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Uniformity of lead isotope ratios in galenas of the so-called conformable class of sulfide deposits indicates that these leads, like those of orthomagmatic deposits in mafic rocks, were derived from a uniform, presumably subcrustal source and were deposited without significant contamination from the radiogenic lead found in crustal rocks. Anomalous, irregular ratios which characterize galenas of fissure-filling deposits result from radiogenic additions during migration of the original lead through crustal material, the degree of contamination reflecting the duration and distance of transit and the composition and thickness of the rocks traversed.
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