Studies On Geology,Geochemistry And Metallogenic Mechanism Of The Chalukou Giant Porphyry Molybdenum Deposit In Heilongjiang,NE China

This work mainly focuses on giant porphyry Mo ore system,and selects the newly-discovered Chalukou porphyry Mo deposit as a case study.The Mo mineralization is zoned vertically from a thin,upper low-grade to a thick,lower high-grade domain.From the core to the boundary,five types of alteration,i.e.,silicic,potassic,sericitic,argillic and propylitic,affected the rocks at the Chalukou ore district.In this study,high-precision geochronology,in-situ sulfur and lead isotope,in-situ sulfide trace elements and multiple isotope systems(multiple sulfur and molybdenum isotopes),have been used to constrain the metallogenic process and mechanism.The main achievements and understandings are as follows.The ore minerals are composed of molybdenite and pyrite.Chalukou Mo mineralization is closely related to potassic and sericitic alteration.The mineralization can be divided into three stages,i.e.,quartz+K-feldspar(magnetite±hematite±pyrite±molybdenite)stage,quartz+molybdenite+K-feldspar+sericite(pyrite±fluorite)stage and quartz polymetallic sulfide stage,in which the second stage is the main mineralization stage.The syn-ore intrusions are characterized by high Si and alkaline,weakly-peraluminous,significant negative Eu anomaly and can be classified as the high fractionated I-type granite,which suggest that they formed in an intra-plate extensional setting.The molybdenite Re-Os geochronology reveals that the bulk of the Mo was deposited in less than 650 Ka(from 147.67±0.10 to 147.04±0.12 Ma),coincident with the emplacement of a fine-grained porphyry at?147 Ma.Our data suggest that the giant Chalukou porphyry Mo deposit primarily formed in a single,short-lived hydrothermal event,in which high flux played an important role.Mo isotope composition shows that the Mo fractionation is mainly controlled by Rayleigh fractionation.The larger Mo isotope fractionation range(from-0.87 to+0.95‰)indicates that more than 80%Mo in the fluids has been precipitated.The complex Mo isotope variation trend of Molybdenite from different stages suggests that Mo isotope fractionation in magmatic-hydrothermal system is controlled by multiple factors,and boiling may be one of them.Coupling of Mo-S isotope(³²S,³³S and ³⁴S)indicates that kinetic fractionation may be responsible for the slightly?³³S anomaly between porphyry(mean=+0.012‰)and molybdenite(mean=-0.052‰).The measured S isotopes of sulfides(from-5.2 to+7.8‰,mean=+2.9‰)are consistent with a magmatic S source;lead isotope compositions of sulfides are consistent with those of magmatic rocks and regional deposits,which indicate that Chalukou Mo mineralization is closely related to the Mesozoic magmatism;sulfide trace element data show that there are obviously differences between the contents of trace elements in different stages of fluid.We conclude that the absolute content of metal in the fluid is not the key factor affecting metal precipitation,but that ore mineral saturation controlled by the system temperature is the real key.Based on the systematic studies,it is proposed that the high fractionated magma produced by long-lived and multiple magmatic evolution is favorable for the pre-enrichment of molybdenum.Chalukou Mo deposit is the product of a short-lived,high flux mineralizing event.Mesozoic magmatism provides the material and fluid sources for mineralization.And multiple factors control the fractionation of molybdenum and multi-sulfur isotopes in magmatic-hydrothermal system,among them,Rayleigh fractionation in a rapid mineralization event is dominant.The identification of the mineralization process for Chalukou giant porphyry Mo system in this region is helpful for ore exploration.