Discrimination of Metamorphic and Metasomatic Processes at the Broken Hill Pb-Zn-Ag Deposit, Australia: Rare Earth Element Signatures of Garnet-Rich Rocks

Abstract

Garnet occurs in a variety of rock types spatially related to the giant stratiform Paleoproterozoic Broken Hill lead-zinc-silver deposit, Australia. 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 rock. The origin of quartz garnetite and garnetite is controversial. Proposed models include: metamorphism of manganiferous sediments formed by submarine hydrothermal processes that mixed with aluminous pelagic sediments; metamorphism of an original detrital sediment; metasomatic mobilization of Mn between the sulfide lenses and the wall rocks either synpeak (D1–D2) or postpeak metamorphism (D3); and reaction of Mn derived from partially melted sulfide orebodies with the surrounding pelitic gneisses. REE patterns of whole-rock samples of garnetite and quartz garnetite show high light rare earth element/heavy rare earth element (LREE/HREE) ratios. Chondrite-normalized REE patterns, based on analyses of garnet in garnetite and quartz-garnetite and some samples of blue quartz gahnite-garnet rock, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques, generally show very low LREE/HREE ratios, flat HREE signatures, and low overall REE contents (<100 ppm). HREE are incorporated in garnet, whereas LREE likely occur in intergranular material between garnet grains and in monazite, zircon, and apatite inclusions in garnet or along grain boundaries. Garnetite and fine-grained quartz garnetite show positive and negative Eu anomalies, respectively. Coarse metasomatic garnet surrounding D3 quartz veins, and metasomatic garnet in quartz garnetite halos around lead-rich orebodies, remobilized quartz garnetite and some blue quartz gahnite-garnet rocks, show positive and negative Eu anomalies, variable HREE, low to moderate LREE/HREE ratios, and arcuate patterns. 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, f O2 conditions, and temperature, whereas arcuate HREE signatures are characteristic of metasomatic garnet that formed postpeak metamorphism. The REE chemistry of garnet is consistent with mineralogical, petrological, and structural studies that demonstrate garnetite and quartz garnetite are metamorphosed sediments that formed by submarine hydrothermal processes.