Marginal facies and compositional equivalents of Bushveld parental sills from the Molopo Farms Complex layered intrusion, Botswana: Petrogenetic and mineralization implications

Abstract

The composition of magmas considered to be parental to layered ultramafic to mafic intrusions are commonly studied using rocks occurring along their marginal portion, in contact with the surrounding country rock. With a size, approximately 1/3rd of the ∼2.05 Ga Bushveld Complex layered intrusion, mafic to ultramafic rocks of the Molopo Farms Complex in Botswana are suggested to have formed from the same magma that formed the Bushveld rocks. Of the different drill cores available, the base of MF38 intersects the country rock. Two other drill cores, MF9 and MF11, were also logged for comparison. Drill core MF38 preserves remobilized country rock at the base, followed up by mafic (norite) to ultramafic (peridotite-pyroxenite) rocks. The norite is quench textured with elongate, skeletal, compositionally zoned orthopyroxene crystals, and contain phlogopite. Significantly, the orthopyroxene mineral chemical composition is similar to that reported for chill margin from the Bushveld Complex. Together with the remobilized country rock beneath, norite at the base of drill core MF38 is interpreted to represent a marginal facies unit. The whole-rock geochemical characteristics of the marginal facies norite are similar to those of chill margin and B1 compositions from the Bushveld Complex, arguing for it to represent the earliest known parental magma composition to the Molopo Farms Complex. A medium-grained phlogopite-bearing pyroxenite occurring higher up the stratigraphy in drill core MF38 is considered compositional equivalent to B1UM from the Bushveld Complex. Further, the whole-rock geochemical characteristics of gabbronorites from drill core MF9 are comparable to B3, considered parental to differentiated end members of the Bushveld Complex. No compositional equivalent to B2 from the Bushveld Complex was observed in the Molopo Farms Complex rocks. Considering the fact that the Molopo Farms Complex is less thick than the Bushveld Complex, and that it preserves mafic and ultramafic rocks showing similarity to the earliest magma (B1, B1UM and chill margins) as well as the most differentiated of the Bushveld magma (B3), it is argued that the Molopo Farms Complex evolved faster. Thus it likely did not form rocks of the Bushveld Complex, which hosts the chromite-bearing layers, known for PGE mineralization. Instead, it is argued that the Molopo Farms Complex is a better target for Cu-Ni sulphide mineralization. A possible sulphide genesis model is presented in relation to the prominent occurrence of magmatic pyrrhotite-pentlandite-chalcopyrite assemblage in the studied rocks, trace amounts of PGM associated with sulphides in the marginal facies norite, and the alteration features associated with sulphides.