Copper Isotope Fractionation and the Evolution of Sulfide Alteration in a Sudbury Tailings Impoundment
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Date
2016-10-18
Authors
Buis, Justin Robert
Journal Title
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Publisher
University of Waterloo
Abstract
The fate of dissolved Cu in the Copper Cliff Central Tailings Impoundment was investigated to better understand the extent of Cu stable isotope fractionation accompanying the oxidation of Cu-bearing sulfide minerals and the retention of Cu through secondary mineral formation. Stable isotope ratios (δ65Cu) were measured in samples of pore water extracted from the oxidized zone of the tailings. Samples of the tailings solids were analyzed using a wide range of analytical techniques including transmitted and reflected microscopy, X-ray fluorescence, selective extractions and synchrotron based X-ray absorption spectroscopy (XAS). Observations were compared to the results from a study conducted at the same location on the tailings at the Copper Cliff Central tailings area, which was completed in 1990, allowing for an evaluation of the physical and chemical effects of sulfide alteration over a prolonged period.
Prolonged sulfide alteration has led to an expansion in the depth of oxidation from 0.8 m below the ground surface (m bgs) in the previous study to a depth of 1.6 m bgs in the current study, sulfide oxidation was modelled using PYROX and results were compared to measurements of gas-phase O2. Analysis of current pore-water chemistry shows the maximum concentration of dissolved Cu (700 mg L-1) occurs near the boundary between the oxidized and unoxidized zones at 1.6 m bgs, with a sharp decline to lower Cu concentrations at greater depths. Analysis of the tailings solids shows an accumulation of Cu (3,000 ppm) at 1.8 m bgs immediately below this sharp decrease in aqueous Cu. Comparison of the aqueous and solid phase concentrations, selective extraction measurements and mineralogical observations indicate that formation and dissolution of covellite (CuS) at the base of the oxidized zone is the main geochemical control on the mobility of Cu.
The dissolution of previously precipitated covellite has led to depletion of aqueous phase δ65Cu (-3.93±0.03‰) above a zone of declining Cu concentrations and an enrichment of δ65Cu (12.01 ±0.50‰) that is attributed to the precipitation of covellite. These observations demonstrate the value of integrating aqueous water chemistry and isotope measurements of field samples to assess the oxidation of Cu-rich sulfide tailings. The Cu isotope fraction observed during covellite formation has the potential for use as an indicator of the natural attenuation of dissolved Cu concentrations by sulfide precipitation as a result of changing redox conditions.
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Keywords
Mine waste, Isotope geochemistry, Sulfide oxidation, Copper isotopes