Distribution and enrichment patterns of selenium in the Ediacaran and early Cambrian strata in the Yangtze Gorges area, South China

The distribution and enrichment patterns of selenium (Se) in the E-D1 strata in the Yangtze Gorges area of South China were obtained. The geochemical characteristics of the significantly and non-significantly enriched strata of Se were analyzed.The observed enrichment factor (EF, relative to the upper continental crust) and concentration coefficient (CC, relative to the similar lithology in Eastern China) both suggest that Se is the most enriched/concentrated (Se-EF=26.97, Se-CC=48.04) among the analyzed 23 trace elements the E-D(1) strata. The normalized enrichment factor (EF', EF after Al or Th normalized) shows Se is secondly enriched (Se-EF'=218.73), which is slightly lower than cadmium (Cd-EF'=288.46) but significantly higher than the third enriched trace element arsenic (As-EF'=97.49). Se concentrations in the E-D(1) strata vary from < 10(aEuro'5) to 30.08 ppm with an arithmetic mean value of 1.35 ppm. Compared to the Nantuo Formation, Se increased 11.78 times in the whole E-D(1) strata and the average EF values are displayed as Shuijingtuo (92.58)> Yanjiahe (54.45)> Doushantuo (24.72)> Dengying (2.48)> Shipai (1.95)> lower Tianheban (1.24) Formations. Se concentrations in the E-D(1) strata are best displayed on natural logarithm normal quantile-quantile (Q-Q) plots and shown as a positive-skewed distribution pattern. The Se significantly enriched (EF > 10) strata sequences mainly include the lower and upper Doushantuo member II (DST-II), top DST-III, DST-IV, the basal and upper Yanjiahe Formation, and lower and upper Shuijingtuo Formation. Geochemical characteristics indicate that Se concentrations in the significantly enriched strata were generally influenced by terrigenous detrital as well as the combined action of single or multiple factors, such as hydrotherm, volcanic debris and deep source. Moreover, pyrite and organic matter promoted the enrichment of Se in the upper DST-II, DST-IV, upper Shuijingtuo Formation and lower DST-II, upper Shuijingtuo Formation, respectively. The Se concentrations in the not significantly enriched strata (except for DST-I, middle Shuijingtuo Formation, Shipai Formation and lower Tianheban Formation) were also influenced by terrigenous detrital, but other enrichment activities (e.g., hydrothermal, volcanic debris, and deep source) were generally insignificant.