This methodology was applied to a case study, the mineralized faults and associated breccia located in the Alligator Rivers Uranium Field (middle Proterozoic Kombolgie Subgroup, Northern Territory, Australia). Faults are cemented by euhedral quartz containing a variety of fluid inclusions with a wide range of ice-melting temperatures, from near 0 °C down to about − 50 °C. The approach has permitted the identification of three types of fluids: an Na-rich brine with variable Mg concentrations, a Ca-rich brine with low Mg concentrations, and a lower salinity fluid. The Ca and Na-brines mixed with each other and a less saline end-member. On the basis of Cl/Br and cation ratios, the Na-brine is interpreted as a primary brine, resulting from the evaporation of seawater. It is inferred that the Na-brine evolved chemically during its interaction with the Ca-rich lithologies from Proterozoic basement, forming subsequently a Ca-brine through Na–Ca exchange reactions. Thus, the fault system, which affects both the base of the Kombolgie sandstone and its basement, represents active drainage zones where different fluid reservoirs were connected, and thus a place where fluid mixing was highly favored.