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Ore Geology/Mineralogy

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 )

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Ore Geology/Mineralogy

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  1. Ore Geology/Mineralogy

  2. Franklin-Sterling Hill Zinc deposits

  3. 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 Grenvillianorogeny) at 1.1 Ga with granite emplacement

  4. 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

  5. Further deposition • Area subsides, deposition of quartzite and then limestone (Kittatinny) in Cambrian • Area uplifted and partially eroded again in Tertiary

  6. 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

  7. 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…

  8. 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 silica

  9. Ore placement (2) • Proto-ore formed from shallow Zn-Fe-Mn rich sediments or oxidation of an existing Zn-Fe-Mn rich sulfidicorebody • Subsequent metamorphism occurred in a largely closed system

  10. 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…

  11. Fluorescence/Phosphorescence

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