EXTREME PGE CONCENTRATIONS IN LOWER CAMBRIAN ACID TUFF LAYER FROM THE KUNYANG PHOSPHATE DEPOSIT, YUNNAN PROVINCE, SOUTH CHINA--POSSIBLE PGE SOURCE FOR LOWER CAMBRIAN Mo-Ni-POLYELEMENT ORE BEDS

Abstract

A study of the platinum group element (PGE) distribution in various lowermost Cambrian rock types of the Kunyang phosphate deposit in south China revealed unusually high PGE concentrations (434 ppb Pt, 142 ppb Pd, 57.97 ppb Rh, <1 ppb Ru, and 0.14 ppb Ir) in a whole-rock sample of acid tuff that is interbedded with phosphorite. Stratigraphically above this unit is an Mo-Ni–polyelement (including PGE and Au) phosphatic and sulfidic black shale. The tuff consists of clastic glass fragments in a matrix of clay minerals (dominantly mixed-layer illite-smectite) and cryptocrystalline quartz with minor dolomite, phosphatic fragments and nodules, and rare pyrite. The clay-quartz fraction revealed elevated Ru (11 ppb) and Ir (1.4 ppb) concentrations, but a depletion in Pt (6.8 ppb), Pd (25 ppb), and Rh (0.35 ppb) relative to the whole rock. LA-ICPMS analyses of pyrite yielded high Ni values (up to 1,243 ppm) and in some grains also elevated Re (up to 576 ppb) and Pd (up to 190 ppb) concentrations. No PGE were detected in the phosphatic nodules. It is very likely that the PGE are in part bound to submicroscopic inclusions in pyrite and/or intermetallic phases bound to the clay fraction. Petrographic observations and isotope data obtained on paragenetically late dolomite veinlets, which yielded C and O isotope ratios that correspond to those of contemporaneous seawater ( δ 13C ranges −2.32 to −2.44‰ V-PDB, δ 18O −10.07 to −9.94‰ V-PDB), do not indicate any hydrothermal overprint. Instead, pyrite formation is ascribed to diagenesis in the reduced environment of black shales, as supported by δ 34S ratios obtained for pyrite between 6.31 and 7.54 per mil CDT and elevated Ni concentrations in the pyrite. If other acid tuffs in the region have similar anomalous concentrations of PGE, they could have played an important role in the origin of the slightly younger Mo-Ni–polyelement (including PGE and Au) phosphatic and sulfidic black shales in the Yangtze platform.