The Copalquin Project lies within the Sierra Madre Occidental physiographic province of north-western Mexico. The project is underlain by andesitic volcanics of the Cretaceous-Tertiary Lower Volcanic Series. A Tertiary granodiorite to monzonite pluton intrudes the andesite and much of the area is capped by Tertiary rhyolite ignimbrites of the Upper Volcanic Series. Mineralisation is thought to be contemporaneous with the eruption of the Upper Volcanic Series.
Semi-continuous low-angle breccia zones have formed within the andesite parallel to the granodiorite contact. These zones include the El Cometa breccia and the Los Reyes breccia. The geometry of these zones is similar to the nearby El Gallo silver deposit of McEwen Mining which is also formed in a series of breccias parallel to the contact between intrusive rocks and Lower Volcanic Series andesite.
A series of high angle normal faults strikes northwest and dips to the northeast including the Refugio, La Soledad and La Constancia structures which host veins mineralised with gold and silver. North-south striking, west dipping faults at San Manuel also host mineralized veins.
Both the low-angle breccias and the high-angle faults host extensive zones of mineralised quartz breccia. It is likely that the low angle zones developed as tectonic breccias during the intrusion of the granodiorite and were later mineralized by hydrothermal activity related to the eruption of the Upper Volcanic Series.
Large areas of argillic alteration occur across the concessions. The alteration forms haloes adjacent to the known structures and large zones where structures have not been identified. Argillic alteration is indicative of widespread penetration of hydrothermal fluids into the surrounding rocks and suggests a long-lived hydrothermal system was active at Copalquin.
The alteration from Refugio to Los Reyes is over 2,000 meters long and from 100 to 400 meters wide. It is expected that the widest zones are related to shallow-dipping portions of the Cometa-Los Reyes structures where the structure is nearer the outcrop surface. Similar alteration is present well to the west at El Platanal and well to the east at Constancia. It cannot be stressed enough that this strong, widespread argillic alteration forming a large-volume halo well out from the veins is the observable geologic characteristic that identifies Copalquin as a major epithermal centre.