Ore–microscopy and Geochemistry of Gold–Silver Telluride Mineralization in Southwestern Hokkaido, Japan

摘要

Tellurium–bearing gold, silver, and base metal assemblages characterize the epithermal mineralization in southwestern Hokkaido, Japan. The telluride deposits in this district consist of quartz–filled fracture systems of Late Miocene to Pliocene age. Most of the telluride mineralization in southwestern Hokkaido consists of vein systems or are massive in form. The dominant host rocks for this deposit type are intermediate to acid volcanic rocks. The gangue and alteration minerals are very fine to fine–grained quartz, chalcedony, anatase, zeolite, sericite, barite, adularia, chlorite, smectite or other clay mineral, while carbonate (calcite and rhodocrosite) and Mn minerals are poorly represented and rarely coexist with tellurides. The country rocks suffered argillic and propylitic alterations, silicification, and kaolinitization. Tellurium–bearing mineralization is related to or co–exists with sulfide and sulfosalt mineralization. The tellurides identified in the studied area consist of native tellurium, hessite, stutzite, petzite, sylvanite, calaverite, tellurobismuthite, tetradymite, Se–bearing tetradymite, tellurantimony, goldfieldite, altaite, coloradoite, tellurite, teineite, and poughite. The first crystallized telluride minerals such as native tellurium, stutzite, petzite, sylvanite and hessite are followed by calaverite, coloradoite, tellurobismuthite, tellurantimony, and altaite, indicating a decline in tellurium activity during telluride deposition with time. Microthermometry of fluid inclusions in quartz and barite indicate that the mineralization formed at moderate temperatures, ranging from 250–200 °C with low average salinity (1.9–5.7 wt% NaCl_(equiv.)). The shift in mineralogy reflects decreases in temperature and fugacity of sulfur, with a concomitant increase in fugacity of tellurium. Further decrease in fugacity of tellurium (coupled with decreasing fugacity of sulfur) resulted in deposition of Au–Ag–Te and other tellurides.