Trace and rare earth element geochemistry of organic-rich sediments in the lacustrine Rukwa rift basin, Tanzania: implications for paleoproductivity, paleosalinity, and redox conditions

Coal and mudstone beds in the Namwele coalfield, lacustrine Rukwa rift basin belong to the Late Carboniferous to Early Permian age. However, their origin, depositional environment, and organic matter enrichment in such a setting have not been well studied. In this study, samples were collected from the field outcrops and analyzed for their hydrocarbon generation potential, trace element (TE), and rare earth element (REE) proxies to examine their origin, paleoproductivity, organic matter enrichment, paleosalinity, and redox conditions. The results show that the total organic contents (TOC) of the investigated samples range from 3.74 to 13.6%, with an average of 7.21%. The anomalous TOC is a proof that sediments deposited in an oxic condition and low sedimentation rate. Concentrations and ratios of various redox indicators such as vanadium, nickel, copper, chromium, uranium, and thorium (V/Cr, V/(V + Ni), Ni/Co, and U/Th) suggest that the organic-rich sediments had strong terrestrial organic matter input and deposited under oxic conditions. The TOC versus redox ratios show negative correlation coefficients proving that oxic conditions are not suitable for organic matter preservation. According to the boron/gallium (B/Ga) ratio, water salinity ranges between 0.2 and 4.15 during sedimentation, indicating a freshwater environment. The more significant content of the trace elements Ni and Cu implies a high flux of organic matter. Lanthanum/ytterbium (La/Yb)n ratio is an indicator for characterizing different enrichment levels between light REE and heavy REE. The results show that samples are enriched in light REE, with (La/Yb)n, indicating a low sedimentation rate. These findings contribute to studies of the same sedimentary environment settings and further extend our knowledge of the potential organic-rich resources in the Rukwa rift basin.