Abstract

Large-scale, carbonate-hosted magnesite (MgCO3) deposits, although rare, occur mainly in Precambrian strata. Although many occurrences have characteristics consistent with penecontemporaneous formation in an evaporative marine setting, the general absence of CaSO4 minerals has precluded the adoption of evaporative marine depositional models. In modern seawater, excess Ca2+ and Mg2+ relative to

\(CO^{2{-}}_{3}\)
and
\(HCO^{{-}}_{3}\)
as well as abundant
\(SO^{2{-}}_{4}\)
require that, upon evaporation, MgCO3 precipitation is accompanied by substantial deposition of CaSO4 minerals. Here we use evidence from a Neoproterozoic magnesite deposit to suggest that differences in Precambrian seawater geochemistry enabled MgCO3 to form in isolation under evaporative conditions. During the Precambrian, precipitation of CaSO4 evaporites was hindered by (1) elevated dissolved inorganic carbon and enhanced precipitation of CaCO3, which limited the availability of Ca2+, and (2) a small marine sulfate reservoir. Because sulfate is an inhibitor to dolomitization, low sulfate concentrations increased the potential for penecontemporaneous dolomitization in marine settings. By utilizing Ca2+, dolomitization served to increase fluid Mg/Ca ratios. In this
\(HCO^{{-}}_{3}\)
-rich but
\(SO^{2{-}}_{4}\)
-poor system, dolomitization coupled with significant evaporative concentration resulted in magnesite formation without coprecipitation of CaSO4 minerals. Decreasing carbonate saturation, progressive oxygenation, and a concomitant increase in sulfate availability during the Proterozoic ultimately led to the development of the more familiar conditions of the Phanerozoic, in which dolomitization was restricted to environments where elevated Mg/Ca ratios could overcome the inhibitory effects of sulfate and significant magnesite deposition was restricted to sabkhas and alkaline lakes.

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