First Geological Description of a Late Proterozoic (kibaran) Metabasaltic Andesite-Hosted Chalcocite Deposit at Omitiomire, Namibia

ABSTRACT
A mafic schist- and gneiss-hosted chalcocite deposit in
KibaranÐage (1,400 Ð 900 Ma) rocks at Omitiomire, Namibia constitutes
a newly recognized copper deposit type in southern Africa.
Chalcocite is hosted by a continuum of (in order of increasing Cu
content): amphibolite to hornblende-biotite gneiss or schist to epidote-
biotite schist. This continuum represents the metamorphosed
equivalents of undeformed to progressively sheared, subaerial
basaltic andesites (52Ð54 wt% SiO2, ~17.8 wt% Al2O3) intercalated
with younger, unmineralized metadacites (~68 wt% SiO2) and minor
metarhyolites (~72 wt% SiO2). Bimodal volcanism (sediments are
absent) suggests a rifted continental tectonic setting for the igneous
precursors. Chalcocite and subordinate digenite, both parts of an
amphibolite facies (~600oC, ~7 kbar) rock assemblage, comprise >
98 modal % of sulfides. Traces of covellite, bornite, chalcopyrite,
and malachite occur. Up to 3 modal % Cr- and Ti-magnetite, probably
derived from retrograde breakdown of amphibolite, are spatially
associated with the highest copper concentrations.
Intermittent exploration at Omitiomire, 120 km NE of
Windhoek, since 1976 included soil geochemical, induced polarization
(IP) and magnetic surveys with percussion and core drilling (60
holes). Chalcocite-bearing lithologies were intersected in nearly all
boreholes. The thickest intersection is 0.47 wt% Cu over 106.1 m
(96.3-202.4 m below surface; borehole OED5). Highest copper
grades (up to 3.23 wt% Cu over 1.40 m) are present in epidote-biotite
schist. Undrilled geochemical and geophysical anomalies suggest the
system is ~15 km long. Geologic and measured resource calculations
indicate at least 300,000 and possibly 600,000 tonnes of copper in the
drill-tested portion. Untested soil copper anomalies suggest a much
larger total geologic resource of copper grading 0.2-1.0 wt%. The
deposit contains almost no carbonate and preliminary metallurgical
leach tests on potential ore indicate acid consumption is very low.
SHRIMP U-Pb ages for zircon cores indicate the following
ages for rocks in the mineralized sequence: amphibolite, 1115±13
Ma; biotite gneiss, 1084±7 Ma; chalcocite-bearing amphibole schist,
1081±10 Ma; foliated tonalite, 1063±9 Ma. Zircon rims and overgrowths
are ~600 Ma (Pan-African), the suspected age of the main
chalcocite-mineralizing event. Mineralized rocks lie in an imbricate
shear plane system (a foreland dipping duplex) formed during the
development of ESE-vergent nappes and thrusts. Originally,
Keweenawan-type basaltic andesite/basalt flows with amygdaloidal,
calcite-filled tops are thought to have contained native copper, copper
oxides and carbonates. Groundwater migration, diagenetic and
metamorphic fluid flow, copper redistribution, deformation, epidote
porphyroblast growth and precipitation of sulfur-poor copper minerals
occurred in and along flattened amygdaloidal tops of lava flows.
These structurally weaker amygdaloidal zones, now represented by
epidote-biotite schists, have a thickness probably less than a tenth of
the original calcite-rich part of the flow. Lava flow interiors are now
amphibolites, unsheared and unmineralized.