Low-sulfidation Epithermal Gold Deposit Research Articles

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.

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.

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.

Epithermal gold mineralization in Cretaceous volcanic belt, SE China: Insight from the Shangshangang deposit

Low sulfidation (LS) epithermal gold deposits within the Southeastern China Fold Belt (SCFB) are generally associated with Early Cretaceous volcanism (91–110 Ma). The Shangshangang (SSG) gold deposit (3 t Au @ 2.7 g/t) is proposed to be a LS ore system hosted by Early Cretaceous volcanic-subvolcanic rocks in the Dongkeng Basin (DVB), northern SCFB. Early Cretaceous felsic volcanic-subvolcanic rocks within the DVB possess high-K calc-alkaline and metaluminous to weakly peraluminous signatures. They show rare trace element (REE) and trace element patterns resembling those of Early Cretaceous A-type granites and rhyolites in the SCFB with significantly Eu negative anomalies (Eu/Eu* = 0.1–0.3). These volcanic-subvolcanic rocks are potentially formed by mixing of melts from crust and Early Cretaceous enriched mantle under an extensional tectonic setting. This is indicated by their negative εHf(t) values (−8.8 to −5.2), young two-stage Hf model ages (TDM2 = 1.32–1.48 Ga), and variable Sr-Nd isotopic compositions.The epithermal event at SSG is represented by three mineralization stages characterized by four distinct quartz vein types. These are: (1) greyish green crustiform comb quartz veins (type A) and smoky grey fine-grained (<0.5 mm) quartz veins (type B) in the early stage, (2) ivory medium- to coarse-grained (>1 mm) quartz veins (type C) in the middle stage, and (3) late white calcite-rich quartz veins (type D). Quartz veins are typically accompanied by hydrothermal alteration with zoning (kaolinite-illite-chlorite zone, adularia-chlorite-quartz zone, and adularia-sericite-quartz zone). Gold mineralization is identified in the early stage and free gold exists as micrometer-sized inclusions of native gold within pyrite. Pb–Zn mineralization dominates the middle stage. Hydrothermal fluids within the ore system are of low salinity (<7 wt% NaCl) and gas-poor. The fluid temperatures decreases from 224 − 316 °C for Au mineralization to 190–261 °C for Pb-Zn mineralization. No indication for fluid boiling is found. The H–O isotopes indicate mixing of magmatic and meteoric fluids and a larger portion of meteoric water at a later stage. The δ34S values of pyrite related to Au mineralization are 1.1–2.0‰ and of sphalerite and galena related to Pb-Zn mineralization range from −1.8 to 2.5‰. The Pb isotopic compositions of sulfides from the early and middle stages are relatively homogeneous and similar with those of subvolcanic rocks within the DVB. Deposit geology, microthermometric result, and isotopic data support a LS epithermal origin for the SSG gold deposit, and therefore suggest a co-existing relationship between low and intermediate sulfidation epithermal gold mineralization within the DVB.

40Ar/39Ar geochronology of the Pongkor low sulfidation epithermal gold mineralisation, West Java, Indonesia

The Pongkor gold deposit located approximately 90 km southwest of Jakarta is well known as the typical low sulfidation (LS) epithermal gold deposit and the largest gold vein deposit in Indonesia. The Pongkor gold mineralisation has been the subject of several geological studies. Determination of the duration of gold mineralisation events, however, is less well understood. The mineralisation events are important for understanding the genesis of ore deposits. This study presents 40Ar/39Ar age results for four adularia samples from Ciguha gold-bearing quartz vein yielding plateau ages of 1.80 ± 0.03 Ma, 2.33 ± 0.32 Ma, and 1.88 ± 0.02 Ma, plus one weighted mean age of 1.95 ± 0.04 Ma. Adularia from the Kubang Cicau gold-bearing quartz veins yielded a 40Ar/39Ar plateau age of 2.02 ± 0.03 Ma. The period of adularia growth in the Ciguha vein may have extended from ca. 2.07 to 1.77 Ma that includes the ages of 1.95 ± 0.04 Ma and 1.88 ± 0.02 Ma. The time interval indicates multiple generations of the adularia. The data suggest that mineralisation of Ciguha may have continued episodically or continuously. The age of adularia from the Kubang Cicau gold-bearing quartz vein (2.02 ± 0.03 Ma) is within the error of the Ciguha vein age and the adularia growth of Kubang Cicau vein reveals at the beginning of Ciguha’s adularia growth. The data suggest that gold mineralisation in the Pongkor deposit for Kubang Cicau and Ciguha veins was initiated at the relatively similar time. The 40Ar/39Ar adularia ages of this study are within error of the previously reported 40Ar/39Ar results (Milesi et al., 1999) but significantly younger than the previously reported K-Ar adularia dating result (Kageyama, 1999 in Rosana and Matsueda, 2002).

Mineralogy and Geochemistry of Gold Ore Low Sulfidation -Epithermal at Lamuntet, Brang Rea, West Sumbawa District, West Nusa Tenggara Province

There are two Artisanal Small scale Gold Mining (ASGM) location in Lamuntet, Brang Rea Subdistrict, West Nusa Tenggara Regency, namely Nglampar and Song location. Nglampar and Song location are included in the low sulfidation epithermal gold deposit system. The research purposes to analyze mineralogy and geochemistry of gold vein deposits and determine system of low sulfidation gold ore in Nglampar, Lamuntet Village. The methods used to determine the mineralogy of gold vein deposits are petrography, mineragraphy and X-ray diffractometer (XRD) analysis, while geochemical analysis using Scanning Electron Microscope (SEM) with Energy Dispersive X-Ray Spectroscopy (EDS), Fire Assay (FA) and Atomic Absorption Spectrophotometry (AAS). The results showed that the minerals contained were quartz (Qz), sericite (Ser), Chalcedon (Chc), chlorite (Chl), pyrite (Py), sphalerite (Sph), galena (Gn) , gold (Au), chalcopyrite (Cp), argentite (Ag), arsenopyrite (Apy), Azurit (Az), Malakit (Mal) and bornite (Bn). Abundant mineral availability such as sphalerite, galena, chalcopyrite and arsenopyrite are characterized by high levels of Zn, Pb, Cu and As the metal in vein deposits. This can be seen on the chemical content of ore in gold vein deposits ie Au 0.1 ppm -27.8 ppm, Ag 3 ppm-185 ppm, Pb 101 ppm - 35,800 ppm, Zn 73 ppm-60,200 ppm, Cu 26 ppm - 1,740 ppm, and As 150 ppm - 6,530 ppm. Based on the results of SEM-EDS analysis shows that the type of gold mineral is the electrum because of the content of Ag&gt; 20%. Based on those characteristics of the mineralogy and geochemistry in this study showed that low sulfidation gold ore in this area is categorized as polymetallic gold-silver system.

Mineralogical characteristics of silver minerals from the Dongyang Gold deposit, China: Implications for the evolution of epithermal metallogenesis

The Dongyang gold deposit is located in Fujian Province in southeastern China, mainly hosted by a NW–SE trending Late Jurassic porphyritic rhyolite, and classified as a low-sulfidation epithermal gold deposit. The units associated with the gold mineralization in this area are highly altered and have undergone sericitic and argillic alteration as well as silicification, with the latter two types of alteration especially associated with gold enrichment. The deposit contains pyrite, marcasite, and arsenopyrite, with lesser amounts of chalcopyrite, galena, sphalerite, and silver minerals. The sulfides and silver minerals within the deposit provide insight into the evolution of the ore-forming fluid and the processes that caused the gold enrichments to form in the study area. In addition, the geochemical similarity of gold and silver means these elements tend to migrate and precipitate simultaneously, in turn indicating that sulfides and silver minerals can provide insights into the processes that concentrate gold. Here, we present chemical compositional, and crystal structural data for sulfides and silver minerals within the Dongyang gold deposit by scanning electron microscopy (SEM), electron microprobe analyser (EMPA), X-ray diffraction (XRD) and laser Raman microprobe, and use these data to gain insights into the characteristics and evolution of the fluids that formed the deposit. We also outline key concepts for future gold deposit exploration in this region. Four types of pyrite in the deposit are recognized based on arsenic content and crystal morphology, evolving from early As-poor pyrite (Py1) to As-rich pyrite (Py2) and/or arsenopyrite, to non-cubic As-poor pyrite (Py3), and finally to cubic As-poor pyrite (Py4), with an inverse correlation between sulfur and arsenic concentrations. Pyrite crystal morphologies vary from earlier pentagonal dodecahedron forms to later cube forms, indicating a decrease in degree of supersaturation (as controlled by the activity of dissolved Fe and sulfide) and/or temperature conditions for pyrite growth. In addition, the presence of As-poor marcasite associated with early pyrite (Py1) suggests that the early ore-forming fluid was relatively acidic. Arsenopyrite is generally concentrated in the shallower parts of the deposit, indicating that later hydrothermal fluids were enriched in arsenic. This study also identified a number of key silver mineral phases that formed in a sequence from chalcopyrite, to a galena–fahlore–allargentum–dyscrasite–polybasite–electrum assemblage, to stephanite, and finally to chlorargyrite. Chalcopyrite replaced by fahlore suggests increased oxygen fugacity and decreased temperature. Fahlore Sb / (As + Sb) (>90%) and Ag / (Ag + Cu) ratios (6%–27%) are indicative of relatively high temperatures (>200 °C), and allargentum is thought to form in elevated pH conditions (pH > 7). Stephanite and chlorargyrite precipitated under relatively low temperature (<200 °C). Finally, the presence of chlorargyrite suggested that silver was transported as chloride complexes, indicating relatively high oxygen fugacity. The sequence of silver mineral precipitation suggests that the ore-forming fluids showed a temporal increase in pH and oxygen activity, and a decrease in temperature and sulfur fugacity. This indicates that the hydrothermal fluids that formed the deposit can be divided into early and late stage fluids, with the former being a relatively higher-temperature and higher‑sulfur fugacity fluid compared with the lower-temperature and higher‑oxygen-fugacity late-stage fluid. Finally, the presence of silver minerals with high gold contents (up to 2 wt%), as well as arsenian pyrite and arsenopyrite, may be useful indicators of prospective areas for gold exploration in this region.

Mineralogy, ore-forming fluids and geochronology of the Shangmachang and Beidagou gold deposits, Heilongjiang province, NE China

The newly discovered Shangmachang epithermal gold (>11t Au; Heilongjiang province, NE China) and Beidagou epithermal tellurium-gold (>5t Au) deposits are located at the northeastern margin of the Central Asian Orogenic Belt and are hosted in Early Cretaceous volcanic rocks. At the Shangmachang deposit, gold occurs largely as electrum microparticles (5–120μm) with a fineness of approximately 401–697 in quartz, calcite, and pyrite. Silver-bearing minerals include electrum, polybasite, and minor argentite. The main mineral paragenetic assemblages consist of pyrite-electrum and galena-polybasite, while the hydrothermal alteration minerals include quartz, calcite, sericite, chlorite, and pyrite. Trace elements in the gangue minerals show that the Al contents of gold-bearing quartz veins are lower than those of pure quartz veins (early stage). Fluid inclusions in quartz and calcite mainly comprise liquid-vapor, and the components contain a liquid phase consisting of Na+, Cl−, and SO42− and a gas phase consisting of N2, CO2, and CH4. The Shangmachang fluid inclusions homogenize at 142.8°–303.0°C and are of low salinity (0.2–8.8wt% NaCl equiv.), while the Au mineralization formed at 150°–225°C. The calcite δ18Ofluid values range from −17.8‰ to −13.1‰, and the carbon isotope compositions range from −8.2‰ to −4.5‰. Zircon U-Pb dating of the Longjiang Formation andesite yields 119.4±1.9Ma, and the granite yields 118.6±1.2Ma, suggesting that local magmatism occurred during the Early Cretaceous. Calcite Sm-Nd dating suggests that the Au mineralization has an age of approximately 113.6±4.0Ma.At the Beidagou deposit, the main mineral paragenetic assemblages consist of sphalerite-galena-hessite and chalcopyrite-altaite-petzite-hessite, while the hydrothermal alteration minerals include quartz, calcite, anhydrite, chlorite, and pyrite. The components of the fluid inclusions in quartz contain a fluid phase consisting of Na+, K+, Cl−, and SO42− and a gas phase consisting of N2, CO2, CH4, and C2H6. The Beidagou fluid inclusions homogenize at 133.8°–370.1°C and are of low salinity (0.2–8.7wt% NaCl equiv.), while the Au mineralization formed at 175°–250°C. The δ18Ofluid and δDSMOW values of the quartz samples from Beidagou range from −8.2‰ to −5.0‰ and −101.6‰ to −92.8‰, respectively. The Al contents of gold-bearing quartz veins are higher than those of pure quartz veins. Rb-Sr dating of ore-bearing quartz veins suggests that the Te-Au-Ag mineralization has an age of about 115.5±4.4Ma.In summary, the interpretation of new data from the geological setting, fluid inclusions, O-H-C isotopes, and geochronology, suggests that the ore-forming fluids predominantly comprised meteoric fluids, while the metal content was derived from a mantle source at both the Shangmachang and Beidagou deposits. A geochemical model suggests that the fluids responsible for quartz deposition had a neutral pH and that adiabatic boiling and mixing triggered the gold, silver and tellurium precipitation. Both Shangmachang and Beidagou are interpreted as low-sulfidation epithermal gold deposits.