Articles | Volume 6, issue 1
Research article
20 Feb 2015
Research article |  | 20 Feb 2015

The rheological behaviour of fracture-filling cherts: example of Barite Valley dikes, Barberton Greenstone Belt, South Africa

M. Ledevin, N. Arndt, A. Davaille, R. Ledevin, and A. Simionovici

Abstract. In the Barberton Greenstone Belt, South Africa, a 100–250 m thick complex of carbonaceous chert dikes marks the transition from the Mendon Formation to the Mapepe Formation (3260 Ma). The sub-vertical- to vertical position of the fractures, the abundance of highly shattered zones with poorly rotated angular fragments and common jigsaw fit, radial structures, and multiple injection features point to repetitive hydraulic fracturing that released overpressured fluids trapped within the shallow crust. The chemical and isotopic compositions of the chert favour a model whereby seawater-derived fluids circulated at low temperature (< 100–150 °C) within the shallow crust.

From the microscopic structure of the chert, the injected material was a slurry of abundant clay-sized, rounded particles of silica, carbonaceous matter and minor clay minerals, all suspended in a siliceous colloidal solution. The dike geometry and characteristics of the slurry concur on that the chert was viscoelastic, and most probably thixotropic at the time of injection: the penetration of black chert into extremely fine fractures is evidence for low viscosity at the time of injection and the suspension of large country rock fragments in the chert matrix provides evidence of high viscosity soon thereafter. We explain the rheology by the particulate and colloidal structure of the slurry, and by the characteristic of silica suspensions to form cohesive 3-D networks through gelation.

Our results provide valuable information about the compositions, physical characteristics and rheological properties of the fluids that circulated through Archean volcano-sedimentary sequences, which is an additional step to understand conditions on the floor of Archean oceans, the habitat of early life.

Short summary
We investigate the composition, physical and rheological properties of fluids at the origin of Palaeoarchean chert dikes in South Africa. The dikes formed by repetitive hydraulic fracturing as overpressured oceanic fluids were released at low temperatures as a siliceous slurry. The gelation capacity of silica conferred the chert precursor a viscoelastic, probably thixotrope behaviour. It is an additional step to understand fluid circulations towards the ocean floor, the habitat of early life.