Metallogenic Study Of Chaganhua Porphyry Mo Deposit In Inner Mongolia:Contribution Of Subduction-modified Fertile Magma Source And Post-collision Extensional Tectonic Setting

Located in Urat Rear Banner, Chaganhua is a typical and newly discovered large scale porphyry Mo deposit in Xing’an Mongolian orogenic belt. There are three main geological units occur in Chaganhua ore district:the Baoyintu Group metamorphic complex, the "X" shaped fractures and the Permian multistage intrusived granites. The Baoyintu Group metamorphic complex is medium-grade metamorphic rock and constitutes the main pre-ore basement. The Permian multistage intrusived granites is mainly composed of Early Permian biotite monzonitic granite and Late Permian porphyritic biotite monzonitic granite, the intrusion of later is controlled by the pre-ore "X" shaped fractures, and lead to the hydrothermal alteration and porphyry Mo mineralization of Chaganhua. The typical alteration includes potassic feldspathization, sericitization, silicification, argillization, epidotization and chloritization. The Mo mineralization is mainly hosted in porphyritic biotite monzonitic granite, and directly associated with the sericitization and silicification alteration. Most of the Mo mineralization occurs as vein and veinlet, based on the assemblage of alteration minerals of these vein and veinlet, they can divided into pre-mineralization quartz venlets, metallogenic quarzt-molybdnite-sericite venlets, quarzt-pyrite-sericite venlets, quarzt-molybdnite-venlets and quartz-wolframite vein and post-mineralization quartz-pyrite venlets.LA-ICP-MS U-Pb dating study shows that most of zircons of plagioclase amphibolite from Baoyintu Group yield206Pb/238U ages ranging from464±5Ma to698±8Ma, and some of them yield207Pb/207Pb ages as old as1995±11Ma. Most of the zircons of two-mica quartz schist yield207Pb/207Pb ages ranging from1135±91Ma to1890±13Ma, and some of them yield206Pb/238U ages as young as633±8Ma. These ages show that the sedimentary time of the protolith of Baoyintu Group may as low as Middle Ordovician. The zircons from monzonitic granite which was deformated and metamorphic with Baoyintu Group yield two weighted mean concordant ages of446.4±1Ma and399.4±1.5Ma, these suggest that the monzonitic granite intruded the Baoyintu Group at Late Ordovician, and the time of metamorphism of Baoyintu Group may as low as Early Devonian. Incorporating the petrogeochemistry studies, we suggest that the protolithes of Baoyintu Group are sedimentary rocks, among which, the protolith of plagioclase amphibolite may be pelite mudstone, dolomite, and the protolith of two-mica quartz schist may be clastic rocks. These sedimentary may deposit in continental marginal arc formed in Late Proterozoic to Early Paleozoic.SHRIMP U-Pb dating study shows that the pre-mineralization biotite monzonitic granite was emplaced at about273Ma, and the mineralization related porphyritic biotite monzonitic granite was emplaced at about273Ma.The emplacement of these granites constitute a long lived and multistaged magmatic evolution cycle, which was evolved from progressive magmatic activity to large-scale intrusion of granite, and waning to small-scale intrusion of ore forming magma, and to the exsolution of ore forming hydrothermal fluid. The mineralization activity is the outcome of this long lived magmatic evolution cycle. The petrogeochemistry studies suggest that these two granite intrusions share the same geochemical feature of highly evolved magma. We suggest that they are the consequence of AFC in the deep magma chamber under Chaganhua porphyry deposit, and their magma source was modified by the Paleozoic sudbuction of oceanic crust, and emplaced in Late Paleozoic extensional tectonic setting. The isotope chemistry studies show enriched feature for these granites, which are different from the low (87Sr/86Sr)i and high ε Nd(t) granites occurring Xing’an-Mongolia orogenic belt.We suggest that these granites are souced from two endmembers:upper crust and EMⅡ.The molybdenite Re-Os dating show that the porphyry Mo mineralization occurred at about243Ma, which was about10Ma later than the emplacement of ore forming magma. This reflects a long duration of hydrothermal activity in Chaganhua deposit, which is agreed with the geological features of chaganhua deposit. These long duration are also supported by the stable isotopes studies, the ore forming fluid have very low δD (-133.9‰) and district positive δ34S (2.97‰), which show intense Rayleigh fractionation process of the ore forming fluid, and these also reflect a long evolution history of the ore forming fluid. Fluid inclusions in quartz of the main stage and late stage of molybdenum mineralization are mainly two phase vapor-liquid inclusions of H2O-NaCl composition with small amount of CO2phase. It’s characterized by moderately low salinities and high homogenization temperatures. Combined with the studies of oxygen isotope thermometer, we suggest that molybdenum began to deposit at653℃,200MPa (approximately7.4km of lithostatic pressure) and mainly deposit at416℃to384℃,65MPa to40MPa (approximately6.5km to4km of hydrostatic pressure). The inclusions data combined with H-O isotope data record an evolution path of the hydrothermal fluid of degassed magma water source from high temperature, high pressure and moderately low salinity to low temperature, pressure and salinity which form Chaganhua porphyry molybdenum deposit, and these studies also suggest that the long fractionation history of ore foming fluid may be one of the important reasons for deposition of molybdnite. The H-O-S-Pb isotope geochemical studies also thow that the ore forming fluid are sourced from magmatic activity, and the ore forming substances are sourced from deep magma, and probably from subcontinental lithospheric mantle and lower crust.The geology and metallogenic studies show that Chaganhua porphyry Mo deposit are comparable to the high F type porphyry Mo deposits occurs in extensional tectonic setting, and suggest Chaganhua is a high F type deposit. Chaganhua is also the oldest large scale porphyry Mo deposit occurring in Xing’an-Mongolia orogenic belt, its formation means the begining of extensive porphyry Mo mineralization along the S-E to NE trending Xing’an-Mongolia orogenic belt. The activity of highly evolved and differentiated magma in post-collision extensional setting may be of great significance, but the Paleozoic subduction-modified fertile magma source may also play a crucial role in the formation of these huge porphyry Mo mineralization belt as follows:(1) Building up the fertile magma source by subduction fluids enriched in incompatible elements in Paleozoic;(2) hydrating the orogenic belt by dehydration melting of subduction fluids in Paleozoic and this highly concentrated wate will be released in the post-collision extensional setting stage, and will contribute the huge volumes of ore forming fluid needed in the formation of this huge porphyry Mo mineralization belt in Xing’an-Mongolia orogenic belt;(3) Thickening the continental lithosphere in by subduction tectonism, the instability of this thickened lithosphere would thin again because of the termination of compressive stress in post-collision extensional setting, which will cause the extensive Crust-mantle interaction.