GARNET ZONING IN THE BROKEN HILL ORE PACKAGE: EVIDENCE FOR METASOMATIC REACTIONS DURING COOLING OF THE PARTIALLY MELTED BROKEN HILL ORE BODY

The Broken Hill ore body is surrounded by rocks containing complex, mostly low-variance assemblages with pyroxenoids, garnet, and quartz: the “ore package”. Plagioclase is absent from the ore package. Unlike pelitic gneiss surrounding the ore package, which contains weakly zoned almandine, the garnet in the ore package is strongly and complexly zoned. The zoning consists of Mn and Ca rich margins and fracture fillings around cores that tend to be richer in almandine. LA-ICPMS analyses of garnets from Broken Hill show that almandine-rich garnets in the pelitic country rock contain ca. 130 ppm REEs, with pronounced negative EU anomalies whereas the Mn- and Ca-rich garnets and the Mn- and Ca-rich overgrowths on almandine contain 30 – 40 ppm REEs with no significant Eu anomalies. These data indicate that REES were also mobile as the garnets grew.

Because garnet has strong affinity for Mn and Ca, the zoning in garnet normally involves Mn and Ca – rich cores with decreasing Mn and Ca outward as these elements are sequestered into garnet by Raleigh fractionation. Our data indicate that the garnets from Broken Hill grew in an environment where the activities of Mn and Ca were increasing. We contend that this took place during the cooling of a polymetallic melt that was formed during granulite metamorphism at Broken Hill. Ca and Mn were compatible with the sulfide melt (CaS and MnS are stable, but rare sulfides) but were incompatible in the galena, sphalerite, and pyrrhotite, which were the major sulfides crystallizing from the melt. Consequently they were released into the country rock where they formed garnet and pyroxenites, depending on the availability of Al. The fluids that were responsible for the transport of Mn and Ca also redistributed REEs. These results mean that: 1) the garnet zoning is prime evidence that the Broken Hill ore package formed during cooling from peak metamorphism, postdating the formation of almandine garnet; it is not a metamorphic version of a metasomatized zone that formed during the primary deposition of the Pb and Zn sulfides of Broken Hill Ore body, and 2) the REE composition of garnets from Broken Hill and other similar metamorphosed ore bodies cannot be used to make inferences about the sea water composition during the formation of the ore body.