The Dolores gold-silver deposit in the Sierra Madre Occidental is a low- to medium-sulfidation epithermal deposit hosted in andesite volcanic rocks of the Lower Volcanic Series of Tertiary age. Mineralization is present in a NNW-striking, steeply-dipping structural zone, and is concentrated in fractures and stock works that lie within and adjacent to latite dikes. The deposit shows characteristics similar to economic deposits in volcanic rocks of Tertiary age in Mexico, South America, Indonesia and elsewhere. Gold and silver mineralization is present in veins, silica stockworks, breccias and replacements. Deeper mineralization tends to be preferentially in high-grade structurally controlled zones that evolve into wider stockworks, veinlets and disseminations at higher elevations. Mineralization is limited to the lowermost portion of the less competent, more permeable, overlying latite flows and tuffs that overlie the andesitic host rocks. Minerals present include galena, sphalerite, stibnite, arsenopyrite pyrite, silver sulfides, native silver and visible gold. Clay-illite-hematite alteration with no significant gold or silver values (commonly referred to as “steam heated” alteration) also is present above and lateral to mineralized zones and is a characteristic feature of this type of epithermal mineralization.
The mineralization considered here occurs in an area over 4,000 meters long and up to 1,000 meters wide, with over 700 meters of vertical extent. The bulk of the deposit is high in the system where feeders widen into breccias and stockworks up to a few hundred meters wide that form halos around the main structures. Grades in the feeders are in the ranges of 10 to 30 g/t gold and 300 to more than 1,000 g/t silver. Disseminated mineralization that lies adjacent to high grade feeder structures at upper levels, has lower grades in the range of 0.3 to 2 g/t gold and up to 150 g/t silver. The width of coherent mineralization commonly is about 200 up to 300 meters wide, and lies in zones that trend NNW and dip steeply to the west.
Some vein and disseminated gold-silver mineralization is known to extend in a NNW and SSE direction beyond the limits of the resource estimate area. This mineralization may be considered exploration potential and may eventually be classed as Resource.