Slide 1Slide 21 billion years agoMetamorphismFurther depositionOre depositOre MineralsOre placement (1)Ore placement (2)Ore model problemsFluorescence/PhosphorescenceOre Geology/MineralogyFranklin-Sterling Hill Zinc deposits1 billion years ago•Basement rocks are a Precambrian oceanic suite of sedimentary rocks•Then a suite of rocks deposited in a rift setting (includes gneiss, marble, amphibolite)•Closing of this rift (associated with Grenvillian orogeny) at 1.1 Ga with granite emplacementMetamorphism•Rocks intensely folded and metamorphosed at temperatures up to 800C and 11-16 km deep (almost 5 kilobars P!) about 1 Ga•Erosional period, rocks uplifted at ~0.01-0.07mm/year•Final exhumation at 650 Ma – deposit was at least partially exposed, with some karst features formingFurther deposition•Area subsides, deposition of quartzite and then limestone (Kittatinny) in Cambrian•Area uplifted and partially eroded again in TertiaryOre deposit•Encased in the Precambrian Franklin marble•Originally the ore stuck out of the ground like a wall – 100-200 feet high in places!•Unique – no other deposits like this are known anywhereOre Minerals•Franklinite ((ZnMnFe)2O4)•Zincite (ZnO)•Willemite (Zn2SiO4)•‘protore’ or proto-ore was likely in place prior to metamorphism (syndepositional) but was changed significantly as the rock was metamorphosed, exhumed, reburied, and metamorphosed again…Ore placement (1)•Proto-ore was a Zn-Fe-Mn dolomite mixed with silica gel and Fe-Mn oxides•Under high P-T metamorphism and de-dolomitization created oxide solid solutions yielding zincite and franklinite, with Willemite formed by reacting zincite with silicaOre placement (2)•Proto-ore formed from shallow Zn-Fe-Mn rich sediments or oxidation of an existing Zn-Fe-Mn rich sulfidic orebody•Subsequent metamorphism occurred in a largely closed systemOre model problems•Very little to no thermodynamic data on the reactions to form zincite, willemite, and franklinite at metamorphic conditions•No other deposit for
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