Geochemistry of lower proterozoic greywackes from the Birim diamondiferous field, Ghana

Abstract

The Birim Diamondiferous field from which most of the alluvial diamonds are recovered is underlain by Lower Proterozoic Birimian metasedimentary and associated mafic rocks. Bulk rock geochemistry was carried out on the sandstones from the metasedimentary rocks to constrain the tectonic setting during the early Proterozoic. Bulk rock geochemical studies on greywackes show that they are characterized by moderately low but variable SiO2 contents, generally high Al2O3 and ferromagnesian element (e.g., Fe, Mg, Cr, V, Co, Sc) contents, and K2O/Na20 commonly less than 1. The geochemical data indicate that the greywackes were deposited in an active continental margin environment. The detritus were mainly derived from the upper continental crust but with an admixture of mafic component. Recent studies have shown that some of the mafics/ultramafics are diamondiferous metakimberlites. The inferred tectonic (arc) setting makes the emplacement of the so-called Proterozoic diamondiferous metakimberlites unlikely, if we are to go by the Clifford's Rule. However, if these ultramafic rocks are really metakimberlites and the source of the diamonds, then the Clifford's Rule may not be applicable in the Ghanaian situation, and the Birim field may be one of the few exceptions to this general rule. This would, then, present a typical example of subduction zone related diamondiferous kimberlites.

Keywords: Birim Diamondiferous field, Birimian, geochemistry, Ghana, tectornc setting

African Journal of Science and Technology Vol. 5(2) 2004: 9-18