Genesis of epithermal Au-Ag mineralization of the Koryu Mine, Hokkaido, Japan

Koryu is an epithermal gold-silver quartz vein deposit n southwestern Hokkaido, Japan. The deposit occurs in an east-west hear zone within Miocene black mudstone. The deposit consists ofeight major veins with a general east-west rike. The three major veins show variable internal structure and texture, in both vertical and horizontal directions. K-Ar ages for adularia from veins 1 and 3 are 0.8 to 1.2 Ma, indicating a Pleistocene age of mineralization. Based on crosscutting relationships and mineral parageneses, the veins appear to have formed during two mineralization epochs. The earlier event is further divided into three stages (E-I,-II,-Ill), whereas even stages can be distinguished in the later event (L-I,-II,-Ill,-IV,-V,-VI,-VII). The wide veins consist of multip16 mineralization stages. Most gold-silver mineralization is associated with the L-III stage. The earlier mineralization stages are characterized bymanganocalcite, johannsenite, and small amounts of ore minerals, whereas the later stages are characterized bylarge amounts of ore minerals. Gangue minerals associated with later ore consist of interstratified chlorite-smectite and microcrystalline quartz alternately banded with adularia nd comb-texture quartz. Gold-silver-bearing minerals include electrum, aeanthite-aguilarite, polybasite-peareeite, pyrargyrite-proustite, jalpaitc, mekinstryite, hessitc, an Ag-Te-Se-S mineral, and tetrahedrite.