Mass Transfer, Oxygen Isotopic Variation and Gold Precipitation in Epithermal System: A Case Study of the Hishikari Deposit, Southern Kyushu, Japan

摘要

Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K_2O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na_2O) content increases, and SiO_2 content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow-mixing model, and the computed results are compared with the distribution of SiO_2 minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10~(-4.2) m/sec, and ca. 0.10 m~2/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain delta~(18)O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold.