Geology, mineralization, alteration, and structural evolution of the Zaozigou Au-Sb deposit, west Qinling Orogen, China

Abstract

The world-class Zaozigou Au-Sb deposit is the largest gold deposit in the West Qinling Orogen. Despite an endowment of nearly 3.8 Moz Au and 0.12 Mt Sb, the geological context of the Zaozigou deposit has not been comprehensively studied or described. New surface and underground mapping of geology, structure, alteration, and mineralization presented here establishes a geological framework with which to evaluate the Zaozigou deposit. The structural evolution of the Zaozigou deposit followed a progressive evolution and rotation of regional stresses related to multiple stages of accretion and ocean-closing within the Qinling Orogen. The tectonic regime at Zaozigou likely initiated as NE-SW compression which developed a fold-and-thrust belt with an axial plane striking roughly 330°. Coincident with the development of the fold-and-thrust belt, fault surface lineations indicate the formation of NE-SW right-lateral strike-slip faults. Many faults and joints of this orientation were sites of emplacement of Triassic porphyry dikes of quartz dacite to diorite composition. The dikes caused extensive contact metamorphism of the surrounding Permian to Triassic clastic sediments of the Gulangdi Formation, thus converting local interbedded flysch carbonate-siltstone meta-sediments to an interbedded marble-hornfels together referred to as slate hornfels. The relative strength of the slate hornfels likely increased the overall competency of the rocks at Zaozigou, allowing for the buildup of higher pore fluid pressures. Pore fluid pressures overcame the strength of the rock package causing fault ruptures as high-angle reverse faults and low-angle extensional veins. Fault-valve fluid pressure cycling is interpreted to continue over an extended period while the dynamic tectonic evolution of the West Qinling Orogen caused a counterclockwise rotation of the principal stress axis. Late-stage faulting crosscuts mineralization and moderately dismembers the deposit. Two main orientations of ore-hosting faults are present at Zaozigou. 1) High angle reverse fault-hosted orebodies are labelled using the “Au” prefix. 2) Low angle fault-hosted extensional vein orebodies are labelled using the “M” prefix. The “Au” orebodies are typified by the Au1 and Au9 orebodies, which trend NE-SW. Field relationships suggest they initiated as right lateral strike slip faults and were reactivated as high angle reverse faults which host multiple generations of cataclastically deformed quartz-stibnite veins, fault gouge, and frictional breccias. “M” orebodies are typified by the M4, M6, M7, and M9 orebodies, which are low angle laminated quartz-stibnite extensional veins. “Au” and “M” orebodies have broad sericitic alteration haloes which host disseminated auriferous arsenopyrite and pyrite. The quartz-stibnite fault-fill and extensional are high grade orebodies up to 56521 ppb Au. Sericite-pyrite-arsenopyrite mineralization precipitated through sericitization of host rocks, and sulfidation of Fe-rich phases. Wall rock hosted disseminated ore is of lower grade, averaging 2936 ppb Au in altered slate hornfels and averaging 4420 ppb Au in altered quartz dacite porphyry dikes. Hydrothermal alteration follows a spatial zonation pattern identified through SWIR from most proximal NH4-illite, to distal illite-chlorite alteration. NH4-illite was only identified in samples taken within the most proximal 10m sericitic alteration haloes around both “Au” and “M” orebodies. Stibnite micro-inclusions observed within wall rock disseminated arsenopyrite crystals indicate that stibnite and arsenopyrite precipitated from a common ore fluid.