Minera Copal

Regional Geology

The Copal Project is located in the Sierra Madre Occidental physiographic province (INEGI 2008), within the sub-province of Mesetas and Cañadas del Sur. The region is predominantly mountainous, with maximum elevations between 1,670 and 1,700 meters above sea level in the Sierra de Álica. The morphology is steep slopes and strong unevenness. The geological environment of the region is located almost entirely within the Sierra Madre Occidental tectonostratigraphic terrane and a few small portions in the Guerrero and Cinturón Volcánico Mexicano terranes.

In the region outcrop lithological units that vary in age from the Upper Jurassic to the Holocene. The base of the stratigraphic column corresponds to a vulcanosedimentary sequence of Upper Jurassic to Lower Cretaceous age, consisting of interbedded limestone, polymictic conglomerate, shale, andesite and sandstone that outcrops to the south along the banks of the Santiago River. It is correlatable with the rocks that make up the Zihuatanejo subterranean, of the Guerrero composite terrain, forming part of a magmatic arc where marine sedimentary rocks interbedded with volcanic rocks were deposited. The vulcanosedimentary sequence is intruded by the granite-granodiorite unit of Upper Cretaceous to lower Tertiary age (laramide batolite), causing slight marmorization, recrystallization and skarn zones and is overlain by small and scarce outcrops of andesitic rocks from the Lower Tertiary, forming both units in the area the Lower Volcanic Complex of the Sierra Madre Occidental. The previous lithological units are unconformably overlain by a sequence of volcanic rocks that mainly includes rhyolitic tuff-ignimbrites generated from numerous and large volcanic calderas, caused by the regression of the magmatic arc, from the east to the Pacific, corresponding to the Upper Volcanic Supergroup of the Sierra Madre Occidental (SMOcc). Intruding the rhyolitic volcanic rocks and the granodioritic intrusive, a subvolcanic porphyritic body of rhyolitic composition is emplaced during the Middle Tertiary in the southern portion of the region. The Upper Tertiary and Quaternary is constituted mainly by basaltic volcanism related to the Cinturón Volcánico Mexicano. The Recent-Quaternary is made up of recent alluvial sediments that mainly include silt and sand.

The most defined regional structural features in the region are controlled by the NW-SE striken fault system, corresponding to the Sierra de Álica semi-graben and the Pochotitán-San Pedro fault system, which in turn are truncated by a system heading NE-SW. These structures have been affected by two deformation events, the first, related to the Laramide orogeny, originated a ductile deformation in the vulcanosedimentary sequence, as well as brittle deformation in the granite-granodiorite unit and andesitic volcanic rocks of the Lower Volcanic Complex of the SMOcc. The second event is attributed to the distensive tectonics that occurred at the end of the Middle Tertiary, which generated a brittle deformation that mainly affected the volcanic units of the Upper Volcanic Supergroup of the SMOcc. It is manifested by the presence of a fault system, of the normal type, oriented mainly in the general direction N-S, NW-SE, ENE-WSW and less frequently E-W, which are cut by a system of lateral faults, with a general NE-SW and E-W orientation, producing a dislocation to the normal fault system, giving rise to a semi-basin type structural arrangement.

The tectonics of the region begins in the Upper Jurassic-Lower Cretaceous, represented by the sedimentation of a vulcanosedimentary sequence correlatable with the Guerrero Terrane. In the Upper Cretaceous to the Lower Tertiary a continental magmatic arc is formed along the western border of America leading to the emplacement of a granitic batholith associated with an intermediate and acid volcanism that originates the Lower Volcanic Complex of the Sierra Madre Occidental. During the Middle Tertiary the magmatic arc migrated to the east causing the Laramide orogeny while its return to the west was characterized by distension and acid volcanism that constitutes the Upper Volcanic Series of the Sierra Madre Occidental. In the upper Tertiary and Quaternary a new compressional tectonic event begins the volcanism of the Cinturón Volcánico Mexicano.