The Flin Flon domain, Trans-Hudson orogen, Canada, is an example of a Proterozoic greenstone belt that hosts a large number of small mesothermal gold occurrences. The greenstone belt consists of tholeiitic to calc-alkaline volcanic rocks (Amisk Group; approximately 1900-1880 Ma) unconformably overlain by molasse-type sedimentary rocks (Missi Group; 1850-1840 Ma). The supracrustal rocks are intruded by gabbroic to granitic rocks ranging in age from synvolcanic to late tectonic (1890-1835 Ma). Metamorphic grade varies from prehnite-pumpellyite facies to amphibolite facies, and peak thermal conditions generally were attained locally during granitoid intrusion. Many of the shear zones and the dominant regional foliation developed during ductile deformation coeval with peak metamorphism. These shear zones were reactivated and mineralized under brittle-ductile conditions during postpeak metamorphic uplift.Mesothermal gold mineralization is hosted in quartz veins that developed at jogs or zones of competency contrasts along brittle-ductile shear zones, in all lithologies in the region. Alteration envelopes, which consist of quartz-carbonate-chlorite-albite-muscovite-pyrite, are usually less than a few meters wide and overprint regional metamorphic assemblages. The veins consist of milky-colored quartz and may also contain tourmaline, ankerite, chlorite, and muscovite. Pyrite and arsenopyrite are the dominant sulfides, and gold grade is proportional to the modal abundance of sulfides.The dominant fluids associated with gold mineralization were H 2 O-CO 2 -NaCl (0.6-14.7 wt % NaCl equiv) in composition, with generally uniform H 2 O/CO 2 phase ratios in a given plane or zone but variable phase ratios between zones. Oxygen isotope mineral pairs indicate temperature of mineralization of between 360 degrees and 420 degrees C, which, when combined with fluid inclusion density estimates, indicate that most of the gold mineralization occurred at a pressure of about 2 kbars. Rapid pressure release in dilatant zones with related phase separation and change in fluid composition and properties, and local fluid-wall rock interaction, are the likely gold-precipitation mechanisms.The O, H, S, C, and Sr isotope compositions of hydrothermal minerals in the shear zones indicate that the mineralizing fluids interacted extensively with Proterozoic metamorphic and igneous rocks similar in composition to those presently exposed at the surface, at high temperatures and at low water/rock ratios. The O and H isotope compositions are compatible with formation of the fluids from devolatilization reactions during prograde metamorphism at depth. The O isotope composition of barren quartz veins varies with the host rocks indicating that these fluids were either locally derived or interacted extensively with the immediate host rocks.An Rb-Sr isochron age of 1760 + or - 9 Ma on tourmaline and muscovite from the Rio deposit is similar to 40 Ar/ 39 Ar plateau ages for muscovites from the premetamorphic Laurel Lake deposit in the Flin Flon domain. This suggests that fluid advection through the Rio deposit was contemporaneous with the thermal event that affected Laurel Lake muscovites. These ages, along with an age from the Tartan Lake mesothermal deposit (1791 + or - 4 Ma) in the Flin Flon domain, indicate that fluid flow related to hydrothermal activity along the shear zones in the western Flin Flon domain occurred periodically over a period of about 30 Ma.These Proterozoic gold occurrences are similar to Archean mesothermal gold deposits in terms of geologic, structural, and tectonic setting, alteration and vein mineralogy, and fluid composition, pressure, and temperature. However, the limited extent of alteration, the low gold content, the isotopic systematics, and the lack of either S-type granites or mantle-derived lamprophyres suggest that the shear zones hosting the western Flin Flon domain occurrences sampled limited quantities of fluids and, thus, were not transcrustal features capable of hosting giant gold deposits.