Hydrothermal cerium oxide in manganese-oxide-cemented breccia, Quadrilátero Ferrífero of Minas Gerais, Brazil

Cerium (Ce) oxide has been found in Mn-oxide-cemented breccia in the Quadrilátero Ferrífero of Minas Gerais, and identified as cerianite-(Ce) on the basis of backscattered-electron imaging and energy-dispersive spectrometry. The Ce oxide compound occurs as nano- to microscale crystals that are dispersed within, or locally concentrated along, colloform bands of cryptomelane, and in open spaces. The breccia shows a prominent positive anomaly of Ce and has enrichment of Mn, Cd and Hg. The occurrence of abundant cryptomelane, a mineral typically formed under near-surface conditions, and its colloform bands that cement angular fragments of vein quartz suggest that Ce oxide directly precipitated as colloidal particles by supersaturation under low-temperature hydrothermal conditions that followed seismic rupturing. This scenario is remarkable because little is known about natural Ce oxide of hydrothermal origin.