The Nabarlek deposit is an unusually high-grade uranium orebody (1.84% U 3 O 8 , 12,000 metric tons) hosted by amphibolite and semipelitic schist of lower Proterozoic age. It is unusual in the Alligator Rivers uranium field in being hosted by amphibolite. The disposition of the ore is controlled by a prominent reverse fault, which is part of a larger zone of reverse faulting. The fault zone is nearly perpendicular to prominent metamorphic S 2 and S 3 fabrics; a geometry which might enhance the fluid-rock ratio. A striking feature of the deposit is the extent and intensity of hydrothermal alteration which extends more than 1 km from the ore. Alteration remote from the ore resulted in pseudomorphous replacement of metamorphic biotite, amphibole, and feldspar, under conditions of a low fluid-rock ratio and mass transfer. Ore is associated with a small desilicification zone within, and superimposed on, this larger envelope. The fluid-rock ratio and mass transfer, resulting mainly in silica loss, were high within this interior zone and produced intense pervasive alteration, leading to an assemblage of phengitic white mica, magnesian chlorite, and hematite. Large volumes of fluid were required to effect the massive desilicification (and deposit such a volume of uranium), given the current knowledge of the solubility of quartz and uraninite.Ore deposition (predominantly as uraninite) postdated intense phengitic white mica alteration. Chlorite, however, is the main gangue mineral, possibly reflecting a change in redox conditions accompanying ore deposition. A redox control on ore deposition is supported by the sparse ocurrence of host-rock graphite which evidently participated in CH 4 -forming reactions. Abundant hematite in the alteration halo suggests the presence of an oxidized fluid (log f (sub O 2 ) approx. -35 at 200 degrees C) which was thus suitable for transporting ore-forming quantities of uranium, probably as chloride complexes. Primary mineralization underwent incongruent dissolution to coffinite, in equilibrium with a vanadian illite, kaolinite, and secondary hematite assemblage (residual ore). Weathering of primary and residual ore produced yellow and green secondary uranium minerals, both pseudomorphous after earlier ore types and as crusting of open goethite-lined fractures.