The northwest portion of the Moon's South Pole-Aitken basin contains an anomalously high abundance of thorium as determined by Apollo and Lunar Prospector gamma-ray spectroscopy. The anomaly's proximity to the antipode of the Imbrium basin has led several investigators to suggest that the anomaly is the result of convergence of thorium-enriched ejecta from the Imbrium impact. Examination of this complex region with new higher-resolution thorium data and several other datasets reveals that a convergence of ejecta cannot explain the anomaly. Alternatively, we propose an indigenous and likely ancient source.