EVIDENCE FOR HIGH-T MN- AND CA-METASOMATISM IN THE BROKEN HILL ORE PACKAGE

The Broken Hill Pb-Zn-Ag ore deposit is surrounded by a characteristic, roughly zoned silicate halo called the ore package. The host rock to the ore body is a pelitic gneiss with the assemblage Kspar-plag-biotite-garnet-sillimanite-cordierite. Adjacent to the ore package the host rock lacks plag and has assemblages ranging from biotite-muscovite-sillimanite-garnet- staurolite to Kspar-biotite-sillimanite- garnet. Inboard of this lie garnet-quartz-biotite and garnet-quartz assemblages. Adjacent to the massive ore are garnetite (monomineralic Mn-Ca-Fe garnet rock) and pyroxenoid rock, with the ore most commonly found within the pyroxenoid rock. X-Ray maps show that the garnet touching biotite in the pelitic horizon close to the ore is typical almandine with garnet-biotite temperatures ranging from 580° to 630°C. Where the garnet is entirely surrounded by quartz the garnet has a thin (< 10mm) rim that is enriched in both Ca and Mn. The garnet-biotite rocks have typical almandine garnets with thicker Ca and Mn-rich rims (up to 50 mm). Ca-reaches a maximum on the inner portion of the rim with Mn reaching a maximum on the outer rim. Garnet from the garnetite has compositions with nearly equal amounts of grossular, spessertine, and almandine. The "reverse" zoning in the garnet in the ore package is strong evidence that the Ca and Mn metasomatism occurred at high temperatures and that the ore package is not an artifact of zoning that was associated with the primary deposition of the ore deposit. The mineral assemblages in the pelitic country rock adjacent to the ore indicate that these assemblages formed during falling temperatures from peak conditions of T=800°C P=5 Kbar. Experiments show that Mn in sulfides does not react with adjacent silicates unless the sulfides have melted. For this reason we contend that the metasomatism was produced by fluids released during crystallization of a sulfide melt during cooling of the Broken Hill ore.