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

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.