Geology,geochronology and fluid characteristics of the Pingqiu gold deposit,Southeastern Guizhou Province,China

The junction of the southeastern Guizhou, the southwestern Hunan, and the northern Guangxi regions is located within the southwestern Jiangnan orogen and forms a NE-trending ∼250 km gold belt containing more than 100 gold deposits and occurrences. The Pingqiu gold deposit is one of the numerous lode gold deposits in the southeastern Guizhou district. Gold mineralization is hosted in Neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold-related structures. Vein types present at Pingqiu include bedding-parallel and discordant types, with saddle-reefs and their down limb extensions dominating but with lesser discordant types. The major sulfide minerals are arsenopyrite and pyrite, with minor sphalerite, galena, chalcopyrite, and rare pyrrhotite, marcasite, and tetrahedrite. Much of the gold is μm- to mm-sized grains, and occurs as fracture-controlled isolated grains or filaments in quartz, galena, sphalerite, pyrite, and wallrock.Three types of fluid inclusions are distinguished in hydrothermal minerals. Type 1 aqueous inclusions have homogenization temperatures of 171–396 °C and salinities of 1.4–9.8 wt% NaCl equiv. Type 2 aqueous-carbonic inclusions yield final homogenization temperatures of 187–350 °C, with salinities of 0.2–7.7 wt% NaCl equiv. Type 3 inclusions are carbonic inclusions with variable relative content of CO₂ and CH₄, and minor amounts of N₂ and H₂O. The close association of CO₂-rich inclusions and H₂O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. The calculated δ¹⁸O_H2O values range from 4.3‰ to 8.3‰ and δD_H2O values of fluid inclusions vary from −55.8‰ to −46.9‰. A metamorphic origin is preferred on the basis of geological background and analogies with other similar deposit types.Two ore-related sericite samples yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 425.7 ± 1.7 Ma and 425.2 ± 1.3 Ma, respectively. These data overlap the duration of the Caledonian gold mineralization along the Jiangnan orogen, and suggest that gold mineralization was post-peak regional metamorphism and occurred during the later stages of the Caledonian orogeny.Overall, the Pingqiu gold deposit displays many of the principal characteristics of the Bendigo gold mines in the western Lachlan Orogen (SE Australia) and the Dufferin gold deposit in the Meguma Terrane (Nova Scotia, Canada) but also some important differences, which may lead to the disparity in gold endowment. However, the structural make-up at deposit scale, and the shallow mining depth at present indicate that the Pingqiu gold deposit may have considerable gold potential at depth.     

Geology and fluid evolution of the Wangfeng orogenic-type gold deposit,Western Tian Shan,China

The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO₂–H₂O inclusions (C-type), pure CO₂ inclusions (PC-type), NaCl–H₂O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO₂–H₂O fluid inclusions with salinities of 1.62 to 8.03 wt.% NaCl equivalent, bulk densities of 0.73 to 0.89 g/cm³, and homogenization temperatures of 256 °C–390 °C. Vapor bubbles are composed of CO₂. The middle-stage quartz contains all four types of fluid inclusions, of which the CO₂–H₂O and NaCl–H₂O types yield homogenization temperatures of 210 °C–340 °C and 230 °C–300 °C, respectively. The CO₂–H₂O fluid inclusions have salinities of 0.83 to 9.59 wt.% NaCl equivalent and bulk densities of 0.77 to 0.95 g/cm³, with vapor bubbles composed of CO₂, CH₄, and N₂. Fluid inclusions in the late-stage quartz are NaCl–H₂O solution with low salinities (0.35–3.87 wt.% NaCl equivalent) and low homogenization temperatures (122 °C–214 °C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO₂–H₂O inclusions are 110–300 MPa and 90–250 MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10 km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO₂. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit.     

Fluid inclusion geochemistry and Ar–Ar geochronology of the Cenozoic Bangbu orogenic gold deposit,southern Tibet,China

Located along the southern part of the Yarlung Zangbo suture zone in southern Tibet, Bangbu is one of the largest gold deposits in Tibet. Auriferous sulfide-bearing quartz veins are controlled by second- or third-order brittle fractures associated with the regional Qusong–Cuogu–Zhemulang brittle-ductile shear zone. Fluid inclusion studies show that the auriferous quartz contains aqueous inclusions, two-phase and three-phase CO₂-bearing inclusions, and pure gaseous hydrocarbon inclusions. The CO₂-bearing inclusions have salinities of 2.2–9.5% NaCl_eq, and homogenization temperatures (Th) of 167–336 °C. The δD, δ¹⁸O, and δ¹³C compositions of the Bangbu ore-forming fluids are − 105.5 to − 44.4‰, 4.7 to 9.0‰ and − 5.1 to − 2.2‰, respectively, indicating that the ore-forming fluid is mainly of metamorphic origin, with also a mantle-derived contribution. The ³He/⁴He ratio of the ore-forming fluids is 0.174 to 1.010 R_a, and ⁴⁰Ar/³⁶Ar ranges from 311.9 to 1724.9. Calculations indicate that the percentage of mantle-derived He in fluid inclusions from Bangbu is 2.7–16.7%. These geochemical features are similar to those of most orogenic gold deposits. Dating by ⁴⁰Ar/³⁹Ar of hydrothermal sericite collected from auriferous quartz veins at Bangbu yielded a plateau age of 44.8 ± 1.0 Ma, with normal and inverse isochronal ages of 43.6 ± 3.2 Ma and 44 ± 3 Ma, respectively. This indicates that the gold mineralization was contemporaneous with the main collisional stage between India and Eurasia along the Yarlung Zangbo suture, which resulted in the development of near-vertical lithospheric shear zones. A deep metamorphic fluid was channeled upward along the shear zone, mixing with a mantle fluid. The mixed fluids migrated into the brittle structures along the shear zone and precipitated gold, sulfides, and quartz because of declining temperature and pressure or fluid immiscibility. The Bangbu is a large-scale Cenozoic syn-collisional orogenic gold deposit     

40Ar/39Ar dating of the Jiehe gold deposit in the Jiaodong Peninsula,eastern North China Craton: Implications for regional gold metallogeny

The Jiehe gold deposit, containing a confirmed gold reserve of 34 tonnes (t), is a Jiaojia-type (disseminated/stockwork-style) gold deposit in Jiaodong Peninsula. Orebodies are hosted in the contact zone between the Jurassic Moshan biotite granite and the Cretaceous Shangzhuang porphyritic granodiorite, and are structurally controlled by the NNE- to NE-striking Wangershan-Hedong Fault. Sulphide minerals are composed predominantly of pyrite with lesser amounts of chalcopyrite, galena, and sphalerite. Hydrothermal alteration is strictly controlled by fracture zones, in which disseminated sulfides and native gold are spatially associated with pervasive sericitic alteration. Mineralogical, textural, and field relationships indicate four stages of alteration and mineralization, including pyrite-bearing milky and massive quartz (stage 1), light-gray granular quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4) stages. Economic gold is precipitated in stages 2 and 3.The Jiehe deposit was previously considered to form during the Eocene (46.5 ± 2.3 Ma), based on Rb-Sr dating of sericite. However, ⁴⁰Ar/³⁹Ar dating of sericite in this study yields well-defined, reproducible plateau ages between 118.8 ± 0.7 Ma and 120.7 ± 0.8 Ma. These ⁴⁰Ar/³⁹Ar ages are consistent with geochronological data from other gold deposits in the region, indicating that all gold deposits in Jiaodong formed in a short-term period around 120 Ma. The giant gold mineralization event has a tight relationship with the extensional tectonic regime, and is a shallow crustal metallogenic response of paleo-Pacific slab subduction and lithospheric destruction in the eastern NCC.     

Fluid inclusion geochemistry and 40Ar/39Ar geochronology constraints on the genesis of the Jianchaling Au deposit,China

The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment.Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO₂ and/or CH₄ (PC-type), NaCl-H₂O (W-type), and NaCl-CO₂-H₂O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360 °C, with salinities up to 15.6 wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220 °C and salinities of 1.1–7.4 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO₂-rich, probably metamorphic hydrothermal to CO₂-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250 °C and salinities of 1.2–12.4 wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354 MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~ 12 km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined ⁴⁰Ar/³⁹Ar isotopic plateau ages of 197 ± 2 and 194 ± 2 Ma, and ³⁹Ar/³⁶Ar-⁴⁰Ar/³⁶Ar normal isochrones of 198 ± 2 and 199 ± 2 Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198 Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200 Ma.     

Geology,geochemistry and ore genesis of the Wenyu gold deposit,Xiaoqinling gold field,Qinling Orogen,southern margin of North China Craton

The Wenyu giant gold deposit is hosted in the Precambrian Taihua Supergroup metamorphic rocks within the Xiaoqinling terrane (Qinling Orogen), on the southern margin of the North China Craton. The mineralization can be divided into three stages: quartz–pyrite veins early, quartz–sulfide veins middle (main), and carbonate–quartz veinlets late, with gold being mainly introduced in main stage. Quartz formed in two earlier stages contains three compositional types of fluid inclusions, i.e. pure CO₂, CO₂–H₂O and NaCl–H₂O, but the late-stage minerals only contain the NaCl–H₂O inclusions. The inclusions in quartz formed in the early, main and late stages yield total homogenization temperatures of 262–417 °C, 236–407 °C and 114–239 °C, respectively, with salinities no higher than 13 wt.% NaCl equiv. Trapping pressures estimated from CO₂–H₂O inclusions are 139–399 MPa and 111–316 MPa in the early and main stages, corresponding to mineralization depths of 14 km and 11 km, respectively. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO₂-rich to CO₂-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ¹⁸O_water values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Wenyu gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic–Early Cretaceous continental collision between the North China and Yangtze Cratons.     

Magma mixing and gold mineralization in the Maevatanana gold deposit,Madagascar

The Maevatanana gold deposit in Madagascar is hosted by Archean metamorphic rocks in quartz–sulfide veins that are structurally controlled by NNW–SSE trending shear zones. Fluid inclusion data show that the trapping conditions in quartz range from 0.87 to 2.58 kbar at temperatures of 269–362 °C. Laser Raman spectroscopy confirms that these inclusions consist of CO₂, SO₂, and H₂O. The δ³⁴S values of the pyrites range from 1.7‰ to 3.6‰, with an average of 2.25‰, supporting a magmatic origin. Noble gases (He, Ne, Ar, Ke, Xe) are chemically inert, thus will not be involved in chemical reactions during geological processes. Also due to the low concentration of He in the atmosphere and the low solubility of He in aqueous fluids, the atmosphere-derived He is unlikely to significantly affect He abundances and isotopic ratios of crustal fluids, ensures that He production should have the typical crust ³He/⁴He ratios. The ³He/⁴He ratios of fluid inclusions in pyrite from the deposit range from 0.06 to 0.12 Ra, while the ⁴⁰Ar/³⁶Ar ratios range from 6631 to 11441. We infer that the ore-forming fluids could have been exsolved from a granitic magma. The oxygen and hydrogen isotope compositions of the ore-forming fluids (1.5‰ ≤ δ¹⁸O_H2O ≤ 7.8‰; –72‰ ≤ δD ≤ –117‰) indicate they were derived from a granitic magma. Four pyrite samples from the gold deposit yield a precise Re–Os isochron age of 534 ± 13 Ma. Given that the post-collisional granites in northern and central Madagascar were derived by melting of sub-continental lithospheric mantle and formed between 537 and 522 Ma, we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas. These data could be accounted for the formation of the Maevatanana gold deposit. First, the shear zones hosting the deposit formed around 2.5 Ga, when the Madagascan micro-continental blocks collided with other continental blocks, triggering large-scale tectono-magmatic activity and forming NNW–SSE trending shear zones. The gold mineralization at Maevatanana is coeval with the crystallization age of the Cambrian post-collisional A-type granitoid plutons in northern and central Madagascar, implying that this deposit is associated with extensional collapse of the East African Orogen. This extension in turn induced asthenospheric upwelling, melting of sub-continental lithospheric mantle. These magmas underplated the lower crust, generating voluminous granitic magmas by partial melting of the lower crust. The mixing magma during tectono-thermal reactivation of the East African Orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced Au–S complexes (e.g., Au(HSO₃)²⁻). The shear zones, which were then placed under extensional collapse of the East African Orogen in the Cambrian, formed favorable pathways for the magmatic ore-forming fluids. These fluids then precipitated gold-sulfides that form the Maevatanana gold deposit.     

Isotope and fluid inclusion geochemistry and genesis of the Qiangma gold deposit,Xiaoqinling gold field,Qinling Orogen,China

The Qiangma gold deposit is hosted in the > 1.9 Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO₂–H₂O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO₂–H₂O inclusions with salinities up to 12.7 wt.% NaCl equiv., bulk densities of 0.67 to 0.86 g/cm³, and homogenization temperatures of 280−365 °C. The early-stage is related to H₂O–CO₂ ± N₂ ± CH₄ fluids with isotopic signatures consistent with a metamorphic origin (δ¹⁸O_water = 3.1 to 5.2‰, δD = − 37 to − 73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO₂–H₂O and aqueous inclusions yield homogenization temperatures of 249−346 °C and 230−345 °C, respectively. The CO₂–H₂O inclusions have salinities up to 10.9 wt.% NaCl equiv. and bulk densities of 0.70 to 0.98 g/cm³, with vapor bubbles composed of CO₂ and N₂. The isotopic ratios (δ¹⁸O_water = 2.2 to 3.6‰, δD = − 47 to − 79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9 wt.% NaCl equiv.) and low homogenization temperatures (145−223 °C). The isotopic composition (δ¹⁸O_water = − 1.9 to 0.5‰, δD = − 55 to − 66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO₂–H₂O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10 km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO₂-rich to CO₂-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ¹⁸O_water values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons.     

Ore-forming process of the Huijiabao gold district,southwestern Guizhou Province,China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO₂ inclusions, CO₂–H₂O inclusions, hydrocarbon inclusions, and hydrocarbon–H₂O inclusions. The ore-forming fluids are characterized by a H₂O + CO₂ + CH₄ ± N₂ system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H₂O + NaCl → H₂O + NaCl + CO₂ + CH₄ ± N₂ → H₂O + NaCl ± CH₄ ± CO₂ with a slight decrease in homogenization temperature and salinity. The δ¹⁸O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_H2O and calculated δ¹⁸O_H2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H₂O–NaCl–CO₂ fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.     

Genesis of Neoproterozoic Au-bearing volcanogenic sulfides and quartz veins in the Ar Rjum goldfield,Saudi Arabia

The Ar Rjum goldfield is an example of late Neoproterozoic Au mineralization that is hosted by submarine arc assemblage and syn-anorogenic intrusive rocks. Apart from ancient workings, recent exploration in the goldfield defined three main targets along 3 km N–S corridor (Um Na'am, Ghazal and Wasema), and indicated that Wasema alone hosts 11.8 Mt @ 2.5 g/t Au. The majority of gold and sulfide mineralization is confined to diorite, where gold content increases with shearing, pyrite–sericite–carbonate alteration and development stockworks of quartz–carbonate–pyrite veins and stringers. Generally, the concentration of gold increases in the diorite samples that experienced variable degrees of hydrothermal alterations near local shear zones. Anomalous gold content (up to 11.76 g/t) in some metachert is the result of the remobilization of volcanogenic lattice-bound (refractory) Au into free Au due to post-metamorphic hydrothermal alterations. The chemistry of pyrite from the mineralized veins and stringers indicates considerable amounts of gold that reaches ~ 0.3 wt.%.Chlorite that co-exists with pyrite in the hydrothermally altered metavolcanics is mostly sheridanite with up to ~ 25 wt.% FeO^t and minor amounts of ripidolite. Chlorite geothermometry suggests that two temperature ranges affecting the area. The first temperature range (290–334 °C) is consistent with regional greenschist facies metamorphism, and the second (306–355 °C) is interpreted to be related to recrystallization-submarine hydrothermal alteration related to the gold mineralization. Stable isotope (δ³⁴S, δ¹⁸O and δ¹³C) data suggest an original volcanogenic arc signature that has been slightly modified by low-grade metamorphism, and finally by the late interaction of hydrothermal fluids. Ore evolution model for the Ar Rjum goldfield includes seafloor sulfide alteration, several deformation episodes and intrusive effects, and in this context the ore resulted from the reduction of seawater sulfates. The gold-rich veins interpreted as orogenic lode deposits are confined to localized shear zones in a syn-orogenic diorite.     

Listvenite-related gold deposits of the South Urals (Russia): A review

The Urals is a complex fold belt, which underwent long geological evolution. The formation of most gold deposits in the Urals is related to the collision stage. In this paper, we review some relatively small listvenite-related gold deposits, which are confined to the large Main Uralian fault zone and some smaller faults within the Magnitogorsk zone. The Mechnikovskoe, Altyn-Tash, and Ganeevskoe deposits are studied in detail in this contribution. They comprise the ore clusters along with other numerous small gold deposits, and constituted the sources for the gold placers exploited in historical time. The gold is hosted by metasomatites (listvenites, beresites) and quartz veins with economic gold grades (up to 20 g/t Au). Listvenites are developed after serpentinites and composed of quartz, fuchsite, and carbonates (magnesite, dolomite) ± albite. Volcanic and volcanoclastic rocks are altered to beresites, consisting of sericite, carbonates (dolomite, ankerite), quartz and albite. Pyrite and chalcopyrite are major ore minerals associated with gold; pyrrhotite, Ni sulfides, galena, sphalerite, arsenopyrite and Au-Ag tellurides are subordinate and rare. Gold in these deposits is mostly high-fineness (>900‰). The lower fineness (∼800‰) is typical of gold in assemblage with polymetallic sulfides and tellurides. The ores have been formed from the NaCl–CO₂–H₂O ± CH₄ fluids of low (∼2 wt% NaCl-equiv.) to moderate (8–16 wt% NaCl-equiv.) salinity at temperatures of 210–330 °C. The oxygen isotopic composition of quartz (δ¹⁸O) varies from 14.7 to 15.4‰ (Mechnikovskoe deposit), 13.2 to 13.6‰ (Altyn-Tash deposit) and 12.0 to 12.7‰ (Ganeevskoe deposit). The oxygen isotopic composition of albite from altered rocks of the Ganeevskoe deposit is 10.1‰. The calculated δ¹⁸O_H2O values of the fluid in equilibrium with quartz are in a range of 5.7–6.3, 4.2–4.6 and 6.3–6.7‰ respectively, and most likely indicate a magmatic fluid source.     

Geology,geochemistry, and genesis of Axi: A Paleozoic low-sulfidation type epithermal gold deposit in Xinjiang,China

Axi is a low-sulfidation type epithermal gold deposit hosted in Paleozoic subaerial volcanic rocks in the western Tianshan orogenic belt, Xinjiang, China. The resource is more than 50 t gold at an average grade of > 4.4 ppm. The deposit occurs in the Tulasu volcanic fault-basin in the Paleozoic active continental margin on the northern side of the Yili-Central Tianshan plate. The host rocks are andesitic volcaniclastic rocks of the Paleozoic Dahalajunshan Formation, and the orebodies occur as veins in annular faults of a paleocaldera. Mineralization at Axi can be subdivided into five stages: quartz and/or chalcedony vein, quartz vein, quartz-carbonate vein, sulfide vein and carbonate vein. There are two types of ore host: quartz vein and altered rocks. Ore minerals are native gold, electrum, pyrite, marcasite, arsenopyrite, hematite, limonite, and trace amounts of pyrargyrite, polybasite, naumannite, cerargyrite, sphalerite, chalcopyrite, tetrahedrite, galena, pyrrhotite and clausthalite; gangue minerals are mainly quartz, chalcedony, illite, calcite, siderite, dolomite, adularia and laumontite. The main wall-rock alteration is silicification and phyllic alteration, carbonatization and propylitization. The deposit is characterized by an enrichment, relative to crustal abundance, of Au, Ag, As, Sb, Bi, Hg, Se, Te and Mo, depletion in base metals (Cu, Pb, and Zn), and a low Ag/Au ratio (0.5–3.7).Three types of fluid inclusions were recognized in quartz from the major mineralization stages: liquid aqueous inclusions, liquid-rich two-phase inclusions and small amounts of vapor-rich two-phase inclusions. Microthermometric measurements indicate that the final ice melting temperatures are − 0.3 to − 4.4 °C, corresponding to salinities of 0.5–6.9 wt.% NaCl equiv. (2.2 wt.% NaCl equiv. in average). The peak temperatures of ice melting varies from − 0.4 to − 1.9 °C, corresponding to salinities of 0.7–3.1 wt.% NaCl equiv. Homogenization temperatures range mainly between 120 and 240 °C, with an average of 190 °C and a maximum of 335 °C. The fluid density is 0.73 to 0.95 g/cm³ and thus the estimated maximum mineralization depth is about 700 m.Hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range: δD_H2O is − 98 to − 116‰ and δ¹⁸O_H2O − 1.8 to 0.4‰. ³He/⁴He ranges from 0.0218 to 0.138 Ra, with an average of 0.044 Ra, indicating that He derived predominantly from crust with negligible mantle He in the ore fluids. By contrast, the ⁴⁰Ar/³⁶Ar ranges from 317.7 to 866.0, suggesting that crust-derived radioactive ⁴⁰Ar⁎ accounts for 7.0 to 66%, and atmospheric ⁴⁰Ar about 43 to 93% in the ore fluids. Hydrogen, oxygen, carbon, sulfur and noble gas isotopes indicate that the ore-forming fluids of the Axi gold deposit consisted predominantly of circulating meteoric water. Ore-forming metals may have derived mainly from the host volcaniclastic rocks of the Dahalajunshan Formation and basement rocks. The occurrence of adularia, platy calcite, and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins, and occurrence of aqueous liquid, and liquid-rich and vapor-rich two-phase inclusions, indicates that boiling of the ore-forming fluid have occurred, leading to supersaturation of the hydrothermal solution and deposition of ore metals. This is the main mineralization mechanism for quartz-vein type ores in Axi. The ore-forming fluid was buffered to a near-neutral pH in a reduced environment during mineralization. The preservation of this Paleozoic Axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit, and minimal amount of erosion after Late Cenozoic uplift.     

Geology and fluid characteristics of the Ulu Sokor gold deposit,Kelantan, Malaysia: Implications for ore genesis and classification of the deposit

The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO₂–CH₄–H₂O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389 °C, salinities of 2.77–11.89 wt.% NaCl equivalent, variable CO₂ contents (typically 5–29 mol%), and up to 15 mol% CH₄. In stage II, gold was deposited at 235–398 °C from a CO₂ ± CH₄–H₂O–NaCl fluid with a salinity of 0.83–9.28 wt.% NaCl equivalent, variable CO₂ contents (typically 5–63 mol%), and up to 4 mol% CH₄. The δ¹⁸O_H2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and − 44 to − 42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-X_CO2 conditions yielded log f_O2 values between − 28.95 and − 36.73 for stage I and between − 28.32 and − 39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ¹⁸O_H2O = 6.8 to 11.9‰, δD = − 77 to − 62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347 °C from a CO₂–CH₄–H₂O–NaCl fluid with a salinity of 5.33 to 11.05 wt.% NaCl equivalent, variable CO₂ contents (typically 9–15 mol%), and little CH₄. The oxygen and hydrogen isotopic compositions of this fluid (δ¹⁸O_H2O = 8.1 to 8.8‰, δD = − 44 to − 32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO₂–H₂O ± CH₄–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483 °C), salinities of 1.02 to 21.34 wt.% NaCl equivalent, variable CO₂ contents (typically 4–53 mol%), and up to 7 mol% CH₄. The oxygen and hydrogen isotopic compositions (δ¹⁸O_H2O = 8.5 to 9.8‰, δD = − 70 to − 58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log f_O2 values from − 28.26 to − 35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ³⁴S values of H₂S of − 2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions.     

Geology,geochemistry and genesis of the Qianfanling quartz-vein Mo deposit in Songxian County,Western Henan Province,China

The Qianfanling Mo deposit, located in Songxian County, western Henan province, China, is one of the newly discovered quartz-vein type Mo deposits in the East Qinling–Dabie orogenic belt. The deposit consists of molybdenite in quartz veins and disseminated molybdenite in the wall rocks. The alteration types of the wall rocks include silicification, K-feldspar alteration, pyritization, carbonatization, sericitization, epidotization and chloritization. On the basis of field evidence and petrographic analysis, three stages of hydrothermal mineralization could be distinguished: (1) pyrite–barite–quartz stage; (2) molybdenite–quartz stage; (3) quartz–calcite stage.Two types of fluid inclusions, including CO₂-bearing fluid inclusions and water-rich fluid inclusions, have been recognized in quartz. Homogenization temperatures of fluid inclusions vary from 133 °C to 397 °C. Salinity ranges from 1.57 to 31.61 wt.% NaCl eq. There are a large number of daughter mineral-CO₂-bearing inclusions, which is the result of fluid immiscibility. The ore-forming fluids are medium–high temperature, low to moderate salinity H₂O–NaCl–CO₂ system. The δ³⁴S values of pyrite, molybdenite, and barite range from − 9.3‰ to − 7.3‰, − 9.7‰ to − 7.3‰ and 5.9‰ to 6.8‰, respectively. The δ¹⁸O values of quartz range from 9.8‰ to 11.1‰, with corresponding δ¹⁸O_fluid values of 1.3‰ to 4.3‰, and δ¹⁸D values of fluid inclusions of between − 81‰ and − 64‰. The δ¹³C_V-PDB values of fluid inclusions in quartz and calcite have ranges of − 6.7‰ to − 2.9‰ and − 5.7‰ to − 1.8‰, respectively. Sulfur, hydrogen, oxygen and carbon isotope compositions show that the sulfur and ore-forming fluids derived from a deep-seated igneous source. During the peak collisional period between the North China Craton and the Yangtze Craton, the ore-forming fluids that derived from a deep igneous source extracted base and precious metals and flowed upwards through the channels that formed during tectonism. Fluid immiscibility and volatile exsolution led to the crystallization of molybdenite and other minerals, and the formation of economic orebodies in the Qianfanling Mo deposit.     

Hydrothermal alteration and ore-forming fluids associated with gold-tellurium mineralization in the Dongping gold deposit,China

The Dongping gold deposit hosted in syenites is one of the largest hydrothermal gold deposits in China and composed of ore veins in the upper parts and altered zones in the lower parts of the ore bodies. Pervasive potassic alteration and silicification overprint the wall rocks of the ore deposit. The alteration minerals include orthoclase, microcline, perthite, quartz, sericite, epidote, calcite, hematite and pyrite, with the quartz, pyrite and hematite assemblages closely associated with gold mineralization. The phases of hydrothermal alteration include: (i) potassic alteration, (ii) potassic alteration - silicification, (iii) silicification - epidotization - hematitization, (iv) silicification - sericitization - pyritization and (v) carbonation. Mass-balance calculations in potassic altered and silicified rocks reveal the gain of K₂O, Na₂O, SiO₂, HFSEs and transition elements (TEs) and the loss of REEs. Most major elements were affected by intense mineral reactions, and the REE patterns of the ore are consistent with those of the syenites. Gold, silver and tellurium show positive correlation and close association with silicification. Fluid inclusion homogenization temperatures in quartz veins range from 154 °C to 382 °C (peak at 275 °C–325 °C), with salinities of 4–9 wt.% NaCl equiv. At temperatures of 325 °C the fluid is estimated to have pH = 3.70–5.86, log fO₂ = − 32.4 to − 28.1, with Au and Te transported as Au (HS)₂⁻ and Te₂^2 − complexes. The ore forming fluids evolved from high pH and fO₂ at moderate temperatures into moderate-low pH, low fO₂ and low temperature conditions. The fineness of the precipitated native gold and the contents of the oxide minerals (e.g., magnetite and hematite) decreased, followed by precipitation of Au- and Ag-bearing tellurides. The hydrothermal system was derived from an alkaline magma and the deposit is defined as an alkaline rock-hosted hydrothermal gold deposit.     

Geology,C–H–O–S–Pb isotope systematics and geochronology of the Yindongpo gold deposit,Tongbai Mountains,central China: Implication for ore genesis

The Yindongpo gold deposit is located in the Weishancheng Au–Ag-dominated polymetallic ore belt in Tongbai Mountains, central China. The ore bodies are stratabound within carbonaceous quartz–sericite schists of the Neoproterozoic Waitoushan Group. The ore-forming process can be divided into three stages, represented by early barren quartz veins, middle polymetallic sulfide veinlets and late quartz–carbonate stockworks, with most ore minerals, such as pyrite, galena, native gold and electrum being formed in the middle stage. The average δ¹⁸O_water values changed from 9.7‰ in the early stage, through 4.9‰ in the middle stage, to − 5.9‰ in the late stage, with the δD values ranging between − 65‰ and − 84‰. The δ¹³C_CO2 values of ore fluids are between − 3.7‰ and + 6.7‰, with an average of 1.1‰. The H–O–C isotope systematics indicate that the ore fluids forming the Yindongpo gold deposit were probably initially sourced from a process of metamorphic devolatilization, and with time gradually mixed with meteoric water. The δ³⁴S values range from − 0.3‰ to + 5.2‰, with peaks ranging from + 1‰ to + 4‰. Fourteen sulfide samples yield ²⁰⁶Pb/²⁰⁴Pb values of 16.990–17.216, ²⁰⁷Pb/²⁰⁴Pb of 15.419–15.612 and²⁰⁸Pb/²⁰⁴Pb of 38.251–38.861. Both S and Pb isotope ratios are similar to those of the main lithologies of the Waitoushan Group, but differ from other lithologic units and granitic batholiths in the Tongbai area, which suggest that the ore metals and fluids originated from the Waitoushan Group. The available K–Ar and ⁴⁰Ar/³⁹Ar ages indicate that the ore-forming process mainly took place in the period of 176–140 Ma, during the transition from collisional compression to extension and after the closure of the oceanic seaway in the Qinling Orogen. The Yindongpo gold deposit is interpreted as a stratabound orogenic-style gold system formed during the transition phase from collisional compression to extension.The ore metals in the Waitoushan Group were extracted, transported and then accumulated in the carbonaceous sericite schist layer. The carbonaceous sericite schist layer, especially at the junction of collapsed anticline axis and fault structures, became the most favorable locus for the ore bodies.     

Geology,geochemistry and age of the Hukeng tungsten deposit,Southern China

The Hukeng tungsten deposit, located in the Wugongshan area in central part of Jiangxi province, South China, is a large-scale quartz-vein wolframite deposit. It is hosted in the Hukeng granitic intrusion. Based on the mineral assemblage and crosscutting relationship of the veins, three mineralization stages are identified, including: (1) quartz–wolframite stage, (2) quartz–fluorite–wolframite stage, and (3) quartz–pyrite–sphalerite–wolframite stage.The homogenization temperatures of fluid inclusions in vein quartz vary from 220 to 320 °C, and the salinities are from 0 to 10 wt.% NaCl equiv.; corresponding densities range from 0.7 to 1 g/cm³. These features indicated that the ore-forming fluids in the Hukeng tungsten deposit have medium temperature, low density and low salinity.The δ¹⁸OSMOW values of quartz range from 10.8‰ to 14.4‰, with corresponding δ¹⁸O_fluid values of 3.7‰ to 7.7‰, and δD values of fluid inclusions of between ? 70‰ and ? 55‰. The combined isotopic data indicate that the ore-forming fluids of the Hukeng tungsten deposit were mainly derived from magmatic water, with some minor input from meteoric water.We have carried out molybdenite Re–Os and muscovite ⁴⁰Ar/³⁹Ar dating to constrain the timing of mineralization. Re–Os dating of six molybdenite samples yielded model ages ranging from 149.1 ± 2.0 to 150.7 ± 3.7 Ma, with an average of 150.0 Ma. The Re–Os analyses give a well-defined ¹⁸⁷Re/¹⁸⁷Os isochron with an age of 150.2 ± 2.2 Ma (MSWD = 0.60). Hydrothermal muscovite yields a plateau ⁴⁰Ar/³⁹Ar age of 147.2 ± 1.4 Ma. ⁴⁰Ar/³⁹Ar age is in good agreement with the Re–Os age. These ages show that the timing of tungsten mineralization occurred at about 150 Ma. Our new data, when combined with published geochronological results from the other major deposits in this region, suggest that widespread W mineralization occurred in the Late Jurassic throughout South China.     

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province,China

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126 Ma Guojialing granite. These deposits collectively formed at ca. 120 ± 5 Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (> 120 t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New ⁴⁰Ar/³⁹Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 ± 4 Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150 Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1 km/m.y. from depths of 25–30 km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126 Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.     

Anatomy of a world-class epizonal orogenic-gold system: A holistic thermochronological analysis of the Xincheng gold deposit,Jiaodong Peninsula,eastern China

Xincheng is a world-class orogenic-gold deposit hosted by the Early Cretaceous Guojialing granitoid in the Jiaodong Peninsula, eastern China. A zircon U–Pb age of 126 ± 1.4 Ma, together with previous data, constrain the emplacement of the Guojialing intrusion to 132–123 Ma. The granitoid underwent subsolidus ductile deformation at >500 °C following its intrusion. The small difference in age between the youngest zircon U–Pb age of unaltered granitoid (~123 Ma) and the ca. 120 Ma ⁴⁰Ar/³⁹Ar ages of sericite, associated with breccias and gold mineralization within it indicate initial rapid cooling from magmatic temperatures to those prevalent during brittle deformation and associated gold mineralization at ~220–300 °C. Evidence of a direct association between granitic magmatism and gold mineralization, such as at least localized near-magmatic depositional temperatures and metal zoning evident in undoubted intrusion-related gold deposits, is absent. The ⁴⁰Ar/³⁹Ar age of ~120 Ma coincides with the mineralization age of many other orogenic-gold deposits along the Jiaojia Fault. Sixteen zircon fission-track (ZFT) ages across the ore and alteration zones range from 112.9 ± 3.4 to 99.1 ± 2.7 Ma. The long period of cooling to the ~100 Ma ZFT closure temperatures recorded here suggests that ambient temperatures for hydrothermal alteration systems lasted to ~100 Ma, possibly because of their focus at Xincheng within the young Guojialing granitoid as it cooled more slowly below approximately 300 °C to 220 °C. However, the restricted number of auriferous ore stages, combined with the presence of cross-cutting gold-free quartz-carbonate veins, indicate that gold itself was only deposited over a restricted time interval at ~120 Ma, consistent with studies of orogenic gold deposits elsewhere. This highlights the complex interplay between magmatism, deformation and the longevity of hydrothermal systems that cause genetic controversies. Based on apatite fission-track (AFT) ages, the Xincheng gold deposit was then uplifted and exhumed to near the surface of the crust at 15 Ma, probably due to movement on the crustal-scale Tan-Lu Fault. Recognition of such exhumation histories along gold belts has conceptual exploration significance in terms of the probability of discovery of additional exposed or sub-surface gold ore bodies as discovery is as much a function of preservation as formation of the deposits.     

Listvenite–lode association at the Barramiya gold mine,Eastern Desert,Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO₂ ± CH₄ ± H₂O) and aqueous-carbonic (H₂O–NaCl–CO₂ ± CH₄) inclusions along intragranular trails, whereas aqueous inclusions (H₂O–NaCl ± CO₂) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ³⁴S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ³⁴S_Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO₂ via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.