Geological, geochemical and geochronological characteristics of the Jiashi sandstone-hosted Cu deposit in the Kalpin fold-thrust belt in front of South Tianshan Orogen and their implications

The fold-thrust belts in front of the South Tianshan Orogen host numerous sandstone-hosted copper deposits and occurrences within Meso-Cenozoic continental red beds. The Jiashi Cu deposit (>2 Mt ore @ 1.2 % Cu in proven reserves) is particularly noteworthy among these sandstone-hosted Cu deposits, and orebodies are hosted in continental sandstone of the Paleogene sedimentary sequences. The copper mineralization at Jiashi is characterized by disseminated, veined, stripped and massive sulfides, which are predominantly composed of chalcocite with minor bornite and chalcopyrite. Chalcocite is inferred to have a hydrothermal origin based on its primary sulfide characteristics, similar compositions of major elements, and analogous δ³⁴S_v-CDT values (−33.34 ‰ ∼ −35.78 ‰) for chalcocite grains from core to rim, although the copper mineralization is superimposed by later supergene oxidation. Eight chalcocite samples were dated using Re-Os isotopic method, yielding an isochron age of 21.2 ± 3.6 Ma, which was regarded as the mineralization age. It is slightly younger than the ore-bearing sandstone of the Paleogene sequences (23.4 ∼ 38 Ma), indicating an epigenetic origin. Based on these geological characteristics and the new mineralization age, we propose that significant sandstone-hosted copper mineralization occurred during the Early Miocene at Jiashi, formed in an intracontinental orogenic environment with coupling of thrust activation and large-scale fluid migration. These findings indicate that the fold-thrust belts in front of the South Tianshan Orogen possess considerable potential for the discovery of additional large-scale sandstone-hosted copper deposits.