Fluid Inclusion Study of Tin-mineralized Greisens and Quartz Veins in the Penouta Apogranite (Orense, Spain)*

J. Mangas and A. Arribas
Department of Geology, Faculty of Marine Sciences, University of Las Palmas de Gran Canaria, Campus de Tafira, 35017 Las Palmas de Gran Canaria (Spain)
Directorate-General XII, Science, Research and Development, Commission of the European Communities, Rue de la Loi 200, B-1049 Brussels, Belgium
*This paper was presented at the ECROFI X Conference. See the following two papers and the Mineralogical Magazine for June 1990.

Abstract: The Penouta deposit is associated with a small Hercynian apogranite stock that intrudes Precambrian-Cambrian gneisses of the Ollo de Sapo Formation. Tin ore occurs as disseminations of cassiterite in the apogranite and as greisenized zones and quartz veins which traverse both the alkaline leucogranite and the surrounding metamorphic country rocks.

A fluid-inclusion study, utilizing microthermometric, crushing tests and Raman spectroscopic techniques on quartz from an intragranitic vein and a greisen of the host rock, indicates that the evolution of fluids was similar in both samples and occurred in the three main stages: The first stage is characterized by complex CO2 (CO2-N2-CH4-H2S) and complex CO2 aqueous (H2O-NaCl-CO2-N2-CH4-H2S) fluids of low salinity (Tm ice > −6 °C), homogenization temperatures between 250 and 410 °C homogenization pressures below 900 bars, and thermobarometric trapping conditions with temperatures below 700 °C and pressures below 3250 bars. These fluids were probably responsible for the greisenization of the apogranite and wall rocks, and the precipitation of cassiterite. The second stage is represented by low-salinity aqueous solutions (H2O-NaCl) with Tm ice ⩾ −4.5 °C, trapped at homogenization temperatures between 110 and 300 °C and homogenization pressures below 100 bars. This stage can be correlated with kaolinization. The third stage is characterized by higher salinity aqueous fluids (Tm ice ⩾ −16.5 °C) containing Na+ and other cations, trapped at homogenization temperatures between 100 and 130 °C and homogenization pressures below 5 bars. These fluids can be associated with the epigenetic or supergene phases of the orebody.

Keywords: fluid inclusions • thermobarometry • apogranite • greisen • quartz veins • cassiterite

Mineralogical Magazine; June 1991 v. 55; no. 379; p. 211-223; DOI: 10.1180/minmag.1991.055.379.10
© 1991, The Mineralogical Society
Mineralogical Society (www.minersoc.org)