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

Abstract

Abstract. A 100 m-thick complex of near-vertical carbonaceous chert dikes marks the transition from the Mendon to Mapepe Formations (3260 Ma) in the Barberton Greenstone Belt, South Africa. Fracturing was intense in this area, as shown by the profusion and width of the dikes (ca. 1 m on average) and by the abundance of completely shattered rocks. The dike-and-sill organization of the fracture network and the upward narrowing of some of the large veins indicate that at least part of the fluid originated at depth and migrated upward in this hydrothermal plumbing system. Abundant angular fragments of silicified country rock are suspended and uniformly distributed within the larger dikes. Jigsaw-fit structures and confined bursting textures indicate that hydraulic fracturing was at the origin of the veins. The confinement of the dike system beneath an impact spherule bed suggests that the hydrothermal circulations were triggered by the impact and located at the external margin of a large crater. From the geometry of the dikes and the petrography of the cherts, we infer that the fluid that invaded the fractures was thixotropic. On one hand, the injection of black chert into extremely fine fractures is evidence for low viscosity at the time of injection; on the other hand, the lack of closure of larger veins and the suspension of large fragments in a chert matrix provide evidence of high viscosity soon thereafter. The inference is that the viscosity of the injected fluid increased from low to high as the fluid velocity decreased. Such rheological behavior is characteristic of media composed of solid and colloidal particles suspended in a liquid. The presence of abundant clay-sized, rounded particles of silica, carbonaceous matter and clay minerals, the high proportion of siliceous matrix and the capacity of colloidal silica to form cohesive 3-D networks through gelation, account for the viscosity increase and thixotropic behavior of the fluid that filled the veins. Stirring and shearing of the siliceous mush as it was injected imparted a low viscosity by decreasing internal particle interactions; then, as the flow rate declined, the fluid became highly viscous as the inter-particulate bonds (siloxane bonds, Si-O-Si) were reconstituted. The gelation of the chert was rapid and the structure persisted at low temperature (T < 200 °C) before fractures were sealed and chert indurated.

Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?

“…Some of the wall rock fragments show evidence of in situ brecciation by the preservation of a jigsaw fit (Fig. 2d;Hofmann and Bolhar, 2007;Ledevin et al, 2014).…”

“…In addition, fragments of surrounding wall rock up to 10 cm in diameter are sometimes found floating in a matrix of chert (Fig. 2d;Hofmann, 2005), suggesting thixotropic properties of the vein-filling chert (Ledevin et al, 2014). Some of the wall rock fragments show evidence of in situ brecciation by the preservation of a jigsaw fit (Fig.…”

“…2b, f and 3). The precipitation of silica in the veins has been attributed to the rapid conversion of colloidal particles of silica into cohesive 3-D networks of highly viscous silica gel that was subsequently transformed to microcrystalline quartz (Ledevin et al, 2014). Some veins show multiple vein fillings of chert and coarsely crystalline quartz, suggesting that both formed at approximately the same time (c.f.…”

“…The silicification is predominantly observed at the top of mafic to ultramafic lava flows at the contact with sedimentary chert horizons (Hofmann and Harris, 2008). A unique feature of these silicified zones is the presence of abundant chert veins that are either stratiform or crosscut the lithological contacts at high angles (Hofmann and Bolhar, 2007;Ledevin et al, 2014;Lowe, 2013;this study). Whereas earlier models interpreted the Simetasomatism as a results of weathering of volcanic flow tops (Lowe and Byerly, 1986), or hydrothermal alteration (Duchac and Hanor, 1987;Hanor and Duchac, 1990), newer field and isotope geochemical studies point to low-temperature hydrothermal processes and fluid-rock interaction with Archaean seawater close to the sediment-water interface (Hofmann and Harris, 2008;Rouchon and Orberger, 2008).…”