| The southern margin of Qinling orogenic belt, there are a lot of Early Paleozoic mafic volcanic rocks, Qiaomaichong gold mines ore syenite porphyry lanes NW invasive basic volcanic formations. In previous studies of Qiaomaichong gold mines element geochemistry, rock chemistry, isotope, inclusion, rare earth elements and other aspects of geochemistry, during the gold mineralization was confirmed leading role Yanshan period brittle- ductile shear deformation and metamorphism, get a group of model ages of 155 Ma possible reaction time of the deposit. Through the syenite porphyry zircon LA-ICP-MS U-Pb dating, Hf isotopes of element geochemistry and explore the syenite porphyry veins intrusion age and rock genesis, numerous mafic rocks diagenetic tectonic environment. combined with the Dabie orogenic belt geological background characteristics, veins invade the era can be divided into two phases, the weighted average age of zircon values were(415 ± 7.3) Ma,(444 ± 5) Ma, the former belongs to the Early Devonian period and the latter for Cambrian to early Ordovician period.The zircon Th/U ratios basic concentrated near the 1, combined with the feature of rare earths, typical Ce is abnormal, the Eu negative anomaly, heavy rare earth concentration, can be speculated that this zircons of magmatic zircon. Is displayed on the geochemical characteristics including higher Si O2 content(> 75%), Rittman index δ = 1.59, low Ti high Al features, Eu and Sr has obvious negative anomaly for the calc-alkaline veins. The syenite porphyry zircon Hf isotope test results get εHf(t) are all positive and the range is large(3.4 to 11.8), indicating magma mainly from depleted mantle, most of them concentrated in the range of 6 to 11, inferred mixing a small amount of crustal material during the magma intrusion to show the crust-mantle mixing features. In the study of geochemical characteristics of mafic rocks, the total amount of rare earth high(ΣREE = 149.62 ~ 321.55 × 10-6), strongly enriched in light rare earth, showing a high degree of differentiation characteristics, samples of large LILE and high field strength elements enrichment than MORB, display with similar OIB characteristics of the mafic surrounding rock, the magma source may be derived from asthenosphere, probably related with the plume.The zircon has dual characteristics of oceanic crust zircon and Mainland zircon, Sample 13QMC-011 and 13QMC-013 the value of zircon two-phase model age range is 913~1284 Ma and 749~1521 Ma, in Meproterozoic to neoproterozoic period. Zircon epsilon of Hf(t) values range were respectively 4 epsilon and 8.4 epsilon unit, shows its crust-mantle mixed source area characteristics. Two phases syenite porphyry veins of epsilon Hf(t) are all positive, indicating that the area indicating that the area from rifting tectonic environment gradually changed into a limited ocean basin environment.In the early Paleozoic era, regional tectonic environment for plate cracking, the asthenosphere intrusion formed by condensation mafic formation, veins complete invasion, the Au element from deep crust to the surface crust, preconcentration and later cut to the formation of Au mineralization Au source provides material elements. Since then the region is ocean basin environment, the formation of a number of sedimentary formations. In the late Caledonian, ending oceanic environmental development, In the late Caledonian, ending oceanic environmental development, Yangtze plate subduction northward happen again. In the Triassic period, the collision occurred with the North China plate; in the Jurassic period, with the enhancement of the role of the collision, the local recurrence of magmatic hydrothermal, provides heat for the syenite porphyry veins in the activation of Au mineralization. Syenite porphyry veins form determined by the times, to determine the area sailed layer era, sailed layer is the primary source material late gold mineralization, which is important for the formation of ore deposits. |
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