Geology ,mineralogy And Geo Chem1stry Of Fluorite Mineralization In The Kerio Valley And Tugen Escarpment Of Kenya
Aljabri, Saleh S
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Investigations were made on the main geological features of the fluorite mineralisation at Kerio Valley South, Tiati Plains and Tugen Escarpment within the Kenya Rift Valley. One mode of fluorite emplacement in the area is identified as replacement of marble and other favourable horizons in fault zones within the Precambrian rocks of the Mozambique Mobile Belt. The other mode of formation is by open-space filling along fissures within the Mozambique belt, the Tertiary sediments and later •.......•..... ....,. Tertiary basalts, phonoli~es and trachytes. The mineralisation resulted f rorn " hydrothermal fluids. Evidence for the replacement and open-space processes is presented and discussed. The Kerio South deposits are situated in an area of more complex faulting than found elsewhere ln the Elgeyo Escarpment. Aggregate en-echelon faults are found with evidence of rotational faults. Two sets of faults are distinguished. The first set is of those sparsely fluoritised, with large displacement and intruded by alkaline dykes. The second set is brecciated fault zones with massive fluoritisation. At K imwa re r , and (iii) Kamnaon there are indications that the fluorspar veins taper with depth, giving way to unreplaced marbles. strong evidence of post-mineralisation faulting is also found in the Kerio south veins. The veins are segmented and offset by faults perpendicular to the primary major faults. The characteristic vesicular texture of the replacement ore is due to volume loss caused by replacement of calcite by fluorite and also the corrosion of kaolinitised feldspar in the host rock. Crustification of the ore and its drusy nature indicates that rep 1acement was a 1so close 1y accompan ied by open space filling. It is suggested that the Kimwarer East Vein originated in part from quartzofeldspathic gneisses. Its unique texture indicates that fluorite replaced kaolin but keeping a relic of the original Precambrian foliation. Mineralisation is lithologi~~lly controlled at the contact of Precambr ian gne isses and Miocene sed iments along the Tugen Escarpment. The sediments were relatively impervious to the ascending mineralising fluids. This is also probably the case for the Tiati depos its if it is assumed that the Tu rkana gr its once covered the entire plain. Throughout the area, the mineral isation consists largely of fluorite deposition followed by silicification. Other minerals are rare except for baryte in Tiati. The mineral isation process was accompanied by argillic wallrock alteration. An acidic environment of the mineralising fluids is inferred. This arose from the CO 2 re 1eased by the rep 1acement process of ca 1cite by fluorite. Sr and Yare enr iched in the fluorspar ve ins as they partially sUbstitute for Ca in the fluorite crystal lattice. Uranium and thorium of hydrothermal origin occur in close association with purple fluorspar breccia at Kerio South. The vi rtua 1 absence of U and Th in other fluorite varieties is notable. Secondary U and Th is found in highly localised, isolated sites in Kimwarer sediments. This was probably mobilised from the primary source by groundwaters and localised under reducing conditions. Samples of both primary and secondary U and Th rich rocks show a distinct enrichment of K, Rb, Y, Sr, Zr, Ti and Nb. -....- ~ In t~e secondary radioactive deposits, also Mo, V and Pb are ~fbund in addition. Fluorine was apparently brought from depths by hydrothermal fluids along fault zones accompanied by Zr, Nb, Y, U, Th and 8. An apparent close association of fluoritisation with high potassic dykes is shown, suggesting a possible common alkaline magma source. Guides for further exploration of fluorspar in the Rift Valley region are pre-Pleistocene faults of moderate displacement, silicified breccia zones, faulted marb 1e and quartz-fe 1dspar hor izons, fau 1ts in gne isses ( v) capped by Miocene sediments, intrusion of high K alkaline dykes, hydrothermally altered zones and areas of radiometric anomaly. Exploration target areas are selected for fluorite and uranium in the area of study .