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Eduncle posted an MCQ
October 19, 2019 • 18:15 pm 0 points
  • IIT JAM
  • Geology (GG)

Which of the following are the characteristics of a typical porphyry copper deposit?

I. Associated with granitic rocks II. Low-grade and high tonnage of ore III. Presence of alteration zones IV. Occurrence in convergent tectonic setting

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    Eduncle Best Answer

    Schematic diagram ofthe tectonic setting of porphyry, epithermal and skarn deposits Image Credi t: MBG
    Tectonic setting of porphyry copper and epithermal ore deposits
    Porphyry copper, skarn and epithermal ore deposits are the most common type of hydrothermal ore deposits. They are genetically linked to convergent tectonic margins characterized by calcalkaline magmatism.
    Porphyry copper deposits originate in or nearby porphyritic intrusives which were saturated by water as they intruded in shallow areas of the crust (less than 6 km depth). The developing hydrothermal systems are characterized by the interaction, to different degrees, of fluids coming directly from the parental intrusive and from meteoric surface waters. 
    Porphyry copper deposit, large body of rock, typically a porphyry of granitic to dioritic composition, that has been fractured on a fine scale and through which chalcopyrite and other copper minerals are disseminated. Porphyry copper deposits commonly contain hundreds of millions of metric tons of ore that averages a fraction of 1 percent copper by weight; although they are low-grade, the deposits constitute important sources because they may be worked on a large scale at low cost. They have been formed by the dissemination of copper minerals in the crushed or fractured periphery of intrusive igneous bodies of such granitic rocks as quartz monzonite and diorite porphyry.
    Strong alteration zones develop in and around granitic rocks with related porphyry deposits. 
    Often there is early development of a wide area of secondary biotite that gives the rock a distinctive brownish colour. Ideally, mineralized zones will have a central area with secondary biotite or potassium feldspar and outward 'shells' of cream or green quartz and sericite (phyllic), then greenish chlorite, epidote, sodic plagioclase and carbonate {prophylitic) alteration. In some cases white, chalky clay (argillic) alteration occurs.

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