Relationships Between Gold and Pyrite at the Xincheng Gold Deposit, Jiaodong Peninsula, China: Implications for Gold Source and Deposition in a Brittle Epizonal Environment

摘要

The Xincheng gold deposit, hosted by the Early Cretaceous 132 to 123 Ma Guojialing-type granitoids in northwest Jiaodong Peninsula, southeast North China craton, formed about 2 billion years later than regional metamorphism of the Archean Jiaodong basement rocks. The Xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold, tellurides, and a variety of sulfides: py(1) as disseminated euhedral to subhedral grains in altered granitoids around quartz veins; py(2) as subhedral grains with brittle cataclastic textures and fractures in quartz-pyrite veins; and py(3) as subhedral, partially corroded crystals in sulfide-rich veins or veinlets. All three generations of pyrite are unzoned and have low trace element contents, including very low lattice-bound gold contents: (py(1): 0.180 ppm; py(2): 0.053 ppm; py(3): 0.060 ppm). Given that there is 10 to 15% pyrite in the ore zone at Xincheng, its very low gold content indicates that it contributes <0.2% of gold to the 7.75 g/t gold in the orebody. Instead, over 99% of the gold is present as discrete electrum and/or gold (total range 0.02-59% silver) grains, which are largely sited in fractures at all scales in pyrite, other ore minerals, and quartz. Importantly, visible gold in py(3) is also sited on solution-corroded pyrite grains. The pyrite textural and geochemical data indicate that it is impossible to derive the high gold-grade orebodies through local remobilization of originally lattice-hound gold in pyrite. Instead, the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a H2O-CO2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement. This concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks. The coupled development of the transtensional, rather than normal transpressional setting, and precipitation of gold within dilational veins and wall-rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor. All deposit characteristics indicate that the Xincheng gold deposit is a member of the epizonal orogenic deposit class.