Fluid inclusions and ore precipitation mechanism in the giant Xikuangshan mesothermal antimony deposit, South China: Conventional and infrared microthermometric constraints

摘要

The giant Xikuangshan antimony deposit, located in the Xiangzhong (central Hunan) Basin, South China, is the largest antimony deposit in the world. Stibnite is the predominant ore mineral, and gangue minerals consist of quartz, calcite, fluorite and barite. The ore mineral assemblages include early-stage quartz-stibnite, fluorite-quartz-stibnite and barite-quartz-stibnite, and late-stage calcite-stibnite. In spite of numerous research works on the Xikuangshan deposit during the past several decades, the signatures of the ore-forming fluids and the ore precipitation mechanisms are poorly constrained. In order to characterize the fluids responsible for antimony mineralization and ascertain the genetic processes in the Xikuangshan ore district, conventional and infrared microthermometric measurements have been performed in this study on fluid inclusions hosted in transparent minerals (quartz, fluorite and barite) and opaque stibnite, respectively. Four types of fluid inclusions were identified for the early-stage mineralization based on their phase assemblages, including type I (pure liquid inclusions), type II (two-phase, liquid-rich aqueous inclusions), type III (two-phase, vapor-rich aqueous inclusions) and type IV (pure vapor inclusions). Stibnite only contains type I and type II inclusions, while all types of fluid inclusions are common in the gangue minerals. The homogenization temperatures and salinities of fluid inclusions in stibnite and associated gangue minerals in this ore district display significant differences. Fluid inclusions in stibnite exhibit relatively low homogenization temperatures (112-323 degrees C) and higher salinities (0.2-15.4 wt% NaCl equiv.) than inclusions in coexisting gangue minerals (119-366 degrees C, 0.2-4.2 wt% NaCl equiv.), usually about 50 degrees C less than the "intergrown" gangue minerals. According to the petrography of fluid inclusions and the relationship between their homogenization temperature and salinity, it was concluded that stibnite and gangue minerals in the Xikuangshan ore district are not co-genetic, instead, they were deposited from two distinctly different hydrothermal solutions. Fluid mixing was responsible for stibnite precipitation, but the precipitation of gangue minerals resulted from fluid boiling due to an abrupt drop in pressure. The Xikuangshan deposit is a typical example of mesothermal deposits, rather than an epithermal deposit as previously considered.