Geochronology, pyrite trace elements, and in-situ S isotopes of the giant Nagengkangqie’er silver deposit in the Eastern Kunlun Orogenic Belt, Northern Tibetan Plateau

Abstract

The Nagengkangqie'er (Nageng) silver deposit, situated in the Eastern Kunlun Orogenic Belt (EKOB), Northwestern China. It contains 5,070 tons of contained silver at a silver grade of 325 g/t. Nonetheless, the genesis of the Nageng silver deposit is still under debate, due to the absence of definitive constraints the mineralization age and the metal sources. The U-Pb data analysis of a post-ore calcite vein established the minimum age of the mineralization at 207 ± 3 Ma, whereas the maximum age of 217 ± 4 Ma was determined from zircon U-Pb data obtained from the altered and mineralized rhyolitic porphyry host-rock. These temporal limitations suggest that the mineralization formed at ca. 217–207 Ma, and it is associated with the post-collisional setting of the Paleo-Tethys evolution in the EKOB. The distinct textural patterns, as revealed by petrographic observations, suggest three types of pyrites: py1 is composed of euhedral pyrite in the early barren quartz vein at stage 1, py2 is represented by subhedral-anhedral grains in massive ore veins at stage 2, and py3 forms euhedral crystals of varying sizes, which are interspersed with early-formed marcasite in the quartz + calcite + silver-bearing sulfosalt-dominated veins at stage 3. In-situ δ34S values of pyrites range between −2.33 ‰ and −0.37 ‰, indicating a dominantly magmatic origin. EPMA studies have revealed that silver in the Nageng deposit predominantly occurs as silver-bearing galena at stage 2, whereas silver is enriched in sulfosalts of stage 3. From py1 to py3, a decrease in the Co-content indicates a decrease in the ore-forming fluid temperatures. The presence of stage 3 sulfide, sliver-bearing sulfosalt, and calcite-quartz in a fracture-filled vein indicates low-pressure conditions during its formation, compared to stages 1 and 2. In summary, the Nageng deposit is a typical magmatic-hydrothermal vein-type Ag-Pb-Zn deposit. Fluid cooling and decrease in pressure, coupled with decreases in fO2 and fS2, are the parameters inferred to have led to a decrease of silver solubility in the hydrothermal fluids and, successively, promoted extensive Ag deposition.