Xiaoqinling District Research Articles

Episodic Au-Mo mineralization events in the Xiaoqinling district, southern margin of the North China Craton

The Xiaoqinling district, located in the southern margin of the North China Craton, is one of China’s most significant gold districts (resources of > 630 t Au). A complex series of overprinting Mesozoic tectonic and related hydrothermal events associated with the metallogenic evolution in this region has led to controversial interpretations and models. Here we present new geological, mineralogical, ore texture, geochronological, and in-situ S isotope data from the Dahu Au-Mo, Qinnan Au-Mo, and Tonggou Au deposits from this region which indicate that the Mo and Au were introduced in two discrete mineralization events. The Mo-bearing mineral assemblage mainly comprises quartz, anhydrite, pyrite, molybdenite, and rutile that yielded an age of 216.1 ± 9.7 Ma at Dahu. In-situ LA-ICP-MS δ34S values for the molybdenite and related pyrite cluster tightly at -10 to -9‰. On the other hand, the Au-bearing mineral assemblage mainly comprises quartz, pyrite, galena, sphalerite, chalcopyrite, native gold, and Te (±Bi) minerals. Hydrothermal rutile and monazite associated with auriferous pyrite yielded ages of 124.7 ± 3.9 Ma (Dahu), 121.5 ± 1.3 Ma (Qinnan), and 122.1 ± 2.0 Ma (Tonggou). The in-situ δ34S values of gold-related sulfides fall within a range of -4 to + 3 ‰. Although Mo and Au ores are both present in the same ore veins, their mineral assemblage, mineralization age, fluid properties and sources are different. Textural relationship shows that the earlier Mo-bearing quartz-anhydrite veins were overprinted by later auriferous fluids. Several lines of evidence support a multiphase mineralization process of Mo and Au, which are genetically linked to the post-collisional extension of the Qinling orogen and the Paleo-Pacific post-subduction tectonics, respectively. The episodic hydrothermal mineralization in Xiaoqinling represents a protracted and complex lode system.

Trace element and S-Pb isotopic compositions of pyrite from the Precambrian metamorphic rocks and their derivative pegmatites in the Xiaoqinling district, southern North China Craton: Implications for possible gold source of the Early Cretaceous gold deposits

The North China Craton is well endowed with numerous Early Cretaceous gold deposits that are mainly hosted in Archean to early Paleoproterozoic metamorphic rocks and late Mesozoic granitoid intrusions intruding the former. Whether or not gold was sourced from those metamorphic rocks remains hotly debated. Here we present trace element and S-Pb isotope data of pyrite from the Neoarchean to early Paleoproterozoic metamorphic rocks in the Xiaoqinling district of southern North China Craton and late Paleoproterozoic pegmatites derived from the former rocks, in an attempt to provide new insights into the gold sources for Early Cretaceous gold lodes genesis in this district. Pyrite from the metamorphic rocks typically occurs as inclusions in rock-forming minerals or disseminated grains roughly distributed along the foliation of the rocks, whereas pyrite from the pegmatites usually occurs as disseminations closely associated with quartz and magnetite. The textural data suggest that pyrite in the metamorphic rocks and pegmatites are of metamorphogenic and magmatogenic origins, respectively. LA-ICP-MS spot analyses show that pyrite both from the metamorphic rocks and pegmatites has Au contents mostly below the detection limit, with minor analyses revealing Au marginally higher than the detection limit. Considering that Au is moderately incompatible and highly siderophile, the very low gold contents in pyrite allow us to infer that the metamorphic rocks contain negligible gold. Pyrite grains from the metamorphic rocks and pegmatites have broadly consistent δ34S values ranging from −2.7 to 31.5 (mean = 17.6) per mil and 6.3 to 17.9 (mean = 14.3) per mil, respectively. Similarly, they have comparable, highly radiogenic lead isotopes, which are 16.859–24.889 for 206Pb/204Pb, 15.355–15.694 for 207Pb/204Pb, and 37.480–43.848 for 208Pb/204Pb. The sulfur and lead isotopes of pyrites from the metamorphic rocks and pegmatites are distinct from those of Early Cretaceous gold deposits hosted by these rocks. Taken together, the pyrite trace element and S-Pb isotope data presented here preclude the Precambrian metamorphic basement rocks as an important gold source for the extensive Early Cretaceous gold mineralization in the Xiaoqinling district and, by inference, other major Early Cretaceous gold provinces across the North China Craton.

Age and mineralization processes of decratonic lode gold deposits in the southern North China Craton: Constraints from trace elements, in-situ S-Pb isotopes and Rb-Sr geochronology of pyrite from the Chen’er gold deposit

The Chen’er gold deposit, with 27 tons of known Au reserves, is one of the typical gold deposits in the Xiaoqinling district along the southern part of the North China Craton. In order to better understand the gold precipitation time and mineralization process, we report in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) trace element analyses, elemental maps, and the Rb-Sr and S-Pb isotopic composition of sulfides from the Chen’er gold deposit with a view to gain insights into the mineralization time and process. Trace element analyses and elemental maps of pyrite show that the content of As is low and has no correlation with Au, indicating As is unlikely to have controlled the Au incorporation into pyrite. Gold mainly occurs as solid solutions in the Py Ⅰ and Py Ⅱ, and as Au-Ag and/or Au-Te sub-micrometer inclusions in Py III. The Rb-Sr isotopic data on sulfides from the Chen’er deposit yielded an isochron age of 129.2 ± 3.4 Ma, which overlaps with the previously reported 40Ar/39Ar age of hydrothermal mica, indicating that the gold mineralization occurred in the Early Cretaceous. The in-situ sulfur isotope values of sulfides from stage Ⅱ to stage III fluctuate significantly (the average value decreases from 4.95 to −0.24‰) and were affected by physicochemical parameters of the mineralization process, especially ƒO2. In addition, the δ34S values are in a narrow range (-1.34 to 5.24‰), indicating a magmatic source for the sulfur. In-situ Pb isotopic compositions of the pyrite and galena of stage Ⅱ and stage III are relatively constant with 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values in the range of 37.264–37.595, 15.364–15.423, 16.992–17.078, respectively, indicating a mixed mantle-crust origin. Based on the new geochronological, geochemical and isotopic data presented in this study, and combined with those from previous studies, we suggest that the lode gold deposits in the Xiaoqinling district are magmatic hydrothermal ore deposits, with metal and ore-forming fluid originating primarily from the mantle, coupled with the cratonic reactivation during the Early Cretaceous.

Geochronology, stable isotopes and pyrite chemistry of the Cangzhuyu gold deposit in the North China Craton: Implications for gold metallogeny of Xiaoqinling

The ore genesis of gold deposits in Xiaoqinling, located in the south margin of the North China Craton (NCC), remains highly debated. Either a magmatic source or a metamorphic origin has been advocated for the ore-forming fluids. In this paper, we report new 40Ar/39Ar dating, in-situ pyrite trace elements and C-H-O isotopic data for the Cangzhuyu deposit, hosted in quartz veins within the Archaean metamorphic rocks to constrain the ore-forming processes. Four stages of mineralization and alteration can be recognized in the deposit, including Stage I: quartz-pyrite veins; Stage II: pyrite-quartz-ankerite veins; Stage III: polymetallic sulfides-ankerite-quartz veins; Stage IV: quartz-carbonate veins. Sericite from stage III gold-bearing veins has a 40Ar/39Ar plateau age of 130.7 ± 1.3 Ma, which is synchronous with the regional gold mineralization event in the Xiaoqinling district. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) trace element analysis shows that Stage I and Stage IV pyrite show similar compositions and rarely contain lattice gold. In contrast, gold mostly occur as native gold or Au-Te-Bi mineral inclusions within Stage III pyrite, which are consistent with their high Ni, Te, Bi contents. The ore-forming fluids equilibrated with ankerite have C-O isotopes of −6.6 to −4.1‰ and 8.8 to 10.8 ‰, respectively. They are consistent with H and O isotopes of sericite that δDfluid ranges from −61.1 to −60‰ and δ18Ofluid ranges from 8.2 to 7.2 ‰, implying a magmatic source. We hence suggest that the Cangzhuyu gold deposit was formed from mantle-derived magmas in a setting of the destruction of NCC and lithospheric thinning.

New constraints on genesis of the Qiangma gold deposit, southern North China Craton: Evidence from rutile U-Pb dating, pyrite trace element composition and S-Pb isotopes

The Xiaoqinling district, situated at the southern North China Craton, is the second largest gold province in China. The genesis of gold deposits in this district, however, has been debated for decades because of lacking unequivocal constraints on mineralization age and gold source(s). In this study, we present ore-related hydrothermal rutile U-Pb dating, pyrite trace element compositions, and S-Pb isotopes of the Qiangma Au deposit that is one of the largest gold deposits in the Xiaoqinling district, to provide new constraints on genesis of the regional gold mineralization. The Qiangma Au deposit is formed by four paragenetic stages consisting of milky quartz—coarse-grained pyrite, grey quartz—fine-grained pyrite, grey quartz—polymetallic sulfides, and quartz-calcite. The gold mineralization is accompanied by intensively hydrothermal alteration assemblage comprising quartz, sericite, sulfides, and calcite. Hydrothermal rutile texturally intergrown with gold-bearing pyrite and sericite yields a U-Pb age of 128.1 ± 7.4 Ma (2σ, MSWD = 0.9), representing the timing of gold mineralization at Qiangma. This age is consistent with the ages of regional felsic to mafic magmas and metamorphic core complexes as results of late Mesozoic tectonic reactivation of the North China Craton. Petrographic investigation and LA-ICP-MS trace element analysis show that gold mainly occurs as native gold with minor amounts of lattice gold within pyrite, which has no correlation with As and Te. Pyrite from the four stages have sulfur isotopes mainly between 1.8 and 5.8 ‰ (mean of 3.7 ‰), suggesting a magmatic affiliation. Pyrite from quartz-polymetallic sulfides stage has lead isotopes of 17.51–17.71 for 206Pb/204Pb, 15.39–15.49 for 207Pb/204Pb, and 38.01–38.53 for 208Pb/204Pb, which are distinctly different from those of Mesoproterozoic clastic and carbonate rocks and late Mesozoic granites, but are broadly consistent with late Mesozoic subcontinental lithospheric mantle derived mafic dikes. Collectively, our new lines of evidence suggest that the Qiangma Au deposit, as well as some gold deposits in the Xiaoqinling district, is most likely related to mantle-derived magmas generated in an extensional setting related to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton.

Geochronology, petrogenesis and oxidation state of the Wenyu igneous suite in the Xiaoqinling district, North China Craton: Implications for lithospheric thinning and magma fertility

Recent studies have shown that the Mesozoic metallogenic event in the southern margin of the North China Craton (NCC) is related to the coeval magma activities generated by the lithospheric thinning and asthenosphere upwelling. However, the reason why some Mesozoic magmatic systems in this area are barren is not clear. In this study, we report new geochronological and geochemical data for a Mesozoic igneous suite of Wenyu monzogranites from the Xiaoqinling area in the southern margin of the NCC, focusing on the petrogenesis and oxidation state with their implications on lithospheric thinning and magma fertility. The Wenyu monzogranites have zircon UPb ages between 141.4 and 122.2 Ma. Whole-rock major and trace element, SrNd and zircon Hf isotope geochemistry suggest that the Wenyu monzogranites were dominantly derived from partial melting of the metamorphic and volcanic rocks from the Neoarchean to Paleoproterozoic, with potential replenishments of mantle-derived magmas triggered by the lithospheric thinning and hot asthenosphere upwelling. Zircon trace element geochemistry suggests a low oxidation state for the Wenyu magmatic system, which probably resulted in early sulfide saturation. Most of the ore-forming chalcophile elements (Au and Cu) were locked in a sulfide phase that was retained in cumulate rocks at depth due to the early sulfide saturation, so that when the magma became fluid saturated at the later stage of magmatic evolution, it did not have access to the Au and Cu. As a consequence, our study suggests that the Wenyu pluton was not prospective for magmatic-hydrothermal Au ± Cu deposits.

Electron back-scattered diffraction and LA-ICP-MS analysis of pyrite from the Dahu lodegold deposit, southern North China craton: Insights into geochemistry and distribution of trace element connection to microstructure of pyrite

The Dahu lodegold deposit is situated in the Xiaoqinling district, hosted in the east-west-trending shear zones, and characterized by auriferous quartz vein-type. Based on the morphology and paragenetic assemblage, three generations of pyrite (termed as first-generation pyrite (Py1), Py2, and Py3) can be classified. The Py2 can be further identified as brittle fractured pyrite (Py2-B) and ductile deformed pyrite (Py2-D). The different generations of pyrites feature distinct contents and distribution patterns of Au and other trace elements. The coarse-grained, euhedral Py1, disseminating in the early formed pyrite-poor quartz veins, contained negligible to extremely low levels of Au and associated trace elements, probably indicating a slow growth rate from a hydrothermal fluid unsaturated with respect to Fe, S, and Au and other trace elements, such as Cu, Pb, Bi, and Te, during the early orestage. Py2, occurring as laminations in the auriferous quartz veins, consistently contained significant amounts of Au, Bi, and Te, with a strong positive correlation between these elements, probably indicating a rapid growth rate and concentrated precipitation process of pyrite due to a hydrothermal fluid supersaturated with respect to Fe, S, Au, and other trace elements during the pyrite-rich auriferous quartz veining. Py3, which precipitated in the polymetallic sulfide orestage, contained a small amounts of Ag and a dramatically decreased content of Au, Bi, Te, and ratios of Bi/Ag. The results of electron back-scattered diffraction analysis on the Py2 revealed that Py2-B was characterized by predominantly brittle deformation with a limited local plastic deformation by dislocation glide on a simple slide system, whereas the Py2-D displayed a significant crystal plastic deformation through a lattice rotation on two separate 〈1 0 0〉 axes and local dislocation creep. Microscopic petrography and laser ablation inductively coupled plasma mass spectrometry analyses revealed that the brittle fracturing-dominated Py2-B generally displayed homogenous distribution patterns of Au and other trace elements and scarce visible gold grains, whereas the plastically deformed Py2-D generally exhibited an inhomogeneous distribution pattern of trace element and commonly contained fine-grained irregular-shaped visible native gold and other sulfide inclusions in the low-angle boundaries and dislocation sites within Py2-D, possibly suggesting the significant influence of ductile deformation on remobilization and reprecipitation of Au and associated trace elements once locked in Py2. The corrosion of Au-Te-Bi-bearing Py2 by the late polymetallic sulfide assemblage led to a liberation of invisible Au and other trace elements and reprecipitation of visible native gold and other Bi–Pb minerals. The results indicate that brittle deformation exhibited no essential effects on the crystal structure and chemical component of pyrite, whereas the plastic deformation can cause the readjustment of crystal lattices and liberation of impurities (e.g., Au, Te, Bi, and Cu) from the crystal lattices of pyrite. The plastic deformation and dissolution–remobilization of pyrite are the important mechanisms responsible for the visible discrete gold grain formation in auriferous quartz veins. Macroscopically, a preferred constant E–W extending orientation and typical lamination structure of the auriferous quartz veins suggest a channeled fluid flowing along the closed and connected fractures under a syn-mineralization S–N oriented extensional tectonic regime related to the Xiaoqinling metamorphic core complex. Here, we report that the microstructures of ductile deformation and brittle fracturing of the auriferous quartz veins are probably indicative of the continuous shearing activation of the ore-hosting normal fault.

Magnetite as an indicator of granite fertility and gold mineralization: A case study from the Xiaoqinling gold province, North China Craton

The Xiaoqinling district in the southern margin of the North China Craton is the second largest gold enriched area in China. The Huashan and Wenyu plutons represent two typical Mesozoic magmatic intrusions in this region and were coeval with the gold mineralization. Here we report the geochemistry and Mössbauer spectral characteristics of magnetite from the two plutons with a view to explore the role of the granitic magmatism in the gold enrichment. The results of EPMA and Mössbauer spectroscopy suggest that the average values of Fe2+/Fe3+ in the magnetite from the two intrusions are lower than the standard value of 0.50, indicating an oxidizing environment. The estimated oxygen fugacity is higher for the Wenyu pluton than that of the Huashan pluton, which is also supported by the occurrence of hematite in the former, and the distinct δEu-δCe anomalies for magnetite from the two intrusions. The Sn content in magnetite indicates lower formation temperatures of the Wenyu pluton as compared to that of the Huashan pluton. The results of LA-ICP-MS analysis demonstrate similar multi-element variation patterns for the two intrusions. Titanium vs. Ni/Cr relationship suggests that the magnetite from the Huashan pluton is of typical magmatic origin, whereas that from the Wenyu pluton shows both magmatic and hydrothermal signature. The Ni/Co ratio of magnetite from the Wenyu granite is higher than that of the Huashan granite, which indicates a deeper source for the Wenyu pluton. The comparatively more favorable physico-chemical conditions, such as higher oxygen fugacity, lower temperature, as well as the involvement of deep-sourced material are envisaged to have provided better metallogenic environment for generating the gold mineralization around the Wenyu pluton.

Discrete mineralization events at the Hongtuling Au-(Mo) vein deposit in the Xiaoqinling district, southern North China Craton: Evidence from monazite U-Pb and molybdenite Re-Os dating

The Xiaoqinling district, the second largest gold producer in China, is situated on the southern margin of the North China Craton and immediately to the north of the Qinling Orogen. The timing and genesis of the gold deposits have been hotly debated because the area has undergone multiple tectono-magmatic events since the stabilization of the North China Craton in the early Paleoproterozoic. Whether they are genetically related to the Triassic orogeny that formed the Qinling Orogen or associated with the early Cretaceous tectonic reactivation of the craton remains controversial. Here we present monazite U-Pb and molybdenite Re-Os dating results from the Hongtuling vein deposit in the eastern part of the Xiaoqinling district to provide new insights into the age and tectonic setting of gold mineralization. The Hongtuling deposit consists of gold veins at the top and minor molybdenum veins at the bottom, with molybdenum veins being locally crosscut by gold veins. Molybdenum veins mainly consist of calcite, quartz, K-feldspar, molybdenite, and pyrite with minor amounts of galena, and a variety of accessory minerals. Gold veins are dominated by quartz and pyrite, with minor to trace amounts of chalcopyrite, galena, sphalerite, and sericite. Monazite grains are well developed both in molybdenum and gold veins, and are intergrown with molybdenite and pyrite, respectively. Monazites from gold vein contain lower REEs contents, more prominent negative Eu anomalies, and higher Th/U ratios than those from molybdenum vein. The textural and geochemical evidence indicates that monazites from both veins precipitated from contrasting ore-forming fluids. Laser ablation ICP-MS U-Pb dating of monazite from the molybdenum vein yields a 206Pb/238U age of 203.5 ± 8.1 Ma (MSWD = 0.23, 2σ), which is statistically indistinguishable from the molybdenite Re-Os age of 204 Ma. In contrast, monazite from the gold vein has a significantly younger 206Pb/238U age of 130.4 ± 5.3 Ma (MSWD = 0.99, 2σ). Results presented here, combined with independent studies, suggest that the molybdenum and gold veins of the Hongtuling deposit, and by inference the other deposits in the district, formed in two discrete mineralization events in the late Triassic and early Cretaceous. The molybdenum mineralization is most likely related to the post-collisional magmatism associated with the Qinling orogeny, whereas gold mineralization occurred in an extensional setting associated with the decratonization of the North China Craton.

Re‐Os Age of Molybdenite from the Checangyu Molybdenum Deposit in the Xiaoqinling District and Its Implication for Metallogeny

Re‐Os Age of Molybdenite from the Checangyu Molybdenum Deposit in the Xiaoqinling District and Its Implication for Metallogeny

Uprising period and elevation of the Wenyu granitic pluton in the Xiaoqinling District, Central China

Xiaoqinling District is an important gold-producing area in China. It ranks second to Jiaodong with regard to gold deposits. The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit, as well as the mineralizing depth and reserved place of gold ore bodies, are significant to ore exploration. Fission-track (FT) analysis of zircons and apatites of granitic rocks from the Wenyu granitic pluton shows that apatite FT (AFT) data modeling indicates a rapid cooling rate of 20°C/Ma from 138 to 120 Ma after emplacement at 138 Ma. Thermal evolution and inversion curves suggest a secondary phase of fast cooling and uprising from 45 to 35 Ma, and 35 Ma, respectively, with a cooling rate of 6.7°C/Ma and a denudation quantity of ∼4.3 km. The last cooling phase took place from <4 Ma, with an average cooling rate of ∼11.3°C/Ma and a denudation amount of 1.3 km. Total exhumation quantity of 5.6 km and uprising elevation of 7.3 km are similar to the estimated results of fluid inclusions from the Dongtongyu and Wenyu gold deposits. The 39Ar/40Ar dating of sericite from the fault planes of the Xunmadao-Xiaohe and Taiyao faults demonstrate two uprising activities of the ore-host metamorphic complex. The Huashan and Wenyu granitic plutons intensively occurred during 77 and 45 Ma, respectively. These data sets are valuable for understanding the uplifting process and for preserving gold ore bodies in the Xiaoqinling area, as well as for further studies on tectonic evolutions of the Taihua Complex and the Qinling-Dabie Orogen.

U–Pb isotope and trace element analysis of columbite-(Mn) and zircon by laser ablation ICP–MS: Implications for geochronology of pegmatite and associated ore deposits

U–Pb isotopes and trace elements of columbite-(Mn) and zircon from an early Cretaceous pegmatite dike in the Xiaoqinling district, North China Craton, were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to better understand the suitability of these minerals for U–Pb geochronology when they contain unusually high U and Th. Columbite-(Mn) grains have high W, Ti, U, Th, and REE contents and yield concordant U–Pb age of 143±1Ma (2σ, n=10) that is interpreted as the emplacement age of the pegmatite dike. In contrast, zircon grains from the same dike show three distinct U–Pb age populations. Nine out of seventeen zircon grains analyzed have textural features typical of magmatic zircons and yield a weighted mean 206Pb/238U age of 143±1Ma (2σ, n=9), identical to that of columbite-(Mn) and thus constrain the timing of pegmatitic magmatism. The second population of zircon is characterized by corroded and zoned textures with the geochemical affinities of magmatic zircon. These zircon grains have a weighted mean 207Pb/206Pb age of 1879±19Ma (2σ, n=5) and are considered to be inherited components derived from Paleoproterozoic basement rocks that are widely distributed in the Xiaoqinling district. A third zircon population is characterized by high porosity and abundant Th–U-rich mineral inclusions (e.g. thorite, uranium oxides), and yield a younger U–Pb age of 127±3Ma (2σ, n=3). These younger zircon grains have elevated Hf, Ca, P, Nb, Ta, and Ti contents and much higher Th/U, LREE/MREE, and LREE/HREE ratios compared to the 143Ma zircon. The textural and geochemical data for the 127Ma zircon grains indicate that they were formed by the hydrothermal alteration of precursor zircon crystallized from the pegmatitic magmas, presumably associated with pervasive hydrothermal flow that led to the formation of numerous early Cretaceous gold deposits in the Xiaoqinling district. Results from this study demonstrate that columbite-(Mn) from the pegmatite dike in the Xiaoqinling district has been resistant to post-magmatic hydrothermal alteration that, however, have disturbed the U–Pb isotopes in some zircon grains from the same dike. Consequently, we propose that columbite-(Mn) is an ideal target for U–Pb dating of pegmatite and associated ore deposits.

Giant Mesozoic gold provinces related to the destruction of the North China craton

Lode gold deposits in Precambrian cratons represent the world's major gold source and were mostly generated during formation and stabilization of the cratons. However, there is an extraordinary exception in the North China craton (NCC), where lode gold deposits formed after prolonged stabilization of the craton. Molybdenite Re–Os and hydrothermal sericite and biotite 40Ar/39Ar dating of major gold deposits from the Xiaoqinling district, southern NCC, bracket their emplacement in the range of 154.1±1.1 to 118.9±1.2Ma (n=23), postdating formation of the craton by more than 1.7 billion years. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (1.52–0.22Ra) and mantle-like Ne isotopes (20Ne/22Ne=10.02−9.22 and 21Ne/22Ne=0.033−0.027), indicating presence of mantle-derived fluids in the ore system. Measured δ34S of pyrite and δD and δ18O of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic/mantle source for sulfur and ore fluids. Gold deposits of similar ages also widely occur in the eastern and northern margins of the NCC, which, together with those in the Xiaoqinling district, have a total reserve of ∼2500t gold, forming the only known giant late Mesozoic gold province in the world's Precambrian cratons. These deposits formed coevally with extensive felsic to mafic magmatism, development of intracontinental rift basins, and exhumation of metamorphic core complexes across the eastern NCC, events interpreted as indicating thinning and destruction of the lithosphere beneath the craton. Rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids, sulfur and, by inference, other ore components to form the giant gold provinces.

The Early Cretaceous Yangzhaiyu Lode Gold Deposit, North China Craton: A Link Between Craton Reactivation and Gold Veining

The Yangzhaiyu gold deposit is one of numerous lode gold deposits in the Xiaoqinling district, southern margin of the North China Craton. Gold mineralization is hosted in Neoarchean to early Paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein-proximal alteration zone. Ore-related hydrothermal alteration is dominated by sericite + quartz + sulfide assemblages close to gold veins, and biotite + quartz + pyrite ± chlorite ± epidote alteration generally distal from mineralization. Pyrite is the predominant sulfide mineral, locally coexisting with minor amounts of chalcopyrite, sphalerite, and galena. Gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with Au-bearing tellurides, mainly petzite and calaverite coexisting with hessite, tellurobismuthite, and altaite. Fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures (175°–313°C) from aqueous or aqueous-carbonic fluids with moderate salinity (5–14 wt % NaCl equiv). δ 34S values of sulfide minerals range mainly from 2.0 to 4.4‰, whereas auriferous quartz vein samples have δ 18O values of 12.4 to 9.6‰, with calculated δ 18OH2O values of 6.0 to 3.2‰. Gold-related pyrite grains yield elevated 3He/4He ratios (1.51-0.32 Ra) relative to crustal reservoirs and mantle-like 20Ne/22Ne and 21Ne/22Ne ratios (9.90-9.68 and 0.029, respectively). The stable and noble gas isotopes thus suggest deep-seated, most likely magmatic and mantle-derived, sources for the ore fluids, sulfur and, by inference, other components in the ore system. 40Ar/39Ar dating of ore-related sericite and biotite separates indicates two episodes of gold genesis at 134.5 to 132.3 and 124.3 to 123.7 Ma. The mineralization ages overlap zircon U-Pb ages of 141.0 ± 1.6 to 125.8 ± 1.4 Ma (2 σ ) for the Wenyu and Niangniangshan monzogranite Plutons and a number of mafic to intermediate dikes intruding these Plutons, all being proximal to the Yangzhaiyu gold deposit. The synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur. The geochronologic data also suggest that gold veining took place billions of years after the stabilization of the North China Craton and associated metamorphism in the Late Archean to early Paleoproterozoic. This contrasts sharply to lode gold deposits in other Precambrian cratons that formed predominantly in Late Archean to Paleoproterozoic, temporarily and genetically related to regional high-grade metamorphism and compressional or transpressional tectonism. Available data have demonstrated that the North China Craton was reactivated in the late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the craton. It is thus suggested that the Yangzhaiyu gold deposit, together with other deposits of similar ages in the Xiaoqinling district, were products of this craton reactivation event. Lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy, fluid, and sulfur required for the formation of the gold deposits.

LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from < 0.01 ppm to 2.2 ppm) but Ni concentrations are rather high (up to 8425 ppm). A four-stage mineralization process is indicated by microscopic and field observations and this can be related to the systematic trace element differences between distinct generations of pyrite. Stage I precedes the main gold mineralization stage; pyrite of this stage has the lowest Au concentrations. Stages II and III contributed most of the gold to the ore-forming system. The corresponding pyrite yielded the highest concentrations of Au and Ni. Our microscopic observations suggest that pyrite in the main gold mineralization stage precipitated simultaneously with molybdenite that has been previously dated as Indosinian (~ 218 Ma by Re–Os molybdenite dating), indicating the Indosinian as the main gold mineralization stage. The Indosinian mineralization age and the geological and geochemical features of these gold deposits (e.g., low salinity, CO 2-rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ 18O and δD data suggestive of mixing between metamorphic and meteoric waters; δ 34S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.

Gold distribution in As-deficient pyrite and telluride mineralogy of the Yangzhaiyu gold deposit, Xiaoqinling district, southern North China craton

The Mesozoic Yangzhaiyu lode gold deposit is situated in the southern edge of the North China craton. Gold mineralization is hosted in Archean amphibolite facies metamorphic rocks, and consists mainly of auriferous quartz veins. Pyrite is the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Based on morphology and paragenesis, there are three generations of pyrite, termed as first generation (G1), second generation (G2), and third generation (G3). They have distinct contents, occurrences, and distribution patterns of gold. The coarse-grained, euhedral G1 pyrite contains negligible to low levels of gold, whereas both invisible and visible gold are present in the fine- to medium-grained G2 pyrite that is characterized by abundance of microfractures and porosities, forming a foam-like texture. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold-bearing tellurides in the G2 pyrite. Visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite, likely resulting from remobilization of invisible gold once locked in the G2 pyrite. The G3 pyrite, invariably intergrown with chalcopyrite, sphalerite, and galena, contains the highest levels of invisible gold. There is a positive correlation between Au, Ag, and Te, indicating that gold occurs as submicroscopic Au-bearing telluride inclusions in the host minerals. Whenever gold, either invisible or visible, is present, As is always below or only marginally higher than the detection limit of LA-ICP-MS. This indicates that As played an insignificant role in gold mineralization. Tellurides are widespread in the auriferous quartz veins, consisting mainly of petzite, calaverite, hessite, altaite, and tellurobismuthite. Native gold commonly occurs as intergrowths with tellurides. Textural evidence indicates a precipitation sequence, in a temporal order, of calcaverite, petzite, altaite, tellurobismuthite, and hessite. Little amount of sulfide phases has been found in association with the tellurides, indicating that tellurides were deposited under low S fugacity (fS2) and/or high Te fugacity (fTe2) conditions. The textural relationships, when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data, permit estimation for logfTe2 during telluride formation, which are −6.8 to −10.8 at 300°C and −9.6 to −17.6 at 250°C. Available geochronological and geochemical data suggest that Te was most likely derived from the late Mesozoic magmatic rocks widespread in the Xiaoqinling district and other parts of the southern North China craton, which were emplaced broadly contemporaneous with gold mineralization at Yangzhaiyu. This study highlights the role of Te and tellurides as important gold scavengers in As-deficient ore fluids.

Hydrothermal Fluid Evolution Associated with Gold Mineralization at the Wenyu Mine, Xiaoqinling District, China

Abstract: The Wenyu mesothermal gold deposit is located in the Xiaoqinling district about 1000 km southwest of Beijing in central China. It occurs in the Late Archean to Early Proterozoic metamorphosed volcanic and sedimentary rocks. Three distinct stages of veins have been identified: (I) gold‐poor quartz–pyrite veins, (II) gold‐rich sulfide–quartz veins, and (III) gold‐poor carbonate–quartz veins. Stage II can be subdivided into IIa and IIb. Gold typically occurs as fracture‐fillings associated with chalcopyrite and galena.Fluid inclusions were examined in quartz samples from veins of both stage I and II. Three types of fluid inclusions are identified: CO2–H2O, CO2–rich, and aqueous inclusions. The first two types are of primary in origin. The last type occurs in two ways: coexisting with CO2–H2O and CO2–rich inclusions and thus primary in origin; and occurring along late healed fractures and hence secondary in origin. CO2–H2O inclusions display progressively decreasing Th and increasing Thco2, from the highest Th (311–408C) and lowest Thco2 (average 18C) in stage I quartz through middle Th (284–358C) and ThCO2(average 25C) in stage IIa quartz to the lowest Th (275–314C) and highest ThCO2 (average 28C) in stage IIb quartz, indicating an evolving H2O–CO2–NaCl fluid system. CO2–rich and primary aqueous inclusions show consistent ThCO2 or Th with their coexistent CO2–H2O inclusions. Whereas the secondary aqueous inclusions in stage I and IIa quartz have almost the same Th and salinity as the primary aqueous inclusions in stage IIb quartz. Comparing with CO2–H2O inclusions, these non–CO2, low salinity aqueous inclusions may come from different origin, most probably meteoric water.Unlike in both stage I and IIa quartz, fluid inclusions in stage IIb do not show evidence of fluid immiscibility. The fact that most of gold is associated with stage IIa and IIb veins and not with veins of stage I which is the main stage of vein formation suggests that gold deposition occurs at the later stage of fluid immiscibility. The continuing phase separation led to the deposition of large amounts of gold at the Wenyu mine.

Geology and Genesis of Orogenic Gold Deposits, Xiaoqinling District, Southeastern China

Orogenic gold vein deposits in the Xiaoqinling district are situated in a basement-cored uplift along the southern margin of the North China craton. The deposits are hosted by Late Archean to Paleoproterozoic amphibolite-facies country rocks, varying in lithology from clastic and chemical sedimentary units to felsic and mafic volcanic rocks and plutons. Absolute and relative age relationships indicate a late Mesozoic emplacement of the lodes, which is subsequent to the deformation associated with the Middle to Late Triassic Qinling orogen along the craton margin. All auriferous quartz veins are hosted in faults. The ores are generally composed of quartz veins with various amounts of pyrite, galena, chalcopyrite, sphalerite, and carbonates. Mineralization can be separated into three distinct stages. Ore fluids are H2O dominant, with approximately 20 to 30 and 10 to 20 mole% CO2 for gold-bearing stage I and II veins, respectively. Vein formation pressures and temperatures were 2.2 kbar and 300 to 370°C for stage I and 1.6 kbar and 250 to 320°C for stage II. The narrow range of δ18O values for ore fluids from the deposits throughout the Xiaoqinling district indicate a common deep fluid source, most likely of magmatic origin. The 34S data suggest sulfur originated from a magmatic fluid of approximately 2 ± 2%, with significant local sulfur contributions leading to large variations in the ore-stage sulfide minerals. The most probable mechanism for the deposition of gold is phase separation caused by the partial loss of volatiles such as CO2 and CH4.

Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins.