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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?

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3 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?

4 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 = % Au Concentration of Au needed to be economically viable as a deposit = few g/t 3 g / 1000kg = 3g/ 1,000,000 g = % 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)!

5 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

6 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

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11 Black smoker metal precipitation

12 Magmatic Ore deposit environments 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

13 Ore deposit environments Sedimentary Placer weathering of primary minerals and transport by streams (Gold, diamonds, other) Banded Iron Formations 90%+ of world s iron tied up in these 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)

14 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 Sulfides (reduced form of S) strongly binds metals many important metal ore minerals are sulfides! Oxides Oxidizing environments form (hydroxy)oxide minerals, very insoluble metal concentrations (especially Fe, Mn, Al)

15 Hydrothermal Ore Deposits Thermal gradients induce convection of water leaching, redox rxns, and cooling create economic mineralization

16 Massive sulfide deposits Hot, briny, water leaches metals from basaltic ocean rocks Comes in contact with cool ocean water Sulfides precipitate

17 Vermont Copperbelt Besshi-type massive sulfide deposits Key Units: Giles Mountain formation More siliciclastic, including graphitic pelite, quartoze granofels (metamorphosed greywacke), hornblende schist, amphibolite Standing Pond Volcanics mostly a fine grained hormblende-plagioclase amphibolite, likely formed from extrusive basaltic rocks (local evidence of pillow structures in St. Johnsbury). Felsic dike near Springfiled VT yielded a U-Pb age of 423± 4 Ma. Waits River formation Calcareous pelite (metamorphosed mudstone), metalimestone, metadolostone, quartzite.