The geological setting of uranium mineralisation in the Rössing Area, Namibia

Abstract

The geology of the Rössing uranium mine area has been mapped and described through compilation and interpretation of all available data, underpinned by field traversing and sampling. The understanding of the stratigraphic and structural setting of granite-hosted uranium mineralisation in the wider Rössing area has in particular been refined. HyMap hyperspectral imagery significantly contributed to resolving the mapped geology in detail. Descriptions of the stratigraphic sequence have been compiled and augmented particularly within the upper parts of the Damaran stratigraphy. A three member diamictite-iron formation stratigraphy is constrained within the Chuos Formation whilst eight members are identified through the Arandis to Karibib Formation sequence. The detailed resolution of the stratigraphy has also resolved many aspects of the structural geology in the area. Pre-D3 extension is expressed as a sheared basement-cover interface, major boudinage of stratigraphic units, and exploitation of such dilational zones by early alaskite intrusions. Within the F2-F4 fold phases identified across the area, major dome and basin and hook geometries result from F2-F3 fold interference. D3 to D4 deformation was progressive where the latter consisted of the development of early, minor north-northeast trending folds and left lateral faulting. Map relationships suggest this was followed by the formation of north-northeast oriented transtensional shear zones highly oblique to pre-D4 litho-structural orientations and into which uranium-bearing granitic melts were focused. Alaskite generation is attributed to migmatisation and partial melting of potassic meta-psammites during lower granulite facies metamorphism. The sequence of alaskite intrusions is correlated with the scheme applied to the nearby Goanikontes uranium deposit with varying confidence. At Rössing SJ the intrusion of voluminous uranium-bearing alaskite melt material was especially facilitated by superimposition of a north-northeast trending shear zone upon both a major F2-F3 fold interference structure and a pre-existing D4 fault zone. Uraniferous alaskites at the SH and SK ore bodies intrude peripheral F3 fold structures superimposed on the F2 folds. In the Z20-Z19-Z17 to Rössing South Zone 1 area, mineralised alaskites occur within the Rössing, Arandis and Karibib Formations, where doming of the Rössing South anticline is also demonstrated to be due to F2-F3 fold interference.