1. Copper is a key material for modern life, being especially important with regards to wiring
and electronics because of its conductive properties. Understanding copper mineralisation
is therefore of key strategic importance to humanity. To understand copper mineralisation,
one must consider its identity under the Goldschmidt Classification of the elements - that
of a chalcophile. Being a chalcophile element, it generally partitions well into chalcogen
minerals - primary associations being S and As. This affinity is greater than that for
oxygen. The minerals formed are therefore relatively dense compared to the crust, and so
chalcophiles are depleted in the crust relative to the bulk earth - though not to the extent of
siderophiles. Copper, along with nearly all mineralisation types primarily is found at sites of
active, or past-active tectonism. Deposits typically require a concentration factor of 80 from
the background continental crust abundance of copper (0.005%). Deposit types include
the three most important - porphyry, clastic sediment hosted and VMS. Deposits of lesser
current importance include layered intrusion deposts, sulphide immiscibility deposits, iron
oxide-copper-gold deposits, and manganese nodules. Secondary enrichment of many of
the aforementioned deposits can result in higher grades and lesser processing costs.
Important Cu ore minerals include the sulphides chalcopyrite, bornite, chalcocite, covellite,
digenite, the hydrated carbonates azurite and malachite, and cuprite, an oxide.
Porphyry deposits are not actually of that high a grade - they can often be economical at
grades as low as 0.2%. Their profitability lies in their immense size and resource
tonnages, and the economies of scale. Their profitability and size has resulted in them
meeting 60% of world demand for copper. Deposits of this type include Bingham Canyon,
Utah (mined for over 100 years), alongside El Teniente and Chiquicamata of Chile. Most
often found in arc volcanic regions, the first step in the mineralisation process is the
creation of an I type, metaluminous, (relatively) oxidised, and water-poor batholith at a
subduction zone, which rises through the crust due to its low relative density. The
batholiths are charged with a specific element profile, given to them through mass transfer
from the subducting slab. The exact method of transfer is currently a matter of debate -
fluid transfer of elements coincidentally allowing for melt creation, or diapirs of low density
LILE rich mélange being included in such melts. In any case, the transfer of LILEs, Pb, the
semi-metals, Cu, Cl, S, and water would appear to be very efficient.
Porphyry Cu deposit can be understood in terms of stages:
1. First boiling - some volatiles lost from a magma that has now been saturated by
interaction with groundwater or through lower crustal pressures
2. Carapace formation - a hardened shell of solidified magma prevents the escape of more
volatiles.
3: Carapace fails - possibly due to fresh melt injection or further crystallisation raising the
pressure.
4: Second boiling - the magmatic volatile phrase separate into metaliferous brines and low
density vapour phases, and these rapidly vacate the intrusion, forming hydrothermal
breccia and a disseminated mineralisation pattern of stockwork veins rich in ore minerals.
The porphyritic texture of the
5: Quick cooling of the volatile phase by groundwater results in a contraction effect,
resulting in sericitic alteration textures being overprinted on those already present
Porphyry copper deposits are often associated with distal epithermal gold-silver deposits.
Their association has lead to their dual economic importance being both a source of gold,
and a possible indicator of ‘blind’ porphyry deposits, which are not surface apparent.

2. The second most important types of copper deposits are clastic sediment hosted, which
account for 25% of world Cu production. Whilst thin, usually < 3m, and never > 30m, they
are very laterally laterally continuous, resulting in huge tonnages and high long term
profitability. One type of sediment hosted copper deposit is best known by the name of its
type-rock - the Kupferschiefer ‘copper shale’ marl of central Europe. The deposits are now
understood to be epigenetic, with early theories focusing on their stratiform nature instead
of overprinting textures. Whether they are syndiagenetic or syntectonic seems to be
deposit specific, the later being proposed for particular Zambian deposits. In any case,
Kupferscheifer deposits are found at the ‘redox boundary’ between red, oxidised
sandstones, and transgressive, reduced, dolomitic mudrocks. The reduction of the fluids
flowing through the sandstone is believed to have been carried out by either pyritic
sulphur, or sulphur found within shale hydrocarbon compounds (e.g. H2S). The second
type is similar, but with the reduced mudrock equivalent being an arkose or conglomerate.
The movement of diagenetic brines which may be introduced via basin scale faults are
hypothesised to bring up footwall and basement leached metal complexes; these are
deposited mainly as disseminated ore.
VMS deposits are relatively well understood in literature. This is mostly because we have
directly observable modern VMS deposit analogues forming today - black and white
smokers and their surroundings. VMS deposits are most often found as multiple lenses
with stockwork bases in ophiolite sequences. This ophiolite association means that
deposits are primarily arc-associated, where ophiolite preservation is more frequent and
complete (particularly in back arcs). That said, many form at MORs, and some may form in
association with hotspot activity. Particularly for the latter, low density oceanic crust
accretion may be important for preservation. It is believed, at any rate, that the formation
mode is as such: A deep, penetrating fault allows for circulation of ocean water (specifically
ocean water, according to Pb isotope data) into rocks undergoing high rates of heat flow
beneath the sea bed. These heated fluids then leach copper, lead, and semi-metals from
the rocks through which they are flowing, and bring them to the surface. It is considered
most likely that the bulk of the metal budget comes from the underlying rocks, and is not
significantly magma-derived (S isotopes). It is also considered likely that the faults must be
in extension to allow for significant transfer of fluid back to the surface. Various lithological
surroundings (pelitic, siliciclastic, mafic, felsic) are associated with specific metal
abundances, though the ‘core set’ of copper, iron, lead and zinc remains dominant.
Significant upgrade of all of the prior systems can be carried out via surface weathering
above the water table, which oxidises the primary copper sulphides. These ‘supergene’
processes can significantly increase the value of a deposit, for copper is cheaply and
easily extracted from more oxidised copper minerals, like cuprite, covellite, chalcocite,
azurite and malachite. The grades of such bodies are often higher than the original
deposit. IOCG deposits are poorly understood but are again fluid associated. Sulphide
immiscibility melts are sources of copper, though they are primarily mined for other metals
like nickel. The same is true of layered intrusions, more mined for Cr and PGE.
In summary, mineralisation of copper happens in strong association with sulphur and other
chalcogen elements due to its chalcophillic tendencies. Plate boundary and basinal
processes enable a variety of genesises, related to either element cycling within the crust
and upper lithosphere, or increased heat flow resulting in strong convection of fluids. Non
magmatic fluids play a key role in the removal of copper from both melt and lithified rock
sources, often as chloride brine complexes. Deposition of these metal complexes is
progressive, and is ultimately based upon the temperature-solubility curve of the mineral
within the fluid, often resulting in zoned deposits in association with other base metals.