ABSTRACT

Many skarn deposits of various ore types occur in the Japanese island arc with genetical relation to Cretaceous and Miocene felsic igneous activities. Carbon and oxygen isotope ratios were determined for interstitial carbonates, mostly calcites, in skarn silicates from 28 representative deposits. The data obtained are compared with the isotopic composition of calcites expected to precipitate from magmatic fluids, on the assumption of isotopic equilibrium at 300–400°C between the carbonate mineral and fluid. The fluids for tungsten, tin and some copper (-iron) deposits in the regions of ilmenite-series granitic activity are characterized by ¹³C-depleted carbon, probably derived from organic materials in sedimentary rocks. In the fluids for most deposits of other types, such as zinc-lead, zinc-copper (-iron) and copper (-iron), magmatic carbon is predominant. Carbon and sulfur isotope values for each deposit show a positive correlation among the studied deposits, suggesting that both carbon and sulfur came from common sources. For many deposits, calculated δ¹⁸O values of the fluids are in the range from +5 to 10‰ (SMOW). Such fluids are either magmatic in origin, or are isotopically equilibrated with felsic igneous rocks. Fluids with ¹⁸O-depleted nature are recognized in some, mainly zinc-lead and zinc-copper (-iron), deposits, suggesting a contribution of surface-derived water to the formation of these deposits.