Research On Metallogenic Process Of The Zhaishang Gold Depsit In Southern Gansu, China

The Zhaishang gold deposit is one important large gold deposit which was found in the West Qinling area. It is located in the western region of Li (County)-Min (County) gold metallogenic belt in the West Qinling, and in the South-Qinling-block between the Shangzhou-Danfeng fault and Mian County-Lveyang fault. The Zhaishang gold deposit is divided into the south ore belt and north ore belt, and each ore body is distributed parallel to the strike of NWW-SEE. The research showed that the mineral assemblage and ore fabric of the two ore belts are similar, which indicated that the south and north ore belts were developed in the same geochemical field. The argentiferous native gold which is found for the first time in the Zhaishang gold deposit is developed closely with telluride, and the chemical analyses released that the elements of gold and tellurium of each ore belt all have better correlate.on, which showed that the gold and telluride complex compounds play important roles in the transportation and precipitation processed of the gold in the Zhaishang gold deposit. A few of intermetallic compounds developed in the Zhaishang gold deposit, and the Cu-Ni-Zn-Sn-Fe intermetallic compounds and the S-bearing alloys are reported for the first time at home and abroad. The appearance of polymetal intermetallic compounds might reflect the environment of rich metals, low sulfur fugacity and low oxygen fugacity, which show that anatectic materials participated the mineralization. With the methods of microthermometry, micro laser raman spectroscopy and GC-MS, it can be concluded that the mineralizing fluid of Zhaishang gold deposit is the system of Ca2+-Na+-Cl--SO42, and the main gas composition is H2O, CO2, N2 and CH4; the temperature of mineralizing fluid is between 93 and 349℃, which is mid-low temperature hydrothermal; the hydrothermal pulse appears obviously in each mineralization period; the fluid salinity is 0.18 to 23.11% and the density is 0.71 to 1.03g/mL; according to the relationship of temperature, salinity and density, the mineralizing fluid in Zhaishang can be divided into three types, and the third one is more relationship with the mineralization. Theδ18OH2O-SMOW of the water for the mineralizing fluid is from +3.43‰to +13.59‰,with average is 7.59‰,and theδDH2O-SMOW is from-73‰to-99‰, and the average is -85.5‰. From the characteristics of the oxyhydrogen isotope evolvement, in the first and second periods to the third period, atmospheric water gradually entered the mineralizing fluid; the mineralizing fluid is influenced by the magmatic fluid. The values ofδ13CPDB are between-2.08‰and +0.85‰,and the average is-0.51‰; the data is concentrated and the carbon isotope ranges not much, which show that the sources of carbon are unified, from the host rocks carbonate strata. The values ofδ34S from sulfur minerals range from +0.2‰to +32.37‰, reflecting the source characteristics of sulfur in marine strata. The characteristics of Lead isotop show that minerals in each vein originated form the same source, which maybe fixed with sedimentary and magmatic lead in Paleozoic. The Pb-Pb dating shows that the ages of sources are between 392.0Ma and 481.0Ma. That is to say, the sources of mineralizing materials are Devonian formation and its underling formation. In summary, the magmatic activity of Caledonian- Hercynian in Paleozoic supply rich materials for the Zhaishang gold deposit, and the frequent structure-hydrothermal activities in the later periods made the mineralizing materials further activate and concentrate to form the Zhaishang gold deposit.