LATE-STAGE METASOMATIC GARNET GROWTH ADJACENT TO THE BROKEN HILL PB-ZN-AG SULFIDE ORE BODY

Zoning patterns in garnets proximal to Pb-Zn-Ag sulfide bodies at Broken Hill, Australia provide compelling evidence for pronounced Mn and Ca metasomatism during cooling from peak metamorphic conditions (5 Kbars, 800C). Garnets in pelitic rocks more than several meters from massive sulfide bodies are compositionally homogeneous or very weakly zoned with slightly elevated spessartine in the cores and almandine on the rims. Cores of garnets in pelites within several meters of massive sulfide bodies are similarly compositionally homogeneous, but these garnets almost invariably have spessartine and/or grossular-rich rims separated from the cores by abrupt boundaries. Some samples contain both larger garnets with compositionally distinct rims (CDRs) and numerous, smaller garnets with compositions indistinguishable from rims on the larger garnets. Muscovite coexisting with spes/gros rims on garnets is heavily embayed in most samples and overgrown by K-feldspar in some samples. Development of CDRs on garnets is most extensive within a few meters of galena - Ag-rich projections of sulfide ore (“droppers”), generally less extensive adjacent to Cu and Pb-rich zones, and least extensive adjacent to Fe-Mn-Zn sulfide bodies. In general, width of rims is inversely proportional to muscovite abundance. Minute sulfide inclusions are common along fractures and within garnets with CDRs.

CDRs grew during highest grade metamorphic conditions with T > 700 C. Spes/Gros-rich rims must have formed later, at lower temperatures and accompanied by metasomatic addition of Mn and Ca to the rocks because (a) sharp compositional discontinuities between cores and rims would not have been preserved at temperatures in excess of 700C, (b) muscovite is clearly a reactant in the production of the rims and it would not have been stable with quartz at T > 700C, and (c) early-formed garnet would have incorporated all available Mn in the earliest stages of garnet growth. Experiments indicate that Mn in sulfides does not react with adjacent silicate unless the sulfides have melted. Metasomatic effects are most pronounced adjacent to the lowest-melting sulfide compositions (droppers) and probably formed when lithophile elements (eg., Ca & Mn) were lost from the crystallizing sulfide melt.