Selenium content and enrichment processes recorded in the Magushan Cu-Mo skarn deposit, Xuancheng ore district, China

Selenium is a semi-metal that is often considered critical and is typically produced as a byproduct of copper production from porphyry-skarn deposits. The Magushan deposit is a skarn Cu-Mo deposit within the Middle and Lower Yangtze River metallogenic belt in China that contains significant amounts of Se mineralization. This study focuses on the distribution and deportment of Se within the deposit, the factors controlling the variations of Se in the sulfides present at Magushan, and the source of Se. The deposit records pre-ore, Cu-Mo, Pb-Zn, and post-ore stages of mineralization, with the majority of Se mineralization associated with the Cu-Mo stage. Except for a very small amount of naumannite (Ag2Se) formed during the Pb-Zn stage, the Se within the Magushan system was mainly incorporated into Cu-Mo stage sulfides by isomorphic substitution for S. The Se content of different sulfides in the system is also likely controlled by variations in distribution coefficients, with Se more likely to be incorporated into pyrite within the Magushan deposit than other sulfides. The sulfides formed during the Pb-Zn stage of mineralization have significantly lower Se concentrations (<50 ppm) than those formed during the Cu-Mo stage of mineralization (molybdenite: 311.9−715.5 ppm Se; chalcopyrite: 325.2−742.3 ppm; pyrite: 221.8−833.5 ppm). This indicates that the Se content of these sulfides is controlled by variations in temperature, oxygen fugacity, and the Se concentration of the hydrothermal fluids that formed the deposit. The Se/S ratios and δ34S values of sulfides from the Magushan deposit (1.1‰−5.4‰ with a mean of 2.5‰; N = 38) also provide evidence of the magmatic sourcing of S and Se. These data also suggest that the assimilation of upper crustal rocks by parental magmas associated with the Se-bearing deposit may be a crucial process for the generation of Se mineralization. In short, the Se mineralization within the Magushan skarn system is related to a proximal porphyritic granodiorite, with the Se within the deposit predominantly hosted by sulfides. The Se concentrations of these sulfides are controlled by variations in temperature, oxygen fugacity, distribution coefficient, and the Se concentration of the hydrothermal fluids that formed the deposit. These characteristics should be used to guide both the exploration for, and the assessment of, the Se potential of skarn systems elsewhere within the Middle and Lower Yangtze River metallogenic belt in China and globally.