Study On Geochemical Kinetics Of Late Paleozoic Strata And Basin-orogen-mineralization In Western Qinling Region

The west Qinling orogenic belt is an important mineralization area in Qinling oregen. Itsformer research works were focused on the Pb-Zn,Au deposits. But all researches showed that theore formation had very important relationship with the evolution of the regional tectonics.Established in scientific research project, this paper did some researches on some important issuessuch as the tectonic evolution of the west Qinling and the relationship between it and the oreforming processes and distribution of these deposits.Qinling microplate was a separated plate between Yangtze plate and North China plate,separated by two main suture zones—Shangdan zone and Mianlue zone. This paper indicated thatthere existed big differences between the east an the west Qinling. So this paper suggested to treatthe West Qinling as an separated geological unit and name it "the west Qinling block".The sedimentary basin in the west Qijnling lied between the Silurian in the south andLiziyuan group in the north. The west Qinling is an unified system composed of the north andsouth basement and the sedimentary basin in the central.This research on the late Paleozoic basin indicated the characteristics of the basin. The basinhad an asymmetry shape as deep and steep in the north and shallow and gentle in the south. Themain growth faults had south forward direction of dip and north forward development. The heatactivity occurred not in the sedimentary center but in the shallow and gentle slope. The basinbelonged to a model of extensional structure producing Metamorphic core complex. The locationof metamorphic core complex was corresponding to the core of Wujiashan anticline. In this paper,we call this kind basin extensional basin. From the upper Devonian, the western part of the westQinling developed foreland basin and from then composed a basin combination of two kinds ofbasins—foreland basin in the north and extensional basin in the south. After late Devonian, theeast part of the west Qinling uplifted constantly and the basin had a west forward constantlydevelopment. In middle Triassic, there took place a violently rift faulting because of subduction bythe old Tethys. The west Qinling basins in all periods belonged to a whole basin evolution system.This paper established the model of basin evolution and analyzed the whole process of theevolution.The nappe and thrust was the main structural pattern of the oregenic movement of the westQinling. The structure in the east part was complex and violence and existed two separated nappeand thrust systems respectively formed in the north and south area. The structure in the west wassimple and a little feeble. There formed a nappe and thrust with a single direction. Followed bycosmically magmatic activities, the strike-slip movement was the main pattern of the postorogenicextension. The distribution of the rock bodies was controlled by two group of growth faultsformed in the period of basin evolution.This paper also discusses the distribution characteristics of the ore-forming elements and thetrace elements, and carries through the contrast work of the content of elements distributed instrata of different periods and different lithology and also of different tectonic positions. Thispaper also researches the affect of nappe and thrust and hydrothermal events to the distributionelements and found that the nappe and thrust could make fluid in the strata move and then carrieda lot of elements toward the forebelt of thrusting. Based onthe experiments of geochemical kinetics, this paper did some research on the mainPb-Zn ore deposit and achieved some important conclusions. In the west Qinling, the mostimportant metamorphic events included three periods. The metamorphic fluid had two originrespectively became from the crust and the mantle. The forming of Changba Pb-Zn ore depositwas related to the magma and hydrothermal fluid in the deep.The vapor could carry metals fromthe deep into upper crust. The regional metamophism and water-heat process could make theore-forming elements move and so they are propitious to form deposits.Some aspects, such as the regularity of metamorphic core complex evolution and thecharacteristics of the growth fault activity in the extensional basin, controlled the forming of theheat-water sedimentary Pb-Zn deposits. This study shows that there is little possibilities to findthis kind of Pb-Zn deposits in the Permo-Carboniferious strata in the west of basin. The possibilityexisted in Devonian, the east part of basin.There were two kinds of gold deposits in the west qinling according to the basic ore-formingmechanism: structure-controled Au-deposits and rock body-controled Au-deposits. The formerwas mainly formed in the tectogenesis and distributed in the violent tectonically deforming area.The latter was mainly related to the rock bodies and was controlled by the structure formed in theintrusion of the rock bodies. Its distribution character was the same of the rocks. In a word, theforming of gold deposits was controlled by the oregenesis in Yinzhi period and thepost-orogenesis extension in Indosinian-Yanshanian period.In sum, this paper systematically investigates some important projects as the tectonicevolution, hydrothermal events, the geochemistry and the mineralization in the whole processfrom basin formation to orogenesis and then to the post-orogenesis extension. It produced a newthought and a new research direction to the research works on the west Qinling, and it wouldprobably promote the geological work on the west Qinling.