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Detailed mapping at the Rosh Pinah massive Zn-Pb-Cu sulphide deposit, southern Namibia (EM-anomaly 12, Eastern Squabble Valley)

Andreas Kamradt (2005)

This mapping project was founded by a cooperation with the mining company Kumba Resources Ltd.. A contract provided a field mapping of geological and structural features of the electromagnetic anomaly 12 on a scale of 1:1000. The mapping area is situated 2 km north of Posh Pinah in the south westernmost part of Namibia, where Kumba Resources Ltd. run a mining complex in the Neoproterozoic Zn-Pb-(Cu-Ag) Rosh Pinah deposit. The host rocks of the base metal mineralization were deposited in the Rosh Pinah graben that was formed as second order basin during continental rifting bound to the opening of the proterozoic Adamastor Ocean at ~ 740 Ma. The graben fillings, which were partly characterized by unstable conditions of deposition, were folded and faulted by the multi-stage tectonic evolution of the Gariep belt, caused by the Adamastor closure at ~ 580 Ma and progressively continental collision. Stratigraphically, the meta-rocks of the mapping area belong to the Rosh Pinah Formation in the upper part of the Hilda Subgroup of the para-authochtonous Port Nolloth Zone.

The geology of the mapping area is dominated by middle greenschist facies metamorphosed siliciclastic rock successions, which consists of mainly meta-arkose beds up to 3 metres thickness and subordinate exposed argillite bands with true thickness up to 8 m, phyllites, microquartzites and tectonic breccias, like cataclastic and hydraulic breccias. Sedimentary features like load cast and fining upwards graded beds has been observed and were used to determine turbitic cycles within the sedimentary succession (Fig. 1).

Fig. 1 Loadcasts, a common sedimentary feature of turbiditic successions display difference of grain size between lower (finer) and upper (coarser) bed. The upper bed is marked by grain size gradation.

Fig. 1 Loadcasts, a common sedimentary feature of turbiditic successions display difference of grain size between lower (finer) and upper (coarser) bed. The upper bed is marked by grain size gradation.

Fig. 1 Loadcasts, a common sedimentary feature of turbiditic successions display difference of grain size between lower (finer) and upper (coarser) bed. The upper bed is marked by grain size gradation.

Namely three deformation stages have affected the rocks in the eastern part of the Port Nolloth Zone. Structural investigations yielded that open to close folds were produced in the eastern and western part of the mapping area during compressive D1-deformation (Fig. 2). Bedding sub-parallel thrusting accompanied this deformation stage and result in two recognized faults. In a second step and progressively deformation, D1 folds were refolded to SSE – NNW trending SW-vergent subhorizonal plunging backfolds by sinistral SE-directed transpression in D2 (Fig. 3).

Fig. 2 Cross section from the western part of mapping area presents the separation of the steep SE-dipping overturned syncline and the open anticline by a sinistral strike-slip with anticlockwise rotation.

Fig. 2 Cross section from the western part of mapping area presents the separation of the steep SE-dipping overturned syncline and the open anticline by a sinistral strike-slip with anticlockwise rotation.

Fig. 2 Cross section from the western part of mapping area presents the separation of the steep SE-dipping overturned syncline and the open anticline by a sinistral strike-slip with anticlockwise rotation.

Fig. 3 Modified field photograph of the syncline-anticline-structure of the cross section above. The sinistral strikeslip fault represents in all probability the Mountain ore body fault, which cut the western limb of the regional-scaled Rosh Pinah anticline.

Fig. 3 Modified field photograph of the syncline-anticline-structure of the cross section above. The sinistral strikeslip fault represents in all probability the Mountain ore body fault, which cut the western limb of the regional-scaled Rosh Pinah anticline.

Fig. 3 Modified field photograph of the syncline-anticline-structure of the cross section above. The sinistral strikeslip fault represents in all probability the Mountain ore body fault, which cut the western limb of the regional-scaled Rosh Pinah anticline.

Brittle deformation and shearing led to the formation of steep NE-dipping faults and influencing of D1-related trusts with renewed brecciation of the previous faulted rocks. The D2 deformation result in a sinistral strike-slip fault system with associated wrenching that is displayed by a steep SE-plunging syncline. Weak effects of a third, rather compressive deformation phase were noticeable by slight crenulation of S2 and double plunging fold axes in the mapping area (Fig. 4).

Fig. 4 Modified photograph of the eastern part in the mapping area and northern elongation shows synclinal (black hinge line) and anticlinal (red hinge line) structures, which form a dome-and-basin structure; bedding traces in metaarkose dotted, bedding traces are supplied with signs for fining upwards.

Fig. 4 Modified photograph of the eastern part in the mapping area and northern elongation shows synclinal (black hinge line) and anticlinal (red hinge line) structures, which form a dome-and-basin structure; bedding traces in metaarkose dotted, bedding traces are supplied with signs for fining upwards.

Fig. 4 Modified photograph of the eastern part in the mapping area and northern elongation shows synclinal (black hinge line) and anticlinal (red hinge line) structures, which form a dome-and-basin structure; bedding traces in metaarkose dotted, bedding traces are supplied with signs for fining upwards.

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