Mineralogy and geochemistry of the sodium metasomatism-related uranium occurrence of Aricheng South, Guyana

Abstract

The Aricheng South uranium occurrence is associated with Na metasomatism that affected the granitoids of the Kurupung Batholith in western Guyana. The mineral paragenesis indicates that late-magmatic albitization was followed by chlorite alteration of biotite. A minor amount of uraninite occurs in fractures in the newly formed albite crystals, often in company of calcite. The main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated in a groundmass of fine-grained hydrothermal zircon. Whole rock geochemistry supports the temporal dissociation of albitization from the main ore stage. Brannerite, zircon, and uraninite are often partially altered to secondary brannerite, zircon, and coffinite, respectively. Stable oxygen (chlorite, calcite) and hydrogen (chlorite) isotope compositions suggest that a highly evolved meteoric fluid, or at least one corresponding to a very high rock/fluid ratio (δ18O of approx. 3.4% to 4‰ and δD of approx. −80‰) may have caused the pre-ore alteration assemblage. The fluids in equilibrium with main ore stage zircon have δ18O of approx. 6.8‰ and appear to be of magmatic origin. The Aricheng occurrence geochemically, mineralogically, thermally, and paragenetically resembles the Valhalla U deposit in northern Australia despite differences between the deposits’ host lithologies, whereas the Lagoa Real and Espinharas U deposits in Brazil have host rock lithology that resembles that of Aricheng.