Low-sulfidation Epithermal Gold Research Articles

Ore Characteristics and Distribution Based on Geological and Geophysical Features of the Tambang Sawah Low Sulfidation Epithermal Gold Prospect in Lebong District, Sumatra Island, Indonesia

Sumatra is recognized as one of the Indonesian islands with a big potential for mineral deposits particularly along the belt known as Neogene Sumatra gold axis. However, the publication of the gold resources is still restricted as regional/district scale maps or limited in some locations and internal company reports, hence, a detailed study on deposit scale remains limited. This study aims to document the deposit geology, ore characteristics and geophysical features of the Tambang Sawah prospect at Lebong district, Sumatra Island. The study was done by geological mapping, sampling, and analyzing of selected samples by petrographic, ore microscopic, X-Ray Diffraction, micro X-Ray Fluorescence and various geochemical analyses to identify ore characteristics. The geophysical survey was carried out using geomagnetic and induced polarization. Ore mineralization is manifested by various alteration types such as argillic, propylitic, and silicification, which suffered andesitic breccia and granite host-rocks. Gold mineralization is in the form of quartz veins with typical opening space filling textures such as colloform, crustiform, cockade, moss, comb, and brecciated. The main sulfides consist of pyrite, chalcopyrite, covelite, sphalerite and galena. Gold commonly occurs as free grains. Highest grade gold of up to 21.3 ppm is associated with colloform and cockade-crustiform quartz veins. Geophysical surveys indicate that gold mineralization is associated with low geomagnetic anomaly areas. The 3D modelling of the IP survey reveals that the ore mineralization may occur at 75m depth. Two target areas located at the northeastern and southwestern portions are recommended for a follow-up detailed exploration based on the mapped ore geology, ore characteristics, and geophysical anomalies.

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.

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 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.

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.

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.

Tectonic-magmatic setting for Early Cretaceous low-sulfidation epithermal gold deposits in the Xing–Meng Orogenic Belt: Constraints from zircon U–Pb and Hf isotopic data of wulaga deposit, NE China

The Xing–Meng Orogenic Belt hosts an amount of low-sulfidation epithermal gold deposits. However, the tectonic-magmatic setting remains problematic, which hinders understanding the factors that control the gold endowment in the region. Wulaga deposit is the largest low-sulfidation epithermal gold deposit in the northeastern Xing–Meng Orogenic Belt. Gold mineralization occurs in the crypto-explosive breccia zone of subvolcanic granodiorite porphyry. Zircon U–Pb ages of three granodiorite apophyses and previous pyrite Rb–Sr dating (113.8 ± 4.4 Ma) indicate granodiorite porphyry and gold mineralization was coeval. The ore-related granodiorite porphyry is moderate SiO2, high-K calc-alkaline, and metaluminous, suggesting an I-type granite. Moreover, Wulaga granodiorite porphyry displays low initial 87Sr/86Sr ratios and positive εNd(t) values with TDM2(Nd) of 799.7–897.4 Ma and εHf(t) values with TDM2(Hf) of 652–785 Ma, indicating that it was derived from partial melting of the Neoproterozoic juvenile mafic lower crust. Ti-in-zircon thermometry, medium–high Sr/Y ratio, high Ba/La, and fO2 value indicate that Wulaga granodiorite porphyry formed at relatively low temperatures (∼700 °C), rich water, and high fugacity within the stability field of garnet in the juvenile lower crust. Combined with ore-related tectonic-magmatic activities in the Wulaga, Dong'an, and Sandaowanzi gold deposits, the Early Cretaceous low-sulfidation epithermal gold deposits in the Xing–Meng Orogenic Belt were formed from magmatic-hydrothermal events triggered by mutual interaction between post-orogenic lithospheric extension related to the closure of the Mongol-Okhotsk Ocean and arc-back extension associated with rollback of the Paleo-Pacific Ocean plate. These data indicate the potential existence of the Early Cretaceous epithermal gold deposits that are related to the contemporaneous igneous activity.

Genesis of the Dongpuzi Gold Deposit in the Liaodong Peninsula, NE China: Constraints from Geology, Fluid Inclusion, and C–H–O–S–Pb Isotopes

The Dongpuzi deposit is an epithermal gold deposit located in the southern margin of the Shaozihe volcanic fault basin in the Liaodong Peninsula. On the basis of fluid inclusion and C–H–O–S–Pb isotope data, a metallogenic model is established for the Dongpuzi deposit. The mineralization at the Dongpuzi deposit has experienced quartz–pyrite (I), quartz–sulfide (II), and quartz–calcite (III) stages. The quartz from ore stage II has liquid-dominated aqueous inclusions, which have homogenization temperatures ranging from 113 to 162 °C and salinities varying from 3.2 to 9.6 wt% NaCl equiv. The quartz from the quartz–calcite stage has decreasing homogenization temperatures (106~143 °C) and salinities (2.7~6.9 wt% NaCl equiv.). The fluid inclusion data indicate that the gold ores were precipitated from low-temperature and low-salinity solutions, with an obvious decrease in temperature and salinity from ore stages II to III. The calculated δ18Owater values for the quartz of ore stage II range from −14.71‰ to −13.31‰, and the corresponding δDwater values range from −103.3‰ to −96.1‰, indicating that the ore-forming fluids could be of a meteoric origin. The calcite from ore stage III has δ13CV-PDB values of −4.5‰ to −4.2‰ and δ18OV-SMOW values of +7.0‰ to +7.4‰, indicating a mantle source for the carbon. The pyrite yielded δ34S values of +4.1‰ to +6.6‰ and Pb isotopes consistent with those of the host trachyte porphyry and volcanic rocks of the Xiaoling Formation, which suggests that the S and Pb in gold ores were dominantly derived from the host trachyte porphyry and volcanic rocks of the Xiaoling Formation, with some combination of Paleoproterozoic metamorphic rocks of the Gaixian Formation. These results, together with the ore geology, indicate that the Dongpuzi deposit is a typical low-sulfidation epithermal gold deposit with important ore-forming materials input from the host trachyte porphyry, volcanic rocks of the Xiaoling Formation, and Paleoproterozoic metamorphic rocks of the Gaixian Formation. The Dongpuzi deposit was formed under an extensional setting related to the Early Cretaceous lithospheric extension and thinning of the eastern North China Craton.

High temperature wall-rock alteration zoning in the Sanjin deposit, Hishikari gold mine, Japan: Implication for exploration of mature mining districts

The Sanjin deposit is one of three major ore zones hosted by Pleistocene quartz-adularia veins being mined at the Hishikari low-sulfidation epithermal gold mine, which has produced more than 242 t of gold at an extraordinary average grade since 1985. Hydrothermal alteration zoning of the Sanjin deposit was examined with respect to mineralogy, geochemistry, fluid inclusion microthermometry, and oxygen and hydrogen isotopes. Clay minerals are dominated by interstratified chlorite-smectite (C/S) and interstratified illite-smectite (I/S) with <20% smectite. Epidote and prehnite are recorded for the first time at Hishikari in the southeastern part of the Sanjin deposit, typically coexisting with chlorite. Trapping temperatures of fluid inclusions from associated ore zone quartz veins typically range from 195 to 230 °C, with higher temperatures prevalent in the southeastern part of the Sanjin deposit. The calculated fluid δ18O and δ2H values from clay minerals and quartz cannot be explained by a simple water-rock interaction or a simple fluid mixing model, since variable isotopic exchange temperature and endmembers have to be considered. This suggests that both water-rock interaction and mixing of fluids occurred between dynamically variable end members during mineralization. The average estimated formation temperature of chlorites in the Sanjin deposit using chlorite geothermometry is 233 ± 19 °C, in agreement with the highest temperature zone in Hishikari, estimated by homogenization temperature of fluid inclusions of ore veins. In addition, the estimated formation temperatures of chlorite in epidote- and/or prehnite-rich altered rocks are higher (avg. 240 ± 17 °C) than those in epidote- and/or prehnite-poor altered rocks (avg. 216 ± 9 °C). Thus, the chlorite-epidote/prehnite assemblage can be an index of a high temperature alteration zone in the Sanjin deposit. Considering the position of the paleo-water table of the three ore zones, these factors are consistent with the formation of the Sanjin deposit proximal to the upflow zone responsible for gold mineralization at Hishikari. We suggest that our approach could be utilized to understand the thermal structure of epithermal gold system, which may be important to explore for blind veins at mature mining districts.

Influence of structures and lithologies in the morphology and localization of Au–Ag mineralization at Martinetas deposit, Don Nicolas mine, Deseado Massif, Argentina

This work describes the structural and lithological controls that affect the distribution, orientation, and precipitation mechanism of mineralization in the Martinetas district, from the structural context and the analysis of the role played by the geological units that host the mineralization. Martinetas is a low sulfidation epithermal gold and silver mining district located in the northeastern sector of the Deseado Massif, Argentina. The district is characterized by numerous quartz veins, sheeted veinlets and breccias, developed in pyroclastic and volcanic rocks of the Bahia Laura Volcanic Complex.The analysis of fractures, faults and host rocks of the vein systems show that the main factors controlling the formation and distribution of the mineralization are the lithology, deformation style and fault kinematics. Their interplay led to positive feedback between the evolution of plane aperture through time, strain localization and the resulting mineralization. Vein orientations are indicative of a NE-SW extensional stress field at the time of hydrothermal fluid ascend, which favored fluid input and pervasive flow within the active fractures and the matrix of the geological units. Subsequently, cyclic channelized fluid flow during repeated fluid input caused extensive veining and numerous filling events. A link between hydrothermal organic matter and the presence of gold in the vein system has been recorded.Knowledge of the structural and lithological controls that set the structure and the distribution of higher grades ore shoots of the mineralization in the Martinetas district constitutes information of high value as an exploratory tool for the region.

Metallogeny of the Late Jurassic Qiucun epithermal gold deposit in southeastern China: Constraints from geochronology, fluid inclusions, and H-O-C-Pb isotopes

On the basis of zircon and apatite U-Pb dating, H-O-C-Pb isotopes, and fluid inclusion data, a metallogenic model is established for the Qiucun gold deposit (gold resource > 10 t, average grade: 4.9 g/t) in the Dehua goldfield in the Mesozoic Coastal Volcanic Belt (southeastern China). Three hydrothermal stages can be distinguished within the Late Jurassic volcanic host rocks: Stage I, dominated by quartz-pyrite, Stage II with a quartz-sulfide assemblage, and Stage III reflected by quartz-calcite. The ore-hosting volcanic rocks were formed at 170.2 ± 0.9 Ma. Distinctly younger U-Pb ages of 162.5 ± 1.0 Ma and 160.5 ± 6.0 Ma obtained on syn-ore hydrothermal zircon and apatite, respectively, clearly indicate that gold mineralization postdates the volcanic host rocks. Fluid inclusion data reveal that the ores were precipitated from low-temperature (275 to 170 °C), low-pressure (<55 bar), and low-salinity (<5 wt% NaCl equiv.) solutions, with a systematic decrease in temperature and pressure from stages I to III. The δ34SV-CDT ratios for gold ores vary between −3.2 and 0.6 ‰. Sulfide separates yielded 206Pb/204Pb ratios of 18.368 to 18.504, 207Pb/204Pb ratios of 15.685 to 15.718, and 208Pb/204Pb ratios of 38.792 to 39.063. These data suggest S and Pb derivation from the host volcanic rocks and perhaps deeper-seated crustal materials. Calcite from Stage III has δ13CV-PDB values of −2.9 to −2.7 ‰ and δ18OV-SMOW values of 8.2 to 10.8 ‰. Calculated δ18Ofluid (−7.6 to −2.7‰) and δDfluid (−119 to −49‰) values document a largely meteoric nature of the mineralizing fluid. All data together point at a Late Jurassic low-sulfidation epithermal gold mineralization with metal remobilization from host volcanic rocks and basement.

Magma evolution and Cu-Au mineralization potential of the Upper Devonian-Lower Carboniferous Tulasu basin, Western Tianshan Orogen (NW China): Apatite U-Pb dating and geochemical perspectives

The large Axi low-sulfidation epithermal gold deposit (reserve: 50 tonnes Au) is one of the most important gold deposits in the Tulasu basin (Western Tianshan Orogen, NW China), and is hosted by the Upper Devonian-Lower Carboniferous Dahalajunshan Formation volcanic rocks. Due to the close relationship between the genesis and spatial distribution of epithermal gold and porphyry copper deposits, whether the latter is buried beneath the Tulasu basin is highly debated.In this study, apatite and zircon grains from nine ore-hosting volcanic rocks were LA-ICPMS U-Pb dated and analyzed for their minor and trace element compositions. Zircon U-Pb dating suggests multiphase magmatism, with the main phase at ca. 370 and 360 Ma. Apatite U-Pb dating results are consistent with those of zircon U-Pb dating and previous zircon dating works (ca. 380 to 350 Ma), demonstrating the accuracy of apatite U-Pb dating. The apatite Cl content varies with age, which shows that it has the potential to record Cl replenishment from the subducting slab into the mantle wedge. Compared with the apatite Cl content from porphyry Cu deposits worldwide, the apatite Cl content from Axi is much (1–2 wt%) lower, suggesting low potential in forming porphyry copper deposits.

Origin of the Kyaukmyet Low-Sulfidation Epithermal Gold Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect is one of the principal epithermal gold prospects in the Monywa District, Central Myanmar; its gold- and base metal-bearing quartz veins contain around 3 g/t gold. Ore minerals are mainly hosted by volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Magyigon Formation. The distribution of magmatic intrusions in the area is controlled by ENE-WSW trending faults; these faults are likely related to ore mineralization. Common ore minerals at the Kyaukmyet prospect include pyrite, sphalerite, galena, chalcopyrite, and electrum. They occur in mineralized crustiform-textured brecciated quartz veins and banded (colloform) and massive quartz veins. Mineralized rock is accompanied by silicification and propylitic and argillic alterations. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, illite/smectite, sericite, and illite. Fluid inclusions in the quartz veins have homogenization temperatures ranging from 148 °C to 304 °C and salinities from 0.35 wt % to 2.75 wt % NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth ranges165-256 m below the paleosurface. The precipitation of gold at the Kyaukmyet prospect may have been formed by mixing large amounts of meteoric fluid with small amounts of magmatic fluid. The coexistence of liquid-rich and vapor-rich inclusions and presence of adularia and bladed calcite indicate that fluid boiling is caused the main mechanism of ore formation. The vein textures, ore mineral assemblages, alteration minerals and fluid inclusion data suggest that the Kyaukmyet prospect is a polymetallic low-sulfidation epithermal gold deposit.

Geology, fluid inclusion and H-O-S-Pb isotope study of the Ganhegou epithermal gold deposit in Yanbian, NE China: Implications for ore genesis and mineralization process

• Ganhegou deposite is a typical low-sulfidation epithermal gold deposit. • The ore-forming fluids were derived from intermediate-acid magmatic water. • The ore-forming materials were derived from Neoproterozoic juvenile lower crust . • Twice boiling are the main mechanism for the precipitation of precious elements. The Yanbian mining district is located at the eastern end of the Central Asian orogenic belt and northeast of the western Pacific continental margin metallogenic belt. It is a porphyry-epithermal Cu-Au mining district related to Mesozoic continental volcanic eruptions and hypabyssal magmatism. To explore the genesis and mineralization process of epithermal gold deposits, we conducted a systematic study of the geology, fluid inclusions and H-O-S-Pb isotopes of the Ganhegou gold deposit, which is expected to be revealed as a medium-sized deposit in the latest exploration. We found the following: (1) The deposit occurs in Early Cretaceous continental andesites of the Jingouling Formation and granodiorite porphyry. The ore belts are controlled by NW-trending volcanic faults. The mineralization process can be divided into the quartz-hematite-arsenopyrite-pyrite stage (I), grey-white quartz-pyrite stage (II), smoky quartz-polymetallic sulfide stage (III), milky quartz-adularia stage (IV) and carbonate stage (V). Gold precipitated in stage III. (2) Vapour-rich-, liquid-rich-, pure-liquid- and pure-vapour-type FIs developed in quartz in stages II, III and IV, and the average homogenization temperatures in these stages are 325 ℃, 273 ℃ and 229 ℃, respectively. The ore-forming fluids belonged to the H 2 O-NaCl ± CO 2 system. (3) The δ 18 O H2O and δD H2O values of stage II-IV range from −9.9 to −5.2‰ and −121.9 to −119.8‰, respectively, indicating that the ore-forming fluids were driven by intermediate-felsic magmatic water and mixed with meteoric water during mineralization. (4) The δ 34 S values of pyrite in stages II and III range from −5.8 to −3.4‰, whereas the values of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are 15.529–15.551, 18.239–18.257 and 38.075–38.153, respectively, implying that the ore-forming materials were derived from Neoproterozoic lower crust. These characteristics indicate that Ganhegou is a low-sulfidation epithermal gold deposit. Mineralization occurred via several main processes: (i) The ore-forming fluids migrated along volcanic faults to suitable positions driven by intermediate-felsic magmatic water. (ii) In stage I, the quartz-arsenopyrite-hematite assemblage formed under relatively high oxidation conditions. (iii) In stage II, the pyrite-hessite-galena-quartz veins formed in the near-neutral environment, which was slightly reduced relative to that of stage I due to the fluid boiling. (iv) In stage III, the gold-bearing quartz veins formed due to the fluid boiling and mixing. (v) In stage IV, the precipitation of quartz-adularia-pyrite veins occurred under near-neutral conditions (pH≈5). (iv) Small-scale carbonate veinlets formed in stage V, signalling the end of mineralization.

Natural and anthropogenic factors influencing hydrochemical characteristics and heavy metals in groundwater surrounding a gold mine, Thailand

This study focused on groundwater quality around a gold mine at the border between the Phichit and Phetchabun provinces by investigating hydrogeological and hydrochemical characteristics and performing a multivariate analysis to determine the impact of natural and anthropogenic processes on groundwater quality. Groundwater samples were collected from 47 wells in the summer and rainy seasons, and hydrochemical parameters were measured on-site and analyzed in a laboratory. The hydrogeologic characteristics of the study area can be categorized into Quaternary floodplain deposits (Qfd), terrace deposits (Qt), weathered fractures volcanic rocks (Vw), volcanic rocks (Vf), and massive volcanic rocks (Vm). According to the Piper Diagram, water types in the summer season are Ca-Na-HCO3 (34.29%), Ca-HCO3 (22.86%), and Na-HCO3 (22.86%), whereas the rainy season primarily features Na-HCO3 (44.74%), and Ca-Na-HCO3 (39.47%). The principal component analysis (PCA) produced seven significant components that explained 71.50% of the cumulative variance. We divide primary contributors to groundwater into two groups. PC1–PC5 (from natural processes) include Na+, K+, Ca2+, and Al (mineral components of potassium feldspar and plagioclase feldspar), Fe, Zn2+, Pb2+, Cu2+, Co2+, and Mn2+ (associated minerals, such as sphalerite (ZnS), galena (PbS), chalcopyrite (CuFeS2), and pyrite (FeS2)), and inclusions in gold ore (low sulfidation epithermal gold deposits). The second source, PC6 and PC7 (mainly from human activities), includes NO3−, and, PO43− from fertilizers in agricultural areas and wastewater discharge from residential areas. This study revealed that natural and anthropogenic processes affected the hydrochemistry of groundwater, but natural processes played a greater role.