The mineralogical properties of goethite and clay minerals from the Shewushan supergene gold deposit have been studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). These results show that in the weathering zone of the Shewushan supergene gold deposit the mineral assemblage is mainly composed of quartz, kaolinite, halloysite, minor illite and goethite. The coexistence of these minerals is apparently indicative of weak laterization. The Al content in goethite from XRD data is ∼10.0%, suggesting formation by weak desilicification. Observation by TEM shows that the flakes of clay minerals with larger particle size usually have extremely rounded outlines, indicating intensive dissolution of clay minerals. Halloysite is derived from the decomposition of kaolinite and the micrographs of curling, tubular, and club-shaped halloysite strongly suggest significant hydration and thus a water-saturated environment. Both the XRD data of goethite and the micrographs of the clay minerals show that the environment in Shewushan is characteristic of high [H2O] activity and high [SiO2] activity. The high dynamic hydraulic conditions may facilitate the downward migration of the primary gold particles during their mechanical concentration, resulting in the accumulation of gold in the lower portion near the water table.

C. R. M. Butt comments: In their recent paper, Lacinska & Styles (2013) described in detail the geological setting, mineralogy and petrology of ‘silicified serpentinite’ in the Hajar Mountains, United Arab Emirates (UAE). They note that their ‘silicified serpentinite’ is essentially the same unit as ‘birbirite’ and other informally named quartz-rich outcrops that overlie ultramafic rocks in this and other regions. Some authors have suggested such silicification to be hydrothermal in origin, but it is now generally accepted to be due to weathering. Lacinska & Styles (2013) concluded that ‘silicified serpentinite’ is a silcrete; this is correct, but it is a purely descriptive term and, without qualification, has no specific genetic implications other than being silica-cemented regolith (Butt & Zeegers, 1992; Eggleton, 2001). The materials illustrated are more correctly termed a silicified saprolite as they preserve the fabric of the protolith.