Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Karykowski, Bartosz T.

doi:10.1007/s00410-017-1380-y

Cited by 16 publications

(9 citation statements)

References 51 publications

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“…Below the PNZ occur interlayered pyroxenite-harzburgitedunite sills that have previously been interpreted to represent the Lower Zone (Van der Merwe 1976;Yudovskaya et al 2013), based on lithological and compositional similarities to LZ rocks in the EBC and WBC where the zone forms a 500-1500 m sequence of predominantly orthopyroxenite, harzburgite and dunite (Karykowski and Maier 2017;Teigler and Eales 1996;Wilson 2012Wilson , 2015. The classical definitions of the boundary between LZ and LCZ established at Cameron Section and Union Section cannot be applied to Turfspruit, or indeed many other Bushveld localities as the Lower Zone shows significant lithological variation within and between limbs.…”

Section: Stratigraphic Correlation Of the Ultramafic Rocks Below The mentioning

confidence: 99%

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

Grobler¹,

Brits²,

Maier

et al. 2018

Miner Deposita

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The Flatreef is a world-class platinum-group element (PGE) deposit recently discovered down-dip from existing mining and exploration operations on the northern limb of the Bushveld Complex. Current indicated resources stand at 42 Moz PGE (346 Mt with 3.8 g/t Pt+Pd+Rh+Au, 0.32% Ni and 0.16% Cu) which, in the case of Pt, is equivalent to~10 years global annual production, making it one of the largest PGE deposits on earth. The grade and thickness of the Flatreef mineralised interval is highly unusual, with some drill core intersections containing up to 4.5 g/t Pt+Pd+Rh+Au over 90 m in drill core. Here, we document the down-dip and along-strike litho-and chemostratigraphy of the Flatreef and its footwall and hanging wall rocks, based on a diamond drill core database totalling > 720 km. At the base of the sequence intersected in the drill cores are up to 700-m-thick sills of ultramafic rocks (dunite, harzburgite, pyroxenite) emplaced into pelitic, dolomitic, and locally quartzitic and evaporitic rocks belonging to the Duitschland Formation of the Transvaal Supergroup. Next is an approximately 100-200-m sequence of lowgrade-sulphide-mineralised, layered mafic-ultramafic rocks containing abundant sedimentary xenoliths and, in places, several chromite seams or stringers. This is overlain by a~100-m-thick sequence of well-mineralised mafic-ultramafic rocks (the Flatreef sensu strictu), overlain by a laterally persistent mottled anorthosite layer at the base of > 1 km of homogenous Main Zone gabbronorite. Based on stratigraphic, lithological and compositional analogies to the layered rocks in the eastern and western Bushveld Complex, we correlate the Flatreef and its chromite bearing footwall rocks with the Upper Critical Zone, notably the interval between the UG2 chromitite and the Bastard Reef as found elsewhere in the Bushveld Complex. This includes recognition of a Merensky Reef correlative. The ultramafic rocks below the main chromitite seam (UG2 correlative) in the Flatreef footwall are correlated with the Lower Critical and Lower zones. However, compared to the western and eastern Bushveld limbs, the studied sequence is strongly enriched in sulphide and PGE, many of the rocks show elevated CaO, K 2 O, Rb and Zr contents, and lateral continuity of layers between drill cores is less pronounced than elsewhere in the Bushveld, whereas ultramafic units are locally considerably thickened. These compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels, intrusion of granitic magmas, and the influence of multiple structural events such as pre-to syn-emplacement regional-scale open folding and growth faults. Evidence for the existence of potholes also exists. In the shallow, up-dip portions of the project area, the entire magmatic sequence below the Main Zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores. This package represents the Platreef as expose...

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Section: Stratigraphic Correlation Of the Ultramafic Rocks Below The mentioning

confidence: 99%

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

Grobler¹,

Brits²,

Maier

et al. 2018

Miner Deposita

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“…The formation of layered intrusions via sill emplacement is likely more widespread than suggested in the literature and not limited to thin peridotite layers (e.g. Karykowski and Maier 2017). The protracted emplacement of smallvolume sills into a developing crystal mush will inevitably have complicated the cooling history of the complex as a whole.…”

Section: Incremental Emplacement Of Peridotite Sillsmentioning

confidence: 98%

“…The varying members within so-called cyclic sequences could very well be intercalated sills and the pre-existing cumulate, irrespective of scale (e.g. Renner and Palacz 1987;Karykowski and Maier 2017). More complex sequences of cumulate might represent the pre-existing cumulate, the intruding sill, and an intermediary (hybrid) lithology formed from the reaction between the two components, which would appear to link the layers by magma fractionation (e.g.…”

Section: Formation Of the Macro-rhythmic Unitsmentioning

confidence: 99%

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Hepworth

Kaufmann

Hecht

et al. 2020

Contrib Mineral Petrol

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The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (± gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large-and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms.

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“…Following Wager (1963), the most commonly used parameter to determine adcumulus status is the bulk rock concentration of incompatible elements such as P 2 O 5 . Accordingly, there should be a correlation between the bulk rock concentration of highly incompatible elements and the mode of minerals grown from the interstitial liquid, but this is not generally observed (McBirney and Hunter, 1995;Meurer and Meurer, 2006;Karykowski and Maier, 2017). As pointed out by McBirney and Hunter (1995), and verified here, there is no such correlation between bulk rock P 2 O 5 and the mode of interstitial minerals in the Skaergaard troctolites ( Figure 6B), due perhaps to the progressive loss or addition of interstitial liquids during solidification (Karykowski and Maier, 2017), or to difficulties in nucleating apatite which ends up crystallizing in channels through which late-stage liquids flowed (Meurer and Meurer, 2006).…”

Section: The Effect Of Fabric On Mush Structure and Adcumulus Statusmentioning

confidence: 99%

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

et al. 2020

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The strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be rearranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320 m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upward through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upward increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events.

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Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Cited by 16 publications

References 51 publications

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

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