Mineralogical factors influencing pyrite weathering rates in the generation of acid rock frainage
Creators: name: Mountjoy, Keith J.
Publisher: name: University of British Columbia
Year: 2009

description: Acid rock drainage prediction involving detailed, quantitative lithological and mineralogical sample characterization is essential to improve interpretation of static and kinetic test data, particularly for geological materials with subequal acid-generating and acid-consuming potentials. The objectives of this study were to: (1) determine whether mineralogical factors influenced pyrite oxidation in the generation of ARD, (2) rank these factors, (3) determine if a quantitative relationship exists between these factors and static and kinetic test results and (4) determine the applicability of image analysis to ARD prediction. Grab and drill core samples of three rhyolite rock types from the Eskay Creek volcanogenic massive sulfide deposit, and monzonite, latite and andesite from the Mount Milligan gold-copper porphyry deposit were tested. Eskay Creek rhyolite samples had an acid generating potential, regardless of the degree of previous natural weathering. Samples from Mount Milligan had an uncertain or acid generating potential, reflecting a more heterogeneous mineral composition. Acid volatilized carbon dioxide analysis determined the relative contribution of carbonates to the neutralization potentials. Eskay Creek rhyolite samples were predominantly devoid of carbonates. Neutralization potentials (NP) were predominantly higher than carbonate neutralization potentials (CaNP) indicating a noncarbonate mineral contribution to the neutralization potential. The weathering of polished thin sections at 37 °C produced mean pyrite oxidation rates ranging from 5.2 to 18.3 x 10⁻⁷ mol/m²/s for Eskay Creek rock types, resulting in the decreasing sequence: rhyolite breccia &gt; rhyolite flow &gt; massive rhyolite. Mount Milligan rock types rates ranged from 4.4 to 33.0 x 10⁻⁷ mol/m²/s, resulting in the decreasing sequence: andesite &gt; latite &gt; monzonite. These rates are similar in magnitude to the published pyrite oxidation rate by ferric iron of 2.7 x 10"7 mol/m2/s. However, evaluation of the leachate chemistry and calculated ferric to ferrous iron ratios support the development of a intermediate stage of oxygen pyrite oxidation, acid generation and metal leaching characterized by pH values between 3.0 and 4.5, redox potentials between 550 and 600 mV (SHE), increased dissolved element concentrations and low but increased ferric to ferrous iron ratios. These conditions were also favorable for bacterial catalysis of pyrite and ferrous to ferric iron oxidation. An elevated operating temperature of 37 °C, a low rock to water ratio, weathering conditions simulating the water-air interface contributed to the elevated oxidation rates obtained in this experiment and account for cumulative sulfate production rates that were 4.8 to 9.6 times higher than traditional humidity cell or column kinetic tests. The following mineralogical factors have been identified as important sample-specific factors influencing pyrite oxidation rates for both Eskay Creek and Mount Milligan: carbonate content, exposed pyrite surface area, pyrite trace element content, degree of pyrite crystallinity and pyrite trace element content. Potential galvanic affects may also have been important, however, post weathering mineralogical analysis is required to confirm this. Dissolved calcium, magnesium, potassium and sodium concentrations indicate the weathering of silicates and carbonates. The cation to sulfate molar ratios ([Ca] + [Mg] + [Na]/2 + [K]/2) / S04) ranged from 0.01 to 0.69 beyond 31 days of weathering, indicating insufficient acid neutralization for most samples.