The possible contribution of a mantle-derived fluid to the Ningwu porphyry iron deposits – Evidence from carbon and strontium isotopes of apatites

Apatite is a characteristic mineral for the iron deposits in the Ningwu region. Apatite shows a range of δ13CPDB values from −9.6‰ to −0.6‰, and most of them with the δ13CPDB values distributed in a range of mantle carbon (i.e., −2‰ to −9‰), except for samples T-06-1 and J-06-3. Apatites in the Ningwu porphyry iron deposits yield a narrow variation range of 87Sr/86Sr values from 0.706326 to 0.707577, similar to those of the volcanic and subvolcanic rocks and higher than that of a typical mantle-derived magmatic apatite (i.e., about 0.7040), indicating that formation of the apatites was involved with crust material contamination. δ13CPDB and 87Sr/86Sr values of the apatite indicate a possible contribution of mantle-derived fluid and material to formation of the Ningwu porphyry iron deposits, supported by regional geology, regional gravities and geochemistry of the volcanic and subvolcanic rocks in the Ningwu region.