Evidence and significance of the oldest Paleoarchean to Mesoproterozoic evaporites

Evaporites are rarely recorded in the Precambrian. In the oldest rocks they are known mostly from pseudomorphs of salt minerals, or can be inferred from other sedimentary and geochemical features. Only in some younger rocks are they present as salt minerals.

About 50 inferred or definitive occurrences of evaporites in Archean through Mesoproterozoic rocks were compiled. These data allow characterisation of the mineralogy and sedimentary environments of the earliest evaporite sediments and insight into their evolution over time.

The earliest documented evaporites are from the Archean eon, with about 15 occurring mostly in the Paleoarchean (3.6–3.2 Ga) and Neoarchean era (2.8–2.5 Ga). 

The earliest Paleoarchean deposits considered as „evaporitic” in origin are bottom-grown barite crystals, formerly interpreted as pseudomorphs after gypsum, and silica pseudomorphs after radiating splays of aragonite in North Pool Chert of the Dresser Formation (3.48 Ga old), Australia. Barite and aragonite presumably crystallized in a volcanic caldera evaporitic basin from brine of both hydrothermal and seawater derivation. However barite, unlike aragonite, cannot be classified as an evaporite mineral due its very low solubility. The other Palaeoarchean evaporites are represented mostly by enigmatic pseudomorphs (after possible gypsum, aragonite, nahcolite, halite, and others). The Archean evaporite crystals are interpreted as precipitated in both marine and non-marine environments, including soils or weathering zones where they could represent terrestrial or pedogenic evaporites.

In the Proterozoic eon the most frequent occurrences are from the Paleoproterozoic (Rhyacyan, Orosirian and Statherian; 2.3–1.6 Ga). Their appearance directly follows the beginning of the Great Oxidation Event in Siderian at about 2.4 Ga. The first abundant evaporites, with mineralogy similar to the present-day marine evaporites (carbonates, Ca-sulphates, halite, and KMg sulphates), appear in the Mesoproterozoic and include several saline giants (evaporites with volume ≥ 1000 km³). The oldest ones are: a) 2.31 Ga old Gordon Lake Formation, Canada, and Kona Dolomite, USA, b) ca 2.0 Ga old Tulomozero Formation, Onega Basin, Karelian craton, Russia (with preserved KMg salts), c) 2.1 Ga old Juderina Formation, Yilgarn craton, Australia. They strongly suggest appearance of marine water very similar to the modern ocean water.

Information about evaporite minerals from the Archean era is uncertain and ambiguous, coming from enigmatic pseudomorphs and geochemical signals. This evidence originates from sedimentary environments that are not widely recognised, including marine, terrestrial, hydrothermal and/or lacustrine environments. Such evidence does not provide a basis for unambiguously characterising the composition of Archean seawater.