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Arsenic speciation in surface water at Lucky Shot Gold Mine, Alaska

Torrance, Keith and Keenan, Helen and Munk, LeeAnn and Hagedorn, Birgit (2011) Arsenic speciation in surface water at Lucky Shot Gold Mine, Alaska. In: Geological Society of America Annual Meeting, 2011-10-09 - 2011-10-12.

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Abstract

Historical gold mining in Alaska has created a legacy of abandoned mine shafts, contaminated land and tailing piles, which continue to impact surface water quality through runoff and leaching of toxic metals, especially arsenic. The Lucky Shot gold mine in Hatcher Pass, South-central Alaska, operated from 1918 until 1942, working gold-bearing quartz veins within a Cretaceous quartz diorite/tonalite intrusion by means of horizontal adits. Arsenopyrite (FeAsS) and pyrite (Fe2S) present in the quartz veins contribute to elevated arsenic levels in water draining from the adits; up to 700µgL-1 or roughly seventy times the United States Environmental Protection Agency (USEPA) Drinking Water Standard of 10µgL-1, although this is rapidly diluted. Lucky Shot is being assessed for future development, driven by the high price of gold, currently around $1,600 per ounce (July, 2011). Water samples were collected from streams, adits and boreholes around the mine and analyzed for major and minor elements using ion-chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). Arsenic species separation was performed in the field to determine the ratio of As(III)/As(V) using anion-exchange chromatography, following the method developed by Wilkie and Hering (1998). It was determined that in the vicinity of the adits, surface water had circum-neutral pH’s and displayed no characteristics of acid mine drainage. Despite being well-oxygenated, As(III) is the dominant arsenic species in the system, accounting for 75% of total arsenic. Oxygen stable isotope studies support the view that the predominance of As(III) can be attributed to the low residence time of water in the system, limiting thermodynamical equilibrium being attained and preferential absorbance of As(V).