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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).


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