Formation of geochemical anomalies in loess-paleosol sequence at the Dahu Gold Mine, central China: A clastic-hydromorphic-gaseous dispersion model

Abstract

Abstract Modern mineral exploration requires a comprehensive understanding of how geochemical anomalies form, particularly where the mineral deposits are covered by unconsolidated sediments. However, the mechanisms that form anomalies vary as a function of cover type and are not well understood. The Dahu Gold Mine in central China is characterized by a lode gold deposit, overlain by a younger loess-paleosol sequence (including saprolite). Thus, the site provides an opportunity to examine how elements associated with primary mineralization have been dispersed through the loess to form anomalies in this poorly-investigated cover type. Fine-grained ( 70\u202fμg/g of Pb vs. the average 30\u202fμg/g of Pb in the Chinese loess. Anomalies in the saprolite are interpreted to have formed by in-situ weathering of the primary mineralized rocks. In contrast, anomalies in the lower Wucheng Loess which correspond to the calcrete horizon were probably formed by the precipitation of metals from ascending Ca (HCO3)2-rich mineralized groundwater. The S N directed horizontal profile shows that anomalies of Au, Cu, Mo and Pb in the upper Lishi Loess which are not in direct contact with the mineralization, spatially coincide with major faults (i.e. the Huashan Frontal Fault) and fractures in the loess-paleosol sequence. Moreover, Pb isotopic ratios documented for the sulphide ores and anomalous and background loess suggest that they are partially derived from the sulphide ores. The anomalies in the upper Lishi Loess are interpreted to be formed by a possible gaseous dispersion process. Elements, probably in metallic form (e.g. nanoparticulate Au), were carried by gases that flowed up faults and fractures to the surface and were precipitated in carbonate-rich clayey loess. Therefore, the formation of anomalies in the loess-paleosol sequence is a synthesis of clastic, hydromorphic and gaseous dispersion processes.