| dc.description.abstract | 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.
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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 . | en |
