Elemental signatures of metamorphic, diagenetic, and pedogenic magnesites from Central Queensland, Australia

Magnesium carbonates record information on water-rock interactions during and after mineral precipitation. The Marlborough Terrane in central Queensland, Australia, contains magnesite-bearing serpentinite highlands surrounded by low-lying sedimentary basins that host authigenic magnesite (MgCO₃). Open pit mines in both settings provide exposures of serpentinites (Gumigil) and Cenozoic sediments and overlying black soils (Yaamba) that host the magnesite and other authigenic phases. The Gumigil mine contains deeply weathered serpentinite hosting metamorphic magnesite veins that formed syn-tectonically; both serpentinite and magnesite are now partially dissolving, silicifying, and ferruginizing. Aqueous Mg²⁺ is being exported into the basins surrounding the serpentinite ridges. The Yaamba magnesite mine in the surrounding plains exposes diagenetic magnesite formation within unlithified alluvial sediments, where ascending magnesium-rich groundwaters replace arkosic sands and silts by magnesite cements, nodules, and pinnacles. Late-stage pedogenic processes at Gumigil and Yaamba drive retrograde transformation of magnesite into geochemically distinct exterior regions of second-generation cryptocrystalline magnesite recording interactions with Fe/Mn-oxides/hydroxides via cerium anomalies, yttrium anomalies and manganese concentrations in zoned magnesites from Yaamba. The complex history of mineral precipitation, dissolution, diagenetic replacement, and supergene alteration is recorded in the major, minor and trace element compositions of magnesites at each site. Serpentinite ridges and magnesite-bearing valley floors in Central Queensland provide a useful analog to the processes that might occur in the ultramafic highlands and carbonated lowlands at Jezero crater, Mars.