Geochemistry Of Ore-forming Processes Of The Xiaoqinling Gold District, Henan Province

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, i.e. the northern ore belt, the middle ore belt and the southern ore belt. Five gold deposits, i.e., the Dahu and Linghu deposits in the northern ore belt and Wenyu, Qiangma and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy, fluid inclusion study, trace element analyses in pyrite, and systematic multi-isotopes (H-O-C-Pb-S) tracing. For the two Yanshanian plutons, we report a systematical zircon U-Pb dating, petrogeochemistry, and Sr-Nd-Hf isotope data in order to constrain their origins. They are of considerable geologic interest, because of their large geodynamic context and likely relationship to Mesozoic lithospheric delamination, but also because they are associated with one of the most important gold districts in China.Wenyu and the Niangniangshan plutons are the main plutonic rocks in the Xiaoqinling district. Both plutons are weakly metaluminous to weakly peraluminous, and can be classified as I-type granites with similar zircon U-Pb ages of 131±1 Ma and 134±1 Ma. They are enriched in Si, K, Ba, and Sr, while depleted in Nb, Ta, P, Ti, Mg, Ni, V and Y relative to primitive mantle, and both have Archaean-type tonalite-trondhjemite-granodiorite (TTG) and adakitic affinity, e.g. enrichment in LREE with high Sr/Y and La/Yb ratios and without Eu anomalies. Both the plutons show relatively high initial 87Sr/86Sr ratios (0.7076～0.7089 for the Wenyu pluton and 0.7074～0.7086 for the Niangniangshan pluton), negativeεNd(t) values (-15.0～-10.1 for the Wenyu pluton and-18.5～-12.2 for the Niangniangshan pluton), and negativeεHf(t) values (-21.9～ -10.4 for the Wenyu pluton and-30.8～-20.8 for the Niangniangshan pluton). All the isotopic compositions indicate old crustal sources for the plutons. Furthermore, low concentrations of MgO, Ni, and V also indicate little or no mantle materials in the source. The local Neoarchaean to Palaeoproterozoic basement rocks are the most likely sources, but their isotopic compositions do not cover the total isotopic range of the plutons, hence other sources may be involved. The occurrence of 1.78 Ga inherited zircon grains on the depleted mantle Hf isotopic evolution line indicates 1.78 Ga underplating mafic rocks to be another possible source. Detailed petrologic and geochemical data indicate that the two plutons derived from partial melting of mixtures of Neoarchaean to Palaeoproterozoic metamorphic rocks and 1.78 Ga mafic lower-crustal rocks at a depth of at least 50 km, with eclogite, garnet amphibolite or amphibolite residues. During the magma crystallization stage, amphibole, apatite and titanite fractionation caused the observed variations of the element compositions. Meanwhile, the existence of 50-km-depth crust indicates that large-scale lithospheric thinning in the North China Craton did not take place prior to the emplacement of these granitoids 130 Ma ago.The Wenyu gold deposit, Henan province, is located in the southern part of the Xiaoqinling gold belt. It is controlled by a brittle-ductile shear zone and hosted in the Taihua group metamorphic complex. The crosscutting relationships and mineral assemblages suggest that the ore forming process includes the early, middle and late stages. Three types of fluid inclusions (Fls) in the quartz of Wenyu gold deposit are identified as CO2-H2O, pure CO2. and H2O-solution. The early-stage quartz contains primary pure CO2 and CO2-H2O with homogenization temperatures clustering at 290～330℃and the salinity of 1.0～9.6 wt% NaCl equiv.. Vapor bubbles are composed of CO2+H2O±N2±CH4 The middle-stage quartz trapped all the three types of FIs. The CO2-H2O FIs is the majority one, and the homogenization temperatures of CO2-H2O and the H2O-solution FIs cluster at 250～290℃, and the salinities are 0.0～12.8wt% NaCl equiv.. Fluid inclusions in late-stage quartz are H2O-solution with low homogenization temperatures (114～239℃) and low salinities (4.2～9.0 wt NaCl equiv.). Minimum trapping pressures estimated from CO2-H2O inclusions are 130～178 MPa and 85～150 MPa for early and middle stages, respectively, corresponding to the ore-forming depth of 4.7～6.5 km and 3.1～5.5 km, respectively. In general, the ore fluids in the early and middle stages are characterized by meso-to hypothermal, CO2-rich and low salinity, which are considered as the nature of metamorphic fluids, whereas the late-stage fluids are low temperature, low salinity and CO2-poor. Fluid immiscibility caused gold precipitation in the middle-stage. Theδ18O data of the fluid range between 4.3%～5.5%,δD -63‰to -47%o. In theδ18O-δD diagram, data of the early stage plot in the metamorphic box, while the data of middle and late stages evolve to the meteoric line, suggesting mixing of metamorphic and meteoric waters. The ore-forming fluid system of the Wenyu gold deposit is the same to that of the orogenic-type. The Wenyu gold deposit is a mesozonal to epizonal orogenic gold deposit. The Wenyu data are comparable with other deposits studied in the preliminary literatures.Pyrite from the Dahu and Yinxin in the northern ore belt and Wenyu gold deposits in the southern ore belt was investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectroscopy (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 age. The Indosinian mineralization age and the geological and geochemical features of these gold deposits 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 in our study 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. Furthermore, the Ni concentrations (4.5～76.1 ppm with the average of 17.4 ppm) in pyrite from the Checangyu Mo deposit which is of granitic hydrothermal origin are much lower than those in pyrite from gold deposits, which is also a good indicator against the granite-related model. Generally low Au concentrations in pyrite are also consistent with those from worldwide orogenic gold deposits.The Pb isotopic compositons of sulfides (206Pb/204Pb 16.9831～17.8070,207Pb/204Pb 15.3968～15.5563 and 208Pb/204Pb 37.4047～38.5123) are close to the data for the host Taihua metamorphic rocks, but totally different from the Yanshanian granites, indicating Pb likely sourced from the Taihua Group. Theδ34S values in the Xiaoqinling district show a large variation (δ13Sv-CDT-13.6～7.5‰), which is similar to that of the Taihua Group metamorphic rocks (δ13SV-CDT-5.4～10.5‰). Theδ34S values of Yanshanian granitic rocks are concentrated in a small range (δ13SV-CDT 1.4～4.3‰) and are not responsible for the S source of the gold ores. The C isotopic compositions of the Fe-dolomite (δ13CV-PDB-3.4～-6.7%o, with the average of-5.1‰) indicate a deep source of carbon, and are consistent with typical orogenic gold deposits. Therefore we speculate Au was also sourced from the host Taihua group.In conclusion, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is related to Indosinian collision between the North China Craton and the Yangtze Craton, the ore forming fluid is metamorphic water and the ore forming materials are all from Taihua Group. During Yanshanian lithospheric thinning process, the gold might be remobilized.