GUANAJUATO DISTRICT REGIONAL GEOLOGY
The Guanajuato District is near the southern edge of the Mexican Central Plateau, where the northwest structures of the Plateau have been abruptly cut by the easterly trend of the Transverse Volcanic Belt. A few kilometers south of Guanajuato are outcrops of only Pliocene, and younger, sediments and volcanics; to the north, northwest, and northeast, the most common rocks exposed are Cretaceous calcareous sediments folded into open and tight northwest-trending folds.
The district lies on the northeast flank of a poorly defined northwest-trending anticline (Wandke and Martínez, 1928). Normal faults parallel to the anticlinal axial trace have dropped the central portions of the anticline downward, and a younger, second set of normal faults formed a series of horsts and grabens trending nearly perpendicular to the axial trace.
The fault parallels to the anticlinal axial trace cut rocks as young as late Oligocene and are mineralized with adularia giving a K-Ar date as 27.4 m.y. (Gross, 1975). Therefore, a late age is assigned to the faulting. This age corresponds well to the taphrogenic stage of the post-Hidalgoan orogeny described by De Cserna (1960).
The oldest rocks of the Guanajuato District are marine organic and calcareous black shales deposited in the Triassic through the Cretaceous Jaliscoan sea (De Cserna, 1960). These are unconformably overlain by post-orogenic continental red beds (Edwards, 1955) and Eocene and Oligocene andesitic and rhyolitic air fall tuffs. Other than alluvium and landslide deposits. The nearest outcrops of post-Oligocene rocks, a basaltic cap at Cerro del Cubilete is about 13 kilometers southwest of Guanajuato.
LOCAL GEOLOGY OF EL PINGUICO
Outcrops in the area of the El Pinguico Property are comprised of Tertiary volcanic formations, called calderones and bufas, and host the El Carmen-El Pinguico vein. The oldest unit in these concessions is the Bufa Formation and consists of rhyolites, tuffs, and rhyolitic breccias. It has a light pink color, a thickness of approximately 500 meters, and hosts most of the mineralization of the El Carmen-El Pinguico vein. The Calderones Formation overlies the Bufa Formation and outcrops in the southeast portion of the mining district of Guanajuato. It consists of massive andesite, tuffs, and andesitic breccias of green color, and has a thickness of approximately 500 meters. In the north exists a dike of rhyolitic composition.