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Lead isotope study of ores and adjacent rocks Reynolds, Peter Herbert

Abstract

The purpose of this thesis is to investigate the isotopic relationships between lead in orebodies and lead in adjacent igneous rocks. Past studies of rocks and ores have been largely uncorrelated, and, in addition, much of the published data is unreliable because of large experimental uncertainties and inadequate interlaboratory calibrations. These data have, however, suggested that most rock-leads originated in a system with a distinctly lower U/Pb ratio than the one associated with certain ore-leads. Samples were obtained for the present study from four selected areas. Both rock-leads and ore-leads were analyzed from Balmat, N.Y. and from Nelson, B.C. Rock-leads from Broken Hill, Australia and from West-Central New Mexico were also studied. The isotopic abundances of ore-leads from these latter two areas have already been determined in this laboratory. Identical experimental techniques were used throughout for both rocks and ores, and a precision of better than 0.10 per cent (one standard deviation) in the measurement of isotope ratios with respect to lead-204 was achieved. A systematic difference in isotopic composition was observed between certain of the above leads. In particular, rock-leads from Balmat and from the Nelson batholith, ore-leads from deposits associated with this batholith, and ore-leads from New Mexico (analyzed by J. Blenkinsop) were apparently derived from a primary system characterized by a present-day value of the U238/Pb204 ratio (the μ-value) equal to 8.7 to 8.8₅. The significance of the isotopic composition of rock-lead from New Mexico was not revealed by the one sample studied. On the other hand, ore-lead from one of the stratiform deposits at Balmat was apparently derived from a primary system with a μ-value of 9.0. This value agrees with the one obtained in this laboratory by R.G. Ostic for stratiform deposits selected by R.L. Stanton in accordance with geological criteria. It is also consistent with A.J. Sinclair's interpretation of the isotopic abundances of lead from the Sullivan mine and from deposits located in the Kootenay arc north and south of the Nelson batholith. In addition, both rock-leads and ore-leads from Broken Hill appear to reflect the existence of this higher μ system. The present study has therefore provided, for the first time, definitive evidence relative to the existence of two distinct distributions of primary μ-values. Several geophysical models are discussed in an attempt to reconcile this difference, and explain in general terms the evolution of lead isotope ratios in the earth. Also, for the first time, analyses of Nelson rocks and ores have provided clear evidence of a genetic relationship between ore deposits and granitic rocks. At the same time, lead isotope abundance patterns in plutonic rocks of batholithic dimensions were investigated.

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