Porphyry copper deposit

Porphyry copper deposits (or copper Porphyries, after English porphyry copper deposits or copper porphyries ) are copper deposits, which in plutonic rocks ( dykes) are bound with a high to intermediate content of silica. The name of the deposits here is slightly misleading, because it refers to the often porphyritic texture of igneous host rocks (large, well-trained single crystals in a fine-grained or glassy groundmass ), not on the microstructure of the copper mineralization itself This type of deposit is usually at subduction zones bound. On the surface, open-minded deposits often have a tertiary age.

General

Given the often enormous volume of mineralized rocks of usually 50 to 100 million tons are porphyry copper deposits, despite their low head grade of usually only 0.4 to 1 % copper, among the most important sources of copper in the world. In addition, small amounts of other metals, such as molybdenum, gold and silver find. These types of deposits was mined successfully for the first time in the 1920s in the southwestern U.S., and since the mass loss has open pit, compared to the classic underground mining, gained more and more importance. Important examples are found especially in the large fold mountains ( orogens ), such as the Andes. These include Chuquicamata in Chile, the largest open pit in the world, Bingham (Utah ), Cerro Colorado ( Panama) Panama or El Chino in New Mexico. The former Mamut Copper Mine in Malaysia belonged to the type of porphyry copper deposits.

Mineralization

The ore occurs very finely distributed in the host rock, usually along fine hairline cracks, sometimes even in larger veins. This type of mineralization is referred to as " Imprägnationserz " (English disseminated ore ). With a larger irregular network of veins is also the term " floor " usual (after English stockwork ), although " Erzstock " the more appropriate translation into German would. Sometimes there ranges from broken rocks with angular, some slightly rounded fragments on ( breccia ). The mineralization of sulfide minerals ( mainly chalcopyrite and molybdenite ) are then especially in the open gaps between the fragments, but also within the fragments themselves fractures are also often filled with sulfides or sulfide- bearing quartz veinlets. Especially high-grade mineralization can be found where intersect several closely spaced joint sets.

The host rocks are mostly irregular or nearly cylindrical, multi-phase intrusions and dykes, which are at least partially is magma chambers or chimneys conveying of former volcanoes. The most common " acidic " are to intermediate plutonic rocks of the granite family. With decreasing silica content, these are granite, granodiorite, tonalite, quartz monzonite, diorite. In addition there exists an intermediate range of diorite monzonite to syenite over. Even non- igneous rock in the immediate vicinity of the intrusions is often mineralized.

Structure of the deposit

A characteristic feature of porphyry copper deposits is a regular sequence of hydrothermal alteration zones around the intrusion. These are formed when overheated, aggressive, mineral-rich hydrothermal solutions flowed through the rugged rocks and responded with the existing minerals. The knowledge of these zoning provides valuable information for the exploration ( prospecting ) and exploration ( exploration ) of such deposits.

• The fresh igneous rock in the center is often (but not always) of a kalireichen metasomatism zone ( potassic zone ) surrounded, in which the feldspars initially formed orthoclase and plagioclase, and various mafic minerals replaced by secondary biotite and orthoclase and / or chlorite were. The inner area of this zone is often erzarm to erzleer.
• Further out, a quartz -sericite zone ( phyllic zone ) closes, which always richer in clay minerals is ( Serizitisierung, or advanced Argillitisierung ) toward the edge. Since it is in this transformation of primary feldspar and biotite to a silica -releasing reaction, a lot of secondary quartz formed ( silicification ). This zone coincides simultaneously with the common occurrence of pyrite. The kupferhaltigsten areas are often located directly within this so-called pyrite envelope, in the transition region for potash metasomatism.
• The zone of intermediate Argillitisierung ( argillic zone ), where especially the newly formed clay mineral kaolinite occurs is not always formed.
• The outer edge is the Propylitisierung ( propylitic zone ) with chlorite, calcite and epidote. This zone, which merges slowly into the surrounding rock, is always trained and can give an indication of the existence of a porphyry copper deposit, even if no copper mineralization is open at the surface.

Formation

The mineralizing phase is often linked ( at a late stage of volcanic cycle ) with the most sophisticated and recent intrusion. The emplacement of the intrusions appears to happen mostly passive, for example, after parts sink from the roof of the magma chamber. It is believed that the tribe still glutflüssige magma rises up to one or two kilometers below the earth's surface, where it gets stuck. Due to the incipient crystallization of anhydrous minerals, the remaining water and other volatile components accumulates more and more in the residual melt. This increases the vapor pressure until it eventually exceeds the surrounding lithostatic pressure. It comes to 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 crevices. Near the earth's surface, the increase in volume of the gas phase is even larger, which would explain the formation of the chimney- like breccia in which the through-flowing hydrothermal fluids would have the rock fragments even partially rounded. The remaining magma can now quickly cool down, resulting in the formation of small and uniform crystals in the matrix that surround the large single crystals formed early, the eponymous porphyritic texture.

Isotope studies on trapped in the rock gases and liquids (fluids) suggest that a substantial part of the hydrothermal solutions, with their high levels of metals and sulfur from the magma itself come. This effect mainly potash metasomatism. Due to the temperature gradient between the growing cold intrusion and the country rock but also fossil ( connate ) water is heated from the surrounding rock and sinking meteoric water from the surface, and included in the hydrothermal cycle, resulting in the formation of the outer alteration zones. In the transition zone between these two hydrothermal systems now are strong gradients in pH, temperature and salinity, which probably causes the precipitation of copper sulfides.

In general, porphyry copper deposits in continental crust has a higher molybdenum content, while they are distinguished in the field of island arcs by a higher gold content. The exact role of the battered by the magma of the lithosphere is still part of scientific discussions.

Make recent studies on the genesis of porphyry deposits as the main factors for the development of significant deposits following points from:

• Multiple -flow of magma in the magma chambers
• A sulfur- saturated magma chamber
• An efficient transfer of metals in hydrothermal solutions.