scholarly journals Chemistry, Morphology and Origin of Magmatic-Reaction Chromite Stringers Associated with Anorthosite in the Upper Critical Zone at Winnaarshoek, Eastern Limb of the Bushveld Complex

2018 ◽  
Vol 59 (8) ◽  
pp. 1551-1578 ◽  
Author(s):  
Roger N Scoon ◽  
Gelu Costin
Keyword(s):  
Bushveld Complex ◽  
Critical Zone ◽  
Eastern Limb

Sill intrusion into pyroxenitic mush and the development of the Lower-Upper Critical Zone boundary of the Bushveld Complex: Implications for the origin of stratiform anorthosites and chromitites in layered intrusions

2020 ◽  
Author(s):  
Reza Maghdour-Mashhour ◽  
Ben Hayes ◽  
Robert Bolhar ◽  
Henriette Ueckermann
Keyword(s):  
Bushveld Complex ◽  
Strontium Isotopes ◽  
Critical Zone ◽  
Superheated Liquid ◽  
Zone Boundary ◽  
Lower Upper ◽  
Box Structure ◽  
Eastern Limb ◽  
Sill Intrusion

Abstract Layered mafic-ultramafic intrusions are the fossilized remnants of magmatic plumbing systems and provide excellent natural laboratories to investigate the processes of magma differentiation and solidification. The Rustenburg Layered Suite is the plutonic mafic-ultramafic part of the Bushveld Complex of South Africa and it has traditionally been assumed to have formed from an upwardly-aggrading (and in-sequence) crystal pile in a melt-dominated chamber. In this study, we present field and petrological observations, complemented with detailed plagioclase mineral chemistry (molar An, LREE and strontium isotopes) for the first stratiform anorthosite layer (MG3F anorthosite) at the Lower-Upper Critical Zone boundary (LCZ-UCZ) in the eastern limb of the Bushveld Complex. We use these data to test the overarching paradigm of a melt-dominated chamber for the magmatic evolution of the Rustenburg Layered Suite. The MG3F anorthosite is immediately overlain by the MG3 chromitite and both are surrounded by pyroxenite. A distinctive ‘egg-box’ structure, consisting of round pyroxenite blocks mantled by chromitite, marks the LCZ-UCZ boundary, and represents an erosional disconformity at the base of the MG3F anorthosite. The MG3F anorthosite is laterally continuous for 100s km in the eastern limb. In the northern-central sector of the eastern limb, the 1.5 m thick MG3F anorthosite is characterized by non-cotectic proportions of foliated plagioclase and chromite chains that lie parallel to the foliation. The MG3F anorthosite is divisible into two sub-layers on the basis of (i) a compositional break in plagioclase molar An, LREE and strontium isotope composition and, (ii) a peak in chromite mode (up to 12 vol%). In the lower half of the layer plagioclase LREE concentrations increase upward, molar An shows a marginal decrease upward and strontium isotopes are relatively homogeneous (87Sr/86Sr2.06Ga 0.7056-0.7057). In the upper half of the layer, plagioclase LREE concentrations decrease upward, molar An shows a marginal increase upward and strontium isotopes show strong inter- and intra-grain variability (87Sr/86Sr2.06Ga 0.7053-0.7064). Strontium isotopes in interstitial plagioclase in the immediate footwall and hangingwall pyroxenites show similar 87Sr/86Sr2.06Ga values to the MG3F anorthosite and decrease with distance from the MG3F anorthosite. In the southern sector of the eastern limb, the 4 m thick MG3F anorthosite exhibits identical stratigraphic compositional trends in terms of molar An in plagioclase. We infer that the MG3F anorthosite formed by two successive sill-like injections of magma into a resident viscoplastic pyroxenitic crystal mush. An initial pulse of plagioclase-saturated melt underwent in situ fractional crystallization, manifested as upwardly decreasing molar An and upwardly increasing LREE in plagioclase in the lower half of the MG3F anorthosite. Sill intrusion caused deformation of the viscoplastic pyroxenite mush and vortices of superheated liquid generated by frictional viscous heating caused disaggregation of the footwall pyroxenitic mush. Disaggregated blocks of pyroxenitic mush reacted with the superheated liquid (a hybrid chromite-saturated melt) to produce chromite-rich rims at the base of the MG3F anorthosite (egg-box structure). A second sill-like injection of magma then entered the chamber that halted in situ crystallization. This sill was a plagioclase slurry that contained isotopically distinct plagioclase laths compared to those present in the previous sill. The upward increase in molar An of plagioclase, and decreasing LREE, may be explained by the slurry becoming more primitive in melt composition with time. The second sill also caused mush disaggregation and renewed the production of a hybrid chromite-saturated melt. Chromite crystals were then mobilized and injected as slurries at the interface between the sill and resident mush towards the back of the flow, culminating in the development of the MG3 chromitite. Our model for the development of the Lower-Upper Critical Zone boundary questions the existence of a melt-dominated chamber and it has implications for the origin of stratiform anorthosites (and chromitites) in crustal magma chambers.

scholarly journals Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa

2015 ◽  
Vol 56 (6) ◽  
pp. 1229-1250 ◽  
Author(s):  
Ilya V. Veksler ◽  
David L. Reid ◽  
Peter Dulski ◽  
Jakob K. Keiding ◽  
Mathias Schannor ◽  
...  
Keyword(s):  
South Africa ◽  
Bushveld Complex ◽  
Critical Zone ◽  
Crystal Mush ◽  
Electrochemical Processes

Re-Os isotope and Pd/Ru variations in chromitites from the Critical Zone, Bushveld Complex, South Africa

1999 ◽  
Vol 63 (6) ◽  
pp. 911-923 ◽  
Author(s):  
Tom E. McCandless ◽  
Joaquin Ruiz ◽  
B.Ivan Adair ◽  
Claire Freydier
Keyword(s):  
South Africa ◽  
Bushveld Complex ◽  
Critical Zone ◽  
Os Isotope

Detection of magnetite in the Roossenekal area of the Eastern Bushveld Complex, South Africa, using multispectral remote sensing data

2020 ◽  
Vol 123 (4) ◽  
pp. 573-586
Author(s):  
M. Twala ◽  
R. J. Roberts ◽  
C. Munghemezulu
Keyword(s):  
Remote Sensing ◽  
Support Vector Machine ◽  
Maximum Likelihood ◽  
Bushveld Complex ◽  
Support Vector ◽  
Classification Algorithms ◽  
Error Matrix ◽  
Geological Processes ◽  
Multispectral Remote Sensing ◽  
Eastern Limb

Abstract Multispectral sensors, along with common and advanced algorithms, have become efficient tools for routine lithological discrimination and mineral potential mapping. It is with this paradigm in mind that this paper sought to evaluate and discuss the detection and mapping of magnetite on the Eastern Limb of the Bushveld Complex, using high spectral resolution multispectral remote sensing imagery and GIS techniques. Despite the wide distribution of magnetite, its economic importance, and its potential as an indicator of many important geological processes, not many studies had looked at the detection and exploration of magnetite using remote sensing in this region. The Maximum Likelihood and Support Vector Machine classification algorithms were assessed for their respective ability to detect and map magnetite using the PlanetScope Analytic data. A K-fold cross-validation analysis was used to measure the performance of the training as well as the test data. For each classification algorithm, a thematic landcover map was created and an error matrix, depicting the user’s and producer’s accuracies as well as kappa statistics, was derived. A pairwise comparison test of the image classification algorithms was conducted to determine whether the two classification algorithms were significantly different from each other. The Maximum Likelihood Classifier significantly outperformed the Support Vector Machine algorithm, achieving an overall classification accuracy of 84.58% and an overall kappa value of 0.79. Magnetite was accurately discriminated from the other thematic landcover classes with a user’s accuracy of 76.41% and a producer’s accuracy of 88.66%. The overall results of this study illustrated that remote sensing techniques are effective instruments for geological mapping and mineral investigation, especially iron oxide mineralization in the Eastern Limb of the Bushveld Complex.

The Lower Zone–Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

2004 ◽  
Vol 45 (6) ◽  
pp. 1209-1235 ◽  
Author(s):  
SONJA BOORMAN ◽  
ALAN BOUDREAU ◽  
F. J. KRUGER
Keyword(s):  
Bushveld Complex ◽  
Critical Zone ◽  
Lower Zone

Hydrogen isotopes in phlogopite indicate crustal fluids in the UG2 chromitite layer, Bushveld Complex

2020 ◽  
Author(s):  
Haoyang Zhou ◽  
Robert Trumbull ◽  
Ilya Veksler ◽  
Johannes Glodny ◽  
Ilya Bindeman
Keyword(s):  
Bushveld Complex ◽  
Minor Component ◽  
Critical Zone ◽  
Econ Geol ◽  
Hydrous Minerals ◽  
Crustal Fluids ◽  
Eastern Site ◽  
Group 2 ◽  
D Values ◽  
Chromitite Layer

<p>The Upper Group 2 (UG2) chromitite layer in the upper Critical Zone of the Bushveld Complex, South Africa, is the world’s largest PGE orebody. The UG2 is typically 0.5 to 1.5 m thick and it consists of 75–90 modal % chromite with interstitial silicate and sulfide minerals. Although a minor component, phlogopite is important because it is a hydrous phase. It has been noted that the UG2 chromitite contains more abundant phlogopite (locally > 1 modal %) than the surrounding pyroxenite layers (Mathez and Mey, 2005). More and more studies suggest that water plays an important role in the UG2 chromite formation and in PGE enrichment or remobilization (e.g., Li et al., 2004; Mathez and Mey, 2005; Schannor et al., 2018). The source of the water is controversial, and this motivated our ongoing study of hydrous minerals in the UG2.</p><p>We determined the chemical composition and hydrogen isotope ratio of phlogopite from the chromitite layer and from the surrounding pyroxenite in drill cores from two sites the eastern and western Bushveld (Nkwe and Khuseleka, respectively). The δD values of phlogopite in chromitite from the eastern site are -38.2 to -25.5‰ (mean = -29.7‰, n = 6). The corresponding values from the western site are similar, with -34.6 to -31.6‰ (mean = -33.2‰, n = 6). The δD values of phlogopite from pyroxenite are more variable, ranging from -43.1 to -26.1‰ for the eastern site (mean = -32.9‰, n = 4) and from -38.7 to -26.1‰ for the western site (mean = -31.7‰, n = 3).</p><p>Published whole-rock δD values for silicate cumulate rocks in the upper Critical Zone are -93 to -55‰ (Mathez et al., 1994), which are similar to mantle values (-70±10%; Boettcher and O'neil, 1980) and are interpreted as magmatic.  In comparison, our δD values of phlogopite from UG2 are much higher and suggest a significant contribution of crustal fluids. Harris and Chaumba (2001) estimated a δD value of -22‰ for paleo-meteoric water in the Bushveld Complex. Given the relative homogeneity of the phlogopite δD data in both sites of the complex, and the primary appearance of the grains in thin section, we argue that the crustal fluids were incorporated in the magma before the crystallization of the UG2 layer. Triple oxygen isotopes will test our hypothesis further.</p><p> </p><p>References: Boettcher & O'neil (1980) Amer. Jour. Sci. 280A, 594–621. Harris & Chaumba (2001) J. Petrol. 42, 1321–1347. Li et al. (2004) Econ. Geol. 99, 173–184. Mathez et al. (1994) Econ. Geol. 89, 791–802. Mathez & Mey (2005) Econ. Geol. 100, 1616–1630. Schannor et al. (2018) Chem. Geol. 485, 100–112.</p>

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