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A sugilite-bearing assemblage from the Wolhaarkop breccia, Bruce iron-ore mine, South Africa: Evidence for alkali metasomatism and 40Ar-39Ar dating

2011; Elsevier BV; Volume: 23; Issue: 4 Linguagem: Inglês

10.1127/0935-1221/2011/0023-2117

1617-4011

John M. Moore, Barbara Kühn, Darren F. Mark, Harilaos Tsikos,

earthquake and tectonic studies

An occurrence of the alkali-rich, Fe-Mn silicate sugilite [KNa2(Fe3+, Mn3+, Al)2Li3Si12O30] is reported from the Wolhaarkop manganese-rich chert breccia at the Bruce iron-ore mine, Northern Cape Province, South Africa. The upper portion of the breccia is dominated by braunite, albite, K-feldspar and aegirine, and locally hosts irregular-shaped void-fills (generally <5 cm) consisting of both zoned and unzoned assemblages of alkali-rich mn-silicates. the zoned void-fills are characterised by marginal needles of a mineral with norrishite-like composition [kmn3+2LiSi4O10(O)2] penetrating into a persistent wall zone that comprises tabular serandite [NaMn2Si3O8(OH)], albite, K-feldspar and lesser granular sugilite. The cores of larger vugs are occupied by masses of interlocking armbrusterite [K5Na6Mn3+Mn2+14(Si9O22)4(OH)10.4H2O] and/or fibrous sugilite. Unzoned assemblages contain a similar mineralogy, comprising albite, K-feldspar, quartz, sugilite, norrishite, serandite, armbrusterite and minor witherite, strontia- nite and kentrolite [PbMn2O2(Si2O7)]. Similarities to sugilite-bearing assemblages at the Wessels mine in the Kalahari Manganese Field suggest that a single regional hydrothermal alteration event in the vicinity of the unconformity between the Olifantshoek and Transvaal Supergroups was the probable genetic cause for both occurrences. 40Ar-39Ar dating of fibrous sugilite, however, yields an age of 620.2 ± 3.3 Ma (2σ) that is distinctly younger than the ages reported for the assemblages from the Kalahari Manganese Field.