Geochemical Characteristics of Selenium and Its Correlation to Other Elements and Minerals in Selenium-Enriched Rocks in Ziyang County, Shaanxi Province, China

Ziyang County, Shaanxi Province, China is a world known selenium (Se)-enriched area, and a severe selenosis incident was reported in Naore Village during the 1990s. This study investigated the geochemical characteristics of Se and its fractions in Se-enriched rocks from Ziyang. Se distribution is uneven, ranging from 0.23 to 57.00 mu g/g (17.29 +/- 15.52 mu g/g). Se content is higher in slate than chert, and even lower in carbonate rocks. Cd, As and V are enriched but Pb is depleted in Se-enriched strata. Se has different correlations both with TS (total sulfur) (R-2=0.59 for chert) and TC (total carbon) (R-2=0.77 for slate, R-2=0.87 for carbonate). Se has significant positive correlations with V (r=0.65), As (r=0.485), Cd (r=0.459) and Pb (r=0.405). The Se level correlates with mineral content, positively with pyrite, chlorite and illite, negatively with albite. Se associated with sulfide/selenide and elemental Se are the predominant fractions of total recovered Se, suggesting that a reducing environment and the formation of sulfides were significant to Se deposition during its geochemical cycle. Although low concentration of bio-available Se (average 5.62%+/- 3.69%) may reduce the risk of Se poisoning in the target area, utilization of Se-rich rock as natural fertilizer should be restricted.