GENESIS OF EPITHERMAL AU-AG MINERALIZATION OF THE KORYU MINE, HOKKAIDO, JAPAN

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 wit h a general east-west strike. The three major veins show variable inte rnal 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, indicat ing a Pleistocene age of mineralization. Based on crosscutting relatio nships 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 distinguish ed in the later event (L-I, II, -III, -IV, -V, -VI, -VII). The wide ve ins consist of multiple: mineralization stages. Most gold-silver miner alization is associated with the L-III stage. The earlier mineralizati on stages are characterized by manganocalcite, johannsenite, and small amounts of ore minerals, whereas the later stages are characterized b y 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, acanthite-aguilarite, polli basite-pearceite, pyrargyrite-proustite, jalpaite, mckinstryite, hessi te, an Ag-Te-Se-S mineral, and tetrahedrite. The presence of vapor-dom inated fluid inclusions in several stages suggests that boiling occurr ed intermittently throughout ore deposition. The formation temperature s of the earlier mineralization epoch (263 degrees-283 degrees C) were slightly higher than those of the later mineralization epoch (246 deg rees-260 degrees C) except for stages E-III-b (223 degrees C) and L-VI I (206 degrees C). Salinities range from 0.5 to 6.0 wt percent NaCl eq uiv, although CO2 concentrations up to 1.4 wt percent in some later st ages account for a portion of the apparent salinity. These data indica te 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 combine d with fluid inclusion studies suggests that boiling of the fluid occu rred repeatedly, leading to silica-supersaturated conditions with resp ect to quartz and resulting in the formation of the various silica tex tures. Recrystallization of silica to quartz occurred throughout vein formation. The stable isotope data combined with parageneses, quartz t extures, and fluid inclusion studies suggest the following model for t he Koryu gold-silver deposits. The veins show two distinct mineralizat ion epochs, an earlier and a later one, which were responsible for typ e 1 and 2 hydrothermal fluids, respectively. Both types are dominantly meteoric water in origin. The early fluid is characterized by relativ ely heavy delta(18)O values (-5.3 to -4.7 parts per thousand) and a te mperature of greater than or equal to 260 degrees C. The later fluid i s characterized by relatively low delta(18)O values (-9.3 to -6.0 part s per thousand) and a temperature of greater than or equal to 250 degr ees 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 wate r, ascended through new conduits, and apparently carried large amounts of gold and silver, although the source of the metals cannot be deter mined at present When the hydrothermal fluids ascended at discrete tim e intervals to the boiling zone (<850 m) during the later epoch, gold and silver were precipitated at 250 degrees C.