Mineralization Characteristics of the Hokuryu Epithermal Au‐Ag Deposit, Hokkaido, Japan

ABSTRACT Veins intersected by a ~600‐m‐long drill core into the Hokuryu epithermal Au‐Ag deposit in the northeastern part of Hokkaido, Japan, were investigated including mineralogy, texture, quartz composition, and fluid inclusion characteristics, with relation to elevation. Host rocks of these veins are flow‐banded rhyolite, pyroclastic breccia, and andesite and are mainly altered by illite and chlorite. The veins display crustiform and massive macroscopic textures. The crustiform veins generally located at higher elevations, contain Ag‐rich and Au‐Ag‐rich bands. The massive veins are mainly present at lower elevations, and they are generally barren of precious metals. The Ag‐rich bands in the crustiform veins contain mineral assemblage of hessite + sphalerite + pyrite + galena. These minerals, together with anhedral to rhombic adularia, exist within the interstices of quartz that display microspherical texture. The Au‐Ag‐rich bands contain mineral assemblage of electrum ± naumannite‐aguilarite associated with quartz with colloform, ghost‐sphere, and ghost‐bladed textures. Bulk compositions of the veins reflect the ore mineralogy observed in which Ag is strongly correlated with Pb, Te, Zn, Cd, and Bi. The barren massive veins are mainly composed of granular quartz. Textural and mineralogical characteristics of Au‐ and Ag‐bearing bands in the crustiform veins indicate amorphous silica and calcite precursors. Together with the coexistence of adularia, these suggest liquid boiling during vein formation. Quartz associated with Au‐ and Ag‐bearing minerals exhibits blue cathodoluminescence (ca. 400 nm wavelength) triggered by high Al impurity probably due to pH and pressure fluctuations or impurity redistribution during recrystallization from metastable amorphous silica. Fluid inclusions hosted in quartz and adularia, which contain negligible amounts of CO 2 , show a modal homogenization temperature range of 260°C–290°C and a salinity range of 1.2–6.9 wt% NaCl equivalent. The homogenization temperatures follow a boiling point curve and indicate an erosion of up to ~340 m for the deposit. The variable salinities in relation to elevation suggest extreme vapor loss during boiling at depth, episodic influx of high salinity fluids from deeper source, or mixing with high salinity fluids along anastomosing fractures at relatively shallower depths.