Chalcophile element depletion in mafic sills of the Mekelle basin, Tigrai: a guide to Ni-Cu-PGE exploration in the Ethiopian flood basalt province

摘要

Doleritic sills from the northern part of the Oligocene flood basalt province of Ethiopia were investigated for their Ni-Cu-PGE potential. The sills intrude Jurassic shales and limestones of the Mekelle basin and are chemically akin high-Ti tholeiitic basalts with OIB-type source characteristics. Detailed geochemistry and petrography allowed subdivision into three compositional groups: hypersthene-bearing dolerites (11.6% to 4% MgO), amphibole-bearing dolerites (～5.2 % MgO) and rutile/apatite-bearing dolerites (5.4 % to 3 % MgO). Hypersthene-bearing dolerites show control of element variation by magmatic fractionation. Amphibole-bearing and rutile/apatite-bearing dolerites deviate from this magmatic trend and, at ～ 5 % MgO, show higher SiO_2, Na_2O, P_2O_5, Ba and Sr and lower Fe_2O_3, TiO_2, Nb, Zr, Y, Cu, Ni, Co and Cr values than the hypersthene-bearing varieties. The distinct geochemical signatures of these dolerites are likely caused by conjunct crustal assimilation and fractional crystallization processes. Depletion of Ni and Cu in the contaminated dolerites suggests that segregation of Cu-Ni sulphides has occured as well. No mineralization outcropping on surface has been found to date and it is likely that assimilation and segregation processes operated prior to sill emplacement in a deeper seated magma chamber. However, the indication of chalcophile element depletion and the low knowledge of the Ni-Cu-PGE potential of the Ethiopian flood basalt province clearly warrants further exploration in the area.