Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

Abstract

New lithological and whole rock compositional data show that the main platinum-group element (PGE) horizons of the Flatreef succession show strong compositional similarities to the Merensky and Bastard reefs of the western Bushveld Complex (WBC), notably in terms of many immobile and incompatible minor and trace elements such as TiO 2 , Zr, Y, and REE. However, Al 2 O 3 , CaO, and Na 2 O contents are markedly lower in the Flatreef, whereas MgO contents are higher. In view of broadly similar silicate mineral compositions in the Flatreef and the WBC reefs, we suggest that the major element compositional differences between the rocks are largely due to higher modal proportions of orthopyroxene and olivine and lower proportions of plagioclase in the Flatreef. The thickness of the mineralised interval is much greater in the Flatreef than in the WBC (several 10 s of m vs~1 m) and the abundance of sulfides in the Flatreef is typically somewhat higher (on average~1.5% vs~1%). These data, complemented by textural observations, are interpreted to reflect enhanced hydrodynamic crystal sorting accompanied by percolation of sulfide melt through incompletely solidified cumulates. Further genetic constraints are provided by metal data: The concentration of Ni (~3000 ppm) in the Flatreef is broadly similar to that in the Merensky Reef of the WBC, but Cu contents are markedly higher (average~1500 ppm vs 700 ppm in the WBC). The concentrations of most PGE are slightly lower (Flatreef~1.5-2 ppm Pt, 100-150 ppb Rh; WBC MR 3.7 ppm Pt, 240 ppb Rh), but Pd has broadly similar contents (Flatreef~1.2-2 ppm; WBC MR 1.54 ppm). The relatively high Cu content of the Flatreef is interpreted as a result of assimilation of Cu sulfides from the sedimentary floor rocks. The reason for the enrichment of Pd relative to Pt, especially in the basal rocks, remains unclear. It could reflect mobilisation of Pd via a fluid phase from the country rocks or the interior of the intrusion, relatively enhanced partitioning of Pd into the sulfides, or relative Pt depletion of the earliest magma pulses in response to Pt alloy fractionation triggered by contamination with reducing country rocks.

Constraining the parent magma composition of the mineralized deep Flatreef at Turfspruit, northern lobe, Bushveld Complex: insights from silicate mineral compositions

“…Since the discovery of PGEs in the Bushveld Complex (Cawthorn, 1999;Scoon and Mitchell, 2004), the bulk of research efforts on this layered intrusion up until the rst decade of the 21st century concentrated on the western and eastern lobes of the intrusion (Eales and Cawthorn, 1996;Cawthorn, 2015). The northern lobe, however, has witnessed a surge in both mining activity and research, particularly for PGEs, since the rst decade of this century (e.g., Scoon et al, 2020;Maier et al, 2021;Yudovskaya et al, 2022). Cawthorn and Walraven (1998) concluded that during emplacement and fractional crystallization of the Bushveld Complex, the intrusion experienced initial temperatures of ca.…”

“…An understanding of ore formation in the northern lobe of has been made more complex, however, due a variety of factors such relatively poor outcrop as well as contamination of Bushveld Complex magmas with country rocks (Buchanan et al, 1981;Buchanan and Rouse, 1984;Cawthorn et al, 1985;Armitage et al, 2002). The occurrence of abundant footwall rafts and xenoliths, together with the disturbance of magmatic stratigraphy in the northern lobe bear testimony to the uniqueness of northern lobe rocks in comparison to those of the western and eastern lobes of the Bushveld Complex (Maier et al, 2021). The discovery of the Flatreef in the southern sector of the northern lobe permitted correlations with other lobes of the Bushveld Complex (Kruger, 1994;Roelofse et al, 2015) through the study of a magmatic stratigraphy less affected by footwall interaction such as strontium isotope ratios (Beukes et al, 2020), for example, and also with the northern sectors of the northern lobe (Buchanan et al, 1981;Gain and Mostert, 1982;Buchanan and Rouse, 1984;Yudovskaya et al, 2011).…”

Introduction to the special issue on the Flatreef PGE-Ni-Cu deposit, northern limb of the Bushveld Igneous Complex

“…The efforts of the organizing committee in carrying out field excursions and coreyard visits are gratefully acknowledged. Maier et al (2020) focus on the petrogenesis of the most highly mineralized portion of the Flatreef, namely, the interval between the MZ and the base of the Merensky Reef. Based on a detailed examination of four drill cores, they propose that the combination of several favourable processes, including country rock assimilation as well as hydrodynamic and hydromagmatic processes, resulted in exceptionally thick mineralized intervals (e.g.…”