Geochemistry of shales of the Middle Buanji Group in the Kimani area,
Southwest Tanzania: Implication to provenance and depositional
environment
Abstract
Major and trace element proxies of the shale samples from the Middle
Buanji Group of the Upper Paleoproterozoic (∼1.67 Ga) are reported in
this paper to decipher the provenance and depositional environment in
the study area. The analytical results of shales in the Middle Buanji
Group indicates relatively low percentage of major oxides compositions
such as; SiO2 (38.84 – 54.26 %),
Al2O3 (6.8 – 10 %),
K2O (2.22 – 3.04 %), TiO2 (0.21 –
0.28 %) and CaO (0.15 – 0.51 %) and moderately high
Fe2O3 (4.34 – 10.4 %) and
P2O5 (1.62 – 2.01 %). The trace
element composition of the analyzed shale samples displays wide
concentration variation such as Mn (29 -19600 ppm), Ti (468 - 35600
ppm), P (370 – 4610 ppm), Ba (400 – 7730 ppm), and S (5 – 2350 ppm),
V (130 – 290 ppm), Zn (5 – 100 ppm), Sr (40 – 160 ppm), As (2 – 70
ppm), and Cr (100 – 250 ppm). Measured proxies of major oxides
Al2O3 /TiO2 (10.86 to
15.31) and K2O/Al2O3
(0.23 – 0.35). Variation of Cr concentrations in the shale samples
indicates diverse source compositions in the study area ranging from;
ultramafic, mafic, intermediate, to feldspar-rich rocks. The measured Mn
values in shales have an average of 2527.65 ppm, and proxies of V/Cr:
0.65 – 1.7, V/ (V + Cr): 0.39 – 0.63, and CuO/Zn: 0.004 – 1.7
elements suggest that shales and dominant clay minerals (illite and
chamosite) were deposited in marine environment under oxidizing
conditions.