In Quadrilátero Ferrífero mercury is used by gold prospectors (garimpeiros) for the separation and purification of fine gold particles. As a result, mercury is lost directly by washing and by volatilization during amalgam burning. It is estimated that in the study area about 78kg of mercury are used annually solely in the streams Água Suja and Gualaxo do Norte (Busher, 1992).
Sediment samples were collected from the streams and placer mining sites in the eastern region of Quadrilátero Ferrífero during summer (rain season) 1992 and winter time (dry season) 1993. The samples were separated in four grain-size classes:
< 2 mm, 2 - 6 mm, 6 - 20 mm and 20 - 63 mm. All these fractions were analysed for total mercury, using an Hg analyser
Hg concentrations of sediments sampled during summer time range from 0.02 ppm to 4.21 ppm and those sampled during winter time from 0.20 ppm to 5.49 ppm. All fractions of sediments taken during winter show higher Hg concentrations than those sampled in summer. A Hg enrichment was observed in the fine fractions and the highest concentrations of this metal were found during both seasons in the grain size fraction < 2 mm. Extremely high Hg values occur in samples from placer mining sites (> 100 ppm in the clay fraction) in the winter time.
Seasonal Hg enrichment factors were calculated by dividing the winter Hg concentrations by the summer Hg concentrations of the grain size fractions. The table shows the Hg enrichment factors for the stream Água Suja. The samples taken from placer mining sites (samples indicated with "a" and "b" ) show very high Hg enrichment factors. Although notable Hg enrichment exists in fine-grained fractions of these sediments, Hg enrichment factors decrease in these fractions indicating a relative higher enrichment of Hg in the coarser fractions.
Busher, D., MS These, T-4305, Colorado School of Mines, 140p (1992).
Table 1: Seasonal Hg enrichment factors for sediments of stream Água Suja
(winter data/summer data).
Sample < 2 mm 2 - 6 mm 6 - 20 mm 20 - 63 mm
AS - 6 3 3 2 2
AS - 6a 78 122 153 124
AS - 7 2 2 4 5
AS - 7a 71 118 100 83
AS - 10a 25 41 47 104
AS - 10b 36 81 79 233
AS - 11 1 2 2 4