Ore Geology Mineralogy Franklin Sterling Hill Zinc deposits 1 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 emplacement Metamorphism 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 forming Further deposition Area subsides deposition of quartzite and then limestone Kittatinny in Cambrian Area uplifted and partially eroded again in Tertiary Ore 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 anywhere Ore 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 dedolomitization created oxide solid solutions yielding zincite and franklinite with Willemite formed by reacting zincite with silica Ore 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 system Ore model problems Very little to no thermodynamic data on the reactions to form zincite willemite and franklinite at metamorphic conditions No other deposit for comparison Fluorescence Phosphorescence
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