Si-O Isotopic Characteristics and Genesis of the Siliceous Rocks from the Daqiao Gold Deposit in West Qinling

The Daqiao gold deposit is situated between the Minxian-Dangchang and Zhouqu-Chengxian regional faults in west Qinling.The ore-body is hosted within the siliceous breccia at the lithological unconformity interface between limestone and argillaceous slate in the lower part of Triassic strata.The breccia shows layered, tabular and lenticular shapes, occurring in the both side of the fault.The ore structures include primarily breccia, quilt-membrane, crusty, vein-stockwork and pore.The ore minerals are mainly pyrite, binarite, pyrrhotite, arsenopyrite, sphalerite, galena and chalcopyrite.The alterations of country rocks are composed of silicification, sericitization and calcilization.The element associations related to the mineralization are Au, Sb, Ag, As, Hg and W.By the oxygen isotope thermometer, the estimated formation temperatures of these siliceous rocks are around 142-218℃, their δ³⁰Si and δ¹⁸O values are -0.7‰-0.4‰ and 12.8‰-17.9‰, respectively, which are similar to the ones of the hot spring gold deposits.Combined with other geological characteristics, it's believed that the siliceous rocks are formed by rock fracturing due to fluid overpressure under the hot-spring environment nearby the periodic fault zone.