Studies On Geological And Geochemical Characteristics And Genesis Of The Qixiashan Pb-Zn Deposit, Jiangsu Province, China

The Qixiashan Pb-Zn deposit is situated in Qixia district, Nanjing City, Jiangsu Province. It is mainly composed of Mian Huadi, Ping Shantou, Hu Zhuashan, Bei Xiangshan, Gan Jiaxiang, Xi Ku and other ore blocks from east to west. The author carried out a detailed study of deep part about Hu Zhuashan ore block. Based on studying and analyzing the previous results and having finely field geological work, through mineralogy, fluid inclusions and isotope geochemical characteristics in depth study, the author discussed the metallogenetic mechanism combined with regional tectonic evolution background to provide more evidence about genesis and deep prospecting of the Qixiashan Pb-Zn deposit.The ore body is mainly controlled by the Silico-calcium lithologic interface, vertical fault, unconformity surface, the ancient karst and concealed intrusion. There are two kinds of Silico-calcium lithologic interface. One is composed by fault wall of F2 and another one by unconformity surface. The ore body is primarily present at the calcium-rich part of the Silico-calcium lithologic interface. Ore minerals include sphalerite, pyrite, galena. Gangue minerals include calcite, quartz, dolomite. The main structures include breccia structure, disseminated structure, massive structure, vein structure, reticular and vein structure, streaked structure, crumb structure. Ore textures include subhedral granular texture, metasomatic relict texture, skeleton crystal texture, emulsion texture, poikilitic texture, reticular texture, crumpled texture, cataclastic texture. Through field work and the microscopic identification, the ore-forming processes of the Hu Zhuashan ore block can be divided into four stages, which are characterized by the pyrite-quartz assemblage, the galena-sphalerite assemblage, the galena-sphalerite-pyrite assemblage, the carbonate-barite-gold(silver) assemblage.Studies of electron-probe micro-analysis(EPMA) have shown that the pyrite, sphalerite ore minerals of Hu Zhuashan ore block are mainly magmatic hydrothermal genesis. Studies of fluid inclusions have shown that the deposit is dominated by vapor-liquid two phase inclusions. The homogenization temperature is 125~294°C while the salinity ω(NaCleqv) is 1.06%~7.59% and the density is 0.84~0.96g/cm3. Laser Raman spectroscopy indicates that the principal gas and liquid components of the fluid inclusions is H2 O and a little CO2. So the ore-forming fluid is a type of low-temperature, low-salinity, low-density H2O-CO2-NaCl system. Studies of hydrogen and oxygen isotope indicates that the ore-forming fluid is a kind of complicated fluid which is derived from magmatic water and is mixed by meteoric water, hot brine, organic water. The ore-forming fluid is dominated by magmatic water in the early stage of mineralization. The proportion of meteoric water is increased which makes the salinity lower constantly along with the mineralization. Combined with sulfur and lead isotope of the previous results, studies have shown that the metallogenic materials of the Qixiashan deposit mainly come from the the mantle and deep crust. It means the metallogenic materials of the Qixiashan deposit mainly come from concealed intrusion and Sinian system strata. From what has been discussed above, the Qixiashan Pb-Zn deposit is a kind of epithermal-mesothermal deposit related to magmatic hydrothermalism fluid.