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

摘要

In the Barberton Greenstone Belt, South Africa, a 100–250 m thick complexof carbonaceous chert dikes marks the transition from the Mendon Formation to the MapepeFormation (3260 Ma). The sub-vertical- to vertical position of thefractures, the abundance of highly shattered zones with poorly rotatedangular fragments and common jigsaw fit, radial structures, and multipleinjection features point to repetitive hydraulic fracturing that releasedoverpressured fluids trapped within the shallow crust. The chemical andisotopic compositions of the chert favour a model whereby seawater-derivedfluids circulated at low temperature (< 100–150 °C) withinthe shallow crust.From the microscopic structure of the chert, the injected material was aslurry of abundant clay-sized, rounded particles of silica, carbonaceousmatter and minor clay minerals, all suspended in a siliceous colloidalsolution. The dike geometry and characteristics of the slurry concur on thatthe chert was viscoelastic, and most probably thixotropic at the time ofinjection: the penetration of black chert into extremely fine fractures isevidence for low viscosity at the time of injection and the suspension oflarge country rock fragments in the chert matrix provides evidence of highviscosity soon thereafter. We explain the rheology by the particulate andcolloidal structure of the slurry, and by the characteristic of silicasuspensions to form cohesive 3-D networks through gelation.Our results provide valuable information about the compositions,physical characteristics and rheological properties of the fluids thatcirculated through Archean volcano-sedimentary sequences, which is anadditional step to understand conditions on the floor of Archean oceans, thehabitat of early life.