Abstract

In several papers published since 1954 by one of us, suggestions have been made concerning the genesis of a certain class of lead-bearing sulfide orebody of the conformable type. Such deposits are thought to have been derived from seaboard vulcanism and to have been emplaced in off-shore, largely volcanic, sequences, during sedimentation and diagenesis. The metals are considered to have come directly from a deep source--probably the basaltic layer or mantle.Recently it has been found that the isotope ratios of leads of several such deposits are apparently "ordinary" and extremely uniform, and the leads have suffered no detectable contamination with crustal radiogenic lead. This suggests that they may indeed have come from deep source and that they have been in contact with crustal rocks for no more than a very short time prior to deposition. In contrast the leads of some vein deposits are anomalous and highly irregular, suggesting significant radiogenic contamination. Apparently the leads of the conformable deposits (and some orthomagmatic leads) have been brought to the surface directly from below the continental crust and rapidly isolated and shielded from crustal materials, whereas those of the veins may have spent a considerable time in contact with the latter.It appears, too, that the occurrence of ordinary and anomalous leads may give important general information on the history of galena-bearing deposits, and that leads from deposits of the conformable type may indicate, with good precision, the age of the sediments enclosing them.

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