Paragenesis of stratiform zinc-lead mineralisation from Besti Gol, Chitral, Hindu Kush, Northern Pakistan

Abstract

Zinc-lead mineralisation in Besti Gol, Chitral is located within the Hindu Kush terrane, 60 km to the northwest of Northern suture that marks the suture zone between the Karakoram plate and the Kohistan island arc. A small suture zone, Tirich Boundary Zone, is located 20 km to the northeast of the investigated area that separates the Karakoram from East Hindu Kush. The zinc-lead mineralisation is mainly confined to a marble horizon within the Jurassic Arkari Formation. The conformability of the sulphide-mineralised zone with the host marble indicates that it has been precipitated in synsedimentary environment and can be classed as a stratiform deposit. The salinities of hydrothermal fluids at Besti Gol range from 2 to 14 wt % NaCl equivalent, indicating that brines generated in a sedimentary basin were responsible for the precipitation of these deposits during diagenesis. Subsequent metamorphism induced distinct textural changes in ore minerals during recrytallisation. Coarse grain size, euhedral morphology and triple junction in the calcite also specify metamorphism. The variation of FeS content in sphalerite indicates crytallisation during sedimentation or a low-pressure-temperature environment followed by high temperature, sulphur and oxygen fugacity during the metamorphism. This was followed by the hydrothermal activities where sphalerite was converted into smithsonite and hydrozincite, and galena into cerussite and plattnerite. The enrichment of the F, Li, As, Rb, Ba, Sc and Zr as compared to average carbonate rocks, coupled with the positive correlation among trace elements such as As with Ba, Sc and Sr within the marble beds, further establishes hydrothermal alteration. The study area has undergone at least two phases of deformation in a collisional environment. It is concluded that the early phases of stratiform zinc-lead mineralisation may have been deposited during the closure of the suture zone, Tirich Boundary Zone, which lies to the SW of the mineralised zone.