Implications of high-precision Re-Os molybdenite dating of the Navachab orogenic gold deposit, Namibia

Abstract

This paper presents and discusses the first Re-Os molybdenite dates from the Navachab orogenic gold deposit, the largest non-Witwatersrand gold resource in southern Africa. At the deposit, a late-kinematic, auriferous, quartz-vein swarm and associated Ca-Mg-Fe-Mn skarn ( sensu lato ) alteration crosscut the Neoproterozoic Damara Sequence, which comprises the upper amphibolite-facies equivalents of continental quartzites, glaciogenic mixtites and pelite – carbonate shelf rocks. Re-Os dating of rare traces of molybdenite in two different types of aplite yielded Cambrian Re-Os ages of 525 ± 2.4 Ma and 523 ± 2.1 Ma (2σ). Molybdenite-bearing, NNE-dipping, extensional auriferous quartz+bismuth+pyrite+pyrrhotite veins with garnet selvage have Re-Os ages of 520 ± 2.1 Ma and 519.4 ± 2.1 Ma (2σ). The 525–520 Ma gold-mineralizing event occurred slightly after the collisional metamorphic peak at 530 Ma, but subsequent gold redistribution and possibly ore mineral melting of the Au-Bi minerals likely occurred at the late-stage thermal peak of the Damaran High-Temperature-Low-Pressure tectonothermal event. This Re-Os dating considerably improves on previous attempts to date the gold-mineralizing event, which comprised U-Pb zircon age determinations on cross-cutting aplite/pegmatite dykes and U-Pb titanite age determinations on a lamprophyre and quartz veins.