Multiphase Methane-Rich Fluid Inclusions in Gold-Bearing Quartz as Illustrated at Pontal (Goias, Brazil)

N. Guilhaumou1, M. Santos2, J. C. Touray3, C. Beny4 and M. Dardenne2
1 Département de Géologie de l'Ecole Normale Supérieure, URA CNRS no 1316, 24 rue Lhomond, 75005 Paris, France
2 Instituto de Geociencias, Universidad de Brasilia, Brasilia, Brazil
3 Laboratoire de Métallogénie et Géochimie minérale, Ecole Supérieure de l'Energie et des Matériaux, URA CNRS no 1366, Université d'Orléans, 45067, Orléans Cédex 02, France
4 G.S. (BRGM-CNRS), 1A, rue de la Férollerie 45071, Orléans Cédex 02, France

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 fO2 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.

Keyworos: gold deposit • quartz lode • fluid inclusions • Raman microprobe • microthermometry

Mineralogical Magazine; June 1990 v. 54; no. 375; p. 257-266; DOI: 10.1180/minmag.1990.054.375.11
© 1990, The Mineralogical Society
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