Potential toxic element mobility ar Roşia Montană gold mine (Metalliferi Mts., Romania)

Roşia Montană, the largest gold mine in Europe, was closed in 2006 after a long mining history, dating back to Roman times. The possibility to re-open the mine is strictly related to finding a solution to severe environmental problems. Waters draining the mine site are characterised by low pH and high concentration of suspended or dissolved potential toxic elements (PTE), leading to severe pollution of the Roşia and Abrud River (BIRD ET AL., 2005). The two principal sources of PTE are the piles of waste rock stored during the old exploitation operations and the the unexploited ore bodies occurring inside the tunnels and in the open pits. This, this study faces the problem of characterising the mineralogical and chemical composition of the Hop dump, one of the main waste-rock dump of the Roşia Montană gold mine. Twenty-five samples were collected on the eastern part of the Hop dump, following a virtual squared grid (knots distance of 25-30 m). Geotechnical, geochemical and mineralogical features of each sample were investigated. Moreover the chemical reactivity was tested by means of static tests, following AMIRA procedure (IWRI and EGI, 2002), and kinetic tests following the “modified EPA method 1312” (SPLP - EPA, 1994). It was assessed, matching field and analytical data with Positive Matrix Factorisation processing, that the waste rocks are composed by two principal lithologies: the first labelled as “andesitic breccia"; the second labelled as “dacite”. A third independent factor was identified and related to the occurrence of “residual ore” in the waste rocks. The concentrations of PTEs into the waste rocks are below the regulatory limits, with the exception of As, which has concentrations up to 10 times higher than the threshold prescribed by international law. As a matter of fact, part of the As can be related to a natural background concentration in dacites, while another part is strictly related to the ore deposit. The SEM analyses showed that a part of the As content is associated to primary minerals occurring within the dacite-rich samples (particularly sulfides) and the remaining part is contained into the secondary authigenic mineral phases (mainly iron oxyhydroxides and oxyhydroxisulfates). Looking at the static tests results and mineralogy, even if all the waste rock samples can produce acid, due to the occurrence of reactive sulfides, only the dacite-rich samples are expected to really generate acid mine drainage (AMD), since the andesite-rich samples contain enough carbonate minerals to determine a acid neutralizing capacity (ANC) higher than maximum potential acidity (MPA). Kinetic tests showed that PTE contents in filtered solutions are generally low and under the law threshold: As ranges between 1 and 7 ppb, Cu ranges between 0 and 98 ppb and Zn ranges between 21 and 570 ppb. pH values of leachates greatly varies, from 2.9 to 8.9, and their sulphate content ranges between 13.5 and 475 ppm thus exceeding the European limit for drinking waters. Comparison between the geochemical features of leached waters and bulk chemistry of waste-rocks shows that the release of As in aqueous solutions is very poor, despite its hazardous concentrations in the solid material (range 80 – 107 ppm). On the other hand, Cu and Zn content in rocks is lower, ranging between 30 and 47 and between 31 and 44 ppm respectively, but they are characterized by a higher geochemical mobility. Given the evidence of field and preliminary analysis on waters, the Hop dump contributes only to some extent in environmental pollution related to AMD processes, whereasit is conceivable that most of the processes take place in the underground tunnels of the mine or in the other waste-rock dumps. [1] BIRD, G., BREWER, P.A., MACKLIN, M. G., SERBAN, M., BALTEANU, D. & DRIGA, B., 2005: J. Geochem. Explor. 86: 26-48. [2] U.S. ENVIRONMENTAL PROTECTION AGENCY, (1994): SW-846 online