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Abstract

A review of known lead isotope variations indicates high probability that the class of stratiform lead-zinc-barite-fluorite deposits includes two distinct types, presumably in consequence of some fundamental difference in genesis. Most deposits in Europe and adjacent areas of the Eastern Hemisphere contain ordinary ore-lead, almost invariant in isotopic composition, that evidently originated during a relatively short span of geologic time in a uniform geologic source, probably the earth's mantle. Most deposits in the central lowland of North America, on the other hand, contain J-lead of variable isotopic composition, part or all of which evidently originated by mobilization of rock-lead from crustal rocks, such as Pre-cambrian basement or enclosing marine sedimentary rocks. Exceptions to the geographic pattern of distribution of the two types—Pine Point in Canada and deposits of Laisvall-type in Sweden and Norway—are likely to prove of special interest.

Future lead isotope studies, if appropriately integrated with other means of field and laboratory study, can help establish whether a real difference exists between two types, whether sources of ore-lead were in the mantle or in the crust, and whether episodes of ore deposition were synchronous with, or distinctly later than, sedimentation and diagenesis of enclosing rocks.

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