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Study of lead isotopes from mineral deposits in southeastern British Columbia and from the Anvil range, Yukon Territory LeCouteur, Peter Clifford

Abstract

The objective of the research was to determine the source and age of emplacement of lead in mineral deposits in three regions of the Western Cordillera — the East Kootenay district, the adjoining Kootenay Arc, and the Anvil Range. Measurements were made of the isotopic composition of lead in 132 samples by gas source techniques and a precision higher than ± 0.16% (2σ) was obtained for all isotope ratios relative to Pb²⁰⁴. In the interpretation of these analyses, the reasonable, but unproved, view that "primary" lead is well-mixed lead from upper crustal rocks was accepted as a working hypothesis. Consequently, correlations of isotopic data with geological occurrence, particularly with stratigraphy, were sought. Galena deposits in the East Kootenay district fall into two distinct isotopic groups; a uniform group and a variable, more radiogenic group. Deposits of the uniform group, including many of the large deposits and all those of stratiform type, are restricted to lower Purcell (Aldridge Formation) rocks and seem to represent a widespread episode of lead-zinc mineralisation 1.2 to 1.4 BY ago. A possible source of this lead is the Aldridge Formation itself, and a possible agent of extraction is connate brine. On the assumption that Aldridge sediments were the source, calculations indicate an age of about 2.6 BY for the provenance of these sediments. In contrast to the uniform leads, the variable leads are found in small veins throughout the Purcell sequence, and probably were emplaced in Mesozoic or Cenozoic times. The isotopic data suggest that these deposits represent lead scavenged from Purcell rocks, perhaps by fluid restricted mostly to fracture systems. Lead isotope analyses reported for other parts of the Belt-Purcell basin are similar to those presented here, permitting speculations on the evolution of lead isotopes over a very large region. For Kootenay Arc deposits, a close correspondence can be demonstrated, between lead isotope compositions and geological characteristics. Structurally concordant deposits, mostly in Cambrian carbonate rocks, differ in lead isotope composition from transgressive deposits. Other workers have suggested on geological grounds that the concordant deposits are significantly older (100 to 500 MY older) than those of transgressive type, and the isotopic data are consistent with this suggestion. Pb²⁰⁶/Pb²⁰⁴ ratios of samples from Slocan City and Sandon camps fit a simple concentric zonal pattern, perhaps related to leaching of lead from country rocks by rising ore-fluid. Lead isotope compositions of four similar stratiform deposits in the Anvil Range area are nearly identical and approximate "primary" leads. The lead may have been derived from the late Proterozoic or Cambrian host rocks at the onset of Cambrian-Ordovician? metamorphism. In conclusion, the writer has demonstrated some close correlations between lead isotope data and observable (i.e. shallow, crustal) geological features. These correlations are regarded as supporting, but not demanding, the conclusion that in all three areas studied the lead (including "primary" leads) is of shallow crustal origin.

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