GEOLOGY & MINERALIZATION

• In the Central Plateau, outcropping rocks that vary in age from Mesozoic to Recent include Mesozoic, volcanic-sedimentary package, sedimentary, and plutonic rocks. Mesozoic metamorphic rocks of very low grade have also been reported. The Mesozoic Era in the region is represented by the Chilitos Formation of the Guerrero Terrain, comprising andesitic volcanic rocks with intercalations of greywacke, radiolarites and lenses of limestone.

 The Cenozoic Era is represented by volcanic rocks, intrusive igneous rocks of acid and intermediate composition and continental conglomerates. The Quaternary includes basalts, lacustrine deposits, alluviums and occasionally layers of evaporates, travertine, sinter, and pebbles.

• The geology in the Caliguey and Santa Clara area is dominated by Quaternary and Tertiary sedimentary units as well as Cretaceous Chilitos Formation volcanic and sedimentary units. Similar formations outcrop in the La Salada area.

Regional Tectonics

• The more significant physiographic features in the region are the result of continental tectonic evolution. The dominant structures in the Central Plateau were formed during the Laramide Orogeny (Early Cenozoic), including anticlinal and synclinal folding and thrusting. A late tectonic phase has affected this territory giving rise to basement folds striking north, northwest, east, and southeast. These events are followed by emplacement of granitic stocks often associated with economic skarn-type mineralisation with the structural conditions for open-space filled type mineralisation by hydrothermal solutions in faults, folds, and bedding planes. An orogenic extension phase during the Tertiary generated a series of normal faults and associated pyroclastic and rhyolitic volcanism. Faulting resulted in horst and graben structures that were later infilled by lacustrine sediments with or without tuffs, travertine, sinter, and basalt flows interleaved with sediments.