The Navachab gold mine in the Damara belt of central Namibia is
characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including
pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold,
bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide
veins and semimassive sulfide lenses that are developed in a
near-vertical sequence of shelf-type metasedimentary rocks, including
marble, calcsilicate rock, and biotite schist. The sequence has been
intruded by abundant syntectonic lamprophyre, aplite, and pegmatite
dikes, documenting widespread igneous activity coeval with
mineralization.
The majority of quartz from the veins has delta(18)O values of 14 to 15
per mil (V-SMOW). The total variations in delta(18)O values of the
biotite schist and calcsilicate rock are relatively small (12-14 parts
per thousand), whereas the marble records steep gradients in delta(18)O
values (17-21 parts per thousand), the lowest values being recorded at
the vein margins. Despite this, there is no correlation between
delta(18)O and delta(13)C values and the carbonate content of the rocks,
indicating that fluid-rock interaction alone cannot explain the isotopic
gradients. In addition, the marble records increased delta(13)C values
at the contact to the veins, possibly related to a change in the
physicochemical conditions during fluid-rock interaction. Gold is
interpreted to have precipitated in equilibrium with metamorphic find
(delta(18)O 12-14 parts per thousand; delta D = -40 to -60 parts per
thousand) at peak metamorphic conditions of ca. 550 degrees C and 2
kbars, consistent with isotopic fractionations between coexisting
calcite, garnet, and clinopyroxene in the alteration halos. The most
likely source of the mineralizing fluid was a midcrustal fluid in
equilibrium with the Damaran metapelites that underwent prograde
metamorphism at amphibolite- to granulite-facies grades. Although there
is no isotopic evidence for the contribution of magmatic fluids, they
may have been important in contributing to the overall hydraulic regime
and high apparent geothermal gradients (ca. 80 degrees C/km(-1)) in the
mine area.