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FAULT-CONTROLLED HYDROTHERMAL ALTERATION OF PALAEOPROTEROZOIC MANGANESE ORE IN WESSELS MINE, KALAHARI MANGANESE FIELD by ,- ALBERT MEIRING BURGER dissertation submitted in fulfillment for the degree of MASTER OF SCIENCE in GEOLOGY in the FACULTY OF SCIENCE at the RAND AFRIKAANS UNIVERSITY SUPERVISOR: PROF. N. J. BEUKES CO-SUPERVISOR: DR. A.S.E. KLEYENSTOBER DECEMBER 1994 ACKNOWLEDGMENTS I would like to thank the following: MINTEK for their financial support. Samancor for their logistical support. Mr. E. Karberg of the analytical chemistry department of Samancor at Hotazel for chemical analyses. The analytical chemistry department at MINTEK for chemical analyses and the use of instrumentation at MINTEK. The geological and administrative personnel of Samancor at Hotazel, especially Mr. R. Arnot, Miss C. Lathy and Mr. O. Mciver for their help and assistance during field and follow-up work. Also the administrative personnel of Samancor at Hotazel and Wessels Mine for making my stay comfortable and smooth running. Patrick and his underground sampling team at Wessels Mine, for assistance during field work as well as advice and guidance during the project. Dr. Hsue-Wen Yeh of the Hawaii Institute of Geophysics in Honolulu, USA for stable isotope analyses. My co-supervisor at MINTEK, Dr. A.S.E. KleyenstOber for his guidance, help and advice, XRD and SEM analyses. The personnel and post graduate students at the Geological department of the Rand Afrikaans University, especially Dr. M. Buxton, Mrs. N. Day, Mr. H. Oirr, Mr. H. Jonker, Mrs. E. Maritz, Mr. V. Morel, Mr. K. Mokgatla (for answering the phone) Prof. C. Roering and Mr. J. Gutzmer for sharing his "manganese knowledge". Mr. D. Selepe and Mr. M. Baloy for cutting and polishing thin sections and Mr. H. Leteane for tea. My parents and brothers for their love, encouragement and financial support. My fiance lIane for her love, encouragement and understanding. My supervisor Prof. N.J. Beukes for his guidance, help and advice which made it possible. All thanks to God who made it possible for everyone of us, through his everlasting love. Hieride tesis is opgedra aan my broer Francois. 25/11/1970 • 10/07/1994 ! Opsomming Die Kalaharimangaanveld, ongeveer 60km noordwes van Kuruman in die Noord­ Kaapprovinsie gelee, bevat die grootste landelike mangaanafsettings ter wereld. Dit bestaan grootliks uit faegraadse sedimentere Mamatwantipe erts (met tot 38 gewigspersent Mn) wat ongeveer 97 persent van die ertsreserwes uitmaak. .Ongeveer 3 persent van die bekende reserwes bestaan uit hoegraadse Wesselstipe erts (met 'n gemiddeld van tussen 42 en 48 gewigspersent Mn) wat voorkom in die noordwestelike deel van die afsetting. fn hierdie gedeelte van die afsetting word die opeenvolging verplaas deur 'n stelsel van noord-suidstrekkende normale verskuiwings. Verskuiwingsones is veryster en die. oorspronklike sedimentere Mamatwantipe erts is hidrotermaal opgegradeer fangs verskuiwings tot hoegraadse Wesselstipe erts. Relikte van Mamatwantipe erts is bewaar in the sentrale gedeeltes van verskuiwingsblokke. Mamatwantipe erts bestaan hoofsaaklik uit braunietlutiet en die mangaankarbonaat kutnahoriet met ondergeskikte kalsiet. In kontras bestaan Wesselstipe erts uit hausmanniet, bixbyiet, brauniet /I en manganiet en ondergeskikte sekondere minerale soos kalsiet, chlinochloor, gaudefroyiet en andradiet. Hidrotermale verandering van Mamatwantipe erts na Wessefstipe erts behels hoofsaaklik loging van 8i02 uit die sisteem en 'n oksidering van mangaankarbonate na mangaanoksiedes. In effens veranderde Mamatwantipe erts is sferiese klein kutnahoritiese konkresies vervang deur hausmanniet en kalsiet. Brauniet is verander na brauniet /I deur 'n loging van 8i02. Die volgende stadium van verandering behels 'n transformasie van brauniet /I na bixbyiet en oksidering van al die mangaankarbonaat na hausmanniet en vorming van kalsiet en andradiet as aarsteenminerale. Die hoogste graad Wesselstipe erts bestaan hoofsaaklik uit hausmanniet. Oit is langs verskuiwingssones gekonsentreer. In die onmiddelike omgewing van verskuiwingsones is die mangaanerts gehematitiseer en van die hausmanniet is ysterryk. Die ysterryke hausmanniet (met tot 15 gewigspersent Fe) is hoogs magneties. In die oorgangsone tussen hoegraadse hausmannietryke Wesselstipe erts en die verysterde sone is 'n sekondfJre fase van massiewe braunietryke erts ontwikkel. Oit is moontlik veroorsaak deur die herpresipitasie van silika, gefoog uit Mamatwantipe erts deur die hidrotermale vloeistowwe. Die hematietryke verskuiwingsones bevat hee/wat ka/siet en tot 'n mindere mate ook kwarts. Normale verskuiwings het blykbaar as kanale vir hidrotermale vloeistowwe gedien. Hidrotermale oplossings, wat verantwoordelik was vir die opgradering van die erts, was blykbaar ryk in tweevalente yster en alkalies om silika te kan loog uit primfJre Mamatwantipe erts. Fe203 is in verskuiwingssones gepresipiteer, terwyl mangaan gemobi/iseer is as Mn2+ om as hausmanniet te presipiteer meer distaal tot die verskuiwingsone. 8i1ikaloging deur die hidrotermale oplossings het veroorsaak dat brauniet verander is na brauniet /I wat op sy beurt weer verander is na bixbyiet. Langs die veranderingsfront, tussen Mamatwantipe- en Wesselstipe erts, was die oplossings oksiderend soos aangedui word deur die verandering van mangaan karbonate na hausmanniet en die vorming van kalsiet. Hidrotermale verandering het blykbaar plaasgevind in 'n tipe van konveksiesel met 8i02 en Ca wat geloog is vanuit Mamatwan-tipe erts en geherpresipiteer is in die nabyheid van die normale verskuiwings in die vorm van sekondere brauniet en kalsiet. Die laterale mineralogiese sonering in die ertse mag moontlikhede inhou vir selektiewe mynbou van verskillende tipes ertse. Die ontdekking van magnetiese hausmanniet in die ertse mag moontlik toepassing vind in geofisiese eksplorasie na hoegraadse Wesselstipe erts, soweI as in die skeiding van verskillende erts grade. Abstract The Kalahari manganese field, situated some 60km northwest of Kuruman in the Northern Cape Province, contains the world's largest known land-based manganese deposits. Low-grade. sedimentary Mamatwan-type are (containing up to 38 weight percent Mn) dominates and constitutes about 97 percent of the are .. reserves. It is composed of finely intergrown braunite and Mn-carbonate. High­ grade Wessels-type are (containing on average between 42 and 48 weight percent Mn) constitutes about 3 percent of the known reserves and occurs in the northwestern part of the deposit. Here the sequence is displaced by a system of north-southstriking normal faults. Fault zones are ferruginized and alongside faults sedimentary Mamatwan-type are has been upgraded by hydrothermal alteration to Wessels-type are. Pockets of Mamatwan-type are are present in cores of fault blocks. Wessels-type are consists mainly of hausmannite, bixbyite, braunite /I and manganite, and subordinate secondary minerals such as calcite, clinochlore, gaudefroyite and andradite. Hydrothermal alteration of Mamatwan-type are to Wessels-type are involved essentially leaching of 5i02 from the system and oxidation of manganese­ carbonates to manganese oxides. In slightly altered Mamatwan-type are kutnahoritic ovoids are replaced by hausmannite and residual calcite remaining and braunite altered to braunite /I through leaching of 5i02. The next stage of alteration involve~ transformation of braunite II into bixbyite and oxidation of all manganese carbonate into hausmannite and formation of calcite or andradite as gangue minerals. Highest grade Wessels-type are is essentially composed of hausmannite. In proximity to fault zones hematitization of manganese are took place and hausmannite is iron-rich. This iron-rich hausmannite (containing up to 15 weight percent Fe) is highly magnetic, as indicated by magnetic susceptibility and other paleomagnetic studies. In the transition zone between hematite-rich fault zones and high-grade.hausmannite-rich Wessels-type are, a secondary phase of massive braunite is developed, as a result of reprecipitation of earlier leached silica. The hematite-rich fault zones may contain abundant calcite and some quartz. Normal faults apparently acted as conduits for hydrothermal fluids. Hydrothermal solutions, responsible for the upgrading of the are, were presumably rich in ferrous iron and alkaline in order to mobilize Mn2+ and turned less alkaline to neutral with relative high temperature (± 300°C), to leach silica from primary Mamatwan-type ores. The solution precipitated ferric iron as Fe203 adjacent to normal faults, while mobilizing manganese as Mn2+ to precipitate it more distal to the fault zone as hausmannite. Leaching of Si02 by the hydrothermal solution caused transformations from braunite into braunite