low sulfidation epithermal deposits. This study, therefore, demonstrates for the first time, the presence of significant low sulfidation epithermal Ag–Au mineralization on the CVL, an intracontinental rift region only previously known for orogenic gold mineralization.Graphical

ALTERATION AND MINERALIZATION STUDIES IN THE JEBLOGAN AREA, KARANGTENGAH, WONOGIRI, CENTRAL JAVA

The research area is located in Jeblogan, Karangtengah, Wonogiri, Central Java. This study aims to investigate the hydrothermal alteration and mineralization. Geological structures present in the research area include faults and fractures. These faults are well-developed trending NE-SW, NW-SE, and N-S as pathways for fluid ascent to the surface, thereby influencing alteration patterns. Mineral alteration assemblages in the research area, based on petrographic, mineragraphy, and XRD analysis, result from hydrothermal processes and include mineral assemblage zones such as Quartz ± Silica ± Pyrite, Alunite + Pyrophyllite ± Zunyite ± Quartz ± Hematite, Illite + Halloysite ± Smectite ± Kaolinite ± Quartz, and Chlorite + Calcite + Epidote ± Pyrite. Mineralization found in the research area includes Chalcopyrite, Sphalerite, Pyrite, Chalcocite, and Galena. Based on the characteristic parameters of the deposit type and laboratory analysis, the deposit type at the research location is low-sulfidation epithermal deposit and intermediate-sulfidation deposit.

Influence of different tectonic settings on fracture formation and fluid flow around upper-crustal magmatic intrusion: insights from numerical modelling

Large porphyry Cu and epithermal Au deposits tend to form in distinct tectonic, porphyry and high-sulfidation epithermal deposits in compressional settings, and low-sulfidation epithermal deposits in extensional settings. Given that the analysis of the shallow metallogenic dynamic processes at the upper-crust scale is insufficient, especially the ore-bearing fracture formation and fluid-focusing mechanism around the mineralizing magmatic intrusion under different tectonic backgrounds, we aimed to study how tectonic settings influence fracture formation and fluid hydrodynamics in and around a hot intrusion. We developed a finite element model coupling thermal-hydrological-mechanical processes to simulate the fracture formation, evolution of fluid velocities, and accumulation of water-rock interactions. The model results show that tectonic compression increases the degree of fracturing, hydrothermal fluid velocities, and water-rock interaction within and laterally around the intrusion; tectonic extension enhances fracturing, hydrothermal fluid velocities, and water-rock interaction at shallow depth. These results confirm that tectonic compression may promote the formation of porphyry Cu deposits, while tectonic extension may promote the formation of shallow hydrothermal deposits. Our model explains the effects of tectonic activity on fracture formation and fluid flow around hot magmatic intrusions in upper crust and deepens our understanding of the relationship between tectonic activity and deposit formation there.

Radiometric Signatures of Gold Mineralization Zone in Pongkor, West Java, Indonesia: A Baseline for Radiometric Mapping Application on Low-Sulfidation Epithermal Deposit

Radiometric mapping could play a prominent role in locating the host rock or alteration that leads to gold mineralization. Nevertheless, in low-sulfidation epithermal gold deposits, the radiometric signatures have to be priorly characterized due to their geometry. It is comprised of a small ore vein system within the large alteration zones. The Pongkor gold mine is a low-sulfidation epithermal deposit and was selected for this purpose. The method started with the surface identification of radiometric signatures on altered and unaltered rocks near Pongkor using portable spectrometers. They are followed by the characterization of the underground mining front, which is comprised of different types of veins and host rocks. The results show that the altered rocks were characterized by a high K% and a low eTh/K ratio. Vice versa, the mineralized veins show low radioelement concentrations. Following the characterization of the geometry of alteration zones and mineralized veins, a study of the relationship between radioelements detected by radiometric mapping and gold pathfinder elements was conducted. Gold pathfinders of Mn, Fe, Zn, As, and Pb were selected for correlation studies with the radioelement. The pathfinders and radioelements were more significantly correlated in veins compared to the host rock. Based on this study, radiometric mapping has the potential and benefit of being applied in the exploration of low-sulfidation epithermal gold deposits. An alteration zone could be delineated by K or eTh/K as an anomaly indicator, and the vein bodies could also be delineated using low K or eTh as an anomaly indicator.

Deep epithermal Au mineralization at Tuanjiegou, northeast China

To better understand the physicochemical features of deeply mineralized intervals of low-sulfidation (LS) Au deposits, the Early Cretaceous Tuanjiegou Au deposit in northeastern China was characterized as it comprises alteration and mineralization styles that reflect deeper portions of the mineralizing system. From shallow to deeper portions of the Tuanjiegou deposit, the mineralization is zoned from sulfide-poor quartz bands through hydrothermal breccia to deep pyrite–marcasite-rich veinlets that occur along hydrofractures within a granodiorite porphyry stock. The zoned alteration halo is centred on the ore zones, and comprises an inner phyllic alteration zone (illite + quartz + muscovite) and an outer intermediate argillic alteration zone (illite ± smectite), with propylitic alteration being limited. Given that apatite fission track ages are extremely sensitive to the ambient temperatures of 60–120 °C, the overlap in the closure dates of magmatic apatite (108.1 ± 3.8 Ma) and zircon (108.2 ± 1.2 Ma) from the granodiorite porphyry is indicative of shallow emplacement of the granodiorite porphyry, likely at depths slightly >0.7 km. A similar paleodepth (900 ± 188 m) was estimated from homogenization temperatures (291 ± 14 °C) of boiling fluids. Preservation of the deep portions of the Tuanjiegou ore system may imply that the productive ore intervals of poorly explored LS epithermal deposits globally are underestimated. Given the evidence for a magmatic origin of the ore-forming fluids at Tuanjiegou, we highlighted that ore-forming fluids are released initially from a water-saturated, oxidizing felsic melt under low-pressure (∼4 km) conditions.

Textural characteristics of ore mineral dendrites in banded quartz veins from low-sulfidation epithermal deposits: implications for the formation of bonanza-type precious metal enrichment

Textural characteristics of ore mineral dendrites in banded quartz veins from low-sulfidation epithermal deposits: implications for the formation of bonanza-type precious metal enrichment

The Role of Selenium and Hydrocarbons in Au-Ag Ore Formation in the Rodnikovoe Low-Sulfidation (LS) Epithermal Deposit, Kamchatka Peninsula, Russia

Gold-silver mineralization in the Rodnikovoe LS epithermal deposit is characterized by selenium speciation. Two main alternating ore assemblages have been identified: silver-aguilarite-acanthite and gold-uytenbogaardtite-acanthite. The former mineral association is intergrown with secondary silver (Ag0.77–0.91), whereas the latter assemblage is closely associated with high-grade gold (Au0.63–0.67). However, both are dominated by Ag0.49–0.56Au0.44–0.51 alloys. The geochemical evolution of the ore-forming system developed in the direction: Fe → Cu; Ag → Au; S → Se; As → Sb. Organic compounds (1 relative %) of both biogenic and thermogenic origin were found in fluid inclusions. These molecules participated in the formation of Ag,Au-complexes and transported noble metals along with selenium. Hydrothermal fluids are characterized by fSe2/fS2 ratios &lt; 1, conditions such that the deposition of selenide minerals is inhibited, except for the naumannite and acanthite series. These conditions allow active entry of selenium into sulfosalts (the selenium substitutes for sulfur).

Timing and ore genesis of the Ciweigou low-sulfidation epithermal gold deposit in the Yanbian Cu-Au district, NE China

The Ciweigou low-17 sulfidation epithermal Cu-Au mining district, NE China, is hosted by Early Cretaceous andesite of the Jingouling Formation. From bottom to top (not explored below the 100 m level), the ore bodies consist of gold-bearing quartz veins, gold-bearing quartz-calcite veins, and calcite veins and the sides are stockwork veins and hydrothermal breccia ore bodies. Five mineralization stages are distinguished: (I) pyrite-sericite, (II) quartz-calcite, (III) grey quartz, (IV) milky quartz-adularia and (V) late carbonate. Au-bearing minerals deposited in stages II and III. Hydrothermal alteration is composed of an inner silicic-pyritic zone, intermediate zone of quartz-sericite-adularia, outer zone of quartz-chlorite-epidote and upper zone of calcite. Systematic fluid inclusion petrography shows that two-phase vapour-rich and liquid-rich inclusions are the dominant fluid inclusions types in the quartz and calcite veins, suggesting that the ore-forming fluids belong to a moderate- to low-temperature (140 °C−320 °C) and low-salinity (0 wt%−14 wt% NaCl-eqv.) H2O-NaCl-CO2 system. HO isotopes (δ18OH2O = -14.0 ‰ to −4.9 ‰ and δD = −123.4 ‰ to −88.1 ‰) indicate that the ore-forming fluids likely originated from mixed fluids driven by residual andesitic magmatic fluid. The precipitation mechanisms are mixing and major fluid boiling. The comprehensive geological and isotopic characteristics show that the Ciweigou deposit is a low-sulfidation epithermal deposit. Six pyrite samples from stages I and II yielded a Rb-Sr isochron age of 102.6 ± 2.2 Ma (MSWD = 1.19), consistent with other low-sulfidation epithermal gold mineralization ages in the Yanbian district and Lesser Xing'an Range. The timing of the Ciweigou gold mineralization at ∼103 Ma postdates the eruption of andesitic magma (106.4 ± 2.1 Ma, zircon U–Pb) and granodiorite porphyry (106.6 ± 1.2 Ma, zircon U–Pb) and diorite porphyry (107.8 ± 1.8 Ma, zircon U–Pb) emplacement, suiting in an extensional environment attributed to the rollback of the subducting Paleo-Pacific Plate.

Hydrothermal-activity stage classification and power-generation potential evaluation of geothermal wells based on a natural analog study on epithermal deposits, in Japan

In this study, we prepare guidelines for hydrothermal-activity stage classification and evaluate the power-generation potential of geothermal wells based on a natural analog study on the hydrogeological and thermal structures and hydrothermal-alteration minerals in a low-sulfidation epithermal deposit. As such, many high-temperature geothermal wells in Japan were classified into four hydrothermal-activity stages (early, middle, late-1, and late-2) based on the developed guidelines. In conclusion, the late-1-stage well is evaluated to supply fluids for power generation with the least economic and environmental risks. In contrast, the geothermal wells in other active stages indicate some of the concerns.

Sulfide Variation in the Coeur Rochester Silver Deposit: Use of Geologic Block Modeling in the Prediction and Management of Mine Waste

Abstract A critical issue in mine waste management is the management and mitigation of acid rock drainage and metal leaching particularly from waste rock. This requires good understanding and characterization of materials most likely to generate acid pH waters and generate leachable metals and metalloids, as well as the presence of minerals most likely to neutralize acidic pH such as carbonates and certain silicates. For low-sulfidation epithermal deposits in silica-rich volcanic rocks, neutralization minerals are rare. Consequently, determination of acid generation is almost entirely a function of sulfide content of the rocks. By understanding the distribution of sulfide minerals within such a deposit, an initial mine waste management plan can be developed. The Rochester mine currently exploits a low-sulfidation epithermal gold-silver deposit and is located on the southern flank of the Humboldt Range in west-central Nevada. The Rochester mine is a conventional open-pit mining operation with cyanide heap leaching and Merrill Crowe processing. Silver and lesser gold mineralization occur associated with sulfide minerals hosted in Permian-Triassic Koipato Group rhyolitic units with extensive quartz-sericite-pyrite alteration. Gangue mineralogy of the deposit is dominated by slow or unreactive silicate minerals with little or no carbonate. Silver-bearing mineralization was controlled by primary hypogene sulfides and secondary supergene oxidation processes, and most of the mineralized material mined to date has been fully or partially oxidized. Oxidation mobilizes and depletes all forms of sulfur from primary sulfide minerals. Due to the low sulfate content and low content of neutralizing gangue phases, the waste rock classification can be based on total sulfur content. Based on the analysis of pulp samples collected from within the mine area, a sulfur block model was developed and compared to the results of standard acid-base accounting and net acid generation tests. This comparison demonstrates the benefit of the block model in identifying zones of potentially acid-generating material in the mined waste. Application of this information to mine planning has allowed more efficient management of acid-generating mine waste by allowing the operator to develop areas of the waste rock facility to receive the waste in a timely fashion. Such an approach is based on good mine geologic and mineralogical characterization and the use of a representative and comprehensive geologic block model.

Fluid boiling during high-grade gold deposition in an epithermal gold-telluride deposit, Guilaizhuang, China

Boiling is a well-established mechanism for precious metal mineralization in low-sulfidation epithermal veins, and is considered a key process in the formation of high-grade bonanza zones in those systems. In contrast, the potential role of fluid boiling in the formation of epithermal gold-telluride veins associated with alkalic porphyries has rarely been discussed. The Guilaizhuang gold-telluride deposit, Shandong Province, China, is an ideal location to investigate the possible role of fluid boiling because the deposit comprises several distinct styles of orebodies, ranging from finely-disseminated gold mineralization hosted in carbonate rocks at depth, through to very high-grade mineralized breccia zones at shallower levels. Here, we examine the relationships between ore-forming fluids and mineralization styles in this deposit, based on paragenesis, vein textures and fluid inclusions. In the deeper carbonate-replacement style orebodies, we found no evidence of fluid boiling and, as such, alternative mechanisms such as fluid mixing or fluid-rock reactions were likely occurring. In contrast, the shallower, high-grade breccias show clear evidence of fluid boiling, which we infer contributed to the formation of exceptional bonanza zones. Our results thus indicate that fluid boiling can play a key role in precious metal mineralization in gold-telluride deposits, analogous to the low-sulfidation epithermal deposits. Furthermore, our results illuminate how contrasting mechanisms of gold mineralization in these deposits (i.e., boiling versus non-boiling) can give rise to the diversity and distribution of gold grades and reserves within an epithermal gold-telluride system.

Characterization and distribution of adularia and other alteration minerals by X-ray diffraction analysis at the El Peñón Au-Ag epithermal deposit, northern Chile

The El Peñón low-sulfidation epithermal deposit, located in the Paleocene metallogenic belt in the Antofagasta region, northern Chile, consists of Au-Ag bearing veins and hydrothermal breccias hosted in volcanic rocks and surrounded by alteration haloes consisting of a series of minerals, such as adularia, carbonates and clay minerals. They are the result of metasomatism generated by the passage of reduced, and near neutral pH, hydrothermal fluids, that transported gold through structures and its interaction with the host rock. We investigated the spatial distribution of these minerals (mainly adularia) in the Aleste vein and its host rock, located in the northern part of the district, to establish their relationship with the Au-Ag ore and, thus, support the identification of new exploration targets. An X-ray diffraction technique to discriminate adularia from other K-feldspars in whole rock samples was developed by the detail study of the diffractogram patterns of an adularia standard. The study of an X-ray diffraction standard pattern allowed the recognition of this mineral by 4 secondary peaks. Our results indicated that adularia occurs in association with felsic volcanic rocks, and it is restricted in intermediate composition units. The spatial distribution of this mineral is correlated with the mineralized zones, being adularia semi-quantitative abundance in the range of 25 to 40% a good indicator of gold mineralization. Sericite and illite occur mainly where adularia is scarce or absent. The spatial distribution of these minerals showed the structural control in the evolution and flow path of the hydrothermal fluids toward the surface. Kaolinite is related to argillic alteration caused by steam-heated fluids, and the advanced argillic alteration associated with later supergene alteration. Chlorite usually is far from the mineralized areas; therefore, it could be a reliable indicator of the margins of the system. The characterization and spatial distribution pattern of the alteration minerals identified by the X-ray diffraction method in the veins of El Peñón deposit show the capability of this type of analyses in determining possible prospection targets.

Constraints of magmatic differentiation on epithermal mineralization at Dongan, NE China: Insights from zircon geochronology, elements and Sr–Hf–Nd isotope geochemistry

Low-sulfidation (LS) epithermal deposits formed under relatively low temperature and pressure conditions are generally associated with volcanic activity. To better understand magmatic processes prior to epithermal mineralization, field geology, petrology, UPb zircon geochronology, whole-rock major, and trace element geochemistry along with HfNd isotopic geochemistry of the cogenetic volcanic rocks at Dongan were assessed. The Early Cretaceous (109.8 to 107.0 Ma) volcanic suite consists of high-K calc-alkaline andesitic-rhyolitic tuff and granite porphyry dikes or/and stocks. Arc magma affinity characterizes the suite with enrichments in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high field intensity elements (HFSEs) and heavy REEs (HREEs). The extrusive andesitic rocks showing enriched mantle features (εNd(t) = + 0.76 to +1.83, εHf = − 3.4 to +3.7, TDM2 = 933 to 1124 Ma) are the result of the hybridization between the mantle and lower crust melts with the contamination of upper crustal materials during eruption or ascending of melts. The intrusive granite porphyry (εNd(t) = +3.81 to +3.9, εHf = −0.3 to +3.7, TDM2 = 901 to 1187 Ma) is geochemically comparable with andesitic rocks and was likely formed by the fractional crystallization of plagioclase and amphibole minerals from cooling andesitic melts at middle–upper crustal levels. Note that the rhyolitic magmas show distinct features of lower εNd(t) (−3.05 to – 2.78), εHf (−0.6 to +1.6) and total REE values (54.78 to 70.09 ppm), older TDM2 (1464 to 1662 Ma) and obvious negative europium anomalies (0.34 to 0.37), suggesting an origin of upper crustal remelting (compositionally equivalent to water-poor felsic granulite or tonalite) that results from the heating of the underlying andesitic magmas. In summary, the sequential emplacement of the ore-related andesitic and granitic melts is attributed to the magmatic processes of the active magma chamber in a continental extensional setting, and the metal-rich bubbles focused at the tops of this chamber due to prolonged differentiation are released to produce epithermal veins at shallower levels (< 1.5 km).

Geological, Fluid Inclusion, H-O-S-Pb Isotope Constraints on the Genesis of the Erdaogou Gold Deposit, Liaoning Province

The Erdaogou gold deposit is located in the conjuncture of North China Carton and Xingmeng orogenic belt. The ore-forming process of Erdaogou gold deposit is divided into three stages (I to III), which are quartz-pyrite stage (stage I), quartz-polymetallic sulfide stage (stage II) and quartz-calcite stage (stage III). Two types of fluid inclusions is distinguished in the ore-forming stage, i.e., aqueous type (W-type) and aqueous-carbonic type (C-type) inclusions. From the stage I to III, the homogenization temperature of fluid inclusions are respectively 334–395, 214–364 and 172–272 °C, with salinities of 7.72 wt.%–11.23 wt.% NaCl equiv., 0.20 wt.%–23.18 wt.% NaCl equiv., and 0.35 wt.% to 5.25 wt.% NaCl equiv. The ore-forming fluids of the Erdaogou deposit have the characteristics of medium-low temperatures, moderate salinities and low densities. And the fluids belong to the CO2-H2O-NaCl system. The values of δ34SV-CDT in sulphide samples at different stages (stages I to III) are between −2.2‰ to 2.3‰, indicating sulfur source from magma volatiles or subvolcanic rock leaching. The lead isotopes suggest that ore-forming metals may be derived from a mixture of lower crust and mantle materials. Oxygen and hydrogen isotope data at Erdaogou indicate that magmatic fluid and meteoric water may both be involved in the hydrothermal system. Based on the geological characteristics, fluid inclusion results, stable and radiogenic isotope results of Erdaogou gold deposit, we believe that the temperature decrease, fluid boiling are the key factors leading to the ore precipitation and the genetic type of Erdaogou gold deposit is low-sulfidation epithermal deposit.

Lithium isotopes and Cu-Au concentrations in hydrothermal alterations from Solfatara Volcano, Campi Flegrei caldera complex, and La Fossa volcano, Vulcano Island, Italy: Insights into epithermal ore forming environments

Hydrothermally-altered rocks collected at Solfatara volcano, Campi Flegrei caldera complex, Italy, are comparable to zones of steam-heated alterations found at low sulfidation epithermal deposits, and volcanic gases collected at Solfatara have temperatures and C-O-H isotopic compositions akin to those forming low sulfidation epithermal deposits. By contrast, hydrothermal alterations collected at La Fossa volcano, Vulcano island, Italy, are comparable to zones of residual vuggy silica formed in high sulfidation epithermal deposits, and volcanic gases collected at La Fossa have temperatures and C-O-H isotopic compositions comparable to those forming high sulfidation epithermal deposits. At Solfatara, amorphous and hydrous opal-A is responsible for shifts in δ7Li values, from +2.2‰ in fresher rocks, to −3.6‰ in most altered rocks, with increases in Au and Cu concentrations (up to 3 ppb and 96 ppm). The increase in Au and Cu concentrations in progressively-altered rocks resulted from the transport of Cu-Au in magmatic-hydrothermal fluids and their partitioning into pyrite, Fe oxides, phyllosilicates, sulfates, and/or opal-A. It is proposed that the combination of opal-A, decreases in δ7Li values, and increases in Cu and Au concentrations represent an exploration vector for low sulfidation epithermal veins. At La Fossa, α-cristobalite is responsible for shifts in δ7Li values, ranging from −0.9‰ in least-altered rocks, to +4.7‰ in most altered rocks, with decreases in Au-Cu concentrations. The decrease in Au and Cu concentrations in progressively-altered rocks may have resulted from the metasomatism of pyrite and Fe oxides, the dissolution of clinopyroxene and opal, and the invasion of the samples by α-cristobalite. The combination of α-cristobalite, increases in δ7Li values, and decreases in Cu and Au concentrations are proposed as proxies for potential high sulfidation epithermal disseminations. Alternating phases of high eruptive activity and quiescent degassing at volcanoes generally, and at Solfatara and La Fossa specifically, suggest that the physicochemical conditions of individual subvolcanic hydrothermal systems should also be alternating, between conditions that are characteristic of low- and high sulfidation epithermal ore-forming environments, and that the related zones of silicification should be alternating between low δ7Li and high Cu-Au values dominated by opal-A, and higher δ7Li and lower Cu-Au values dominated by α-cristobalite.

Textural Characteristics of Barren and Mineralized Colloform Quartz Bands at the Low-Sulfidation Epithermal Deposits of the Omu Camp in Hokkaido, Japan: Implications for Processes Resulting in Bonanza-Grade Precious Metal Enrichment

Abstract The Miocene low-sulfidation epithermal deposits of the Omu camp in northeastern Hokkaido, Japan, are small past-producers of precious metals and represent significant exploration targets for high-grade Au and Ag ores. The quartz textures of ore samples and the distribution of ore minerals within quartz veins were studied to identify the processes that resulted in the bonanza-grade precious metal enrichment in these deposits. In the high-grade vein samples, which are crustiform or brecciated in hand specimen, ore minerals exclusively occur within colloform quartz bands. High-magnification microscopy reveals that ore-bearing colloform bands consist of fine-grained quartz exhibiting relic microsphere textures and quartz having a mosaic texture that formed through recrystallization of the microspheres. The presence of relic microspheres is evidence that the microcrystalline quartz hosting the ore minerals formed through recrystallization of a noncrystalline silica precursor phase. The ore-hosting colloform bands composed of agglomerated microspheres alternate with barren colloform quartz bands that are composed of fibrous chalcedonic quartz and mosaic quartz formed through recrystallization of the chalcedony. The findings of this study are consistent with previous models linking bonanza-grade precious metal enrichment and the formation of bands of noncrystalline silica in low-sulfidation epithermal veins to episodic vigorous boiling or flashing of the hydrothermal system in the near-surface environment.

Type of Gold Deposit in Arinem Cisewu and Its Surrounding Garut Regency, West Java, Indonesia

Gold deposit based on the mineralization of gold and other ore minerals in Arinem area and its surrounding was found in the quartz vein. The research area is in Arinem area and its surrounding, Garut Regency, West Java. Arinem and the vicinity was the location for early stage of research program of mineralization in Papandayan area. This area has good mineralization, and according to previous studies, is a low sulfidation area with the presence of pyrite, chalcopyrite, galena and sphalerite (Antam, 2014). The presence of galena and sphalerite mineral has suggested that instead of low sulfidation, the area may be in intermediate sulfidation zone state supported by other data such as quartz breccia, banded quartz and shear as control structure. Lithology in the area consists of breccia Quaternary and lava unit of Jampang formation in Miocene and unit of andesite and basaltic in Quaternary and intrusion of dacite, andesite and diorite. The alteration and mineralization of research area was classified as propylitic alteration zone with the presence of chlorite, epidote and calcite mineral, argillic alteration zone with montmorillonite, kaolinite mineral, silicic alteration zone with the presence of quartz-sericite, a bit of calcite mineral. The observed ore minerals are pyrite, chalcopyrite, galena, sphalerite. The geological structure was controlled by horizontal fault in almost north-south direction of N 170O-180O E and northeast – southwest horizontal fault in N 40O – 50O E, while the mineralization zone was controlled or following fault pattern of N 170O - 175O E. The epithermal deposit of the study area can be classified as intermediate sulfidation epithermal deposit for zones with galena and sphalerite mineral particularly on several locations near fault zones and brecciated quartz. Meanwhile, the study area is generally low sulfidation epithermal type deposit.

The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment

The Ixtacamaxtitlán area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Trans-Mexican Volcanic Belt (TMVB). In this paper, a new 40Ar/39Ar age (2.87 ± 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite ± opal ± cristobalite ± smectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TMVB, a relevant feature for regional exploration. A ~12 My gap is established between the formation of the Cu-Mo-Au porphyry/skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping. Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage—that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous. Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastomoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of &lt;1 µm thick interwoven filaments in close association with hyphae (about 2.5 µm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are not commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Hydrogeochemical modeling constrains pH to between ~3.2 and ~3.6 and temperature to between 53 and 75 °C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling.

A cold-water trap as an essential process for the generation of low-sulfidation epithermal deposits: Geological and numerical studies of the Hosokura deposit, northern Japan

Although low-sulfidation epithermal deposits, including the Hosokura deposit (Ag–Pb–Zn) in the Miyagi Prefecture, northern Japan, are globally significant metal sources, their ore deposition mechanisms are not thoroughly understood . This study aimed to construct a genetic ore deposition model for this deposit by focusing on the richest vein, the Fuji, and test the model using a numerical simulation of the formation and transitions of this fossil geothermal system. It was proposed that the mixing of neutral hydrothermal fluids (~260 °C) ascending through the vein with oxygenated cold groundwater caused early stage mineralization and the transport of hydrothermal fluids into the surrounding host rocks, forming a hydrothermal alteration halo around the vein that results in silicification, adularia alteration, and argillization. The progress of this alteration decreased the permeability of the halo and generated a mushroom-shaped low-permeability alteration halo (LPAH) in permeable zones surrounding the vein. The low-permeability and insulating properties of the LPAH increased the fluid temperature, the vein fluid boiled, and consequently the middle stage mineralization occurred. After a further decrease in LPAH permeability, hydrothermal brecciation resulting from increasing fluid pressure occurred at the top of the vein, resulting in late-stage mineralization. A TOUGH2 numerical simulation tested these proposed processes: early stage cooling of hydrothermal fluids by mixing with groundwater, the development of vapor in the vein by the formation of an LPAH during the middle stage, and an increase in fluid pressure leading to hydrothermal brecciation during the late-stage. The results of the simulation confirmed that a cold-water trap is an essential process for the generation of low-sulfidation epithermal deposits. Mushroom-shaped LPAHs, silicification zones, and brecciated veins caused by cold-water traps have been found in representative low-sulfidation epithermal deposits in Japan, such as the richest Au deposit, the Hishikari. Therefore, identification of cold-water-trap footprints can contribute to the exploration of this type of deposit.