Transcript Lecture 20 - Ore deposits

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
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
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
Cuprite, Cu2O
Elemental copper
Chalcocite, Cu2S
Chalcanthite, CuSO4*5H2O
Malachite, Cu2CO3(OH)2
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, SO42-) is
very soluble
• A different kind of mineral is
formed!
Ely Mine