Rhenium-osmium isotope constraints on the age and source of the platinum mineralization in the Lower Cambrian black rock series of Hunan-Guizhou provinces, China

摘要

The shale at the bottom of the Lower Cambrian black series in South China is rich in nonferrous metals, noble metals, rare and dispersed elements and radioactive elements, known as "polyelement-rich bed". In order to date and trace the source of the platinum group elements, 6 samples were collected from the Zhongnan region of Guizhou Province, and the Ganzi-ping-Sancha region of Hunan Province. The contents and isotopes of Re and Os were measured. Re and Os are positively correlated with each other and the correlation coefficient between ~(187)Os/~(188)Os and ~(187)Re/~(188)Os is 0.99856. On the ~(187)Os/~(188)Os-~(187)Re/~(188)Os diagram, 6 sample points are well-distributed along the (542+-11) Ma fitting isochron in high coincidence and with 0.84 +- 0.12 as the initial ~(187)Os/~(188)Os value. The result displays that the age value of the major mineralization of Re, Os and other platinum group elements is (542 +-11) Ma, which is identical with the stratigraphical age of the wall rocks; the 'polyelement beds' in Hunan and Guizhou provinces bear high isochroneity; the differences of the forming times of different materials in the 'polyelement bed' of the same area do not surpass the error limit permissible for Re-Os date fitting; there exists no distinct time interval between the formation of the diagenetic veinlets and that of the 'polyelement bed', or the Re-Os isotope composition has not been distinctly changed by the former process; basaltic crust might be the major source of Re, Os, other platinum group and trace elements in the Lower Cambrian 'polyelement bed' of Hunan and Guizhou provinces.