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Isotopic investigation of conformable lead deposits. Ostic, Ronald George

Abstract

The purpose of this thesis is to investigate the isotopic composition of primary leads. The leads analysed were selected by R. L. Stanton in accordance with his geologic description for conformable deposits. It was anticipated that, of all available terrestrial leads, these were the least likely to have been modified isotopically by crustal processes. Detailed analyses and interpretations for fifty-six samples from nine different districts in Eastern Australia, Tasmania, Canada and New Zealand are presented. A precision of better than ±.05 per cent in the measurement of the Pb²⁰⁶/Pb²⁰⁴, Pb²⁰⁷/Pb²⁰⁴ and Pb²⁰⁸/Pb²⁰⁴ ratios was achieved for all samples by means of an intercomparison technique derived from that used by P. Kollar and R. D. Russell, and improved methods for reducing the data. As expected, the geologic criteria used by Stanton to identify conformable deposits have been found to be sufficient to identify deposits which are very uniform in isotopic composition. The findings of this research indicate that the criteria, although remarkably good, are not completely adequate for identifying primary leads. In particular, leads from two conformable deposits (Manitouwadge and Rosebery) were found to be anomalous, and those from two other deposits (Hall's Peak and Bathurst) may be anomalous. The single-stage lead model was found to be a very good approximation for isotopic development of the leads from the remainder of the conformable deposits studied by the writer, as well as for leads analysed by other workers at the University of British Columbia and assumed to be primary. These results suggest that primary leads have formed in a region for which there are very narrow limits of ±.6% and ± .8% to the variations in U²³⁸/Pb²⁰⁴ and Th/U ratios. They substantiate the hypotheses of Russell and Stanton that: (1) primary leads lie very closely to a unique single-stage growth curve in a plot of the ratios Pb²⁰⁷/Pb²⁰⁴ against Pb²⁰⁶/Pb²⁰⁴, and in a plot of Pb²⁰⁸/Pb²⁰⁴ against Pb²⁰⁶/Pb²⁰⁴; (2) leads away from this growth curve are anomalous. From the fit of the conformable leads to a single-stage growth curve, values of 4.52 ± .03 Gyr. and 4.54 ± .02 Gyr. have been calculated for the age of the earth. With the identification of a unique growth curve for primary leads, severe restrictions are placed on anomalous lead interpretations. These are illustrated in the interpretations of anomalous leads studied by the writer.

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