DISCRIMINATION OF METAMORPHIC AND METASOMATIC PROCESSES AT THE BROKEN HILL LEAD-ZINC-SILVER DEPOSIT, AUSTRALIA: RARE EARTH ELEMENT SIGNATURES OF GARNET IN GARNET-RICH ROCKS

Garnet occurs in a variety of rock types spatially related to the stratiform Paleoproterozoic Broken Hill deposit, which was the largest accumulation of lead-zinc-silver mineralization in the world (280 million tons of 10% Pb, 8.5% Zn, and 148 g/t Ag). The deposit was metamorphosed to granulite facies conditions and subjected to at least five periods of deformation (D1-D5). Sulfides at Broken Hill are commonly associated with quartz garnetite, garnetite, and blue quartz-gahnite-garnet rocks (BQGGR). The origin of quartz garnetite and garnetite is controversial and includes: metamorphism of manganiferous exhalites/inhalites mixed with aluminous pelagic sediments; metamorphism of an original detrital sediment; metasomatic interaction between the deposit and the wall rocks either syn-peak (D1-D2) or post-peak metamorphism (D3); and reaction between partially melted orebodies and the surrounding pelitic gneisses. Previously published REE patterns for whole rock samples of garnetite show chondrite normalized high light rare earth element/heavy rare earth element (LREE/HREE) ratios. Normalized REE patterns, using LA-ICP-MS techniques, of garnet in garnetite, quartz garnetite, and some BQGGR generally show very low LREE/HREE ratios, flat HREE signatures, and low overall REE contents (< 100 ppm). Garnetite and fine-grained quartz garnetite show positive and negative Eu anomalies, respectively. HREEs are incorporated in garnet whereas LREEs occur in intergranular material between garnet grains. Coarse metasomatic garnets surrounding D3 quartz veins, and metasomatic garnet in quartz garnetite halos around lead-rich orebodies, remobilized quartz garnetite and some BQGGR, show positive and negative Eu anomalies, variable HREEs, and low to moderate LREE/HREE ratios. The REE signature of garnet in garnetite and quartz garnetite is characteristic of metamorphic garnet with Eu anomalies reflecting relative inputs of detrital to hydrothermal components in a premetamorphic protolith, fO₂ conditions and T, whereas arcuate normalized HREE signatures are characteristic of metasomatic garnet that formed post-peak metamorphism. The REE chemistry of garnet is consistent with mineralogical, petrological and structural studies that demonstrate garnetite and quartz garnetite are metamorphosed exhalites/inhalites.