Manganese-rich garnet rocks associated with the Broken Hill lead- zinc-silver deposit, New South Wales, Australia.
Rocks rich in manganoan garnet are spatially and genetically related to metamorphosed sulfide deposits and represent exploration guides. At the subject deposit, three varieties of garnet-rich rock are found at wallrock margins or as blocks within ore. Quartz-bearing garnetite, the most abundant, contains up to 80 pct garnet. Garnetite contains 80%-95% garnet and occurs on the margins of the lead lodes and as blocks in the a lode. "Garnet envelope," a narrow zone of garnet-rich rock at ore body margins, is variable, from discordant to a prograde schistosity, or is concordant to a retrograde schistosity in the wallrocks, or surrounds late-stage quartz-fluorite veins. The precursors to quartz-bearing garnetite and garnetite formed when mn-rich emanations from hydrothermal hot springs mixed with pelagic clays and were subsequently metamorphosed. The "garnet envelope" apparently was formed during a retrograde metamorphic event by the reaction of fluids from the ore horizon and al-rich wall rock. Equilibrium calculations reveal that fe oxides and sulfides are stable at lower oxidation potentials than mn minerals. It is shown that fe minerals normally precipitate from solution before mn minerals, regardless of the nature of the aqueous species present. This fractionation of fe and mn can explain the increase in mn/fe from the stratigraphic footwall relative to the hanging wall of the Broken Hill Deposit.