Abstract

Koryu is an epithermal gold-silver quartz vein deposit in southwestern Hokkaido, Japan. The deposit occurs in an east-west shear zone within Miocene black mudstone. The deposit consists of eight major veins with a general east-west strike. 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, -III), whereas seven stages can be distinguished in the later event (L-I, -II, -III, -IV, -V, -VI, -VII). The wide veins consist of multiple mineralization stages. Most gold-silver mineralization is associated with the L-III stage.The earlier mineralization stages are characterized by manganocalcite, johannsenite, and small amounts of ore minerals, whereas the later stages are characterized by large amounts of ore minerals. Gangue minerals associated with later ore consist of interstratified chlorite-smectite and microcrystalline quartz alternately banded with adularia and comb-texture quartz. Gold-silver-bearing minerals include electrum, aeanthite- aguilarite, polybasite-pearceite, pyrargyrite-proustite, jalpaite, mckinstryite, hessite, an Ag-Te-Se-S mineral, and tetrahedrite.The presence of vapor-dominated fluid inclusions in several stages suggests that boiling occurred intermittently throughout ore deposition. The formation temperatures of the earlier mineralization epoch (263 degrees -283 degrees C) were slightly higher than those of the later mineralization epoch (246 degrees -260 degrees C) except for stages E-III-b (223 degrees C) and L-VII (206 degrees C). Salinities range from 0.5 to 6.0 wt percent NaCl equiv, although CO 2 concentrations up to 1.4 wt percent in some later stages account for a portion of the apparent salinity. These data indicate that maximum P total of the ore fluid was 31 to 68 bars, equivalent to 430 to 850 m below the paleowater table.Quartz morphology combined with fluid inclusion studies suggests that boiling of the fluid occurred repeatedly, leading to silica-supersaturated conditions with respect to quartz and resulting in the formation of the various silica textures. Recrystallization of silica to quartz occurred throughout vein formation.The stable isotope data combined with parageneses, quartz textures, and fluid inclusion studies suggest the following model for the Koryu gold-silver deposits. The veins show two distinct mineralization epochs, an earlier and a later one, which were responsible for type 1 and 2 hydrothermal fluids, respectively. Both types are dominantly meteoric water in origin. The early fluid is characterized by relatively heavy delta 18 O values (-5.3 to -4.7ppm) and a temperature of > or = 260 degrees C. The later fluid is characterized by relatively low delta 18 O values (-9.3 to -6.0ppm) and a temperature of > or = 250 degrees C. Type 1 fluids may have circulated deeply and leached Ca and Mn, which were precipitated as manganocalcite and johannsenite during the earlier mineralization epoch. Type 2 fluids mixed with shallower water, ascended through new conduits, and apparently carried large amounts of gold and silver, although the source of the metals cannot be determined at present. When the hydrothermal fluids ascended at discrete time intervals to the boiling zone (<850 m) during the later epoch, gold and silver were precipitated at 250 degrees C.

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