Tectonic deformation styles and dynamics of gold-polymetallic deposits in Fengtai apart-pull basin of Shaanxi Province in relation to gold mineralization

Abstract：The studied apart-pull basin was formed during the oblique-direction subduction and collision of the Huabei and Yangtze Plates. The tectonic deformation history, styles, dynamics, records of tectonic lithofacies and large-scale migration of fluids in the apart-pull basin constitute some of the most important problems in the continental dynamics, and the study of these problems are helpful to the detailed investigation of the mineralization regularity and the controlling factors of polymetallic deposits, Carlin-type gold deposits and similar ore deposits in the West Qinling orogeny. The Late Paleozoic apart-pull basin in Shaanxi is the high-density area which hosts not only Sedex-type polymetallic deposits but also Carlin-type gold deposits and similar deposits. Tectonic deformation history, styles and dynamics of the Fengxian-Taibai apart-pull basin and the driving factors of fluid migration in the basin were studied by the authors based on the tectonic lithofacies. Sequences of tectonic deformation for the apart-pull basin have been classified into three main deformation stages although it has more complicated tectonic deformation history. The first stage was tectonic inversion and superimposition of hydrothermal fluid in Carboniferous-Middle Triassic period (DS1), the second stage was compressive and shrinking deformation of the basin, faulting and folding, hydrothermal superimposition by magmatic intrusion, and brittle-ductile shear deformation (DS2) during the Indo-China main orogeny resulting from the full-scale continent-continent collusion, and the third stage was tectonic taphrogeny and brittle compressed deformation of the basin during the Yanshanian intracontinent orogeny (DS3). Styles of tectonic inversion and superimposition of hydrothermal fluids during the Carboniferous-Middle Triassic period (DS1) included layering shear rheology tectonics, zone of hydrothermal brecciation, and hydrothermal fluid superimposed lithofacies in the Devonian fill-up of the basin. At the same time, Wenjiangsi-Liufengguan apart-pull and taphrogeny basin of the second-order was formed in the southern part of the basin. Inside the apart-pull basin, associations of tectonic styles are composed of multiple folds and compression-shear faults, brittle-ductile shear zone, thrusting-napping structure, recoil faults and up-recoil-ramp fault. However, thrusting-napping fault zones with the ramp-faulting style in conversion direction were formed on both sides of the apart-pull basin. These deformation tectonic associations resulted in driving force for the basin fluid in the apart-pull basin, and coupled with regional hydrothermal fluid superimposition with stable hydrothermal field supported by Xiba compound medium-acidic magmatic intrusion in the eastern part of the apart-pull basin. Different styles of tectonic deformation might have had different controlling roles in localization of gold and polymetallic deposits in the apart-pull basin. Polymetallic deposits were probably mainly controlled by “M-W”- type multiple anticline-fault associations. However, zonation of gold and polymetallic deposits was controlled by the ramp-faulting area of up-ramp-faulting style. The Carlin-type and similar gold deposits were controlled by “W-M”-type multiple syncline plus brittle-ductile shear zone. Moreover, the Baguamiao gold deposit characterized by multiple stages of gold mineralization, a deposit similar to Carlin-type gold deposit, was controlled by overturned syncline and brittle-ductile shear zone, while the Wenjiangsi gold deposit, a Carlin-type gold deposit, was controlled by syncline and brittle fault zone.