Formation timing and genesis of Madiu fluorite deposit in East Qinling, China: Constraints from fluid inclusion, geochemistry, and H-O-Sr-Nd isotopes

The Heyu fluorite ore district is newly discovered in the East Qinling metallogenic belt (EQMB), central China. In this district, the large Madiu fluorite deposit contains five Mesozoic granite-hosted orebodies. The deposit may have formed at shallow depth and occurred as continuous veins controlled by a series of N-S- or NE-trending, steeply dipping faults. The fluorite mineralization can be divided into two stages (I and II). Fluid inclusions in stage I fluorite homogenized at 132.2 degrees C to 342.5 degrees C (peak at 170 degrees C-210 degrees C) with salinities of 0.18 to 4.43 wt% NaCl equiv, whilst those in stage II fluorite homogenized at 122.2 degrees C to 305.7 degrees C (peak at 130 degrees C-170 degrees C) and salinities of 0.18 to 3.71 wt% NaCl equiv. Raman spectroscopic analysis of the fluid inclusions reveals that they comprise predominantly H2O. The rare earth element (REE) characteristics indicate that the deposit is of hydrothermal origin. The delta O-18(H2O) (-8.8 parts per thousand to -0.5 parts per thousand) and delta D-H2O (-69.0 parts per thousand to -61.9 parts per thousand) values show a subhorizontal trend, suggesting that the oxygen isotopic compositions were modified by fluid-rock interactions. Fluid inclusion and stable isotopic data suggest that the ore-forming fluids belong to a meteoric water-sourced, medium-low temperature, and low salinity NaCl-H2O system. Initial fluorite Sr-87/Sr-86 ratios (0.708879-0.709682) resemble those of the Heyu batholith. The water-rock reaction, which leached Ca from the host rock and increased the fluid pH, was likely causative to the ore precipitation. Geochemical and mineralization features suggest that the Madiu fluorite deposit is best classified as a fault-controlled hydrothermal vein-type fluorite deposit. The Sm-Nd isochron age of 118.9 +/- 7.8 Ma and its regional age correlation suggest that the fluorite mineralization occurred under regional extension or lithospheric thinning setting and that the fluorite mineralization was an important component of the Early Cretaceous mineralization event in the EQMB.