Lead isotopic feature of the Besshi-type deposits and its bearing on the ore lead evolution.

Abstract

Additional lead isotope measurements have been made on pyrite samples from the Mesozoic and Paleozoic Besshi-type deposits and on a galena sample from the Proterozoic stratiform mineralization in the Flin Flon area, Canada. The supplemented data for the Besshi-type mineralization confirm, on the one hand, the previous observation that leads in this type of deposit are isotopically variable and are consistently deficient in 207Pb relative to the major lead (conformable) ore system, and reveal, on the other hand, that, in some cases, a small but significant isotopic variation exists even within a single deposit. The isotopic pattern of these deposits, which is similar to that of the ocean-ridge basalts, is in striking contrast with a uniform isotopic composition and its conformity with the evolutionary pattern of the major lead orebodies found in one of the Mesozoic deposits examined, and thus reinforces the previous view that the lead in most Besshi-type deposits may have been derived mainly from the mantle. The 207Pb deficiency relative to the major lead ore system and other two remarkable features common to most Besshi-type deposits, close association with mafic magmatism and generally Pb-impoverished nature, are also recognized in the Flin Flon area. Similar examples of stratiform deposits seem to exist in other areas. The Besshi-type and other 207Pb deficient deposits appear to indicate the presence of a mantle-derived ore-lead system. The mineralization of such deposits seems to occur most commonly in Archean to early Proterozoic and late Paleozoic to Mesozoic terrains, but might occur also in other simatic environments, whereas the large Pb-rich orebodies presumably of crustal origin may only be found in Proterozoic to Phanerozoic sialic environments. The current concept of global tectonic history appears to be generally consistent with the view of two major evolutionary systems of stratiform ore leads.