Abstract: In the Pontal auriferous lode, dominant saccharoidal quartz is associated with oligoclase, biotite, hornblende, tremolite-actinolite, sulphides (less than 2%), and disseminated native gold. Four main types of fluid inclusion have been distinguished based on their habit, distribution and spatial relation-ship with gold particles. Type S are primary multiphase large sized (100 to 200 µm) inclusions with homogenization temperatures (V + L → L) between 350 and 450°C. They contain siderite and/or calcite and graphite-like microcrystals as daughter phases. Commonly associated with these inclusions are tiny (50 to 100 µm) solid inclusions of biotite or actinolite. In most of these inclusions, only CH₄ has been detected in the vapour phase. However some noticeable exceptions were observed (CO₂/CH₄ ratio near 0.85). Type C inclusions are later than gold and occur disseminated in quartz or along trails that crosscut quartz grain boundaries. They may contain nahcolite daughter crystals. CO₂/CH₄ ratios range from 0.0 to 0.5. Homogenization temperatures vary from 150 to 300°C. Type V are mainly gaseous CH₄-H₂O inclusions. They may occur as small-sized inclusions directly associated with gold particles. Type L are aqueous two-phase inclusions of late secondary origin.

The scattering of the CO₂/CH₄ ratios could be related to fluctuations of the oxygen fugacity that triggered gold precipitation at the time of trapping. These variations of f_O2 with time could reflect unbuffered fluid-rock interaction with respect to redox conditions during quartz deposition.

Finally, gold deposition is interpreted to have occurred at elevated temperature (500°C and pressure, compatible with boundary conditions between greenschist and amphibolites facies.