Post on 19-Dec-2015
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