Degao Zhai

Fluid evolution of the Jiawula Ag–Pb–Zn deposit, Inner Mongolia: mineralogical, fluid inclusion, and stable isotopic evidence

The Jiawula Ag–Pb–Zn deposit lies in the renowned Ag–polymetallic metallogenic province of northern China. The origin of this structurally controlled ore body is linked to fluid migration and mineralization along cogenetic fault systems. Sulphur isotopic compositions suggest that the ore-forming aqueous solutions were derived mainly from deep magmatic fluids. Hydrogen and oxygen isotopic compositions indicate that these fluids were magmatic during early stages of ore formation and meteoric during late-stage mineralization. Lead isotopic compositions indicate that this metal was derived mainly from a mantle source, and to a lesser extent from a crustal source. Collectively, the isotopic data indicate that formation of the Jiawula Ag–Pb–Zn deposit was ultimately a reflection of late Yanshanian (140–120 Ma) volcanic–subvolcanic hydrothermal activity. The addition of meteoric water to these magmatic hydrothermal fluids created favourable conditions for mineralization. During ore formation, metallogenesis took place in a relatively open, non-equilibrium system under conditions of low δ34S∑S and an intermediate oxidation state. Microthermometric study of fluid inclusions indicates homogenization temperatures of 180–260°C. Salinities, densities, pressure, and depth of ore-forming fluids ranged from 0.18 to 12.62 wt.% NaCl eqv., 0.637 to 0.976 g/cm3, 3.44 to 162.05 bar, and 0.5 to 1.5 km, respectively. Laser Raman studies of single-phase fluid inclusions show that the ore-forming fluids belong to the H2O–NaCl system. Analysis of bulk chemical compositions of fluid inclusions indicates that the ore-forming fluid can be classified as the Na+–Ca2+– –Cl− fluid type. All obtained geochemical data demonstrate that the ore-forming fluids of the Jiawula Ag–Pb–Zn deposit are medium- to low-temperature, medium- to low-pressure, medium- to low-salinity, and low-density fluids. Based on their compositions, they can be classified into two end-members: magmatic hydrothermal fluid and meteoric water. The key factors allowing for metal transport and precipitation during ore formation include the sourcing of magmatic fluids with high contents of metallogenic elements and the mixing of these hydrothermal fluids with meteoric waters resulting in the formation of a large Ag–Pb–Zn deposit. In terms of genetic type, the Jiawula deposit can be regarded as a volcanic–subvolcanic hydrothermal vein Pb–Zn–Ag ore deposit.

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling, China: insights from LA-ICP-MS zircon U–Pb ages, Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

ABSTRACT West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO2 (63.59–76.22%) and low P2O5 (0–0.2%) concentrations, a high K2O/Na2O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous εHf(t) (−4.425 to 1.067 for Xujiaba and −4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.