Garnet-rich Rocks and their Genetic Relationship to the Broken Hill Lead-Zinc-Silver Deposit, New South Wales, Australia

The stratiform Paleoproterozoic Broken Hill deposit, Australia, is the largest accumulation of lead-zinc-silver in the world (280 million tons of 10% Pb, 8.5% Zn, and 148 g/t Ag). It is located within a rift in an upward-coarsening sequence of clastic metasediments into which Paleoproterozoic mafic and felsic melts have intruded, probably while the sediments were wet. The deposit was metamorphosed to granulite facies conditions (750-800^oC and 5-6 kb) and was subjected to at least five periods of deformation (D₁-D₅). Garnet occurs in a variety of rock types, including sillimanite-bearing pelitic schist, felsic gneisses (e.g. feldspar-quartz-garnet-biotite rock [Potosi Gneiss]), pegmatite, banded iron formation, and the stratigraphic equivalents of sulfide rocks, as a hanging wall syn- to late-D₂ halo to sulfides. Garnet also occurs as discordant halos on the margins (up to 50 cm wide) of post-D₃ epidotized dolerite dikes, lode pegmatite, quartz veins (so-called garnet envelope), and massive sulfides. However, the garnet-rich rocks that are the best indicator to each of the six main orebodies sulfides are quartz-garnetite and garnetite. Quartz-garnetite and garnetite are massive or banded and contain a variety of accessory silicates, sulfides, phosphates, and oxides. Laminae in these rocks are predominantly coplanar with S₀ and S₁ and garnetite is also associated with F₂ and F₃ sulfide projections (droppers). Some droppers contain epidotized dolerite clasts suggesting post-D₃ formation. Coarse garnets (up to 1 cm in diameter) in one massive metasomatic variety of quartz garnetite cross-cut laminated fine-grained massive quartz garnetite and garnetite. This remobilized variety, as well as discordant gahnite-bearing quartz garnetite, likely formed during D₂ or D₃. Retrograde shear zones (D₄, D₅ and possibly later deformation events) contain transposed cataclastic sulfide masses with a quartz-garnet rim thereby showing that sulfide mobilization and formation of quartz-garnet envelope occurred over 1,100 million years with every deformation and intrusive event at Broken Hill. Over 1000 garnet analyses reveal that garnet compositions in ore, garnetite, and quartz-garnetite are generally enriched in Fe in C and B lodes, in Ca in 2 lens, in Mn in 3 lens and A lode, and both Fe and Mn in 1 lens. However, considerable variations in Ca, Mn, and Fe contents of garnet occur in 2 lens in the central part of the deposit. Garnets are generally homogeneous in composition but can show Mn-enriched metasomatic overgrowths. We are opposed to the concept recently proposed in the literature that garnetite formed as a residue of partial melting of the deposit. Instead, we suggest that the Fe and Mn component of garnetite and quartz garnetite are products of exhalation or inhalation at or near the sea-floor. Some varieties of these rocks, including garnet envelope, formed by metasomatic processes throughout the long prograde and retrograde metamorphic history affecting the deposit.