Structurally Controlled Deposition of Gold During Inversion of Sedimentary Basin: A Case Study of Sediment-Hosted Orogenic Gold in the Gaolong Deposit, Youjiang Basin, South China Block

Abstract The Youjiang basin underwent late Paleozoic expansion and Triassic inversion, resulting in scattered Paleozoic isolated carbonate platforms surrounded by Triassic turbidites. The Gaolong gold deposit is representative of sediment-hosted orogenic gold deposits formed along isolated platforms in the central-southern basin, where three stages of deformation were identified. The pre-ore D1 deformation in Gaolong features a WNW-trending brachy-anticline and NNE-trending dextral strike-slip shearing within the competent carbonate platform, overthrust of the turbidites onto the carbonate platform along the lithological contacts, and WNW-trending thrusts and related asymmetric tight folds in incompetent turbidites. Kinematic analyses revealed that northeast-southwest to north northeast–south southwest regional shortening occurred. The U-Pb dating of the synkinematic calcite constrains the D1 deformation age of 240.5 ± 5.2 Ma. Syn-ore D2 deformation is associated with north-south shortening and features NE-striking sinistral and NW-striking dextral strike-slip mineralized faults that cut through the D1 structures. Gold orebodies formed in the stratigraphic-structural traps, like tight folds, thrusts, and strike-slip faults in turbidites at the less compressed and permeable eastern and western edges of the platform. Gold mineralization in those edges is mainly invisible gold in the rims of zoned pyrite and arsenopyrite in quartz veins, hydraulic breccia, and altered turbidites with ore-related silicification, sulfidation, sericitization, and minor carbonatization. The inferred stress field and published bitumen Re-Os age suggest that the D1 and D2 deformations were formed during Triassic inversion and record a change in direction of the propagation process of basin inversion. The post-ore D3-1 deformation is characterized by west-northwest–east-southeast shortening, whereas the latest post-ore D3-2 deformation represents extension. The north-northeastward basin inversion progressively propagated from the southernmost toward the northernmost of the Youjiang basin during the Middle to Late Triassic. This inversion involved horizontal crustal shortening and vertical thickening, leading to overpressure of deep fluids and triggering large-scale vertical fluid migration via regional inverted preexisting faults. These faults served as conduits, linking the deep source with shallower traps in the fold-and-thrust belt. At shallow depths, gold deposition during compression is controlled by lithological and rheological contacts along competent isolated platforms, which provide better permeability and Fe-bearing reactants that facilitate the precipitation of gold-bearing sulfide. The geochronology of progressive basin evolution and regional sediment-hosted gold mineralization reveals a coupled spatiotemporal relationship between them. Both the sedimentary facies and structural architecture preparation during basin formation and the following structural inversion and lithological-rheological conditions during basin inversion jointly controlled gold deposition. The identified processes for the formation of sediment-hosted orogenic gold deposits can be used to guide mineral exploration in inverted sedimentary basins worldwide.