New Zealand Journal of Geology and Geophysics abstracts
Mobilisation and attenuation of arsenic around gold mines, east Otago,
New Zealand
D. CRAW
D. CHAPPELL
A. REAY
D. WALLS
Geology Department and Environmental Science
Programme
University of Otago
P.O. Box 56
Dunedin, New Zealand
Abstract Arsenic is released into the environment by
decomposition of natural arsenopyrite in gold-bearing veins in east Otago.
Natural release is slow, but is accelerated by mining activity. Waters from old
mine adits have pH of 6-7, compared to normal groundwater pH of 7-8, and adit
waters have arsenic concentrations up to 4 ppm compared to background
levels of c. 0.01 ppm. Tailings disposed about 80-90 years ago into
what is now a wetland at Barewood have arsenic concentrations of up to
33 000 ppm, principally in the form of the secondary arsenic mineral
scorodite. The maximum dissolved arsenic concentration in the Barewood tailings
is 0.75 ppm at pH of c. 5.5. Dissolved arsenic release at that pH may
be restricted to levels near 1 ppm by the formation of scorodite with
solubility and/or kinetic controls on scorodite redissolution. Modern mining
activity in the Macraes area has resulted in localised dissolved arsenic
concentrations in excess of 200 ppm at pH near 10. Evaporation of this
water produces scorodite precipitates. Dissolved arsenic released from
arsenopyrite decomposition is attenuated by fine-grained minerals,
predominantly phyllosilicates, in soils around weathered veins, and in wetlands
downstream from mine workings. Lithic soils can hold at least 50-100 ppm
As on the fine fraction (c. 1 um), and wetland soils can hold
hundreds to thousands of parts per million on their fine fractions. Both soil
adsorption and scorodite mediation slow the release of arsenic into the
environment, allowing more effective dilution by downstream waters to safe
arsenic levels.
Keywords arsenic; scorodite; gold; mining; attenuation;
pollution
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
373-383
0028-8306/00/4303-0373 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (2006K); (scanned from paper original: notes about this process)
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