USING CU ISOTOPES TO STUDY FLUID-ROCK INTERACTION IN THE BUTTE MINING DISTRICT, MONTANA

Recent studies have shown relatively large variations in the Cu isotopic composition of minerals (δ⁶⁵Cu ≈10‰) from Cu ore deposits. Quantifying the fractionation mechanisms that lead to the isotopic variation in ore deposits may aid in our understanding of the geochemical processes (e.g. mineral precipitation and dissolution, sorption) that influence Cu cycling in these regions. Here we present Cu isotopic data from water and mineral specimens across the Butte Mining District, Butte, MT. Our aim is to show the potential of using Cu isotopes to assess the cycling of Cu related to the supergene enrichment, to determine sources of Cu in stream and ground waters, and to evaluate the degree of Cu-sulfide weathering throughout the mining district.

Cu-bearing sulfide minerals were collected from within the active mine and from an archived collection of hand samples from the underground workings. We collected waters from the Berkeley Pit lake, monitoring wells near the lake, and stream samples from Silver Bow Creek and the Upper Clark Fork River downstream of the Butte mine. The δ⁶⁵Cu values were measured using a Thermo Neptune MC-ICP-MS (2σ error of 0.14‰).

Primary chalcocite (Cu₂S), chalcopyrite (CuFeS₂), and bornite (Cu₅FeS₄) samples from 700 to 1500 meters depth have δ⁶⁵Cu values that range from -0.30 to 0.37‰, whereas supergene chalcocite closer to the surface has values between 5.07 and 5.64‰. δ⁶⁵Cu values of Berkeley Pit waters and groundwater samples surrounding the pit lake range from 1.56 to 2.32‰. A stream water sample collected 5 km downstream from the Berkeley Pit has a value of 1.22‰ and another at 25 km has a value of 1.19‰. Surficial Cu-salt samples from weathered mine tailings deposited along the stream approximately 30 km and 60 km below Butte have values of 0.40‰ and 0.32‰ respectively.

A comparison of δ⁶⁵Cu values of primary minerals with the secondary chalcocite suggests the occurrence of multiple supergene enrichment events. However, we conclude from our isotopic results that the dominant source of aqueous Cu in the immediate mine district is from rapid oxidative weathering of primary sulfide minerals. Downstream from Butte, the lower δ⁶⁵Cu values of the water indicate that either Cu scavenging by sorption to Fe oxides is occurring or that the Cu source is from the dissolution of highly weathered minerals.