Light Hydrocarbons in Fluid Inclusions and Their Constraints on Ore Genesis: A Case Study of the Songxi Ag (Sb) Deposit, Eastern Guangdong, China

Abstract

The Songxi deposit is a newly discovered large Ag (Sb) deposit. By using a suite of high‐vacuum quadrupole gas mass spectrometer systems, the authors have recognized many kinds of light hydrocarbons in fluid inclusions of minerals. These hydrocarbons are mainly composed of C1‐C4 saturated alkanes, while the contents of C2‐C4 unsaturated alkenes and aromatic hydrocarbons are quite low, suggesting that the metallogenic processes have not been affected by magmatic activities. Chemical equilibrium studies show that these hydrocarbons may be a mixture of organic gases generated by microorganism activity and those by thermal cracking of type‐II kerogens (kukersite) in sedimentary host rocks, and the former may constitute more than two‐thirds, implying that microorganism might have played an important role in the metallogenesis. The equilibrium temperature of the latter is about 300°C, which is much higher than the geothermal temperature at the estimated depth of metallogenesis. Thus, the light hydrocarbons generated by thermal cracking of kerogens probably originated in the deep part of the sedimentary basins and then migrated through a long distance to shallower horizons of the basin. Based on the composition of light hydrocarbons in fluid inclusions, the authors infer that the Songxi deposit was formed in a continental rift. The analytical data presented in this paper support from one aspect the genetic model that the Songxi deposit may be a sedimentary hot brine transformed deposit instead of a submarine basic volcanic exhalation and low‐medium temperature volcanic hydrothermal fluid filling deposit proposed by most previous researchers.