bushveld igneous complex
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2021 ◽  
Vol 59 (6) ◽  
pp. 1339-1362
Author(s):  
Malose M. Langa ◽  
Pedro J. Jugo ◽  
Matthew I. Leybourne ◽  
Danie F. Grobler

ABSTRACT The UG-2 chromitite layer, with its elevated platinum-group element content, is a key marker horizon in the eastern and western limbs of the Bushveld Igneous Complex and the largest platinum-group element chromite-hosted resource of its kind in the world. In contrast, much less is known about its stratigraphic equivalent in the northern limb, the “UG-2 equivalent” (UG-2E) chromitite. Recent studies on chromite mineral chemistry show similarities between the UG-2 and sections of the UG-2E, but also that the UG-2E was partially contaminated by assimilation of local metasedimentary rocks. Here, we provide a detailed characterization of sulfide minerals and platinum-group minerals in a suite of samples from the UG-2E and compare the results with data obtained from a reference suite of samples from the UG-2. Results from petrographic observations, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, quantitative evaluation of materials by scanning electron microscopy, and δ34S isotopes show that: (1) sulfide minerals in the UG-2E and UG-2 consist mainly of pentlandite-chalcopyrite-pyrrhotite, but pyrrhotite is significantly more abundant in the UG-2E and almost absent in the UG-2; (2) iron contents in pentlandite from the UG-2E are significantly higher than in the UG-2; (3) platinum-group element contents within sulfide minerals are different between the two chromitites; (4) UG-2E platinum-group minerals are dominated by arsenides and bismuthotellurides, and by alloys and platinum-group element-sulfide minerals in the UG-2; (5) sulfide mineral chemistry and δ34S values indicate some crustal contamination of the UG-2E; and (6) sulfide mineral and secondary silicate mineral textures in both the UG-2E and UG-2 are indicative of minor, millimeter- to centimeter-scale, hydrothermal alteration. From our observations and results, we consider the UG-2E chromitite in the northern limb to be the equivalent to the UG-2 in the eastern and western limbs that has been contaminated by assimilation of Transvaal Supergroup footwall rocks during emplacement. The contamination resulted in UG-2E sulfide mineral elemental contents and platinum-group mineral types and abundances that are distinct from those of the UG-2 in the rest of the Bushveld.


Author(s):  
Wolfgang D. Maier ◽  
Marina Yudovskaya ◽  
Pedro Jugo

AbstractMore than 30 years ago, Cox and Singer (1986) suggested that magmatic platinum-group element (PGE)-Ni-Cu deposits are amongst the best understood of ore deposits, yet the origin of PGE mineralization in the Bushveld Igneous Complex (BIC) remains controversial after a century of study. In the northern limb of the BIC, the unravelling of ore formation proved particularly difficult due to relatively poor outcrop, which is typically affected by contamination of the intruding magmas with the host rocks and expressed in the form of abundant xenoliths, footwall rafts and disturbance of magmatic stratigraphy. In this thematic issue, we present contributions on the Flatreef, a recently discovered world-class PGE-Ni-Cu deposit constituting a downdip extension of the mineralized unit of the Platreef of the northern limb. Two deep shafts are currently being sunk, making the Flatreef one of the most significant new mine development on the Bushveld in several decades.


2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Marco L. Fiorentini ◽  
Craig O’Neill ◽  
Andrea Giuliani ◽  
Eunjoo Choi ◽  
Roland Maas ◽  
...  

AbstractLarge-scale mantle convective processes are commonly reflected in the emplacement of Large Igneous Provinces (LIPs). These are high-volume, short-duration magmatic events consisting mainly of extensive flood basalts and their associated plumbing systems. One of the most voluminous LIPs in the geological record is the ~ 2.06 billion-year-old Bushveld Igneous Complex of South Africa (BIC), one of the most mineralised magmatic complexes on Earth. Surprisingly, the known geographic envelope of magmatism related to the BIC is limited to a series of satellite intrusions in southern Africa and has not been traced further afield. This appears inconsistent with the inferred large size of the BIC event. Here, we present new radiometric ages for alkaline magmatism in the Archean Yilgarn Craton (Western Australia), which overlap the emplacement age of the BIC and indicate a much more extensive geographic footprint of the BIC magmatic event. To assess plume involvement at this distance, we present numerical simulations of mantle plume impingement at the base of the lithosphere, and constrain a relationship between the radial extent of volcanism versus time, excess temperature and plume size. These simulations suggest that the thermal influence of large plume events could extend for thousands of km within a few million years, and produce widespread alkaline magmatism, crustal extension potentially leading to continental break-up, and large ore deposits in distal sectors. Our results imply that superplumes may produce very extensive and diverse magmatic and metallogenic provinces, which may now be preserved in widely-dispersed continental blocks.


Author(s):  
Evan Keir-Sage ◽  
Matthew I. Leybourne ◽  
Pedro J. Jugo ◽  
Danie F. Grobler ◽  
Cédric C. Mayer

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.


Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 379 ◽  
Author(s):  
Leonidas Vonopartis ◽  
Paul Nex ◽  
Judith Kinnaird ◽  
Laurence Robb

The stanniferous granites of the Zaaiplaats Tin Field are part of the A-Type Lebowa Granite Suite, within the greater Bushveld Igneous Complex of northeast South Africa. The tin field comprises three granites: (1) the Nebo, a leucocratic, equigranular biotite granite; (2) The brick-red hypidiomorphic Bobbejaankop granite, which is extensively microclinized with chloritized biotite and characteristic synneusis-textured quartz; and (3) The variably altered roof facies of the Bobbejaankop granite known as the Lease microgranite. The Bobbejaankop and Lease granites were both extensively mined for cassiterite until 1989. The cassiterite is hosted in disseminations, miarolitic cavities, and within large hydrothermal, tourmalinized, and greisenized pipes and lenticular ore-bodies. An extensive petrological and whole-rock XRF and ICP-MS geochemical study, has provided new insight into the magmatic and magmatic-hydrothermal mineralization processes in these granites. Trace elements and Rayleigh Fractionation modelling suggest the sequential fractionation of the Nebo granite magma to be the origin of the Bobbejaankop granite. Incompatible elemental ratios, such as Zr/Hf and Nb/Ta, record the influence of internally derived, F-rich, hydrothermal fluid accumulation within the roof of the Bobbejaankop granite. Thus, the Lease granite resulted from alteration of the partially crystallized Bobbejaankop granite, subsequent to fluid saturation, and the accumulation of a magmatic-hydrothermal, volatile-rich fluid in the granite cupola. The ratio of Nb/Ta, proved effective in distinguishing the magmatic and magmatic-hydrothermal transition within the Bobbejaankop granite. Elemental ratios reveal the differences between pre- and post-fluid saturation in the mineralizing regimes within the same pluton. Thus highlighting the effect that the location and degree of hydrothermal alteration have had on the distribution of endogranitic tin mineralization.


2020 ◽  
Author(s):  
Simon Tapster ◽  
Iain McDonald ◽  
Dave Holwell ◽  
Danie Grobler

&lt;p&gt;Models for the formation of the Rustenberg Layered Suite of the Bushveld Igneous Complex continue to be debated. The consensus timescale over which magmatism took place has reduced hand in hand with advancements in geochronological techniques and data precision. The most recent studies by double spiked (&lt;sup&gt;202&lt;/sup&gt;Pb-&lt;sup&gt;205&lt;/sup&gt;Pb) zircon CA-ID-TIMS U-Pb have indicated emplacement in less than 1 Myrs [1][2]. Increasing analytical precision has also seemingly permitted individual magmatic layers to be resolved, leading to the &amp;#8220;out of sequence sill&amp;#8221; emplacement model [2], albeit contested [3].&lt;/p&gt;&lt;p&gt;We present two new high-precision zircon dates obtained from two continuous core intervals collected &amp;#160;&lt;4m apart in a single Ni-Cu-PGE rich pyroxenite unit in the Turfspruit section of the Platreef, Northern Limb of the Bushveld Complex [4]. Grobler et al. [5] correlate this pyroxenite with the Merensky Cyclic Unit of the Upper Critical Zone in eastern and western limbs. Assuming the recommended zircon &lt;sup&gt;238&lt;/sup&gt;U/&lt;sup&gt;235&lt;/sup&gt;U of Hiess et al. [6] without uncertainties propagated as per previous studies e.g. [1][2], the age interpretations of these two samples define a minimum and maximum temporal interval between 1.01 &amp;#177;0.16 Myrs and 1.28 &amp;#177;0.22 Myrs that brackets, or overlaps with, the entirety of previous dates from all preceding studies. The pyroxenite is continuous, without intrusive contacts, and the stratigraphically lower sample produces an apparently younger zircon age than the overlying sample. &amp;#160;It seems highly unlikely the entire longevity of the Bushveld&amp;#8217;s magmatic evolution was apparently captured within this 4 m section. Therefore, it now seems highly improbable that the Bushveld was emplaced and cooled in less than 1 Myrs, as the current paradigm states [1].&lt;/p&gt;&lt;p&gt;The older date from the Platreef now aligns the isotopic age relationships with the field observations of the overlying Main Zone, in contrast to the interpretation of Mungall et al. [2]. The new dates alone neither support nor contradicts the &amp;#8220;out of sequence&amp;#8221; sill emplacement model. Rather they merely indicate that melt related process that crystallised zircon was protracted within narrow vertical intervals, and that future work should acknowledge this potential complexity. It raises questions which age of event(s) introduced or modified sulfides within the ore bearing horizon. This requires greater integration of the geochronological record with ore textures at a high sampling density.&lt;/p&gt;&lt;p&gt;However, there also remains a substantial, yet previously overlooked caveat to all geochronological interpretations presented thus far; &amp;#8220;out of sequence&amp;#8221; sills in particular. This caveat is that the variations in the &lt;sup&gt;238&lt;/sup&gt;U/&lt;sup&gt;235&lt;/sup&gt;U between samples over observed magnitudes of variations in zircon [4] could account for any offsets in &lt;sup&gt;207&lt;/sup&gt;Pb/&lt;sup&gt;206&lt;/sup&gt;Pb dates interpreted as real temporal differences. This issue remains to be tested.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;[1] Zeh A et al. (2015) EPSL 418:103-114; [2] Mungall J et al. (2016) Nat. Coms. 13385; [3] Latypov R et al. (2017) South African Jour of Geol. 120.4, 565-574; [4] Nodder SM (2015) MESci dissertation, Cardiff University, 257pp; [5] Grobler D et al. (2019) Min Dep 54, 3-28; [6] Hiess J et al. (2012) Science 418,103-114&lt;/p&gt;


2019 ◽  
Vol 122 (4) ◽  
pp. 489-504 ◽  
Author(s):  
J.E. Johnson ◽  
S.M. Webb ◽  
C.B. Condit ◽  
N.J. Beukes ◽  
W.W. Fischer

AbstractManganese-bearing minerals in ancient strata provide a particularly informative record of the redox potentials of ancient Earth surface environments due to the high specificity of species that can oxidize Mn(II). However, little is known about how this sedimentary archive might have been altered by processes occurring long after lithification, including the effects of metamorphism, fluid mobilization, and metasomatism. We investigated Mn mineralization across known metamorphic gradients in the Kaapvaal craton, South Africa, in Archean and early Paleoproterozoic age carbonate-, shale-, and iron formation-bearing marine strata. We sampled contemporaneous strata that record the drowning of the Campbellrand-Malmani carbonate platform and a transition to iron formation deposition in a range of localities, from two metamorphosed (greenschist and above, affected by the intrusion of the Bushveld igneous complex) and four better-preserved (sub-greenschist) deep subsurface drill cores. To evaluate the geochemistry and mineralization tied directly to petrographic textures and cross-cutting relationships, we combined bulk geochemistry with light and electron microscopy and synchrotron microprobe X-ray absorption spectroscopy and imaging to produce Mn speciation maps at the requisite micrometer length scales for these textures. Samples with lesser degrees of post-depositional transformation contained minor amounts of Mn(II) in early diagenetic marine carbonate cements and detrital carbonate grains, while metamorphosed samples typically contained Mn concentrated into a combination of coarse-grained and vein-filling carbonate phases (ankerite, siderite, and rhodochrosite), garnet and amphibole. Chemical imaging analyses of these more metamorphosed samples show that Mn is held by phases and textures that mineralized post-deposition and lithification, demonstrating that Mn was mobilized – at least locally – by metasomatic fluids, although it is difficult to distinguish whether this Mn was original to these strata or was introduced secondarily. Our results confirm that Mn can be mobilized and therefore caution should be applied when interpreting Mn enrichments in sedimentary rocks, especially when Mn enrichment is not geographically extensive and coincides with metamorphic processes.


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