Origin of ultramafic-hosted vein magnesite deposits

Ore Geology Reviews 7, no.3 (1992) p. 155-191

Genetic questions about ultramafic-hosted vein deposits are summarized with the aid of flow charts. Most vein magnesite is attributed to an influx of metamorphic-degassing CO₂ and CH₄ into groundwater which flows through serpentinite or peridotite. The source of CO₂-CH₄ is considered to be deeper than 10 km although vein magnesite precipitation generally has occurred within just a few hundred metres of the Earth's surface. Vein deposits are considered to be genetically related to massive bodies but the massive bodies typically contain less ¹²C. The ¹²C-rich CO₂, which has produced vein deposits, is attributed to a fluid phase which rose until it reached upper-crustal groundwater. CO₂-enriched water is envisioned to have been expelled regionally due to tectonic processes, leaving little mineralogical evidence of its 

expulsion through nonultramafic rock. CO₂-rich water which happened to encounter either serpentinite or peridotite presumably has produced magnesite vein deposits and a common silicate byproduct, nontronite.