The origin of the Tongkeng-Changpo tin deposit,Dachang metal district,Guangxi, China: clues from fluid inclusions and He isotope systematics

Tongkeng-Changpo is the largest tin deposit within the giant Dachang polymetallic tin ore field in Guangxi, southern China, which is part of a large skarn system associated with Cretaceous granitoids. The Tongkeng-Changpo mineralization consists of veins and stockworks in the upper levels and replacement stratiform orebodies (mantos) at lower levels. Based on textural relationships, three major mineralizing stages can be recognized: stage I with cassiterite, sulphides, stannite, tourmaline, and quartz; stage II with cassiterite, sulphides, sulphosalts, quartz, and calcite; and stage III with calcite as the main phase. The study of fluid inclusions has shown that there are two main fluid types: CO₂ and NaCl-H₂O. Homogenization temperatures are 270 to 365°C, 210 to 240°C, and 140 to 190°C for stages I, II, and III, respectively. Salinities range from 1 to 7 wt.% NaCl equiv. in the early ore stage and 3 to 10 wt.% NaCl equiv. in the late stages. Laser Raman Spectroscopy indicates that the inclusion fluids in stages I and II were of carbono-aqueous composition, with minor amounts of CH₄ and H₂S, whereas those in stage III were aqueous. Helium isotopic analyses of inclusion fluids indicate that the ³He/⁴He ratios in the ore veins are in between 1.2 to 2.9 Ra (Ra = 1.4 × 10⁻⁶, modern atmospheric ratio), and range from 1.6 to 2.5 Ra in the stratiform orebodies. This range of ³He/⁴He ratios is significantly higher than that of crustal fluids (0.01–0.05 Ra). The similar characteristics of fluid inclusions and their He isotopic composition, as well as age constraints, indicate that the ore veins and stratiform orebodies of the Tongkeng-Changpo deposit formed from the same hydrothermal system, likely related to granite intrusions of the Mesozoic Yanshanian tectono-thermal event. In addition, the high R/Ra ratios indicate a mantle contribution in the ore fluids.     

Genesis of the Maoling gold deposit in the Liaodong Peninsula: Constraints from a combined fluid inclusion,C-H-O-S-Pb-He-Ar isotopic and geochronological studies

The large tonnage Maoling gold deposit (25 t @ 3.2 g/t) is located in the southwest Liaodong Peninsula, North China Craton. The deposit is hosted in the Paleoproterozoic metamorphic rocks. Four stages of mineralization were identified in the deposit: (stage I) quartz-arsenopyrite ± pyrite, (stage II) quartz-gold- arsenopyrite-pyrrhotite, (stage III) quartz-gold- polymetallic sulfide, and (stage IV) quartz-calcite-pyrrhotite. In this paper, we present fluid inclusion, C-H-O-S-Pb-He-Ar isotope data, zircon U-Pb, and gold-bearing sulfide (i.e. arsenopyrite and pyrrhotite) Rb-Sr age of the Maoling gold deposit to constrain its genesis and ore-forming mechanism. Three types of fluid inclusions were distinguished in quartz-bearing veins, including liquid-rich two-phase (WL type), gas-rich two-phase (GL type), and daughter mineral-bearing fluid inclusions (S type). Fluid inclusions data show that the homogenization at temperatures 197 to 372 °C for stage I, 126 to 319 °C for stage II, 119 to 189 °C for stage III, and 115 to 183 °C for stage IV, with corresponding salinities of 3.7 to 22.6 wt.%, 4.7 to 23.2 wt.%, 5.3 to 23.2 wt.%, and 1.7 to 14.9 wt.% NaCl equiv., respectively. Fluid boiling was the critical factor controlling the gold and associated sulfide precipitation at Maoling. Hydrogen and oxygen stable isotopic analyses for quartz yielded δ¹⁸O = ?5.0‰ to 9.8‰ and δ D = ?133.5‰ to ?77.0‰. Carbon stable isotopic analyses for calcite and ankerite yielded δ¹³C = ?2.3‰ to ?1.2‰ and O = 7.9‰ to 14.1‰. The C-H-O isotope data show that the ore-forming fluids were originated from magmatic water with meteoric water input during mineralization. Hydrothermal inclusions in arsenopyrite have ³He/⁴He ratios of 0.002 Ra to 0.054 Ra, and ⁴⁰Ar/³⁶Ar rations of 1225 to 3930, indicating that the ore-forming fluids were dominantly derived from crustal sources almost no mantle input. Sulfur isotopic values of Maoling fine-grained granite range from 6.‰1 to 9.8‰, with a mean of 7.7‰, δ³⁴S values of arsenopyrite from the mineralized phyllite (host rock) range from 8.9‰ to 10.6‰, with a mean of 10.0‰, by contrast, δ³⁴S values of sulfides from ore vary between 4.3‰ and 10.6‰, with a mean of 6.8‰, suggesting that sulfur was mainly originated from both the host rock and magma. Lead radioactive isotopic analyses for sulfides yielded ²⁰⁶Pb/²⁰⁴Pb = 15.830–17.103, ²⁰⁷Pb/²⁰⁴Pb = 13.397–15.548, ²⁰⁸Pb/²⁰⁴Pb = 35.478–36.683, and for Maoling fine-grained granite yielded ²⁰⁶Pb/²⁰⁴Pb = 18.757–19.053, ²⁰⁷Pb/²⁰⁴Pb = 15.596–15.612, and ²⁰⁸Pb/²⁰⁴Pb = 38.184–39.309, also suggesting that the ore-forming materials were mainly originated from the host rocks and magma. Zircon U-Pb dating demonstrates that the Maoling fine-grained granite was emplaced at 192.7 ± 1.8 Ma, and the host rock (mineralized phyllite) was emplaced at some time after 2065.0 ± 27.0 Ma. Arsenopyrite and pyrrhotite give Rb–Sr isochron age of 188.7 ± 4.5 Ma, indicating that both magmatism and mineralization occurred during the Early Jurassic. Geochronological and geochemical data, together with the regional geological history, indicate that Early Jurassic magmatism and mineralization of the Maoling gold deposit occurred during the subducting Paleo-Pacific Plate beneath Eurasia, and the Maoling gold deposit is of the intrusion-related gold deposit type.     

Geology,fluid inclusions,and geochemistry of the Zhazixi Sb–W deposit,Hunan, South China

The Zhazixi Sb–W deposit in the Xuefeng uplift, South China, exhibits a unique metal association of W and Sb, where the W orebodies are hosted by interlayer fractures and the Sb orebodies are contained within NW-trending faults. This study proposes that the W and Sb mineralization took place in two separate periods. The mineral paragenesis of the W mineralization reveals a mass of quartz, scheelite and minor calcite. The mineral assemblage of the Sb mineralization developed after W mineralization and consists of predominantly quartz and stibnite, and small amounts of native Sb, berthierite, chalcostibnite, pyrite, and chalcopyrite. Fluid inclusions in quartz and coexisting scheelite are dominated by two-phase, liquid-rich, aqueous inclusions at room temperature. Microthermometric studies suggest that ore-forming fluids for W mineralization are characterized by moderate temperatures (170–270 °C), low salinity (3–7 wt% NaCl equiv.), low density (0.75–0.95 g/cm³), and moderate to high pressure (57.2–99.7 MPa) and these fluids experienced a cooling and dilution evolution during W mineralization. Ore-forming fluids for Sb mineralization are epithermal types with low temperatures (150–230 °C), low salinity (4–6 wt% NaCl equiv.), moderate density (0.82–0.94 g/cm³), and high pressure (42.2–122.5 MPa) and these fluids display an evident decline in homogenization temperature during Sb mineralization. Laser Raman analyses of the vapor phase indicate that the ore-forming fluids for both W and Sb mineralization contain a small amount of CO₂.The ore-forming fluids for Sb mineralization are identified as predominantly originating from the continental crust, as suggested by the low ³He values (0.009 × 10⁻¹² cc.STP/g) and ³He/⁴He ratios (0.002–0.056 Ra) as well as high ³⁶Ar values (1.93 × 10⁻⁹ cc.STP/g) and ⁴⁰Ar/³⁶Ar ratios (909.5–2279.7). The source of S is identified to be the Neoproterozoic Wuqiangxi Formation, as traced by the δ³⁴S_V-CDT values of stibnite (3.1–9.4‰). The ²⁰⁸Pb/²⁰⁴Pb (37.643–40.222), ²⁰⁷Pb/²⁰⁴Pb (15.456–15.681), and ²⁰⁶Pb/²⁰⁴Pb (17.093–20.042) ratios suggest a mixture of lower crustal and supracrustal Pb sources.It is thus concluded that the ore genesis of the Zhazixi Sb–W deposit is related to the intracontinental orogeny during the early Mesozoic. Fluid mixing is considered to be the critical mechanism involved in W mineralization, whereas a fluid cooling process is responsible for Sb mineralization. Furthermore, the absence of Au is attributed to the low Σa_s content in Sb-mineralizing fluids.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

Tectono-sedimentary evolution of active extensional basins controlling the deposition of the Middle Miocene Kareem Formation,southwestern Gulf of Suez,Egypt

The Naozhi Au–Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry–epithermal gold–copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas–liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO₂-bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7–406.7, 230–338.8, and 154.6–308 °C, with salinities of 2.40–7.01 wt% NaCl_eq, 1.73–9.47 wt% NaCl_eq, and 6.29 wt% NaCl_eq, respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6–400 °C, with salinities of 39.96–46.00 wt% NaCl_eq; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO₂ and H₂O, with H₂O gradually increasing and CO₂ being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have ³He/⁴He ratios of 0.03–0.104 Ra, ²⁰Ne/²²Ne ratios of 9.817–9.960, and ⁴⁰Ar/³⁶Ar ratios of 324–349. These results indicate that the percentage of radiogenic ⁴⁰Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5–91.2 % atmospheric ⁴⁰Ar; (5) the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁶Pb/²⁰⁴Pb ratios of sulfides are 18.1822–18.3979, 15.5215–15.5998, and 38.1313–38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.

     

He–Ar isotope geochemistry of iron and gold deposits reveals heterogeneous lithospheric destruction in the North China Craton

The North China Craton (NCC) provides a classic example for extensive destruction of the cratonic lithosphere. The Mesozoic magmatism which contributed to the decratonization of the NCC was also accompanied by the formation of a variety of mineral deposits. In order to gain further insights into the cratonic destruction process, typical iron and gold deposits are investigated here. Helium–argon isotopic data on pyrite, from typical skarn iron deposits of the Beiminghe and Fushan in the Han-Xing district of the central NCC, and the Linglong and Canzhuang gold deposits in the Jiaodong district in the eastern NCC, are presented in this paper. The ³He/⁴He, ⁴⁰Ar/³⁶Ar and ⁴⁰Ar/⁴He ratios show generally uniform patterns within the individual deposits and reveal a complex evolutionary history of the ore-forming fluids with varying degree of crust–mantle interaction. The ore-forming fluids associated with the gold mineralization at the Jiaodong mine have higher content of fluids of mantle origin with mantle helium ranging from 1.24% to 18.02% (average 6.73%; N = 18). In contrast, the ore-forming fluids related to the iron ore deposits contain less mantle contribution with mantle helium ranging from 0.12% to 4.96% (average 1.29%; N = 10). Our results suggest complex and heterogeneous crust–mantle processes associated with the magmatism and metallogeny, where the lithosphere of the eastern NCC was subjected to more extensive thinning and destruction as compared with that in the western part, consistent with the observations from geophysical studies in the region. Our study demonstrates that fluids associated with the Mesozoic metallogenic processes in the NCC provide useful insights into the geodynamics of destruction and refertilization of the cratonic lithosphere.     

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.     

Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits

Fluid origins in the sandstone-hosted Pb-Zn class of ore deposit have been investigated in three deposits from Scandinavia; Laisvall, Vassbo and Osen. The deposits studied are hosted by autochthonous Cambrian sandstones that preserve a near original structural relationship to the underlying Precambrian basement, enabling the role of basement interaction to be assessed.Mineral samples have been collected from across the paragenetic sequence: sphalerite, galena, pyrite, fluorite and barite, of impregnation and related joint-hosted mineralization. Fluid-inclusion halogen (Cl, Br and I) and noble gas isotope (⁴⁰Ar, ³⁶Ar, ⁸⁴Kr) compositions were determined simultaneously by noble gas mass spectrometry of irradiated sample splits. Complementary He isotope analyses are obtained from nonirradiated splits of the same samples.³He/⁴He values at Laisvall and Osen are highly radiogenic, 0.02 Ra, and the ⁴He/⁴⁰Ar* ratio extends to values greater than the crustal production value of 5, characteristic of low-temperature crustal fluids. At Vassbo, a slightly elevated ³He/⁴He ratio of 0.1-0.3 Ra is compatible with a very minor mantle component (1%-4%) suggesting a distal source for the basinal brine-dominated fluid.Br/Cl molar ratios 3.2-8.2 × 10⁻³ are greater than the present seawater value of 1.54 × 10⁻³ and correspond with I/Cl molar ratios in the range 64-1600 × 10⁻⁶. The upper limits of both the I/Cl and Br/Cl values are amongst the highest measured in crustal fluids. Together, the data indicate acquisition of salinity by the evaporation of seawater beyond the point of halite saturation and subsequent fluid interaction with I-rich organic matter in the subsurface. The data are compatible with the independent transport of sulfate and sulfide and indicate that fluids responsible for joint-hosted mineralization were distinct to those responsible for impregnation mineralization.All three deposits preserve fluids with ⁴⁰Ar/³⁶Ar in the range of 6,000-10,000 and fluid inclusion ⁴⁰Ar* concentrations of >0.02-0.05 cm³cm⁻³. Fluid-inclusion ⁴He concentrations are also extremely elevated with maximum values of ∼0.1 cm³cm⁻³ in Laisvall fluorite and sphalerite. The high ⁴⁰Ar/³⁶Ar values, together with the high ⁴He and ⁴⁰Ar* concentrations, result from a very long premineralization crustal residence time on the order of 100-200 Ma.Together, the noble gas and halogen data are compatible with a Caledonian mineralization event (∼425 Ma) caused by mixing of two or more, long-lived, hydrothermal basinal brines and pore fluids at the sites of mineralization. The data suggest negligible recharge of the basinal brines by meteoric water and indicate extensive fluid-basement interaction before mineralization. The similar noble gas composition of each deposit, suggests that similar processes operated at all three deposits and favors a single-pass fluid-flow model for mineralization.     

Geology,geochronology, geochemistry and ore genesis of the Wangu gold deposit in northeastern Hunan Province,Jiangnan Orogen,South China

The Wangu gold deposit in northeastern Hunan, South China, is one of many structurally controlled gold deposits in the Jiangnan Orogen. The host rocks (slates of the Lengjiaxi Group) are of Neoproterozoic age, but the area is characterized by a number of Late Jurassic–Cretaceous granites and NE-trending faults. The timing of mineralization, tectonic setting and ore genesis of this deposit and many similar deposits in the Jiangnan Orogen are not well understood. The orebodies in the Wangu deposit include quartz veins and altered slates and breccias, and are controlled by WNW-trending faults. The principal ore minerals are arsenopyrite and pyrite, and the major gangue minerals are quartz and calcite. Alteration is developed around the auriferous veins, including silicification, pyritic, arsenopyritic and carbonate alterations. Field work and thin section observations indicate that the hydrothermal processes related to the Wangu gold mineralization can be divided into five stages: 1) quartz, 2) scheelite–quartz, 3) arsenopyrite–pyrite–quartz, 4) poly-sulfides–quartz, and, 5) quartz–calcite. The Lianyunshan S-type granite, which is in an emplacement contact with the NE-trending Changsha-Pingjiang fracture zone, has a zircon LA-ICPMS U–Pb age of 142 ± 2 Ma. The Dayan gold occurrence in the Changsha-Pingjiang fracture zone, which shares similar mineral assemblages with the Wangu deposit, is crosscut by a silicified rock that contains muscovite with a ca. 130 Ma ⁴⁰Ar–³⁹Ar age. The gold mineralization age of the Wangu deposit is thus confined between 142 Ma and 130 Ma. This age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of NE-trending extensional faults, the emplacement of Late Jurassic–Cretaceous granites and the formation of Cretaceous basins filled with red-bed clastic rocks in northeastern Hunan, which forms part of the Basin and Range-like province in South China. EMPA analysis shows that the average As content in arsenopyrite is 28.7 atom %, and the mineralization temperature of the arsenopyrite–pyrite–quartz stage is estimated to be 245 ± 20 °C from arsenopyrite thermometry. The high but variable Au/As molar ratios (>0.02) of pyrite suggest that there are nanoparticles of native Au in the sulfides. An integration of S–Pb–H–O–He–Ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks, possibly driven by hydraulic potential gradient created by reactivation of the WNW-trending faults initially formed in Paleozoic, with possible involvement of magmatic and mantle components channeled through regional fault networks. The Wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic, epithermal and Carlin-type gold deposits, and may be better classified as an “intracontinental reactivation” type as proposed for many other gold deposits in the Jiangnan Orogen.     

Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China

The Jinshan orogenic gold deposit is a world-class deposit hosted by a ductile shear zone caused by a transpressional terrane collision during Neoproterozoic time. Ore bodies at the deposit include laminated quartz veins and disseminated pyrite-bearing mylonite. Most quartz veins in the shear zone, with and without gold mineralization, were boudinaged during progressive shear deformation with three generations of boudinage structures produced at different stages of progressive deformation. Observations of ore-controlling structures at various scales indicate syn-deformational mineralization. Fluid inclusions from pyrite intergrown with auriferous quartz have ³He/⁴He ratios of 0.15–0.24 Ra and ⁴⁰Ar/³⁶Ar ratios 575–3,060. δ¹⁸O_fluid values calculated from quartz are 5.5–8.4‰, and δD values of fluid inclusions contained in quartz range between −61‰ and −75‰. The δ¹³C values of ankerite range from −5.0‰ to −4.2‰, and ankerite δ¹⁸O values from 4.4‰ to 8.0‰. The noble gas and stable isotope data suggest a predominant crustal source of ore fluids with less than 5% mantle component. Data also show that in situ fluids were generated locally by pervasive pressure solution, and that widespread dissolution seams acted as pathways of fluid flow, migration, and precipitation. The in situ fluids and fluids derived from deeper levels of the crust were focused by deformation and deformation structures at various scales through solution-dissolution creep, crack-seal slip, and cyclic fault-valve mechanisms during progressively localized deformation and gold mineralization.     

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.     

Geological,fluid inclusion,and O–C–S–Pb–He–Ar isotopic constraints on the genesis of the Honghuagou lode gold deposit,northern North China Craton

The Honghuagou Au deposit is located in the Chifeng-Chaoyang region within the northern margin of the North China Craton. The auriferous quartz veins are mainly hosted in the mafic gneiss and migmatite of the Neoarchean Xiaotazigou Formation along NNW- and NE-striking faults, with pyrite as the predominant ore mineral. The gold mineralization process can be divided into two stages, involving stage I quartz-pyrite and stage II quartz-calcite-polymetallic sulfide. Three types of fluid inclusions (FIs) have been identified in the Honghuagou deposit, namely, carbonic inclusions, aqueous‑carbonic inclusions, and aqueous inclusions. Quartz of stage I contains all types of FIs, whereas only aqueous inclusions are evident in stage II veins. The FIs of stages I and II yield homogenization temperatures of 275–340 °C and 240–290 °C with salinities of 3.4–10.7 wt% and 1.4–9.7 wt% NaCl eqv., respectively. The ore-forming fluids are characterized by medium temperature and low salinity, belonging to the H₂O–NaCl–CO₂ system. The δ¹⁸O_H2O values of the ore fluids are between 2.1‰ and 5.9‰, within the range of enriched mantle-derived fluids in the North China Craton. The carbon isotope compositions of calcite (δ¹³C_PDB = −4.4‰ to −4‰) are also similar to mantle carbon. He-Ar isotope data (³He/⁴He = 0.38–0.44 Ra; ⁴⁰Ar/³⁶Ar = 330–477) of fluid inclusions in pyrite indicate a mixed crustal and mantle source for the ore-forming fluids. Whereas, S-Pb isotope compositions of sulfides reveal that ore metals are principally derived from crustal rocks. On the basis of available geological and geochemical evidence, we suggest that the Honghuagou deposit is an orogenic gold deposit.     

Magmatic-dominated fluid evolution in the Jurassic Nambija gold skarn deposits (southeastern Ecuador)

The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad_20–98) with subordinate pyroxene (Di_46–92Hd_17–42Jo_0–19) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500°C to >600°C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100–120 MPa, corresponding to about 4-km depth. Lower-temperature (510–300°C) and moderate- to low-salinity (23–2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513°C to 318°C and salinity from 1.0 to 23 wt.% eq. NaCl) and δ¹⁸O values between 6.2‰ and 11.5‰ for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50–100 MPa, in line with the evidence for CO₂ effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO₂ effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by δ¹⁸O values of 0.4‰ to 6.2‰ for fluids depositing quartz (below 350°C) in sulfide-rich barren veins. Low-temperature (<300°C) meteoric fluids (δ¹⁸O_water between −10.0‰ and −2.0‰) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100°C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202–74.5°C) with δ¹⁸O values of −0.5‰ to 3.1‰ and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl₂, trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO₂ in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin.     

Helium and argon isotopic compositions of the Longquanzhan gold deposit in the Yishu fault zone and their geological implications

The Longquanzhan gold deposit hosted in granitic cataclasites with mylontization of the foot wall of the main Yishui-Tangtou fault. 3He/4He ratios in fluid inclusions range from 0. 14 to 0. 24 R/Ra,close to those of the crust-source helium. 40Ar/36Ar ratios were measured to be 289-1811, slightly higher than those of atmospheric argon. The results of analysis of helium and argon isotopes suggested that ore-forming fluids were derived chiefly from the crust. The δ18O values of fluid inclusions from vein quartz range from -1.78‰ to 4.07‰, and the δD values of the fluid inclusions vary between -74‰ and -77‰. The hydrogen and oxygen isotope data indicated that the ore-forming fluid for the Longquanzhan gold deposit had mixed with meteoric water in the process of mineralization. This is consistent with the conclusion from the helium and argon isotope data.     

A sulphur isotopic study of the Navia gold belt (Spain)

The Navia gold belt is located in the West Asturian-Leonese Zone of the Iberian Variscan Orogen. The host rocks of the mineralization are quartzites, sandstones and black shales of Cambro-Ordovician age. The gold belt extends along 35 km and has five major veins: Penedela, Encarnita, Fornaza, Carmina and S. Jose. The ores belong to at least four associations having contrasting mineralogies and textures. The δ³⁴S values for individual mineral phases reflect the polyphase metallogenic history. The older association (Stage 1) is Fe-Mn-rich and is made up of spessartine, grunerite-dannemorite and quartz, with magnetite, pyrrhotite and chalcopyrite as metallic phases. The mineralization of Stage 1 is followed by the As-rich Stage 2 with quartz, arsenopyrite and pyrite. The δ³⁴S values for pyrite range from 14.9 to 19.9 per mil (n = 16), and for arsenopyrite from 13.2 to 17.3 per mil (n = 7). The observed isotopic homogeneity likely implies isotopic equilibrium at the scale of the gold vein. Stage 3 contains a coarse-grained base metal sulphide-rich association. The δ⁴S values for sphalerite range from 16.4 to 20.6 per mil (n= 16), and for galena from 17.0 to 18.7 per mil (n = 11). δ³⁴S_sp > δ³⁴S_gl suggests that the sulphur isotopic fractionation of the ore-forming system had reached equilibrium. The youngest crosscutting mineral association (Stage 4) consists of Pb-Sb sulphosalts, bornite, electrum and quartz. The δ³⁴S values for sulphosalts range from 9.7 to 15.8 per mil, showing the lightest results of the Navia sulphides.The relatively tight clustering of δ³⁴S values of the Au-related sulphides, and the results of fluid inclusions and paragenetic studies, can be interpreted to indicate that the hydrothermal fluids of the last three stages were dominated by H₂S. In the H₂S predominant field, sulphide minerals precipitating from solutions would exhibit δ³⁴S values similar to the δ³⁴S_ΣS value of the ore fluid. The heavy δ³⁴S_ΣS of the Navia fluids is consistent with leaching of sulphur from the host rocks. The main sulphur source could be diagenetic pyrite from the siliciclastic rocks of the Cabos and Luarca Formations, which exhibit δ³⁴S values from 8.3 to 21.2 per mil. An additional sulphur-source in Stage 3 would be the leaching of disseminated sphalerite and galena present in Cambrian carbonates.     

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     

Timing and mechanism of gold mineralization at the Wang'ershan gold deposit,Jiaodong Peninsula,eastern China

The Wang'ershan gold deposit, located in the southern Jiaojia goldfield, is currently the largest gold deposit hosted within the subsidiary faults in Jiaodong Peninsula, with a gold reserve of > 60 t gold at a grade of 4.07 g/t Au. It is hosted in the Late Jurassic Linglong biotite granites and controlled by the second-order, N- to NNE-trending Wang'ershan Fault (and its subsidiary faults) which is broadly parallel to the first-order Jiaojia Fault in the goldfield. Gold mineralization occurs as both disseminated- and stockwork-style and quartz–sulfide vein-style ores, mainly within altered cataclasites and breccias, and sericite–quartz and potassic alteration zones, respectively. Mineralization stages can be divided into (1) the pyrite–quartz–sericite stage, (2) the quartz–pyrite stage, (3) the quartz–sulfide stage, and (4) the quartz–carbonate stage.Two sericite samples associated with the main ore-stage pyrites from pyritic phyllic ores of the deposit with weighted mean plateau ⁴⁰Ar/³⁹Ar age of 120.7 ± 0.6 Ma and 119.2 ± 0.5 Ma, respectively, were selected for ⁴⁰Ar/³⁹Ar geochronology. On the basis of petrography and microthermometry, three types of primary fluid inclusions related to the ore forming event were identified: type 1 H₂O–CO₂–NaCl, type 2 aqueous, and type 3 CO₂ fluid inclusions (in decreasing abundance). Stage 1 quartz contains all three primary fluid inclusions, while stages 2 and 3 quartz contain both type 1 and 2 inclusions, and stage 4 quartz contains only type 2 inclusions. The contemporaneous trapping, similar salinities and total homogenization temperature ranges, and different homogenization phases of type 1 and type 2 inclusions indicate that fluid immiscibility did take place in stages 1, 2 and 3 ores, with P–T conditions of 190 to 85 MPa and 334 to 300 °C for stage 1 and 200 to 40 MPa and 288 to 230 °C for stages 2 and 3. Combined with the H–O–C–S–Pb isotopic compositions, ore-forming fluids may have a metamorphic-dominant mixed source, which could be associated with the dehydration and decarbonisation of a subducting paleo-Pacific plate and characterized by medium–high temperature (285–350 °C), CO₂-bearing (~ 8 mol%) with minor CH₄ (1–4% in carbonic phase), and low salinity (3.38–8.45 eq. wt.% NaCl). During mineralization, the fluid finally evolved into a medium–low temperature NaCl–H₂O system. Au(HS)₂⁻ was the most probable gold-transporting complex at Wang'ershan, due to the low temperature (157–350 °C) and near-neutral to weakly acidic ore fluids. The reaction between gold-bearing fluids and iron-bearing wall-rocks, and fluid-immiscibility processes caused via fluid–pressure cycling during seismic movement along fault zones that host lode-gold orebodies, which led to breakdown of Au(HS)₂⁻, are interpreted as the two main precipitation mechanisms of gold deposition.In general, the Wang'ershan deposit and other deposits in the Jiaojia camp have concordant structural system and wall-rock alteration assemblages, nature of orebodies and gold occurrence conditions, as well as the similar geochronology, ore-forming fluids system and stable isotope compositions. Thus gold mineralization in the Jiaojia goldfield was a large-scale unified event, with consistent timing, origin, process and mechanism.     

Monte Ollasteddu, a new gold discovery in the Variscan basement of Sardinia (Italy): first isotopic (40Ar−39Ar, Pb) and fluid inclusion data

The Monte Ollasteddu deposit represents a major gold discovery in the Variscan basement of southeastern Sardinia. Gold occurs in late-Variscan extensional brittle structures hosted by meta-volcanic, and subordinately meta-sedimentary, rocks. The vein mineralogy is dominated by quartz; arsenopyrite is the main sulphide. Reconnaissance ⁴⁰Ar–³⁹Ar dating gives ages around ∼260 Ma on K-feldspar from mineralized veins, whereas metamorphic white mica from the host rock gives ages clustering at ∼307 Ma. The best age estimate for biotite from a nearby leucogranite body is 286.3±2.2 Ma. The Pb isotope signature of ore and gangue minerals is entirely consistent with literature data for Variscan deposits of Sardinia, and for European Variscan gold deposits. Fluid inclusion data point to the presence of both CO₂-bearing and CO₂-free fluids, with homogenization temperatures ranging from 220 to 415°C, with low-to-moderate salinities (0.4–6.2 wt% NaCl equivalent). Monte Ollasteddu shows several features similar to European Variscan gold deposits; however, the age of mineralization might post-date granitoid intrusion by as much as 30 Ma, being instead coeval with very late calc-alkaline basaltic dykes, marking the transition to a post-orogenic, pre-Tethyan geodynamic setting. Electronic Supplementary Material Supplementary material is available for this article at     

Hydrothermal alteration associated with Mesozoic granite-hosted gold mineralization at the Sanshandao deposit,Jiaodong Gold Province,China

The Sanshandao gold deposit (reserves of more than 200 t Au and average grade of 3.96 g/t), located at northwestern edge of the Jiaodong Peninsula, eastern North China Craton, is one of the largest gold deposits in the Jiaodong gold province. In this deposit, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids; mineralization and alteration are largely controlled by the regional Sanshandao–Cangshang fault. Host granitic rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Activities of the Sanshandao–Cangshang fault created structurally controlled permeability allowing for infiltration of hydrothermal fluids, leading to diffusive K-feldspar alteration on the two fault planes. Later, large scale diffusive sericitization symmetrically developed across the main fault, and partially overprinted the earlier K-feldspar alteration. Following the sericitization, relatively small scale silicification occurred, but now it is only retained in the hanging wall of the main fault. Subsequently, the fault gouge formed as a “barrier layer”, which is impermeable for later fluids to move upward. After that, strong pyrite–sericite–quartz alteration occurred only in the footwall of the main fault, and was accompanied by gold precipitation. The last stage carbonation and quartz-carbonate veins marked the waning of gold-related hydrothermal activity. Mass-balance calculations indicate complex behaviors of different types of elements during fluid–rock interaction. Most major elements were affected by intensive mineral replacement reactions. As expected, the fluid-mobile elements, LILE and LREE, generally show moderate to high mobility. It is notable that even the commonly assumed fluid-immobile elements, such as HREE and HFSE, tend to be changed to various degrees. In addition, Y–Ho, Zr–Hf and Nb–Ta fractionations are observed in altered domains. Studies on alteration assemblages and fluid inclusions suggest that the ore-forming fluids were characterized by low salinity (≤ 8.4 wt.% NaCl equiv.), moderate temperature (300–400 °C), weakly acidic (pH: 3–5), and relatively reducing (log f_O2: ~–28) characteristics. In this type of fluids, gold was most likely transported as Au(HS)₂⁻ complex. With alteration going on, log (a_K⁺/a_H⁺) of fluids generally decreased due to significant formation of secondary K-bearing minerals. In addition, there might be a decrease of f_O2 from pre-gold alteration stage to the main gold mineralization stage, and decrease of f_O2 was probably one of the factors controlling gold precipitation. The Sr and Nd isotopic compositions of hydrothermal minerals, combined with previous H–O and He–Ar isotopic studies, indicate that the hydrothermal fluids were mainly derived from crustal sources (e.g., degassing of felsic magmas and meteoric water), but with involvement of mantle derived components. The gold mineralization event just coincided with reactivation of the North China Craton, as marked by asthenosphere upwelling, voluminous igneous rocks, and high crustal heat flow, which may have provided sufficient heat energy and fluid input required for the formation of the gold deposits.     

Age and metal source of orogenic gold deposits in Southeast Guizhou Province,China: Constraints from Re-Os and He-Ar isotopic evidence

The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits(Pingqiu and Jinjing) are investigated, including combined Re-Os dating and the He-Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400 ± 24 Ma,with an initial ~(187)Os/~(188)Os ratio of 1.24 ± 0.57(MSWD = 0.96). An identical isochron age of 400 ± 11 Ma with an initial ~(187)Os/~(188)Os ratio of 1.55 ± 0.14(MSWD = 0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial ~(187)Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element(REE), trace elements, Nd-Sr-S-Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by ~3 He/~4 He ratios ranging from 5.3 × 10~(-4) Ra to 2.5 × 10~(-2) Ra(Ra = 1.4 × 10~(-6), the ~3 He/~4 He ratio of air), 40 Ar=/~4 He ratios from 0.64 × 10~(-2) to 15.39×10~(-2), and ~(40)Ar/~(36)Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin,evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic ~4 He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.