Sukumaland Greenstone Belt Research Articles

Refold structures of the Archaean Nyanzaga gold Project, Sukumaland Greenstone Belt, Tanzania: Precursors to the gold-bearing fault system

This study presents a complete investigation into the structural-hydrothermal evolution of the Nyanzaga and Kilimani gold deposits within the Neoarchean Sukumaland Greenstone Belt, Tanzania Craton. The first compressional stage, D1a, creates upright E-W folds in response to N-S-oriented shortening. The second stage, D1b, refolds the previously formed structures during ongoing compression/transpression under N-S-oriented shortening. Due to the brittle nature of late deformation, during D1c, a set of faults emerges, with the NW-SE ones exhibit dextral shearing and control the formation of normal N-S vertical faults.A protracted hydrothermal history is recorded, beginning with a disseminated silicate minerals- and pyrite-rich, gold-barren stage associated with the D1a and D1b deformations, followed by the development of a gold-endowed vein system during progressive D1b and D1c stages. Gold was mainly concentrated within vertical NW-SE (Kilimani) and N-S (Nyanzaga) faults. Our findings challenge traditional polyphase deformation models presented in the other gold deposits of the area, by proposing a progressive and continuous deformation-hydrothermal history. In addition, we highlight the influence of refold structures on fault geometry, which is crucial in concentrating gold mineralization under continuous N-S compression and transpression.

Petrogenesis and tectonic setting of TTG rocks from the Katoro area, Sukumaland Greenstone Belt, NW Tanzania: Evidence from zircon U-Pb geochronology, Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes

The northern part of the Tanzania Craton is mainly comprised of several low-grade metamorphic granite-greenstone belts that are exposed on the east and south shores of Lake Victoria. Due to their gold-endowment, these greenstone belts have been the main focus of research and were interpreted to form in various tectonic settings involving back-arc basin, island arc or continental arc, and oceanic plateau. However, the TTG gneiss, as the dominant component of Archean crust, has received far less attention from the scrutiny of modern analytical methods than its counterpart, the mafic volcanics of the greenstone belts. Here, we present a comprehensive geochemical dataset including zircon U-Pb age, zircon Lu-Hf isotope results, as well as bulk-rock geochemical compositions and Sm-Nd isotopic analyses for TTG suites from the Katoro area in northwestern part of Tanzania. Zircon U-Pb dating for three TTG samples yielded 207Pb/206Pb ages of 2688 ± 19 Ma, 2717 ± 19 Ma and 2695 ± 15 Ma, respectively, and the results fall within the peak crustal growth period in northern part of the Tanzania Craton. The Katoro TTGs have relatively high contents of SiO2 (70.77–74.69 %) and Na2O (4.74–5.16 %), high ratios of Sr/Y (59.49–167.25) and (La/Yb)N (17.27–50.72), but low MgO (0.24–0.70 %) and K2O (1.71–3.26 %) contents, low Nb/Ta ratios (8.50–16.60), and most of them belong to the trondhjemitic series rocks. They are characterized by strong enrichment of LILEs and LREEs with slightly positive Eu anomalies (Eu/Eu*=1.05–1.38), but depleted in Nb-Ta and Ti, as well as heavy REEs and Y. They have positive εHf(t) values (average εHf(t) = +1.90 ± 0.50 or + 2.07 ± 0.33) and εNd(t) values (+1.19 to + 2.40) but lower than the contemporaneous depleted mantle, which is inconsistent with a subducting slab signature, and it is proposed that the Katoro TTGs most likely originated from partial melting of rutile-free, garnet- and amphibole-bearing lower mafic crust of an oceanic plateau. The results presented here suggest that the tectonic evolution in this part of Tanzania Craton needs to invoke a scenario in which crustal growth caused by subduction-related modification of an oceanic plateau.

Identification of hydrothermal paleofluid pathways, the pathfinders in the exploration of mineral deposits: A case study from the Sukumaland Greenstone Belt, Lake Victoria Gold Field, Tanzania

Hydrothermal fluids play a key role in the process of metalliferous mineralization such as gold deposits. The modern exploration indicators for such deposits are tectonic structures and characteristic alteration minerals observed as detectable halos adjacent to mineral deposits. Tectonic fractures are the conduits to these hydrothermal fluids and thus control the spatial locations for the formation of mineral deposits. Along crustal structures, hydrothermal fluids commonly induce mineral alteration in the adjacent wall rocks depending on the physical–chemical conditions. These alteration patterns, which are the pathfinders for the proxies in the modern mineral exploration, can be detected by innovative application of combined remote sensing techniques. The study area has experienced intense tectonic deformations, which resulted to two major sets of structures, the NW–SE and NE–SW-trending structures. The knowledge-based analysis applied to SRTM data was useful in identifying crustal lineaments, which the above two set of structures, truncating lithological units of the Sukumaland Greenstone Belt were identified. The Feature Oriented Principal Component Selection (FPCS) together with the GIS functions applied to Landsat 7ETM+ data, were useful to enhance signals from hydrothermal alteration minerals. Results have revealed that the Sukumaland Greenstone Belt is intensively fractured, in a systematic pattern, and has apparently been “injected” with large volumes of hydrothermal fluids. Both processes together have resulted in the systematic and structurally controlled hydrothermal alteration patterns. In this study linear alteration patches are interpreted to represent the hydrothermal paleofluid pathways. Alteration patches coincide spatially with regional and local tectonic structures and are consistent with major gold occurrences and gold mines. This study indicate that careful analysis of SRTM and Landsat ETM+ data can identify crustal lineaments, the likely hydrothermal paleofluid conduits, which may lead to the discovery of potential ore deposits.

Structural analysis, metamorphism, and geochemistry of the Archean granitoids-greenstones of the Sukumaland Greenstone Belt around Geita Hills, Northern Tanzania

Greenstone rocks, which include Banded Iron Formations (BIFs), tuffs, volcanic flows (basalt, andesite and rhyolite), and clastic sedimentary rocks (shale-mudstone, greywacke-sandstone and conglomerate), crop out around Geita Hills and are flanked by granites and granodiorites. BIFs and tuffs occupy larger area than other lithological units, which crop out as patches. Structural analysis indicates that layers of green-stone rocks are folded and display a regional fold axis with an attitude of 320o/40o. Low-grade metamorphic mineral assemblages (actinolite-epidote-chlorite in basalts and muscovite-epidote-chlorite in granitoids) are common in these rocks; this indicates a regional metamorphism at greenschist facies. However, BIFs and basalts are locally metamorphosed to epidote-amphibolite and amphibolite facies. Basalts belong to the tholeiite series whereas granites, diorites and rhyolites belong to the calcalkaline series. Chondrite normalized rare earth element pattern of basalt is flat and plot slightly below the average N-MORB values suggesting the enrichment of the light rare earth elements, which means that mantle magma source was an E-MORB. Granitoids and rhyolites have strong affinities to the continental arc source magma displaying strong enrichments in the LREEs with (La/Sm)N values ranging between 2.53 and 3.95 in rhyolites and between 4.08 and 5.40 in granitoids. The granitoids are classified as the I-type synorogenic metaluminous granites and granodiorites. Geochemical signatures suggest that the Geita Hills basalts erupted at the enriched mid ocean ridge setting of the back arc setting, and the granites, granodiorite and rhyolite formed in a volcanic arc setting particularly the continental arc.

The origin of late archaean granitoids in the Sukumaland greenstone belt of Northern Tanzania: geochemical and isotopic constraints

Granitoids intruding the late Archaean sequences of the Sukumaland Greenstone Belt of northern Tanzania belong to two distinct geochemical suites. Suite 1 is characterised by Na2O/K2O > 1 (1.04 – 4.67), high Sr/Y (56 – 204) and Ba/Rb ratios (6.1 – 27.1) and low Rb/Sr ratios (0.08 - 0.25). The rocks are enriched in Sr (405 – 1264 ppm) and depleted in Yb (0.17 – 0.93 ppm) and Rb (56 – 132 ppm). On chondrite-normalised REE diagrams, the rocks display highly fractionated patterns characterised by relative LREE enrichment ((La/Yb)N = 23 – 128 and (Gd/Yb)N = 3.10 – 8.54) and lower concentrations of the HREE (YbN = 0.80 – 4.45). On primitive mantle-normalised spidergrams, Nb and Ti, together with P and Y are depleted relative to adjacent elements. The major and trace element characteristics of Suite 1 are comparable to those of typical Archaean TTG suites and High Silica Adakites (HSA). Suite 2 granitoids are characterised by Na2O/K2O Tanzania Journal of Science Vol. 32 (1) 2006: pp. 75-88

Geochemistry of the Neoarchaean mafic volcanic rocks of the Geita area, NW Tanzania: Implications for stratigraphical relationships in the Sukumaland greenstone belt

Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at ∼2823 Ma. These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.

Ion microprobe zircon U–Pb dating of the late Archaean metavolcanics and associated granites of the Musoma-Mara Greenstone Belt, Northeast Tanzania: Implications for the geological evolution of the Tanzania Craton

Ion microprobe zircon U–Pb ages from metavolcanic and associated granitic rocks of the late Archaean Musoma-Mara Greenstone Belt (MMGB) of northeast Tanzania reveal that the oldest mafic volcanism in the belt occurred at 2676–2669 Ma followed by felsic volcanism at ∼2668 Ma. The felsic volcanism was coeval with the emplacement of the oldest pulse of massive granitoids that is dated at 2668 Ma. The youngest volcanic episode, represented by a volcanic horizon in the largely sedimentary Kavirondian Supergroup that overlies the greenstone sequence with a marked unconformity, occurred at ∼2667 Ma. A younger phase of post-orogenic granites concluded the magmatic evolution of the MMGB at ∼2649 Ma. Our age data suggests that the entire volcano-sedimentary sequence in MMGB was emplaced in a relatively short time interval between ∼2676 and ∼2667 Ma. It also shows that contrary to arguments based on the degree of deformation, the foliated granites and some amphibolite rafts enclosed in them do not constitute the basement to the greenstone sequence. The data further shows that volcanism in the MMGB was younger than the ∼2820 Ma age of volcanism in the Sukumaland Greenstone Belt (SGB) to the far southwest and the ∼2720 Ma age of volcanism in the nearby Kilimafedha Greenstone Belt (KGB) to the south. The age of granitic magmatism (ca. 2.69–2.55 Ga) in the three belts was, however, largely coeval. Granitic magmatism of this age has also been reported in different parts of the Tanzania Craton suggesting that it was responsible for the late Archaean crustal growth and marks the beginning of a period of stability (or of cratonization).

Nd isotopic, petrologic and geochemical investigation of the Tulawaka East gold deposit, Tanzanian Craton

New isotopic (Sm–Nd) and geochemical data are presented for volcano-sedimentary and granitic lithotypes of the Tulawaka East gold deposit in the Late Archean Sukumaland Greenstone Belt of the Tanzanian Craton. The data help to characterize the geological environment of the greenstone belt/gold deposit and the crustal evolution of the Tanzanian Shield. The volcano-sedimentary units were intruded by leucogranites and aplitic dykes that were deformed and metamorphosed to amphibolite facies grades. The Au mineralization occurs in thrust faults at the contact between the aplite dykes and the volcano-sedimentary units. The metavolcanic rocks are largely basaltic to intermediate in composition with negative Nb anomalies and flat rare earth element (REE) profiles varying from 10 to 100× chondritic values. Initial ɛNd 2.8 Ga values vary from +0.9 to +5.3 and Nd model ages ( T dm) range from 2.8 to 3.0 Ga. The metasedimentary rocks are generally enriched in light REE (100× chondrite) with weak negative Eu anomalies and ɛNd 2.8 Ga values vary from 0.0 to +2.6. The leucogranites are garnet-rich and peraluminous with strong negative anomalies in Eu and Ti and with flat REE profiles (10–100× chondrite). The aplite dyke is also peraluminous with similar negative Eu and Ti anomalies and a flat light REE, but is depleted in heavy REE. The similarity in composition with the leucogranite suggests that the two are cogenetic and formed by fractionation of a more mafic parental magma. The combination of the flat REE element patterns and negative Nb anomalies among the mafic volcanic rocks suggests formation as an immature (island?) arc at ca. 2.8 Ga. However, Nd isotope signatures in the metasedimentary rocks provide evidence of an older crustal sedimentary provenance (ca. 3.0–3.1 Ga) and proximity of a continental influence. The age of the leucogranite and aplite dyke are constrained by a Sm–Nd garnet-whole-rock age of ca. 2.5 Ga, indicating that the shear zone-hosted gold mineralization is younger than 2.5 Ga. The range of isotopic compositions found in the Tanzanian Shield overlap with those of the Zimbabwe Craton to the south.

Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland Greenstone Belt, northwestern Tanzania

The late Archaean volcanic rocks of the Rwamagaza area in the Sukumaland Greenstone Belt consists of basalts and basaltic andesites associated with volumetrically minor rhyodacites and rhyolites. Most basalts and basaltic andesites yield nearly flat patterns (La/Sm CN = 0.89–1.34) indicating derivation by partial melting of the mantle at relatively low pressure outside the garnet stability field. On primitive mantle normalized trace element diagrams, the basalts and basaltic andesites can be subdivided into two groups. The first group is characterised by moderately negative Nb anomalies (Nb/La pm = 0.51–0.73, mean = 0.61 ± 0.08) with slight enrichment of LREE relative to both Th and HREE. The second group is characterised by nearly flat patterns with no Nb anomalies (Nb/La pm = 0.77 ± 0.39). The observed Nb and Th anomalies in the Rwamagaza basalts and basaltic andesites, cannot be explained by alteration, crustal contamination or melt–solid equilibria. Rather, the anomalies are interpreted, on the basis of Nb–Th–La–Ce systematics, as having formed by partial melting of a heterogeneous mantle consisting of variable mixtures of components derived from two distinct sources. These sources are depleted mantle similar to that generating modern MORB and an LREE-enriched and HFSE-depleted source similar to that feeding volcanism along modern convergent margins. The rhyolites are characterised by high Na 2O/K 2O ratios (>1) and Al 2O 3 (>15 wt.%), low HREE contents (Yb = 0.24–0.68 ppm) leading to highly fractionated REE patterns (La/Yb CN = 18.4–54.7) and large negative Nb anomalies (Nb/La pm = 0.11–0.20), characteristics that are typical of Cenozoic adakites and Archaean TTG which form by partial melting of the hydrated basaltic crust at pressures high enough to stabilize garnet ± amphibole. The Rwamagaza basalts and basaltic andesites are geochemically analogous to the Phanerozoic Mariana Trough Back Arc Basin Basalts and the overall geochemical diversity of Rwamagaza volcanic rocks is interpreted in terms of a geodynamic model involving the interaction of a depleted mantle, a melting subducting oceanic slab in a back arc setting.

Dating basaltic volcanism in the Neoarchaean Sukumaland Greenstone Belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton

Abstract Metabasalts from the Sukumaland Greenstone Belt of north western Tanzania yield a whole rock Sm–Nd isochron age of 2823±44 Ma (initial e(Nd)=2.7, MSWD=1.24). This age, which is interpreted as dating the eruption of the oldest mafic volcanics in the belt, is at the 95% confidence level indistinguishable from 2780±3 and 2808±3 Ma single zircon U–Pb ages previously reported from stratigraphically higher rhyolitic pyroclastic rocks from the southern margin of the belt. The age equivalency suggests that the entire ∼5–7 km thick greenstone sequence that has been traditionally classified into a predominantly mafic lower part overlain by an upper part in which felsic volcanics and BIF predominate, was emplaced within a relatively short time interval not exceeding ∼44 Ma. The Sm–Nd age of the metabasalts is significantly older than a published zircon U–Pb age of 2680±3 Ma obtained from a migmatitic gneiss on the southernmost fringe of the Sukumaland Greenstone Belt. This corroborates previous evidence that high-grade metamorphism in the Tanzania Craton postdates emplacement of the greenstones and is most likely associated with the regional emplacement of the large granitic bodies that intrude and flank the greenstones.

The geochemistry of banded iron formations in the sukumaland greenstone belt of Geita, northern Tanzania: evidence for mixing of hydrothermal and clastic sources of the chemical elements

Major and trace element compositions of samples of Banded Iron Formations (BIF) from the Neoarchaean Sukumaland Greenstone Belt of Geita in northern Tanzania reveal that the BIF precipitated from hydrothermal solutions. Fe-Ti-Al-Mn systematics suggest that the hydrothermal deposits have been contaminated, by up to 20% by weight, with detrital material having a composition similar to modern deep-sea pelagic clays. SiO2 and Fe2O3 contents are 48.2 to 88.5% and 8.9 to 49.1% respectively. Al2O3 contents lie between 0.33 and 2.1% and show no correlation with either Fe2O3 or SiO2. Al2O3 is, however, positively correlated with Ti, Ga, Hf, Rb,Th, Zr and Sr but not with CaO, the alkalies and the total Rare Earth Elements (REE). The other major element oxides are generally present in negligible amounts. The samples are characterised by mean Zr/Hf and 144Sm/143Nd ratios of 48± 5 (2 SE) and 0.10±0.01 (2 SE) respectively, similar to mean upper continental crustal values. Shale-normalised REE patterns are nearly flat, except for small positive Eu and very slight negative Ce anomalies and reveal that, compared to average upper crust, the abundances of the REE in the BIF are up to an order of magnitude lower. Chondrite-normalised patterns are characterised by light (L) REE enrichment, flat to slightly depleted heavy (H) REE, slightly positive Eu anomalies and very small negative Ce anomalies. The HREE-depleted patterns are similar to patterns derived from granite-dominated upper continental crust and indicate that the bulk of the REE in the Geita BIF can not have been derived from a mixture of Neoarchaean sea water and bottom hydrothermal solutions. The trace element data, and the REE in particular, indicate that, despite their relatively low proportions, granitic detritus probably derived from contemporaneous felsic flows and pyroclastics are the cause of the dominant trace element geochemical signature of the BIF. Tanzania Journal of Science Volume 27 (2001), pp. 21-36

Isotopic age data of single zircons from the Archaæan Sukumaland Greenstone Belt, Tanzania

This study presents precise U Pb single zircon age data for three rhyolitic metavolcanic units: one trachyandesitic porphyry, presumably of extrusive origin; one palæosome of a migmatitic gneiss; and one lamprophyric dyke cross-cutting banded iron formation (BIF) of the Upper Nyanzian Supergroup in the Archæan Sukumaland Greenstone Belt, Tanzania. The obtained ages partly contradict the expected field stratigraphical relationships. The isotopic ages of the felsic metavolcanics give a good age bracket for the deposition of the Nyanzian Greenstone Belt units. The older felsic metavolcanics yield ages of 2808±3 Ma and 2780±3 Ma, respectively, whereas the younger felsic metavolcanics provide a minimum age of 2654+15 −13 Ma for the greenstone belt. The porphyritic trachyandesite, intercalated with oxide BIF at Geita, yield an age of 2699±9 Ma and the lamprophyre dyke an age of 2644±3 Ma, respectively. The data for the lamprophyre imposes a maximum age on the Au mineralisation at Geita Mine, since the dyke has been mineralised together with adjacent BIF. The palæosome of a migmatitic gneiss, possibly a basement flanking the greenstone belt to the south at Kahama, has been dated at 2680±3 Ma. Previous models advocating the pre-existing, significantly older, Dodoman basement gneiss within the Archæan Sukumaland Greenstone Belt of northern Tanzania cannot be supported. Emplacement or complete reworking of the granitic protolith coeval with greenstone belt volcanism appears more likely.

Implications of Pb-isotopic compositions at the Geita gold deposit, Sukumaland Greenstone Belt, Tanzania

Pb-isotopic determinations have been made of mineralised and unmineralised Banded Iron Formations (BIF) from the Geita gold deposit, Sukumaland Greenstrone Belt, north-western Tanzania and also of barrent BIf some 80 km south-east of Geita. Indistinguishable dates of 2721+198−230 Ma and 2696+127−140 Ma have been obtained for the mineralised and unmineralised BIF respectively. In contrast, Pb-data from barren BIF from the Geita gold deposit indicate a significantly younger date of 1788+400−548 Ma. The older dates are consistent with existing age date (in particular galena and sulphide model age data as well as recent UPb zircon age data) from the Nyanzian in Tanzania and sorrounding regions, and are interpreted as the age of sedimentation of the BIF. This implies that the gold mineralisation at Geita, for which a maximum age limit of 2644±3 Ma is imposed by a pre-mineralisation lamprophyre dyke, must be epigenetic in origin. The younger date obtained from the barren BIF approximates the ages found for numerous dykes and sills of Ubendian age which intrude the BIF in the Geita region but insufficient data is available to fully evaluate the significance of this date.

Genetic aspects of the Geita and Jubilee Reef Archean BIF-hosted gold deposits, Tanzania

The Sukumaland Greenstone Belt (new name), which is located in north-western Tanzania hosts a large number of gold deposits, prospects and occurrences. The Archean stratigraphy comprises an older basement of granitic gneisses (Dodoman System) overlain by mafic and ultramafic volcanics which are succeeded by felsic volcanic rocks and a banded iron formation (BIF), predominantly as oxide facies. The volcanics and BIF together make up the Nyanzian System. The uppermost part of the Archean stratigraphy (Kavirondian System) consists of conglomerates and quartzites which rest unconformably on the older units. The sequence has been intruded by both syn- and post-orogenic granitoids and by several generations of felsic and intermediate dykes and sills. Gold mineralisation is abundant in all stratigraphic units except for the granitic rocks. Six types of mineralisation can be distinguished. These are BIF-hosted, hear zone type, (quartz-) vein type, clastic sedimenthosted alluvial/eluvial, and massive sulphide type deposits. The last is represented by only one deposit in this area.