A stable-isotope study of magnesites from the Rum Jungle Uranium Field, Australia: Implications for the origin of strata-bound massive magnesites

Abstract

The contrast between the scanty magnesite deposition in modern sedimentary environments and the large strata-bound magnesites of the past constitutes a “magnesite problem” which, unlike the “dolomite problem”, has so far attracted little attention from the geologists. Magnesite beds up to 700 m thick from the Lower Proterozoic rocks of the Rum Jungle Uranium Field, Northern Territory of Australia, offer attractive circumstances for studying the isotope systematics of coarse-crystalline magnesite and coexisting dolomite. Magnesites yield δ 18O and δ 13C compositions of + 11.2 to + 14.6‰ (SMOW) and + 1.0 to + 5.8‰ (PDB), respectively. Dolomites yield δ 18O- and δ 13C- values of + 11.7 to + 21.9‰ and − 4.0 to + 2.9‰, respectively, and show an 18O enrichment gradient away from the magnesite. Fluid-inclusion homogenization temperatures, used in conjunction with the isotope determinations and an approximate magnesite-water equation, indicate that magnesites were formed in the temperature range of 100–200°C from fluids that became progressively enriched in 18O. The results support an epigenetic-metasomatic replacement model of dolomite by magnesite under the influence of invasive hot Mg-rich basinal fluids. Strata-bound massive magnesites, which abound particularly in Archean-Proterozoic rock sequences, generally preserve the sedimentary structures but do not retain primary isotopic compositions and are therefore unsatisfactory indicators of oceanic isotope evolution and ocean paleotemperatures.