Paper Number: 3938
Geology of the Archean Busia-Kakamega Greenstone Belt, Western Kenya
Sharp, T.R.1, Poulsen, H.2 and Tosdal R.3
1Acacia Mining, Kisumu, Kenya, tsharp@acaciamining.com
2Consulting geologist, Ontario, Canada
3Consulting geologist, Folly Beach, SC, USA
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The Busia-Kakamega Greenstone Belt is situated on the northeast extremity of the Lake Victoria Goldfield within the Archaean Tanzania Craton. It is bounded to the south by the Neogene Kavirondo Rift and to the east by the Late Proterozoic East African Orogen. To the west it transitions into the granitoids and gneisses of the Uganda basement complex. Gold was discovered in the belt in the 1930s, which led to a rush and development of several mines. The largest was the Rosterman Mine, which produced 259,000 ounces of gold between 1935 and 1952. No commercial gold mining is currently underway in the belt and the use of modern exploration has been limited.
Acacia Mining is presently undertaking a comprehensive Au exploration programs across 2000 km2 of exploration tenements within the Belt. To assist with targeting, Acacia Mining has completed extensive regional scale lithostratigraphic mapping and lithogeochemistry programs. In addition, selected geochronological samples were collected for U-Pb Shrimp analysis of zircon. This work has allowed the revision of the work completed by the Geological Survey of Kenya in the 1930s-1950s (e.g. [2]; [3]; [4]; [5]; [1]). A new stratigraphic and structural framework has been developed and is presented here.
Busia-Kakamega Greenstone Belt comprises an overall northeastward facing sequence of volcanic and sedimentary rocks intruded by granitoids. The oldest rocks are pre-2750 Ma Samia Hills Group in the west through the 2700-2670 Ma Ndori group and 2670-2660 Ma Yala group, eastward into the 2660 Ma Kavirondo group. The volcanic units, and their intrusive equivalents, include rocks of komatiiic, tholeiitic, calcalkaline and high potassium adakitic composition: the adakitic rocks within the Yala River group are intercalated with sandstone and conglomerate which has been included in the Kavirondo group in the past. The komatiitic suite hosted by the Ndori Group includes high-Mg basalts and ultramafic volcaniclastic rocks. The granitoids can be separated into two groups: an older calcic syn-volcanic suite predating deposition of the Kavirondo group sedimentary rocks and a younger circa 2650 Ma high potassium suite which post-dates the Kavirondo group.
Most of the rocks throughout the Busia-Kakamega Greenstone Belt show remarkably little strain at the outcrop scale. Exceptions are occurrences of mafic volcanic meta-tectonites. Field relations suggest these meta-tectonites are passively intruded by the circa 2670 Ma Assembo granitoid, which provides a minimum age for this early deformation. Rocks of the younger Kavirondo group are locally folded and in places cut by a steep cleavage in outcrops. The inferred circa 2660 Ma age of the Kavirondo group provides a maximum age for this younger cleavage and a clear indication that there are at least two distinctive generations of structures in the belt. ENE faults are inferred across the belt from several lines of evidence. These faults are discordant features, most likely representative of younger deformation. The major structural grain of rocks in the central part of the belt is NE, truncated by the North striking intrusive contacts of the circa 2655 Ma Maragoli and Mumias external batholiths.
References:
[1] Huddleston A (1954) Geological Survey of Kenya Report 28.
[2] Murray-Hughes R (1933) Geological Survey of Kenya Report 3.
[3] Pulfrey W (1946) Mining and Geological Department of Kenya Report 9.
[4] Pulfrey W (1938) Geological Survey of Kenya Report 7.
[5] Pulfrey W (1936) Kenya Colony, Geological Magazine 73: 26-38.