Element Copper (Cu) is a pinkish-orange colour element found in Group 11, Atomic Number 29, d-block, Mass 63.546 particularly useful in all aspects of electrical appliances. Geologically, copper occurs in Carbon bearing shale (Congo and Zambia), commonly as the copper porphyry deposit type as well as massive sulphides deposits.
Copper porphyry largely occurs in Chile with the biggest world deposit located in Chuquicamata. Other deposits include the Bingham (USA), Grasberg (Indonesia), Oyu Tolgio (Mongolia).
Geology of copper porphyry
Copper porphyry mostly occurs around active continental margin and is usually part of a volcanic arc experiencing active volcanism. The magmatic fluids produced are blocked at some depth to the earths surface where the melt releases volatile (aqueous fluids) which over saturate the fluids. The fluids react with country rocks to form low sulphidation zones where minerals like Gold and Silver are deposited, high sulphidation zones where Gold and Copper are deposited and lastly copper porphyry including some gold and Molybdenum are formed at depths very close to the buried magmatic body. This process is summarized by the retrograde boiling reaction
The solubility of fluids in magmatic melts depend on pressure, so the fluid will escape from melt at the proximal (near surface) part. At very high depths, depending on density rock Vs fluid density, we have the prominence of lithostatic pressure which changes to hydrostatic pressure at shallow depths as we move towards the earth’s surface due to interconnected pore spaces.
The Physical consequences of this reaction is expansion which leads to brecciation and stock works (narrow vein network). The chemical consequences result from hot water generated from dissolution of water soluble rock components like salts within the system. These have an enormous transporting capacity for metals which are carried by the fluids in a metal complex from e.g. [Cl–] which will carry Fe, Cu, Pb, Zn and Sn or [HS–] which carries Au and allows more solubility of the metals. Sulphur present in the system will be dissolved in the fluid phase.
The movement of the magmatic fluids towards the earth’s surface crystalizes anhydrous minerals making the fluid to be more saturated in the hydrous phase. Pressure also falls, reducing the solubility making the fluids to dissolve salts, forming metal complexes, transporting and depositing them (resulting in retrograde boiling and its consequences). The physical fracturing of rocks occurs during expansion of water which is trying to escape the melt.
The brecciated zone will experience the precipitation of minerals amongst which Chalcopyrite (CuFeS3) which is a typical cupper mineral as well as pyrite (FeS2). Quartz which is highly soluble in high temperature fluids will be precipitated in cooler areas of the brecciation (cracks) the reason why it forms veins.
Other copper ores include Chalcosite, generated by the destruction of chalcopyrite (weathering under acidic conditions) and deposition by supergene precipitation (20°C).
Some of the alterations that occur in hydrothermal environments include
Other Cupper Deposits
Lithostratigraphic elements for stratiform Cupper deposits
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