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

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.Pasˇava et al.(Econ Geol 105:1047–1056, 2010) reported anomalously high PGE concentrations in this section, up to576 9 10-9(434 9 10-9Pt, 142 9 10-9Pd) 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 NiMo-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 Bed5 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 than0.90 9 10-9, and Pt ? Pd concentrations of no higher than0.70 9 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.

U–Pb zircon age of the base of the Ediacaran System at the southern margin of the Qinling Orogen

Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive d13 C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deepwater basinal cap carbonate section with strong negative δ^(13) C values in the southern margin of the Qinling Orogen,Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma(1 r, MSWDCE= 0.31,ProbabilityCE= 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides ageochronological constraint for the Ediacaran successions in the Qinling Orogen.