Abstract

Geochemical investigations of a widely dispersed Late Proterozoic impact ejecta horizon and its host shales (Bunyeroo Formation, Australia) have provided strong evidence for low-temperature mobilization of the platinum group elements (PGE), including Ir. PGE anomalies are associated with both the ejecta horizon (Ir up to 2.0 ppb, Pt up to 270 ppb) and the green shales that envelop it. Several PGE anomalies (0.073-0.45 ppb Ir, 3.11-314 ppb Pt) also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence (average red shale background of 0.019 ppb Ir and 0.89 ppb Pt). All green shale horizons analyzed, regardless of their stratigraphic position, have relatively high levels of Ir and other PGE. The nonextraterrestrial, postdepositional origin of the PGE at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin, permeable, green beds in a predominantly red shale sequence, their association with enrichments in Cu-V-Zn-Ni, and their nonchondritic PGE interelement ratios. The impact ejecta horizon has a more chondritic PGE geochemistry consistent with a meteoritic origin. A redox precipitation model similar to that invoked for redbed Cu-U-V deposits is proposed to explain the PGE anomalies in the green shales.

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