The weathering of sulfide-bearing mineral deposits, exposed either by natural erosion or by mining to oxygen and water, can produce natural acid-rock drainage (ARD) and mining-related acid-rock drainage (acid mine drainage, AMD), respectively. The composition and extent of acid drainage is highly variable, depending on many factors, such as the composition of ore bodies and related host rocks, climate, mining operations and processing (Nordstrom and Alpers, 1999). Due to the low pH and high concentrations of contaminants, acid drainages can severely degradate surface and groundwater systems, as well as soils. At Furtei (Sardinia, Italy), a high-sulfidation epithermal gold deposit consists of pyrite and enargite with minor amounts of luzonite, tennantite, and chalcocite. The oxidized cap mainly consists of iron oxy-hydroxides, gypsum, jarosite, halotrichite and scorodite. The gold deposit was exploited by open pit from 1997 to 2003. The Au and Ag were recovered from oxidized ores by cyanidation. Sulfide ores were processed by flotation to recover Cu concentrates. Total production was about 4 t of Au, 6 t of Ag, and 1500 t of Cu (Cidu et al., 2013). Pre-mining conditions showed waters (flow <0.1 L/s) with pH down to 2.3, a typical example of ARD characterized by high salinity (26 g/L) with predominant sulfate (20 g/L) and very high amounts of dissolved contaminants (in mg/L): Al 2000, Fe 1700, Mn 35, Cu 29, Zn 11, Ni 4, Co 3, Cr 1, As 0.4, and Cd 0.1 (Cidu et al., 1997). During exploitation and after the mine closure, the number of sites with contaminated water increased. Contaminated waters showed pH and chemical compositions similar to those observed under pre-mining conditions, but much higher concentrations of Cu, Zn, As and Cd, respectively up to 180, 97, 5, and 1.7 mg/L (Da Pelo et al., 2009; Cidu et al., 2013). The seepage from the tailings impoundment is collected in a well and back pumped into impoundment. The highly contaminated water in the open pits and drainages from the pyrite-rich waste dumps may flow downstream from the mine under heavy rain periods and pose a hazard to agricultural areas.
From acid-rock drainage to acid-mine drainage: extreme contamination in waters at the Furtei Au-mine (Sardinia, Italy)
Biddau R.;Cidu R.;Da Pelo S.
2018-01-01
Abstract
The weathering of sulfide-bearing mineral deposits, exposed either by natural erosion or by mining to oxygen and water, can produce natural acid-rock drainage (ARD) and mining-related acid-rock drainage (acid mine drainage, AMD), respectively. The composition and extent of acid drainage is highly variable, depending on many factors, such as the composition of ore bodies and related host rocks, climate, mining operations and processing (Nordstrom and Alpers, 1999). Due to the low pH and high concentrations of contaminants, acid drainages can severely degradate surface and groundwater systems, as well as soils. At Furtei (Sardinia, Italy), a high-sulfidation epithermal gold deposit consists of pyrite and enargite with minor amounts of luzonite, tennantite, and chalcocite. The oxidized cap mainly consists of iron oxy-hydroxides, gypsum, jarosite, halotrichite and scorodite. The gold deposit was exploited by open pit from 1997 to 2003. The Au and Ag were recovered from oxidized ores by cyanidation. Sulfide ores were processed by flotation to recover Cu concentrates. Total production was about 4 t of Au, 6 t of Ag, and 1500 t of Cu (Cidu et al., 2013). Pre-mining conditions showed waters (flow <0.1 L/s) with pH down to 2.3, a typical example of ARD characterized by high salinity (26 g/L) with predominant sulfate (20 g/L) and very high amounts of dissolved contaminants (in mg/L): Al 2000, Fe 1700, Mn 35, Cu 29, Zn 11, Ni 4, Co 3, Cr 1, As 0.4, and Cd 0.1 (Cidu et al., 1997). During exploitation and after the mine closure, the number of sites with contaminated water increased. Contaminated waters showed pH and chemical compositions similar to those observed under pre-mining conditions, but much higher concentrations of Cu, Zn, As and Cd, respectively up to 180, 97, 5, and 1.7 mg/L (Da Pelo et al., 2009; Cidu et al., 2013). The seepage from the tailings impoundment is collected in a well and back pumped into impoundment. The highly contaminated water in the open pits and drainages from the pyrite-rich waste dumps may flow downstream from the mine under heavy rain periods and pose a hazard to agricultural areas.
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