Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/gexplo Alkalinity generation from weathering of accessory calcite and apatite and acid drainage neutralization in an Archean granitoid waste rock Jef B. Langman a, ⁎ , Sean Sinclair b,d , Richard T. Amos c , David Wilson d , Carol J. Ptacek d , David C. Sego e , Leslie Smith f , David W. Blowes d a Department of Geological Sciences, University of Idaho, Moscow, ID 83844, USA b Diavik Diamond Mines, Inc., Yellowknife, NWT X1A3T1, Canada c Department of Earth Sciences, Carleton University, Ottawa, ON K1S5B6, Canada d Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON N2L3G1, Canada e Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G1H9, Canada f Dept. of Earth, Ocean, and Atmospheric Sci., University of British Columbia, Vancouver, BC T6G1H9, Canada ARTICLEINFO Keywords: Accessory calcite Granitoid weathering Alkalinity generation Acid rock drainage 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 frst 25weeks of the 80-week experiment. This period represents decomposition of readily available carbonates (infllings) and phosphates (euhedral grains) that release acid-consuming CO 3 and PO 4 into solution. The total alkalinity—represented by the release of Ca—produced during the frst 25 weeks was compared to the total acidity—represented by the release of SO 4 —produced during the frst 25weeks of a previous humidity cell experiment with an acid-generating (Type III) waste rock from the same site. The easily weathered carbonates andphosphatesofthetonalite+pegmatitehavethepotentialtoproducesufcientnetalkalinitytoconsumethe net acid generated from the early weathering of sulfdes in the Type III waste rock. Aqueous extractions per- formed on post-experiment samples of the tonalite + pegmatite indicate remaining carbonate and phosphate minerals that were not readily available at the experimental ≤6.3 mm size. The heterogeneity of carbonate content and availability in granitic country rock at the mine site are refective of the variability of accessory calcite in granitoids and indicate a continued need for site-specifc determination of alkalinity-generating re- actions with the weathering of waste rock. 1. Introduction The environmental impact of acid rock drainage (ARD) from the oxidative dissolution of sulfdic waste rock is a global and continuing problem for the mining industry (Egiebor and Oni, 2007; Simate and Ndlovu, 2014). The disposal of potentially ARD-generating waste rock can be a substantial economic factor in mine operations and mine closure, but the use of local resources to inhibit/lessen ARD can mini- mize costs for waste rock disposal and protection of the surrounding environment. The oxidative dissolution of the primary iron sulfde minerals—pyrrhotite [Fe( 1-x )S] and pyrite [FeS 2 ]—and Fe-containing sulfde minerals (e.g., arsenopyrite [FeAsS], chalcopyrite [CuFeS 2 ], and pentlandite [(Fe,Ni) 9 S 8 ]) can produce sufcient acid (H + ) to mobilize metals and form potentially harmful ARD (Blowes et al., 2014; Nordstrom, 2011a, 2011b). The generation of ARD may be controlled, or its efects minimized, through natural or engineered alkalinity-gen- erating (acid-consuming or acid-bufering) systems (Skousen et al., 2017). The balance of acidity/alkalinity reactions associated with the oxidation of Fe sulfdes (Eqs. (1)–(5), examples for pyrrhotite and pyrite) and dissolution of carbonate/phosphate minerals (Eqs. (6)–(11), examples for calcite [CaCO 3 ] and hydroxylapatite [Ca 5 (PO 4 ) 3 (OH)]) within a weathering waste rock is the main factor in generating ARD https://doi.org/10.1016/j.gexplo.2019.106341 Received 26 March 2019; Received in revised form 6 July 2019; Accepted 18 July 2019 ⁎ Corresponding author. E-mail address: jlangman@uidaho.edu (J.B. Langman). Journal of Geochemical Exploration 205 (2019) 106341 Available online 19 July 2019 0375-6742/ © 2019 Elsevier B.V. All rights reserved. T