Mercury contents in rocks, soil and stream sediments were determined to investigate the origin of the geochemical anomaly found in sediments of the Iguaçu River as indicated in previous studies. Except for its headwaters in the Curitiba area with Precambrian basement rocks and Cenozoic sediments of the Curitiba Basin, the Iguaçu River lies across the Paraná Basin comprising Paleozoic to Mesozoic sedimentary and volcanic rocks. In the study area, the geological units include basaltic and dacitic rocks of the Serra Geral Formation (Cretaceous) and black shales of the Irati Formation (Permian-Triassic). Total mercury was measured by thermal decomposition with atomic absorption spectrometry and Zeeman correction in volcanic rocks (0.2 to 0.4 ng/g), black shales (286 and 430 ng/g), soil (average of 103±57 ng/g, grain size <177 μm) and sediment (average of 37±18 ng/g, grain size <177 μm). The elevated mercury and lost on ignition values enrichments in rock-soil profiles was assessed by mass balance calculation. The results point out to a geogenic origin of mercury in the Iguaçu Valley where no human-induced mercury inputs to the environment were observed.254437447BELLIENI, G., COMIN-CHIARAMONTI, P., MARQUES, L.S., MELFI, A.J., NARDY, A.J.R., PAPATRECHAS, C., PICCIRILLO, E.M., STOLFA, D., Petrogenetic aspects of acid and basaltic lavas from the Paraná Plateau (Brazil): Geological, mineralogical and petrochemical relationships (1986) Journal of Petrology, 27 (4), pp. 915-944BIZZI, L.A.SCHOBENHAUS, C.GONÇALVES, J.H.BAARS, F.J.DELGADO, I.M.ABRAMS, M.B.LEÃO NETO, R.MATOS, G.M.M.SANTOS, J.O.S. (Coords.), Geologia, Tectônica e Recursos Minerais do Brasil: Sistema de Informações Geográficas - SIG. Rio de Janeiro: CPRM (Programa Levantamento de Recursos Minerais do Brasil), 2001. 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