Processes of high-T fluid–rock interaction during gold mineralization in carbonate-bearing metasediments: the Navachab gold deposit,Namibia |
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| Authors: | A Dziggel K Wulff J Kolb FM Meyer |
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| Institution: | 1.Institute of Mineralogy and Economic Geology,RWTH Aachen University,Aachen,Germany;2.Geological Survey of Denmark and Greenland (GEUS),Copenhagen K,Denmark |
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| Abstract: | The Navachab gold deposit in the Damara belt of central Namibia is hosted by a near-vertical sequence of amphibolite facies
shelf-type metasediments, including marble, calc-silicate rock, and biotite schist. Petrologic and geochemical data were collected
in the ore, alteration halos, and the wall rock to evaluate transport of elements and interaction between the wall rock and
the mineralizing fluid. The semi-massive sulfide lenses and quartz–sulfide veins are characterized by a complex polymetallic
ore assemblage, comprising pyrrhotite, chalcopyrite, sphalerite, and arsenopyrite, native bismuth, gold, bismuthinite, and
bismuth tellurides. Mass balance calculations indicate the addition of up to several orders of magnitude of Au, Bi, As, Ag,
and Cu. The mineralized zones also record up to eightfold higher Mn and Fe concentrations. The semi-massive sulfide lenses
are situated in the banded calc-silicate rock. Petrologic and textural data indicate that they represent hydraulic breccias
that contain up to 50 vol.% ore minerals, and that are dominated by a high-temperature (T) alteration assemblage of garnet–clinopyroxene–K-feldspar–quartz.
The quartz–sulfide veins crosscut all lithological units. Their thickness and mineralogy is strongly controlled by the composition
and rheological behavior of the wall rocks. In the biotite schist and calc-silicate rock, they are up to several decimeters
thick and quartz-rich, whereas in the marble, the same veins are only a few millimeters thick and dominated by sulfides. The
associated alteration halos comprise (1) an actinolite–quartz alteration in the biotite schist, (2) a garnet–clinopyroxene–K-feldspar–quartz
alteration in the marble and calc-silicate rock, and (3) a garnet–biotite alteration that is recorded in all rock types except
the marble. The hydrothermal overprint was associated with large-scale carbonate dissolution and a dramatic increase in CO2 in the ore fluid. Decarbonation of wall rocks, as well as a low REE content of the ore fluid resulted in the mobilization
of the REE, and the decoupling of the LREE from the HREE. The alteration halos not only parallel the mineralized zones, but
may also follow up single layers away from the mineralization. Alteration is far more pronounced facing upward, indicating
that the rocks were steep when veining occurred. The petrologic and geochemical data indicate that the actinolite–quartz–
and garnet–clinopyroxene–K-feldspar–quartz alterations formed in equilibrium with a fluid (super-) saturated in Si, and were
mainly controlled by the composition of the wall rocks. In contrast, the garnet–biotite alteration formed by interaction with
a fluid undersaturated in Si, and was mainly controlled by the fluid composition. This points to major differences in fluid–rock
ratios and changes in fluid composition during alteration. The alteration systematics and geometry of the hydrothermal vein
system are consistent with cyclic fluctuations in fluid pressure during fault valve action.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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| Keywords: | Navachab gold deposit Fluid– rock interaction Rare earth element mobility Orogenic gold mineralization |
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