Metallogeny of Late Paleozoic lode gold mineralization of western Tien Shan: From orogenic shortening to intracontinental strike-slip shearing

Abstract

Extending eastward from the Kyzylkum desert of Uzbekistan, through Tajikistan, Kyrgyzstan and continuing into western China, the late Paleozoic Tien Shan orogen in the southwestern Central Asia Orogenic Belt (CAOB) hosts one of the world's richest gold provinces. The >10,000-metric ton (t) gold endowment of the Tien Shan province occurs within lode gold deposits, which have been variably classified as Kyzylkum-type, mesothermal, orogenic or intrusion-related. The metallogeny of the Late Paleozoic lode gold mineralization in the Tien Shan province comprises two distinct episodes related to the accretionary evolution of the western CAOB. The initial 325 to 305 Ma mineralizing episode is interpreted to be related to regional orogenic shortening and associated contractional deformation during terminal subduction of the Turkestan Ocean. This episode resulted in the formation of sheeted auriferous veins, veinlets and stockworks that are controlled by thrust-related brittle fault systems and associated fracture networks (e.g., Unkurtash and Katebasu). The subsequent 290 to 280 Ma metallogenic episode is the most regionally extensive. It produced some of the world's largest gold accumulations (e.g., Muruntau, Kumtor, Zarmitan etc.) that account at present for more than 97 % of the bulk gold resources of the province. This episode is interpreted to be the product of widespread intracontinental strike-slip shearing that immediately followed the amalgamation between the Tarim–Karakum cratons and the Kazakhstan–Yili–Central Tianshan terranes. Mineralization of this latter episode is generally controlled by ductile shearing with predominantly shear-hosted and fracture-filled gold-bearing veins. Lode gold deposits of the Tien Shan show some unique characteristics compared to typical orogenic or intrusion-related gold deposits. The gold lodes are hosted by Proterozoic–Paleozoic metamorphic carbonaceous flysch and/or syn- or post-collisional granitoids. Hydrothermal alteration associations display diversity that depends upon the varying crustal level of the gold mineralization and its spatial distance from granitoid intrusions. The H-O-C and HeAr isotope ratios suggest a dominantly metamorphic fluid with variable contribution of magmatic water and/or mantle-derived deep fluids in a medium-temperature, CO₂-rich, low-salinity H₂O–CO₂–NaCl system. Available geological, sulfur and lead isotope data indicate multiple potential sources for the ore metals and their source reservoir that include metamorphic carbonaceous flysch, granitoid intrusions or the deep mantle. Compilation of apatite fission track (AFT) data suggests that the western segment of the Tien Shan in Central Asia underwent less extensive post-ore exhumation, whereas its eastern segment in China was subjected to a more complex and prolonged exhumation history. Post-ore differential uplifting and exhumation played an important role in forming the heterogeneous endowment of the different segments of the Tien Shan gold province.