Fahlore minerals of the tetrahedrite-tennantite series are common constituents of a variety of mineral deposits. The chemical formula of tetrahedrite (TTR) is Cu6[Cu4(Fe,Zn)2]Sb4S13 and can host high quantities of several trace elements such as Ag, As, Bi, Cd, Cu, Fe, Hg, Mn, Se, Te, Zn. The mineral deposit of the Mt. Avanza (Carnic Alps, Friuli Venezia Giulia, Italy) is an ancient fahlore mine near Forni Avoltri village, where tetrahedrite was the main ore mineral extracted for the recovery of Cu and Ag (Barago et al., 2023). In the (Zn-Hg-rich) tetrahedrite (TTR) minerals, mercury (Hg) is present in the 1.48 – 8.66 % wt. range (Casari, 1996), thus posing potential environmental risks due to the dispersion of Hg due to weathering and transport processes. In this work, thermodesorption technique coupled with continuous determination by atomic absorption spectrometry (AAS) was used for the determination of Hg species in different matrices (ore minerals, waste rocks, soils and stream sediments). This technique is based on the gradual heating of a sample, from ambient temperature up to 700 °C, which releases Hg at different temperature intervals depending on its chemical form. Thus, a single Hg species that desorbs at a specific temperature may be identified via a thermogram of the sample. In addition, X-Ray Powder Diffraction (XRD) was used to identify the main mineral phases in the same samples. The aim of this study was to identify the desorption temperature of Hg corresponding to TTR mineral and to recognise additional chemical forms of Hg associated to different environmental samples nearby the fahlore mine site. Once TTR minerals were manually separated from samples of the main waste rock piles and identified via XRD, they were desorbed to identify the desorption temperature of Hg, which is occurred at 437 ± 11.5 °C. However, by applying the thermodesorption technique to soils, sediments, and waste rocks enriched of TTR minerals, usually with total Hg concentrations < 1000 mg/kg, a second release peak was evidenced at lower temperature, varying between 200 - 250 °C. Two hypotheses were considered to explain this second peak: the first could be related to lithogenic sources, being Hg associated with Devonian to Permian rock-forming minerals, the second to Hg-bearing secondary minerals as alteration products of tetrahedrite primary sulfosalt minerals.
Identification of mercury species via thermodesorption in environmental samples from a tetrahedrite historical Cu-Sb(-Ag) mining site (Mt. Avanza, Friuli Venezia Giulia, Italy)
Barago N.;Petranich E.;Floreani F.;Pavoni E.;Covelli S.
2023-01-01
Abstract
Fahlore minerals of the tetrahedrite-tennantite series are common constituents of a variety of mineral deposits. The chemical formula of tetrahedrite (TTR) is Cu6[Cu4(Fe,Zn)2]Sb4S13 and can host high quantities of several trace elements such as Ag, As, Bi, Cd, Cu, Fe, Hg, Mn, Se, Te, Zn. The mineral deposit of the Mt. Avanza (Carnic Alps, Friuli Venezia Giulia, Italy) is an ancient fahlore mine near Forni Avoltri village, where tetrahedrite was the main ore mineral extracted for the recovery of Cu and Ag (Barago et al., 2023). In the (Zn-Hg-rich) tetrahedrite (TTR) minerals, mercury (Hg) is present in the 1.48 – 8.66 % wt. range (Casari, 1996), thus posing potential environmental risks due to the dispersion of Hg due to weathering and transport processes. In this work, thermodesorption technique coupled with continuous determination by atomic absorption spectrometry (AAS) was used for the determination of Hg species in different matrices (ore minerals, waste rocks, soils and stream sediments). This technique is based on the gradual heating of a sample, from ambient temperature up to 700 °C, which releases Hg at different temperature intervals depending on its chemical form. Thus, a single Hg species that desorbs at a specific temperature may be identified via a thermogram of the sample. In addition, X-Ray Powder Diffraction (XRD) was used to identify the main mineral phases in the same samples. The aim of this study was to identify the desorption temperature of Hg corresponding to TTR mineral and to recognise additional chemical forms of Hg associated to different environmental samples nearby the fahlore mine site. Once TTR minerals were manually separated from samples of the main waste rock piles and identified via XRD, they were desorbed to identify the desorption temperature of Hg, which is occurred at 437 ± 11.5 °C. However, by applying the thermodesorption technique to soils, sediments, and waste rocks enriched of TTR minerals, usually with total Hg concentrations < 1000 mg/kg, a second release peak was evidenced at lower temperature, varying between 200 - 250 °C. Two hypotheses were considered to explain this second peak: the first could be related to lithogenic sources, being Hg associated with Devonian to Permian rock-forming minerals, the second to Hg-bearing secondary minerals as alteration products of tetrahedrite primary sulfosalt minerals.Pubblicazioni consigliate
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