Strontium Isotopic Composition in Barite from the Minamishiraoi Kuroko-type Deposit, Southwestern Hokkaido

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Strontium isotope ratios in barites were analyzed for a better understanding of the geochemical feature of the hydrothermal solution responsible for the formation of the Minamishiraoi massive barite deposit. The <SUP>87</SUP>Sr/<SUP>86</SUP>Srr ratios(0.70556-0.70647) in barites were found to be between 0.70450 of andesite, the footwall rock of the deposit and 0.7088 of seawater of the Miocene age. The isotope ratios obtained showed a lower range than that of the Hokuroku Kuroko deposits. Sr isotope ratios ranged between 0.70561 and 0.70576 for barites from both the sulfide-rich ores of a stratiform orebody and the stockwork ores of a silicified zone in the Minamishiraoi deposit. On the other hand, barites from baritepredominated ores, the principal constituent of stratiform orebody had slightly higher ratios(0.70556-0.70647) than the former ones. The compositional differences of the Sr isotope among the ore types can be ascribed to temporal and spatial changes of the contribution of seawater to the barite precipitation taking place on and beneath the seafloor. It is considered that strontium isotope and oxygen isotope characteristics of the Minamishiraoi barite resulted from the isotopic compositional deviation of ore forming fluids and the temperature difference of ore formation from the typical Kuroko deposits.
資源地質学会の論文