Geological characteristics, tectonic setting and preliminary interpretations of the Jilau gold-quartz vein deposit, Tajikistan

The southern Tien Shan metallogenic province of General Asia hosts a number of important gold resources including the Jilau gold-quartz vein system in western Tajikistan. These deposits were formed at the late stages of conti nent-continent collision in association with subduction-related magmatism, metamorphismm and continental margin deformation attributed to the Central Asian Hercynian Orogeny. Jilau is hosted by a Hercynian syntectonic granito id intrusive that was emplaced into bituminous dolomite country rocks. Econ omic mineralisation is associated with a dilational jog within a high-angle , oblique dextral-reverse slip shear zone that was undergoing brittle-ducti le deformation. The orebody takes the form of shear-zone subparallel quartz veins and lenses that emanate from a steeply plunging ore shoot of veins a nd stringers within a silicified and sulphidised granodiorite core. It is t hought to have formed by a dynamic process in which fluid flow was governed by a fault-valve mechanism. Numerous cycles of fluid pressure build-up, fa ult failure, jog dilation, fluid flow, phase separation of low salinity H2O -CO2-CH4(-N-2) fluids, and sealing took place. Gold appears together with s cheelite and bismuth minerals predominantly as inclusions in arsenopyrite i n quartz veins and altered wall-rock, and is mainly associated with quartz containing Fluid inclusions enriched in CH4. The correlation between high g old grades and high CH4 concentrations suggests that components of the mine ralising fluids were derived from, or passed through, the reducing, carbona ceous rocks in the contact aureole of the intrusive. The occurrence of Au a nd W in an adjacent Hercynian skarn deposit and in the Jilau orebody, infer s that the ore metals in both these systems were ultimately derived from a magmatic source.