Geology, isotope geochemistry, and ore genesis of the Yindonggou Ag-Au(-Pb-Zn) deposit, Hubei Province, China

摘要

The Yindonggou Ag-Au(-Pb-Zn) deposit is hosted by metamorphosed volcanic rocks of the ca. 740-760Ma Wudangshan Group in the Proterozoic Wudang Block of the southern part of the Qinling Orogen, central China. The deposit consists of a series of mineralized quartz veins located in the Yindongyan Anticline. Based on the mineral assemblages and cross-cutting relationships of quartz veins, the deposit can be divided into:(1) early fine-grained quartz-sphalerite-galena veins; (2) fine-grained quartz-silver-gold veins containing minor amounts of pyrite; (3) coarse-grained quartz veins with minor amounts of galena, sphalerite, and chalcopyrite; and (4) late ankerite-quartz veins. Most of the Pb-Zn mineralization formed during the early (Stage 1) veins followed by the deposition of Ag-Au mineralization in the Stage 2 veins. The δ~(18)O value for the ore-forming fluids decreases from 6.6-9.4‰ in the Stage 1 veins through 3.6-4.9‰ in the Stage 2 veins to -1.2‰ to 0.4‰in the Stage 3 veins (the δ~(18)O values could not be determined for the Stage 4 veins). Furthermore, the δD values are -74‰for the Stage 1 veins, -95‰ to -56 ‰for the Stage 2 veins, and -48‰ to -73‰ for the Stage 3 veins. The δ~(13)C values for ankerite in the Stage 4 veins are between -2.9‰and -1.1‰. The δD vs. δ~(18)O_(H_2O) plot for these values indicates that there was a shift from metamorphic fluids during the formation of the early veins to meteoric fluids during the formation of the later veins at the deposit. The H-O-C isotope systematics also indicate that the ore fluids forming the deposit were probably initially sourced from metamorphic dehydration of volcanic-carbonate rocks in the ca. 740-760Ma Wudangshan Group and with time gradually mixed with meteoric water by Stage 4. The δ~(34)S values for sulphides from the deposit range from -0.9‰ to 7.1‰ in the Stage 1 veins, 3.8‰ to 5.0‰ in the Stage 2 veins, and 2.4‰ to 11.3‰ in the wallrocks. Sulphides from the mineralized Stage 1 veins yield ~(206)Pb/~(204)Pb ratios of 16.44-16.6, ~(207)Pb/~(204)Pb ratios of 15.25-15.5, and ~(208)Pb/~(204)Pb ratios of 36.4-36.98. Five pyrite samples from the Stage 2 veins yield ~(206)Pb/~(204)Pb ratios of 16.475-16.529, ~(207)Pb/~(204)Pb ratios of 15.346-15.395, and ~(208)Pb/~(204)Pb ratios of 36.49-36.616. Both the S and Pb isotope ratios are between the ratios for units in the Wudangshan Group and mantle but differ from other lithological units in the Wudang Block, which suggest that the mineralized fluids interacted with both the Wudangshan Group and deep-seated sources. Thus, we suggest that the original ore-forming fluids are metamorphic in origin, and the metal deposition resulted from fluid mixing. From the characteristics observed, the Yindonggou Ag-Au(-Pb-Zn) deposit can be classified as an orogenictype deposit generated during the Triassic Qinling Orogeny resulting from northward oceanic plate subduction along the Mian-Lue Suture.