Mineralogy and isotope geochemical characteristics for Xiaozhen copper deposit, Langao County, Shaanxi Province and their constraint on genesis of the deposit

The Xiaozhen copper deposit is a typical deposit in the North Daba Mountain area polymetallic metallogenic belt. With the exception of geological prospecting research, however, few studies have been reported on the genesis of the deposit. This paper focuses on the mineralogical and isotope geochemical characteristics for the Xiaozhen copper deposit. Detailed field-work and microscope observations suggest that the ore's structural fabric and mineral assemblages feature a medium-low-temperature hydrothermal pattern of metallogeny. The results of electron microprobe analysis indicate that chalcopyrite atomic ratio aMe/S = similar to 0.95-1.03, which suggests that they may not be of sedimentary origin. In addition, the S content of the pyrite is from 50.57 wt% to 53.16 wt% that is less than standard the value of 53.45 wt%, the Fe/S value is from 0.878 to 0.955, and the Ni content in pyrite is high even up to 0.38 wt%. These results, combined with Ni-Co graphic projection, indicate that the pyrite is of magmatic hydrothermal origin. Moreover, most of the sphalerites contain Se, and their calculated w(FeS) content ranging from 4.60 wt% to 10.37 wt% suggest magmatic origin and a formation temperature of approximately 300 A degrees C. Further, the content of chi(Fe) is from 46.55 wt% to 46.97 wt% for pyrrhotites, suggesting they are clino-pyrrhotites. This result, combined with mineral assemblages and the Fe-S system diagram, indicates a mineralization temperature of approximately 310 A degrees C. The delta S-34 values of the sulfide range from 2.6aEuro degrees to 11.5aEuro degrees, indicating multisource properties. The Pb-208/Pb-204 has wider ratios (similar to 38.897-39.918), reflecting that they contain abundant radiogenic lead. The Pb isotope tectonic setting and genetic classification plot show that the ore lead was provided mainly by the upper crust in addition to a few magma materials. The analytical results of fluid inclusions in the chalcopyrite show that the 3He/4He ratios are similar to 0.268-0.326 R/Ra, which is higher than the crustal ratios but grossly lower than that of the mantle fluid and that the Ar-40/Ar-36 ratios are higher than the argon isotopic composition of the atmosphere. Moreover, the He-Ar isotope system suggests that the source of the ore-forming fluids is predominantly meteoric low-temperature underground water, with a small amount of mantle fluid. In summary, this study has determined that the Xiaozhen copper deposit is a medium-temperature magmatic hydrothermal deposit. This finding is combined with metallogenic age to briefly describe the mineralization process.