Reflectance spectroscopy as an effective tool to reconstruct late diagenetic and hydrotheramal alteration processes in Youjiang basin

The Shuiyindong gold deposit in Youjiang basin is the largest Carlin-like gold deposit in China. The fine-grained nature of the deposit and the limited study of pre-ore geological event(s) have posed significant problems in the study of hydrothermal processes and gold exploration. Here we present results of short-wave infrared (SWIR) spectroscopy analysis, combined with conventional geochemical and mineralogical methods, to characterize hydrated mineralogy of host rocks and ores with various degrees of mineralization. These data provide insights into the alteration processes of gold mineralization and a useful guide for gold exploration. Our results show that (1) Pre-ore late diagenesis causes illitization of smectites (weathering products of E'meishan basalt) and ferroan dolomitization in Longtan formation argillites and limestones, which is a critical step in making the rocks susceptible to later gold mineralization; (2) The gold deposition mechanism is dominantly sulfidation of Fe in diagenetic Ferroan dolomite and Fe-bearing illite/I-S. Gold mineralization in strata-bound limestones is closely associated with Fe-poor dolomite and/or Al-rich illite; (3) SWIR spectroscopy is an effective and efficient method to identify calcite, ferroan dolomite, Fe-poor dolomite and semi-quantify their relative abundances, and constrain crystal-chemistry and crystal-structure of illite/I-S formed during late-diagenesis and hydrothermal processes. (4) The combination of peak positions of absorptions around 2200 nm and 2330 nm of impure limestones with different alteration degrees shows that the hydrothermal and mineralization processes evolve from illitization of I-S to dissolution of calcite and then to dolomitization of ankerite. Reflectance spectroscopy is an effective tool to reconstruct late diagenetic and hydrothermal alteration processes in Youjiang basin. On a grander scale, this approach shows how infrared spectroscopy and remote sensing can be applied to mineral exploration throughout the Sola System because of the ability to identify and characterize crystal-chemistry of and spatial relationships among key indicator minerals in alteration systems.