Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization |
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| Authors: | I S McCallum M W Thurber H E O'Brien B K Nelson |
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| Institution: | (1) Department of Geological Sciences, Box 351310 University of Washington Seattle, WA 98195, USA e-mail: mccallum@u.washington.edu, US |
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| Abstract: | Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from
the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between
sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic
equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement”
radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids
derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which
cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally
derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility
that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism
cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation
between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples
that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios
consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals
(serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic
Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides
during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise
leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization
and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over
a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the
time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of
platinum group elements (PGE) within the ore zone.
Received: 30 December 1998 / Accepted: 16 June 1999 |
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