Origin and age of paleoproterozoic conglomerates and sandstones of the Tarkwaian Group in Burkina Faso, West Africa

The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup which is considered to be part of a juvenile crust formed during the Eburnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, chert and schist), embedded in an acid volcaniclastic matrix, During a sinistral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding. Fold axes are nearly horizontal, parallel to a stretching lineation and oriented 035-215 degrees. A typologic study of zircons in the volcaniclastics, coupled with radiometric datings, enables the volcanism to be characterized and its age to be determined. The typologic study has shown that: (1) zircons from the rhyolitic pebbles, the matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to alkaline evolutionary trend implying three main episodes of volcanism; (3) the morphology of this zircon population is very similar to those usually observed in post-orogenic metasediments or sediments. The tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the: course of a major period of crustal growth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments. The radiometric study has shown that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma, Three main volcanic episodes may be defined 2170, 2150 and 2124 Ma; the last one was probably contemporaneous with the sedimentation. Deformation of Tarkwaian rocks took place between 2124 +/- 9 Ma (the age of the youngest inherited zircon) and 1991 +/- 12 Ma (the age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fits well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also recognized in South America. The Tarkwaian conglomeratic rocks of Burkina Faso do not have gold concentrations similar to those in Ghana and Guyana. With respect to the Tarkwaian rocks of Ghana, where gold is concentrated in the quartz pebbles, they also differ because: (1) there is a volcaniclastic contribution; (2) the zircons from the Tarkwaian rocks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing. With respect to Guyana, the rocks are very similar and probably have the same origin; the short transport distance may not allow a secondary concentration of gold. These differences, at least, may help explain the lack of economic gold concentration in the Tarkwaian metasediments.