7/27/2019 Band_77_151_159

1/9

Mitt, sterr. geol. Ges. 771984 S. 151-1592Taf. Wien, Dezember 1984

Notes on the Geology and Mineral Resources of the SouthernKenyan CoastBy A. D . HORKEL * )

with the collaboration ofW. NEUBAU ER, G. NIEDERMAYR, R. E . OK EL O, J . K. WAC HIRA , and

W . W E R N E C KWith 2 plates

AbstractThe r if t ing and break-up of the Gondwana continent dominate the sedimentaryand structural evolution of the southern Kenyan coast. Jurassic rifting transformedan intracratonic Permo-Triassic basin filled with continental elastics into a marginalmarine basin located at the trailing edge of the African plate. An alkaline intrusivecomplex is related to struc tures w hich suggest that a failed rift arm of a triple junctionformed in association with the opening of the Indian Ocean.The m ineral deposits , characteris t ic of the metallogenetic env ironme nt com m only

associated with rifting, and include apart from insignificant sedimentary mineraloccurences a major Nb-rare earth deposit associated with a carbonatite complex aswell as several minor vein-type Pb-Zn-Ba mineralizations.Zusammenfassung

Das Riften und Auseinanderbrechen des Gondwanakontinents sowie das ffnendes Indischen Ozeans beherrschen die sedimentre und tektonische Entwicklung derkenyanischen Kstenregion sdlich von Mombasa.Ein intrakratonisches permo-triassisches, mit kontinentalen Sedimenten geflltesBecken wird im Zuge des Rif tens von Gondwanaland in ein marines Randbeckenumgewandelt . Strukturen, die an ein nicht voll entwickeltes Grabensystem einerTriple Junction" erinnern, kontroll ieren einen intrusiven Alkalikomplex.Die Lagerstt ten entsprechen der generell mit Grabensystemen verbundenenmetal logenet ischen Fazies . Neben unbedeutenden sedimentren Vorkommen vorwiegend von Steinen und Erden exist ieren eine bedeutende Niob-Seltene ErdenLagerstt te in einem Karbonati t-Komplex sowie mehrere gangfrmige Blei- Zink-Schwerspat-Vererzungen.*) Authors adresses: Dr. Walter NEUBAUER, Dipl . - Ing . Dr . Werfr ied WERNECK and Dr. Mag. Alexande r D . HORKEL: Austrominera l Ges . m. b . H . , Pr inz-Eugen-Str . 8-10 ,

A-1040 Vienna, Austria.J im K. WACHIRA, BSC. (Hon.) and Richard E. OKELO, BSC. (Hon.) : Mines andGeological Department, P. O. Box 30009, Nairobi, Kenya.Dr . Gerha rd NIEDERMAYR: Naturhis tor isches Museum, Burgr ing 7 ,A-1014 Vienna, Austria.

7/27/2019 Band_77_151_159

2/9

152 Alexander D. Horkel

Table of ContentsAbstract 151Zusammenfassung 1511. Introd uctio n 1522. Regio nal Ge olog y . . . . 1522.1 Gen eral Geolog ical Setting 1522.2 Stratigraphy and Sedim entology 1532.2.1 Du rum a G rou p (Permo-Triassic) 1532.2.2 M arine Jurassic-C retaceou s Sediments 1562.2.3 Plio-Pleistocen e Dep osits 1562.3 The Jom bo-M rima Alkaline Com plex 1572.4 Structural Ge olo gy : 1573. Econ om ic Geolo gy 157

3.1 Sedim entary M ineral Occu rences 1573.2 Hy dro the rm al Pb-Z n-B a M ineralizations 1583.3 M ineralizations associated with Alkaline Ro cks 158Acknowledgements 159References 1591. Introduction

The Kenyan coast south of Mombasa contains one of the world's major potentialniobium/rare earths deposits and several mineral occurences of lesser importance.Past mineral exploration was restricted to exploration of this Nb/rare earth prospect,to diamond exploration, and to some limited mineral reconnaissance for base metaldeposits. Systematic regional mineral exploration was therefore justified and apertaining programme was implemented to this end from 19761978 within theframew ork of a bilateral explo ration project funded b y the gov ernm ents of Ken yaand Austr ia , wi th AUSTROMINERAL ac t ing as execut ing agency.Th e general geological concept of the metallogenetic env ironm ent presen ted in thispaper served as basis for the systematic appraisal of the area's mineral potential(A U ST R O M IN ER A L 1978); i t is derived from a comp ilation and review of the

previously existing geological data (BAKER 1953 who focussed on the igneous rocksin the area, CASWELL 1953 & 1956 w ho dealt extensively with stratigrap hy, sedimen tology and paleontology, COETZEE & EDWARDS 1959), supplemented by regionalgeological mapping (shown in Plate 1).2. Regional Geology

2.1 General Geological SettingThe regional geological and metallogenetic setting of the southern Kenyan coast is

dom inated by the r if ting and break-u p of the Paleozoic Gon dw ana con tinent and thedevelopment of the Indian Ocean (EMBLETON & VALENCIO 1977).Proterozoic gneisses of the Mozambique belt ( P O H L & H O R K E L 1980) form thebasement of an intracratonic basin, filled with continental Permo-Triassic elastics.

7/27/2019 Band_77_151_159

3/9

N ote s on the Geo logy and Mineral Resources of the Southern Kenyan Coast 153

Rifting during the early to middle Jurassic, presumably preceeded partly by up-doming along the incipient rift, transformed it into a marine marginal basin at thetrailing edge of the African plate. An alkaline intrusive complex in the south isclosely associated with structures suggestive of a failed rift arm of a minor triplejunction, formed as a consequence of the opening of the Indian Ocean. Generalstratigraphy and tectonic evolution are summarized in Plate 2.

2.2 Stratigraphy and Sedimentology2.2.1 Durum a Group (Perm o-Triassic)

Most of the area is underlain by continental Permo-Triassic sediments assigned tothe Duruma Group ("Duruma Sandstone Ser ies" CASWELL 1953), which is generallyconsidered as the Kenyan equivalent of the Karroo system of southern Africa. TheDuruma sediments essentially comprise elastics (grits, arkosic sandstones, andshales), accumulated under lacustrine, sub-aerial conditions with minor marineingressions in a broad, roughly NNE-SSW trending intracratonic trough, whichformed towards the end of the Paleozoic within the Proterozoic gneisses of this partof the Gondwana cont inent .

During the initial development stages of this trough, downwarping was fairlyrapid, and the basal Taru formation (exposed inland of the area investigated) consistsmainly of coarse-grained, poorly sorted arkoses. Intercalated shales contain freshwater fauna. Intraformational reworking and sedimentary structures are widespread,and indicate rapid denu dation and short transport w ith subsequent re-deposit ion in ahigh-energy lacustrine environment.

Towards the top, the grain size of the Taru sediments decreases, and the generallyfine-grained Maji-ya-chumvi formation overlies the Taru sediments along a slightunconformity. Current bedding and ripple marks are common and indicate deposition in shallow water. A basal sequence, composed of fissile dark shales with thinintercalations of sandy siltstones, frequently shows rain pits and desiccation crackstogether with appreciable amounts of precipitated salts. It therefore indicates aperiod of arid climate during which the trough apparently dried-up frequently. Thisfacies is terminated by a marine ingression which deposited fish-bearing shalesfollowed by flaggy, fine-grained argillaceous sandstones, siltstones, and shales with awealth of sedimentary structures (cross-, current-, and convolute bedding, rippledlamin ations, slu mp folding, etc.); they contain Triassic fauna indicating the return ofbrackish and even fresh-water conditions.During the deposit ion of the Mariakani formation, which conformably overliesthe May i-ya-chum vi bed s, erosion became m ore intensive and a rhyth mic successionof fine to medium grained sandstones and impure shales was deposited, probablyagain under lacustrine conditions. Massive, fine-grained sandstones with distinctmottling prevail in the basal part of the formation, flaggy arkosic sandstones with

usually well developed cross-bedding in the upper part.More pronounced erosion persisted during the deposit ion of the Mazeras formation, which unconformably overlies the Mariakani formation. I t s tarts with coarsegrained, cross-bedded arkoses, with lenses of grits and minor siltstone/shale

7/27/2019 Band_77_151_159

4/9

1 5 4 Alexander D. H o r k e l

N N

1 '

PLATE I

PLIO-PLEISTOCENE

JURASSICCRETACEOUSGROUP

Legend

RECENT | | A l l uv i umg U y C o r a l r e ef('' ''.'l K i l i nd i n i f o rm a t i on[Tpi J M arga r i n i f o rm a t i onF j u = l U p pe r J u r as s ic f o r m a t i o n s ! u n d i f f l| ^ " - | K ib i o ng o n i f o r m a t i o n| i ; I j Kam be f o rm a t i on

DURUMAGROUP( PERMO-TRIASSIC)GEOLOGICAL MAPOF THE

SOUTHERN KENYAN COAST

[ l:W':-.\ M azeras f o rm a t i onj H s 8 l M ar i akan i f o rm a t i on[ I U I L o w lr M a j ' - y a - c h u m v i f o r m a t io n

I y +1 Alka l ine Igneo us rocks-~~~ Volca nic dy ke

Volcanic vent * K i m be r l i t i c d i a t rem eif- Mneral deposits

\ / Syncl ine_ - Fa u l t s and l i n e a m e n t s

Plate 1: Geological map of the Southern Kenyan Coast

7/27/2019 Band_77_151_159

5/9

UTHOLOGY STRATIGRAPHY TECTONICEVENTS M I N ER ALOCCURRENCES

3

POST-REEF DEPOSITSALLUVIUMCORAL REEFKIUNDINI Fm. L I M E S T O N E I S I LI C A S A N D )M A R G A R I N IFm . I GEM. PLACERS

UPPER JURASSICTO NEOCOMIANFORMATIONSKIBIONGONI Fm.KAMBE Fm.

M A Z E R A S F m .

MARIAKANI Fm.UPPER MAJI-YA-LOWER CHUMVI Fm.

ALKALINE IGNEOUSINTRUSIONI AGE UNCERTA IN!

MAJOR FAULTINGAND RIFTINGU P - D O M I N G

I K IMBERL ITIC D IATREMESINIOBIUM / RARE EARTHS

LIMESTONE"

SUBSIDENCEDOWN WARPING

INIT IAL FAULTING