Using δ65Cu and δ34S to determine the fate of copper in stream waters draining porphyry mineralization: Implications for exploration targeting

The fate of metals, such as Cu, in stream waters draining porphyry mineralization is commonly controlled by several natural processes such as sorption, microbial processes, and ligand availability. Isotopes of Cu offer a novel approach to understanding these processes and determining metal sources within complicated mineralogical systems. Drainages at the Casino Cu-Au-Mo porphyry deposit, Yukon, Canada exhibit circumneutral (pH > 5) in Casino Creek and natural acid rock drainage (pH < 3.5) in Proctor Gulch with the precipitation of schwertmannite (Fe3+). Isotopic systems δ65Cu and δ34Ssulfate indicate different metal sources, with signatures of both hypogene and supergene mineralization. Waters from Proctor Gulch contain δ65Cu values (< −0.5 ‰) consistent with supergene Cu sources from the leached or oxide portion of the mineralization. Comparatively, drainage in the upper part of Casino Creek contains a δ65Cu composition (> 0.5 ‰) characteristic of Cu sourced from hypogene sulfide mineral oxidation. Variation in metal sources is similarly supported by aqueous δ34Ssulfate values in the stream waters, which suggest mixing of S derived from a sulfide mineral phase and a much heavier sulfate mineral (e.g., gypsum or anhydrite). Isotopic fractionation of Cu in the dissolved (<0.45 μm) phase presents two predominant controls on Cu dispersion. The natural acid conditions in Proctor Gulch favor the preferential co-precipitation of 63Cu with schwertmannite but could be influenced by intracellular assimilation or adsorption by microbes, which also has been shown to preferentially favor 63Cu. Copper isotopic fractionation results in a gradient of increasing δ65Cu values in waters downstream. In Casino Creek, higher pH conditions favor the precipitation of Fe(OH)3 and the preferential adsorption of 65Cu, resulting in decreasing δ65Cu values downstream. Copper concentrations in stream waters remain elevated (up to 4.1 μg/L) above ambient background (1.9 μg/L) levels up to 11 km downstream of the deposit. Given the abundance of surface water in many parts of northern Canada, hydrogeochemical prospecting using broad scale stream water catchment analysis is clearly a viable greenfield exploration methodology.