Host rock peculiarities and influence of major structures on gold and base metal sulfide mineralization in northern Pakistan

Abstract

For decades, the streams and rivers in northern areas of Pakistan are being explored for gold but the source rock for primary gold is yet to be located. Recent geological surveys have identified several areas for follow-up study for Cu-Au mineralization in Chitral and Gilgit-Baltistan regions. The rocks exposed at various localities are variably altered and/ or mineralized and intensely deformed, which mostly include diorite, granodiorite, greenschists, limestones and undeformed volcanic rocks and granitoids of the Kohistan island arc. The results of petrographic and X-ray diffraction (XRD) analyses of a variety of representative rock samples from a number sites have revealed that mineralization in the Chitral and Gilgit-Baltistan regions is dominantly in the form of pyrite, tetrahedrite, chalcopyrite and galena with subordinate amounts of magnetite, malachite and azurite. The associated/ gangue minerals include albite, quartz, actinolite, biotite, chlorite, epidote, calcite, garnet, illite and titanite (sphene). Diffraction patterns of glycolated samples did not show the existence of swelling clay (e.g., smectite). Sericitic, chloritic and carbonatic alterations have been observed in the studied altered rocks. Mass balance calculations indicate that most of the sulfide-bearing samples are enriched in Cu, Au, Sb, As and Pb and depleted in the rest of the metals reflecting differences in elements mobility during the ore-forming processes. Some of the mineralized rocks especially limestone have experienced a significant volume change due to mobilization of some of the major element oxides e.g. CaO, Fe2O3, SiO2 and ore-forming components e.g. Cu and Pb. The mineralization is of epithermal type in most of the investigated sites, however a few areas show ore formation though orogenic processes and very rarely the ore deposit displays features of the volcanogenic massive sulfide type. Some of the areas are quite promising with respect to their ore element contents and the highly mineralized zones lie in close proximity to major structures in the region. Besides, results from the current study indicate that the mineralization shows close spatial association and hence a strong genetic link with strike slip faults especially where they bend and merge together.