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by Dave Craw


Sb has similar chemical behaviour to arsenic, in that it has 3+ and 5+ oxidation states, and forms oxyanions in solution. Sb occurs at low background levels in New Zealand rocks (typically 0.01-0.1 mg/kg), distinctly lower than arsenic (2-20 mg/kg). Sb is readily mobilised from background rocks by hydrothermal waters, and some is transported to hot springs and near-surface geothermal systems in the North Island, accompanying As and Hg. Ancient mesothermal vein systems in the South Island also contain variable amounts of Sb, and some minor Sb mining has occurred historically. Sb occurs at elevated levels (typically 10-100 mg/kg), but is subordinate to arsenic, in most mesothermal gold deposits in the South Island. Stibnite (Sb2S3) is the most common Sb sulfide mineral, and this mineral commonly occurs with arsenopyrite (FeAsS). Arsenopyrite can have up to % level Sb in solid solution, and stibnite can have % level As in solid solution. Stibnite oxidises to a variety of Sb oxide minerals, principally valentinite (SbIII 2O3) and stibiconite (SbIIISbV 2O6OH) under surficial conditions. These oxides are soluble and dissolved Sb up to 50 mg/kg can arise, as SbVO3 - or SbV(OH)6 -. Dissolved Sb is readily attenuated by, for example, high surface area iron oxyhydroxide, with empirical Kd of ca. 105 L/kg. Without this type of attenuation, elevated dissolved Sb can be traced for kilometres or even hundreds of kilometres from point sources.

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