Dynamics of Late Paleozoic Pullapart Basin and Its Relationship with Mineralization of Goldpolymetallic Deposits in FengxianTaibai Counties, Shaanxi Province,China
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摘要: 采用沉积盆地构造—古地理位置恢复重建和构造—岩相学等新方法研究认为，凤太晚古生代沉积盆地属于受板块斜向俯冲碰撞动力学控制下的拉分盆地。在中泥盆世初期，凤太沉积盆地被周缘垂向基底隆起分隔，其成盆构造动力学主要受四组同生断层，晚古生代沉降中心和沉积中心不断发生迁移。中泥盆世中期在盆地北部形成了北西向沉降中心和沉积中心，晚泥盆世末期沉积盆地萎缩，沉降中心和沉积中心收缩于沉积盆地中心。石炭纪沉降中心和沉积中心从盆地中心迁移到盆地四周边缘的同生断裂带附近，在沉积盆地北侧边缘商丹带南侧，形成了石炭纪—早三叠世与俯冲消减带有关的楔状沉积充填体。在凤太拉分盆地中形成的近东西向、北北东向、近南北向和北西向网状同生断裂带系统共同控制了凤太泥盆纪拉分盆地形成与演化过程。其中，商丹带（西段）、礼县—凤县—凤镇—山阳同生断裂带（中段）和酒奠梁—镇安—板岩镇同生断裂带（西段）三个主控同生断裂带不但在泥盆纪期间对于凤太泥盆纪拉分盆地形成具有显著控制作用，而且石炭纪—早三叠世拉分盆地演化过程也具有十分重要的控制作用，石炭纪—早三叠世同生断裂带发生构造反转并控制了沉降中心和沉积中心。采用沉积盆地动力学和构造—岩相学等新方法研究认为，在凤太晚古生代拉分盆地具有分级特征，西部凤县二级盆地为金—多金属成矿集中区，东部太白二级盆地为金矿成矿集中区。在八方山—银母寺三级拉分盆地中，八方山和银母寺等多金属矿床与八卦庙超大型金矿床具有矿田尺度上成矿分带，主要由于三级盆地、同生断裂、热水沉积岩相和构造热流体叠加岩相控制了矿田和矿床尺度上金与多金属成矿分带。凤太拉分盆地北部和东部金矿矿源层和初步富集成矿形成主要与泥盆纪钠长岩相和钠质热水沉积岩相有关，并受钠长碳酸质角砾岩—铁白云石钠长石角砾岩等石炭纪构造—热流体岩相叠加；凤太拉分盆地南部温江寺三叠系浊积岩系中热水硅质岩相和层状英安质凝灰岩是卡林型金矿重要赋矿层位；凤太拉分盆地中部热水沉积—改造型铅锌矿主要与硅质岩相和菱铁矿铁白云岩相等热水沉积相密切有关。Abstract: FengxianTaibai sedimentary basin could be the Paleozoic pullapart basin which might be controlled by obliquedirected subduction and collision of the Qinling and Yangtze Plates based on new research methods, i.e., reconstruction and restoration of geotectonicpaleogeographic position and tectonic lithofacies. During the initial stage of the Middle Devonian period, FengxianTaibai sedimentary basin was enclosed and separated by marginal vertical baseuplifting, and dynamics of basin was controlled by four groups of synsedimentary faults, resulted in continual migrations of subsidence and sedimentary centers in the Late Paleozoic basin. During the middle stage of the Middle Devonian period, subsidence and sedimentary centers trending northwestdirected might be located in north part of FengxianTaibai basin while subsidence and sedimentary centers trending northwestdirected shrink in the center part of FengxianTaibai pullapart basin during the Late Devonian period. However, the Carboniferous subsidence and sedimentary centers were migrated from the center to the zones of marginal synfaults of FengxianTaibai pullapart basin, and the Carboniferous to the Early Triassic wedgeshaped sedimentfilling bodies related to zone of the plate subduction and collision was located at the southern part the Shangdan suture in the northern margin of FengxianTaibai pullapart basin. Finally, formation and evolution of FengxianTaibai pullapart basin might have been controlled by the NW, NNE, NS and NWtrending synsedimentary faults in the FengxianTaibai Devonian pullapart basin. The west part of Shangdan, the middle part of LixianFengxianFengzhenShanyang, and the west part of JiujianliangZhenanBanyanzhen synsedimentary faults were of most important controls not only in the formation of the FengxianTaibai Devonian pullapart basin but also in the evolution of the Carboniferous period to the Early Triassic pullapart basin. Tectonic inversion of synsedimentary faults controlled subsidence and sedimentary centers during the evolution of the basin from the Carboniferous period to the Early Triassic period. The FengxianTaibai Paleozoic pullapart basin can be divided into three different level basin based on the study of basin dynamics and tectonic lithofacies. The secondlevel, Fengxian basin is mineralization concentrated area of goldpolymetallic deposits in the west part of FengxianTaibai pullapart basin while the secondlevel, Taibai basin is mineralization concentrated area of gold deposits in the west part of FengxianTaibai pullapart basin. In the thirdlevel, BafangshanYinmushi basin, Bafangshan and Yinmushi polymetallic deposits and Baguamiao gold are the mineralization zoning at the scale of orefield and of deposits, which might be controlled by threelevel basin, synsedimentary faults, hydrothermal sedimentary lithofacies, and superimposed lithofacies of tectonic hydrothermal fluids. In the north and east part of the Paleozoic pullapart basin, formation of gold source beds and preliminary enrichment might be related to albiteite lithofacies and albitehydrothermal sedimentary lithofacies, and they may be superimposed by albitecarbonate breccia and Fedolomitealbite breccia formed by the Carboniferous tectonic hydrothermal fluids while the Triassic hydrothermal cherts and layered dacitic tuffs hosted Carlintype gold deposits in Wenjiangsi area in the south part of the Paleozoic pullapart basin. SEDEXtype leadzinc deposits may mainly related to chertic lithofacies and sideriteferrodolomitite lithofacies.
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