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 de-dolomitization 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