CUSI
[Geology]
The rocks that crop out in the area are mainly volcanic rocks of the Sierra Madre Occidental (Campa U.F.M. 1999) of the Chihuahua area, predominantly volcanic rocks of Eocene-Oligocene age, continental deposits of the Upper-Middle Tertiary, and in smaller proportion, Quaternary deposits. The geologic column comprises a sequence of volcanic rocks, with a minimum thickness of 750 m in “La Bufa” mountain, but possibly thicker since its base is not exposed nor it has been reached by the deepest mining operations. Volcanic rocks comprise deposits from the Lower Volcanic Series, the base of which is represented by felsic lavas and tuff that McDowell (1979) dated at 37 Ma by U-Pb method (Stimac, 1983). They unconformably overlie a sequence of agglomerates and andesitic lavas (37 Ma, Stimac, 1983), with a smaller amount of intercalated volcanoclastic sediments and andesitic tuffs.
The Upper Volcanic Series unconformably overlies the earlier series and is represented by felsic, intermediate and mafic volcanic rocks with an age range of 36-27 Ma (Stimac, 1983). The mafic rocks originated during periods of faulting and are represented by andesitic basalts that are correlated with the andesitic-basaltic unit “Rancho Viejo”, dated by Stimac (1983) at 28 Ma. The rhyolitic rocks, such as tuffs, breccias and ignimbrites, belong to the Sierra Madre Occidental Superior Volcanic Group.
During the Miocene, basin-and-range faulting accompanied deposition of continental deposits comprising polymictic conglomerates. Quaternary deposits include poorly-sorted terrace gravels.
The rocks are altered by monzonitic intrusions and cut by rhyolitic dykes associated with Ag-Pb-Zn-Cu-Au mineralization.
The structural pattern of the region is dominated by basin-and-range faulting, which gave the region its present morphology.
The structures are usually high-angle normal faults. There are three structural systems: 1) ENE striking faults; 2) a NNW-trending structural system; and, 3) a NW-trending structural system. The main mineralized structures of the district are associated with the first system. The second system is made of normal faults with 200 m displacement. Finally, the third system includes the faults that originated during the post-volcanism period (27Ma).
Mineral deposits occur as hydrothermal fissures filling type, mainly silver veins with Pb, Zn, Cu and Au.
The length of the structures varies between 500 to 2,000 m, with high grade zones and also barren sections. Alteration is mainly silicification, oxidation and cloritization.