A number of sediment-hosted, Carlin-type/-like gold deposits are distributed in the Youjiang basin of SW China. The gold ores are characterized by high As, Hg, and Sb contents but with low base metal contents (Cu+Pb+Zn < 500–1000 ppm). The Nibao deposit is unique among these gold deposits by having tennantite–tetrahedrite-series minerals in its ores. The deposit is also unique in being primarily hosted in the relatively unreactive siliceous pyroclastic rocks, unlike classic Carlin-type gold deposits that are hosted in carbonates or calcareous clastic rocks. In this study, we have identified tennantite-(Zn), tennantite-(Hg), and tetrahedrite-(Zn) from the tennantite–tetrahedrite-series mineral assemblage. The tennantite-(Zn) can be further divided into two sub-types of Tn-(Zn)-I; and Tn-(Zn)-II;. Tn-(Zn)-I; usually occurs in the core of a Tennantite–tetrahedrite composite and appears the darkest under the SEM image, whereas Tn-(Zn)-II overgrows on Tn-(Zn)-I and is overgrown by tetrahedrite-(Zn). Tennantite-(Hg) occasionally occurs as inclusions near the uneven boundary between Tn-(Zn)-I and Tn-(Zn)-II. An appreciable amount of Au (up to 3540 ppm) resides in the tennantite–tetrahedrite-series minerals, indicating that the latter is a major Au host at Nibao. The coexistence of tennantite–tetrahedrite-series minerals and Au-bearing pyrite indicates the Nibao ore fluids were more oxidized than the Carlin-type ore fluids. The tennantite–tetrahedrite series at Nibao evolved from Tn-(Zn)-I through Tn-(Zn)-II to tetrahedrite-(Zn), which is likely caused by Sb accumulation in the ore fluids. This indicates that the Nibao ore fluids may have become more reduced and less acidic during Au precipitation.

A suite of subduction-related early Triassic igneous rocks crops out in the Pingxiang area of the Dian-Qiong suture (DQS) in southwest China; this suite represents an important geological record of subduction and closure of the Paleo-Tethys Ocean. In this study, we report geochronological, geochemical, and Nd-Hf isotope data for newly discovered rhyolites and biotite granites in the Pingxiang area. We use these data to constrain their emplacement ages, origins, and geodynamic implications. Zircon U-Pb dating indicates that the rhyolites and biotite granites were emplaced at 251–250 Ma and 249 Ma (early Triassic), respectively. The rhyolites and biotite granites have elevated FeO^T/(FeO^T + MgO) (0.78–0.89) and 10,000 × Ga/Al (2.83–4.11) ratios, with geochemical affinities to A₂-type granites. These rocks are enriched in some large-ion lithophile elements (e.g., Rb, Th, and Ba) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti), indicating their formation in a subduction-related arc. The rhyolites and biotite granites have negative whole-rock ε_Nd(t) (−11.5 to −9.7) and zircon ε_Hf(t) (−14.5 to −6.2) values, suggesting that these magmas were derived from an ancient crust-dominated source. Geochemical and Nd isotope data reveal that the peraluminous A₂-type rhyolites and biotite granites were derived by partial melting of felsic crustal rocks under low-pressure and high-temperature conditions. By integrating all the available data with the regional tectonic evolution of the southwestern Youjiang Basin and adjacent regions, we attribute the generation of the peraluminous A₂-type rhyolites and biotite granites to the extensional setting that existed during oceanic subduction, which was induced by roll-back of the Paleo-Tethys oceanic lithosphere at 251–249 Ma. This study indicates that subduction-related magmatism related to Paleo-Tethys oceanic lithosphere was still active in the early Triassic.

Introduction: The North Qilian orogenic belt, as the Northern branch of the original Tethys tectonic domain, is important for reconstructing the tectonic evolution of the ancient Tethys. However, the tectonic history of the North Qilian orogenic belt remains controversial. This study addresses this issue from a geochemical perspective.Methods: In this study, a comprehensive analysis of the geochronology, whole-rock geochemistry, clinopyroxene mineral geochemistry, zircon Ti crystallization temperature, and gabbromagma temperature and pressure in the Yushigou ophiolite of the North Qilian orogenic belt was conducted to provide constraints on its tectonic evolution.Results and Discussion: Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating results reveal that the gabbros have ages of 519 ± 3 Ma and 495 ± 4 Ma, belonging to the Cambrian period. Most of the studied gabbros exhibited geochemical characteristics of tholeiitic basaltic rocks with normal mid-ocean ridge basalt and island arc tholeiite dual geochemical affinities. The gabbros are interpreted to have formed by a high degree of partial melting of the depleted mantle spinel lherzolite. These results suggest that the back-arc basin of the North Qilian tectonic belt may have evolved to a relatively mature stage from 519 to 495 Ma. Overall, this study contributes to our understanding of the tectonic evolution of the North Qilian orogenic belt through geochemical analyses.