扬子板块北缘马元铅锌矿床有机地球化学特征及成矿作用

近年来,有机质与金属成矿关系研究备受关注。扬子板块北缘马元矿床铅锌矿体分布与古油藏、沥青在空间上叠置,是探索有机质与铅锌成矿关系的理想地区。矿区发育分散有机质、沥青、气态烃类等不同赋存形式的有机质,生物标志化合物特征明显,以低碳数正构烷烃为主,具浅海环境藻类来源特征,均处于过成熟演化程度。沥青²⁰⁶ Pb/²⁰⁴ Pb值为17.946~18.071,207 Pb/204 Pb值为15.593~15.703,²⁰⁸ Pb/²⁰⁴ Pb值为37.812~38.072,Pb模式年龄为612~478 Ma,结合油-源对比结果,沥青主要来源于下寒武统郭家坝组烃源岩,郭家坝组烃源岩经历了较强的还原环境和缺氧的高盐度环境。各类有机质C同位素组成一致,δ¹³ C值为-35.3‰~-31.4‰,与区内闪锌矿流体包裹体中甲烷、乙烷等C同位素一致,显示有机成因碳典型特征。综上所述,马元铅锌矿床成矿与有机质关系密切,古油藏热裂解和热化学硫酸盐还原(TSR)作用提供了还原硫,富含Pb、Zn的盆地卤水与富含CH₄、H₂S的有机还原性流体混合是其重要成矿机制。

Geochemical Characteristics of Organic Matters and Metallogenesis of Mayuan Pb-Zn Deposit in the Northern Margin of Yangtze Plate,China

The relationship between organic matter and metal mineralization has attracted much interest in recent years.The spatial relationships between paleo-oil reservoirs,asphalt and Mayuan Pb-Zn deposit,which is an ideal area in the northern margin of Yangtze plate for studying the relationship between organic matter and Pb-Zn mineralization,are superimposed.The mineralized area is characterized by organic matter,asphalt,and hydrocarbons.The biomarker combinations have low-carbon n-alkanes characteristic of an algae source in the shallow sea environment,and all of them are over-mature evolution.The Pb isotope compositions of the asphalt are 17.946-18.071²⁰⁶ Pb/²⁰⁴ Pb,15.593-15.703207 Pb/²⁰⁴ Pb,and 37.812-38.072208 Pb/²⁰⁴ Pb with the model ages of 612-478 Ma,which indicate that the asphalt has a Cambrian source.The probable source is Lower Cambrian Guojiaba Formation deposited in a strongly reducing and anoxic highly saline environment,as deduced from the oil-source correlation.The C isotope compositions of various organic matter are the same withδ¹³ C values of-35.3‰--31.4‰,which are consistent with the C isotope compositions for methane and ethane in sphalerite fluid inclusions.All these data are typical for organic carbon.The mineralization of Mayuan Pb-Zn deposit is therefore genetically related to organic matter,and the thermal cracking of the paleo-oil reservoir and TSR produces reduction sulphur.It is deposited in a reducing environment involving the mixture of Pb-Zn-rich brine and fluids containing CH₄ and H₂S.