Minerogenetic mechanism of the Songxi silver-antimony deposit of northeastern Guangdong—Ore-controlling role of organic matter

Organic geochemistry and comparisons of characteristics of the organic matter in wall rocks of the ore-controlling strata and ores of the Lower Jurassic Songling black shale formation and the related Songxi silver-antimony deposit of northeastern Guangdong have been studied in this paper. The results show that the Lower Jurassic Songling shale formation is a suite of biologic-rich and organic-rich ore-bearing marine sedimentary rocks. Micro-components of the organic matter in the Songling black shale formation consists primarily of algae, amorphous marine kerogen, solid bitumen, and pyrobitumen. The thermal evolution of organic matter is at the over-maturity stage. There is a general positive correlation between total organic carbon (TOC) and metallogenetic elements such as Ag and Sb in the black shale formation. Organic matter in the host rocks in the Songxi ore deposit played a role in controlling the silver-antimony depositing environment during the forming process of the black shale ore-bearing formation. In the absence of vitrinite, the relative level of thermal maturity calculated by solid bitumen reflectance indicates that the ore-forming temperature of the Songxi silver-antimony deposit was about 150–170°C, which was considered as an epithermally reworked ore deposit. The roles of organic matter in the formation of the Songxi ore deposit are a primitive accumulation of the metallogenetic elements (Ag, Sb) in the sea-water cycle system for ore source and a concentration of metals by ion exchange or chelation as well as reduction of the oxidized metals.