Presentdayoredeposition in the geothermalsystems of kenya—i. the mwananyamalahotsprings, northeast of jombohill, coastprovince

Abstract

The Mwananyamalageothermal field extends over an area of 4 km x 11 km in a region of faulted Permo—Triassic sandstones. The sandstones consist of deltaic arkoses, and fluvio-lacustrine shales, siltstones and sandstones. Alkaline intrusives of Upper Cretaeous age outcrop 3–10 km from the hotsprings. Monchiquite dykes are associated with two of the four hotsprings in the area.

The hotsprings discharge slightly alkaline NaC1-NaHC03 waters. Maximum surface discharge temperatures are 56–76°C. Calculated reservoir temperatures are 172–180°C. Maximum heavy metal concentrations in the discharge waters are 1.3 ppm Pb, 0.2 ppm Zn, 0.2 ppm Cu and 2.2 ppm Fe. Massive pyrite is currently being deposited at or near the surface at three of the hotsprings. The pyrite contains up to 2% Ag, 0.9% La, 30 ppm Pb, 5 ppm Zn and 4 ppm Cu.

At one of the hotspring localities, malachite is ubiquitous. The malachite-bearing rocks contain, on average, 2.4% Cu and 2.2% Pb.