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Quantitative interpretations of anomalous lead isotope abundances Kanasewich, Ernest Raymond

Abstract

A new method has been proposed for determining the age of lead sulfide mineralization from anomalous lead Isotope abundances. The anomalous leads are most readily recognized by the linear relationship of the isotope ratios on any compositional diagram. The method assumes that the initial stage of lead isotope development occurred in a system having a uniform distribution of uranium, thorium and lead. This accords with experimental results obtained by the writer and other research workers at the University of British Columbia that single-stage isochrons as proposed by Houtermans are either very short or do not exist. Consider t₁ to be a time of tectonic activity during which ordinary leads were differentiated from a deep source and either concentrated to form a lead deposit at this time or disseminated throughout the upper crust. Contemporaneously uranium and thorium bearing solutions were incorporated into the same environment. At time t₂ tectonic activity recurred in the area. Some of the ordinary leads were remobilized and became contaminated with radiogenic lead to form anomalous lead deposits. Existing lead isotope analyses yield the following results. At Broken Hill, Australia, t₁ is 1600 million years, while t₂ is 510 ± 80 m.y. For Goldfields, Saskatchewan, t₁, is 2015 m.y. while t₂ is 560 ± 250 m.y. At Sudbury, Ontario, t₁ is 1730 m.y. and t₂ is 870 ± 280 m.y. For lead deposits around the Ozark Dome area, including Joplin and Bonne Terre, Missouri, t₁ is 1350 m.y. and t₂ is about 115 m.y. For leads in west-central New Mexico, t₁ is 1490 m.y. and t₂ is about 69 m.y. The errors in the estimates are generally quite large but the values quoted are consistent with available geological and chronological data. The simplest type of anomalous leads which can occur is a mixture of two ordinary leads. This has only been found to occur on the boundary between two geological provinces. Examples of this type have been identified in the Cobalt area, Ontario and in the Baltic Shield along the border between the Fenno-Karelides and Svecofennides. A large proportion of anomalous lead Isotope measurements may be interpreted on the basis of the two-stage models outlined above. Extension of this model to an n-stage system is considered.

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