scholarly journals Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

2020 ◽  
Vol 61 (3) ◽  
Author(s):  
Roger N Scoon ◽  
Gelu Costin ◽  
Andrew A Mitchell ◽  
Bertrand Moine
Keyword(s):  
Bushveld Complex ◽  
Crustal Contamination ◽  
Large Degree ◽  
Critical Zone ◽  
Main Zone ◽  
Initial Ratio ◽  
Isotopic Data ◽  
Floor Rock ◽  
Merensky Reef ◽  
Northern Limb

Abstract The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2·055 Ga-old Bushveld Complex. The unit contains the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite–melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2). Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr87/Sr86 initial ratios and ϵNd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low ϵNd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high ϵNd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr87/Sr86 initial ratio (average of 0·71113) and a markedly low ϵNd value (average of -11·35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.

Some geochemical constraints upon models for the crystallization of the upper critical zone-main zone interval, northwestern Bushveld complex

1986 ◽  
Vol 50 (358) ◽  
pp. 567-582 ◽  
Author(s):  
H. V. Eales ◽  
J. S. Marsh ◽  
A. A. Mitchell ◽  
W. J. de Klerk ◽  
F. J. Kruger ◽  
...  
Keyword(s):  
Bushveld Complex ◽  
Rock Type ◽  
Critical Zone ◽  
Primary Phase ◽  
Main Zone ◽  
Sr Isotope ◽  
Mafic Rocks ◽  
Merensky Reef ◽  
Geochemical Constraints ◽  
Significant Factors

AbstractRatios between elements Mg, Fe, Co, Cr, Ni, V, and Sc are consistently different in mafic rocks of the upper critical zone, and those above the Bastard unit. Within the 300 m section above the Merensky Reef, 87Sr/86Sr ratios increase from c.0.7063 to c.0.7087, irrespective of rock type. Decoupling of Mg/(Mg + Fe2+) ratios and the Ca contents of plagioclase, and wide variations in the proportions of anorthosite within the Bastard, Merensky, and Merensky Footwall units, are inconsistent with anorthosite formation by simple fractional crystallization of magma batches of limited volume. Conversely, significant differences in Sr-isotope ratios show that these anorthosites could not have shared a common parental liquid. These data are used to develop a model whereby (a) the 300 m column above the critical zone represents the mixing of liquids of isotopically and geochemically discrete upper critical and main zone lineages, (b) mafic layers of the Bastard, Merensky, and Merensky Footwall units crystallized from discrete injections of primitive, mafic liquid while (c) the leucocratic upper parts of these units crystallized during progressive hybridization of liquid residua, which remained after significant separation of mafic phases, with a supernatant column representing the liquid residua of earlier cycles, and (d) the buoyancy of plagioclase, and enlargement of the primary phase volume of plagioclase consequent upon an increase in An/Ab ratio of hybrid liquids, were significant factors in the generation of anorthositic layers.

scholarly journals Assessing the extent of local crust assimilation within the Flatreef, northern limb of the Bushveld Igneous Complex, using sulfur isotopes and trace element geochemistry

2020 ◽  
Author(s):  
Evan Keir-Sage ◽  
Matthew I. Leybourne ◽  
Pedro J. Jugo ◽  
Danie F. Grobler ◽  
Cédric C. Mayer
Keyword(s):  
Trace Element ◽  
Primitive Mantle ◽  
Trace Element Geochemistry ◽  
Isotopic Data ◽  
Igneous Complex ◽  
Merensky Reef ◽  
Northern Limb ◽  
Geochemical Parameters ◽  
Pge Mineralization ◽  
Bushveld Igneous Complex

Abstract The proximity to metasedimentary footwall rocks relative to platinum group element (PGE) mineralized intrusive rocks in the northern limb of the Bushveld Igneous Complex (BIC) has resulted in complex local contamination in the intrusions. To assess the extent of incorporation of non-magmatic material and its effects on PGE mineralization, major element, trace element, and S isotopic data were collected from drill core UMT094 on the Turfspruit farm, where core logging has shown that the mineralized Platreef, forming the Flatreef deposit, is located stratigraphically well above local sedimentary footwall rocks. The S isotopic data combined with whole rock geochemistry data (including CaO/Al2O3, (V/Ti)PM, (Ni/Cr)PM, S/Se, loss on ignition) were used to assess incorporation of a range of local footwall material. The δ34S data show a steady decrease from the footwall assimilation zone (δ34S typically + 8 to + 9‰, maximum 12‰) to near constant δ34S values (δ34S < + 4‰) below the main PGE reef. Similar values have been documented for the Merensky Reef in the eastern and western limbs of the BIC (δ34S ~ 0 to + 3.5‰). Other geochemical parameters, such as S/Se and CaO/Al2O3, also match the ranges documented for the Merensky Reef elsewhere in the BIC. In addition, parameters such as whole rock V/Ti, normalized to primitive mantle (V/Ti)PM, are shown to be useful indicators of contamination and the type of contaminant with 1 < (V/Ti)PM < 2 for uncontaminated magmatic units; [V/Ti]pm > 2 for shale assimilation; and [V/Ti]pm < 1 for carbonate assimilation. The results suggest that the main PGE mineralization in the Flatreef deposit formed without significant in situ contamination and that the primary mechanism of PGE mineralization in the Platreef at Turfspruit was no different than the mechanism that generated the Merensky Reef in the eastern and western limbs of the BIC.

Concentrations of Te, As, Bi, Sb (TABS) and Se in the Marginal Zone of the Bushveld Complex: evidence for crustal contamination and the nature of the magma that formed the Merensky Reef

2020 ◽  
Author(s):  
Eduardo Mansur ◽  
Sarah-Jane Barnes
Keyword(s):  
Marginal Zone ◽  
Bushveld Complex ◽  
Crustal Contamination ◽  
Sulfide Minerals ◽  
Sulfide Liquid ◽  
Liquid Component ◽  
Merensky Reef ◽  
Chalcophile Elements ◽  
Magmatic Sulfide ◽  
Primary Magmas

&lt;p&gt;The association of platinum-group elements (PGE) and the chalcophile elements Te, As, Bi, Sb and Sn (TABS) has been documented in several magmatic sulfide deposits. These groups of elements are either hosted within sulfide minerals, or combine to form discrete platinum-group minerals (PGM) associated with sulfide minerals. However, the concentration of TABS in parental magmas from which magmatic sulfide deposits formed was still missing. This study presents the distribution of TABS and Se in B-1, B-2 and B-3 rocks of the Marginal Zone of the Bushveld Complex. These rocks have been proposed as representative of the parental liquids from which the Bushveld Complex crystallized, thus allowing us to assess the concentration of Se and TABS in the liquids from which some of the largest PGE deposits in the world have formed. Concentrations of As and Sb in the initial Bushveld liquid (B-1) are significantly higher than in primary magmas, whereas the Se and TABS of later magmas (B-2 and B-3) are similar to primary magmas. We attribute the difference due upper crustal contamination of the B-1 magma, whereas the B-2 and B-3 magmas were most likely contaminated with a plagioclase-rich residuum formed upon the partial melting of the upper crust. Moreover, we modeled the concentrations of the TABS in the Merensky Reef using a mixture of two of the magma types present in the Marginal Zone (the B-1 and B-2) as the initial silicate liquid. The modeled concentrations closely resemble the measured values obtained for a section across the Merensky Reef at the Impala mine. This supports the B-1 and B-2 mixture as an appropriate initial liquid for the crystallization of the Merensky Reef. The modeling also shows that the distributions of Se, Te and Bi across the Merensky Reef are controlled by the sulfide liquid component. In contrast, As and Sb distributions are influenced both by the amount of silicate melt component in the cumulates and the sulfide liquid component. This is because Se, Te and Bi are moderately to strongly chalcophile elements, but As and Sb are only slightly chalcophile elements. Consequently, the effect of crustal contamination for elements with high partition coefficients between sulfide and silicate liquid (Te, Bi and Se) is obscured by the interaction of sulfides with a large volume of silicate magma. Therefore, the concentrations of these elements are higher in samples with greater proportions of sulfide minerals. In contrast, for elements with lower partition coefficients (As and Sb), the whole-rock concentrations are not upgraded by the presence of sulfide minerals, and thus the effect of crustal contamination can be more readily assessed.&lt;/p&gt;

Re-evaluation of magma compositions and processes in the uppermost Critical Zone of the Bushveld Complex

1996 ◽  
Vol 60 (398) ◽  
pp. 131-148 ◽  
Author(s):  
R. Grant Cawthorn
Keyword(s):  
Bushveld Complex ◽  
Hanging Wall ◽  
Experimental Studies ◽  
Critical Zone ◽  
Element Abundances ◽  
Merensky Reef ◽  
Low Viscosity ◽  
Silicate Magma ◽  
Geochemical Study ◽  
Restricted Mobility

AbstractA detailed geochemical study is presented of the uppermost Critical Zone, especially of the footwall and hanging wall to the Merensky Reef, at Impala Platinum Mines in the Bushveld Complex. The approximately 100 m-thick sequence below the Merensky Reef consists of 13 distinct layers which have sharp boundaries. They are adcumulates with varying proportions of cumulus plagioclase, orthopyroxene and chromite.Experimental studies on the composition of coexisting orthopyroxene liquid indicate that the magma which produced this sequence contained between 4 and 6% MgO. The magma from which the Merensky Reef formed was more evolved than the footwall magma.Significant variations exist for both the En content of orthopyroxene and mg# number of whole-rock analyses in short vertical sections. Pyroxenite and norite always have higher values than anorthosite. Extremely sharp breaks in these values correlate with changes in modal proportions, and argue against both significant fractionation within the studied interval, and infiltration metasomatism. Quantitative modelling shows that the entire footwall section could have contained pyroxene with a uniform primary composition of En82, and that all the variation now observed reflects the effect of reaction with trapped magma.Two independent methods for determining the proportion of trapped liquid are presented, based on mg# number and incompatible element abundances. Both yield a uniform proportion in all samples of approximately 10%. Immiscible sulphide liquid from the Merensky Reef can be shown to have infiltrated downwards for <5 m, despite its high density contrast with silicate magma, very low viscosity and low crystallization temperature. Residual silicate magma would have had even more restricted mobility. The migration of residual liquid or fluid through pothole structures in the floor of the Merensky Reef is not supported by the present data.

Compositional cyclicity in a pyroxenitic layer from the Main Zone of the Western Bushveld Complex: evidence for repeated magma influx

1996 ◽  
Vol 60 (398) ◽  
pp. 149-161 ◽  
Author(s):  
Andrew A. Mitchell
Keyword(s):  
Bushveld Complex ◽  
Mineral Chemistry ◽  
Critical Zone ◽  
Central Portion ◽  
Main Zone ◽  
Iron Enrichment ◽  
Southern Sector ◽  
Modal Layering ◽  
Layer Mineral ◽  
Textural Evidence

AbstractLacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, 1100 m above the Main Zone — Critical Zone contact, a 10 m thick pyroxenitic layer was intersected in an exploration borehole from the southern sector of the western Bushveld.Within the pyroxenitic layer, mineral chemistry defines a series of five cycles of upward Mg and Cr enrichment in pyroxenes, and Ca enrichment in plagioclase. The mineral chemistry, supported by textural evidence, suggests the influx of successive surges of magma. Sustained streaming of magma gave rise to adcumulate textures in the central portion of each cycle, with orthocumulate textures at bases and tops of cycles representing waxing and waning stages of magma surges.

Iron-rich ultramafic pegmatite replacement bodies within the Upper Critical Zone, Rustenburg Layered Suite, Northam Platinum Mine, South Africa

2002 ◽  
Vol 66 (6) ◽  
pp. 895-914 ◽  
Author(s):  
D. L. Reid ◽  
I. J. Basson
Keyword(s):  
Mobile Phase ◽  
Bushveld Complex ◽  
Bulk Composition ◽  
Critical Zone ◽  
Crystal Mush ◽  
Merensky Reef ◽  
Ti Oxides ◽  
Mode Of Occurrence ◽  
Chromitite Layer ◽  
Layered Sequence

Abstract Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainlyon geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterallyand upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.

Unusual Fe-Ti-Cr spinels from discordant bodies of iron-rich ultramafic pegmatite at the Amandelbult Platinum mine, northwestern Bushveld Complex

2002 ◽  
Vol 66 (6) ◽  
pp. 857-879 ◽  
Author(s):  
R. N. Scoon ◽  
H. V. Eales
Keyword(s):  
Bushveld Complex ◽  
Analytical Data ◽  
Critical Zone ◽  
Northwestern Part ◽  
Main Zone ◽  
Spinel Type ◽  
Oxide Melt ◽  
Complex Process ◽  
Compositional Gradients ◽  
Stratigraphic Height

Abstract Spinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite. The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.

The Upper Critical Zone of the Rustenburg Layered Suite in the Swartklip Sector, north-western Bushveld Complex, on the farm Wilgerspruit 2JQ: I. Stratigraphy and PGE mineralization patterns

2019 ◽  
Vol 122 (2) ◽  
pp. 117-142 ◽  
Author(s):  
A.A. Mitchell ◽  
J. Henckel ◽  
A. Mason-Apps
Keyword(s):  
Binary System ◽  
Bushveld Complex ◽  
Critical Zone ◽  
Total Thickness ◽  
The South ◽  
Chemical Erosion ◽  
Merensky Reef ◽  
The Past ◽  
Pge Mineralization ◽  
North Western

Abstract The Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations. Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.

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