Reconnaissance assessment of the Stenian-Tonian granitoids of southern Tanzania for metal resources by using geological remote sensing and geochemical techniques

Abstract

This study presents the use of geological remote sensing and geochemistry to determine spatial extents, geochemical characteristics and petrogenetic origin of Ndanda-Masasi metagranitoids in order to explore for important metals (Sn, W, Ta and Nb) in the metagranitoids. Geological remote sensing was done using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, and major and trace elements were analysed using a combination of Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Inductively Coupled Plasma Emission Spectrometer (ICP-ES). It has been revealed that the metagranitoids and other rocks in the Ndanda-Masasi area crop out in a larger spatial coverage than how it is depicted in available geological maps. Moreover, the meta-granitoids have variable SiO2 contents, low Mg, Ni and Cr concentrations and moderate to large Fe-numbers. They classify mainly as magnesian- and ferroan-type granites and minor granodiorite and quartz monzonite. The chondrite normalized Rare Earth Elements (REE) patterns of the rocks show slight to strong enrichment of light REE over middle REE and heavy REE ((La/Yb)N = 4.2–289.1), due to their variable contents of MREE and HREE. The rocks show flat patterns of middle REE and heavy REE and have negative Eu-anomalies (mean Eu/Eu* = 0.7), with the exception few rocks in some localities yielding positive Eu-anomalies. They are characterised by variable Sr/Y (4.7–356.7), Nb/Ta (2.6–44.5) and well correlated trends with negative slopes of SiO2 vs MgO, TiO2, CaO, and Al2O3 diagrams. These geochemical features reflects a most likely magmatic formation of the rocks in the presence of plagioclase, hornblende, garnet and rutile, or/and fractionation of the phases at variable crustal depths. Numerous myrmekites forming convex grain boundaries around K-feldspar in the rocks indicate Ca- and Na-metasomatism as an important process affecting the rocks in their evolution. The omnipresent retrograde assemblage of titanite, chlorite, epidote and muscovite forming at the expense of plagioclase, hornblende, biotite and opaque (Fe-Ti-oxides?) reflects their formation during uplifting history of the rocks. Contrary to granitoids of similar age such as those of the Karagwe-Ankolean Belt, which are metalliferous, our reconnaissance survey found that the Ndanda-Masasi meta-granitoids have uneconomic concentration of most important metals.