USING FIELD, EXPERIMENTAL AND THEORETICAL DATA TO PREDICT PIT-LAKE CHEMISTRY; CAN IT BE DONE AND HOW SHOULD THE UNCERTAINTY BE ADDRESSED?

Understanding site-specific geochemical and hydrologic conditions are critical in determining the “success” of long-term pit quality predictions. Traditional testing procedures for acid base accounting (ABA) include static tests which measure the amount of acid generating elements, mainly sulfide sulfur and this is compared to the amount of acid neutralizing minerals, mainly carbonates and the material is then classified as acid generating, potentially acid generating, neutral, or alkaline and these static tests are further confirmed by kinetic tests where needed. This methodology generally works well in carbonate poor environments but in carbonate rich or carbonate hosted rocks the long term water quality predictions have additional challenges and uncertainty. Long-term water quality is controlled by a competition between the rate of sulfide oxidation and the rate of carbonate dissolution followed by subsequent possible geochemical reactions, i.e., precipitation and /or sorption reactions. Rates are a function of surface area, water exposure, specific mineralogy of the carbonate material, specific mineralogy of the sulfide material, potential biologic activity, among other factors. One of the challenges of making an accurate prediction is obtaining representative samples from drill cores or cuttings. Standard kinetic tests are useful in constraining some of the uncertainty but the ultimate pit water pH and associated trace elements that are going to be in the “final” pit water can be a challenging prediction.

Two very different pit lake water quality predictions will be compared and discussed from Peru where some of these carbonate hosted sulfide deposit challenges have been identified and uncertainty attempted to be reduced. Different grain size ABA results were compared in multiple samples to determine if the finest fractions had any different sulfide sulfur to carbonate ratios and thus the sulfide minerals having more potential to control long-term water quality. The prediction of the long term water quality in carbonate rich rocks needs to recognize and possibly account for the difficulty in making long-term water quality predictions and a contingency for long term water management needs to be considered from mine operations through closure.