Genesis of the Xishadegai Mo deposit in Inner Mongolia, North China: Constraints from geology, geochronology, fluid inclusion, and isotopic compositions

Abstract

The Xishadegai Mo deposit is a medium‐sized deposit located in the northern margin of the North China Craton. The Mo mineralization is structurally controlled, and spatially and temporally related to the Xishadegai felsic intrusive rocks. Ore bodies mainly occur as quartz veins/veinlets in altered granitic rocks associated with potassic, phyllic, argillic, and fluorite alterations. The ore‐forming process can be divided into 3 stages: Stage I K‐feldspar‐quartz ± molybdenite, Stage II quartz‐pyrite‐molybdenite‐muscovite ± fluorite, and Stage III quartz‐fluorite ± muscovite. Four types of fluid inclusions were distinguished in smoky grey and dark grey quartz of the main‐ore stage (II), including two‐phase aqueous inclusions, CO2‐H2O inclusions, daughter mineral‐bearing multiphase inclusions, and minor vapour aqueous inclusions. The fluid inclusions in smoky grey and dark grey quartz are homogenized at temperatures of 195–350 °C and 191–291 °C, respectively, with calculated salinities of 3.9–11.1% NaCleq and 31.5–33.0% NaCleq, respectively. The ore‐forming fluids belong to a H2O‐CO2‐NaCl system characterized by abundant CO2, moderate to high temperature, and low to high salinity. The δ18OH2O and δD values of ore‐stage quartz vary from −0.2‰ to 0.9‰ and from −120‰ to −104‰, respectively, indicating that the ore‐forming fluids were evolved from magmatic water and gradually mixed with significant amounts of meteoric water. Sulphur and lead isotopic compositions indicate that the ore materials were mainly derived from magmatic sources. Zircon LA‐ICP‐MS U–Pb dating on the mineralized porphyritic moyite yielded a weighted mean age of 235.1 ± 2.0 Ma, corresponding to the Triassic postcollisional setting following the closure of the Paleo‐Asian Ocean between the Siberian Plate and the North China Craton. The εHf(t) values and TDM2 ages range from −15.0 to −12.8 and from 2.2 to 2.1 Ga, respectively, suggesting that the Xishadegai granite was mainly generated by melting of Paleoproterozoic crustal components. Collectively, evidence from geology, fluid inclusion, H‐O‐S‐Pb isotopes, and geochronology suggests that the Xishadegai deposit could be classified as a magmatic–hydrothermal vein Mo deposit. Phase separation (immiscibility and boiling) was the most likely mechanism for ore deposition.

Late Palaeozoic tectonic transformation of the North Qinling Orogenic Belt: Insight from the geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Carboniferous magmatic rocks

“…In the study area, the Qinling Group, as the main stratum in the lower crust of the NQB, mainly consists of various gneiss and schist, and the main protolith is a set of near‐source rapid accumulation greywacke with low maturity (You et al, 1991). The Paleoproterozoic Nd‐Hf two‐stage model age of the granite shows a significant temporal correlation with the formation age of Qinling Group with gneiss U–Pb age of 2267–2172 Ma (Zhang et al, 1994). Furthermore, the Sr‐Nd isotope composition and geochemical characteristics also show an affinity with gneiss in the Qinling Group (Figures 6a,b and 10a).…”

“…It comprises gneiss, amphibolite, marble and metapelite, representing the Precambrian crystalline basement (Diwu et al, 2014;Zhang et al, 1994;Zhang et al, 2001). The gneiss and amphibolite of the Qinling Group formed in the Paleoproterozoic (Zhang et al, 1994). Furthermore, the HP-UHP metamorphic rocks distributed in the Qinling Group show a peak metamorphism age of 511-484 Ma and two phases of retrograde metamorphism age of 476-440 Ma and 426-400 Ma (Chen et al, 2015;Kang et al, 2022;Liu, Liao, et al, 2013).…”

“…The Qinling Group lies between the northern Erlangping Group and the southern SDSZ. It comprises gneiss, amphibolite, marble and metapelite, representing the Precambrian crystalline basement (Diwu et al, 2014;Zhang et al, 1994;Zhang et al, 2001). The gneiss and amphibolite of the Qinling Group formed in the Paleoproterozoic (Zhang et al, 1994).…”

“…Earlier research has revealed that the formation of the QOB was associated with two‐stage convergent events along the Shangdan and Mianlue suture zones in both the Early Palaeozoic and Mesozoic (Dong & Santosh, 2016; Dong, Zhang, Hauzenberger, et al, 2011; Dong, Zhang, Neubauer, et al, 2011; Jiang et al, 2022; Wang et al, 2013; Zhang et al, 2001). The North Qinling Orogenic Belt (NQB), as a prominent tectonic unit of QOB, preserves abundant ocean–continent transition records in the Palaeozoic (Dong et al, 2003; Zhang, 1995; Zhang et al, 1994). Therefore, it is considered to be a key position for studying the Palaeozoic evolution of the QOB.…”