Structural control and genesis of the Oligocene Zhenyuan orogenic gold deposit, SW China

The Zhenyuan gold deposit with >50t gold reserve is located within the Jinshajiang–Ailaoshan suture zone that formed through the closure of Paleo-Tethyan ocean in late-Permian. This suture separates the Simao and South China blocks. A west-dipping high velocity zone down to ca. 250km beneath the Simao block in the seismic tomography suggests that the South China lithosphere was underthrust westward. The NNW-trending Ailaoshan–Red River shear zone is situated to the east of Ailaoshan suture. The shearing in this belt, generally following a regional mantle upwelling and crust extension event, was initiated at 32Ma and culminated at ca. 27Ma. This shearing was induced by the lateral extrusion and large clockwise rotation of the Simao block during the India–Eurasia continental collision and the underthrusting of South China block. The Zhenyuan ore deposit consists of the Donggualin and Laowangzhai orebody clusters. The Donggualin orebodies are controlled by the NW-striking shear faults, whereas most orebodies in the Laowangzhai are dominated by NE- and ENE-striking transcompressional faults. It is observed that the NE- and ENE-striking faults are crosscut by the NW-striking faults. It is deduced that the former were primarily formed as nearly NS-trending faults during the regional extension or initial shearing stage at ca. 32Ma. The primary faults were then re-oriented to the current strike concomitant with the rotation of the Simao block, and further cut by the NW-trending shear faults, which are subsidiary to the Ailaoshan–Red River shear zone. The rock types in the ore deposit including slate, meta-sandstone, limestone, lamprophyre, and meta-mafic to ultramafic units, which were all mineralized by the infiltration of ore-bearing fluids into extensional fractures. The low temperature ore mineral assemblage (pyrite–stibnite–arsenopyrite), enriched element association, and ranges of S and Pb isotopic compositions in the two ore clusters are broadly similar, indicating that the mineralization is contemporary despite the different formation times of ore-controlling structures. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of hydrothermal pyrite are close to those of the lower crust. Pyrite δ34S values show a wide range with a peak near 0 per mil. The S and Pb isotopic compositions, together with previously published H–O–He–Ar isotopic data, suggest that the ore fluid is mainly metamorphic. The data also suggest the input of mantle volatiles, consistent with mantle upwelling. The control of the shear zones on mineralization favors the interpretation that the Ar–Ar isochron age ~27Ma of phlogopite in the mineralized lamprophyre represents the mineralization age. This mineralization age implies that the South China block had started westward underthrusting before ~27Ma. The Zhenyuan ore deposit is considered to be an orogenic type, formed in a transitional stage from mantle upwelling to continental underthrust. The continental underthrusting drove the release of gold-charged metamorphic fluids and formation of the Zhenyuan ore deposit.