Most samples collected from the Veta Madre in this study show some evidence of
boiling (>750 out of 855 samples). Most of the samples that do not show evidence of
boiling consist of only rhombic calcite which is not thought to be precipitated during
boiling. With distance away from the Veta Madre, the Ag and Au grades drop of
drastically (Fig. 16 B, C). The only feature that is recognized in 4 samples collected along
a traverse perpendicular to the vein is jigsaw texture silica (Fig. 16A). The rapid drop-off
in boiling intensity (and Ag and Au grades) with distance from the mineralized vein has
both advantages and disadvantages for exploration. The disadvantage is that the target
area that can be defined using these features has relatively small lateral extent on the
surface. Conversely, if an exploration sample collected from the surface contains good
evidence for boiling, it is likely that this represents the heart of the system and defines the
location where mineralization should occur if the system is mineralized.
As noted earlier, and as observed in modern geothermal systems, once boiling
begins at depth it usually continues to the surface. Additionally, the best gold grades
occur at the base of the boiling zone where upward migrating fluids begin to boil.
Therefore, good evidence of boiling in surface samples indicates that the base of the
boiling zone where precious metal mineralization is mostly likely to occur, is beneath the
present surface and should be given high priority for drilling to explore the subsurface,
even if precious metal grades in the surface samples are poor. At Guanajuato, an angled
drill hole from the surface into the Veta Madre showed a systematic increase in boiling
intensity factor as the vein was approached (Fig. 16D), even though the metal grades in
the drill core were well below economic values.