Continental-scale stream sediment geochemical mapping in southern China: An insight into surface processes and tectonic framework

Abstract

Continental-scale stream sediment geochemical data from the 76-element GEochemical Mapping (76-GEM) project covering southern China were analyzed in this contribution. 5244 composite stream sediment samples at the scale of ca. 1 sample per 500 km2 were used. The focus has been directly put on element associations of: (1) Light Rare Earth Elements (LREEs, i.e. La, Ce, Pr, Nd, Sm and Eu); (2) Heavy Rare Earth Elements (HREEs, i.e. Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y); (3) Iron Group Elements (IGEs, Ti, V, Cr, Mn, total Fe2O3 (FeT), Co and Ni); (4) Platinum Group Elements (PGEs, i.e. Ru, Rh, Pd, Os, Ir and Pt) and (5) Mo, Sn, W and Bi. Given that geochemical data are typical of compositional (closed) data, the five associations were represented by the sum values of centered logratio (clr) transformed data of elements in the predefined groups. It presents the first-time the continental-scale patterns of representative element associations in southern China. Maps were compared to independent datasets of lithology, tectonics and topography to discover processes governing the distribution.The five element associations could reflect the processes of: (1) weathering of dominantly felsic intrusive rocks (LREEs and HREEs); (2) weathering of mafic/ultramafic rocks (IGEs and PGEs); (3) weathering of dominantly felsic intrusives and hosted mineralization (Mo-Sn-W-Bi) and (4) anthropogenic and possible geogenic additions (Mo-Sn-W-Bi and PGEs). A general good agreement was found between maps of element associations and maps of surface geology and tectonics. The clr patterns are interpretable geologically and respond reasonably well to the processes such as lithological control and secondary transportation.As indicated by the clr patterns, two prominent tectonic units with controversial boundary in southern China, i.e. the Yangtze and Cathaysia blocks can be geochemically demarcated by the spatial patterns of REEs, IGEs and PGEs at this scale. The distinct patterns across the Jiangshan-Shaoxing and Pingxiang-Chaling faults (or the Shi-Hang Belt) suggested a felsic end-member to the southeast and a mafic end-member to the northwest. The spatial patterns of IGEs, PGEs and Mo-Sn-W-Bi also reflected that lithology is in dominant control of the distribution rather than local mineralization at this scale. Future studies of this dataset regarding single elements and certain element associations of particular interests (e.g., Cd, Ni and REEs) are also needed, which may provide new insights into environmental issues, mineral resources and tectonic framework in southern China.