Study On Metallogenesis And Prospecting Direction Of Late Jurassic Porphyry Cu(mo) Deposits In The Northern Segment Of The Great Xing'an Range

The Late Jurassic porphyry Cu deposits in the northern segment of the Great Xing'an Range have not been reported until recent years.These contemporaneous porphyry Cu deposits provide a good example for the study of geological characteristics of deposits,characteristics and evolution of ore-forming fluids,diagenetic and metallogenic ages,petrogenesis,source of ore-forming materials,and regional tectonic evolution.The Xiaokelehe Cu(Mo)porphyry deposit,the Huoluotai Cu(Mo)porphyry deposit,and the Fukeshan Cu(Mo)porphyry deposit in the northern segment of the Great Xing'an Range are selected as the research objects in this paper.Based on the study of geological characteristics,diagenetic and metallogenic chronology,petrogeochemistry,isotope geochemistry,and fluid inclusions(FIs)of these typical deposits,combined with regional geological,magnetic anomalies,and remote sensing interpretation data,this paper discusses the diagenetic and metallogenic ages,the characteristics and evolution of ore-forming fluids,regional tectonic evolution,the petrogenesis and source of ore-forming materials,metallogenic model and prospecting direction.These studies will provide further theoretical basis for late Mesozoic regional metallogenic regularity and prospecting.The mineralization of the three typical deposits occurs mainly as disseminations and veinlets in the Late Jurassic(ca.149Ma)medium-acid porphyry bodies.The ore-forming rock of the Xiaokelehe and Huoluotai Cu(Mo)deposits is granodiorite porphyry,and the ore-forming rock of the Fukeshan Cu(Mo)deposit is quartz diorite porphyry.The alteration zonation of the Fukeshan Cu(Mo)deposit is not obvious due to superposition of multiple post-metallogenic magmatism.In addition to the Fukeshan Cu(Mo)deposit,the wall rock alteration of the Xiaokelehe and Huoluotai Cu(Mo)deposits has obvious zonation from inside to outside.The alteration types of these deposits are mainly potassic,silicified,chlorite–sericite,phyllic,and propylitic alteration,which are similar to those of typical porphyry deposits.Based on fluid inclusion thermometry,laser Raman spectroscopy and H-O isotope composition,it is found that the ore-forming fluid from the Xiaokelehe and Huoluotai Cu(Mo)deposits was an H₂O-NaCl-CO₂ system in stage I and an H₂O-NaCl system from stages II to IV.Fluid boiling occurred during mineralization.The ore-forming fluid of the Fukeshan Cu(Mo)deposit from stages I to IV was an H₂O-NaCl system,and fluid boiling occurred in stage I.The initial ore-forming fluids of these deposits were magmatic in origin,which have the characteristics of high temperature,high salinity,and high oxygen fugacity.The ore-forming fluids were gradually mixed with meteoric water in the late mineralization stage,and homogenization temperatures decreased from stages I to IV.In this study,most sulfide minerals are present in the S-type FIs during different mineralization stages,indicating that Cu and Mo are mostly transported as complexes in the brine phase.Taking the Xiaokelehe Cu(Mo)deposit as an example,this paper discusses the metal precipitation mechanism of typical Late Jurassic porphyry Cu deposits in the northern segment of the Great Xing'an Range(NSGXR),and suggests that fluid cooling is an important mechanism to induce mineral precipitation.In addition,other factors also affect the mineral precipitation.The decrease of temperature,magnetite precipitation,and potassic alteration promote the production of H₂S in the fluid,and the high activity of H₂S leads to the mineral precipitation.Fluid boiling results in greater distribution of ore-forming elements(e.g.,Cu and Mo)into the brine,forming mineral-rich fluids.Degassing during fluid boiling will also result in precipitation of some minerals.Incursions of cooler meteoric water into the magmatic fluids may have also assisted in the temperature decrease and further promoted mineral precipitation.The S-Pb-Re isotopic characteristics of the Late Jurassic porphyry Cu(Mo)deposits in the NSGXR indicate that the source of ore-forming materials is characterized by crust-mantle mixing.It is considered that Cu may have mainly originated from the subducting oceanic crust,and the mantle wedge may provide some of the Cu,with a small amount of crust-derived ore-forming materials mixed in.In addition,the volcanic rocks(wall rock)from Late Jurassic porphyry deposits in the NSGXR may provide some minerals(e.g.,Mo and Pb).The Early–Middle Jurassic(ca.200–160Ma)and late Late Jurassic(ca.149Ma)ore-bearing adakitic rocks in the NSGXR were generated by partial melting of a subducted oceanic slab,with involvement of marine sediments in the source.Owing to flat-slab subduction of the Mongol–Okhotsk oceanic lithosphere,the Early–Middle Jurassic adakitic rocks rarely reacted with mantle peridotite during ascent through the thin mantle wedge.However,the late Late Jurassic ore-bearing granodiorite porphyry interacted with mantle peridotite adequately during ascent through the relatively thick mantle wedge.The Late Jurassic Huoluotai diorite porphyry(146.1±1.3Ma)formed by partial melting of a lithospheric mantle wedge modified by fluids released from a subducted oceanic slab,and that it underwent limited crustal contamination.The Early Early Cretaceous Huoluotai granite porphyry(142.2±1.5Ma)most likely formed by partial melting of thickened mafic lower continental crust.The Early–Middle Jurassic adakitic rocks in the NSGXR which were generated by partial melting of a subducted oceanic slab suggest that flat-slab subduction of the Mongol–Okhotsk oceanic lithosphere in the NSGXR might have began in the Early Jurassic.Based on the age data of the Jurassic–Early Cretaceous magmatite in the NSGXR,it is found that there was a period of magmatic quiescence of?10 Myr(160–150Ma)in the NSGXR,indicating that flat-slab subduction of the Mongol–Okhotsk oceanic slab in the NSGXR entered a late stage at this time.The Late Jurassic–early Early Cretaceous(ca.150–133Ma)magmatite and associated deposits in NE China are mainly concentrated in the Great Xing'an Range(the western part of the Songliao Basin),suggesting a genetic relation to the evolution of the Mongol–Okhotsk Ocean rather than the Paleo-Pacific Ocean.The Late Jurassic porphyry Cu(Mo)deposits in the NSGXR were formed in a tectonic transition phase,i.e.,an extensional tectonic setting after protracted flat-slab subduction of the Mongol–Okhotsk oceanic slab,which was likely the result of tearing and collapse of the Mongol–Okhotsk oceanic slab.The intense compressional tectonism in the NSGXR during the early Early Cretaceous demonstrates that the eastern Mongol–Okhotsk Ocean has closed in the early Early Cretaceous.Based on the above research results,combined with the regional stratigraphic,structural,and magmatic evidence,as well as previous studies,this paper reconstructs a comprehensive model for the Jurassic–Early Cretaceous(ca.200–106Ma)tectonomagmatic evolution of the NSGXR.This model can be divided into the following five stages from early to late:flat-slab subduction of the Mongol–Okhotsk oceanic slab(ca.200–160Ma),a period of magmatic quiescence(ca.160–150Ma),tearing and collapse(ca.150–145Ma)of the Mongol–Okhotsk oceanic slab,crustal thickening(ca.145–133Ma),and lithosphere delamination(ca.133–106Ma).Based on the systematic analysis of magnetic anomalies and remote sensing data in the NSGXR,the prospective model of magnetic anomalies and remote sensing information for late Mesozoic porphyry deposits in the NSGXR is established in this paper.The results show that weak positive magnetic anomaly and positive and negative magnetic anomaly gradient zones are important indicators for searching for late Mesozoic porphyry deposits in the NSGXR.The edge of high frequency region of the remote sensing linear-circular structural convergence positions is also important for finding porphyry deposits.For the further prospecting ideas of typical porphyry deposits in the study area,this paper suggests that the internal contact zones of the granodiorite porphyry and the deep part of the south side of the granodiorite porphyry in the Xiaokelehe Cu(Mo)mining area has great prospecting potential.The mineralization of the Huoluotai Cu(Mo)deposit extends to the deep part of the southeast to a certain extent.The mineralization of the Fukeshan Cu(Mo)deposit extends to the deep part of the south to a certain extent.