Mineralogical and fluid inclusion features of rock alterations in the Seigoshi gold-silver mining district, western part of the Izu Peninsula, Japan

Abstract

Propylitic and advanced argillic alterations occur in the Seigoshi gold-silver mining district, in the western part of the Izu Peninsula, Japan. The propylitic alteration has the following zonal arrangement from the deeper portion to the shallower portion: an epidote-prehnite-K-feldspar-chlorite zone; a wairakite-laumontite zone; and a stilbite-heulandite-montmorillonite zone. Lateral and vertical zoning is conspicuous in the advanced argillic alteration. The inner zone is silica-rich and this grades laterally and vertically through alunite- to clay-rich zones. Fluid inclusions from the epidote-prehnite-K-feldspar-chlorite zone are liquid-dominated and filling temperatures of this zone are in the range of 225–285°C. Filling temperatures for the zeolite zone are variable, being in the range of 240–380°C. This wide range suggests that boiling of the fluids was responsible for the zeolite zone. Filling temperatures of the advanced argillic alteration range widely from 210° to 430°C. This wide range and the coexistence of liquid- and vapor-dominated fluid inclusions in a given sample suggest that liquid-vapor separation simultaneously occurred during the advanced argillic alteration process. It is deduced that the ranges of gaseous fugacity are quite different for each type of alteration. For instance, f S 2 and f O 2 of the advanced argillic alteration are estimated to be higher than those of the propylitic alteration. Based on the alteration mineral assemblage, chemical composition and mode of occurrence of alteration minerals, fluid inclusions, estimated ranges of gaseous fugacities, and comparison of these features with those of active geothermal systems, it is concluded that the propylitic alteration minerals were probably precipitated due to the loss of gases such as CO 2, and that the formation of advanced argillic alteration was caused by subsurface mixing of volcanic gas and/or condensed hot water with groundwater. These coexisting propylitic and advanced argillic alterations are commonly found in the other AuAg mining districts in Japan.