Porphyry copper deposit
Porphyry copper deposits (or copper porphyries , after English porphyry copper deposits or copper porphyries ) are copper - deposits that at plutonic rocks ( intrusions ) with high to intermediate content of silicon dioxide are bonded. The name of the deposits is slightly misleading, because it refers to the often porphyry structure of the igneous host rocks (large, well-formed single crystals in a fine-grained or glassy matrix), not to the structure of the copper mineralization itself. This type of deposit is usually on Subduction zones bound. On the surface, open-minded deposits often have a tertiary age.
General
Because of the often enormous volume of the mineralized rocks of usually 50 to 100 million tons, porphyry copper deposits, despite their low ore content of usually only 0.4 to 1% copper, are among the most important copper sources in the world today. There are also small amounts of other metals, such as molybdenum , gold and silver . This type of deposit was successfully mined for the first time in the Southwest of the USA in the 1920s, and since then, mass mining in open-cast mining has become increasingly important compared to classic underground mining . Important examples can be found especially in the great fold mountains ( orogenes ), such as the Andes. These include Chuquicamata in Chile , the largest open pit mine in the world, Bingham Canyon Mine (Utah, USA), Cerro Colorado (Panama) in Panama and El Chino in New Mexico . The former Mamut Copper Mine in Malaysia also belonged to the type of porphyry copper deposits.
Mineralization
The ore occurs very finely in the host rock, mostly along fine hairline cracks, sometimes also in larger veins. This type of mineralization is referred to as "impregnation ore " (English disseminated ore ). With a larger irregular network of capillaries, the term "floor" is common (after English stockwork ), although "ore body" would be the more appropriate translation into German. Occasionally, areas of broken rock with angular, sometimes slightly rounded fragments ( breccias ) appear. The mineralization from sulphide minerals (especially chalcopyrite and molybdenite ) is then located especially in the open crevices between the fragments, but also within the fragments themselves. Crevices are also often filled with sulphides or with sulphide-containing quartz veins. Particularly high-grade mineralization is found where several closely spaced fissures intersect.
The host rocks are mostly irregular or approximately cylindrical, multiphase intrusions and rock dikes , which are at least partially magma chambers or conveyor chimneys of former volcanoes. The most common are “acidic” to intermediate deep rocks of the granite family. With decreasing silica content, these are granite , granodiorite , tonalite , quartz monzonite , diorite . There is also an intermediate series from diorite to monzonite to syenite . Even non-igneous rock in the immediate vicinity of the intrusions is often mineralized.
Construction of the deposit
A characteristic feature of the porphyry copper deposits is a regular sequence of hydrothermal alteration zones around the intrusion. These were created when overheated, aggressive, mineral-containing, hydrothermal solutions flowed through the rugged rock and reacted with the existing minerals. Knowing these zoning provides valuable information for the exploration ( prospecting ) and exploration ( exploration ) of such deposits.
- The fresh igneous rock in the center is mostly (but not always) surrounded by a potassium-rich metasomatosis zone ( potassic zone ) in which the primarily formed feldspars orthoclase and plagioclase , as well as various mafic minerals, are displaced by secondary biotite and orthoclase and / or chlorite were. The inner area of this zone is often ore poor to ore empty.
- Further outward a closed quartz - sericite zone ( phyllic zone ), which towards the edge always rich in clay minerals is (Serizitisierung or advanced Argillitisierung). Since this conversion of primary feldspars and biotite is a silica-releasing reaction, a lot of secondary quartz is also formed (silicification). This zone coincides with the most common occurrence of pyrite . The areas containing the most copper are often located directly within this so-called pyrite shell, in the transition area to the potash metasomatosis.
- The zone of intermediate argillic zone , where the newly formed clay mineral kaolinite occurs, is not always formed.
- The outer edge is formed by the propylitic zone with chlorite, calcite and epidote . This zone, which slowly merges into the host rock, is always formed and can give an indication of the existence of a porphyry copper deposit, even if no copper mineralization is exposed on the earth's surface .
Emergence
The mineralizing phase is often related to the most differentiated and recent intrusion (late in the volcanic cycle). The intrusions apparently mostly take place passively, for example after parts of the roof of the magma chamber sink in. It is believed that the still glowing trunk magma rises to a kilometer or two below the surface of the earth, where it gets stuck. As the water-free minerals begin to crystallize, the remaining water and other volatile components increasingly accumulate in the residual melt. This increases the vapor pressure until it finally exceeds the surrounding lithostatic pressure. There is a sudden outgassing of the magma, with a corresponding increase in volume and the characteristic small-scale disruption of the host rock, through the finest cracks and fissures. The increase in volume of the gas phase is even greater near the earth's surface, which would explain the formation of the chimney-like breccias in which the hydrothermal solutions flowing through would have even partially rounded off the rock fragments. The remaining magma can now cool down quickly, which leads to the formation of small and uniform crystals in the matrix, which enclose the large single crystals that formed early, the eponymous porphyry structure.
Isotope investigations on gases and liquids ( fluids ) enclosed in the rock suggest that a substantial part of the hydrothermal solutions, with their high contents of metals and sulfur, originate from the magma itself. These mainly cause the potash metasomatosis. Due to the temperature gradient between the cooling intrusion and the secondary rock, fossil (connate) water from the secondary rock and sinking meteoric water from the earth's surface are heated and included in the hydrothermal cycle, which leads to the formation of the outer alteration zones. In the transition zone between these two hydrothermal systems, there are now strong gradients in pH , temperature and salinity , which probably causes the precipitation of the copper sulfides.
In general, porphyry copper deposits in the continental crust have a higher molybdenum content , while they are characterized by a higher gold content in the area of island arcs . The exact role of the lithosphere penetrated by the magma is still part of scientific discussions.
Recent studies on the genesis of porphyry deposits identify the following points as the main factors for the formation of important deposits:
- multiple subsequent flows of magma into the magma chambers
- a sulfur-saturated magma chamber
- an efficient transfer of the metals to hydrothermal solutions.
Individual evidence
- ^ WL Griffin, GC Begg, Suzanne Y. O'Reilly: Continental-root control on the genesis of magmatic ore deposits , nature geoscience, October 13, 2013, doi: 10.1038 / ngeo1954
- ↑ Jamie J. Wilkinson: Triggers for the formation of porphyry ore deposits in magmatic arcs , nature geoscience, October 13, 2013 doi: 10.1038 / ngeo1940
literature
Anthony M. Evans: Mineral Deposition Studies . Ferdinand Enke Verlag, Stuttgart 1992. ISBN 3-432-99801-5 .