Title: Sub-surface geology, petrology and hydrothermal alteration of Menengai geothermal field, Kenya

Abstract

Menengai is a trachytic central caldera volcano in the Kenya rift valley with
abundant high-temperature geothermal activity. The field is currently in its initial
stages of development for geothermal energy in Kenya following Olkaria and Eburru
fields. Regional surface geology of Menengai is largely composed of late Quaternary
volcanics. The building of a 200,000 year old trachyte shield volcano was followed
by piecemeal subsidence through two paroxysmal eruptions 29,000 and 8,000 B.P
to produce a caldera of about 84 km2 in size that has subsequently been largely filled
by recent trachyte lavas. Twenty four exploration and production wells, some of
them hotter than 390°C, have been drilled in Menengai caldera by Geothermal
Development Company (GDC). The aim of this study is to reveal the evolutionary
history of the Menengai volcano and to describe its igneous lithostratigraphy and
secondary mineralization in order to characterize hydrothermal processes within the
field. Analytical methods used include binocular microscope analysis, petrographic
analysis, X-ray diffractometer analysis and inductively coupled plasma (ICP-ES)
analysis. Studies of drill cutting from wells MW-02, MW-04, MW-06 and MW-07
have provided information on the stratigraphy, hydrothermal alteration and present
state of the geothermal system. Petrochemistry of these wells revealed subsurface
lithostratigraphy that includes, consolidated pyroclastic tuff, pyroclastics, basalt,
trachybasalt, phonolite, trachy-andesite, syenitic intrusive units, while trachyte
constitutes over 90% of the total.
Reaction of geothermal fluids with the host rocks has resulted in a progressive
hydrothermal alteration with increasing depth. A number of studies have recognized
characteristic alteration zones at Menengai based on distribution of key index
alteration minerals. These zones, in order of increasing alteration grade, are zeolitesmectite,
quartz-illite and wollastonite-actinolite zone. The study reveals that
Menengai evolved a complex compositional variation with time as a result of magma
fractionation, possibly in combination with crustal contamination and hydrothermal
activities en route to the surface. The study defines the geology of the Menengai into,
post-caldera, syn-caldera, upper and lower pre-caldera volcanics. Petrography and
mineral chemistry of the basaltic and trachytic end members of the Menengai rocks
indicate that two or more distinct magma types were involved in the formation of the
volcano.
This study reveals that magma exists beneath the caldera floor, at depth slightly over
2000 m within the summit area. This is evidenced by fresh glassy, quenched cuttings
at these depths in wells MW-04 and MW-06. This implies that the thickness of the
geothermal reservoir above the magma-containing domain may be no more than 1.5
km from the hydrostatic surface at ~400 m.