Geochronology, geochemistry, fluid inclusion and C, O and Hf isotope compositions of the Shuitou fluorite deposit, Inner Mongolia, China

Abstract

The Late Mesozoic (i.e. 150–120Ma) marks one of the most active periods of magmatism in the Southern Great Xing'an Range (SGXR), associated with large-scale mineralization. Here we investigate the large Shuitou fluorite deposit located within the SGXR mineralized belt. The ore bodies strike nearly NS and discontinuously extend for 10km long. Zircon U–Pb dating of the associated intrusion (the Herimu Tai granite) in the southern part of the area yields an emplacement age of 136.9±1.7Ma whereas Sm–Nd isotopic data on the fluorite ore yield an isochron age of 132±11Ma. Fluid inclusions in the fluorite show low homogenization temperatures (140–220°C), salinities (0.27wt% NaCl eqv – 2.8wt% NaCl eqv) and densities (0.86–0.891g/cm3). The inclusions are mainly composed of H2O, along with minor C3H6 and other organic components. The associated calcite has δ13CV-PDB varying from −7.1‰ to −6.3‰ (average −6.73‰) and δ18OV-SMOW from −2.8‰ to −0.7‰. The field, petrologic and isotopic data from our study leads to the following salient findings. 1) The major minerals in the ore deposit include fluorite, quartz and calcite, which are of hydrothermal origin and have a uniform source and therefore the ore deposit is considered to be of hydrothermal vein-type. 2) The ore-forming fluids can be mainly attributed to NaCl–H2O system with moderate to low temperature, low salinity and low density. There is no evidence for post-magmatic hydrothermal fluid. The ore-forming fluid shows a meteoric water source, with water-rock reaction as the main mechanism for fluorite precipitation. 3) The deposit was formed in an extensional setting within the Central Asian Orogenic Belt and is closely associated with the late Jurassic to early Cretaceous polymetallic events. The nearly synchronous magmatic events in the study area suggest that these might have provided the heat source for fluorite mineralization.