AN Qi1, ZHOU Jia-xi2, 3 *, XU Lei1, YANG Xing-yu1, REN Hou-zhou1, LU Mao-da1
(1. No. 104 Geological Team, Bureau of Geology and Mineral Exploration and Development of Guizhou Province, Duyun 558000, China;
2. School of Resource Environment and Earth Science, Yunnan University, Kunming 650504, China;
3. Stake Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China)
Abstract: The discovery of the Maozhachang (Maomaochang-Zhazichang) Pb-Zn deposit is a successful case of deep ore prospecting in NW Guizhou in recent years. The Maozhachang deposit, which is adjacent to the Fulaichang, Tianqiao and Banbanqiao deposits, in Hezhang County, Guizhou Province, is located in the central-eastern part of the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province in the southwestern Yangtze block. Surface placers and oxidized ores were mined in this deposit in the past years. The newly discovered sulfide ore bodies occurred in Carboniferous carbonate rocks in stratiform, lenticular or veining with obvious strata-bound features. In this work, based on the systematic mineralogical observation, the in situ Pb isotopic geochemistry of galena by using femtosecond laser ablation multi receiver plasma mass spectrometer (fs LA-MC-ICP-MS) has been studied. The results show that in-situ Pb isotopic compositions of galena are varied within a narrow range, with 206Pb/204Pb ratios varying from 18.755 to 17.775, 207Pb/204Pb ratios varying 15.780 to 15.799, and 208Pb/204Pb ratios varying from 39.413 to 39.477, respectively, indicating a single source of ore metal or a highly homogenized mixture of ore-forming fluids. By comparing with the Pb isotopic ratios of possible source rocks of the Pb-Zn mineralization, we found that the Pb metal in the Maozhachang Deposit could be mainly sourced from both the basement metamorphic rocks and the cover sedimentary rocks, with the contribution of sedimentary rocks higher than that of the basement rocks. In addition, the comparison among Pb isotopic ratios of sulfides of the Maozhachang and Fulaichang, Tianqiao and Banbanqiao deposits shows that the Pb isotopic ratios are subsequent gradually decreased from the Maozhachang to Banbanqiao, suggesting that with the evolution or transport of ore-forming fluids, the contribution proportion of high Pb isotopic ratios’ sediments to the fluids was gradually decreased. Since the basement rocks are rich in ore-forming metals, the degree of their contribution could be positively related to the size of the deposit. Therefore, it is believed that there is still great prospects for prospecting Pb-Zn resources in depth of the Maozhachang deposit and in Maozhachang-Banbanqiao area.
Keywords: fs LA-MC-ICPMS in-situ Pb isotopes; source of ore-forming elements; evolution of hydrothermal fluids; the Maozhachang Pb-Zn deposit; the NW Guizhou Pb-Zn metallogenic district
