Geochemical and petrological study of eastern limb of Rustenburg Layered Suit (Bushveld mafic complex)

South Africa’s Bushveld Complex is the most significant and important example of layered mafic complex in the world. It is an iconic geological site, where many generation of geologists were ventured to understand the phenomenology of intrusive magma crystallization process. The interst in Bushveld Compelex expanded since the discovery (Merensky, 1925) of the world’s large reserve of platinum (and platinum elements group). The Complex comprises four exposed sectors - the eastern limb, the western limb, the far western limb and the northern limb, with a fifth limb, the southeastern Bethal limb, obscured by younger sediments. These sectors are formed by mafic-ultramafic layered suite at the base, a granite suite and a sequence of heterogeneous predominantly felsic volcanic rocks of the Rooiberg Group. Both extrusive and intrusive Bushveld magmatism occurred with a time span of a few million years around 2057±3 (Mungall et al., 2016). The majority of the ore deposits are, however, restricted to the intervening group of ultramafic-mafic rocks, or Rustenburg Layered Suite. The Rustenburg layered Suite comprises a package of rocks which range in composition from dunite to diorite. This layered suite is subdivided into marginal, lower (LZ), critical (CZ), main (MZ) and upper (UZ) zones, although their exact boundaries have been the subject of much debate (e.g. Kruger, 2005). Despite the countless published papers since the Twenties of the previous century, there is no consensus yet on the details of its mode of formation. However, it is generally assumed that the layered rocks represent an upward-aggrading pile of crystals deposited on the floor of a vast, long-lived and repeatedly replenished magma chamber (e.g. Mungall et al., 2016). In 1998 the field excursion poposed by the programme of four-yearly International Vocanic Congress (IAVCEI), held in Cape Town (SA), was focused on the eastern limb of the Rustenburg Layered Suite. Petrologists of the Unviersity of Ferrara (Luigi Beccaluva and Franca Siena) attended the field excursion and collected 31 samples representing all the main rock types of each zone (dunites, orthpyroxenites, pyroxenites, anorthosites). This rock collection remained unworked for many years and just recentely were resumed by the authors of this contribution. Here, the initial stage of a geochemical and petrological study of eastern limbof Rustenburg Layered Suite is presented. On the basis of preliminary bulk geochemistry (major and trace elements) and mineral modal distribution, the majority of the samples are cumulates: dunites, orthopyroxenites, peridotites norite, anorthosites, gabbros; a few metasedimentary rocks of the Transvaal basement are also included. The general order of appearance (and disappearce) of cumulus minerals suggests a multiple a crystallization processes in an open melt-filled chamber. Kruger F.J. (2005) - Filling the Bushveld Complex magma chamber: lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti-V-magnetitite deposits. Miner. Deposita, 40, 451-472. Merensky H. (1925) - How we discovered platinum. Mining Ind. Mag. South. Africa, 1, 265-266. Mungall J.E. et al. (2016) - U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa. Nat. Commun., 7, 13385.