A comparative mineralogical and geochemical study of sulphide mine-tailings at two sites in New Mexico, USA

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Boulet, Michelle Paulette
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A comparative study of sulphide mine-tailings from two sites near Silver City in southwest New Mexico has shown the need for environmental monitoring in a geological context. The Cyprus-Pinos Altos and Cleveland deposits consist of Cu and Zn skarn mineralization in the Pinos Altos Mountains of New Mexico. Primary ore minerals in both deposits include chalcopyrite, sphalerite and galena. The Cyprus-Pinos Altos tailings (CPAT) are stored in a lined, bermed impoundment. They are dominantly water-saturated and there are no secondary phases. The grains are not cemented and show no evidence of primary-mineral dissolution. The Cleveland mill tailings (CMT) were deposited in a valley at the headwaters of an ephemeral stream. They are highly oxidized and differentially cemented. They have undergone numerous wet/dry cycles, resulting in extensive oxidation. The Cyprus-Pinos Altos and the Cleveland deposits ha similar primary mineralogical assemblages and geochemical signatures. The advanced age and well drained nature of the CMT have promoted extensive oxidation and dissolution of primary phases. In contrast, the saturated condition of the CPAT and the short time since their deposition have resulted in limited alteration of primary minerals. While Pb is relatively immobile, Cd is easily mobilized out of the tailings in dissolved forms. Low mobility of Pb within the tailings and low concentrations of Pb in acidic drainage indicate that this element may be less of a threat to the environment than other, more mobile, toxic elements (e.g., Cd) that are present in lower concentrations in tailings. (Abstract shortened by UMI.)