Multiple Episodes of Tectono-Thermal Disturbances in the Huayangchuan U-Nb-Pb Polymetallic Deposit in the Xiaoqinling Region, Central China and Their Significances on Metallogeny

The Huayangchuan U-Nb-Pb polymetallic deposit is hosted in a suite of Neoarchean to Paleoproterozoic gneisses in the Xiaoqinling region, central China. The deposit is composed mainly of discrete small-scale carbonatite veins emplaced largely along shallow-lying brittle fractures. Various regimes of deformation from ductile flow to brittle fracturing are documented in the deposit and its adjacent areas. These include an earliest phase of contracting deformation (D1) manifested as well-developed boudinages, abundant relict rootless folds and penetrative compositional bands, i.e., gneissosity generally trending E-W, a subsequent phase of disperse dextral ductile shear (D2) along the E-W-trending gneissosity, and a following phase of general NE-SW compression (D3) resulting in local contraction deformation bands and abundant sinistral ductile shear along the E-W-trending gneissosity, all of which are affected by an overwhelming NNE-SSW compression (D4) with hemi-ductile to brittle deformation behaviors. The deformation after D4 is generally brittle and may arise from an earlier regime of dominant compression (NW-SE) (D5) and a later regime of extensive extension (D6). Multiple thermal events are superimposed on gneisses as revealed by 40Ar/39Ar dating on biotite collected from various lithologies in the deposit and its adjacent areas, which yields ages ranging from Late Triassic (238-203 Ma), through earliest Cretaceous (139-134 Ma) and middle Cretaceous (117-92 Ma), to latest Cretaceous (72 Ma) and even early Cenozoic (53 Ma). This sequence of thermal disturbances unlikely predates the D4 deformation given that it is the last ductile deformation experienced by the Precambrian rocks. Ore-bearing carbonatite veins have their shallowly lying attitudes consilient with the D4 and D5 deformation and ages compatible with the first two thermal overprints. Accordingly, a two-stage mineralization model is proposed for the Huayangchuan polymetallic deposit with an early phase of ore veining in the Late Triassic to latest Jurassic during the nearly N-S collision between the South and North China blocks and a late phase of ore veining or more possibly reworking in the Early Cretaceous related to the Yanshanian orogeny due to a generally NW-directed subduction of the Paleo-Pacific oceanic plate beneath the eastern Asia continent.