• journal_title：European Journal of Mineralogy
• Contributor：John M. Moore ; Barbara K. Kuhn ; Darren F. Mark ; Harilaos Tsikos
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2011/0023-2117
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：23
• issue：4
• firstpage：661
• section：Articles

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, 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. 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.