Certain trends in alteration are commonly observed near Archean mesothermal gold deposits. This study was designed to monitor the changes in composition of fluid inclusions using solid probe mass spectrometry by: (1) establishing and verifying systematic changes in the composition of the fluid during fluid-rock interaction, (2) characterizing the composition and behavior of fluids in the vein and the wall rock in order to establish a link between them, (3) elucidating new insights on the role of fluid immiscibility in gold deposition, and (4) scrutinizing the results for potential use in exploration.Three representative sites of gold mineralization were chosen: a large deposit with appreciable amounts of free gold (Sigma mine in Val d'Or), a second deposit with a smaller amount of free gold (Norbeau mine in Chibougamau), and a third area of gold mineralization with altered wall rock and little veining (Tadd prospect in Chibougamau). Fluid inclusions were carefully selected to sample different alteration zones and the gold mineralization at each site. In most of the cases, varying degrees of unmixing of CO 2 -H 2 O were noted.Mass spectra results show that during alteration all generations of inclusions at all sites contained CO 2 and H 2 O. For the alteration-hosted deposit a decrease in X (sub CO 2 ) could be mapped outward from the mineralized zone. The most striking result was that fluid inclusions associated with the main mineralizing episode at Sigma showed a nearly complete segregation into CO 2 and H 2 O end members. This can be interpreted as the result of extreme fluid pressure fluctuations. This characteristic was absent from the Norbeau veins where free gold exists in very limited amounts. A test on a high-grade ore shoot from the Doyon mine resulted in a pattern similar to Sigma. We suggest extreme pressure fluctuations were limited in vein-hosted deposits like Norbeau and gold was precipitated essentially from fluid-rock mineral interaction. Unmixing was the most likely cause of free gold deposition. Mineralizing systems may have undergone major fracturing events at time of free gold deposition.This study suggests high CO 2 /H 2 O ratios are related to gold mineralization in altered rock-hosted deposits but that the ratio may be an unreliable guide in vein-hosted deposits because of the variable unmixing pattern of CO 2 -H 2 O fluids. Decrepitometry may prove a useful and rapid method for identifying CO 2 -H 2 O unmixing and bonanza gold potential.