Short Wavelength Infrared Spectral Characterization of the Mineralogy of Gokona and Nyabigena Andesite-Hosted Gold Deposits in North Mara, Tanzania

The mineralogy of Gokona and Nyabigena gold deposits in Tanzania have been studied using Short Wavelength Infrared (SWIR) Spectrometry technique with an aim to fingerprint hydrothermal system responsible for the formation of the deposits and establish pertinent mineralogical signatures for gold deposition. Gokona and Nyabigena deposits are hosted in porphyritic andesite to basaltic andesite that contains plagioclase phenocrysts, pyroxene, hornblende and Fe-Ti oxides as primary minerals. Other minerals are actinolite, chlorite, epidote, albite and hematite, which indicate metamorphism of the rock under greenschist facies conditions. Hydrothermal alteration of the rock produced chlorite, sericite, carbonates (ankerite, siderite and calcite), pyrite and quartz, which is consistent with formation of the minerals in mid-crustal levels (< 5 km). Depth of absorption features of SWIR spectra for chlorite and sericite from borehole samples revealed intense formation of the minerals in the deposits. Sericite is proximal to ore zones, less crystalline and mainly muscovitic to phengitic in composition, whereas chlorite occurs distal to the ore zones, and is mainly intermediate (Fe-Mg) to Fe-rich in composition. These phyllosilicates indicate both pH and temperature control during hydrothermal alteration and provide signatures that can be targeted in exploration to extend mine lifespan. Keywords: Hydrothermal alteration; Short Wavelength Infrared Spectrometry; Gold exploration; Musoma-Mara Greenstone Belt; North Mara mine

Crustal evolution and hydrothermal gold mineralization in the Katuma Block of the Paleoproterozoic Ubendian Belt, Tanzania

Orogenic belts are regions of the Earth’s continental crust, in which regional metamorphic rocks occur that may have formed in the deep crust of an orogen. Studies of metamorphic belts are thus important to an understanding of processes that are taking place in roots of mountain belts. In order to determine the formation and crustal evolution of the linear Paleoproterozoic Ubendian Belt in Tanzania and the age and origin of its precious base metal deposits, the rocks in the Katuma Block of the northwestern Ubendian Belt were geochemically and petrologically investigated and their U-Pb zircon and U-Th-total Pb monazite ages determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometer and an Electron Probe Micro Analyser, respectively. Internal textures of the zircon grains in combination with their U-Pb ages indicate that the magmatic formation of the protoliths of the orthogneisses and metabasites of the Katuma Block occurred mostly in the Neoarchean (ca. 2.71-2.64 Ga) but also during the Paleoproterozoic (2.05-1.94 Ga). These two periods are separated for about 600 Ma and are interpreted as active continental margin stages at the border of the Tanzania Craton, as deduced by the calc alkaline nature and trace element geochemistry of the metabasites and orthogneisses. The granulite facies corona assemblages in metabasites consisting of garnet, clinopyroxene, quartz and hornblende replacing magmatic orthopyroxene and plagioclase indicate that the magmatic protoliths experienced a near isobaric cooling after their intrusion into the deep crust during a tectonically quite period (< 2.64 Ga). The detrital zircon grains from the metasediments of the Katuma Block gave ages ranging between 2.64 and 2.05 Ga, similar to the magmatic formation ages of rocks of the Katuma Block, suggesting that the Katuma Block itself was most likely the source for the detritus. The time interval of sedimentation is constrained by the oldest age of the detrital zircon at about 2680 Ma and the age of the first metamorphism of the sediments at about 1960 Ma. This event is documented by the age of metamorphic zircon rims and that of monazite cores. The stage of sedimentation and the near isobaric cooling of the metabasites in the deep crust is ascribed to a passive continental margin stage of the Tanzania Craton between 2.65 and 2.05 Ga. In contrast to the post magmatic cooling nature of the Archean metamorphism of the metabasites, the common occurrence of sillimanite pseudomorphs after cm sized kyanite crystals in migmatitic metapelites provides evidence for two stages of prograde metamorphism associated with Paleoproterozoic crustal thickening events. An early stage of metamorphism took place in the kyanite stability field whereas the subsequent peak metamorphism is characterised by the stability of the mineral assemblage sillimanite-garnet/cordierite-K-feldspar. Pseudosection modelling of the XMg ratios for garnet in combination with GASP barometry revealed that the formation of the compositionally homogenous cores of garnet porphyroblasts formed at conditions of about 7 kbar and 770 ̊C. The formation of late stage plagioclase coronas around garnet in metapelites and the decrease of XMg and of the spessartine component in rims of garnet porphyroblasts point to a near isothermal uplift after peak metamorphism and thus to a crustal thickening event that preceded the peak metamorphism. The U-Th-total Pb monazite ages suggest that during the Paleoproterozoic time the Katuma metapelites experienced two separate metamorphic events at about 1.96 Ga and 1.84 Ga. As the two ages of monazite growth zones (cores and rims) are found in monazite grains of the rock matrix and in inclusions in garnet porphyroblasts, the garnet growth must have occurred during or after the second metamorphic event at 1840 Ma. This interpretation is in agreement with the increasing depletion of HREE and Y in the monazite rims indicating concurrent growth of the monazite rims and the garnet porphyroblasts. The second, high-grade event at ca. 1840 Ma is correlated with the formation of the Ubendian Belt during the collision between the Tanzania Craton and the Bangwelu Block. The first metamorphic event at ca. 1960 Ma that preceded the collision for about 120 Ma is attributed to the kyanite grade metamorphism during accretionary processes and associated calc alkaline magmatism (2.05-1.94 Ga) along the active continental margin of the Tanzania Craton. U-Th-total Pb dating of hydrothermally altered monazite grains from hydrothermally altered metapelites hosting the Au-Cu-Pb bearing veins of the Mpanda Mineral Field in the Katuma Block yielded a Mesoproterozoic age (1171 ± 17 Ma). This age coincides with the first, amphibolite grade metamorphism of metasediments in the Wakole Block adjoining the Katuma Block to the southwest. The obtained age provides a link between the metamorphism of the Wakole metasediments and the generation of hydrothermal fluids responsible for the formation of the gold copper lead bearing veins in the Katuma Block.Tief erodierte Orogene sind Regionen der kontinentalen Erdkruste, in denen regionalmetamorphe Gesteine vorkommen, die sich in der tiefen Kruste gebildet haben. Studien an solchen metamorphen Gürteln sind daher wichtig für das Verständnis von Prozessen, die in Wurzeln von Gebirgsgürteln stattfinden und zur Entschlüsselung von Krustenbildungsvorgängen und der Krustenentwicklung. In dieser Arbeit geht es um die Entschlüsselung der orogenen Entwicklung eines linearen paläoproterozoischen Gebirgsgürtels, dem Ubendian Belt von Tanzania, der unter anderem auch eines der ältesten in einem Gebirge aufgeschlossenen Eklogitvorkommen der Erde enthält. Um die Bildung und Krustenentwicklung dieses linearen Ubendian Belts, sowie das Alter und die Entstehung der dortigen Lagerstätten an edlen und unedlen Metallen zu bestimmen, wurden die Gesteine des Katuma-Blocks des nordwestlichen Ubendian Belts geochemisch und petrologisch untersucht. Zudem wurde das Alter von Zirkon (U-Pb) und Monazit (U-Th-total Pb) unter Verwendung von Laser Ablation Inductively Coupled Plasma Massenspektrometrie beziehungsweise der Elektronenstrahlmikrosonde. Interne Texturen der Zirkonkörner in Verbindung mit ihren U-Pb-Altern deuten darauf hin, dass die magmatische Bildung der Protolithe der Orthogneise und Metabasite des Katuma-Blocks hauptsächlich im Neoarchaikum (ca. 2.71-2.64 Ga), aber untergeordnet auch während des Paleoproterozoikums (2.05-1.94 Ga) stattfand. Diese zwei Perioden liegen etwa 600 Ma auseinander und werden als Stadien mit aktivem Kontinentalrand an der Grenze des Tansania Kratons interpretiert, wie aus der kalkalkalischen Zusammensetzung und den Spurenelementmustern der Metabasite und Orthogneise abgeleitet werden kann. Die granulitfaziellen Säume in Metabasiten, bestehend aus Granat, Klinopyroxen, Quarz und Hornblende, welche magmatisch gewachsenen Orthopyroxen und Plagioklas ersetzen, lassen darauf schließen, dass die magmatischen Protolithe nach ihrer Bildung in der tiefen Kruste eine nahezu isobare Abkühlung während einer tektonisch ruhigen Phase (< 2.64 Ga) erlebten. Die detritischen Zirkonkörner aus den Metasedimenten liefern Alter von 2.64 bis 2.05 Ga, ähnlich den Bildungsaltern der Gesteine des Katuma-Blocks, was andeutet, dass der Katuma-Block selbst die wahrscheinliche Quelle des Sediments war. Der Zeitraum der Sedimentation kann eingeschränkt werden durch das höchste Alter detritischer Zirkone bei ca. 2680 Ma und dem Alter der ersten Metamorphose der Sedimente bei ca. 1960 Ma. Dieses Ereignis wurde datiert durch das Alter von metamorphen Zirkon-Säumen und das von Monazit-Kernen. Die Phase der Sedimentation und der nahezu isobarer Abkühlung der Metabasite in der tieferen Kruste wird einem Stadium mit passivem Kontinentalrand des Tansania Kratons zwischen 2.65 und 2.05 Ga zugeordnet. Im Gegensatz zur Art der post-magmatischen Abkühlung der archaischen Metamorphose der Metabasite, liefert das häufige Vorkommen von Sillimanit-Pseudomorphosen nach cm-großen Disthenkristallen in migmatitischen Metapeliten Beweise für zwei Stadien prograder Metamorphose verbunden mit paläoproterozoischer Krustenverdickung. Ein frühes Stadium der Metamorphose fand innerhalb des Disthen-Stabilitätsfeldes statt, wogegen der anschließende Metamorphose-Peak durch die Stabilität der Mineralparagenese Sillimanit-Granat/Cordierit-K-Feldspat charakterisiert wird. Pseudosection Modellierung der XMg-Verhältnisse für Granat zeigen, dass sich die Bildung der homogen zusammengesetzten Kerne von Granatporphyroblasten bei Bedingungen von 7 kbar und 770 ̊C ereignete, was vereinbar ist mit den Ergebnissen der GASP-Barometrie. Die Bildung von späten Plagioklas-Säumen um Granat in Metapeliten und die Abnahme von XMg der in den Rändern von Granatporphyroblasten, lassen auf eine nahezu isotherme Heraushebung nach der Peak-Metamorphose schließen und damit auf eine Krustenverdickung, die der Peak-Metamorphose vorangegangen sein muss. Die U-Th-total Pb Monazit-Alter lassen vermuten, dass die Metapelite des Katuma-Blocks während des Paleoproterozoikums zwei verschiedene Metamorphoseereignisse bei ca. 1.96 Ga und 1.84 Ga durchlebten. Da zwei Wachstumszonen von Monazits (Kerne und Säume) sowohl bei Monazitkörnern in der Gesteinsmatrix als auch bei Einschlüssen in Granatporphyroblasten gefunden werden, muss das Granatwachstum während oder nach dem zweiten metamorphen Ereignis bei ca. 1840 Ma stattgefunden haben. Diese Interpretation ist wird unterstützt von der zunehmenden Verarmung an HREE und Y in den Monaziträndern, die auf ein gleichzeitiges Wachstum der Monazitsäume und der Granatporphyroblasten hinweist. Das zweite, hochgradige Ereignis bei ca. 1840 Ma steht somit in Verbindung mit der Entstehung des Ubendian Belts während der Kollision zwischen dem Tansania-Kraton und dem Bangweulu-Block. Das erste metamorphe Ereignis bei ca. 1960 Ma, welches sich ca. 120 Ma vor der Kollision ereignete, wird einer Metamorphose bei Disthen-Stabilität zugeschrieben, die während Akkretions-Prozessen und damit verbundenem Kalk-alkalinem Magmatismus (2.05-1.94 Ga) entlang des aktiven Kontinentalrandes des Tansania-Kratons stattfand. U-Th-total Pb-Datierung an hydrothermal alterierten Monazitkörnern von hydrothermal überprägten Metapeliten, welche die Au-Cu-Pb-haltigen Adern des Mpanda Mineral Field im Katuma Blocks enthalten, liefern ein mesoproterozoisches Alter (1171 ± 17 Ma). Dieses Alter stimmt überein mit der ersten amphibolit-aziellen Metamorphose von Metasedimenten im Wakole-Block, der nach Südwesten an den Katuma-Block angrenzt. Das ermittelte Alter liefert eine Verbindung zwischen der Metamorphose der Metasedimente im Wakole-Block und der Freisetzung hydrothermaler Fluide, welche für die Entstehung der Gold-Kupfer-Blei haltigen Adern des Katuma-Blocks verantwortlich sind

Reconnaissance Exploration for Gold in the Misaki Area within the Iramba-Sekenke Greenstone Belt, Central Tanzania

Combinations of geological, geophysical and geochemical techniques have been used to explore for gold deposits at Misaki within the Iramba-Sekenke Greenstone Belt in Singida region. The Misaki area is occupied by different rocks including homogeneous coarse-grained granite containing numerous xenoliths (&gt; 5 vol. %), tonalite, K-rich granite, pegmatite and dolerite dykes. All granitic rocks have been intruded by dolerite dykes, cut by epidote veins and are found juxtaposed to a tonalitic rock. Granitic rocks contain mainly anhedral to subhedral K-feldspar, plagioclase, quartz, biotite, hornblende, muscovite and opaque minerals, whereas tonalite contains similar minerals with relatively large amounts of plagioclase and mafic minerals (&gt; 10 wt. %; biotite, hornblende and opaque minerals) and no K-feldspar. The rocks are weakly foliated to massive and poikilitic. K-feldspar is perthitic, whereas plagioclases have cores with rim overgrowths. Chlorite, epidote and sericite in the rock formed at the expense of primary minerals under greenschist facies conditions. Geophysical datasets managed to identify lineaments that crosscut different rocks at Misaki, from which three (3) major structural trends have been recognized, which are NE-SW, NW-SE and ESE-WNW. Results from radiometric data mapped different lithological units by their different radiometric element contents (U, Th and K), distinguishing areas occupied by sediments or sedimentary rocks and those underlain by granitoid rocks. Soil geochemical survey have identified gold anomalies of up to 0.2 ppm Au that in parts show strong affinity with Pb. Bismuth and arsenic were also found to be associated with Au in the soil. Gold anomalies when overlaid with magnetic lineaments indicate an association of gold with NE-SW trending lineaments. Results of this work call for a follow-up detailed geological mapping that would involve trenching and sampling of unweathered rocks, and documentation of geological structures to uncover potential gold deposits of the Misaki area within the Iramba-Sekenke Greenstone Belt. Keywords: Gold exploration; Iramba-Sekenke; Greenstone Belts; Integrated prospecting

Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania

International audienceOldoinyo Lengai is the only volcano on Earth currently erupting natrocarbonatites, of which the source and genesis remain controversial. Cognate xenoliths and fumaroles were sampled at the summit of Oldoinyo Lengai, and deep crustal xenoliths from Oltatwa maar, in 2010 and 2014, after the 2007-2008 sub-Plinian eruption. The summit cognate xenoliths provide direct information on the isotopic composition of the mid-crustal magma chamber that was active during the 2007-2008 explosive eruption. Cognate xenolith-hosted pyroxenes from Oldoinyo Lengai have an average 3 He/ 4 He = 6.58 ± 0.46 R A , similar to values from nearby silicate volcanoes (4.95-7.30 R A), and reflecting a sub-continental lithospheric mantle (SCLM) signature. This similarity implies that Oldoinyo Lengai carbonatites form from a similar mantle reservoir as the nearby silicate volcanoes. We identify SCLM, metasomatized by fluids/melts derived from the depleted convective mantle, as the common source of magmas in the Arusha volcanic province. Fumarole measurements highlight that fumarolic 3 He/ 4 He values have been relatively constant since at least 1988, indicating that dramatic changes to the crater region morphology during the 2007-2008 eruption did not affect the architecture of the hydrothermal system, which is probably connected to the crustal magma chamber(s). Moreover, the similarity between 3 He/ 4 He values from the mid-crustal magma chamber (6.58 ± 0.46 R A) and fumaroles (7.31 ± 0.24 R A) of Oldoinyo Lengai attests that helium is not subjected to atmospheric contamination or crustal assimilation during transport to the surface

The impact of complex volcanic plumbing on the nature of seismicity in the developing magmatic Natron rift, Tanzania

Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10-month temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6827 earthquakes with ML -0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ~17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3 - 10 km) beneath Gelai, including two swarms. The deepest seismicity (~down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment

The nyerereite crystal structure: a possible messenger from the deep Earth

Carbonates in the system Na2CO3-CaCO3 are nowadays suggested as having a wide stability field at conditions of the mantle transition zone. Our structural analysis of nyerereite, which has limited stability fields at ambient conditions, and its similarities with already known carbonates that are stable at high-pressure conditions, allowed us to propose that nyerereite likely undergoes phase transitions at both high-pressure and high-temperature conditions. This supports the hypothesis that nyerereite takes part in carbon transportation from the mantle/deep crust toward the surface, with important implications for the deep carbon cycle associated with carbonatites. K-free nyerereite [Na2Ca(CO3)(2)] was synthesized both at hydrothermal conditions and from the melt. The structure of nyerereite was refined as a three-component twinned structure in the centrosymmetric Pbca space group with ratios of the three twinning components 0.221(3):0.287(3):0.492(3). Twinning at micro- and nano-level can introduce some minor structural deformations that influence the likely occurrence of the inversion center as one of the symmetry elements in the nyerereite structure. Based on the automated topological algorithms, we show that nyerereite has a unique crystal structure, not having analogs among the known structures, except for the structure with a similar composition K2Ca(CO3)(2) fairchildite. A comparison between the centrosymmetric Pbca nyerereite structure and that of aragonite (CaCO3, Pmcn space group) reveals two main scenarios for the high-pressure form of Na2Ca(CO3)(2): (1) polysomatic relations as the interlayering of the high-pressure polymorph Na2CO3 and CaCO3-aragonite, and (2) high-pressure structure with ninefold-coordinated Na and Ca sites resembling that of aragonite. Our discussion heightens the interest in the high-pressure behavior of the nyerereite structure and strengthens the hypothesis about the possibility for nyerereite to be stable at high-pressure/high-temperature conditions