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Effects of metamorphism and metasomatism on manganese mineralogy: Examples from the Transvaal Supergroup

Johnson, J. E. and Webb, S. M. and Condit, C. B. and Beukes, N. J. and Fischer, W. W. (2019) Effects of metamorphism and metasomatism on manganese mineralogy: Examples from the Transvaal Supergroup. South African Journal of Geology, 122 (4). pp. 489-504. ISSN 1996-8590. doi:10.25131/sajg.122.0034.

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Manganese-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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Fischer, W. W.0000-0002-8836-3054
Additional Information:© 2019 Geological Society of South Africa. Underlying data used in figures can be found in the University of Michigan Deep Blue online repository doi:10.7302/3928-tw22. We are grateful to anonymous reviewers who improved our manuscript. We would like to thank Bertus Smith for help with field work in South Africa, Courtney Roach for X-ray microprobe assistance and trouble-shooting, Chi Ma for help with EBSD measurements, and George Rossman for helpful mineralogical consultations and assistance with Raman spectral data collection and interpretation. We also express our gratitude to the Lewis and Clark Foundation for funding field research and core sampling in South Africa, and acknowledge a David and Lucile Packard Foundation grant to W.W.F., NSF Division of Earth Sciences award EAR-1349858, and the Agouron Institute for drilling support. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract DE-AC02-76SF00515.
Funding AgencyGrant Number
Lewis and Clark FoundationUNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Agouron InstituteUNSPECIFIED
Department of Energy (DOE)DE-AC02-76SF00515
Issue or Number:4
Record Number:CaltechAUTHORS:20191218-153250846
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Official Citation:J.E. Johnson, S.M. Webb, C.B. Condit, N.J. Beukes, W.W. Fischer; Effects of metamorphism and metasomatism on manganese mineralogy: Examples from the Transvaal Supergroup. South African Journal of Geology ; 122 (4): 489–504. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100365
Deposited By: Tony Diaz
Deposited On:19 Dec 2019 00:01
Last Modified:16 Nov 2021 17:53

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