
 The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments.
 We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.

The depositional environment of the Koeris Formation in the Aggeneys-Gamsberg ore district, South Africa


 The chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it.
 Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established.
 The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets.
 The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.

New constraints on the age of ore at Black Mountain mine, Bushmanland Ore District, South Africa

Four previously unrecognized neotectonic fault scarps in southwest Namibia are described. These relatively straight, simple but segmented structures are 16–80 km long and have measured vertical separations of 0.7–10.2 m. We estimate that each is capable of producing earthquakes of Mw 6.4 or greater, indicating that large earthquakes may occur despite limited cumulative displacement. There is strong evidence that some of these scarps were formed by repeated earthquakes. Comparison with aeromagnetic and geological maps reveal that the normal faults reactivate major crustal weaknesses that are orientated north‐south and northwest‐southeast and perpendicular to the local gravitational potential energy gradient. The presence of these structures in an area with a limited record of instrumental seismicity suggests that the Mmax of this region may be much larger than generally assumed. They highlight the necessity of incorporating information from fault studies into probabilistic seismic hazard assessments in this region, in a similar way to other stable continental regions such as Australia. The fact that such major structures have gone hitherto unrecorded suggests significant further research is needed to characterize these sources of hazard. The identification of an apparent cluster of large magnitude neotectonic earthquakes in the area may be related to the exceptional preservation potential of scarps rather than indicating an area of comparatively rapid deformation. If this interpretation is correct, then these scarps represent an important indication of the potential seismic hazard across the region, and the occurrence of infrequent large‐magnitude seismicity on similar structures should be considered throughout southwestern Africa.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2022TC007693

Exceptional Scarp Preservation in SW Namibia Reveals Geological Controls on Large Magnitude Intraplate Seismicity in Southern Africa

Four previously unrecognized neotectonic fault scarps in southwest Namibia are described. These relatively straight, simple but segmented structures are 16–80 km long and have measured vertical separations of 0.7–10.2 m. We estimate that each is capable of producing earthquakes of Mw 6.4 or greater, indicating that large earthquakes may occur despite limited cumulative displacement. There is strong evidence that some of these scarps were formed by repeated earthquakes. Comparison with aeromagnetic and geological maps reveal that the normal faults reactivate major crustal weaknesses that are orientated north-south and northwest-southeast and perpendicular to the local gravitational potential energy gradient. The presence of these structures in an area with a limited record of instrumental seismicity suggests that the Mmax of this region may be much larger than generally assumed. They highlight the necessity of incorporating information from fault studies into probabilistic seismic hazard assessments in this region, in a similar way to other stable continental regions such as Australia. The fact that such major structures have gone hitherto unrecorded suggests significant further research is needed to characterize these sources of hazard. The identification of an apparent cluster of large magnitude neotectonic earthquakes in the area may be related to the exceptional preservation potential of scarps rather than indicating an area of comparatively rapid deformation. If this interpretation is correct, then these scarps represent an important indication of the potential seismic hazard across the region, and the occurrence of infrequent large-magnitude seismicity on similar structures should be considered throughout southwestern Africa. 

Tectonic evolution of the Aggeneys-Gamsberg Ore District, South Africa, and implications for the geodynamic setting of the Namaqua Sector

A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from ∼1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at ∼1350 Ma occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at ∼1290–1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at ~1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between ∼2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 1350–1240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at ∼1210 Ma (peak conditions: 4.5–6 kbar and 770–850 °C), associated with voluminous, predominantly S-type granitoid orthogneisses between ∼1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until ∼1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a ‘hot orogen’ developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts. Ductile D2 deformation resulted from “bottom-driven” tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at ∼1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between ∼1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080–1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at ∼990 Ma. 

C. Spencer

Papers

A continental back-arc setting for the namaqua belt: Evidence from the kakamas domain