Mineralization In The Migori Greenstone Belt, Macalder, Western Kenya

Abstract

Macalder copper sulphide deposit is located to the western end of the Migori greenstone belt in Kenya at the northern margin of an intensely dislocated metamorphosed Archaean volcanogenic sedimentary sequence known as the Nyanzian system. It is a copper sulphide deposit interpreted as having a metasomatic origin with hydrothermal solutions interacting with and modifying a banded iron formation. Copper, zinc, gold, and silver have all been sporadically mined from the ore bodies in the period from 1935 to 1966.

The ore body occurs as sheets, veins, and lenses located at the contact between metabasalts, banded iron formation and greywackes, and also at fold hinges and along faults and fissures.

The process of ore formation occurred in two distinct stages:

1 The deposition of magnetite occurred during the metamorphism of Nyanzian volcanosedimentary sequences. Parts of this sedimentary succession were iron-rich giving rise to horizons of banded iron formation. 2 At a later stage, sulphide ore formation developed as sheets, veins, and lenses at the stratigraphic boundaries between banded iron formation, sandstone, and metabasalts. The magnetite in the banded iron formation is corroded and represents remobilization of iron-rich horizons by circulating hydrothermal fluids.

The sulphide ores are characterized by four main mineral assemblages: quartz, sphalerite-pyrite, pyrrhotite–chalcopyrite, and carbonate. Each mineral assemblage is characterized by its own paragenetic association. Mineralization occurred in various stages at temperatures between 245–271°C. The presence of native bismuth further confirms the relatively low temperatures of ore formation.