Difference in the nature of ore-forming magma between the Mesozoic porphyry Cu-Mo and Mo deposits in NE China: Records from apatite and zircon geochemistry

Abstract

Porphyry Cu-Mo and Mo deposits are the most important source of Mo in the world, but factors controlling their difference remain enigmatic. In recent years, many Mesozoic porphyry Cu-Mo and Mo deposits have been discovered in NE China, which provides a good opportunity to explore the difference between the Cu-Mo and Mo mineralization. In this region, porphyry Cu-Mo deposits (Wunugetushan, Jinchanggou) are distributed along the subduction zone, while the porphyry Mo deposits (Luming, Fu'anpu, Daheishan) are far away from the subduction zone. On the basis of whole rock geochemical studies, we present a comprehensive investigation on the apatite and zircon from Mesozoic typical porphyry Cu-Mo deposits and Mo deposits, in order to reveal the nature of parent magma constraints on Cu-Mo and Mo mineralization. Whole-rock geochemistry, in-situ apatite Sr-Nd and zircon Hf isotopic compositions from these granites in porphyry Cu-Mo and Mo deposits indicate that these granites originated from a juvenile crustal source. Apatites from the porphyry Cu-Mo mineralized granites show relatively high Cl and low F contents, accompanied by enrichment of fluid-mobile elements Ba and Sr, indicating that the magmatic source of porphyry Cu-Mo deposits were modified by subducted slab-derived fluids. In comparison, apatites from the porphyry Mo mineralized granites have high F and low Cl contents with correspondingly high F/Cl ratios, suggesting that these granites mainly formed in juvenile crustal dehydration self-metasomatism under the intra-continental environment. Additionally, apatites from porphyry Cu-Mo mineralized granites have higher OH contents than those from porphyry Mo mineralized granites, indicating that the original melt of porphyry Cu-Mo system was more hydrous, consistent with their higher (Ce/Nd)/Y and lower Dy/Yb ratios of zircon. It is here proposed that the formation of porphyry Cu-Mo deposits in NE China are related to partial melting of the juvenile crust, which metasomatized by slab-derived Cl-enriched fluids with carry significant amounts of Cu, whereas the porphyry Mo deposits are related to partial melting of juvenile crustal material with released F-enriched fluids by the dehydration of hydrous minerals.