ABSTRACT

The Yanahara pyritic ore deposits, Okayama-Ken, was investigated for the distribution of the sulfur isotopes, cobalt, selenium and nickel. The Co/Fe ratio in sulfides, mostly pyrites, varies in the range (0.02-34.7) × 10^-4 and the Se/S ratio in the range (0.32-38.4) × 10^-5, respectively, while δ³⁴S values with respect to the meteoritic sulfur are almost uniformly distributed around the average, +2.8‰. Cobalt in the Kabu ore body, one of the biggest in the deposits, is distributed in a well defined zonal structure, the Co/Fe ratio decreasing from the south-eastern margin (Gari-Ko) toward the western side of the ore body. Despite of the significant intra-ore body fractionation of cobalt and selenium, the weighted averages of the Co/Fe and Se/S ratios in the Yanahara deposits are very similar to those in the crust, while the Ni/Fe ratio in the ore deposits is much smaller than that in the crust. These features in the isotopic and trace element chemistry of the deposits are shown to be compatible with those theoretically expected on the basis of fractional crystallization of an ore magma. An ore magma may have been separated from a later differentiate of the Yanahara intrusives. When pyrrhotite coexists with pyrite, cobalt seems to be enriched in pyrite, whereas the opposite trend is apparent for selenium. However, the enrichment seems to occur only at the contact of the two minerals, and short range chemical exchange or rearrangement of the trace elements around the boundary is suspected during and/or after the formation of pyrrhotite.