EXPLORATION IN A VOLCANOPLUTONIC CENTER AT GUANAJUATO, MEXICO

In the northern part of the Guanajuato district, a Cretaceous volcano- sedimentary sequence has tectonically been overridden by thrust sheets that contain basic and ultrabasic rocks of ophiolite affinities and t onalite of a continental magmatic are, all of Mesozoic age. Subsequent ly, the volcano-sedimentary sequence was intruded by an early Tertiary granitoid batholith. Subeconomic base metal and polymetallic veins an d two examples of stratiform sulfides have been emplaced in the mixed sequence and deformed by tectonic events of Laramide age. Later, follo wing erosion and preserved in the central and southern part of the dis trict, the continental Guanajuato Conglomerate was overlain by a seque nce of Oligocene volcanic rocks, related to a volcano-plutonic center which exhibits several characteristics of a caldera, formed near the i ntersection of regional northeast and northwest extensional fracture s ystems of mid-Oligocene age. Caldera fill includes a base surge deposi t and up to 360 m of rhyolitic fill, megabreccia, and peripheral rhyol ite domes. The northern and central parts of the volcanic center and b eyond have been cut by three major, but local, northwest-striking, sil ver-rich, fissure vein systems accompanied by proximal silicification and widespread propylitization of host rocks and less extensive crossc utting phyllic and argillic alteration facies. Previous investigations have shown that three mineralization stages occurred, with gold to si lver ratios which vary from 1/72 to 1/214 in ores that have recently b een exploited. In the economically important Lower zone, four overlapp ing, vertically stacked ore assemblages occur, in the lower three of w hich adularia is common. There is a deep base metal-rich assemblage be low the 1,800-m elevation associated with a monzonite intrusion, follo wed upward by the main silver-bearing assemblage between the 1,800- to 2,350-m elevation. Recognition of this vertical zonal arrangement led to a major discovery on the southeastern part of the Veta Madre in 19 68 that includes silver-rich, fissure-filling, and stockwork mineraliz ation. The uppermost gold-rich, silver sulfosalt, and selenium assembl age is located in transverse, northeast-striking veins between the 2,2 50- and 2,550-m elevation. In the past, fluid inclusion data suggested that precious metals were deposited above a boiling level attended by repeated flashing, deposition, and sealing. But more recent investiga tions conclude that the boiling level was above the eroded surface tha t contains the Lower zone except for the gold-rich Cube mine whose upp er workings are above the reconstructed boiling level. Of major econom ic significance in the district was the discovery during 1978-1983 of the important, uppermost northeast-striking, gold-rich ores in the San Nicolas and adjacent veins, perpendicular to the previously known nor thwest-striking Sierra vein system in the Cube mine. These veins cut t he Peregrina rhyolite dome located in the wall of the collapse structu re. Additionally, several parallel gold-rich veins in the eastern part of the district parallel or occupy northeast-trending fractures that control a latite dike swarm. The San Nicolas vein has an average gold to silver ratio of approximately 1/18 and the ore occupies the highest stratigraphic, structural, and topographic part of the district. All four ore assemblages of the Lower zone were emplaced by hydrothermal f luids of mid-Oligocene age. In contrast, Intermediate zone subeconomic mineralization discovered in 1988 occurs in northeast- and northwest- striking fractures and breccias in the northwestern part of the distri ct and beyond the caldera limits. It is characterized by proximal sili cification and advanced argillic alteration and widespread propylitiza tion. A suite of trace elements in anomalous amounts includes Au, Ag, Cu, Hg, As, and Sb, occurs in a 200-m interval above the Lower zone, a nd is probably mid- to late Oligocene in age.