Spessartine rich garnet-bearing metasomatic rock within jos biotite granite, north of Bukuru, Nigeria

Abstract

Garmets with approximately 50 mole% almandine, 40 mole% spessartine, 10 mole% grossular, with still more spessartine rich rims, were found in the dark grey rocks “impregnated” with fine biotite (lepidomelane) flakes. The rocks form a zone about 60 meters long and 10 meters wide within the Jos biotite granite close to the contact with the younger Rayfield-Gona biotite granite complex. Fluorite, sphalerite, galena, small amounts of other sulphides and cassiterite are associated with the rocks. Transitions from dark grey rocks through slightly altered light grey granites to the unaltered light granite country rock, without considerable textural changes were observed. The dark grey rocks are poorer in SiO 2 and richer in almost all other oxides than the typical Jos biotite granite. The rocks are supposed to be the result of hydrothermal metasomatism, possibly in two episodes one garnet-bearing the other biotite bearing, which altered the Jos biotite granite under the influence of fluids originating most probably from the Rayfield-Gona biotite granite intrusion. The concentration of Mn could have taken place in the process of magma differentiation.