Mineralogy and Geochemistry of the Asanje Iron Ore Deposit within the Mayamaya-Hombolo Belt, Dodoma Region, Central Tanzania

This paper presents the mineralogy and chemical composition including iron ore impurities to ascertain its suitability for industrial applications. The Asanje iron ore deposit is hosted in Precambrian volcano-sedimentary rocks in the Mayamaya-Hombolo Belt in Dodoma region, within the Lake Nyanza Superterrane. Iron ores are found in two parallel ridges trending NW-SE. Ridge I occurs as vein and banded hematite-type ore and Ridge II as massive-type ore. A total of 24 ore samples were analysed for major and trace elements by XRF and petrographic studies by optical microscopy and XRD. The XRD results revealed that hematite and goethite are the main components and quartz is the gangue mineral in the iron ores. The XRF data shows the Fe2O3t content ranges from 20.8 to 87.3 wt% with an average of 52.7 wt%. The average concentrations in wt% of impurities such as SiO2, Al2O3, P2O5 and S are 37.9, 0.9, 1.2, and 0.2, respectively. By comparison, Ridge II has greater iron content (30.5–87.3 = Avg. 64.66 wt% Fe2O3t) than Ridge I (20.8–78.22 = Avg. 48.68 wt% Fe2O3t). Based on the chemical composition, the quality of iron ore is categorised as low to medium grade, and can be used in metallurgical and cement industries. Keywords:    Mineralogy, Geochemistry, Asanje Iron ore deposit, Mayamaya-Hombolo Belt, Tanzani

Mineralogical and geochemical characteristics of graphite-bearing rocks at Chenjere Area, south-eastern Tanzania: Implications for the nature and quality of graphite mineralization

This study focused on the mineralogical and geochemical characteristics of graphite-bearing rocks at Chenjere area by conducting surface geological mapping, geochemical and petrographical analyses with the intention of characterizing the nature of graphite occurrence in these lithological units. This paper presents results of the flake size, grade and mineralization extension of graphite in the graphitic gneiss. Field observations, petrographic investigation and comparison with other studies indicate that graphite occurrence at the Chenjere area is of sedimentary origin which fall under syngenetic type. The petrographic study revealed that nature of metamorphism of rocks in the Chenjere area is of high grade (amphibolite facies). Both field observations and petrographic studies indicate that minerals in the rock associated with graphite include quartz, feldspar (mostly K-feldspar) and biotite. Further, the mineralized zones are concordant to the rest of the lithologies of the area and biotite gneiss is forming the hangingwall and footwall. The rocks’ foliation is generally NE striking and dipping SE with the dip amount ranging from 30 to 60 degrees. The graphite mineralization at the study area occurs as medium to coarse grained crystalline, flake type graphite with long axis of up to 1000 micrometres in size. Graphite flakes are disseminated and oriented in the host rock that represents a normal metamorphic fabric. Geochemical results indicate that graphite contents in the host graphitic gneiss range from 3.03 wt.% to 16.00 wt.%. Mineralogy and texture of the graphite at Chenjere area meet the standards required for industrial applications in various advanced technologies. Keywords: Graphite Mineralization, Flake Graphite, Chenjere Are

Probabilistic seismic hazard analysis for Northern Tanzania Divergence Region and the adjoining areas

This paper presents the seismic hazard levels for the Northern Tanzania Divergence (NTD) and adjoining areas by using area seismic source zones. The 15 source zones were considered based on the major geological and tectonic features, faulting style, and seismicity trends. For each source, earthquake recurrence parameters were computed by using the earthquake catalogue with events compiled from 1956 to 2011. The peak ground accelerations (PGA) and spectral accelerations (SA) at 0.2 and 2.0 second, respectively, were computed for a 10% probability of exceedance in 50 years at sites defined by a 0.1° x 0.1° grid. The recurrence parameters of 15 zones and attenuation relations developed by Akkar et al. (2014) and Chiou and Youngs (2014) were integrated into a logic tree. Obtained results that are presented as hazard maps show strong spatial variations ranging from 60 to 330 cm/s/s for PGA, from 100 to 650 cm/s/s at 0.2 sec and from 6 to 27 cm/s/s at 2 sec for 475 years mean return period and 5% damping. Hazard levels depict the general tectonic setting of the study area with the western (Eyasi-Wembere) and central (Natron-Manyara-Balangida) rift segments having relatively high PGA values compared with the eastern Pangani rift. This work provides indications of seismic hazards to policymakers and planners during planning and guidelines for earthquake-resistant design engineers. Keywords: Homogeneous Earthquakes Catalogue; GMPE; PSHA; NT

Geology of part of the eastern margin of the Tanzanian Craton in the Mpwapwa area and its relationships with an off-craton, high-grade supracrustal gneiss sequence (Mpwapwa Group) of possible Palaeoproterozoic age

A number of traverses have been undertaken across a ca. 45 km section of the north-south oriented eastern margin of the Tanzanian Craton between Dodoma and Mpwapwa, Central Tanzania. The boundary is a SE-dipping zone of high strain between about 1 and 2 km wide. The rocks of the eastern craton are uniformly composed of coarse-grained grey granodioritic, migmatised orthogneisses which are heterogeneous at outcrop scale, but are regionally homogeneous. The orthogneisses have no regionally consistent fabric and foliations are variably oriented at outcrop scale. However, there is a gradual increase in strain eastwards towards the edge of the craton, manifest as an increasingly strong, regionally consistent, SE-dipping foliation. This strain increase eventually leads to mylonitic and porphyroclastic planar fabrics and strong, uniformly SE-plunging, linear fabrics. The kinematics of the high-strain mylonites show a consistent top-to-the-NW sense of movement. The frontal thrust zone grades laterally into steep sinistral and dextral oblique strike-slip shear zones to the north and south respectively. The contact is a single wide thrust zone in the north and south section of the studied area, with an imbricate belt in the central part. To the east, the cratonic rocks are in contact with a high-grade supracrustal succession, termed here the Mpwapwa Group in the light of uncertain regional correlations (= “Isimani Suite”?). It consists of a thick sequence of leucocratic quartzo-feldspathic gneisses and migmatites, semi-pelitic two mica-kyanite-garnet schists/gneisses, quartzites, marbles and calc-silicate rocks and abundant metabasic layers. There appears to be an east-west zonation of Mpwapwa Group lithological units, with most of the quartzites, calcareous rocks and pelitic schists/gneisses tending to occur close to the craton margin, with semi-pelitic gneiss/migmatite to the east, along with interlayered repetitions of bimodal acid, quartzo-feldspathic leucogneisses and mafic gneisses (amphibolite, mafic garnet amphibolite). Mineral assemblages, as evidenced by garnet-kyanite in pelitic rocks, garnet-clinopyroxene in some metabasites suggest metamorphism under moderate to high pressure amphibolite facies, as might be expected at the base of a thrust stack and resulting crustal thickening. Possibly, therefore, the Mpwapwa Group was deposited in a rifted passive-margin setting at the edge of the Tanzanian Craton, with shallow marine environments at the immediate continental margin and bimodal volcanic rocks more distally. During collision orogeny, thrusting took place at this rifted margin, inverted the Mpwapwa Group basin and transported the supracrustal rocks over the craton margin, an event which telescoped, but did not obliterate the original depositional zonation. The group may thus be viewed as a parauthochthonous succession. The rocks were intruded by plutons of largely unfoliated biotite granite, two-pyroxene charnockite and tonalite, the ages of which are unknown. With the above hypothesis and uncertainties in mind, a suite of samples are undergoing U-Pb zircon dating in order to constrain the timing of these events

Lithospheric modification by extension and magmatism at the craton-orogenic boundary: North Tanzania Divergence, East Africa

We present a joint analysis of newly acquired gravity and teleseismic data in the North Tanzanian Divergence, where the lithospheric break-up is at its earliest stage. The impact of a mantle upwelling in more mature branches of the East African Rift has been extensively studied at a lithospheric scale. However, few studies have been completed that relate the deep-seated mantle anomaly detected in broad regional seismic tomography with the surface deformation observed in the thick Archaean Pan-African suture zone located in North Tanzania. Our joint inversion closes the gap between local and regional geophysical studies, providing velocity and density structures from the surface down to ca. 250 km depth with new details. Our results support the idea of a broad mantle upwelling rising up to the lithosphere and creating a thermal modification along its path. However, our study clearly presents an increasing amplitude of the associated anomaly both in velocity and density above 200 km depth, which cannot be solely explained by a temperature rise. We infer from our images the combined impact of melt (2-3 per cent), composition and hydration that accompany the modification of a thick heterogenous cratonic lithosphere are a response to the hot mantle rising. The detailed images we obtained in density and velocity assert that Archaean and Proterozoic units interact with the mantle upwelling to restrict the lithosphere modifications within the Magadi-Natron-Manyara rift arm. The composition and hydration variations associated with those units equilibrate the thermal erosion of the craton root and allow for its stability between 100 and 200 km depth. Above 80 km depth, the crustal part is strongly affected by intruding bodies (melt and gas) which produces large negative anomalies in both velocity and density beneath the main magmatic centres. In addition to the global impact of a superplume, the velocity and density anomaly pattern suggests a 3-D distribution of the crust and mantle lithospheric stretching, which is likely to be controlled by inherited fabrics and enhanced by lateral compositional and hydration variations at the Tanzanian craton-orogenic belt boundary

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

International audienceRifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15–18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE–SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8–9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup

Fault-magma interactions during early continental rifting: Seismicity of the Magadi-Natron-Manyara basins, Africa

International audienceAlthough magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2-D studies. We analyze seismicity data from a 13 month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3-D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87±0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early-stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies