Fig 3 - uploaded by Jeff B. Chaumba
Content may be subject to copyright.
Geologic map and cross-section of the Zvishavane Ultramafic Complex (after Laubscher, 1964, 1968) showing sample locations and sample number utilized in this study.
Source publication
The Zvishavane Ultramafic Complex (ZUC) in the south central part of the Zimbabwe craton is comprised of coarse-grained serpentinites, metadunites and metagabbros and hosts Africa's reserves, and largest mine, of high-grade chrysotile asbestos. Magnesiohornblende, actinolite, plagioclase (An 0.6-41.9), augite, diopside and clino-zoisite constitute...
Contexts in source publication
Context 1
... ZUC is located near the town of Zvishavane (Fig. 2), and it covers a surface area of 15 km by 2.5 km ( Fig. 3;Martin, 1978). The complex trends NW-SE and dips at 40-60° to the SW ( Fig. 3; Laubscher, 1963Laubscher, , 1964Martin, 1978). Within the Shabanie Mine area, the dips were likely obtained from contacts of the different lithologies that were intersected by diamond drilling as none of these workers describe how their reported dips were determined. The main lithologic units of the ZUC encountered during this ...
Context 2
... sampling purposes, geologic maps of Laubscher (1964,1968) were used in order to obtain samples constituting a variety of lithologies. Samples obtained for this investigation ranged from serpentinites, metadunites and medium-grained metagabbros. Sample locations are shown in Fig. 3 and sample descriptions and global positioning system co-ordinates are provided in Table 1. Some representative field pictures of sampled areas for serpentinites (Fig. 4a), metadunites (Fig. 4b), and metagabbros ( Fig. 4c) are also shown. On outcrops of both serpentinites and metadunites, cross-fractures of various scales are visible ...
Context 3
... of significant influx of silica-rich hydrothermal fluids characterize environments where asbestos formed, which include processes that can be driven by regional metamorphism, contact metamorphism, or magmatic hydrothermal systems (Virta, 2005b). In the case of ZUC, the faulting within this body and the enclosing Zvishavane gneiss complex (Fig. 3) may have been caused by emplacement of the "younger" ...
Citations
... This may be one of the possible reasons why we did not identify rodingites in the ZC in this study. The occurrence in the ZUC of calcium-rich minerals diopside and epidote (Chaumba and Mamuse 2023), which also form part of the mineral assemblages in rodingites (e.g. Bach and Klein 2009), suggest that rodingites may also occur in the ZUC although they were not identified in our suite of samples. ...
... In the MIC, however, chromite cores are rimmed by ferrian chromites indicating that chromite modification was incomplete as the chromite investigated occurs as economic chromite seams (Chaumba 2018). The chromites were likely modified during contact metamorphism accompanied by serpentinization at temperatures between 500 and 800°C and pressures of less than 2kb (Chaumba and Mamuse 2023). The high-grade chrysotile asbestos deposits at Shabanie Mine were inferred by these investigators to have been formed during the hydrothermal circulations accompanying the intrusions of granitic batholiths in the vicinity of Zvishavane. ...
The Zvishavane Ultramafic Complex (ZUC) is a metamorphosed ultramafic-mafic body exposed within the Zvishavane gneiss complex in south central Zimbabwe. The complex hosts Africa's largest mine and reserves of high-grade chrysotile asbestos. Rock types of the ZUC range from serpentinites, metadunites, and metagabbros. Accessory chromites, which are altered to ferrian chromites, are hosted in metadunites and serpentinites. Chromium number, Cr# (100*Cr/(Cr + Al)), is extremely high, and ranges from 90.6 to 99.5. Low magnesium number, Mg# 100*Mg/(Mg + Fe ²⁺ ), ranges from 5.1 to 8.4.
Most ZUC ferrian chromites occur as inclusions or marking the outlines of relict olivine crystals, and in veinlets traversing the serpentinites. On 100*Fe ³⁺ /(Al + Fe ³⁺ ) versus 100*Fe ²⁺ /(Mg + Fe ²⁺ ) diagram, the chromites all plot in the magnesioferrite field, and in the ferrian chromite field on a Fe ³⁺ -Cr-Al ternary diagram. On wt% Al 2 O 3 versus wt% Cr 2 O 3 and 100*Cr/(Cr + Al) versus 100*Mg/(Mg + Fe ²⁺ ) plots, chromites all plot in the modified trend fields. The bulk of the chromites were metamorphosed at temperatures of ≤500°C, and no samples experienced temperatures exceeding 550°C. A plot of Fe ³⁺ /(Fe ³⁺ + Fe ²⁺ ) versus wt% MnO is indicative of hydrous metamorphism of ZUC chromites. It is interpreted that extensive modification of the ZUC chromites occurred during hydrothermal alteration, under conditions more oxidising than quartz-fayalite-magnetite but less than magnetite-hematite, which accompanied the formation of high-grade chrysotile asbestos for which the ZUC is famous. Four granitic plutons exposed within 50km of ZUC were likely the source of hydrothermal fluids responsible for extensive hydrothermal alteration, which likely led to the formation of high-grade chrysotile asbestos, and almost completely modifying the chromites in the process. Compositions of accessory chromites in chrysotile asbestos meta-ultramafic bodies elsewhere are likely to be extremely modified.
Products of geological processes such as rock formations, unconformities, structures, minerals, fossils, and landforms, represent unique records of the evolution of the Earth. These form a coherent picture showing how the Earth evolved. The picture becomes blurred with antiquity. Consequently, there are challenges in gathering information from the Archaean, the period during which the foundations of the Earth were laid down. The 2.7 Ga Belingwe Greenstone Belt in Zimbabwe has proved to be valuable because it has some of the best-preserved Archaean stratigraphy in the world. An unconformity between sialic basement and supracrustal rocks of the greenstone belt, and exotic rocks such as komatiites and stromatolites, are immensely contributing towards the knowledge about the evolution of the young Earth and the beginning of life. The frequent use of the Belingwe Greenstone Belt examples to explain geotectonic processes of the early Earth give testimony to the importance of this structure. Interpretation of some of the features of the greenstone belt is sometimes controversial, which forms areas of endless research to better understand the Archaean Era. It is for these reasons that arguments are presented for consideration of the Belingwe Greenstone Belt as a national Geoheritage site.
