An evaluation of the Upper Zone sequence in core NP1 and its regional significance to the northern limb of the Bushveld Igneous Complex

Abstract

The Bushveld Complex was emplaced by repeated injection of magma ranging in volume from small to large pulses (Eales and Cawthorn, 1996). It consists of five limbs, namely; northern limb, eastern limb, western limb, far western limb as well as south-eastern limb and is variably enriched with ores ranging from chromium and platinum group elements (PGE) to vanadium (Eales and Cawthorn, 1996). The Complex extends north of the Hout River Shear Zone (van der Merve, 1976; Vermark and van der Merve, 1986) that was recently proven at the Waterberg Project area (Huthmann et al., 2016; Kinnaird et al., 2017). This research was proposed to further prove the existence of the Bushveld Complex north of the Hout River Shear Zone. The task was to evaluate how the Non Parella sequence (NP1) correlates with the known northern limb (south of the Hout River Shear Zone), the Villa Nora fragment and the Waterberg sequence as well as its affiliation with the Bushveld Upper Zone. Furthermore, Huthmann et al(2017) stated that there is a lack of stratigraphic connection between rocks north and rocks south of the Hout River Shear Zone and this research aimed to evaluate this. The 2500 m deep NP1 borehole was drilled by Rand Mines Ltd in 1991 on the Non Parella farm located north of the Hout River Shear Zone and west of the Waterberg Project area. The entire core was logged as the Upper Zone by the Rand Mines exploration team. To further evaluate the NP1 sequence, core logging and sampling were carried out in April 2017 at the Council for Geoscience core library. Core logging was followed by the petrographic description of thin sections and whole rock XRF analysis undertaken at the University of the Witwatersrand in 2017. Sr isotopic analysis of fresh, unaltered plagioclase grains using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) was also performed at the University of Johannesburg in June 2018. The stratigraphy of the NP1 core is made up of Waterberg sedimentary rocks and dolerite sills from surface to a depth of 1312 m below surface. From 1312 m to 2158 m further below surface is the fine-grained to porphyritic gabbronorite package. The lower unit of the NP1 sequence from 2158 m to end of hole at 2500 m consists of magnetite and olivine-rich gabbroic rocks. These olivine-rich gabbroic rocks are olivine-bearing gabbro, plagioclase-phyric (plagio-phyric) olivine gabbro, and olivine gabbro to troctolite, and the section includes a 3.5 m interlayer of mottled anorthosite. The gabbronorite section includes a number of subvertical feldspar-quartz-magnetite veins and feldspar veins while the zone of the transitional contact between gabbronorite and olivine gabbro includes metasedimentary xenoliths. NP1 whole rock geochemistry does not reveal the upwards Si, Fe, and P enrichment expected for the Bushveld Upper Zone as identified by von Gruenewaldt (1973) in the eastern and western limbs and by Ashwal et al (2005) in the northern limb. It rather reveals an opposite trend consistent with the visible increase in magnetite, fayalitic olivine and apatite contents towards the lower part of the NP1 intersection within the gabbroic rocks. The trace and major element compositions of NP1 gabbronorite appeared to be sharing similar characteristics with the compositions of low and high Ti volcanic rocks of the Dullstroom Formation (Buchanan et al., 1999) indicating that the NP1 gabbronorite package could belong to the Dullstroom Formation and not the Upper Zone as it was initially logged by the Rand Mines exploration team. Sr isotope analysis of plagioclase in situ was carried out to verify the NP1 sequence affiliation with the Upper Zone. The initial Sr ratios of plagioclase from NP1 gabbronorite range from 0.7042 to 0.7066 and are not consistent with the Upper Zone signature of 0.7063 to 0.7079 in the northern limb (Mangwegape et al., 2016) and 0.7065 to 0.7075 at the Waterberg Project area (Huthmann et al., 2017). In contrast, the initial Sr isotope ratios of plagioclase from NP1 olivine-rich gabbroic rocks, ranging from0.7064 to 0.7073, are consistent with an origin from the Upper Zone melts. This interpretation is also supported by the elevated while variable abundances of apatite, magnetite, and olivine within the NP1 gabbroic rocks that is consistent with the Upper Zone rock compositions elsewhere in the northern limb and the rest of the Bushveld Complex. Field relations between the Bushveld Complex and Rooiberg Group whereby the unconformable emplacement of the Rustenburg Layered Suite (RLS)cross-cut the Rooiberg Group, specifically the Damwal and Dullstroom Formations, across the Bushveld Complex (Buchanan et al., 2004: Buchanan, 2006) provides more evidence that the NP1 gabbronorite package indeed belong to the Rooiberg Group. Dullstroom Formation rocks form the roof of the RLS in the NP1 sequence. The intrusive relationships between the RLS and its roof are supported by evidence of metamorphism within the NP1 gabbronorite section that include the presence of recrystallised sieve-texture porphyroblasts of clinopyroxene and phlogopite as well as granoblastic texture and metamorphic associations of hornfels. Considering the analytical results from this research and evidence from literature, the NP1 borehole intersected an earlier unseen top contact (at 2158 m) of the Upper Zone with the metamorphosed rocks of the Dullstroom Formation. This correlation is consistent with the similarities in the mineral assemblages, the similar whole rock geochemical compositions, the identical Sr isotope composition of plagioclase, evidence of metamorphism and the occurrence of sedimentary xenoliths in the up-section of the NP1 core. The NP1 sequence further confirms the existence of the Bushveld extensive magmatic basin north of the Hout River Shear Zone. The absence of magnetitite layers at the NP1 sequence favours a provisional stratigraphic connection rather with the Waterberg Project area (located north of the Hout River Shear Zone) than with the Villa Nora sequence and the northern limb (both located south of the Hout River Shear Zone) supporting an earlier hypothesis on a lack of stratigraphic continuity between RLS sequences north and south of the Hout River Shear Zone (Huthmann et al., 2017)