Geochemical and fluid evidences for fluorine-rich magmatic-hydrothermal origin of the giant Chalukou Mo deposit in the northeast China

Abstract

The Chalukou deposit is a world-class porphyry Mo deposit in the Great Hingan Range, northeast China. Fluorites were analyzed by scanning electron microscope-cathodoluminescence (SEM-CL), microthermometry, and laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) to characterize the composition and temperature of mineralizing fluids. There are four types of veins in Chalukou deposit that can be categorised as A vein (quartz–fluorite–magnetite–K-feldspar vein), B vein (quartz–fluorite–molybdenite vein), D vein (quartz–fluorite–sphalerite–galena–pyrite vein) and post-mineralization vein. The CL images of fluorites are characterized by heterogeneous texture in A vein, but homogeneous texture in B and D veins. Fluorites in A vein have total rare earth elements (REE) contents of 10–4963 ppm and temperatures of 370–410 °C, which show similar geochemical features with ore-bearing porphyry. In contrast, fluorites in B and D veins exhibit total REE contents of 4.6–41.3 ppm and temperatures between 257 and 300 °C, showing the decreasing trends of total REE contents and temperatures from early to late stage. Fluid inclusions of four types of veins are composed of one-phase, two-phase and daughter mineral-bearing multiphase with decreasing trend of temperatures from 450 °C to 120 °C and salinities from 53.0% to 0.7 wt% NaCl equiv. during the evolution. It is proposed that high-fluorine fugacity has a considerable influence on metal solubility and migration in the early stage of the Chalukou deposit. Temperature decreasing and fluid mixing may be the key factors of metal precipitation during the Mo mineralization stage, but the effect of fluorine is not obvious.