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Magmatic-Hydrothermal Ore Deposits
Porphyry Deposits
Porphyry deposits are a type of magmatic-hydrothermal deposit
and are subduction zone related. They normally host copper
(chalcopyrite, bornite), gold (in Cu phases), tin (cassiterite -
SnO2), tungsten (wolframite) and molybdenum (molybdenite - MnS2).
All porphyries are associated with granites / granitic rocks, in
particular, porphyritic granite, from which the deposit gets its
name. Porphyritic granites contain large phenocrysts (crystals
formed in the magma chamber) and fine groundmass indicating rapid
cooling after phenocryst formation. Porphyry: large, low grade
metal deposit associated with granite. Epigenetic: ore
mineralisation added to a previously existing rock (e.g. porphyry
deposit). Syngenetic: host rock and ore mineralisation formed at
the same time (e.g. banded iron formations).
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Most porphyry deposits have very large tonnage but low grade.
Significant amounts of metal and other elements (Cu, Au, Cl, S)
come out of volcanoes in gases. Cl and S are the most popular
ligands - elements that make metals soluble, for example AuHS. They
are very wet, unlike mafic rocks
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Associated with island arcs andSubduction zones
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- the first stage in the formation of Porphyry copper deposit is
the intrusion of a sub-volcanic magma to a depth ~ 4 km. The magma
type is I-type (e.g. granite I-type magma) and thus has high
volatile contents (H2O, CO2, Cl, etc).I-magma intrusion
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-in the second stage, the sub-volcanic magma chills against the
country rocks, thus crystallizing magma close to the country
rockmagma crystallization
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separation of magmatic fluid- in stage three, magmatic fluids
(hydrothermal fluid or water volatile content) separate during the
crystallization. This process is known as the second boiling.
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In this stage, pressure starts to build-up as the magmatic fluid
boils to form steam, producing increase in volume. This process is
known as the first boiling.pressure build-up
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In stage 5, the pressure generated by first boiling results to
the fracturing of the crystallized magma and country rocks as the
pressure build-up is greater than pressure of the country
rocks.fracturing and formation of stock work
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In stage 6, the fracture of the crystallized magma and country
rocks results to rapid fluid escape into the fracture network known
as stock work; deposition of ore mineral in the stock work, as the
magmatic fluid contains copper mineral. This stage is also part of
second boiling.
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Requirements for the formation of porphyry CuI-type (e.g.
granite / granadiorite I-type magma) and thus has high volatile
contents (H2O, CO2, Cl, etc).crystallization at low pressures to
form anhydrous phases (~4 km); intermediate depthExsolvation of
fluids at a certain pressurefirst boiling and second boiling they
wont sulfur-saturate (because Cu will stay with sulfur)
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In the last stage, the magmatic fluid may undergo phase
separation into low density vapour and brine phases. The dense
brine will tend to pond at the top of the intrusion. The potassic
alteration develops close to the core of the system and propylitic
alteration further out.
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Porphyries and water
Water is the crucial factor in forming porphyry deposits Wet
magmas can travel higher in the crust than dry magmas, however, as
soon as they reach a pressure low enough to exsolve water, they
stop and crystallise in place, whereas dry magmas move
incrementally, fractionating (crystallising) on the way up. The
addition of water to granitic systems causes melting to occur at a
much lower temperature than it otherwise would, that is, the
liquidus moves to a lower temperature. A substance that causes
melting to occur at a lower temperature than normal is a flux.
Other examples are CO2, boron, and fluorine (topaz and tourmaline
are common minerals in granitic pegmatites).
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The Albite-H2O system is a good example of this as it is simple
and reflects the behaviour of all rock-water systems.
The maximum melting temperature of albite is ~1100oC at 1
atmosphere (rising with increasing pressure). As more water is
added to the system (5%, then 10%) (red lines) the liquidus moves
to a lower temperature (blue lines).
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Granites in porphyry systems are fractured due to the release of
water. This water then carries away all ore-forming elements, and
deposits them some distance above / away. This is why a dry granite
is worthless when it comes to forming porphyries. Chlorine, which
dissolves in the melt, is also carried away when the water exsolves
and forms compounds with metals such as copper and tin.
If you start crystallising at low pressure, hydrous phases are
formed. These phases take water out of the magma, so that at the
end you are only crystallising anhydrous phases. The result is that
the magma doesn't become saturated in water, and a porphyry is
unable to form.
If you crystallise the magma at high pressure, however,
anhydrous phases form, so the magma becomes water saturated! The
term used to describe the depth at which porphyry deposits form is
hypabyssal, which means intermediate depth
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Another important concept with respect to porphyry ore formation
is boiling. Boiling is what concentrates the ore metals in the
fluid and causes them to be deposited. First boiling is
decompression saturating the magma in water which then exsolves
(just take P down ). Second boiling is saturation of magma by water
caused by the crystallisation of anhydrous phases . Usually a
combination of both occurs, and the whole process can be summed up
as:
H2O in granite > saturate > exsolve fluid >boil
(concentrate) > deposit
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Alteration
Wall rock alteration is always present around porphyry deposits.
When water exsolves from the granitic magma, it causes the
surrounding rocks to crack and a water saturated carapace (a shell
around the magma) is formed. The released water is extremely hot
and is able to alter the rocks around the granite
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Hot fluid passing through the rock not only changes the
composition of the rock (alteration) but this in turn changes the
composition of the ore-bearing fluid. The changes in rock and fluid
compositions causes several alteration zones to form around the
igneous rock. These are described in order from innermost to
outermost alteration:
Potassic (K-metasomatism): Very high temperature fluid.
K-feldspar replaces most other minerals. Other secondary minerals
include sericite and biotite. This type of alteration is
particularly indicative of porphyry deposits. Phyllic (acidic):
Characterised by quartz-sericite-pyrite assemblage. Argillic:
Characterised by kaolinite (clay). Propylitic: As the fluid has
cooled significantly by this stage, this type of alteration can be
found all over the world and so is not very indicative of any
particular deposit. It is characterised by
chlorite-epidote-carbonate.
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The following alteration reactions occur (in order): K-feldspar
to sericite (consuming H+): 3KAlSi3O8 + 2H+ > KAl3Si3O10(OH)2 +
6SiO2 + 2K+ Sericite to kaolin (H-metasomatism, hydrogenating):
2KAl3Si3O10(OH)2 + 2H+ + 3H20 > 3Al2Si2O5(OH)4 + 2K+ Hydrogen
comes from the ore-forming reaction: CuCl2 + FeCl2 + H2S + 1/4O2
> CuFeS2 + 1/2H2O + 3H+ + 4Cl-
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Ore is found in the potassic and phyllic zones, where boiling
occurs. Aluminium is not a very mobile element, and normally the
only way to increase its abundance is to take everything else away
from it. As you remove potassium and iron, you increase alumina.
Alkalis (K, Na, etc) are easily remobilised and deposited near the
core, hence potassic alteration. The next rocks out are affected by
fluid that has lost its potassium but is rich in hydrogen (H+), and
is therefore acidic. This rock is more aluminium rich, and
muscovite is produced. Finally chlorite and epidote are produced in
the outer rocks. Hydrous phases are not made initially because the
water is too hot.
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Different types of porphyries
All porphyries are formed in the same way. So how do you make
different metal deposits? It turns out that it is not so much the
type of melt but the melt's history that forms different deposits,
specifically, the magma's oxidation state. It is also important to
remember Goldschmidt's rule - an element must have the same valency
and size to replace another element.
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Magnetite is found in oxidised magmas while ilmenite is found in
reduced magmas. Copper deposits form from oxidised granites and are
not fractionated Tin deposits, on the other hand, form from reduced
granites and are highly fractionated, meaning that the magma spent
a lot of time crystallising during its ascent and as a result
altered the melt composition.
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Why don't oxidised magmas make tin deposits? In an oxidised
magma the valency of tin is 4+. So there must be something that
takes Sn4+ out of the magma easily. An example of a mineral Sn4+ is
compatible in is sphene - CaTiSiO5. Ti generally has a valency of
4+, so tin substitutes readily into the mineral to make molailite -
CaSnSiO5 . DSn4+sphene/melt = 70; DSn2+xals/melt < 1, so while
Sn4+ is more compatible in a mineral, Sn2+ prefers to stay in the
melt and so forms tin deposits.
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How to make a porphyry (Cu, Mo, Sn, W, Au...):
need a wet granite (~6.4 wt% H2O, Xwm ~ 0.5) must crystallise at
low enough pressure to form anhydrous ph ases, but high enough to
prevent explosion exsolve fluid at the right pressure don't
sulfur-saturate (Cu will stay with sulfur) appropriate fO2
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How not to make a porphyry:
have a dry granite (anorogenic, found at centres of continents)
crystallise deep (at high P) crystallise at very low P (let it
erupt) sulfur-saturate wrong fO2
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EXPLORING FOR COPPER
The concentration of a metal in an ore is called its grade.
Grade is usually expressed as a weight percentage of the total
rock. For example, 1000 kilograms (kg) of iron (Fe) ore that
contains 300 kg of iron metal has a grade of 30%:Grade = (kg metal
/ kg rock ) x 100 Most of the world's copper comes from porphyry
cooper deposits located primarily in South America, New Guinea,
Indonesia, the United States, and Canada.
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Vertical cross section showing a porphyry copper deposit as it
occurs deep within the earth. (Modified from Evans, 1980)
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In addition to forming ore deposits, this circulating water can
form large bodies of altered rocks surrounding the stocks known as
alteration zones. Minor copper mineralization can be formed away
from the stocks within thin planar bodies known as veins. However,
this mineralization does not usually contain enough copper to be
considered ore.
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Exploration Techniques One important technique is geologic
mapping. A geologic map shows the distribution of the various rocks
at the surface of the earth. In the case of porphyry copper
deposits, geologists know that such deposits usually form on the
outer edges of the igneous stocks and within alteration zones. Once
a map is constructed, the geologists can focus their exploration
activity in these areas.
Another common exploration technique is called geochemical
exploration Another commonly used geochemical exploration technique
is soil geochemistry. Geologists establish a sampling grid over an
area of interest
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One difficulty in using sediment and soil geochemistry to
explore for ore deposits is the occurrence of anomalies related to
human activities. Construction of bridges often produces high
concentrations of metals in sediments. Pollution from industry or
landfills can impart high metal content to soils, streams, or the
atmosphere. Such geochemical anomalies produced by human activities
can be confused with anomalies that might indicate the presence of
ore deposits.
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Paleogene Magmatism
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Golden QuadrilateralNeogene magmatism
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