Tectonic setting of the Matchless Belt pyritic copper deposits, Namibia

Univ. Witwatersrand, Dep. Geol., Wits, South Africa

Massive sulfide Cu-Zn(-Ag-Au) deposits are sporadically developed along the 350-km strike length of the late Proterozoic Matchless amphibolite belt in the Damara orogen in central Namibia. The amphibolites represent metamorphosed tholeiitic basaltic rocks formed in an extensional tectonic setting. Samples from the eastern part of the belt generally show affinities to ocean ridge basalts whereas samples from the western part are more highly fractionated and show characteristic features of within-plate basalts. During emplacement, the western part of the Matchless belt was floored by thinned and stretched continental crust. In the eastern part, newly formed oceanic crust is indicated, apparently in a graben or pull-apart subbasin. The massive sulfide deposits thus formed in a tectonic setting of advanced continental rifting and initial ocean-floor spreading.Analogous tectonic settings, basaltic rocks, and sulfide deposits are found in the present-day Red Sea and Gulf of California. The similarities in geology and geochemistry between the Matchless deposits and the besshi-type deposits in the Sanbagawa belt of Japan suggest that the latter also formed in an extensional tectonic setting, i.e., in an advanced rift environment.

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