week 4 Flashcards
Cu content in igneous rocks
- common igneous rocks generally have a low abundance of ore minerals
- e.g. the low Cu content of most igneous rocks
~ 50 ppm in ultramafic rocks
~ 100 ppm in mafic rocks
~ 25 ppm in felsic rocks
what are the three processes of ore formation in magmatic systems
- accumulation and concentration of ore minerals in magma chambers during progressive magma crystallisation e.g. chromite, magnetite and vanadium in layered mafic intrusions
- separation of two immiscible melts in magma e.g. Ni-sulphide ores in komatite lavas
- incorporation of a mineral that occurs at a specific depth in the earth
e.g. diamond deposits in kimberlites, which form at the graphite-diamond transition in the mantle
what are layered mafic intrusions
- LMIs are thought to be related to plumes and/or continental rifting. they cool slowly and the mafic magma has a relatively low viscosity
- low viscosity means high-density minerals (e.g. olivine and pyroxene, >3 g cm^-3) crystallising from the melt sink whereas less dense minerals (e.g. feldspathoids, <2.5 g cm^-3) float.
describe ore minerals in LMIs
- the processes that produce the sub-horizontal, well-ordered layering in most LMIs do not account for the occasional development of monomineralic layers of ore minerals (e.g. chromite or vanadium-rich magnetite) found in many LMIs around the world
- many mafic intrusions contain ~0.5-1m thick layers of near-monomineralic chromite (FeCr2O4) or magnetite (Fe3O4) that extend laterally for tens of km, representing huge reserves of Cr and Fe-V ore.
chromium - added to steel to reduce corrosion (makes it stainless steel)
vanadium - added to steel for strength
describe the Bushveld Complex, South Africa
- has the world’s largest LMI, covering an area of ~67,000 km^2
- contains >75% of the world’s chromite reserves
- chromite = iron chromium oxide (FeCr2O4)
other economically valuable metals include: - iron
- tin
- titanium
- vanadium
- platinum group metals (PGMs): Pt, Pd, Os, Ir, Rh, Ru
describe komatiites and Ni sulphide ores
komatiites - ultramafic lavas with MgO > 18% and end up to 30% MgO. volcanic equivalent of peridotite
- high eruption temps of 1400 degrees C to >1600 degrees C
- predominantly Archean in age
- discovered in the Komati River, Barberton Archean greenstone belt, South Africa
- Komatiites are predominantly Archean due to higher heat production at that time. heat production from decay of radioactive isotopes was ~2-3x greater in the Archean than today and the mantle was ~100 - 250 degrees C hotter
describe Ni sulphide deposits in Australia
- IN 2019, Australia was the largest holder of economic nickel resources, with ~24% of global resources
- most is produced from mines within the Archean craton of Western Australia
- 82% of its nickel production derives from komatiite deposits associated with Archean greenstone belts
how did the sulphur get into the komatiites
- assimilation of sediment by thermally-eroding komatiitic lava channels enhanced the sulphide content within the lava and promoted local sulphide saturation
describe sulphide-silicate immiscibility
- chalcophiles such as Ni were scavengedd from the turbulent, flowing komatiite lava by immiscible sulphide globules
- the dense globules sank and accumulated as massive sulphide ore along the bottom of the channels
summary of Ni sulphide ore formation
- the high T of komatiite lavas combined with turbulent flow made them efficient at eroding a substrate composed of rocks with a lower melting temperature
- assimilated sulphur-rich sediment combined with Ni from the komatiite to form an immiscible liquids that settled to the bottom of the channels
describe surficial and supergene ore formation
- once metals have been concentrated in the crust and exposed at the surface, they are commonly subjected to further concentration by chemical weathering
- this process often leads to the creation of a viable ore deposit and many ores would not be mineable without enrichment in the surficial environment
e.g. - laterite Ni-CO deposits
- bauxite deposits (aluminium)
- porphyry copper deposits
describe chemical weathering
- the driving agent for ore formation is the downward percolation of meteoric water (O2 and H2O) into near-surface rock
- rainwater has low concentrations of solutes and is undersaturated with respect to all minerals so minerals will readily dissolve into fresh meteoric water
- weathering leaches the more soluble chemical components from the rock and enhances porosity
- a proportion of the dissolved material is carried away from the site of weathering in solution to rivers and oceans
describe lateritic deposits
- laterite is an intensely-weathered regolith that forms in warm, humid, tropical regions. rich in Fe and Al, they develop by prolonged weathering of the underlying parent rock
- mostly composed of Fe, Al, Ti and Mn oxides because these are the least soluble components of rocks undergoing chemical weathering. primary source of aluminium ore (bauxite) but can also host metals such as Ni, Mn, Au, Cu and PGE
describe bauxite deposits
- bauxite is the main source of Al. formed from a mixture of three hydrates of alumina: mainly gibbsite (Al2O3.3H2O) + diaspore and boehmite (Al2O3.H2O)
- it results from tropical weathering of Al silicate rocks e.g. granite or arkose sandstone
- usually mixed with Fe oxides, giving it a deep red colour
- bauxite must contain sufficiently high levels of Al2O3 and suitably low levels of Fe2O3 and silica to be economically extractable. supply is dominated by Australia
describe Ni-CO laterite deposits
- Ni-Co laterite account for >60% of the global Ni supply and 20-30% of the Co supply
- much shallower than magmatic sulphide Ni deposits - easier and cheaper to mine
- they form by intensive tropical weathering of Ni-bearing, olivine-rich mafic rocks such as dunite, peridotite and komatiite
- typically contain ~0.3% Ni. enrichment zone can contain >3% Ni
- due to many years of exploitation of Ni sulphide ores, they are being depleted and/or are more expensive to mine so companies are increasingly turning to Ni laterites
- but although these near-surface ores are relatively attractive from a mining perspective, they are generally mineralogically complex and challenging to process
describe the supergene enrichment of PCDs
- hypogene sulphides (e.g. chalcopyrite, bornite) are chemically unstable in high oxidation state environments at or near the Earth’s surface and are dissolved during weathering
- Cu is released into solution and may be leached out of the ore body or precipitated in secondary minerals, typically oxides, carbonates or new sulphides
- the secondary minerals form either at the site of dissolution of the primary ore minerals or elsewhere in or around the ore body
describe the leached cap (gossan) and leached zone
- near the surface, a primary sulphide deposit can be oxidised to form hydroxides and sulphuric acid
4FeS2 + 15O2 + 14H2O -> 4Fe(OH)3 + 8H2SO4
pyrite + oxygen + water -> iron hydroxide + sulphuric acid - if Cu is present (e.g. in chalcopyrite), a similar reaction produced Cu sulphate. this is take up in solution and percolates downward. the remaining Fe forms insoluble hydroxide or oxide (e.g. hematite) that remains at the surface forming the red leached cap or ‘gossan’
4CuFeS2 + 17O2 + 10H2O -> 4Fe(OH)3 + 4H2SO4 + 4CuSO4
chalcopyrite + oxygen + water -> iron hydroxide + sulphuric acid + copper sulphate
describe the oxide zone
- just above the water table, native Cu and Cu oxides (cuprite, tenorite), carbonates (malachite, azurite), sulphates (gypsum) and silicates are produced
- 2CuSO4 + 2Na2CO3 -> Cu2(CO3)(OH)2 + 2Na2SO4 + CO2
- copper sulphate + sodium carbonate -> malachite + sodium sulfate + CO2
describe the secondary sulphide enrichment zone
the sulphate-bearing water percolates down until it reaches reducing conditions below the water table, when metals are precipitated out e.g.
5FeS2 + 14CuSO4 -> 7Cu2S + 5FeSO4 + 12 H2SO4
pyrite + copper sulphate -> chalcocite + iron sulphate
CuFeS2 + CuSO4 -> 2CuS + FeSO4
chalcopyrite + copper sulphate -> covellite + iron sulphate
describe sulphur-oxidising bacteria
- present in the Gossan and Leached zones
- sulphur-oxidising bacteria are lithoautrotrophic aerobic bacteria. they obtain their energy from the oxidation of Fe and use this energy to fix CO2 into cell biomass for growth
- they have been used to speed up acid mine leaching for a long time but it wasn’t clear whether they naturally formed in supergene enrichment regions
- it is estimated that this increases the reaction rate at least five times compared to just abiotic chemical reactions
describe sulphate-reducing bacteria
extremophile archaea - microbes that love extremes
- the opposite process is carried out by sulphur-reducing anaerobic bacteria, such as Extremophile archaea
- these are organic compounds that convert sulphates to hydrogen sulphide (H2S)
as a byproduct of metabolism
- copper then reacts with the H2S to form, e.g. covellite (CuS):
Cu2+ + H2S -> CuS(s) +2H+
what is the supergene goldilocks zone
for supergene enrichment to occur,
- there must be precipitation and the ground should not be frozen
- it is important that any bacteria are not dried or frozen - hot climates are better for bacteria to survive
- not too much water (everything is flushed away) or tectonic uplift (the deposit is eroded away)
