The formation of antitaxial calcite veins with well-developed fibres, Oppaminda Creek, South Australia

Abstract

Of the several vein types with elongate crystals, we distinguish a class of veins with well-developed fibres, exposed at Oppaminda Creek (South Australia). These calcite veins are characterised by very limited competition between growing crystals, which leads to the development of fibrous crystals with smooth boundaries and which can attain extreme length–width ratios. Fibres are often curved and track the opening trajectory. The veins are also characterised by their outwards, antitaxial growth, starting from a narrow median zone. These median zones contain wall rock inclusions and a distinctly different, non-fibrous internal microstructure. Such veins continue growing as long as extension is suitably oriented relative to the vein orientation. Multiple, cross-cutting vein sets can thus grow simultaneously and can together track over 90° rotation of the fibre growth direction. The veins formed by calcite precipitation on both their outer surfaces. We suggest that the term ‘antitaxial’ should be redefined as applying only to those veins that have two simultaneous growth planes at their outer surfaces. Antitaxial veins grow without further fracturing, which sets them apart from other veins that grew by precipitation inside a crack, and which only have one single growth plane at the time. The veins at Oppaminda Creek are about 585 Ma in age, and therefore formed at an estimated 4–6 km depth. Scanning electron microscope observations revealed about 1-μm-sized structures, which we interpret as fossils of microbes that lived inside the veins at the time of their formation, and which may even have played a role in the vein formation. These structures would represent the first finds of fossilised life forms of such a combined age and depth.