A total of 268 thermal spring samples were analyzed for total soluble As using reduced molybdenum-blue; 27 of these samples were also analyzed for total Sb using name atomic absorption spectrometry. At Yellowstone the Cl/As atomic ratio is nearly constant among neutral-alkaline springs with Cl > 100 mgL^-1, and within restricted geographic areas, indicating no differential effects of adiabatic vs. conductive cooling on arsenic. The Cl/As ratio increases with silica and decreases with decreasing Cl/ ΣC0₃; the latter relationship is best exemplified for springs along the extensively sampled SE-NW trend within the Lone Star-Upper-Midway Basin region. The relationship between Cl/As and Cl/ΣC0₃ at Yellowstone suggests a possible rock leaching rather than magmatic origin for much of the Park's total As flux. Condensed vapor springs are tow in both As and Cl. Very high Cl/As ratios (>1000) are associated exclusively with highly diluted (Cl < 100 mgL^-1) mixed springs in the Norris and Shoshone Basins and in the Upper White Creek and Firehole Lake areas of Lower Basin. The high ratios are associated with acidity and/or oxygen and iron; they indicate precipitation of As following massive dilution of the As-bearing high-Cl parent water.

Yellowstone Sb ranged from 0.009 at Mammoth to 0.166 mgL^-1 at Joseph's Coat Spring. Within basins, the Cl/Sb ratio increases as the Cl/ΣC0₃ ratio decreases, in marked contrast to As. Mixed springs also have elevated Cl/Sb ratios. WHITE (1967) and WEISSBERG (1969) previously reported stibnite (Sb₂S₃), but not orpiment (As₂S₃), precipitating in the near surface zone of alkaline geothermal systems.