Precambrian evaporites

The oldest documented chevron halite is 850Ma and the oldest bedded anhydrite is 1.2Ga, beyond that only evaporite pseudomorphs are available to study. So, beyond the 850 Ma record of oceanic chemistry changes established by halite inclusions, one can trace halite other Precambrian evaporites such as gypsum, anhydrite and trona, nahcolite back through time by documented occurrences of their pseudomorphs. But this type of documentation by its nature is patchy and gives indirect clues as to a chemical scenario for the Precambrian world’s paleo-oceans and brine?


Pseudomorphs, especially of halite hoppers, occur in marine rocks as old as Archaean, but are far more common, as are the actual salts, in Proterozoic strata. Halite or its pseudomorphs characterise areas of widespread marine chemical sedimentation from the Archaean to the present. CaSO4 pseudomorph distribution is more enigmatic. In the 1980s and 1990s, the oldest documented CaSO4 pseudomorphs were thought to pseudomorphically preserved in cm-sized growth-aligned barytes and cherts in 3.45 Ga metasediments in the Pilbara/North Pole region of Western Australia. They were interpreted as replacing primary bottom-nucleated gypsum and occur in volcaniclastics, in association with what are possibly the world’s oldest stromatolites.  Similar growth-aligned baryte crystals, which were also interpreted as likely primary gypsum pseudomorphs, occur in the Nondweni greenstones in South Africa, some 3.4 Ga.


Sequences in both regions are now completely silicified or barytised. At the time they were first documented, the recognition of what were considered shallow-water Early Archaean gypsum pseudomorphs at North Pole, Pilbara Craton, caused a re-evaluation of models of a totally reducing Archaean atmosphere. The presence of free sulphate in surface brines of the Archaean world was thought to imply an at least locally oxygenated hydrosphere. Gypsum precipitating in Archaean ocean waters also meant calcium levels in the ocean waters were in excess of bicarbonate, as is in the modern oceans. The presence of free-standing gypsum on the seafloor is incompatible with any model of the Early Archaean ocean as a “soda lake.”


However, in both the Pilbara and the South African sequences there are no actual calcium sulphate evaporites preserved, only growth-aligned crystal textures now preserved as baryte or chert. Textures in baryte ore from Frasnian sediments in Chaudfontaine, Belgium, are near identical to those observed at North Pole, Australia. The Belgian barytes are primary sea-bottom precipitates with no precursor mineral phase). Some workers in the Pilbara feel that the growth-aligned baryte in this region is also a primary precipitate, formed in the vicinity of hydrothermal vents. As such, it is not secondary after gypsum. A similar hydrothermal discharge model has been developed for aligned barytes in the Barberton Greenstone belt.



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