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).

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

Associated with island arcs andSubduction zones

- 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

-in the second stage, the sub-volcanic magma chills against the country rocks, thus crystallizing magma close to the country rockmagma crystallization

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.

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

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

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.

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)

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.

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).

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).

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

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

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

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.

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-

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.

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.

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.

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.

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

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

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.

Vertical cross section showing a porphyry copper deposit as it occurs deep within the earth. (Modified from Evans, 1980)

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.

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

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.

Paleogene Magmatism

Golden QuadrilateralNeogene magmatism

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