Utilization of pre-existing competent and barren quartz veins as hosts to later orogenic gold ores at Huangjindong gold deposit, Jiangnan Orogen, southern China

Most, if not all, orogenic gold deposits form during the late stages of deformation. In some cases, there is barren pre-mineralization hydrothermal alteration which may be part of a single progressive hydrothermal event or a temporally distinct event relative to deposition of gold. At the Huangjindong orogenic gold deposit in the Jiangnan Orogen of southern China, there are two phases of vein generation, the earlier of which is largely barren. Structural analysis, petrography, mineralogy, and in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) gold mapping of different generations of gold-bearing sulfides reveal the relationship between different phases of hydrothermal alteration and quartz veins. Pre-existing barren and brittle quartz veins, which are controlled by tight folds and thrust faults, provide favorable competent hosts for later overprinting by gold-bearing ore-bearing fluids that infiltrated during ongoing reverse-sinistral fault movement. These overpressured fluids caused hydrofracturing to complete brecciation of segments of the earlier quartz veins and associated pre-existing breccias, with deposition of auriferous quartz-sulfide veins and breccia cements or matrices within them. Although fluid inclusion data are non-definitive, fluid pressure fluctuations are interpreted to have resulted in phase separation within the ore fluid which was primarily responsible for destabilization of gold complexes and deposition of native gold with other invisible gold-bearing ore-related sulfides formed by sulfidation reactions, in hydrofractures and breccia matrices. Thus, competent barren quartz veins in fold/thrust belts may locally host superimposed gold mineralization and provide favorable targets for gold resources.