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Recently, several fluid inclusion studies have been conducted along the Sierra

vein system that parallels the Veta Madre in the GMD. In the Cubo Mine the San Nicolas

vein is oriented perpendicular (east-west) to the Veta Madre vein (Fig. 2). Most fluid

inclusions from this vein homogenize at about 250°C (Girnius, 1993), and evidence for

boiling was only found in samples from the western-most portion of the vein. Girnius

(1993) interpreted these data to indicate that deposition was the result of both fluid

mixing and boiling. More recently, Abeyta (2003) studied the San Nicolas vein and

reported homogenization temperatures of 172 – 282°C and salinities of 0 – 3 wt% NaCl

equivalent. Abeyta (2003) noted the presence of coexisting liquid-rich and vapor-rich

inclusions as well as bladed calcite in the veins and interpreted this to indicate boiling.

Perhaps one of the best examples of the close association between boiling and

precious metal deposition in the epithermal environment is provided by the modern

geothermal systems in New Zealand. There, a new back-pressure plate was installed in

the high-pressure part of a well in the Ohaaki-Broadlands geothermal field. The back-

pressure plate is located at the point where high temperature one-phase liquid geothermal

fluids “flash” (boil) to produce steam at the wellhead. After 44 days the plate was

removed and the scales were analyzed. The scale was mostly chalcopyrite, containing

about 4-6 wt% Au and 2.5-30 wt% Ag. Brown (1986) concluded that the sudden flashing

that converts hot water to steam resulted in metal precipitation.

Fluid inclusion studies and studies of active geothermal systems summarized

above document the generally recognized spatial and temporal relationship between

boiling and precious metal mineralization. This relationship is consistent with theoretical

and experimental studies of gold solubility. Gold solubility at 250°C as a function of fluid


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