Fluid associated with W and Ag-Au deposits of the Mirandela area, NE Portugal : an example of peri-granitic zoning

In the Mirandela area, tungsten (Pedra Luz and Lombo da Veiga) and silver-gold (Freixeda) ore deposits are found which constitute a presumed example of zoning associated with Hercynian leucogranites. This peri-granitiç zoning was checked using microthermometry and Raman spectroscopic analysis of fluid inclusions in the prebearing quartz veins. In the Pedra-Luz and Lombo da Veiga scheelite-bearing quartz veins, fluids are rich in almost pure CO2 (ZCO2, 91-100 % ; XCO2, 13-28 mole %), but no CO2 has been detected at Freixeda. However, small amounts of CH4 and N2 have been detected in the latter deposit (ZCH4, 62-100 % ; XCH4, 0.42 mole % ; ZN2, 0-38 % ; XN2, 0.26 mole %). The fluids of the three ore deposits have low densities and salinities (Pedra-Luz d=0.64, 4 wt. % NaCl eq. ; Lombo da Veiga d=0.52, 3.5 wt. % NaCl eq. ; Freixeda d=0.73, 3.8 wt. % NaCl eq.). The trapping pressures are close to 700 bars in the W ore deposits and close to 300 bars in the Ag-Au deposit. The probable trapping temperatures range from 325 to 350 °C at Pedra-Luz, 375 to 400 °C at Lombo da Veiga and 310 to 330 °C at Freixeda. Assuming that the fluid phase was in equilibrium with graphitic material present in host metasedimentary rocks, fO2 is estimated between 10-21 and 10-20 bar, close to the Ni-NiO buffer on the basis of the equilibrium C+O2 ⇌ CO2. At Freixeda, a lower fO2 closer to the Q-F-M buffer is indicated (occurrence of CH4).