| dc.description |
THE idea that the lead in many large ore bodies originates from a uniform, deep-seated, possibly upper-mantle, source region<sup>1</sup> has proved of great value, particularly in the recognition and interpretation of the systematics of â anomalousâ (â multi-stageâ ) lead systems. When these so-called â single-stageâ or â primaryâ lead deposits came to be examined in greater detail<sup>2</sup>, however, the situation turned out to be not quite so simple; it led Kanasewich and Farquhar<sup>3</sup>, for example, to conclude that â 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 leadâ uraniumâ thorium systemsâ . Other indications of the need for re-examination of current ideas about the â primary leadâ system are the many geophysical data which suggest lateral inhomogeneities in the upper mantle<sup>4</sup>, supported by isotopic data which suggest long-term inhomogeneities in its U/Pb ratio<sup>5</sup>, and the contention by Hobbs and his co-authors<sup>6</sup> 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 asserted<sup>7</sup>. |
