Unveiling the Wealth of Ore Deposits and Minerals

1. Ores • Principally we discuss ores as sources of metals • However, there are many other resources bound in minerals which we find useful • How many can we think of?

2. Ore Deposits • A deposit contains an unusually high concentration of particular element(s) • This means the element(s) have been concentrated in a particular area due to some process • What sort of processes might concentrate these elements in one place?

3. Gold  Au • Distribution of Au in the crust = 3.1 ppb by weight  3.1 units gold / 1,000,000,000 units of total crust = 0.00000031% Au • Concentration of Au needed to be economically viable as a deposit = few g/t  3 g / 1000kg = 3g/ 1,000,000 g = 0.00031% Au • Need to concentrate Au at least 1000-fold to be a viable deposit • Rare mines can be up to a few percent gold (extremely high grade)!

4. Ore minerals • Minerals with economic value are ore minerals • Minerals often associated with ore minerals but which do not have economic value are gangue minerals • Key to economic deposits are geochemical traps  metals are transported and precipitated in a very concentrated fashion • Gold is almost 1,000,000 times less abundant than is iron

5. Economic Geology • Understanding of how metalliferous minerals become concentrated key to ore deposits… • Getting them out at a profit determines where/when they come out

6. Ore deposit environments • Magmatic • Cumulate deposits – fractional crystallization processes can concentrate metals (Cr, Fe, Pt) • Pegmatites – late staged crystallization forms pegmatites and many residual elements are concentrated (Li, Ce, Be, Sn, and U) • Hydrothermal • Magmatic fluid - directly associated with magma • Porphyries - Hot water heated by pluton • Skarn – hot water associated with contact metamorphisms • Exhalatives – hot water flowing to surface • Epigenetic – hot water not directly associated with pluton

7. Geochemical Traps • Similar to chemical sedimentary rocks – must leach material into fluid, transport and deposit ions as minerals… • pH, redox, T changes and mixing of different fluids results in ore mineralization • Cause metals to go from soluble to insoluble • Sulfide (reduced form of S) strongly binds metals  many important metal ore minerals are sulfides!

8. Hydrothermal Ore Deposits • Thermal gradients induce convection of water – leaching, redox rxns, and cooling create economic mineralization

9. Ore deposit environments • Sedimentary • Placer – weathering of primary mineralization and transport by streams (Gold, diamonds, other) • Banded Iron Formations – 90%+ of world’siron tied up in these (more later…) • Evaporite deposits – minerals like gypsum, halite deposited this way • Laterites – leaching of rock leaves residual materials behind (Al, Ni, Fe) • Supergene – reworking of primary ore deposits remobilizes metals (often over short distances)

10. Ore Deposit Types I • Placer uranium gold • Stratiform phosphate • Stratiform iron • Residually enriched deposit • Evaporites • Exhalative base metal sulphides • Unconfornity-associated uranium • Stratabound clastic-hosted uranium, lead, copper • Volcanic redbed copper • Mississippi Valley-type lead-zinc • Ultramafic-hosted asbestos • Vein uranium • Arsenide vein silver, uranium • Lode Gold

11. Ore Deposit Types II • Clastic metasediment-hosted vein silver-lead-zinc • Vein Copper • Vein-stockwork tin, tungsten • Porphyry copper, gold, molybdenum, tungsten, tin, silver • Skarn deposits • Granitic pegmatites • Kiruna/Olympic Dam-type iron, copper, uranium, gold, silver • Peralkaline rock-associated rare metals • Carbonatite-associated deposits • Primary diamond deposits • Mafic intrusion-hosted titanium-iron • Magmatic nickel-copper-platinum group elements • Mafic/ultramafic-hosted chromite