Genetic links between multistage gold mineralization and magmatism in the Tanjianshan goldfield, northern Tibetan Plateau: A geochronological and geochemical perspective of dikes and pyrite

Abstract Gold deposits in the Tanjianshan goldfield, located at the northern margin of the Qaidam basin (northern Tibetan Plateau), are not only hosted in metamorphic rocks, but also spatially coexist with abundant mineralized, pyrite-bearing mafic-intermediate dikes. Thus, they provide a unique opportunity to reveal links between gold mineralization and magmatism, and differentiate the sources of ore-forming fluids, which remain a subject of debate. Our previous study identified an early gold preconcentration event at ca. 429 Ma, and a main gold mineralization event with possible contribution of magmatic fluids, but the timing of both the main stage of gold mineralization and the corresponding magmatism remain unclear. Here, we present detailed textural, geochronological, and geochemical investigations of the pyrite-bearing mafic-intermediate dikes, as well as the pyrite grains, at Tanjianshan. The diorite porphyrite dike displays a zircon U-Pb age of 404.9 ± 2.9 Ma and εNd(t) values of −1.4 to 1.3. It dominantly originated from partial melting of the Precambrian era metamorphic rocks that host pyrite grains (Py1). Pyrite that crystallized from the dioritic magma (Pydp) thus shows similar trace elemental features and δ34S (+15.1‰ to +16.8‰) as Py1 from the early gold preconcentration stage. The mineralized diorite yields a zircon U-Pb age of 124.4 ± 1.4 Ma, which constrains the lower time limit of the main gold mineralization. The gabbro, which developed simultaneously with the late-stage mineralization, has a similar age of 124.9 ± 1.4 Ma. Therefore, the main late-stage mineralization was constrained to a short period at ca. 125 Ma. The mineralized diorite and gabbro have εNd(t) values of 0.7 and −5.4 to 5.1, which potentially suggests their mantle origin. Pyrite grains disseminated in the diorite (Pyd) show high contents of metal elements and δ34S (+6.6‰ to +7.7‰) consistent with that of Py3–Py5 from the main-stage gold orebodies, and pyrite grains from the gabbro (Pyg) have δ34S (+9.0‰ to +10.0‰) similar to that of the late-stage Py6. Gold and arsenic expelled from the Pyd during dissolution were reconcentrated along the margin of the same grains, forming native gold and arsenopyrite, which contributed to a marked upgrading of gold grades and the overall gold budget. We propose that the ore-forming fluids of the main- and late-stage mineralization were exsolved from the Early Cretaceous magma. The multistage gold mineralization of the Tanjianshan goldfield formed intrusion-related gold deposits in the Yanshanian intracontinental orogenic setting.