Textures of auriferous quartz-sulfide veins and 40Ar/39Ar geochronology of the Rushan gold deposit: Implications for processes of ore-fluid infiltration in the eastern Jiaodong gold province, China

Abstract

The Rushan gold deposit, located in the eastern part of the world-class Jiaodong gold province, contains the largest gold resource within a single fault-controlled quartz vein in China. Processes involved in auriferous quartz vein deposition are not clear, and the precise gold mineralization age remains controversial for this deposit. New 40Ar/39Ar dating data on sericite from ores and altered rocks suggest that the Rushan gold deposit formed between ca. 122 Ma and 117 Ma, which overlap or are slightly younger than those of gold deposits in northwestern Jiaodong province. The age differences between samples from different locations indicate that the quartz veins on the margin of the orebody may be younger than those in the center, suggesting a single and protracted gold mineralization event lasting for about 5 My. Differences between gold mineralization ages and emplacement/cooling ages of the ore-hosting Kunyushan granite imply that they are not genetically related and that the Kunyushan granite is unable to provide any thermal input to the mineralization system. This also applies to the Sanfoshan monzonite as it marginally postdated gold mineralization and lacks any spatial association with it.Internal textures of high-angle shear veins and low-angle extension veins indicate that episodic fluctuations from supralithostatic to hydrostatic fluid pressure triggered the multiple ore-controlling fault ruptures induced by breach of supralithostatic fluids under a seismogenic regime. In this model, the controlling fault starts to slip with both reverse and sinistral kinematics, and dilatational zones are created. The ore-forming fluids then migrate upwards along the fault, due to the large pressure difference between the dilatational zones and the deep parts of the fault, causing hydraulic fracture in the host rocks and earlier quartz veins. Large volumes of quartz, pyrite, and other ore components are precipitated due to fluid immiscibility caused by pressure fluctuations, and new quartz veins form along both sides of the earlier quartz veins. The fault is now sealed by these new quartz veins and the fluid pressure starts to build until the next breach cycles. Orebodies with complex internal textures were thus deposited incrementally in these cycles, closely fitting the classic fault-valve model, in this case under a weak regional NW-SE transpressional stress.