Geochemistry of the Potassic Basalts from the Bufumbira Volcanic Field in Southwestern Uganda

Bufumbira volcanic field is the southernmost of the four Ugandan small Pleistocene to Recent volcanic fields within the western branch of the East African rift system. The rocks consist of silica undersaturated and vesicular basalts with numerous primary structures. The rocks consist of basanites, leucitites, olivine basalts, trachytes, tephrites, trachyandesites and andesites. The basalts are picritic in the northern part of the field where they are dominated by olivine and are clinopyroxene rich in the southern part of the field. Leucite and plagioclase are common in the groundmass in varying proportions by volume for the entire field. Type 1 dunite and wehrlite upper mantle xenoliths characterize the northern part of the field whereas type II clinopyroxenite xenoliths are common in the southern part of the field. The various basalts are low in SiO2 wt %, Al2O3 wt % and Na2O wt % but high in MgO wt %, TiO2 wt %, CaO wt %, K2O wt % with K2O/Na2O = 1.08 to 2.07. These are potassic belonging to the kamafugite series. Plots discriminate two geochemical trends corresponding to the picritic and clinopyroxene rich basalts. The diagram of Na2O + K2O wt. % against SiO2 wt. % enables various rocks to plot in the designated fields for the different alkaline basalts. The field is enriched in trace, light rare earth (LREE) and high field trace elements (HFSE) where La/Yb = 31 – 55. The petrographic and geochemical studies elucidate enrichment of the upper mantle by both mineralogical (modal) and cryptic (geochemical) metasomatism.Keywords: Basalts, Enrichment, Mantle, Metasomatism, PotassicTanz. J. Sci. Vol 36 201

Impact of industrial activities on heavy metal loading and their physico-chemical effects on wetlands of lake Victoria basin (Uganda)

The diverse functions of wetlands are being adversely affected by human activities. This paper discusses the impact of these activities on heavy metal loading in different media within the wetlands. Water and sediment/soil samples were taken from areas with active industrial activities and from an area where there is no industrial activity. Sources of water pollution include effluents from a brewery (high pH) and areas associated with tanning activities, sewage treatment plant andformer copper smelter (high electric conductivity values). Effluents from a battery assembly plant, water treatment plant, pharmaceutical industry and former copper smelter have relatively high Pb values (up to 1.4 ppm) otherwise most heavy metal concentrations are below maximum acceptable limits for water. This calls for mitigation measures. In sediments, high heavy metal values were associated with battery and metal fabricating industry (Pb), operations involving Zn scrap (Cd), former Cu smelter (Cu, Pb, Ni and Co), tannery and pharmaceutical industry (Cr), and soap and cosmetics industry (Hg). Fish have low levels of Hg. However, fish is known to bioaccumulate Hgthrough methylation, may cause increase in the Hg in the food chain. Industries which release effluents with high heavy metal contents should treat them before discharge

The potential for gold mineralisation in the Greenstone belt of Busia district, south eastern Uganda

Busia district in south eastern Uganda is one of the areas in the country with a history of potential for gold mineralisation and exploitation. Gold was first discovered in the Archaean greenstone belt of Busia by Davies in 1932. Only small mining operations, mostly artisan in character, have taken place in the district since then except the newly opened mines by Busitema Mining Company. However, all recent work in the area indicates potential for gold exploitation. Both quartz vein-hosted and Banded Iron Formation (BIF)-hosted gold deposits occur in Busia district. The results of Electron Microprobe (EMP) analysis of 10 rock samples from the quartz vein deposits and AAS analysis of 10 rock, 27 soil and 18 stream sediment samples taken from Busia district are presented in this work.Gold occurs as electrum or native gold inclusions in pyrite with fineness in the range of 451 – 863; and as invisible gold in sulphide minerals (up to 5700 ppm in pyrite). The main ore minerals are pyrite, magnetite, ilmenite, galena, pyrrhotite, chalcopyrite, covellite and rutile. Gold values of the stream sediment samples average 7.7 ppm Au, the highest being 38.6 ppm Au. Soil sampling was done in the three areas proposed for follow-up by Mroz et al. (1991), of which 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). There is a strong correlation between the distribution of lead and that of gold.Key words: Busia district, Archaean greenstone belt, quartz vein-hosted gold deposits, banded iron formation-hosted deposits, sulphide

Patterns and origin of igneous activity around the Tanzanian craton

Tertiary and later igneous activity is common on and around the Tanzanian craton, with primitive magma compositions ranging from kimberlites and varieties of picrites through nephelinites, basanites and alkali basalts. This review focuses on elucidating the conditions of origin of the melts, addressing the question of the state and involvement of the Tanzanian cratonic lithosphere in magma genesis. The Tanzanian craton is anomalous with a surface elevation of >1100. m reflecting buoyancy supported by a subcratonic plume whose effects are seen in the volcanics of both western and eastern rift branches. Magmatism on the craton and at its edge has high K/Na and primitive melts show fractionation dominated by olivine. Slightly further from the craton pyroxene fractionation dominates and K/Na ratios in the magmas are lower. Off-craton melts are nephelinites, basanites and alkali basalts with low K/Na. Potassium enrichment in the melts correlates with the occurrence of phlogopite in mantle-derived xenoliths, and also with carbonate in the magmas. This is attributed to melting at >140. km depths of mixed source regions containing phlogopite pyroxenite and peridotite, whereby the carbonate is derived from oxidation of diamonds concentrated near the base of the cratonic lithosphere. Mixed source regions are required by arrays of radiogenic isotopes such as Os and Sr in the volcanic rocks. The temporal progression of lamproites to phlogopite. +. carbonate-rich rocks to melilitites, nephelinites and alkali basalts seen during the erosion of the North Atlantic craton are seen around the Tanzanian craton as the coeval occurrence kimberlites, kamafugites and related rocks, nephelinites and alkali basalts showing spatial instead of temporal variation. This is due to the different stages of development of rifting around the craton: in northwestern Uganda and northern Tanzania, K-rich volcanism occurs at the craton edge, whereas nephelinites, basanites and alkali basalts occur where shallower level melting occurs in well-developed rifts. The presence of pyroxenite in the melt sources in the rifts is seen as high Ni in volcanics with intermediate Mg#, and melting of underplated alkali basaltic material may explain the production of voluminous flood phonolites in the eastern rift branch.18 page(s

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U-Pb age determinations (2637-2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age

Geochemical Constraints on the Origin of Primitive Potassic Lavas in the Eastern Virunga Volcanic Province

Abstract Young mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data from monogenetic cinder cones near Bufumbira, Uganda, in the Virunga Volcanic Field: 87Sr/86Sr = 0.7059–0.7079, εNd = −6.5 to −1.3, εHf = −6.3 to +0.9, 208Pb/204Pb = 40.1–40.7, 207Pb/204Pb = 15.68–15.75, and 206Pb/204Pb = 19.27–19.45. Olivine phenocrysts from the Bufumbira lavas have 3He/4He = 6.0–7.4 RA. The isotopic data, in conjunction with major and trace element systematics, indicate that primitive Bufumbira magmas are derived from two different metasomatized lithospheric source domains. Melts generated by lower degrees of melting record greater contributions from ∼1 to 2 Ga isotopically enriched garnet‐amphibole‐phlogopite pyroxenite veins within the lithosphere. As melting progresses, these vein melts become increasingly diluted by melts that originate near the lithosphere/asthenosphere boundary, shifting the isotopic compositions toward the common lithospheric mantle (CLM) proposed by Furman and Graham (1999, https://doi.org/10.1016/s0024-4937(99)00031-6). This ∼450–500 Ma source domain appears to underlie all Western Rift volcanic provinces and is characterized by 87Sr/86Sr ∼ 0.705, εNd ∼ 0, εHf ∼ +1 to +3, 206Pb/204Pb ∼ 19.0–19.2, 208Pb/204Pb ∼ 39.7, and 3He/4He ∼ 7 RA. Basal portions of the dense subcontinental lithospheric mantle may become gravitationally unstable and founder into underlying warmer asthenosphere, exposing surfaces where melting of locally heterogeneous veins produces small‐volume, alkaline mafic melts. Mafic lavas from all Western Rift volcanic provinces record mixing between the CLM and locally variable metasomatized source domains, suggesting this style of melt generation is fundamental to the development of magma‐poor rifts