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?

http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf

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?

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)!

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

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

http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf

Black smoker metal precipitation

http://oceanexplorer.noaa.gov/explorations/02fire/background/hirez/chemistry-hires.jpg

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

• 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)

Ore deposit environments

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)

Hydrothermal Ore Deposits• Thermal gradients induce convection of water –

leaching, redox rxns, and cooling create economic mineralization

Massive sulfide deposits• Hot, briny, water

leaches metals from basaltic ocean rocks

• Comes in contact with cool ocean water

• Sulfides precipitate

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.