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Terje Bjerkgård, Geological Survey Of Norway (Norway)
Holly J. Stein, AIRIE Program (United States)
Bernard Bingen, Geological Survey Of Norway (Norway)
Iain H.C. Henderson, Geological Survey Of Norway (Norway)
Jan Sverre Sandstad, Geological Survey Of Norway (Norway)
Amandio Moniz, Direccâo Nacional de Geologica, Mozambique (Mozambique)
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Gold mining has been carried out since 1990 along Lake Niassa in the Niassa Gold Belt (NGB) in northernmost Mozambique, at a rate of about 5 t/year. The NGB is hosted in the mainly metasedimentary Txitonga Group, divided in two formations. The lower M'Popo Formation comprises in addition to metasediments, a bimodal, mainly mafic, magmatism with within-plate geochemical signature. A zircon U-Pb age for a metarhyolite dates deposition of the sequence at 714+/-17Ma. In the west, the Txitonga Group overlies the Paleoproterozoic high grade Ponta Messuli Complex along a southeast dipping tectonic contact. In the east, it abuts against sediments of the Karoo Supergroup. The Ponta Messuli Complex is correlated with the Usagaran and Northern Irumide belts in Zambia and Tanzania, and is interpreted as part of the margin of the Congo Craton during the Neoproterozoic. The Txitonga Group is interpreted as a rift basin formed at the margin of the Congo-Tanzania Craton, similar to the Katangan Lufilian Arc, and parts of the Bukoban and Malagarazi supergroups in Tanzania and Burundi.
Gold occurs in quartz veins in metagabbro and metasediments formed as extensional structures related to regional strike-slip NE - SW-trending Pan African shear. The deposits are classified as mesozonal and metamorphic in origin, based on their occurrence, alteration of the host rock and relation to regional structure and fabric.
Re-Os isotopic data on sulfides suggest two periods of gold deposition: A pyrite-chalcopyrite assemblage yields a Pan-African age of 483+/-72Ma, dating deposition of the quartz veins. Reworking of particularly chalcopyrite, took place at 112+/-14Ma, during Lower Cretaceous Gondwana dispersal. The c.483Ma assemblage yields a chondritic initial 187Os/188Os ratio of 0.123+/-0.058, implying a juvenile ore fluid source, possibly involving the hosting Neoproterozoic metagabbro.
The shear zones have a similar trend and greenschist-facies grade as the 444 ± 5Ma Macaloge shear-zone situated to the south of the Karoo graben-structure. These strike-slip structures could be related to a regional phase of late Pan-African extension, which is well-defined further east in Mozambique. Very similar events affects the extensive Mwembeshi structure in Zambia and Malawi. Northwards, the structures bounding the NGB continue as a major lineament extending to the northernmost part of Lake Niassa, where it links up with the Livingstone fault and the extensive Rukwa fault structure in Tanzania.
We propose that a mega-scale structure extends from the Mwembeshi and Chimaliro shear zones in Zambia and Malawi, via the shear zones related to the NGB in Northern Mozambique, to the Rukwa Shear Zone in Tanzania. A number of vein-quartz mesothermal gold deposits, are genetically linked to this structure, and the structure should have a great potential for finding new gold deposits.
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