THE BASAL GABBRO UNIT AND THE IDENTITY OF THE PARENTAL MAGMA OF THE UITKOMST COMPLEX, BADPLAAS, SOUTH-AFRICA

The Uitkomst Complex in Mpumalanga Province, South Africa, is a layere d ultrabasic to basic intrusion that is subdivided into six lithologic al units, which are, from bottom to top, the Basal Gabbro, Lower Harzb urgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite, and Gabbronorite Units. The Basal Gabbro Unit, consisting of an aphanitic chill zone that grades upwards into phaneritic gabbroic rocks, shows a n abrupt transitional relationship with the overlying, dominantly oliv ine-orthopyroxene cumulate of the Lower Harzburgite Unit. Chemical ana lyses of phaneritic rocks of the Basal Gabbro Unit reveal a considerab le variation between samples. This variation is particularly pronounce d in sulphur and the chalcophile elements. With the help of principal component analysis it is illustrated that this chemical variation is l argely attributed to in situ fractionation of a sulphide, silicate, an d chromite fraction, in order of importance. Furthermore, it is shown that the parental magma of the Uitkomst Complex (referred to as the Ui tkomst Bu magma) is chemically identical to a mixture of Bushveld B2 m agma and a fractionated variant of Bushveld B1 magma. After emplacemen t and probably in the late stages of crystallization, the Uitkomst Com plex has been subjected to intense hydrothermal alteration which cause d a redistribution of the mobile elements like Ca, Sr, and Rb. The evi dence presented supports the existence at deeper levels of an intermed iate magma chamber, which allowed fractional crystallization of the BI magma.