STRATIGRAPHY OF TANZANIA

STRATIGRAPHY OF TANZANIA

STRATIGRAPHY AND MINERAL OCCURRENCE OF TANZANIAGreenstone beltsArchaean greenstone belts are the oldest major goup of well preserved volcano-sedimentary basins and can therefore give us information on early crustal evolution. They occur in many shield areas and have ages ranging from 3500 to 2500 Ma.Greenstone beltsIn Tanzania, six greenstone belts have been identified in the Nyanzian Supergroup, namely, Sukumaland, Kilimafedha, Musoma-Mara, Shinyanga-Malita, Nzega and Iramba-Sekenke.Greenstone belts are very important source of mineralization. They are one of the main depositaries for Au, Ag, Cr, Ni, Cu and Zn.High grade terranesHigh (metamorphic) grade granulite-gneiss belts are another imporant Archaean crustal province. The high grade terranes comprise of gneissic complexes and batholiths which are dominantly tonalite, trondhjemite, and granodiorite, the so called TTG suite. In Tanzania, the Dodoman comprises an Archaean high grade terrane.High grade terranesMost gneissic complexes (TTG) in high grade terranes yield ages of between 3.8 - 2.5 Ga with a significant cluster between 2.7 - 3.1 Ga.Most Archaean high grade terranes experienced high-grade metamorphism reaching up to granulite facies.Unlike Archaean greenstone belts, the high grade terranes are not particularly endowed in mineral resources High grade terranesRecent Geochronological evidence shows that Dodoman rocks records episodes of magmatism that date as far back as 3.6 4.0 Ga whereas greenstones in Lake Victoria are dated at 2.8 2.67 Ga General stratigraphy Geology of Tanzania can generally be subdivided into FOUR tectono-stratigraphic units:Archaean Tanzania Craton (central Tanzania high grade terrane and northern Tanzania low grade granite greenstone terrane)General stratigraphy Paleo- to Neo-proterozoic mobile belts surrounding the Archaean Tanzania Craton (Ubendian Usagaran, Karagwe Ankolean, Bukoban, Pan African Mozambique belt). Paleozoic to Mesozoic sedimentary basins (e.g. Karoo)Cenozoic sedimentary and volcanic formations (e.g. Neogene volcanics of northern and southern Tanzania).General stratigraphy The greenstone sequence has been traditionally assigned into the Nyanzian Supergroup; with the Nyanzian itself being subdivided into lower and upper units.Lower Nyanzian = predominatly comprised of mafic volcanics, gabbros, subordinate felsic flows General stratigraphy Upper Nyanzian = composed of BIF, chert, felsic pyroclastics, carbonaceous shales, sandstones, siltstones and greywacke.The Nyanzian is unconformably overlain by the coarse clastic sedimentary rocks of the Kavirondian Supergroup (basal conglomerate, sandstones, grits and are interbedded with thin horizons of volcanic rocks).Mineralization and styles of mineralizationGreenstone belts are the major source of metallic ore deposits including Au, Ag, Cu, Ni, Co, and Cr). About 90% of Au production in Tanzania comes from the greenstone belts of lake Victoria. Styles of Au mineralization in the lake Victoria greenstone belts include:Mineralization and styles of mineralizationQuartz vein hosted mineralization associated with shear zones (Bulyankulu, Tulawaka, Busolwa Mawemeru). Mineralization in this category is hosted in metavolcanic and volcano-sedimentary rocks.Mineralization and styles of mineralizationBanded Iron Formation (BIF) hosted mineralization associated with shear zones (Geita, Golden Ridge). Mineralization in this category can also be categorised as strata-bound as is the case with that of Golden Ridge.Mineralization and styles of mineralizationGranitic intrusives hosted mineralization associated with shear zones (Nyabirama - North Mara, Buzwagi)Clastic sedimentary rocks (sandstones, siltstones) hosted mineralization associated with shear zones (Golden Pride, Nyanzaga).Karagwe - Ankolean Supergroup K-A is best developed in the 3 NW districts of Karagwe, Biharamulo and Ngara. An outlier outcrops as the Kigoma quartzite on the western shores of Lake Tanganyika.Characterized by a basal sequence of conglomerate and sandstone with some amygdaloidal basaltic rocks passing up into several siliciclastic cycles:Each cycle consists of shales and arenites at its base The Bukoban Group It comprises of sediments traditionally regarded as Bukoban sandstone group. These sediments are cross-cut by the gabbro intrusives which yielded Ar-Ar ages of 1379 10 Ma and 1355 10 Ma. Similar ages (1360 20 Ma and 1340 9 Ma) are reported for gabbro intrusives that cross-cut the Nkoma-Kavumwe group sandstones of SE Burundi. Bukoban Supergroup Typical Bukoban succession is characterized by shales, siltstones, sandstones, arkoses, ortho-quartzites, dolomitic limestones with cherts, stromatoliths and occassional ooliths. These sediments are associated with amygladoidal flood basalts and flood andesites.Bukoban Supergroup Occurrence in Tanzania Northwestern to western Tanzania between Ngara district and Kigoma (Busondo Masontwa, Kigonero Flags and Uha groups). Southwestern Tanzania in the Uwanji sub-district of Njombe better exposed at Chimala (Buanji group). Northeast Tanzania in the Serengeti district (Ikorongo group).Pan African Mozambique Belt The Mozambique Belt is a Neoproterozoic orogen that stretches from southern Mozambique through East Africa to the Arabian Peninsula(Arabian-Nubian Shield, ANS). Mozambique belt towards the south in Kenya, Tanzania, Malawi and Mozambique is characterized by higher grade of metamorphism (amphibolite to granulite facies), less juvenile material and increased reworking of older crustal components.Pan African Mozambique Belt The type area of the MB orogenic belt is in northern Mozambique and the major rock types are multiply deformed biotite-hornblende gneisses and migmatities largely derived from granitoid precursors, marbles, amphibolites, and post-kinematic granitoids and pegmatites. Pan African Mozambique Belt The Mozambique belt in Tanzania has traditionally been termed a gemstone belt because of the abundance of a variety of gemstones found in this belt.Recent exploration efforts by the companies, however, shows that the MB is also prospective for gold mineralization.22/4/99