Peng Zhu

The Chutuan and Jiashan fluorite deposits are situated in the Donghai–Linshu area within the southwestern segment of the Sulu ultrahigh–pressure metamorphic belt. Both deposits share similar mineralization characteristics, with fluorite veins strictly controlled by fault structures and associated with mineral assemblages comprising fluorite, barite, quartz, and calcite. Two mineralization stages have been identified in both deposits: Stage I (quartz–fluorite–barite stage), representing the main ore–forming event, and Stage II (quartz–barite–calcite stage). This study focuses on integrated geochemical and geochronological analyses of fluorite from Stage I, providing new constraints on the genesis and metallogenic processes of these deposits. Direct Sm–Nd isotopic dating of fluorite yields an isochron age of 104 ± 16 Ma, indicating that mineralization occurred during the late Early Cretaceous. Fluid inclusion and stable isotope studies reveal that the ore–forming fluids constitute a complex hydrothermal system characterized by a wide temperature range (112–324 °C) and variable salinities (0.18–21.87 wt% NaCl eq.). The H–O isotopic compositions exhibit a distinct latitudinal trend, supporting a dominant meteoric water component. However, the presence of high–temperature, high–salinity fluid inclusions, along with a shift in some δD values towards the magmatic water field, suggests episodic mixing between meteoric water and deep–seated magmatic–hydrothermal fluids. Sr–Nd isotopic data (⁸⁷Sr/⁸⁶Sr = 0.711785–0.713424; εNd(t)= −27.7 to −27.5) potentially demonstrate that the ore–forming materials (Ca and REEs) were not derived from coeval magmatic rocks. Instead, they were primarily leached from the Precambrian Donghai Group metamorphic complex through extensive water–rock interaction. Based on these findings, the Chutuan and Jiashan deposits are classified as hydrothermal vein–type systems. Fluorite precipitation was governed by a combination of fluid cooling, water–rock interaction, and fluid mixing. Finally, a metallogenic model is established, offering important insights into the genesis of fluorite mineralization in the Sulu Orogenic Belt and analogous geological settings in eastern China.

Recent discoveries of fluorite–barite deposits in the Donghai–Linshu area in northern Jiangsu Province, China, underscore the region’s mineral potential, yet detailed geological investigations remain limited. In this study, we examined monzonite and quartz monzonite from drill cores in the Jiashan mining area using petrography, U–Pb zircon dating, zircon trace element geochemistry, whole-rock geochemistry, and zircon Lu–Hf isotopes. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb analyses were conducted to constrain the crystallization ages of the monzonite (127.06 ± 0.54 Ma and 126.83 ± 0.75 Ma) and quartz monzonite (127.2 ± 0.5 Ma and 128.59 ± 0.62 Ma) to the Early Cretaceous, marking a significant magmatic event. Many of the zircons contain inherited Neoproterozoic cores (718–760 Ma and 800–860 Ma), indicating the assimilation of deep crustal materials of this age. The monzonite is metaluminous, with moderate SiO₂ (61.61–62.41 wt.%), high alkalis (Na₂O + K₂O = 7.48–7.92 wt.%), and A/CNK = 0.72–0.91. The quartz monzonite has higher SiO₂ (66.26–68.18 wt.%) and alkalis (8.32–9.33 wt.%). Both rock types exhibit similar trace and rare earth element patterns: enrichment in large-ion lithophile and light rare earth elements, depletions in Nb, Ta, and Ti, no significant Zr-Hf depletion, and weak negative Eu anomalies (δEu ≈ 0.84–1.00). Their low Zr + Nb + Ce + Y contents, Ga/Al ratios, and TFeO/MgO ratios indicate that they have an I-type granite affinity. The Early Cretaceous zircons have highly negative ε_Hf(t) values (−33.7 to −23.5) and ancient two-stage model ages (2622–3247 Ma), which are consistent with derivation from Archean crust. The inherited Neoproterozoic zircons have younger Paleo–Mesoproterozoic T_DM2 ages. The evidence suggests that both intrusions were mainly generated by partial melting of ancient Archean basement, with minor mantle input. The magma generation was likely triggered by crustal anatexis induced by the underplating of mantle-derived magmas in an extensional tectonic regime, coeval with Early Cretaceous magmatism in the Sulu orogen.