ANTIMONY MOBILITY AND SPECIATION ASSOCIATED WITH THE OXIDATION OF STIBNITE-BEARING WASTE ROCK FROM THE EASTERN TINTINA GOLD PROVINCE, ALASKA AND YUKON

The factors controlling the speciation and transport of Sb in post-mining environments are poorly understood. We have examined antimony speciation associated with the oxidation of stibnite (Sb₂S₃)-bearing waste rock in the eastern Tintina Gold Province (TGP). The TGP is a 150,000 km² area comprised of >15 gold belts and districts in western Yukon and east-central Alaska, in a region bounded by the Tintina and the Denali-Kaltag fault systems and underlain by Paleozoic metasedimentary rocks and mid-Cretaceous granitoids. Late-stage, fine- to coarse-grained stibnite, occurring in trace to minor amounts, is common within many of the ca. 90 Ma precious metal-bearing veins and stockworks in this region.

Field measurements (pH, specific conductance, temperature), 0.45-mm filtered water samples, and <63-mm fraction stream sediments were collected at numerous sites within and downstream from waste rock at abandoned mine sites. These sites included three gold mines (e.g., Scrafford, McCarty, and Chatham Creek,) within the Fairbanks mining district in Alaska, and the Keno Hill Pb-Ag-Zn deposits in the Yukon. Water and sediment samples were analyzed for major, minor, and trace elements by ICP-mass spectrometry and for major anion compositions by ion chromatography. The stream waters ranged in pH from 5.5-8.5, with specific conductance from 17-694 mS/cm, and can be described as Ca-HCO₃^- type waters.

Our results show that the Sb concentrations in the <63-mm stream sediments range broadly from 4 to 5700 ppm, and correlate well with As, Fe, and Al. Dissolved (<0.45 mm) Sb concentrations were found to range from <0.3-634 mg/L; however, the correlation with As, Fe, and/or Al is inconsistent. Antimony concentrations were an order of magnitude higher in both sediments and waters when draining exposed waste rock and tailings, compared to buried or re-vegetated waste-rock and tailings. Results of x-ray absorption spectroscopy analyses (XAFS and XANES) of the <63-mm fraction stream sediments, and of secondary mineral weathering surfaces indicate that Sb in the stream sediments is in the form of Sb(V) and most likely adsorbed onto iron hydroxide, whereas secondary weathering phases appear to be a mixed Sb(III)/Sb(V) oxide. In contrast, As in stream sediments has been observed as both As(III) and As(V), thus indicating that Sb and As are not behaving in a chemically similar manner.