“Primary” Leads

Abstract

THE idea that the lead in many large ore bodies originates from a uniform, deep-seated, possibly upper-mantle, source region¹ has proved of great value, particularly in the recognition and interpretation of the systematics of “anomalous” (“multi-stage”) lead systems. When these so-called “single-stage” or “primary” lead deposits came to be examined in greater detail², however, the situation turned out to be not quite so simple; it led Kanasewich and Farquhar³, for example, to conclude that “no infallible geologic criteria have been found for independently distinguishing ordinary or single-stage leads from those which were produced in two or more distinct lead–uranium–thorium systems”. Other indications of the need for re-examination of current ideas about the “primary lead” system are the many geophysical data which suggest lateral inhomogeneities in the upper mantle⁴, supported by isotopic data which suggest long-term inhomogeneities in its U/Pb ratio⁵, and the contention by Hobbs and his co-authors⁶ that the seemingly stratiform nature of the deposit at Broken Hill, NSW, may be not quite so directly the result of sedimentary processes as has been asserted⁷.