Discussion on the PGE anomalies and source materials of K-bentonite (Bed 5) in the Lower Cambrian Meishucun section, Yunnan

Abstract

The Meishucun section in Yunnan is the stratotype section for stratigraphic correlation of the Lower Cambrian strata across the Yangtze Block. Known for enriched small shelly fossils, it is a prominent section for investigating the Early Cambrian phosphogenic event. Pasava et al. (Econ Geol 105:1047–1056, 2010) reported anomalously high PGE concentrations in this section, up to 576 × 10−9 (434 × 10−9 Pt, 142 × 10−9 Pd) for the total PGE concentrations of a K-bentonite sampled from the bottom of Bed 5. This finding can illustrate two significant statements: (1) in addition to the attested polymetallic Ni-Mo-PGE ore layer, another potential PGE enrichment layer exists with PGE concentrations up to the mineralization level; and (2) acid volcanics have high PGE contents overturning conventional views. To inspect whether the anomalous PGE concentration is pervasive, we investigated Bed 5 of the Meishucun section systematically, and sampled from a profile with a thickness of 3.5 m. The major and trace element geochemistry indicate the Bed 5 K-bentonite is derived from acid volcanic ash. PGE concentrations were determined repeatedly by isotope dilution-ICP-MS using improved digestion technique (Qi et al., in J Anal At Spectrom 26:1900–1904, 2011), and were duplicated by fire assay method. The results showed that each sample had total PGE concentrations of less than 0.90 × 10−9, and Pt + Pd concentrations of no higher than 0.70 × 10−9. Combined with the petrological and mineralogical features, and trace and rare earth element analyses, it is inferred that no generality of PGE enrichment exists in Bed 5 and that the anomalous PGE concentration is likely due to the nugget effect of volcanic ash modified by currents in a shallow coastal environment.