CONSTRAINTS ON THE FORMATION OF THE GIANT DAHEISHAN PORPHYRY MO DEPOSIT (NE CHINA) FROM ACCESSORY MINERAL GEOCHEMISTRY

There are more than 80 porphyry Mo deposits in NE China with Mesozoic ages, which have been accepted to form mainly in a continental arc setting related to the subduction and subsequent slab rollback of the Paleo-Pacific oceanic plate. The Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, with a total Mo reserve of 1.09 Mt. To better understand the factors that have controlled the Mo mineralization in Daheishan, the geochemical and isotopic compositions of the ore-related intrusions, and the magmatic accessory minerals from the Daheishan intrusions, were investigated to quantify the magma oxidation states, magmatic water contents, as well as volatile and metal concentrations. These results could provide insights into the roles that the magma has played in the formation of the deposit.

Magmatic apatite and titanite from the causative intrusions show similar ε_Nd(t) values from -1.1 to 1.4, corresponding to a restricted range of T_DM2 ages from 1040 to 840 Ma, indicative of a relatively uniform magma source from the melting of dominantly the Neoproterozoic lower crust. The Ce and Eu anomalies, and (or) Ga contents of the magmatic accessory minerals have been used as proxies for the magma redox states, and the results suggest that the ore-forming magmas are highly oxidized. This is also consistent with the high Fe₂O₃/FeO ratios (1.3–26.4) of the whole-rock samples. The Daheishan intrusions have relatively high Sr/Y ratios of 40–94 with adakitic signatures. The accessory minerals are characterized by relatively high 10000*δEu/Y ratios, high apatite Sr/Y ratios, high titanite La/Yb ratios, and low zircon Dy/Yb ratios. These geochemical characteristics suggest that the fractionation of amphibole rather than plagioclase is dominant in the crystallization of the original magmas, which further indicates high water contents of the ore-related magmas. The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from other mineralized, subeconomic or barren intrusions. Furthermore, the Monte Carlo modelling indicates that a magma volume of ~260 km³ with ~12 ppm Mo was required to form the 1.09 Mt Daheishan deposit. The present study suggests that a relatively large-sized magma chamber with high oxygen fugacities and high water contents are essential for the generation of the giant Daheishan porphyry Mo deposit, whereas the melt compositions, including the magmatic sulfur and Mo concentrations, might be less important.