BIOGEOCHEMISTRY OF ANTIMONY AND ARSENIC IN HYDROTHERMAL FLUIDS AND OPALINE DEPOSITS FROM EL TATIO GEYSER FIELD, CHILE

At 4400 m altitude, the El Tatio geysers in Northern Chile comprise one of the largest known geyser zones. The hydrothermal waters support abundant communities of mat-forming thermophilic microorganisms, and siliceous geyserite encrusts outflow channels and geyser pools, forming thick siliceous terraces. Water, core samples, and microbial mat samples were collected from hydrothermal features within the basin. Geochemical analyses of opaline deposits (geyserite) and sediment samples consisted of HF dissolution for bulk trace metal analysis, sequential extraction to determine the availability and bonding character of metals, microprobe analysis, and laser ablation-ICP-MS. Filtered water samples were analyzed for dissolved metal(oid)s, including As and Sb species. Unfiltered samples were digested and analyzed for total metals. Geochemical evolution of the water and geyserite precipitation was modeled using Geochemist's Workbench. The geysers lie in three major drainage basins, and at a discharge of 10 cfs, produce near-boiling, reduced, circum-neutral pH, Na-Cl water. The hydrothermal waters contain extremely high concentrations of As (0.6 mM) and Sb (0.034 mM). Upon discharge, As and Sb in the hydrothermal fluids oxidize rapidly to As(V) and Sb(V). The As:Sb atomic ratio in the springs is ~10-20, while the As:Sb atomic ratio further downstream is ~30. Silica in some geyserite, which also contains abundant cyanobacterial filaments, is finely laminated (~10 - 100 μm thick) with varying Sb content between laminations (~0.5% to 3.5 % as Sb₂O₅). The As in the geyserite is HCl- and NaH₂PO₂-extractable, but Sb-minerals are found to only dissolve in HF. While As appears to behave conservatively on the basin-scale, it interacts with Fe-oxide colloids in the hydrothermal waters. Sb is not conservative and precipitates with the silica, probably as an Sb-oxide (Sb₂O₄, Sb₄O₆, or Sb₂O₃). High night-time oxidation rates of As(III) indicate that microbes are controlling As oxidation. Sb oxidation-rate analyses were inconclusive; however, the partitioning of Sb into geyserite and the periodicity of the laminations indicate there are changes in the controls of Sb biogeochemistry. Microbial oxidation of Sb(III) appears to be bioenergetically favorable, and mass fractionation analysis of ¹²¹Sb and ¹²³Sb stable isotopes may indicate microbial activity as a control in Sb geochemistry.