ABSTRACT

Lead isotope ratios measured for selected pyrite samples from pre-Cenozoic bedded cupriferous iron sulfide (Besshi-type) deposits in Japan show a considerable variation, contrasting with the previous results on the Cenozoic stratiform (Kuroko-type) deposits. The data obtained for two major tectonic units in Southwest Japan indicate that the Paleozoic ores from the Outer Zone are distinctly more enriched in ²⁰⁶Pb than the ores from the Inner Zone. In the Outer Zone, the lead isotope ratios appear to be essentially uniform within the area of Shikoku Island where occur principal Paleozoic deposits. Leads less enriched in ²⁰⁶Pb are found in another area of the Outer Zone, suggesting a variability in isotopic composition along the whole extension of the Outer Zone. The Outer Zone Paleozoic leads tend to be more radiogenic than the Cenozoic Kuroko leads. A genetic link of lead between these two types of stratiform ores may thus be ruled out. A certain evolutionary relationship, presumably in an island-arc environment, is possible between the Kuroko and the Inner Zone Paleozoic leads. Available geochemical data for volcanic rocks associated with the Outer Zone deposits suggest that the ore leads of this zone may have been derived from a rift-zone igneous activity. Compared with “conformable” ore leads, the stratiform ore leads from Japanese Paleozoic terrains are more variable in isotopic composition. Of the Outer Zone leads, the Shikoku Island group is distinctly more enriched in ²⁰⁶Pb than the “conformable” ore leads of similar ages. It is suggested that there may be more than one class of evolutionary system in the stratiform ore leads.