Genesis of the Meishan iron oxide–apatite deposit in the Ningwu Basin, eastern China : Constraints from apatite chemistry

The Meishan iron oxide-apatite deposit is located in the Ningwu volcanic basin in eastern China. The deposit comprises massive and brecciated ores in the main orebody, located at the contact between a gabbro-diorite porphyry and biotite-pyroxene andesites, and sub-economic stockwork and disseminated ores. Among the four stages of alteration and mineralization, apatite coexists with magnetite, andradite, and quartz in massive magnetite ore and occurs in disseminated magnetite ore, coexisting with magnetite, siderite (after diopside), and quartz for Stage 2 iron mineralization. Apatite is also present in the altered gabbro-diorite porphyry. The apatites from the massive and disseminated magnetite-apatite ores are fluor- and hydroxyl- variety. Those from the altered gabbro-diorite porphyry show extensive solid solution between the hydroxyl-apatite, fluor-apatite, and chlor-apatite end members. The apatites at Meishan record oxidized states during formation of the mineral deposit. The SO3 contents of apatite in the Meishan deposit mostly vary from 0.4 to 1.2 wt%, with the highest value of 4.97 wt%. MnO contents in apatites from our study are less than 0.17 wt%, and most values are below detection limit, indicating that the apatite formed in a high integral(O2) magmatic-hydrothermal fluid. The strong Eu depletion in apatites at Meishan resulted from the fractionation of plagioclase. The gabbro-diorite porphyry, magnetite, and apatite show similar LREE-enriched patterns with significant negative Eu anomalies for apatite and magnetite, and no Eu anomaly for the gabbro-diorite porphyry. The gabbro-diorite porphyry and Stage 2 apatite associated with iron mineralization have also similar primitive mantle-normalized trace element patterns, suggesting that the mineralization was related to gabbro-diorite porphyry. The REE pattern with significant negative Eu anomalies in apatite from the Meishan deposit is comparable with that of other Kiruna-type deposits elsewhere in the world.