Marginal facies and compositional equivalents of Bushveld Parental sills from the Molopo farms complex layered intrusion, Botswana: petrogenetic and mineralization implications

Abstract

Parental magma composition to layered ultramafic to mafic intrusions are commonly studied using rocks occurring along their contact with the surrounding country rock. These include sills, dykes and/or rare chill margins. In comparison to the well-studied ~2.05 Ga Bushveld Complex layered intrusion, no study is available on the parental magma composition of the Molopo Farms Complex layered intrusion in southern Botswana. With a size, approximately 1/3rd of the Bushveld Complex, mafic to ultramafic rocks of the Molopo Farms Complex 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. 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 margins from the Bushveld Complex. Together with the remobilized country rock beneath, norite at the base of MF38 is interpreted to represent a marginal facies unit. Close similarity with whole-rock geochemical characteristics of chill margin and B1 compositions from the Bushveld Complex argues for the marginal facies norite to represent the earliest known parental magma composition to the Molopo Farms Complex. In contrast to the typical cumulate textured ultramafic rocks, a medium-grained phlogopite-bearing pyroxenite occurs higher up the stratigraphy in MF38, and is considered compositional equivalent to B1UM from the Bushveld Complex. Geochemical exercise also indicates that the gabbronorites from MF9 preserve compositions equivalent 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 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 and 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. The studied samples, including the marginal facies unit, hosts a sulphide assemblage of pyrrhotite, pentlandite and chalcopyrite with traces of pyrite and sphalerite, the association similar to that typically found in layered mafic to ultramafic rocks.