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ABSTRACT:

The precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform-building process during deposition of the 2560-2520 Ma Campbellrand-Malmani carbonate platform, South Africa. Aragonite fans and fibrous coatings are common in unrestricted, shallow subtidal to intertidal facies. They are also present in restricted facies, but are absent from deep subtidal facies. Decimetre-thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies. The proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0% to > 65% depending on the facies. Subtidal facies commonly contain 20-35%in situ precipitated carbonate, demonstrating that Neoarchaean sea water was supersaturated with respect to aragonite, carbonate crystal growth rates were rapid compared with sediment influx rates, and the dynamics of carbonate precipitation were different from those in younger carbonate platforms. The abundance of aragonite pseudomorphs suggests that sea-water pH was neutral to alkaline, whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans.