Geochemistry of Apatite from the Apatite‐rich Iron Deposits in the Ningwu Region, East Central China

Abstract

Abstract: Four types of apatite have been identified in the Ningwu region. The first type of apatite is widely distributed in the middle dark colored zones (i.e. iron ores) of individual deposits. The assemblage includes magnetite, apatite and actinolite (or diopside). The second type occurs within magnetite‐apatite veins in the iron ores. The third type is seen in magnetite‐apatite veins and (or) nodules in host rocks (i.e. gabbro‐diorite porphyry or gabbro‐diorite or pyroxene diorite). The fourth type occurs within apatite‐pyrite‐quartz veins filling fractures in the Xiangshan Group. Rare earth elements (REE) geochemistry of apatite of the four occurrences in porphyry iron deposits is presented. The REE distribution patterns of apatite are generally similar to those of apatites in the Kiruna‐type iron ores, nelsonites. They are enriched in light REE, with pronounced negative Eu anomalies. The similarity of REE distribution patterns in apatites from various deposits in different locations in the world indicates a common process of formation for various ore types, e.g. immiscibility. Early magmatic apatites contain 3031.48–12080 times 10−6 REE. Later hydrothermal apatite contains 1958 times 10−6 REE, indicating that the later hydrothermal ore‐forming solution contains lower REE. Although gabbro‐diorite porphyry and apatite show similar REE patterns, gabbro‐diorite porphyries have no europium anomalies or feeble positive or feeble negative europium anomalies, caused both by reduction environment of mantle source region and by fractionation and crystallization (immiscibility) under a high oxygen fugacity condition. Negative Eu anomalies of apatites were formed possibly due to acquisition of Eu2+ by earlier diopsite during ore magma cooling.The apatites in the Aoshan and Taishan iron deposits yield a narrow variation range of 87Sr/86Sr values from 0.7071 to 0.7073, similar to those of the volcanic and subvolcanic rocks, indicating that apatites were formed by liquid immiscibility and differentiation of intermediate and basic magmas.