Fe-bearing minerals and implications for gold mineralization for the Wangu deposit in Central Jiangnan Orogen

Hydrothermal alteration with bleaching of host rocks is the most important prospecting indicator for gold deposits in the Jiangnan Orogen Belt. The alteration has been identified as pre-ore carbonate (siderite)-sericitization and the Fe of siderite in the alteration zone is derived from the host rocks rather than fluids. In addition, such alteration decreases in intensity and width with depth and gold mineralization also occur in the non-bleached rocks, casting doubt on the reliability of the prospecting indicator. Detailed petrographic work and SEM analysis on the Wangu deposit indicate that there are two types of siderites, i.e., Sd1 and Sd2. Among them, Sd1 grains are relatively small and distributed along the planes of unaltered host rocks, while Sd2 grains, only occurring in the altered slates, are commonly larger. Both types of siderites were altered by auriferous fluids, producing porous cores and minerals such as pyrite, quartz, and ankerite. Compared with unaltered parts, the altered parts have lower Fe, but higher U, Pb, and REE. In addition, Sd1 and Sd2 are similar in Mn, Na, V, and Sr concentrations but different in Fe and Mg. The occurrence and geochemical compositions of both siderites indicate that Sd1 could be transformed into Sd2 by pre-mineralization alteration through dissolution-reprecipitation. Chlorite is another important Fe-bearing mineral in the host rocks, and EPMA analysis suggests that it is ripidolite with relatively high Fe contents. Consequently, chlorite can also provide Fe to form the pre-ore carbonate(siderite)-sericitization. Geochemical modeling demonstrates that both ripidolite and siderite can result in sulfidation and therefore gold precipitation. As a result, this study demonstrates that pre-ore alteration with characterized bleaching is not a prerequisite for gold mineralization despite of its prominent features. Due to the presence of Fe-bearing Sd1 and chlorite, gold mineralization could still occur through sulfidation in the unaltered rocks.