The Maw Zone REE deposit is located in the southern part of the basin between Key Lake and the McArthur River unconformity-related uranium (URU) deposits, stratigraphically about 130 meters above the basal unconformity and confined to the MFd member of the Manitou Fall Formation. The spatial association of this deposit with URU deposits led geologists to consider a genetic relationship between each. The present study aims to further understand the genesis of the Maw Zone and its relationship with URU deposits through detailed petrography, U-Pb isotopic dating of xenotime, O isotopic analyses of tourmaline and coexisting quartz, and microthermometry, Raman spectroscopy and gas analysis of fluid inclusions. Petrographic studies indicate that the tourmaline (mainly of magnesiofoitite composition), syntaxial quartz and drusy quartz in the Maw Zone is partly similar to those found in the McArthur River deposit in terms of composition, texture, and paragenetic position. Xenotime is shown to have formed after significant compaction rather than in early diagenesis, and it is paragenetically comparable with uranium mineralization in the McArthur River deposit. Secondary ion mass spectrometer (SIMS) analysis of oxygen isotopes of co-existing drusy quartz and tourmaline suggests that the average temperature of the hydrothermal fluid is 163 °C, and the average δ18OVSMOW of the fluid is -1.8 ‰. U-Pb dating of xenotime by sensitive high-resolution ion microprobe (SHRIMP) indicates that the REE mineralization is of the same age as the major primary unconformity-related uranium mineralization in the eastern Athabasca Basin. Fluid inclusion analysis on drusy quartz indicates the presence of liquid-rich biphase inclusions, vapor-dominated inclusions, and solid phase-bearing inclusions. The co-existence of these inclusions with variable vapor/liquid ratios within individual fluid inclusion assemblages (FIAs) suggests fluid immiscibility and heterogeneous trapping. Homogenization temperatures of the liquid-dominated biphase inclusions measured from FIAs and isolated fluid inclusions range from 77 °C to 178 °C and 55 °C to 162 °C respectively. The range of salinity is from 24.8 to 33.6 wt.% NaCl + CaCl2 for FIAs and from 8.1 to 33.6 wt.% NaCl + CaCl2 for isolated inclusions. The microthermometric data are generally comparable to those reported for URU deposits. Based on the assumption of fluid immiscibility and low concentrations of non-aqueous volatiles (<2 mole%) as analyzed by mass spectrometry for bulk fluid inclusions, fluid pressures were calculated to be less than 200 bars. All these results suggest that the Maw Zone REE deposit was formed from the same hydrothermal systems associated with the unconformity-related uranium deposits, possibly in a shallow environment involving hydrothermal fluids from the basin and other sources. The absence of uranium mineralization in the Maw Zone may be due to the mineralizing fluid precipitating uranium to form URU deposits before arriving at Maw Zone, lack of reducing agents at Maw Zone, or a combination of both. However, the possibility that the ore-forming fluid for the REE mineralization at Maw Zone was initially low in U, thus distinct from the mineralizing fluid for the URU deposits, cannot be ruled out. In the latter case, fluids from different sources were involved in the formation of the URU and REE deposits, but one of the fluids, which is responsible for the tourmaline alteration, may be the same. Keywords: Maw Zone REE deposit, xenotime, unconformity-related uranium deposits, petrography, oxygen isotope, U-Pb dating, fluid inclusion, shallow environments, Athabasca Basin