PETROLOGY AND ECONOMIC MINERAL INVESTIGATION IN KAR GAH VALLEY, KOHISTAN ISLAND ARC, GILGIT, PAKISTAN

Abstract

This study deals with petrology and economic mineral investigation in Kar Gah Valley, Gilgit. The thesis area was situated approximately five kilometers towards west of Gilgit city and bounded by latitude ~ 35⁰ 45‟ to 36°00‟ longitude ~ 74°00 to 74°15‟ E. The rocks exposed in the Kar Gah Valley represent igneous and metamorphic rocks belonging to Kohistan island arc. Igneous rocks such as diorites, granodiorites, granites (Kohistan Batholith), basalts, dolerite dykes and metamorphites (paragneisses, slates, marble and green schist) were the lithological constituents of the Kar Gah Valley. The metamorphic rocks were grouped as the Jaglot group comprising the Thelichi and Gilgit formations and Gashu-Confluence volcanics in between them. The Kohistan Batholith intrudes the Jaglot group. Sulphide mineralization is noticed in a zone that extends from Shingai Gah and terminates at Chelili Gah within the Thelichi formation, which comprises basalts, basaltic dykes, dolerites, slates, siliceous schist and marble metamorphosed into greenschist metamorphic facies. Heavy metal analysis in altered rocks of the studied area reveal concentrations of gold, silver, copper, cobalt, lead, zinc, manganese, nickel, chromium and cadmium out of which gold, silver and copper have relatively enriched. Petrographic studies reveal hydrothermal alteration in the rocks. The minerals include amphibole, plagioclase, as primary minerals and chlorite, epidote, biotite, muscovite, sericite, calcite, clayey material as secondary minerals formed after the alteration of volcanic rocks. Quartz occurs as primary and secondary mineral in all the rock types. Opaque includes oxides and sulphides. XRD analysis also confirms the petrographic study. Based on field and laboratory study, the mineralization seems to be polymetallic vein type and fracture controlled. It is suggested that polymetallic mineralization in the area is due to postcollisional granitoid intrusions within the Thelichi formation and the Kohistan Batholith after the ceasing of subduction processes.