KINETIC AND ISOTOPIC EQUILIBRATION OF A NEUTRAL MINE DISCHARGE WITH ATMOSPHERIC CO2(G): CASE STUDY OF A GOLD MINE DISCHARGE AT THE CRIPPLE CREEK MINING DISTRICT, COLORADO, USA

We made chemical and mineralogical analyses and assessed the evolution of dissolved inorganic carbon (DIC) concentrations and carbon isotopic composition of DIC (δ¹³C_DIC) for a distance of 568 m at the end of an 8 km tunnel that drains a gold mine. The δ¹³C of the aqueous samples and carbonate precipitates were continuously enriched from the tunnel exit to 318 m and then remained nearly constant thereafter. The DIC evolution caused the precipitation of carbonate and spatial increases in pH, decreasing concentrations of Ca²⁺ and DIC and continuous enrichment in the δ¹³C_DIC and the carbonate precipitates. The isotopic fractionation during DIC evolution was initially by kinetic process from CO_2(g) outgassing which was followed by equilibrium fractionation from carbon exchange between DIC and atmospheric CO_2(g). The combined kinetic and equilibrium fractionation in the aqueous phase caused the δ¹³C of the carbonate precipitates to be enriched by 3.5 ± 0.3 ‰ by 318 m. Thus, carbonate evolution to isotopic equilibrium with atmospheric CO_2(g) was achieved after 318 m of flow. Chemical equilibrium with atmospheric CO_2(g) was never achieved as the DIC continuously decreased at pCO₂ conditions higher than atmospheric even at isotopic equilibrium with atmospheric CO_2(g). Scanning electron microscopy-backscattered electron (SEM-BSE) images showed the presence of diatoms within calcite precipitates. Diatoms preferentially use the lighter ¹²CO₂ during photosynthesis and can potentially enrich the mine discharged in the heavier ¹³CO₂ in addition to facilitating the precipitation of calcite. If the effect of the photosynthetic activities of the diatoms was responsible for the enrichment in the δ¹³C_DIC of the mine discharge, then the δ¹³C_DIC after 318 m would have continuously been enriched which is not observed, suggesting that abiotic processes are the main control on calcite precipitation.