Geology, structural evolution and controls of hydrothermal gold mineralization in the Eastern Karagwe-Ankole fold belt, North Western Tanzania

Koegelenberg, Corne (2016-03)

Thesis (DSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: In central-east Africa, the north-western margin of the Archaean Tanzania Craton (TC) is overlain by imbricated, low grade, volcano-sedimentary rocks of the Karagwe-Ankole belt’s (KAB) Eastern Domain (ED). Centred in the ED, vast stretches of a sheared and Au mineralized basement-cover contact are exposed along margins of the Mugera-Nyakahura (MN) inlier. To date, no detail research has been done in the area and the regional geology has been described from only broad reconnaissance studies. As part of an exploration project the first high resolution geological maps of key prospects and larger, more encompassing, scale maps of the basement-cover region was compiled. Mapping was supplemented by a regional scale structural traverse of the ED and selected sampling for analysis of micro-structures, geochronology and oxygen isotopes. The collective structural data has indicated that basement gneisses of the MN inlier may be considered as part of a forethrusted tectonic wedge caused by regional top-to-the-SE thick skinned thrusting. Above and in front of the wedge diagnostic back-thrusts and the reversal of kinematic fabrics in weak, often graphitic, metapelitic rocks of the Muyaga Group depict top-to-the-NW, hinterland-directed, tectonic transport along the main “roof” detachment. To the east under-thrusting of the coarse clastic Bukoba Group by the Muyaga Group has also created a distinct triangle zone at the frontal termination of the KAB. 39Ar-40Ar muscovite ages of detachment mylonites in the easternmost, and latest developing, parts of the KAB has constrained timing of D2 to at least 1326 ± 10 Ma. This age corresponds with the youngest end of the main phase of granite plutonism and mafic dyke emplacement in the KAB (1380 – 1328 Ma) and may point towards a Mesoproterozoic collisional event between the Congo- and Tanzania Cratons. U-Pb detrital zircon ages of the Muyaga- and Bukoba Groups have indicated uplift, erosion and subsequent reworking of Muyaga Group sediments and layered volcanics into the Bukoba basin after 1780 Ma, but before 1568 Ma. The Bukoba Group thus correlates with the Bwezigoro Group in SW Uganda, indicating the presence of an extensive Paleo- and/or Mesoproterozoic foreland basin overlying the western margin of the TC and Uganda Block. Lastly, controls of D2 fluid flow and mineralization along the low angle phyllonitic detachment are linked to NE trending ramp structures that were most favourable for the initiation of slip and development auriferous quartz vein networks. Upwards into the Muyaga Group progressive fold amplification and eventual fold-lock of second order anticlines, cored by competent and chemically re-active ferruginous mafic sills, are responsible for the late-kinematic development of auriferous quartz veins. Oxygen isotope values of D2 quartz veins and host rocks have indicated that fluids are derived from dehydrated clastic sediments of the Kagera Supergroup and, as such, may suggest that gold associated with greenstones of the TC have not been remobilized during D2 fold-and-thrust development in the ED. Collectively these findings greatly enhance the understanding of the geological evolution of the KAB’s easternmost parts and provides future research and exploration with a much improved geological background.

AFRIKAANSE OPSOMMING: In Sentraal- en Oos-Afrika is die noord-westelike rand van die Argeïese Tanzanië Kraton (TK) oorlê deur geimbrikeerde, lae-graadse, vulkaan-sedimentêre gesteentes van die Karagwe-Ankole plooistelsel (KAP) se Oostelike Segment (OS). Sentraal in die OS is daar groot lengtes van ‘n gouddraende skuifskeur kontak tussen vloer- en bedekking gesteentes wat blootgestel is aan die rante van die Mugera-Nyakahura (MN) venster. Daar was nog geen navorsing in die omgewing gedoen nie en die regionale geologie is slegs vanuit breedvoerige verkenning beskryf. As deel van ‘n eksplorasie projek is die eerste hoë resolusie kartering van sleutel prospekte en groter, meer omvattende, skaal kaarte van die skuifskeur kontak vervaardig. Kartering is aangevul deur ‘n regionale skaal struktuur dwars-snit van die OS en selektiewe monsters vir die analiese van mikro-strukture, geokronologie and suurstof isotope. Algehele struktuur data het aangedui dat basale gneisse van die MN venster wel deel mag wees van ‘n voor-stootverskuifde tektoniese wig gedurende regionale bo-na-die-SO dik vel stooting. Bo-op en voor die wig is kenmerkende terug-stote en die omkeer van kinematiese foliasies in swak, somtyds grafietise, meta-pelitiese gesteentes van die Muyaga Groep ‘n aanduiding van bo-na-die-NW, agterland gestuurde, tektoniese vervoer op die hoof “dak” vloeroorskuiwing. Na die ooste is onder-stoot van die Bukoba Groep deur die Muyaga Groep verantwoordelik vir die vorming van ‘n “driehoek” sone and die voorkant van die KAP. 39Ar-40Ar muskoviet ouderdomme van vloeroorskuiwing miloniete in die mees oostelike, en laasgevormde, dele van die KAP het D2 vervorming beraam tot en met 1326 ± 10 Ma. Die ouderdom vergelyk met die jongste fase van granite plutonisme and mafiese gang indringing in die KAP (1380 – 1328 Ma) en mag aanduidend wees van ‘n Mesoprotersoïese botsing tussen die Congo- en Tanzanië Kratons. U-Pb ditrale zirkoon ouderdomme van die Muyaga- en Bukoba Groepe het aangedui dat tektoniese oplig, erosie en gevolglike herwerk van Muyaga sedimente en vulkaniese lae binne in die Bukoba kom in na 1780 Ma, maar voor 1568 Ma. Die Bukoba Groep vergelyk dus met die Bwezigoro Groep van SW Uganda, wat aanduiding gee tot ‘n uitgebreide voorland kom oor die westerlike rand van die TK en Uganda blok. Laastens, is die beheer van D2 vloeistof migrasie en mineralisasie langs die milonietiese vloeroorskuiwing gekoppel aan NO strekkende skuinstes wat mees gepas is vir begin van skuifskeer glip en ontwikkeling van goud-yster draende kwarts aar netwerke. Opwaarts in die Muyaga Groep is aanhoudende vou-versterking en uiteindelike vou-sluiting van sekondêre antiklinale, gekern met harde en chemiese-reaktiewe plate, verantwoordelik vir die laat-kinematiese onwikkeling van goud-yster draende kwarts-are. Suurstof-isotoop waardes van D2 kwarts-are en gasheergesteentes dui aan dat vloeistowwe afkomstig is van gedehidreerde klastiese sedimente van die Kagera Supergroep en, gevolglik, aandui dat goud geassosieer met groenstene van die TK nie gemobiliseer was gedurende D2 verforming nie. In die geheel het al die bevindinge grootliks bygedra tot die verstaan van die geologiese ontwikkeling van die KAP se OS en verskaf dit toekomstige navorsing en eksplorasie met ‘n verbeterde geologiese agtergrond.

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