• Principally we discuss ores as sources of
• 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%
• 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 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
Black smoker metal precipitation
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
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)
Geochemical Traps
• Similar to chemical sedimentary rocks – must leach
material into fluid, transport and deposit ions as
• 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
• 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,
– 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,

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