Geochemical Characteristics and Their Geologic Significance of the Lower Jurassic Ridang Formation Host Strata from the Zhaxikang Sb-Pb-Zn-Ag Polymetallic Ore-concentrated District, South Tibet
LI Guan-qing1,2, CHENG Wen-bin3, ZHANG Yong-mei1, ZHANG Yan4, LV Peng-rui5, DAI Hong-zhang1, ZHANG Xing-guo2, XIA Bao-ben2
1. School of Earth Science and Resources, China University of Geoscience, Beijing 100083, China;
2. Regional Geological Survey Team, Tibet Bureau of Geology and Exploration, Lhasa 851400, China;
3. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China;
4. Shandong Institute of Geological Science, Jinan 250013, China;
5. Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054, China
Abstract:
The Lower Jurassic Ridang Formation is an important ore-hosting layer and ore-source bed in the Zhaxikang Sb-Pb-Zn-Ag polymetallic ore-concentrated district in South Tibet. The initial enrichment mechanism for ore-forming elements of the Ridang Formation has been discussed in this study. Geochemical study shows that the contents of SiO2, MgO, Rb and Zr in various rocks from the Ridang Formation are similar to the world's average shale on terrigenous background, and thus, the continental input could not serve as a major factor for the enrichment of ore-forming elements. For these rocks, the Fe3+/TFe ratios are less than 0.5, the enrichment factor (EF) values of U, V, Cr, Co and Ni are less than 5, and Ce has no obvious negative anomaly, reflecting that they formed in the condition of oxidized to sub-oxidized bottom water during sedimentation and the ore-forming elements could not be concentrated by authigenous minerals under the condition of anoxic-sulfurized bottom water. The EF values of P and Ba that serve as indicators of productivity are less than 1, suggesting a low to normal ancient productivity condition and reflecting that the ore-forming elements are not the products of biological enrichment under high productivity condition. The enrichment of As, Sb and Ag, locally positive Eu anomalies, as well as the existence of altered andesite and hydrothermal sediments in the reginal Ridang Formation suggest that seafloor hydrothermal activity is likely responsible for the enrichment of ore-forming elements in the Ridang Formation of the Zhaxikang ore-concentrated district.
Keywords: seafloor hydrothermal activity element enrichment Ridang Formation Zhaxikang Sb-Pb-Zn-Ag polymetallic ore-concentrated district South Tibet
