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

作者：Moore John M; Kuhn Barbara K; Mark Darren F; Tsikos Harilaos^*

来源：European Journal of Mineralogy, 2011, 23(4): 661-673.

DOI：10.1127/0935-1221/2011/0023-2117

• sugilite
• alkali metasomatism
• manganese
• breccia
• South Africa
• Ar-40/Ar-39 dating
• armbrusterite

An occurrence of the alkali-rich, Fe-Mn silicate sugilite [KNa2(Fe3+, Mn3+, Al)(2)Li3Si12O30] 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 [KMn23+LiSi4O10(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+Mn142+(Si9O22)(4)(OH)(10 center dot)4H(2)O] and/or fibrous sugilite. Unzoned assemblages contain a similar mineralogy, comprising albite, K-feldspar, quartz, sugilite, norrishite, serandite, armbrusterite and minor witherite, strontianite 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. Ar-40-Ar-39 dating of fibrous sugilite, however, yields an age of 620.2 +/- 3.3 Ma (2 sigma) that is distinctly younger than the ages reported for the assemblages from the Kalahari Manganese Field.

• 出版日期2011-7