Fluid inclusion evidence on the origin of some Australian unconformity-related uranium deposits

Abstract

Fluid inclusions in altered and unaltered host rocks and hydrothermal quartz, calcite, and dolomite from the unconformity-related uranium deposits of Koongarra, Nabarlek, and Jabiluka (Northern Territory, Australia) were examined using standard microthermometry, laser Raman microprobe, and scanning electron microscopy. Decrepitated gases were analyzed in a mass spectrometer. The relationship of fluid inclusions to alteration and mineralization was established by occurrence restricted to the alteration zones around the deposits and the presence in fluid inclusions of solid phases which also occur as megascopic phases in the alteration.Secondary, halite-saturated fluid inclusions in altered host-rock quartz and in early hydrothermal quartz veins are believed to relate to hydrothermal alteration associated with ore deposition. These inclusions are identical to primary inclusions in silicified and boron-metasomatized Kombolgie Formation sandstone above the Jabiluka 2 deposit (Durak et al., 1983) and hence sample post-Kombolgie fluids. The trapped fluid is saline and calcium rich (ca. 23 equiv wt % CaCl 2 ) and sulfate poor (SO (super -2) 4 < 0.001 m). No gases were detected in the vapor phase of such inclusions and the median vapor disappearance temperature is 140 degrees C. Primary inclusions in postore dolomite veins and secondary inclusions in host rock and hydrothermal quartz sample postore hydrothermal fluid. This fluid is hypersaline and also calcium rich (typically 35 equiv wt % CaCl 2 ) and is saturated with as many as 11 solid phases, including dolomite and iron-rich chlorite. Vapor bubbles in these inclusions contain methane and minor N 2 and the median vapor disappearance temperature is 127 degrees C. Aromatic hydrocarbons detected by mass spectrometry are possibly related to this population.The calcic brines are thought to have been terrestrial in origin and derived from the oxidized red-bed sediments overlying the deposits (MacArthur basin). Ore deposition was a response either to reduction of the ore-bearing fluid by direct interaction with graphite and ferrous iron in the wall rocks or by mixing with a methane and higher hydrocarbon-bearing brine, represented by postore fluid inclusions. Ore deposition and related hydrothermal alteration occurred between 200 degrees and 300 degrees C under a depth of cover of ca. 2 km. This temperature is consistent with independent temperature constraints and suggests an anomalous thermal gradient during ore formation.