Hg Contamnation from New World Gold & Silver Mine Tailings
75
• In the majority of tailings Hg is present as metallic Hg. Water-soluble Hg is
generally less than 0.1% of the total Hg content in tailings (Andrade et al. 1988;
CETEM 1989; Lacerda et al. 1990; Ramos and Costa 1990; Lechler and Miller
1993). A larger fraction of Hg forms is only extractable with aqua regia,
indicating that these forms are associated with residual minerals or as
elemental Hgo (Ching and Hongxiao 1985). A fraction of Hg lost to
tailings may also be amalgamated with gold grains (Lane et al. 1988; Fuge
et al. 1992).
• Mercury distribution in tailings is a function of the distribution of mining
activities in the area rather than any geochemical process. In areas where old
preconcentration ponds existed, Hg concentration can reach a few micrograms per gram of tailings material, while the mean Hg content in the whole
tailings body is similar to local background values (Prokopovich 1984; CETEM
1989)·
• Different mining techniques may result in different Hg concentration and
distribution in tailings. Typically, these concentrations range from less than
0.5 ~lg g-I to 4900 ~lg g-I (dry weight) (Andrade et al. 1988; Ramos and Costa
1990; Lechler and Miller 1993). For example, when Hg amalgamation is
performed in specific sites in the mining field, these sites will show the highest
Hg concentrations in the tailings (CETEM 1989). When the amalgamation step
is performed during the gravimetric concentration, as in riffles, Hg tends to be
spread through tailings, showing a more even distribution (CETEM 1992).
More uniform distribution of mercury concentrations in tailings can also
occur when sodium chlorine is used in the extraction process (Lechler 1993).
• Mercury present in gold mining tailings can represent an important source of
contamination even centuries after the cessation of the mining operations
(Lane et al. 1988; Fuge et al. 1992; Bycroft et al. 1992; Lechler and Miller 1993).
The four characteristics listed result in a very low reactivity and bioavailability
of Hg in tailings, but turn these deposits into a long-lasting contamination
source.
A detailed study has been carried out on the Hg concentrations in the Tanque
dos Padres tailings, central Brazil, obtained from the analysis of a 200 sample
points grid (CETEM 1989). The Hg distribution was extremely inhomogeneous,
showing background values in over 95% of the samples «0.20 ~lg g -I). The few
samples with anomalous high Hg content (4 to 25 ~lg g-I) showed no distribution
pattern and were not correlated with any geochemical or geological aspect of the
tailings, but correspond to former amalgamation ponds at the mining site
(CETEM 1989). Prokopovich (1984) measured Hg concentrations ranging from
0.036 to 0.2 ~lg g -I in over 60 samples of last -century dredge tailings in the
Folsom-Natomas gold field, California. In three samples randomly taken,
however, Hg concentrations reached 37.5; 21.5, and 22 ~lg g-I.
The distribution of mercury in gold and silver mine tailings hampers any
generalization. Tailing deposits should be considered as unique, at least in
relation to the spatial distribution of Hg concentrations. However, the
mechanisms of Hg release from tailings are probably very similar, regardless
75
• In the majority of tailings Hg is present as metallic Hg. Water-soluble Hg is
generally less than 0.1% of the total Hg content in tailings (Andrade et al. 1988;
CETEM 1989; Lacerda et al. 1990; Ramos and Costa 1990; Lechler and Miller
1993). A larger fraction of Hg forms is only extractable with aqua regia,
indicating that these forms are associated with residual minerals or as
elemental Hgo (Ching and Hongxiao 1985). A fraction of Hg lost to
tailings may also be amalgamated with gold grains (Lane et al. 1988; Fuge
et al. 1992).
• Mercury distribution in tailings is a function of the distribution of mining
activities in the area rather than any geochemical process. In areas where old
preconcentration ponds existed, Hg concentration can reach a few micrograms per gram of tailings material, while the mean Hg content in the whole
tailings body is similar to local background values (Prokopovich 1984; CETEM
1989)·
• Different mining techniques may result in different Hg concentration and
distribution in tailings. Typically, these concentrations range from less than
0.5 ~lg g-I to 4900 ~lg g-I (dry weight) (Andrade et al. 1988; Ramos and Costa
1990; Lechler and Miller 1993). For example, when Hg amalgamation is
performed in specific sites in the mining field, these sites will show the highest
Hg concentrations in the tailings (CETEM 1989). When the amalgamation step
is performed during the gravimetric concentration, as in riffles, Hg tends to be
spread through tailings, showing a more even distribution (CETEM 1992).
More uniform distribution of mercury concentrations in tailings can also
occur when sodium chlorine is used in the extraction process (Lechler 1993).
• Mercury present in gold mining tailings can represent an important source of
contamination even centuries after the cessation of the mining operations
(Lane et al. 1988; Fuge et al. 1992; Bycroft et al. 1992; Lechler and Miller 1993).
The four characteristics listed result in a very low reactivity and bioavailability
of Hg in tailings, but turn these deposits into a long-lasting contamination
source.
A detailed study has been carried out on the Hg concentrations in the Tanque
dos Padres tailings, central Brazil, obtained from the analysis of a 200 sample
points grid (CETEM 1989). The Hg distribution was extremely inhomogeneous,
showing background values in over 95% of the samples «0.20 ~lg g -I). The few
samples with anomalous high Hg content (4 to 25 ~lg g-I) showed no distribution
pattern and were not correlated with any geochemical or geological aspect of the
tailings, but correspond to former amalgamation ponds at the mining site
(CETEM 1989). Prokopovich (1984) measured Hg concentrations ranging from
0.036 to 0.2 ~lg g -I in over 60 samples of last -century dredge tailings in the
Folsom-Natomas gold field, California. In three samples randomly taken,
however, Hg concentrations reached 37.5; 21.5, and 22 ~lg g-I.
The distribution of mercury in gold and silver mine tailings hampers any
generalization. Tailing deposits should be considered as unique, at least in
relation to the spatial distribution of Hg concentrations. However, the
mechanisms of Hg release from tailings are probably very similar, regardless