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?

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

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.

Minerals associated with economically recoverable metals

• Elemental forms• Sulfides• Oxides• Carbonates• Sulfate salt

Elemental copper

Chalcocite, Cu2S Chalcanthite, CuSO4*5H2OMalachite, Cu2CO3(OH)2

Cuprite, Cu2O

Sulfides Part 1

• Substitution into sulfides is very common

• As and Se substitute for S very easily

• Au can substitute in cation sites (auriferrous minerals)

• Different metals swap in and out pretty easily Cu and Fe for instance have a wide range of solid solution materials

Sulfide Minerals• Minerals with S- or S2- (monosulfides) or

S22- (disulfides) as anionic group

• Transition metals bonded with sulfide anion groups

Iron Sulfides• Mackinawite – FeS

• Greigite – FexSy

• Pyrite – FeS2 (cubic)

• Marcasite – FeS2 (orthorhombic)

• Troilite – FeS end member

• Pyrrhotite – Fe1-xS (slightly deficient in iron)

• Arsenopyrite – FeAsS

• Chalcopyrite – CuFeS2

Other important sulfides• Galena – PbS

• Sphalerite/wurtzite – ZnS

• Cinnabar – HgS

• Molybdenite – MoS

• Covellite – CuS

• Chalcocite – Cu2S

• Acanthite or Argenite – AgS

• Stibnite – Sb2S3

• Orpiment – As2S3 ; Realgar – AsS

Sulfides are reduced minerals what happens when they contact O2?

• This is the basis for supergene enrichment and acidic mine drainage

Actively Oxidizing Pyrite

• FeS2 + 3.5 O2 + H2O Fe2+ + 2 SO42- + 2 H+

• FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+

• 14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O

• Sulfur species and H+ generation:– FeS2 + 2 Fe3+ 3 Fe2+ + ¼ S8 + 0 H+

– FeS2 + 7 Fe3+ + 3 H2O 8 Fe2+ + 0.5 S4O62- + 6 H+

AMD neutralization• Metals are soluble in low pH

solutions – can get 100’s of grams of metal into a liter of very acidic solution

• HOWEVER – eventually that solution will get neutralized (reaction with other rocks, CO2 in the atmosphere, etc.) and the metals are not so soluble but oxidized S (sulfate, SO4

2-) is very soluble

• A different kind of mineral is formed!

Ely Mine