WEI Chen1,2, YAN Zai-fei1, HUANG Zhi-long1*, HU Yu-si1,2
(1. State Key Laboratory of Ore Deposit Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China)
Abstract: The Nayongzhi deposit, located in the southwestern margin of the Yangtze Plate, is a newly discovered large lead-zinc deposit in Guizhou Province with an estimated lead-zinc metal resources over 1.35Mt. Its ore bodies occurred as stratiform, stratiform-like, and veins in dolomite of the Lower Cambrian Qingxudong Formation. The ore generally contains predominant minerals of sphalerite, galena, and dolomite, and minor minerals of pyrite and calcite. Dolomite is the most important gangue mineral in this deposit. It was formed in various stages throughout the ore-forming process. Therefore, geochemical characteristics of the ore-stage hydrothermal dolomite can provide very important information for revealing the origin and evolution process of the ore-forming fluid. The results show that the ore-stage hydrothermal dolomites (HD1) contain total REE contents ranging from 61.79×10-6 to 63.03×10-6, with enriched LREE, relatively obvious negative Eu anomalies (δEu values ranging from 0.63 to 0.66), and weak positive Ce anomalies (δCe values ranging from 1.28 to 1.33). The HD1 samples have different total REE contents comparing to the ore-bearing country rocks, but have relatively similar REE distribution patterns with enrichment of LREE. The concentric bands of light and dark in the cathodoluminescence (CL) images of the HD1 samples indicate that the chemical composition of the hydrothermal fluid could have been changed and/or pulsing changed at least eight times during the entire dolomitization process. Combining with REE distribution patterns and diagrams of Y vs. REE and Y vs. Sm of the HD1, we have suggested that the ore-forming fluid could be a mixture of multiple source fluids, derived from host carbonate rock, underlying strata, and basement rocks, respectively. The growth bands of the HD1 crystals could be closely related to the dissolution of host dolomite and/or the sulfide precipitation. In addition, the characteristics of CL images and REE distribution patterns of the HD1 are obviously different to those of the post-ore stage hydrothermal dolomite (HD2). These differences can be used as important geochemical parameters for prospecting Pb-Zn resources in the Wuzhishan area.
Keywords: hydrothermal dolomite; cathodoluminescence; REE; prospecting indicators; the Nayongzhi Pb-Zn deposit; Northwestern Guizhou
