Marine origin for Precambrian, carbonate-hosted magnesite?
Marine origin for Precambrian, carbonate-hosted magnesite?
Geology (Boulder) (December 2003) 31 (12): 1101-1104
- Adelaide Geosyncline
- alkaline earth metals
- Australasia
- Australia
- bedding
- C-13/C-12
- calcium
- carbon
- carbonate rocks
- carbonates
- carbonatization
- chemically precipitated rocks
- depositional environment
- dolomitization
- dolostone
- evaporites
- geochemistry
- isotope ratios
- isotopes
- lithofacies
- lithostratigraphy
- magnesite
- magnesium
- marine environment
- metals
- Neoproterozoic
- planar bedding structures
- Precambrian
- Proterozoic
- sedimentary rocks
- sedimentary structures
- South Australia
- stable isotopes
- upper Precambrian
Large-scale, carbonate-hosted magnesite (MgCO (sub 3) ) 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 CaSO (sub 4) minerals has precluded the adoption of evaporative marine depositional models. In modern seawater, excess Ca (super 2+) and Mg (super 2+) relative to (CO (super 2-) ) (sub 3) and (HCO (super -) ) (sub 3) as well as abundant (SO (super 2-) ) (sub 4) require that, upon evaporation, MgCO (sub 3) precipitation is accompanied by substantial deposition of CaSO (sub 4) minerals. Here we use evidence from a Neoproterozoic magnesite deposit to suggest that differences in Precambrian seawater geochemistry enabled MgCO (sub 3) to form in isolation under evaporative conditions. During the Precambrian, precipitation of CaSO (sub 4) evaporites was hindered by (1) elevated dissolved inorganic carbon and enhanced precipitation of CaCO (sub 3) , which limited the availability of Ca (super 2+) , 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 Ca (super 2+) , dolomitization served to increase fluid Mg/Ca ratios. In this (HCO (super -) ) (sub 3) -rich but (SO (super 2-) ) (sub 4) -poor system, dolomitization coupled with significant evaporative concentration resulted in magnesite formation without coprecipitation of CaSO (sub 4) 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.
