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

作　　者：Lecai Xing Mingzhong Zhou Liang Qi Zhilong Huang 

机构地区：[1]State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences [2]University of Chinese Academy of Sciences [3]School of Geographical and Environmental Sciences,Guizhou Normal University

出　　处：《Chinese Journal Of Geochemistry》2015年第3期346-361,共16页中国地球化学学报

基　　金：supported by National Science Foundation of China (Nos. 41072054, 40963002)

摘　　要：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.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., were in J Anal At Spectrom 26：1900-1904, 2011）, and 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.

关 键 词：元素异常 下寒武统 膨润土 剖面 梅树 铂族 云南 酸性火山岩 

分 类 号：P534.41[天文地球—古生物学与地层学] X825[天文地球—地质学]