Reinterpretation of Quartz Textures in Terms of Hydrothermal Fluid Evolution at the Koryu Au-Ag Deposit, Japan

Abstract

Spatial and temporal variations in quartz textures and the types of quartz-hosted fluid inclusions in the no. 3 vein of the Pleistocene Koryu epithermal Au-Ag deposit were studied. Bonanza-grade ore zones contain multiple generations of quartz with six distinct textures—crustiform, comb, microcrystalline, colloform, cockade, and platy. Early-formed parts of the vein are mainly composed of comb quartz that is overgrown by later quartz with a variety of textures, including comb, microcrystalline, and colloform; the latter two are the most common textures of later parts of the vein. Comb, microcrystalline, and colloform textures can be traced along each mineral band at the scale of the ore deposit, suggesting the existence of uniform silica-supersaturated conditions during the deposition of the band. Interpretation of petrographic features of fluid inclusions in quartz at the same mine level reveals three types of fluid conditions—intense boiling (flashing), gentle boiling, and nonboiling conditions. The presence of comb quartz texture corresponds to fluid inclusion evidence for gentle boiling or nonboiling conditions. The presence of microcrystalline and colloform quartz textures indicates intense boiling, as suggested in previous studies, although the secondary fluid inclusions in earlier-formed quartz only correspond to fluid inclusion evidence for intense boiling. A fluid inclusion microthermometric study using fluid inclusion assemblages in comb quartz trapped under both gentle boiling and nonboiling conditions shows hydrothermal temperature fluctuates mostly between 243° and 268°C. The relationship between boiling conditions and quartz textures suggests that the intensity of boiling of silica-supersaturated hydrothermal fluid fluctuates at a given depth over time. The close association between precious metal content and microcrystalline and colloform quartz supports the idea that the metals precipitated due to intense boiling. Intense boiling at the base of the boiling zone likely was the main mechanism for bonanza precious metal precipitation, whereas physical transportation of the metals by gently boiling fluids was only a minor mechanism for local precipitation of precious metals at Koryu. On the basis of the observations of this study and previous paleodepth data, quartz textures are overprinted; a deep assemblage of the early comb quartz, formed at >500-m depth below the paleowater table, was overprinted by a late microcrystalline and colloform quartz assemblage formed at <500-m depth. Such textural variations may be attributed to a decrease in the depth of the water table caused by nearby volcanic eruptions.