Study on the evolution of ore-formation fluids for Au-Sb ore deposits and the mechanism of Au-Sb paragenesis and differentiation in the southwestern part of Guizhou Province, China

摘要

Ore deposits (occurrences) of Au, As, Sb, Hg, etc. distributed in Southwest Guizhou constitute the important portion of the low-temperature metallogenic domain covering a large area in Southwest China, with the Carlin-type Au and Sb deposits being the most typical ones. In this paper the Au and Sb ore deposits are taken as the objects of study. Through the petrographic analysis, microthermomitric measurement and Raman spectrophic analysis of fluid inclusions in gangue minerals and research on the S and C isotopic compositions in the gold ore deposits we can reveal the sources of ore-forming materials and ore-forming fluids and the rules of ore fluid evolution. Ore deposits of Au, Sb, etc. are regionally classified as the products of ore fluid evolution, and their ore-forming materials and ore fluids were probably derived mainly from the deep interior of the Earth. Fluid inclusion studies have shown that the temperatures of Au mineralization are within the range of 170-361℃,the salinities are 0.35 wt%-8 wt% NaCl eq.; the temperatures of Sb mineralization are 129.4-214℃ and the salinities are 0.18 wt%- 3.23 wt% NaCl eq.; the ore-forming fluid temperatures and salinities tend to decrease progressively. In the early stage (Au metallogenic stage) the ore-forming fluids contained large amounts of volatile components such as CO2, CH4, N2 and H2S, belonging to the H2O-CO2-NaCl fluid system; in the late stage (Sb metallogenic stage) the ore-forming fluids belong to the Sb-bearing H2O-NaCl system. The primitive ore-forming fluids may have experienced at least two processes of immiscibility: (1) when early ore-bearing hydrothermal solutions passed through rock strata of larger porosity or fault broken zones, CO2, CH4, N2 would escape from them, followed by the release of pressure, resulting in pressure release and boiling of primitive homogenous fluids, thereafter giving rise to their phase separation, thus leading to Au unloading and mineralization; and (2) in the late stage (Sb metallogenic stage ) a large volume of meteoric water was involved in the ore-forming fluids, leading to fluid boiling as a result of their encounter, followed by the drop of fluid temperature. As a result, the dissolubility of Sb decreased so greatly that Sb was enriched and precipitated as ores. Due to differences in physic-chemical conditions between Au and Sb precipitates, Au and Sb were respectively precipitated in different structural positions, thus creating such a phenomenon of Au/Sb paragenesis and differentiation in space.