The genesis and characteristics of the gold mineralisation in the area between river Malaba and river Solo in Busia district, South Eastern Uganda.

Abstract

The present study was carried out in Busia district, in eastern Uganda and covered an area of about 100 km2. It involved rock, soil and stream sediment sampling and field observations (e.g. structures, terrain, etc.). The area has two types of gold deposits: quartz vein-hosted deposits and Banded Iron Formation (BIF) -hosted deposits. In the BIF-hosted deposit type, no ore minerals are visible to the naked eye and any gold mineralisation therein is perhaps disseminated.

Thin section studies show that the rocks in the area are metabasalts, quartz diorites, quartz monzodiorites, monzodiorites, monzogranites syenogranites and banded quartzites (BIF). Polished sections of the quartz vein-hosted type were also studied by reflecting microscope and Electron Microprobe (EMP). The main ore minerals are pyrite, magnetite and ilmenite but pyrrhotite, chalcopyrite, covellite, galena and rutile are also present. EMP analyses of ore minerals indicate that gold in the Busia quartz vein deposits occurs as electrum or native gold inclusions in pyrite with fineness in the range of 451 – 863. It also occurs as invisible gold in the sulphide minerals. Pyrrhotite has the highest values of invisible gold (up to 4.33 wt % Au) but its volume proportion is insignificant. Invisible gold in pyrite however may contribute significantly to the total gold budget of the Busia quartz vein deposits since pyrite is abundant. Fluids intruding the basaltic country rocks through NW-trending fractures led to the formation of the quartz veins and the precipitation of the sulphides and the gold. This points to an epigenetic origin for the quartz vein hosted mineralisation. The Buteba granite is unlikely to be the source of the mineralising fluids. Isotope/ REE studies and age determination of the mineralisation need to be done to determine the genesis of the mineralising fluids.

Stream sediment, soil and rock samples were analysed chemically by AAS and XRF methods. Of the three areas proposed for follow-up by Mroz et al. (1991), the Tira area has the first priority soil anomaly (up to 4.8 ppm Au) followed by the Osapiri area (up to 2.8 ppm Au). Stream sediment anomalies are found on rivers Nakola, Tira and Aget. Generally, the stream sediments have higher gold concentrations (up to 38.6 ppm Au) than the soil samples. Lead shows the distribution most similar to that of gold in the rock, soil and stream sediment samples and so is the best indicator element for gold.