The aims of this project on prediction and prevention of acid rock drainage from a geological and mineralogical perspective are: 1) to clarify the susceptibility of different types of ore deposits to acid rock drainage problems; 2) to investigate the relative weatherability of common sulphide minerals; and, 3) to examine the role of a field geologist in acid rock drainage assessment, especially during mineral exploration.
Based on a literature survey and supplementary field investigations, seven common types of metallic mineral deposits occurring in western Canada are ranked according to acid rock drainage potential. The susceptibility of a deposit to develop acid rock drainage problems is largely determined by the net acid-generating potential of the prevalent mineral assemblages, accessibility of the reacting minerals to the weathering agents and potential mediation by iron-oxidizing bacteria. Generally, the alteration mineral assemblages about an orebody reflect well the acid rock drainage potential of the relevant rock types. A formula for assessing acid rock drainage potential based on field estimates of mineral abundance is suggested.
To determine the relative weatherability of common sulphide minerals, a kinetic-type weathering experiment using polished thin sections as starting materials has been conducted. Detailed examination of the prevalent sulphides in the sections with petrographic and scanning electron microscopes, augmented by electron microprobe analysis and determination of leachate chemistry by induction coupled plasma mass spectrometry (ICPMS) indicate the importance of trace element composition in affecting the stability of individual sulphides. Where different sulphides are in contact with each other, electrochemical processes are likely to be operative, further complicating the relative reactivity of the sulphides. Thus, a general weathering sequence for the common sulphides may not exist except at a local scale. To predict the rate of acid generation and metal release associated with sulphide oxidation, further work to clarify the interdependence of geochernical reactions, microbial mediation and electrochemical processes involved is required.
Well-trained to collect and interpret mineralogical and geological data, a field geologist can readily assess the acid rock drainage potential of geologic materials based on field observations. A practical checklist for field assessment of acid rock drainage potential during mineral exploration is suggested and the tasks of a geologist in acid rock drainage assessment at various phases of a pre-mine development project are briefly described.
In conclusion, in as much as acid rock drainage is a product of natural water-rock interaction, its prediction and prevention relies on a thorough understanding of the local geology and mineralogy. The involvement of a geologist in acid rock drainage assessment in every stage of a mining project may lead to a significant reduction in the cost of tackling the serious environmental problem and is thus highly recommended.
