Geological And Geochemical Characteristics And Genesis Of The Daqiao Gold Deposit In Gansu Province

The Daqiao gold deposit is located between two faults: the Minxian-Dangchang-Liangdang and Zhouqu-Chengxian. The ores mainly exist in the lower lithologic layer of the lower part of Triassic(Ta-1), which is composed of metamorphic quartz sandstone, metamorphic siltstone and clastic breccias. The ore bodys are mainly of bedded, lenticular and saddle shaped. The ore minerals are pyrite, marcasite and arsenopyrite, the main gangue mineral is quartz, and the main wall rock alterations are silicification and pyritization. Basing on the research on geological characteristics of the deposit, this thesis focus on the rocks(ores) on the characteristics of major elements, trace elements, rare earth elements, sulfur isotope, diagenetic fluids and age, and achieves the following understandings:(1) According to the research on breccias geology and geochemical characteristics, we suggest that the silicified breccias are controlled by the faults around and the crash of rock caused by faulting. Silicified breccia has obvious characteristics of cement, acidic magmatic hydrothermal provides the source of SiO2, the silicified breccia is formed by silicification cementation.(2) Geochemical characteristics of the rocks(ores) indicates that stratum is of a gold-rich feature, and is the main source of ore forming materials; there is material exchang between the rock surrounded and the magma, which provides source of SiO2; Carbon in silicified breccia is in favor of gold enrichment.(3) The related research on sulfur isotope δ34S indicates that the sulfur mainly comes from the formation, and gets influenced by the magma with the characteristics of mixed sulfur. The study of fluid inclusions shows that the inclusions possess the characteristics of low-middle temperature,low salinity and shallow depth. The ore-forming fluids are of the NaCl-H2O-CO2 system, and laser Raman shows the gas phase component in the fluids is mainly of CO2 and a small amount of CH4 and H2 O.(4) The Zircon age data of the granodiorite indicates that the magma formed in 228 ~206Ma, as a part of the late Indosinian magmatic activities, and the correlation between the dikes and the mineralization is close., the dikes provided the heat source and SiO2 in the mineralization and played a role in the activation of gold and extracted some gold from the formation. The Daqiao gold deposit might be caused by the late Indosinian regional large scale metallogenesis of the West Qinling.(5) This thesis draws a conclusion considering the Daqiao gold deposit as one of sedimentary-reformation type. The terrigenous detritusearly produced by early weathering denudation makes initial enrichment of the gold. Gold from the formation is the main source of ore-forming materials of the deposit. Indosinian tectonism and magmatism make the activated and transformed gold in the dikes, then let it precipitates in favorable position of structure, and then forms the gold deposit.