NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE DEPOSIT

Bulk measurements of neutralization potential in mine wastes may not be relevant to the evolution of solution chemistry depending on the availability of acid-neutralization capacity (ANC) as a function of time. Reliance on ANC from silicates is controversial, but seldom studied with long-term, empirical data.

We examine the availability of ANC in two samples, VR71213 (< 0.2 % S_tot) and VR90125, (8.13 % S_tot) from a small, high-grade, Ni-Cu magmatic sulfide deposit. The ore is pentlandite >> chalcopyrite in pyrrhotite-dominated, massive to disseminated sulfides hosted by peridotite. Long-term (288 – 317 weeks) column leaching shows that waste intervals in the peridotite will generate neutral drainage, whereas ore-grade rocks evolve through a series of pH-buffered states to mildly acidic effluents. The nature and concentrations of dissolved metals in leachates are functions of the sulfide minerals and their grades, as the pH range evolves during sulfide oxidation.

Small concentrations of interstitial carbonate are important to the initial leaching period, but late-time effluents show that weathering acids are being neutralized by reaction with silicates. In VR71213, the long-term weathering acid is H₂CO₃; in VR90125 it is dominantly H₂SO₄. Mg²⁺ is the dominant solution cation, and Mg/Si ratios in effluents are intermediate between fosterite and lizardite , suggesting congruent dissolution of olivine and serpentine at long times.

Available ANC for the long-term leaching sequences was evaluated as [eq. H⁺ neutralized •m^-2 rock surface leached • week^-1], calculated from the equivalents of [Ca²⁺ + Mg²⁺ + Na⁺ + K⁺] released to solution by titration of the gangue silicates by weathering acid. For weathering by H₂CO₃, the available ANC is calculated from VR71213 as 1.3 E-4 eq. H⁺ per m² per week at near pH 8. For VR90125, after early carbonates have dissolved, the available ANC increases from 5.8 E-4 to 9.2 E-4 eq. H⁺ per m² per week as pH falls from 6 to 4. The available ANC increases with the strength of the dominant weathering acid and with proton flux from sulfide oxidation.

The long-term solution chemistry of the sulfide waste is controlled by the available ANC provided dominantly by olivine and lizardite. These silicate minerals neutralize acid rapidly enough to control the longevity of neutral drainage in these rocks.