Insights into the formation of thucholites

Thucholites are unique organic features from igneous and sedimentary rocks composed of a U-C-rich interior enclosed by an organic outer shell. Here, we evaluated thucholites from the Ediacaran Salt Range Formation of the Upper Indus Basin in Pakistan using field emission scanning electron microscopy (FESEM) and nanoscale secondary ion mass spectroscopy (NanoSIMS). The thucholites can be divided into two types: 1) Ovoid to round structures (14–200µm in diameter) comprised of a single U-C-rich core (averaging ~16% of thucholite by area) surrounded by a mantle of organic matter (OM) within a fine-grained clay mineral matrix; and 2) Complex ovoid structures (23–100µm in diameter) with multiple U-C-rich cores (Averages ~26% of the entire thucholite by area) surrounded by an OM-rich mantle within a chert matrix. U-rich minerals within the thucholite cores consist of clusters of fine, needle-like structures (avg. length 339.2 nm) that are similar to U-rich minerals found in environmental remediation studies of U-impacted environments. NanoSIMS measurement transects across thucholite masses show that both the 34S/32S and the 13C/12C isotopic ratios decrease going from the rim to the core. Lighter C and S isotopic values in the core support a biological fractionation mechanism as opposed to radiolytic polymerization for the mantle. This data was collected as part of an effort to determine how thucholites form in sedimentary sequences and to understand their significance (i.e., paleoenvironment, impact to potential energy and mineral resources) in the geologic record.