Alkalinity generation from weathering of accessory calcite and apatite and acid drainage neutralization in an Archean granitoid waste rock

Abstract

Abstract Formation of acid rock drainage is a temporal balance of acid-generating and alkalinity-generating reactions during mineral weathering. Accessory calcite and apatite are commonly present in granitoid rocks, and their weathering can be a source of net alkalinity. To evaluate the acid-consuming capability of the calcite and apatite group minerals in a granitoid waste rock, a humidity cell experiment was conducted with a mix of a high carbonate tonalite and a low carbonate pegmatite representative of the country rock surrounding a kimberlite pipe at the Diavik Diamond Mine in Canada. The alkalinity generated from the tonalite + pegmatite was evaluated through release of Ca and Sr, which indicated a primary alkalinity-generating period during the first 25\u202fweeks of the 80-week experiment. This period represents decomposition of readily available carbonates (infillings) and phosphates (euhedral grains) that release acid-consuming CO3 and PO4 into solution. The total alkalinity—represented by the release of Ca—produced during the first 25\u202fweeks was compared to the total acidity—represented by the release of SO4—produced during the first 25\u202fweeks of a previous humidity cell experiment with an acid-generating (Type III) waste rock from the same site. The easily weathered carbonates and phosphates of the tonalite + pegmatite have the potential to produce sufficient net alkalinity to consume the net acid generated from the early weathering of sulfides in the Type III waste rock. Aqueous extractions performed on post-experiment samples of the tonalite + pegmatite indicate remaining carbonate and phosphate minerals that were not readily available at the experimental ≤6.3\u202fmm size. The heterogeneity of carbonate content and availability in granitic country rock at the mine site are reflective of the variability of accessory calcite in granitoids and indicate a continued need for site-specific determination of alkalinity-generating reactions with the weathering of waste rock.