TSK 11 Göttingen 2006 Laukamp et al.
 Structural control of fluid
 flow on a carbonate plat-
 form margin: an example
 from the Otavi Mountain-
 land, Namibia Vortrag
 Carsten Laukamp1 Volker Petzel2
 Thilo Bechstädt1
 Fault inversion and alteration influence
 fluid pathways over the period of de-
 formation and thereafter. Relationships
 between the formation and site of ore
 deposits can be established if the stages
 of deformation can be linked to spe-
 cific hydrothermal events. In order
 to find new indications on the gene-
 sis and distribution of known and even-
 tual further base metal mineralisation
 in the Otavi Mountainland (OML) in
 northern Namibia, we started a cement-
 stratigraphic and detailed structural in-
 vestigation.
 The OML is positioned in the north-
 eastern part of the Damara belt, on
 the northern tip of a foreland fold and
 thrust belt. The ENE-trending belt in
 northern central Namibia results from
 the pan-African collision of the Congo
 with the Kalahari Craton. The pan-
 African tectonic evolution of the OML
 is summarised in Fig. 1. Sedimen-
 tary lithologies in the OML consist of
 the Neoproterozoic Damara sequence
 with the siliciclastic and volcanoclastic
 Nosib Group at the bottom, overlain
 by the Otavi Group carbonates and the
 molasse-like Mulden Group. Deposition
 proceeded from Cryogenian to late Edi-
 acaran or early Cambrian (Frimmel et
 al. 2004, amongst others).
 1 Geologisch-Paläontologisches Institut, Uni-
 versität Heidelberg, Im Neuenheimer Feld 234
 2 Geological Survey of Namibia, 1 Aviation
 Road, Windhoek, Namibia
 Figure 1: Tectonic evolution of the OML
 (ages after Frimmel et al. (1996), Frimmel
 (2004), Goscombe et al. (2004), Haack and
 Martin (1983), Haack et al. (1980), Hoff-
 mann et al. (2004))
 The present study focuses on the north-
 western OML, southwest of the mining
 town of Tsumeb. The dominating struc-
 ture in this area is the NW-SE striking
 Guinas Fault. The Guinas Fault is a
 shear zone that divides an area, which
 is different in sense of stratigraphic fea-
 tures, the grade of pre-, syn- and post-
 Damaran deformation and the type and
 grade of mineralisation. The northern
 area is enriched by Cu-rich base metal
 sulphides, whereas the southern area is
 almost barren.
 The carbonate successions in the Guinas
 Fault area are part of the Tsumeb Sub-
 group (upper Otavi Group, lithozones
 T4–T8). North of the Guinas Fault
 the massive dolomites of the Hütten-
 berg Formation (T6–T8) are uncon-
 formably overlain by conglomerates and
 sandstones of the lower Mulden Group.
 South of the Guinas Fault T6 and lower
 1
Laukamp et al. TSK 11 Göttingen 2006
 T7 show almost the same stratigraphy,
 but instead of the laminated dolomites,
 there are thin laminated and thick bed-
 ded limestones with intercalated mass
 flow breccias. Total thickness of the
 T7 in the south is about 600m, but
 in the north only about 35m (Petzel,
 1993). This huge difference in thickness
 could either be caused by (1) tectonic
 nappe stacking in the south, (2) lateral
 facies change obliterated by the thrust-
 ing or (3) due to growth faulting during
 the deposition of the Hüttenberg Forma-
 tion. Mass flow breccia lenses embed-
 ded in the laminated limestones of the
 T7 lithozone exist only southwest of the
 Guinas Fault. Clasts of carbonate sed-
 iments from the northeastern area are
 enclosed in the mass flows. Restricted
 to the northeast there are algal reefs
 pointing to shallow marine conditions,
 whereas the mass flows and the dark
 limestones and shales might have been
 deposited at a slope just south of the
 Guinas Fault. Therefore the origin of
 the Guinas Fault as a growth fault along
 an unstable slope at the southwestern
 margin of a carbonate platform is pos-
 sible. Similar growth faults have been
 reported from the central OML.
 Differences in the deformation south
 and north of the Guinas Fault are even
 bigger. South of the Guinas Fault our
 study examined a wide range of Dama-
 ran deformations. Bedding parallel first
 cleavage S1 is marked by thin chert lay-
 ers, which are isoclinal folded around
 an NW–SE axis. The second cleav-
 age cuts S1 and small scale thrusts
 and nappe stacking evolve. Therefore
 nappe stacking is evident south of the
 Guinas Fault, but only on a minor
 scale and tectonic thinning of the lime-
 stone units might have partly neutral-
 ized the greater thickness of the stacked
 Figure 2: transect of the Guinas Fault and
 the upper Tsumeb Subgroup (Hüttenberg
 Fm., T6–T8); not to scale
 T7-litho-units. A major lateral facies
 change is not likely, because the small
 dimension of thrusting is not sufficient
 to obliterate the major facies change.
 The hanging wall of the Guinas Fault
 itself is a thrust horizon, which is made
 up of a highly silicified oolitic dolomite.
 Silification happened in a late stage or
 even after the main phase of the north-
 eastward thrusting (D2) along this hori-
 zon.
 A transect through the Guinas Fault
 (Fig. 2) shows a displacement of the car-
 bonates of the upper Tsumeb Subgroup
 (T6–T8, Hüttenberg Formation) along
 the Guinas Thrust. The carbonates are
 thrusted over the Mulden siliciclastics,
 indicating that the thrusting is of an
 Early Cambrian age developed during
 D2. Karst pipe structures, filled by
 Mulden siliciclastics, are common hosts
 for mineralisation in the northern OML,
 like the Tsumeb Pipe, which is deformed
 by Damaran tectonics. Karstification
 down to the middle Tsumeb Subgroup
 occurs preferred at fractures, which are
 related to the syn-D2 folding event. The
 Guinas Fault has been involved again
 during the late Damaran uplift (D3), as
 minor normal faulting along the floor
 thrust of the silicified thrust horizon can
 be referred to D3.
 2
TSK 11 Göttingen 2006 Laukamp et al.
 Applying our data to the base metal
 mineralisation in the northwestern
 OML, we find implications for the
 formation and alteration of ore bodies.
 Deposition of primary base metal
 mineralisation has started at the latest
 during the deposition of the lower
 Mulden siliciclastics (ca. 580-540Ma)
 and before the peak metamorphism of
 the Damaran orogenesis in the OML
 (ca. 535 Ma). Ages of around 530Ma
 (Kamona et al. 1999) may indicate an
 upper age of the primary mineralisation
 or its syn-Damaran remobilisation
 (Fig. 1). Adequate conduits for min-
 eralising fluids could have been the
 Guinas Fault/Thrust itself, strati-
 graphic horizons of higher porosity in
 the Tsumeb Subgroup and karst struc-
 tures in the upper levels (Fig. 2). In a
 later stage of the Damaran orogenesis
 (Fig. 1) hydrothermal fluids could have
 moved along these pathways to the area
 north of the Guinas Fault, whereas the
 distribution to the south was prevented
 by the syn-D2b silicified Guinas Thrust
 horizon that acted down to the lower
 Tsumeb Subgroup as a dam for the
 ascending fluids (Fig. 2). The result
 is a highly-deformed, almost barren
 area in the south, the inverted Guinas
 Growth Fault and the silicified Guinas
 Thrust Horizon in the center and the
 less deformed Uris-Tsumeb-mining
 area with all the base metal sulphide
 deposits, as a precursor for further
 supergene Cu- and V-enrichments in
 the north.
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