Genesis And Constraints From Deep Processes For Antuoling Porphyry-type Molybdenum Deposit

Tuoyun volcanic group was formed while several volcanoes erupted nearly same time in Tuoyun basin in southwest Tianshan Mountain. These volcanoes can be divided into two types: central-eruption-type an d flood-basalt-type. A series of volcanic apparatus were formed including calderas,volcanic necks and cone sheets.The volcanic face of lava face, cone sheet face and volcanic neck face were found in Tuoyun basin. Tuyon volcanic rocks are classified as alkali basalts with low in silicon and high in alkali. The products of both primary magma and evolved magma were found in Tuoyun basin. Basaltic magma ascended rapidly because of hosting peridotite xenoliths, which means magma was not contaminated by crustal materials. Partial melting source of Tuoyun basaltic magma, whose one of residual minerals is phlogopite, has underwent complex process, such as metasomatism of mantle fluid enriched CO2, multi-stage separation of mantle partial melting source, interaction of deep fluid pools and fractional crystallization in upper area of partial melting mantle source (melting column). SHRIMP U-Pb age of sample from cone sheet is 48.1±1.6Ma, however, zircon ages of 3 basaltic rocks from lava face and volcanic neck face shape a complex age assemblage. According to depth inversion of partial melting source by using mantle melting column model, the lithosphere thickness under Tuoyun basin at 48.1Ma is 84.3km. Tuoyun basalts is products of Tuoyun lithosphere delamination, which means a mixed fluid system formed in the complex process of mantle partial melting source and interaction of deep fluid pools was triggered to discharge, and ascended rapidly along the lithospheric fault.Lithosphere depth evolution of north part of north China Craton and Taihang Mountain orogen area in Mesozoic-Cenozoic was received by employing mantle melting column model. Lithosphere depth evolution in two areas in Mz-Cz is a heterogeneous process, for instance, there are three distinct types lithosphere depth history for north section, middle section and south section of Taihang Mountain orogen in Mz-Cz. Antuoling basalt occurred as basaltic dyke with 1.0~2.5m in depth, and was formed in early Cretaceous (122.31Ma) by K-Ar whole rock dating. Peridotite xenolith, assemblage of carbonate and megacrysts of anorthoclase, corundum and phlogopite were found in basalt. Antuoling basalt with 46.50-50.20% in SiO2 content has suffered slightly alternation of carbonatization, chloritization and serpentinization of olivine, and chloritization and carbonatization of clinopyroxene. The compositional points of Antuoling basalt in Zr/TiO2-Nb/Y diagram fall into alkaline basalt field. According to mantle melting column model, the melting column locates from 82.5km to 75.3km in depth, which means that the lithosphere depth under Antuoling was 75.5km when Antuoling basaltic magma erupted in 122.31Ma. So Antuoling basalt is production of lithosphere delamination of North Taihang Mountain in Mesozoic. Combined with composition and ages of Nandaling basalt and Nankou mafic dykes, the lithosphere depth evolution under Antuoling-Nandaling-Nankou was established, a process of a small size delamination, thickening, delamination and keeping stable. It becomes deep dynamic condition for constructing Antuoling Mo deposit model.Normal faults extensively distribute in Antuoling Mo deposit, and they have formed a grid-shape faults system. Shear joint develops comprehensively with diverse occurrence. There is a complex igneous assemblage, and divided into two groups: porphyry body and dykes. Antuoling porphyry body is composed mainly of quartz-monzonite porphyry in central and granodiorite porphyry in outer. Dykes type are basalt, diabase, lamprophyre, diorite porphyrite, granodiorite porphyry, monzonite porphyry, syenite porphyry, granite porphyry and aplite. SHRIMP U-Pb age results for quartz-monzonite porphyry and granodiorite porphyry are 141.3±1.5Ma and 139.4±1.7Ma, respectively. Petrography and major elements features of Antuoling porphyry body has suggested that fluid enriched in potassium has been injected in the process of porphyritic igneous rock. They shows positive anomaly for Rb and K, negative anomaly for Th, Nb,Ta, P and Ti, smoothed REE pattern with enrichment of LREE and the depletion of HREE, and characterized by adakite. Oscillatory zonings of plagioclase phenocryst in diorite porphyrite has become the proof of magma mixing and mafic magma injection during the process of porphyritic rock formation. Lamprophyre occurred in Antuoling Mo deposit was intruded in early Cretaceous, and they shows negative anomaly for Nb, Ta and Ti, smoothed REE pattern with enrichment of LREE and the depletion of HREE. Antuoling lamprophyre occurred as dykes, and olivine xenocrysts and carbonate can be found in lamprophyre, has indicated lamprophyre magma ascended rapidly, and their geochemistry features is controlled by source composition and fraction of partial melting.Favorable mineralized areas at Antuoling Mo deposit are mainly located in the transition belt of higher-lower resistivity anomaly, i.e., the alternation belt of outer granodiorite porphyry. Furthermore, the formation of more extensive mineralization is tightly related to transition belt of higher-lower resistivity in deeper. The 3D ore bodies shape built by Micromine in Antuoling Mo deposit is irregular and complex. Re-Os model ages of molybdenite are from 144.91±0.54 Ma to 146.11±0.71Ma. Fluid inclusions in Antuoling Mo deposit are composed mainly of H2O-rich and CO2-rich types, whose homogenization temperature peak is from 220℃to 260℃. In the process of mineralization, the high temperature fluids while mineralization progressed have been injected because the homogenization temperature of fluid inclusions in molybdenum quartz is higher than those located in quartz before mineralization. According to Pb isotope of molybdenite and pyrite, the mineralization fluids come from lower crust-upper mantle.Huge deep fluid was discharged, which triggered by the process of Antuoling lithosphere delamination. Polyporphyritic texture is an important petrological indicator for finding concealed ore body, which has been proofed in the exploration process at Antuoling Mo deposit. Antuoling porphyritic molybdenum deposit is a successful example for metallogenic theory on the transmamagtic fluids, and metallogenic model was displaced.