Constraints On The Formation Of The Jurassic Porphyry Mo Mineralization In The Lesser Xing'an-Zhangguangcai Range,Northeastern China

Northeastern China is an important Mo resource region in China,with nearly 100 Mesozoic porphyry(or skarn)Mo deposits.These are thought to have formed mainly related to the subduction of the Paleo-Pacific oceanic plate and subsequent slab rollback in the Mesozoic.In this study,the geochemical and isotopic compositions of the ore-related intrusions and their magmatic accessory minerals in the Daheishan and Huojihe deposits in the Lesser Xing'an-Zhangguangcai Range region were investigated to provide insights into the nature of the source magmas and clarify the possible mechanisms controlling Mo mineralization.Zircon U-Pb dating shows that the biotite monzogranite from the Huojihe deposit formed at181.6±0.6 Ma,which is identical to the previously reported zircon U-Pb and molybdenite Re-Os age,indicating that the Mo mineralization is probably related to the intrusion.The granitic samples share homogeneous geochemical and Sr-Nd isotopic compositions,reflecting a uniform magma source.The Sr-Nd-Hf isotope results suggest that the causative intrusions were generated from a mixing model between mantle-derived basaltic melts and the Precambrian lower crust.The low Ga and Ce and high Eu contents in the magmatic apatite demonstrate that the original magmas were oxidized,which is also supported by the high zircon Ce/Ce*(22–568)and Eu/Eu*(0.38–0.71)values.Magmatic apatite and titanite from the causative intrusions in Daheishan show similar?_Nd(t)values from-1.1 to 1.4,which may have similar magma source with Huojihe,but with more involvement of mantle material.The Ce and Eu anomalies of the magmatic accessory minerals and estimated?FMQ range(+1.8 to+4.1)suggest that the ore-forming magmas in Daheishan are highly oxidized.This is also consistent with the high whole-rock Fe₂O₃/Fe O ratios(1.3–26.4).The Daheishan intrusions display negligible Eu anomalies,relatively high Sr/Y ratios(40–94)with adakitic signatures;they also have relatively high Sr/Y ratios in apatite and titanite.All these suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas,which further indicates a high magmatic water content(>5 wt%).Magmatic sulfur concentrations of the Daheishan and Huojihe deposits were calculated using two available partitioning models for apatite from granitoids,and the results(9–125 ppm for Daheishan and 20–100 ppm for Huojihe)are indistinguishable from other mineralized,subeconomic and barren intrusions.Using the sulfur concentration data,a minimum volume of10–50 km³magma has been suggested to be necessary to produce the Huojihe Mo deposit based on a simple mass balance modelling.Besides,the Mo concentration in the original magma has also been roughly estimated to be 2-10 ppm based on the magma size and the contained Mo in Huojihe.Furthermore,Monte Carlo modelling has also been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit,and the result reveals that a magma volume of?280 km³with?10 ppm Mo was required to form the Mo ores containing1.09 Mt Mo in Daheishan.The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a large-size porphyry Mo deposit like Daheishan and Huojihe,whereas a specific magma composition(high Mo and/or S concentrations),might be less critical.In NE china,the compression in Jurassic due to the Paleo-Pacific plate subduction provided a favorable condition for accumulation of large amounts of magmas at depth crust,which gave rise to the formation of large-to giant-sized Mo deposits(e.g.,Daheishan and Huojihe)in this region.