deposition of adularia and sericite. In addition, a sample from the Rayas mine collected at
an elevation of 1705 m showed evidence of boiling and homogenization temperatures
from 290 to 385 ºC. Twelve other samples from the Rayas mine contained vapor-rich
inclusions but these were interpreted to have formed by necking down (Buchanan, 1979).
Two boiling horizons were observed in the Las Torres Mine. The shallow horizon was
interpreted to represent “normal” hydrostatic boiling, whereas the deeper horizon was
thought to represent flashing of the hydrothermal fluids when the impermeable seal
fractured. The boundary between the boiling zone (above) and the non-boiling zone
(below) was located at an elevation of about 1800 m (Buchanan, 1979).
In contrast to the observations of Buchanan (1979; 1980), Mango (1988) and
Mango et al. (1991) found no evidence of boiling in fluid inclusions in quartz, calcite,
and sphalerite from the Rayas Mine. Stable isotope analyses indicated that the precious
and base-metals were deposited from meteoric water (Mango, 1988). Homogenization
temperatures of fluid inclusions range from 230 to 305 ºC and have a salinity of 1.2 wt.
% NaCl equivalent. Results from gas analysis showed 0.3 to 2.1 mole % CO2, 0.06 to 0.8
mole % H2S, and less than 1 mole % CH4, H2 and CO in the inclusions (Mango et al.,
1991). Mango et al. (1991) reported that boiling did not occur at any of the levels studied
at Rayas, and suggest that if boiling of the hydrothermal fluids did occur it was at higher
stratigraphic levels which have since been eroded. These workers also report that up to
850 m of erosion may have occurred above the Rayas orebody, and Buchanan (1981)
indicated that boiling only occurred to a depth of 650 m below the surface at the time of
mineralization. It should be noted that Rayas contains more base metal sulfides than some
other mines in the GMD (i.e., Las Torres), and less gold than others (i.e., El Cubo).