Abstract

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 tonalite of a continental magmatic arc, all of Mesozoic age. Subsequently, the volcano-sedimentary sequence was intruded by an early Tertiary granitoid batholith. Subeconomic base metal and polymetallic veins and two examples of stratiform sulfides have been emplaced in the mixed sequence and deformed by tectonic events of Laramide age.Later, following erosion and preserved in the central and southern part of the district, the continental Guanajuato Conglomerate was overlain by a sequence of Oligocene volcanic rocks, related to a volcano-plutonic center which exhibits several characteristics of a caldera, formed near the intersection of regional northeast and northwest extensional fracture systems of mid-Oligocene age. Caldera fill includes a base surge deposit and up to 360 m of rhyolitic fill, megabreccia, and peripheral rhyolite domes. The northern and central parts of the volcanic center and beyond have been cut by three major, but local, northwest-striking, silver-rich, fissure vein systems accompanied by proximal silicification and widespread pro-pylitization of host rocks and less extensive crosscutting phyllic and argillic alteration facies.Previous investigations have shown that three mineralization stages occurred, with gold to silver ratios which vary from 1/72 to 1/214 in ores that have recently been exploited. In the economically important Lower zone, four overlapping, vertically stacked ore assemblages occur, in the lower three of which adularia is common. There is a deep base metal-rich assemblage below the 1,800-m elevation associated with a monzonite intrusion, followed 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 1968 that includes silver-rich, fissure-filling, and stockwork mineralization. The uppermost gold-rich, silver sulfosalt, and selenium assemblage is located in transverse, northeast-striking veins between the 2,250- 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 investigations conclude that the boiling level was above the eroded surface that contains the Lower zone except for the gold-rich Cubo mine whose upper workings are above the reconstructed boiling level.Of major economic 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 northwest-striking Sierra vein system in the Cubo mine. These veins cut the Peregrina rhyolite dome located in the wall of the collapse structure. 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 fluids 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 district and beyond the caldera limits. It is characterized by proximal silicification and advanced argillic alteration and widespread propylitization. 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, and is probably mid- to late Oligocene in age. The Upper zone, associated with a range front fault of northwest orientation, consists of sinter deposits of Recent age and active hot springs, characterized by H 2 S in escaping fluids that have a water temperature of 96.4 degrees C. These later and apparently unrelated geothermal systems occupy the highest part of the epithermal system, and with the Intermediate zone deposits, represent a shift of mineralizing activity from within the volcanic center and central part of the district, northwestward and peripheral to the economic orebodies.In the past, the source of metals in the Guanajuato ores has variously been attributed to processes related to granitic intrusions or leaching of Tertiary volcanic rocks or the Mesozoic sedimentary strata. More recently, oxygen, carbon, and lead isotope studies suggest that ore deposition is related in part to fluids of magmatic composition which mixed with isotopically exchanged meteoric water that circulated through the volcanic pile. This interpretation is supported by the recognition of volcano-plutonic activity in the form of a partially eroded and structurally deformed caldera that overlaps the ore-forming epoch. Spatially, the deep base metal-rich ore and the gold-rich assemblage of the Lower zone are related to plutonic phases adjacent to and within the collapse feature, respectively. Overall, the mineralizing epoch at Guanajuato is contemporaneous with widespread mid-Tertiary magmatic activity and formation of fissure-vein metallic deposits in northern Mexico, attributable to subduction processes.

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