Mineralogy and geochemistry of diorites and associated hydrothermal sulfide mineralization of Gawuch Formation in Drosh area, Chitral, northern Pakistan
Keywords:
Petrochemistry; Sulfide-mineralization; Gawuch Formation; ChitralAbstract
Sulfide mineralization in Drosh-Shishi area is related to hydrothermal activity which is mainly associated with altered diorite and quartz veins of Gawuch Formation. This Formation comprises variably metamorphosed volcanics and sediments intruded by plutons of diorite and granodiorite composition. Three varieties of diorites are distinguished in the studied area. These include 1) diorites, 2) altered diorites and 3) gneissose diorites. All types of diorites are mineralogically similar having major constituent minerals like plagioclase, orthoclase, amphibole and quartz. Calcite, chlorite, and epidote occur as minor secondary minerals while muscovite, biotite and ore occur as accessory phases.
Detailed geochemistry, mainly based on major and trace elements, suggests that the magmatism responsible for Gawuch Formation was calc-alkaline and contains strong subduction component. Sulfide mineralization occurs in association with diorites and quartz veins, along foliation planes, as dissemination and as supergene enrichment. Tetrahedrite, chalcopyrite, pyrite and galena are dominant ore minerals with subordinate amounts of sphalerite, magnetite, malachite and azurite. Electron microprobe analyses show the pure nature of these ore phases. A variable gain and loss of MnO, K2O, Na2O and P2O5 are noticed in mineralized diorite which shows an enrichment of Cr, Zn, Mo, Cd, Ag and Au. Both mineralized diorites and mineralized quartz veins have a high δ18O signature, suggesting the involvement of isotopically heavy ore forming fluid in the alteration and copper mineralization.
References
Bard, J.P., Maluski, H., Matte, P.H., Proust, F., 1980. The Kohistan sequence; Crust and mantle of an obducted island arc. Geological Bulletin University of Peshawar, 13, 87-94.
Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. The interpretation of igneous rocks. Allen and Unwin, London.
Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Sciences, 8, 523-548.
Khan, M.A., Jan, M.Q., Windley, B.F., Tarney, J., Thirlwall, M.F., 1989. The Chilas mafic-ultramafic igneous complex; the root of the Kohistan island arc in the Himalaya of northern Pakistan. Geological Society of America, Special Paper. 232, 75-94.
Khan, T., Khan, M.A., Jan, M.Q., 1994. Geology of a part of the Kohistan terrain between Gilgit and Chilas, Northern Areas, Pakistan. Geological Bulletin University of Peshawar, 27, 99-112.
Maniar, P.D., Piccoli, P.M., 1989. Tectonic discrimination of granitoids. Bulletin Geological Society of America, 101, 635-643.
Miyashiro, A., 1974. Volcanic rock series in island arc and active continental margins. American Journal of Sciences, 274, 321-355.
Pearce, J.A., Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters, 19, 290-300.
Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for tectonic interpretation of Granitic rocks. Journal of Petrology, 25, 956-983.
Petterson, M.G., Windley, B.F., 1986. Petrological and Geochemical evolution of the Kohistan arc-batholith, Gilgit, N. Pakistan. Geological Bulletin University of Peshawar, 19, 121-149.
Petterson, M.G., Windley, B.F., 1991. Changing source regions of magmas and crustal growth in the Trans-Himalayas: Evidence from the Chalt volcanics and Kohistan batholith, Kohistan, northern Pakistan. Earth and Planetary Science Letters, 102, 326-341.
Pudsey, C.J., Coward, M.P., Luff, I.W., Shackleton, R.M., Windley, B.F., Jan, M.Q., 1985. Collision zone between the Kohistan arc and the Asian plate in NW Pakistan. Transactions of the Royal Society of Edinburgh, Earth Sciences, 76, 463-479.
Pudsey, C.J., Maguire, P.K.H., 1986. Magnetic profiles across the northern suture, Kohistan, NW Pakistan. Geological Bulletin University of Peshawar, 19, 47-60.
Shah, M.T., Shervais, J.W., 1999. The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan. Journal of Asian Earth Sciences, 17, 459-475.
Tahirkheli, R.A.K., Jan, M.Q., 1979. A preliminary geological map of Kohistan and the adjoining areas, N. Pakistan. Geological Bulletin
University of Peshawar, 11, (in pocket).
Tahirkheli, T., Khan, M.A., Shah, M.T., 2005. Geochemistry and petrogenesis of metavolcanic rocks in Gawuch and Drosh formations, Chitral, northern Pakistan. Geological Bulletin University of Peshawar, 38, 189-202.
Tahirkheli, T., Shah, M.T., Khan, M.A, 1998. Oxygen Isotope signature of the hydrothermal copper mineralization in Drosh-Shishi area, Chitral, Pakistan. Acta Mineralogica Pakistanica, 9, 111-116.
Tahirkheli, T., Shah, M.T., Khan, M.A., 1997.
Mineralogical, Isotopic and fluid inclusion studies of the hydrothermal copper mineralization in Drosh- Shishi area, Chitral, Pakistan. Proceedings of National Symposium on economic geology of Pakistan, Islamabad, 59-71.
Wilson, M., 1989. Igneous Petrogenesis, A Global Tectonic Approach. London Unwin Hyman.
Zeitler, P.K., Johnson, N.M., Naseser, G.W., Tahirkheli, R.A.K., 1982. Fission-Track evidence for quaternary uplift of the Nanga Parbat region, Pakistan. Nature, 298, 255-257.
