Mineralogical characterization of the thrusted manganese ore above the Blackridge Thrust Fault, Kalahari Manganese Field: The footprint of the Mukulu Enrichment

• Published in: The island arc 2019-01, Vol.28 (1), p.n/a
• Main Authors: Vafeas, Nicholas A., Viljoen, Karel S., Blignaut, Lauren C.
• Format: Article
• Language: English
• Subjects: Apatite; Arsenic; Boreholes; Enrichment; Exhibits; Geological faults; Hausmannite; Kalahari; Mamatwan; Manganese; Manganese ores; Mukulu; Oxides; supergene; Thrust faults; Wessels
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12280

The thrusted manganese ore above the Blackridge Thrust Fault is a predominantly unstudied ore‐type and as such, there is an urgent need to petrographically characterize it. A total of 80 samples were collected from the thrusted lower manganese ore bed of six boreholes on Mukulu in the north of the Kalahari Manganese Field. Each sample underwent a range of optical and electron petrographic analyses in order to effectively characterize three dominant stages of alteration, namely Stages 1, 2 and 3. Similar to the hydrothermally‐altered Wessels‐type manganese ore, braunite II is the predominant mineral phase present during Stage 1 alteration, and alters to bixbyite with increasing proximity to normal faults at the onset of Stage 2 alteration. The final stage of alteration is exhibited by the development of hausmannite and jacobsite adjacent to the ferruginised zone at the normal fault plane. Apatite precipitation, in abundances previously undocumented in the Kalahari Manganese Field, is a hallmark feature of the newly identified Mukulu Enrichment. This supergene‐related enrichment process contributed to the significant abundance of arsenic‐bearing apatite in the ore, whilst residually enriching the ore‐hosted Fe and Mn oxides. The thrusted manganese ore of the Kalahari Manganese Field, South Africa, exhibits high‐grade textural alteration similar to the hydrothermally‐altered Wessels‐type ore in the area. Continued supergene alteration at surface resulted in the precipitation of significant abundances of arsenic‐bearing fluorapatite during the newly identified Mukulu Enrichment.