Epithermal Gold Deposits Research Articles

Middle Devonian Evandale porphyry Cu-Mo (Au) deposit, southwestern New Brunswick, Canada: Analysis of petrogenesis to potential as a source for distal intrusion-related epithermal gold mineralization

The 391.2 ± 3.2 Ma (U-Pb on zircon) Evandale Granodiorite of southwestern New Brunswick is quite a unique intrusive rock in the southern part of the northeastern Appalachians. This research examines the relationship between the porphyry Cu-Mo-(Au) and epithermal systems with Evandale Granodiorite and environs. This Middle Devonian multi-phase I-type granite intrudes along the northern boundary of the deformed Silurian sedimentary and mafic volcanic rocks of the Mascarene Basin. This granitoid is identified as an oxidized I-type system with adakitic geochemical signatures and associated porphyry Cu-Mo-(Au) mineralization, possibly linked to slab failure sourced from decompression melting of the lower crust caused by post-collisional uplift, similar to the Magaguadavic Granite to the southwest. The Evandale adakitic intrusion is divided into two phases that consist of (1) a calc-alkalic coarser phase ranging from medium- to coarse-grained, seriate to porphyritic granodiorite to monzogranite with metaluminous to peraluminous features, and (2) the later stage alkali-calcic fine-grained (to aplitic) locally porphyritic phase ranging from a monzogranite to syeno-granite with peraluminous characteristics. The primary mineralogy of the intrusive rocks consists of K-feldspar + albite + plagioclase + quartz + biotite + hornblende + apatite ± muscovite ± zircon, along with several alteration phases, such as biotite, chlorite, sericite, and calcite. Geochemical analyses (using INAA, ICP-MS, and XRF) indicate that both phases of the Evandale granodiorite show a high concentration of Cu (108 ppm) and Au (33 ppb) associated with the presence of pyrite, chalcopyrite, and arsenopyrite in the fine-grained dyke samples. Up to 6 ppm Mo was contained within the granitic phase, whereas lesser amounts of Mo were confined to the fine grained to aplite phases. Microprobe analyses of primary biotite as both phenocrysts and groundmass shows 0.55 avg. wt.% Cl and 0.21 avg. wt.% F, which are relatively concentrated compared to other high-grade porphyry deposits. Hydrothermally altered rocks, veins, and sulfides associated with the porphyritic suite result from buoyant magmatic-hydrothermal fluids from a deeper intrusion exsolving fluids (quenched porphyry related), depressurizing, and reacting with the host coarser grained granodiorite and even some of the dykes locally. Based on the geothermometry and geobarometry of amphibole (actinolite-magnesiohornblende), the crystallization temperature and pressure of the Evandale coarse-grained granite and aplite are 674–755 °C, 0.2–0.7 kbar (20–70 MPa) and 838 °C, 0.42–1.4 kbar (42–140 MPa), respectively. This is consistent with biotite formation at temperatures ranging from 670° to 760 °C, based on Ti-in-biotite thermometry. Also, zircon saturation temperatures of the earlier coarse-grained porphyritic granites and later fine-grained porphyry to aplitic dykes are 736 °C and 787 °C, respectively. Spatially and temporally associated gold deposits beside Evandale and within this region of southwestern New Brunswick suggests that these oxidized I-type intrusions are a potential source for distal, low sulfidation epithermal gold deposits in the region.

The main stages of ore formation of the Svetloe epithermal gold deposit (Khabarovsk Region) based on the study of the mineral composition of metasomatic rocks and ores

The study of the spatiotemporal relationships of metasomatites and ores of various zones of the epithermal Au-Ag deposit Svetloe (Khabarovsk Region) using a traditional mineralogical and petrographic approach made it possible to establish several stages of ore formation confined to two paleovolcanic structures of the central type. The early stage corresponds to the moderately acidic (IS) type of mineralization, the second stage of ore formation corresponds to the acidic sulfate (HS) type.Gold mineralization associated with the early stage of ore formation is represented by two main types: Au-Ag-Te and Au-Ag. The late stage of mineralization is characterized by the predominance of redeposited high-grade gold, as well as the presence of other native elements: tellurium, selenium, sulfur, and bismuth.

Distinguishing of areas for conducting the GDP (GMM)-200 for gold, based on the compilation and analysis of the distribution regularities and forecast maps for the black-shale and epithermal type large-volume gold deposits of the Russian Federation at scale 1:2 500 000

The article is devoted to problems of distinguishing the areas for conducting the GDP (GMM)-200 oriented for the black-shale and epithermal type large-volume gold deposits. The approach is based on compilation of the distribution regularity and forecast maps of the Russian Federation at a scale of 1:2 500 000, specialized for these gold deposits, and their analysis. Regional forecast criteria of the gold ore objects and the character of their manifestation over the territory of the Russian Federation are considered. Recommendations are given for setting up the GDP (GMM)-200 programs for map sheets favorable for localization of areas promising for revealing the large-volume black shale and epithermal gold deposits. The forecasted resources of gold and silver of the Far Eastern Federal District and the Arctic zone of the Russian Federation are presented.

Digital Tools for the Promotion of Geological and Mining Heritage: Case Study from the Thames Goldfield, Aotearoa, New Zealand

The geoheritage value in the Thames District, North Island, New Zealand, is notable for its over 50 epithermal gold deposits associated with the Coromandel Volcanic Zone, significant to the region’s geological, cultural, and mining heritage. This case study was conducted in collaboration with the Thames School of Mines and Mineralogical Museum to develop a series of web-based applications for public outreach and an accessible museum experience through the utilization of specimens from the mineralogical museum. This research applies a conceptual framework of ex situ geoheritage to explore links between local geology with cultural and mining heritage. Minerals and rock specimens collected for the Thames School of Mines Mineralogical Museum were used to create 3D virtual models demonstrating the epithermal mineralization in the Thames Goldfield. Outputs of this project consist of two digital products, including a digital mineral and rock repository and the dissemination of the geological collection through integrating the photogrammetric models into a user-friendly outreach, ArcGIS Storymaps, to depict the geoheritage relationship of the specimens to regional gold mining, and at the same time, to be developed to be implemented in geoscience education and communication. The results of this paper are intended to promote the use of digital tools for enhancing and raising awareness of the geoheritage values of the Thames Goldfield. This approach has relevance for Papua New Guinea and the Pacific Islands in raising awareness of geological, mineral, and mining heritage within widely distributed and often isolated communities across island archipelago nations.

Formation–exhumation history of the Carboniferous Axi epithermal gold deposit in the Chinese Western Tianshan based on zircon U–Pb and pyrite Re–Os geochronology and (U–Th)/He zircon–apatite thermochronometry

The Central Asian Orogenic Belt represents a Late Paleozoic archipelago, but crustal growth and the reworking and exhumation of the individual microcontinental massifs remain poorly constrained. Here, we use the Axi epithermal deposit to examine continental preservation and exhumation in the Central Asian Orogenic Belt of the Chinese Western Tianshan region. Zircon U–Pb dating and geochemistry demonstrate that the andesitic host rock formed via the incremental addition of magma in an Andean-type magmatic arc setting at 362, 354 and 342 Ma. Pyrite Re–Os data and textural evidence reveal two mineralization events at 355 and 332 Ma. Zircon (U–Th)/He data reveal temperatures of c. 180°C until 317.8 ± 9.8 Ma, which is interpreted to record the timing of exhumation of the andesite and gold ore bodies prior to their burial by Carboniferous-age sediments. Further sedimentary concealment continued until the Late Mesozoic when the system was re-exhumed between 148.6 ± 8.6 and 120.0 ± 13 Ma at a rate of c. 9.8 m Ma −1 , as shown by apatite (U–Th)/He data. Collectively, the geochronology/thermochronology data show that the Chinese Western Tianshan region records a transition from compressional to extensional tectonism during the Late Paleozoic and Late Mesozoic. The shallow epithermal mineralization was protected from erosion by post-mineralization deposition. Supplementary material: Trace element and zircon U–Pb data for the andesitic host are available at https://doi.org/10.6084/m9.figshare.c.6640014

The Umm Matierah gold prospect: Mineralogical and geochemical characteristics of a potential low-sulfidation epithermal gold deposits, southeastern Arabian Shield, Saudi Arabia

The Umm Matierah gold prospect is located in the southeastern part of the Arabian Shield, at the northernmost tip of the Jabal Ishmas-Wadi Tathlith gold belt. Detailed mineralogical and geochemical investigations indicated that the studied metavolcanics show lithological varieties of meta-alkali basalt, meta-andesite, meta-trachyandesite, and meta-dacite. These rocks are foliated and hydrothermally altered (bleached), indicating a low-temperature propylitic alteration, affected by breccia veins and veinlets, and irregular stockwork. The Umm Matierah gold deposit is characterized by quartz-adularia-sericite-chlorite-carbonate alteration assemblage. The ore minerals of the Umm Matierah gold prospect are dominated by pyrite and arsenopyrite, with minor amounts of sphalerite. The sulfides have no preferred host rock; however, they are mainly present within the veins and veinlets and at the contacts with host rocks; they are also associated with the quartz-rich breccias. Minute gold grains are traced at the contact between the inner pitted and the outer clear zones of large pyrite crystals. Gold and sulfide enrichment do not exceed 10 vol% of the whole rock and are correlated with the thickness of extensive alteration zones that also show an ultimate association of chlorite with sulfide minerals. Compositionally, the studied rocks show 6.47 ppm average gold, are relatively rich in K, Ag, As, Sb, and W, and are relatively poor in Al, Na, Cu, Cr, Ni, Nb, Y, and Rb. The host rocks range in composition from ultrapotassic, shoshonitic, high-K calk-alkaline, to calk-alkaline end member, with transitional environmental signature from intraplate to oceanic island arc. These compositional features suggest that these rocks may have been derived from island source and subsequently slightly fractionated and contaminated during ascent and/or slightly affected by hydrothermal alteration. The host rocks display strong positive Eu, and negative Th, Nb, and Sr anomalies in keeping with the upper continental crustal pattern. There is a general enrichment of the LILEs and the LREEs relative to the MREEs. Collectively, our data, suggest that gold mineralization at Umm Matierah gold prospect, is a possible candidate for a low-sulfidation epithermal style of mineralization, spatially associated with the distal intrusion.

Nanoscale Distribution of Elements in Gold: Examples from Contrasting Deposit Types

Abstract Quantifying the distribution and nature of trace elements in native gold is a novel approach to understanding the genesis of gold deposits and has implications for metallogenic processes, mineral exploration, and metallurgical processing. Most previous research on the formation of gold deposits has utilized proxies for gold, rather than gold itself, to infer its source, transport, and depositional mechanisms, despite the difficulty in establishing unequivocal relationships among the proxies and gold. This approach has left many questions regarding the formation of high-grade coarse gold. This contribution presents results on the composition and distribution of trace elements in native gold using electron probe microanalysis and, for the first time, atom probe tomography. Gold grains selected are from a deformed and metamorphosed Archean orogenic gold deposit (Preston Mine) and a young, undisturbed epithermal gold deposit (McLaughlin mine) which are endmembers in terms of deposit type and geologic time. Results show that Cu, Ag, and Hg are homogenously distributed in gold while Sb is not, which raises questions regarding the substitution mechanisms of large radii metallic elements in gold. These data provide new insights into the incorporation of trace elements in gold, as well as primary and secondary processes related to the formation of high-grade gold deposits through time.

Sulfide Trace Element Signatures and S- and Pb-Isotope Geochemistry of Porphyry Copper and Epithermal Gold-Base Metal Mineralization in the Elatsite–Chelopech Ore Field (Bulgaria)

The Elatsite–Chelopech ore field in the northern part of the Panagyurishte district in Central Bulgaria comprises numerous spatially associated porphyry copper and epithermal gold deposits and prospects. In addition to the mineralization and alteration features, trace elements, lead and sulfur isotope signatures of sulfide minerals from porphyry copper, base metal and gold-base metal deposits/prospects have been studied. LA-ICP-MS analyses of pyrite, arsenopyrite and sulfosalt minerals validate them as major carriers for Au, Ag, Sb, Se and Co. Pyrite from the three types of mineralization has specific geochemical characteristics. Pyrite from the porphyry copper deposits/prospects has generally lower total trace element content compared to pyrite from the epithermal prospects, except for Se, Co and Ni. Pyrite from the base metal and gold-base metal veins is enriched in As, Au, Ag, Sb and Pb. In pyrite from the base metal deposits, Co and Ni have contents comparable to the pyrite from the porphyry copper deposits, while pyrite from the gold-base metal veins shows lower Co and Ni. Arsenopyrite from these deposits shows similar features. Similarly, sphalerite from the gold-base metal veins also has lower Co content compared to sphalerite from the base metal veins but higher In and Cu contents. In addition to the close spatial relationships between the Elatsite and Gorna Kamenitsa porphyry Cu deposits and Negarstitsa-West and Dolna Kamenitsa base metal prospects, as well as similarities in the mineralization and alteration styles, the lead isotopic (206Pb/204Pb = 18.61–18.68, 207Pb/204Pb = 15.64–15.65 for porphyry and 206Pb/204Pb = 18.55–18.67, 207Pb/204Pb = 15.64–15.68 for base metal) and sulfur isotopic (δ34S values of −3 to +1‰ for porphyry and δ34S values of −1.7 to +3.5‰ for base metal) signatures of sulfides support the idea of a genetic link between these two types of deposits. The porphyry and base-metal mineralization result from a common major ore-forming event during the Late Cretaceous, corresponding to deep/higher-temperature and shallower/distal/lower-temperature environments, respectively. In particular, more radiogenic lead (206Pb/204Pb = 18.41–18.47, 207Pb/204Pb = 15.67–15.76) and slightly different sulfur isotopic compositions (δ34S values of +3.5 to +10.6‰) of sulfides from the distal gold-base metal veins of Kordunsko Dere, Svishti Plaz and Shipkite might be a consequence of the interaction of the ore-forming fluids with an external older crustal and isotopically positive S source. Alternatively, a different fluid source/event for the formation of these gold-base metal veins may be suggested.

Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Abstract. The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 ∘C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for 40Ar / 39Ar geochronology to constrain the timing. 39Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of 39Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol−1. Application of Dodson's recursion determined closure temperatures that range from 400–560 ∘C for a cooling rate of 100 ∘C Ma−1. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at ∼2.25 and ∼2.00 Ma.

Enrichment of precious metals associated with chalcopyrite inclusions in sphalerite and pyrite

Abstract The results of investigations into enrichment of precious metals in sphalerite and pyrite from the Maluntou epithermal gold deposit, China, are reported. The obtained data suggest intimate associations of Au- and Ag-bearing nanoparticles with chalcopyrite inclusions in sphalerite and pyrite. The origins of chalcopyrite inclusions involved different hydrothermal processes, including recrystallization-driven phase separation from parent chalcopyrite-sphalerite solid solutions and replacement of pre-existing pyrite in the presence of Cu-bearing fluids. The chalcopyrite blebs/lamellae follow sphalerite {111} planes, which define a shared sulfur layer for both chalcopyrite and sphalerite. This study indicates that mixing and boiling during the evolution of ore-forming fluids for the Maluntou deposit are key processes for the abnormal enrichment of precious metals in sphalerite and pyrite. The chalcopyrite micro/nano inclusions enhanced enrichment of precious metals in sphalerite provides new insights into the controls on the enrichment of precious metals in sulfides.

Tellurium and gold enrichment aided by melts and pyrite crystallization kinetics: Insights from the Yongxin gold deposit, northeast China

The Yongxin gold deposit, in the southern part of the Duobaoshan polymetallic metallogenic belt (DPMB), northeast China, is an epithermal gold deposit with 21 tons of identified gold reserves. Gold mineralization is mainly hosted in the Carboniferous syenogranite and mylonite. This deposit shows a pronounced tellurium signature which is characterized by the occurrence of abundant tellurides including tetradymite, tsumoite, hessite, altaite, petzite, volynskite and Te-Ag alloy. However, the enrichment mechanism of Te-Au in this deposit is still poorly understood. Pyrite, as the predominant sulfide mineral and main host of Au-Te mineralization, was investigated using a combination of ore microscopy, electron probe microanalysis (EPMA) and in‐situ laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Based on morphology, texture, paragenesis and trace element concentrations, we identify three generations of pyrite, including coarse-grained euhedral Py1, fine-grained vein Py2 crosscutting Py1 and anhedral aggregated Py3 coexisting with other sulfides. The results shows that Py3, featured by abundant grain boundaries and porosities, is volumetrically the most abundant among the three, and is the main host of native gold and telluride. LA-ICP-MS analyses of pyrite indicate that the Py1 and Py2 contain low levels of Au, Ag, Te, Bi and other elements, whereas the Py3 has the highest concentrations of Au, Ag, Te and Bi. Arsenic concentrations are constantly low or below the detection limit in all of stages of pyrite and have no discernable correlation with Au. However, there are remarkable positive correlations between Te, Bi and Au suggesting that Te-Bi-rich melts, as gold scavenger, play a critical role in governing gold distribution patterns and leading to high concentrations in the Yongxin gold deposit. This interpretation is supported by the petrographic observations which shows that native gold commonly coexists with tetradymite (Bi2Te2S), indicating a strong geochemical affinity between Te, Bi and Au. Petrographic study, X-ray and LA-ICP-MS element mapping of pyrite show that tellurium commonly occurs along the grain boundaries of Py3, which demonstrate that the precipitation of Te is closely related to crystallization of Py3. In crystallography, the crystallization of minerals is a process that impurity elements are continuously eliminated by the eigenelements. During crystallization of Py3, Te, as impurities, are displaced from the inside of Py3 and finally precipitate at the grain boundaries. Consequently, crystallization kinetics of pyrite might be responsible for the enrichment of Te in the Yongxin gold deposit.

Magmatic degassing controlled the metal budget of the Axi epithermal gold deposit, China

Abstract From integrated textural and compositional studies of auriferous and barren pyrite/marcasite in the epithermal Axi gold deposit, China, we have identified a relationship between multiple gold mineralizing events, mafic magma recharge, and fluid-rock reactions. Three generations of pyrite (Py1–3) and four generations of marcasite (Mar1–4) record episodic gold mineralizing events, followed by silver-copper-lead-zinc-cadmium enrichment. The gold mineralizing events are recorded by high concentrations of subnanometer-sized gold in Py1, Py3, and Mar3 (max. = 147, 129, and 34 ppm, med. = 39, 34, and 12 ppm). Based on previous Re-Os age determinations of pyrite and U-Pb zircon ages of the andesitic wallrock, these gold events slightly postdate pulsed mafic magma recharge and represent the incursion of Au-As-S-rich magmatic volatiles into circulating meteoric water. Silver-Cu-Pb-Zn-Cd enrichment in Py2, Mar2, and Mar4 are consistent with quiescent degassing and gradual Ag-Cu-Pb-Zn-Cd enrichment in an evolved felsic magma. Barren Mar1 records the dominance of meteoric water and a limited magmatic fluid contribution. High-Co-Ni-V-Cr-Ti contents in porous cores of Py1 and Mar2 are attributed to wall rock alteration and dissolution-reprecipitation. The results provide convincing evidence that the metal budget (especially for Au, Ag, Cu, Pb, Zn, Sb) of the hydrothermal fluids and sulfides in epithermal systems are controlled by the influx of magmatic fluids and associated magma, whereas the enrichment of certain fluid-immobile elements, such as Co, Ni, V, Cr, and Ti, is caused in part by fluid-rock interaction.

The petrogenesis and metallogenesis of the Kalchuyeh epithermal gold deposit, central Iran: Constraints from geochemistry, fluid inclusion, and H-O-S isotopes

The petrogenesis and metallogenesis of the Kalchuyeh epithermal gold deposit, central Iran: Constraints from geochemistry, fluid inclusion, and H-O-S isotopes

Geophysical characterization of a low sulfidation epithermal gold and silver deposit, Mendoza, Argentina

The combined analysis of aeromagnetic and gamma-ray spectrometry data, allowed us to determine and differentiate the responses of hydrothermal rocks units and low sulfidation epithermal gold systems, in the south area of the San Rafael Block, in the Mendoza province. The data processing approach was focused on mapping geological limits and structures, and their response over the gold mineralization area, linked to hydrothermal zones. To do this, filtering algorithms, such as vertical derivative, tilt derivative, logistic function, and analytic signal were applied to the aeromagnetic data. For the analysis of gamma-ray spectrometry data, images of K, eU, and eTh concentrations, K/eTh ratios, and an F-parameter were used. The analytic signal filtering of the data reveals relatively low magnetic intensity zones associated with hydrothermally altered rock areas. The radiometric data delineate local anomalies that reflect enrichment with high potassium values in the deteriorated zones. A broad K/eTh anomaly correlates with the extension of the magnetic low intensity zones, indicating a generalized potassium enrichment in the Don Sixto area. These results indicate that the main structural directions are arranged in NW-SE strikes and the secondary ones in N–S strikes. These lineaments are spatially correlated with the most significant structures in the study area, evidencing that the mineral deposits located in the study area respond to the main lineaments. In addition, the main geophysical characteristics allowed us to identify the zone of hydrothermal alteration related to the low sulfidation epithermal gold and silver deposit Don Sixto, which would be related to high levels of K/eTh ratios and low intensity magnetic zones.

Low sulfidation epithermal gold mineralization at Tambang Sawah, Lebong, Bengkulu Province, Indonesia

The Tambang Sawah area, Lebong, Bengkulu Province, is geologically located in the Barisan mountain range and the Sumatran fault zone and is composed of igneous rocks of the Sunda – Banda magmatic arc. This study is aimed to elucidate geological framework and characteristics of the low sulfidation epithermal gold at Tambang Sawah. Gold mineralization is hosted by the lithology of granite and andesitic breccia rocks, and controlled by faults, especially the Ketahun fault which forms a fracture and extensional zone. Hydrothermal fluid filled the structure found to form gold-bearing quartz veins. Hydrothermal alteration recognized at the study area is a typical type of alteration found in epithermal gold deposits, namely, argillic, silicified and propylitic. The characteristic sulfide minerals are low sulfidation epithermal gold deposits in the Sawah Mine area which are found, pyrite, chalcopyrite, covelite, sphalerite and galena with vein textures of colloform banded, cockade-crustiform, moss, and brecciated texture. Gold is interpreted to be coevally deposited with sulfides within quartz veins. Gold grade in rocks and veins varies from 0.07 ppm to 21.3 ppm.

Genesis and Evolution of Hydrothermal Fluids in the Formation of the High-Grade Hishikari Gold Deposit: Carbon, Oxygen, and Sulfur Isotopic Evidence

The Hishikari low-sulfidation epithermal gold (Au) deposit in Kyushu, Japan, is world-famous for its premium ore. It has been hypothesized that magmatic contributions to the hydrothermal fluid during early stages of mineralization is possible, even if the hydrothermal fluids for many Au occurrences near the Hishikari deposit are of meteoric origin and are influenced by basement sedimentary rocks. The purpose of this study is to obtain constraints on the genesis and evolution of hydrothermal fluids in the formation of the high-grade Hishikari Au deposit by carbon and oxygen isotope ratios of calcite-bearing samples. Since the microanalysis of carbon and oxygen isotope ratios in every 12 μm of the calcite-bearing sample along the growth direction (corresponding to 10 years of the Hishikari mineralization) scatter in a particular range, the fluid evolution might not be a gradual change from a magmatic to a meteoric origin. Alternatively, a rapid turnover of two fluids might be happening locally. The average sulfur isotope ratio of hydrothermal pyrite is similar to that of the adjacent magma. However, according to the secondary ion mass spectrometry (SIMS) microanalysis, local pyrite with extremely low sulfur isotope ratios may interact with basement sedimentary rocks. Unlike other epithermal Au deposits in the vicinity, rapid local mixing of the magmatic-origin deep fluid and meteoric-origin fluid reacted with organic matter containing basement sedimentary rocks might cause gold precipitation at the Hishikari deposit.

GEOLOGY AND ORE CHARACTERISTICS OF LOW SULFIDATION EPITHERMAL GOLD MINERALIZATION AT TAMBANG SAWAH, LEBONG, BENGKULU PROVINCE

Abstract –– Tambang Sawah Area, Lebong Regency, Bengkulu Province, is geologically located in the Barisan mountains and Sumatra fault zone. The prospect of Tambang Sawah area lies in the physiography of the central Sumatran mountains ranges composed by the igneous rocks of the Sunda – Banda magmatic arc. This study is aimed to understand the deposit geology and ore characteristics of the gold deposit at the study area. Quartz vein mineralization of Tambang Sawah occurred in granite rock intruded into andesitic breccia rocks, with the geological structure of mineralization control in the form of joint, breccia, dike, and fault, namely the Ketahun fault that forms extentional fracture zones which were filled by hydrothermal fluid in form of quartz veins. Hydrothermal alterations that appear at the study site are typical types of alterations found in epithermal gold deposits, namely, argillic, silicified and propylitic. Sulfide minerals markers of low sulfidation epithermal type gold deposits found, namely, pyrite, calcopyrite, covelite, sphalerite and galena with colloform vein texture, cockade-crustiform, moss, and brecciated texture. Gold is interpreted to be derived from the deposition of sulfide minerals and the deposition of quartz veins, the level of gold in rocks and veins varies greatly with an average 4,8 ppm. Based on the results of ore geochemical analysis of positively correlated gold with elements Ag, Cu, Pb and Zn.
 Keywords: Mineralization, low sulfidation epithermal gold, Tambang Sawah, Lebong, Bengkulu.

A case of Te-rich low-sulfidation epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Tellurium-bearing low-sulfidation epithermal Au-Ag deposits are significant producers of gold, silver, and potentially strategic elements if mineral processing methods are optimized for recovery. Although these deposits are generally related to alkaline magmatism, our study documents an unusual occurrence of Te-rich low-sulfidation epithermal systems in the North Heilongjiang Belt in northeast China that is spatially and temporally associated with calc-alkaline magmatism and tectonic extension in a continental arc‐setting.In the North Heilongjiang Belt, Te-bearing Au-Ag deposits are usually sited in dilatant zones, mostly along extensional NW structures or at their intersections with deep‐seated NE-striking faults such as the Nenjiang-Heihe and Jiayin-Mudanjiang lineaments. Of these deposits, Sandaowanzi is well‐known for its bonanza gold grades. These faults localized andesitic to rhyolitic volcano-plutonic centers that evolved from mantle‐derived mafic melts to intermediate compositions due to differentiation and crustal assimilation as documented by their mineralogy and Sr, Nd, and Pb isotope compositions. During assimilation of country rocks, fluids containing 3He and other volatiles derived from mantle magmas displaced or mixed with external groundwater containing radiogenic 4He derived from country rocks. In this belt, the bulk metal content of Au, Ag and Te was probably introduced by deep mantle‐derived mafic‐intermediate calc‐alkaline intrusions. However, our study does not exclude input of metals leached from underlying metasedimentary rocks and older Te‐bearing Au mineralization. The noble gas, hydrogen, oxygen and lead isotope compositions of fluid inclusions and ore minerals suggest that the deposits formed by mixing between magmatic fluids and convecting meteoric ground waters containing lead leached from surrounding country rocks. At deeper levels, isotopic evidence suggests that Au-Ag-Te (Bi) precipitated with pyrite, quartz, sericite, and carbonate minerals due to mixing with Te-rich fluids. At shallower levels, Au and Ag precipitated contemporaneously with base‐metal sulfides, hydrothermal quartz and sericite during episodic boiling as evidenced by silica morphology and mineral textures.The relatively uniform spacing of epithermal systems along NW- and NE-trending structures in the North Heilongjiang Belt was used to identify unexplored spaces within this mineral belt that are prospective for concealed low-sulfidation epithermal gold deposits. Although some of the Au-Ag deposits in this belt are enriched in Te, there is a lack of coeval alkaline igneous rocks that are commonly associated with Te-bearing gold deposits elsewhere in the world (e.g., Cripple Creek, Emperor, and Lihir Island).

The solubility and speciation of gold in ammonia-bearing aqueous solutions at elevated temperature: An experimental study

The discovery, using short wavelength infrared radiation (SWIR), that some low-sulfidation epithermal gold deposits are associated with hydrothermal alteration involving ammonium minerals, particularly buddingtonite ((NH4)AlSi3O8), has led to the suggestion that the gold may have been transported as a complex involving ammonia (NH3). The speciation of gold in ammonia-rich solutions, however, has received little attention from experimentalists and, consequently, it is not known whether gold forms stable complexes with NH3 and whether such species could contribute significantly to the transport of gold in epithermal ore-forming systems. To resolve this issue, we have conducted experiments to investigate the speciation and solubility of gold in ammonia-bearing solutions at temperatures of 225 and 250 °C under vapour-saturated water pressure with oxygen fugacity buffered by the assemblage magnetite-hematite. Based on the results of these experiments, gold does form stable species with ammonia at elevated temperature. The gold is interpreted to have been dissolved predominantly as Au(NH3)OH0 in solutions with a NH30 concentration of 0.005 ∼ 0.49 m and a pH(T) of 3.29 ∼ 7.47. This species formed via the reaction, Au (s) + NH3 (aq) + H2O = Au(NH3)OH(aq) + 1/2 H2 (g) The logarithms of the equilibrium constant for this reaction are −8.95 ± 0.84 and −7.71 ± 0.85 for 225 and 250 °C, respectively. In order to determine whether this species is important for the transport of gold in epithermal systems, we modeled the speciation of gold in a fluid at conditions typical of those epithermal gold depositing fluids. This modeling shows that the solubility of gold as AuHS0 and Au(HS)2- is much higher than that as Au(NH3)OH0 except in alkaline ammonia-bearing fluids. The association of gold mineralization with hydrothermal alteration involving the formation of ammonium-bearing minerals in the deposits is therefore probably unrelated to the transport of gold as a Au-NH3 complex but simply reflects the high concentration of NH4+ in the ore-forming fluid.

Geochemistry, geochronology and fluid inclusion studies of the Dongji epithermal gold deposit in Fujian province, south‐east China

The Dongji epithermal Au deposit formed in the Early Cretaceous is located in the Dongkeng volcanic Basin of the South China Block. The orebodies of the deposit are mainly hosted in the Jurassic volcanic–subvolcanic rocks. Here, detailed zircon U–Pb geochronology, major‐trace elements, and fluid inclusion data of the Dongji deposit are reported. At least two epochs of magmatic activity were identified at Dongji, including the Late Jurassic rhyolitic crystal tuff lava (165.3 ± 1.8 Ma to 162.8 ± 2.6 Ma) and quartz porphyry (156.0 ± 1.9 Ma), and the Early Cretaceous dacitic crystal tuff lava (108.4 ± 5.7 Ma) and purplish‐red crystal tuff lava (98.1 ± 0.8 Ma). The Late Jurassic volcanic–subvolcanic rocks are characterized by high SiO2 content, enriched LILE (large ion lithophile element) elements, depleted HFSE (high field strength element) elements, and obvious P, Ti and Eu anomalies. These geochemical characteristics suggest that the volcanic–subvolcanic rocks were generated by partial melting of the lower crust and controlled by fractional crystallization of Ti‐bearing phases, apatite, and plagioclase. In comparison, the Early Cretaceous volcanic–subvolcanic rocks are characterized by high K contents, and arc‐like signatures (e.g., enriched Rb, Th, K, and depleted Nb, Ta, Ti), indicating a partial melting of crust–mantle interaction under an extensional tectonic setting. The ore‐bearing dacitic crystal tuff lava and the caprock of purplish‐red tuff lava constrain the timing of gold mineralization in the range of 108–98 Ma. The Au mineralization at Dongji is divided into the early stage of quartz‐pyrite association and the late main mineralization stage of quartz‐polymetallic sulfide association. The fluid inclusion microthermometry results show that the ore‐forming fluids at the early mineralization are characterized by enrichment of liquid‐rich two‐phase inclusions and CO2‐bearing aqueous inclusions. The fluids with moderate homogenization temperatures (292–382°C) and low salinities (5.4–10.0 wt% NaClequiv) were possibly exsolved from magmatic rocks. Nevertheless, fluid inclusions in the late‐stage contain liquid‐rich two‐phase inclusions and pure liquid phase inclusions. The homogenization temperatures (222–309°C) and salinities (2.4–8.3 wt% NaClequiv) of the late‐stage fluid inclusions are relatively lower than those of the early stage, probably indicating a fluid mixing of magmatic and meteoric fluids. Based on the above, we propose that fluid mixing is the key factor facilitating Au precipitation in the Dongji deposit, probably applicable to other Au deposits in the Dongkeng volcanic Basin.