Stratigraphy, hydrothermal alteration, formation temperature and petrology of the Domes area, greater Olkaria volcanic complex

Abstract

Wells OW-905A, OW-910 and OW-917 have been drilled in the Domes area of the Olkaria geothermal field. Well OW-905A has been drilled to the west while OW-910 in the central part of the Domes field inside the caldera. OW-917 has been drilled on the eastern margins of the ring structure. Two wells, OW-905A and OW-910 reveal seven stratigraphic units; basalt, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, rhyolite and tuff. Well OW-917 cuts across two stratigraphic units; rhyolite and trachyte. Syenitic, basaltic and micro-syenitic intrusives are also encountered in the three wells. The meausred formation temperature is consistent with the abundance and depth distribution of alteration mineral assemblages. Well OW-910 has high abundance of high temperature alteration minerals, OW-905A has moderate abundance while OW- 917 has low abundance of these minerals. Temparature of 225°C is recorded at shallower depth (1200 m.a.s.l) in well OW-910. This is consistent with the the location of the upflow zone and the heat source in Domes area. In OW-905A, 225°C temperature is recorded at 600 m.a.s.l. This well is located in a recharge zone. Well OW-917 has 225°C temperature recorded at 200 m.a.s.l. This well is located in a down-flow zone. High permeability is observed in wells OW-905A and OW-910 while well OW-917 shows poor permeability. The basalt-trachyte suite in the Greater Olkaria Volcanic Complex (GOVC) have been generated through fractional crystallisation and magma mixing. Rhyolites were mainly generated through fractionation of trachytes and partial melting of syenites. The GOVC plumbing system is composed of independent discrete magma chambers and conduits. In these chambers, magma underwent modification before being erupted. The GOVC has had different episodes of eruptions.