查看更多>>摘要：Phosphate-rich sedimentary rocks (phosphorites) host significant concentrations of rare earth elements (REE, including Y) that are critical for development of 'green' technologies and renewable energy supply. Cambrian (ca. 505 Ma) phosphorites from the epicontinental Georgina Basin can contain up to 0.5 wt% REE, marking them as among the most REE-enriched phosphorites globally. This contrasts with previous work that proposed generally poor REE endowment of Cambrian phosphorites across the world. Here, we report orders of magnitude variation in REE concentration across the northern, central and southern parts of the Georgina Basin. We use principal component analysis (PCA) on centred-log ratio (CLR) transformed whole rock geochemical data to show that the southern phosphate prospects of the basin (Ardmore, Duchess, Phosphate Hill) have higher concentrations of elements that substitute into the major primary phosphate mineral carbonate fluorapatite (CFA) (P2O5, CaO, Na2O, REE, Sr and possibly Ba), while those of the central and northern prospects (Lily and Sherrin Creek, Barr Creek, DTREE, Paradise South and Paradise North) show a greater contribution of terrigenous material characterised by a higher concentrations of elements typically excluded from the CFA lattice (SiO2, TiO2, Al2O3, K2O, MgO, Zr, Nb, Cs, Ga and Rb). REE concentrations are up to two orders of magnitude higher in the southern prospects than in the central and northern prospects, while phosphorites from all prospects display modern seawater-like REE patterns. It is unlikely that global secular changes of seawater chemistry through time are the dominant control on the REE content of the Georgina Basin phosphorites, given the relative short time span of phosphogenesis (ca. <5 m. y.), the restricted interaction of the epicontinental and open sea waters, and the order of magnitude variations in REE concentration on a localised basin scale. Instead, variations in REE concentrations across the Basin are likely due to: 1) textural differences, with apatitic non-pelletal mudstones dominating the northern and central prospects, and largely pelletal (granular) phosphorites in the south; 2) local seawater composition that reflected the proximal basement rocks and riverine/groundwater input. Higher concentrations of REE in the southern prospects may be explained by surface and groundwater interaction with A-type granite and schistose metasedimentary basement rocks. We suggest, therefore that REE prospectivity of phosphatic sedimentary rocks is not primarily controlled by temporal variations in global seawater chemistry, but rather by local sedimentological and source region factors.
NSTL
Elsevier