Cycling of rare earth elements at the Precambrian-Cambrian boundary

Abstract

The Great Unconformity at the Precambrian-Cambrian boundary marks a global erosion surface, and a time gap which in places exceeded a billion years. The weathered sub-Cambrian rocks include abundant granites and pegmatites. These rocks and others were mineralized in several regions. The basal Cambrian sediments show that rare commodities including gold and rare earth elements (REEs) were concentrated from sub-Cambrian sources by both chemical and physical processes. The clay-rich unconformity in Europe demonstrates the weathering of Palaeoproterozoic pegmatites (~1.2 billion years older) and liberation of strontium and REEs to reprecipitate as authigenic phosphate minerals. This is consistent with a global strontium isotope excursion at the Precambrian-Cambrian boundary. The global extent of sub-Cambrian granites and pegmatites indicates a possible exploration play for REEs. More generally, the abundance of ores exposed on the surface globally, and examples of early Cambrian enrichment, indicate that the surface has high potential for exploration of rare elements.