Weather-enhanced sulphide oxidation accelerates carbonate dissolution and releases CO₂ into the atmosphere. Yet over extended geological timescales, transitions from CO₂ degassing to sequestration may also occur for ultramafic/mafic magmatic minerals. Ultramafic/mafic mine tailings provide a unique opportunity to monitor carbon balance processes because mine waste undergoes instantaneous and rapid chemical weathering, which shortens the duration from CO₂ degassing to absorption. In this study, we analysed 30 ultramafic/mafic (V-Ti-Fe ore) mine tailings ponds with varying cessation times in the Panxi region, China. These sites have outcrops of mega-mine waste from ~60 years of mineral excavation and dressing. Based on the analysis of anions, cations, saturation simulations, and ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ³⁴S isotopic fingerprints from mine tailings filtrates, it was found that the dissolution load of mine tailings may depend significantly on early-stage sulphide oxidation. Dolomite dominates chemical weathering (accounting for ~79.2% of the cationic load), despite the abundance of ultramafic/mafic minerals in tailings (~5.1% cationic flux sources from silicate weathering and ~15.7% from exotic input). In addition, owing to sulphuric-carbonate weathering, the filtrates undergo deacidification along with sulphide depletion. The data in this study suggests that pristine V-Ti-Fe tailings ponds should undergo CO₂ degassing in the first two years, after which they can fix atmospheric CO₂ along with SO₄^2- at levels less than ~3000 μmol/L in the filtrates (or TDS ≤ 700 mg/L). Our results permit improved monitoring of weathering transitions and carbon balances in ultramafic and mafic rocks.

Ultramafic/mafic weathering,Sulphide oxidation,CO2 transition,V-Ti-Fe mine tailings