Iron formations and associated amphibolites in the southern part of the Damara Orogen of South West Africa/Namibia

Abstract

Abstract provided by author:

In the southern part of the late Proterozoic Damara Orogen of South West Africa/Namibia, iron-rich rocks and massive sulphide deposits (iron formation, magnetite quartzite, pyritic copper deposits) are associated with mafic schist and amphibolite. The rocks have undergone metamorphism up to amphibolite grade and intense structural deformation (isoclinal and recumbent folding, thrusting)

In the Chuos Formation of the Southern Margin Zone iron formations were precipitated in two distinct stratigraphic positions during early continental rifting accompanied by intense block faulting. The lower Chuos iron formation consists of magnetite-quartz rock and is interbedded within a thick sequence of diamictite ("pebbly schist"). In the upper Chuos Formation, oxide, silicate, carbonate and minor sulphide facies iron formation occurs in a lithological sequence of predominantly quartz-mica schist, quartzite, mafic schist and amphibolite. The closely associated mafic rocks are metamorphic equivalents of tholeiitic basalts and minor mafic intrusives. Mafic igneous activity and chemical precipitation of upper Chuos iron formation are considered contemporaneous and cogenetic. The basaltic rocks are interpreted to have been erupted along normal fault planes. The iron formations are believed to be of exhalative origin related to hydrothermal convection caused by raised geothermal gradients along deep-reaching faults during rifting and magmatic activity

The Matchless Member in the Southern Zone (Trough Zone) is interbedded within a thick succession of metamorphosed semi-pelitic to pelitic sediment and comprises metamorphosed tholeiitic basalt and gabbro. The magmatic event that led to the deposition of the Matchless Amphibolite Belt is considered to have taken place during advanced rifting and the formation of a rapidly subsiding depositional basin (Khomas Trough). The varying geochemical compositions of the metabasites along strike of the Matchless Belt are interpreted to indicate their eruption through thinned and stretched continental crust and the possible local formation of a sub-basin underlain by oceanic crust. Strata-bound pyritic copper, deposits and magnetite quartzite associated with the Matchless Belt are considered to be of volcanogenic-exhalative origin derived from hydrothermal convection caused by high heat flow in a tectonic environment of advanced continental rifting and initial ocean floor spreading