In situ major and trace element compositions of apatites from Luanchuan orecluster: Implications for porphyry Mo mineralization

Abstract

The effects of volatile concentrations and oxidation state of magma on the Mo mineralization potential have been explored through petrographic studies and in situ electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of apatites from coeval economically Mo- mineralized granite porphyry and quartz diorite, and sub-economically mineralized granite porphyry at Luanchuan orecluster. The occurrence of abundant K-feldspar xenocrysts, possibly entrained from granitic magma, in quartz diorite implying a magma mingling process. Studied apatites are fluorapatites or fluor-hydroxyapatites with decreasing OH concentrations from economically mineralized quartz diorite (0.30–0.49 apfu) through mineralized granite porphyry (0.14–0.41 apfu) to sub-economically mineralized granite porphyry (0.01–0.11 apfu). Chondrite-normalized REE patterns of all the apatites are characterized by LREE enrichment relative to MREE and HREE, and negative Eu anomaly, similar to their host magmas, indicating a melt composition control. Strontium concentration of studied apatites are probably controlled by the crystallization of plagioclase or alkali feldspar. Mineralized quartz diorite formed at a relatively reduced environment compared to other rocks, but mineralized granite porphyry and sub-economically mineralized granite porphyry formed at a similar oxidation state, based on the SO3, Eu, Ce, and Mn content variation of apatites. The OH-contents of apatites, combined with published log (H2O/HF) ratios of melt in equilibrated with biotites, reflect that economically mineralized intrusions have higher water contents than the sub-economically mineralized intrusions. In a porphyry Mo system, a high magma water content is probably a key parameter controlling Mo mineralization. Mafic magma may supply water (and heat) to ore-forming granitic magmas, enhancing their Mo mineralization potential.