Abstract

The Proterozoic Willyama Complex hosts metamorphosed massive lead-zinc-silver sulfide deposits (including the main Broken Hill lode) distributed across three prograde regional metamorphic zones ranging from middle amphibolite to granulite facies. FeS contents of sphalerites coexisting with pyrrhotite are consistent within a single sample but range widely within a deposit and across the complex showing no correlation with metamorphic grade. Primary (metamorphic) pyrrhotite was iron rich and now consists of hexagonal pyrrhotite with exsolved troilite. Although most primary pyrrhotites were altered to monoclinic pyrrhotite, pyrite, marcasite, or mixtures of these minerals during retrograde or postmetamorphic events, sphalerites were not affected significantly. Closely spaced samples from No. 3 Lens at North mine in the main Broken Hill lode indicate that the domains of sulfide equilibria are less than 30 cm and may not be much larger than the dimension of a polished section.The partitioning of FeS between sphalerite and hexagonal pyrrhotite is a function of P, T, and a FeS . The determination of a FeS for the bulk compositions of pyrrhotites together with analyses of coexisting sphalerites permits an estimation of pressure during regional metamorphism provided that the pyrrhotites have not been altered and the temperature of sulfide equilibration is known. Four samples from the main Broken Hill lode in granulite rocks give mean pressures of 8.1 + or - 0.6 kb for an assumed temperature of final sphalerite equilibration of 650 degrees C, as suggested from sulfur isotopic data, or 9.9 + or - 0.7 kb at 800delta C which is the estimated peak metamorphic temperature.

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