Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

Abstract

This study was aimed to identify the main processes involved in the release of metals using sulfur isotopes and those acting on the fractionation of copper isotopes. We determined sulfur and copper isotopic ratios in rocks, tailings, precipitates and soils samples from the historical mining district of Taxco, Southern Mexico. Ratios of sulfur isotopes are reported in notation of δ per mil, and standardization was based on international standards OGS-1 and NBS123. Only copper ratio isotopes were studied in pyrite (Py), sphalerite (Sph) and galena. Results are reported as δ65Cu=((65Cu/63Cusample/65Cu/63CuNIST976standar)−1)∗1000. Error for all the analyzed samples was 0.01‰ and any mass bias was corrected trough bracketing method. The sulfur isotopic value of a sample from the Mexcala shale Formation was −7.7‰. The δ34S values in tailing ponds ranged from −2.6 to 1.2‰, in precipitates from −7.2 to −3.2‰, and in soils from −0.8 to 4.5‰. Isotopic δ34S values in rocks evidenced a magmatic influence of sulfides present in the Mexcala Formation. Oxidation of sulfides contained in tailings and precipitates was observed related with their low δ34S values close to zero, oxidation followed the order: sphalerite>pyrite>pyrite rich in copper>chalcopyrite>galena. In soils, oxidation is not the main process as supported by their positive δ34S values and low sulfate content. Results of δ65Cu values in rocks were −0.09 to 0.54‰; in sulfides values were δ65Cu=0.18‰ (Py) and δ65Cu=−0.20‰ (Sph), concentration of copper in galena was not enough to get a reliable isotopic value. In tailings δ65Cu ranged from −1.19 to 0.91‰ and in precipitates from −0.36 to 0.92‰; in soils values were between 0.26 and 1.72‰. Results in rocks indicate an igneous influence on copper provenance. The δ65Cu values in Py and Sph were within the range of terrestrial primary sulfides (from −1 to 1‰) confirming its igneous provenance. Processes related with fractionation of copper isotopes were oxidation of tailings and reprecipitation of 65Cu in tailings with a high content of Fe oxyhydroxides. Enrichment in 65Cu in precipitates and soils is favored by organic matter presence that retains 63Cu. Erosion of rocks and tailings is the main source of copper being rocks the initial and tailings the end members. The isotopic studies on environmental research provide a useful tool on: 1) The behavior of copper and sulfur isotopes in environments where both, natural and anthropogenic activities, are present. 2) Understanding surficial processes of dispersion of metals and identifying their source in mining areas surrounded by villages and agricultural soils.