Mineralisation of atmospheric CO2 in hydromagnesite in ultramafic mine tailings – Insights from Mg isotopes

In this study we present the first Mg isotope data that record fate of during mineralisation atmospheric CO2 in ultramafic mine tailings. At Woodsreef Asbestos Mine, New South Wales, Australia, weathering waste sequesters significant amounts hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Mineralisation above-ground, sub-aerially stored tailings is driven by infiltration rainwater dissolving from bedrock minerals Hydromagnesite, forming on surface tailings, has lower ?26Mg (?26MgHmgs = ?1.48 ± 0.02‰) than serpentinised harzburgite (?26MgSerpentinite ?0.10 0.06‰), bulk (?26MgBulk ?0.29 0.03‰) and weathered containing authigenic clay (?26MgWeathered +0.28 0.06‰). Dripwater (?26MgDripwater ?1.79 co-existing ?2.01 0.09‰), a tunnel within nodular magnesite [MgCO3] (?26MgMgs ?3.26 0.10‰) have surficial fluid (?26Mg ?0.36‰) hydromagnesite. Complete dissolution source minerals, or formation Mg-poor products weathering, expected to transfer into solution without alteration isotopic composition. Aqueous geochemical modelling saturation indices, along with Rayleigh distillation mixing calculations, indicate 26Mg-depletion drip water, relative result brucite and/or precipitation secondary Mg-bearing silicates cannot be assigned dissolution. Our results show main mineral sources (silicate, oxide/hydroxide carbonate minerals) are isotopically distinct composition fluids thus precipitating reflects both Mg-rich phases. The consistent enrichment depletion 26Mg carbonates, respectively, underpins use presented compositions as tracer pathways rocks.