Distributions of selenium and related elements in high pyrite and Se-enriched rocks from Ziyang, Central China

Abstract

Pyrite-bearing volcanic tuff and carbonaceous slates of the Lower Cambrian Lujiaping Formation have been considered the source of selenium (Se) in high-Se soils in the area of the Naore Village of the Ziyang County, China, where acute Se toxicity occurred in the 1980s. For geochemical exploration, the accumulation and distribution of Se and trace elements within the pyrite grains are of great indicative significance for understanding the possible origin and forming mechanism of the Se enrichment. In this work, 7 rock samples and 89 pyrite grains were collected and analysed to investigate the element micro-distribution within the pyrite grains using EPMA and LA-ICP-MS. The whole-rock analysis shows that the pyrite-bearing Lower Cambrian rocks in Ziyang are commonly enriched in Se, As, Mo, Cd, Cr, Ni, Sb, Cu and Ba. In situ microanalysis indicates that Se (30–1880 μg/g) and As (160–2960 μg/g) concentrations within the pyrite grains are 1–2 orders of magnitude higher than those in the host rocks (Se: 0.56–73.1 μg/g, As: 2.56–195 μg/g) and that the Se contents are negatively correlated with pyrite mineral sizes. Grain-scale element mappings of fresh Se-enriched pyrites reveal two types of Se and As distribution patterns (type-I: high-Se 120–1870 μg/g and low-As <150–1580 μg/g cores, type-II: low-Se 60–680 μg/g and low-As <150–1310 μg/g cores) with alternating Se or As contents during the growth of zoned pyrite. The texture of pyrite element zoning reflects multi-stage hydrothermal pulses with changing trace element concentrations. Type-II pyrite formed later than type-I pyrite. Genetic analysis confirms that Se-enriched strata were strongly affected by hydrothermal submarine deposition and that Se in pyrite was mainly derived from hydrothermal fluids. Secondary hydrothermal erosion led to the formation of high-Se “fractures” (Se content 19.3 wt%). In summary, utilizing pyrite as an indicator for the Lower Cambrian Lujiaping Formation Se-bearing strata extends our understanding of selenium geochemical processes and highlights the mechanistic aspects of Se.