Granitoid intrusions of the Boundary Igneous Complex separate segments with different ages of high-grade metamorphism in the Kaoko Belt, NW Namibia. Two granitoids of this complex were dated at 575 ± 10 Ma (secondary ionization mass spectrometry; SIMS) or 571 ± 9 Ma (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) and 562 ± 11 Ma (SIMS) or 572 ± 4 Ma (LA-ICP-MS), respectively. The age of granulite-facies metamorphism in the eastern part of the Western Kaoko Zone was established at 549 ± 5 Ma (SIMS) by analysing metamorphic overgrowths of older (
c
. 1850–1000 Ma) zircons from melt segregations in amphibolites. The coastal part of the Western Kaoko Zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine-grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks. Zircons from felsic members of two bimodal suites have SIMS ages of 805 ± 4 Ma and 810–840 Ma, respectively, that are interpreted as dating their respective igneous protoliths. Melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 ± 5 Ma (SIMS) or 645 ± 5 Ma (LA-ICP-MS) and 629 ± 6 Ma (SIMS) or 630 ± 5 Ma (LA-ICP-MS), respectively. The later age is indistinguishable from the age of 630 ± 4 Ma (SIMS) or 625 ± 10 Ma (LA-ICP-MS) obtained from melt patches present in overlying metagreywackes. The available age data suggest that the Boundary Igneous Complex masks the suture between the Coastal Terrane and the rest of the Kaoko Belt. Ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 Ma.
The Omitiomire Cu deposit (resource of 137 Mt at 0.54% Cu) in the Ekuja Dome of the Damara Belt in Namibia is hosted by an anastomosing, low-angle Pan-African (ca. 520 Ma) shear zone system developed around an older (ca. 1100-1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. High-grade ore shoots preferentially formed along contacts between tectonically interleaved biotiteepidote-quartz-chalcocite schists and felsic gneisses, and are directly related to an increase in the number and cumulative thickness of thin, contact-parallel mineralized shear zones. Alteration and mineralization are associated with elevated concentrations of K 2 O, Cr, Rb, S, and Cu and a loss of Na 2 O, CaO, and MgO. Oxygen isotope fractionation for quartz-biotite, quartz-feldspar, and quartz-amphibole mineral pairs support equilibrium temperatures of between 500 and 650 °C during the fluid/rock interaction. Mineral separates from amphibole-biotite gneisses and mineralized schists have similar ranges in δ 18 O values of about 1.2 to 2 ‰ relative to VSMOW. Coexisting minerals are arranged in an order of increasing δ 18 O values from biotite, to epidote, amphibole, and quartz, suggesting that the Omitiomire Shear Zone was a rock-dominated system. Similarly, H-isotope results for mineral separates from biotite-epidote schists and amphibole gneisses do not show any reversals for D/H fractionations, with δD values of between −48 and −82 ‰, typical of metamorphic-magmatic rocks. The homogeneous and low δ 34 S values (−6.1 to −4.7 ‰ CDT) are compatible with a local redistribution of sulfur from magmatic rocks and interaction with sulfur derived from metamorphic fluids of metasedimentary origin. The relatively low fluid/rock ratios and elevated Cu values (>1500 ppm) from unaltered amphibolite point to a local redistribution of an earlier (late Mesoproterozoic) Keweenaw-type Cu mineralization into later Pan-African shear zones during the exhumation of the Ekuja Dome. The timing, polyphase evolution, and tectonic setting of the Omitiomire deposit show remarkable similarities with the large Cu deposits of the Domes Region in the adjoining Lufilian Arc of northern Zambia. This suggests the presence of a much larger, regionally significant Cu province extending from central Namibia, through northern Botswana, and into Zambia.
KeywordsDamara Belt . Cu . Mesoproterozoic dome . Pan-African . Shear zone . Stable isotopes . Fluid/rock ratio . Lufilian Arc * Shawn Kitt
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.