VOLCANOLOGICAL AND TECTONIC EVOLUTION OF A COMPLEX OLIGOCENE CALDERA SYSTEM, GUANAJUATO MINING DISTRICT, CENTRAL MEXICO

The pre-Oligocene geologic history of the District includes development of an intra-oceanic island arc and forearc basin and their accretion to the western edge of North America during Middle and Late Mesozoic time, intrusion of these rocks by a tourmaline-rich Paleocene granite batholith, and block-faulting with consequent formation of Late Eocene alluvial fan deposits and intercalated mafic lavas. Complex interactions between continued extension and felsic to intermediate volcanism between 37 Ma and 27 Ma produced the rocks and structures which host the mineral deposits. Although the ore bodies have been exploited for nearly 500 years, basic volcanological analysis of the rocks hosting the deposits is needed to identify future exploration targets. Eruptions began with the emplacement of the Bufa Ignimbrite and its phreatoplinian precursor the Loseros Formation. Piecemeal subsidence following these events created a caldera of complex graben type, and the later Calderones and Cedro Formations were emplaced in shallow waters impounded in this closed basin. The Calderones tuffs and tuff-breccias were erupted from a ring dike and from a long-lived dome field, and possibly from other sources. They include block-and-ash flows derived from repeated dome collapses, ignimbrites discharged from fissures including the ring dike, lahars, and phreatomagmatic breccias related to the interaction of andesite dikes with water-saturated distal (locally reworked) ignimbritic tuffs. Chloritization is pervasive in the Calderones tuffs, affecting fiamme, clasts and matrix. Contacts with the overlying Cedro andesites are gradational, and bases of Cedro flows are locally pillowed. A conspicuous megabreccia makes an arcuate outcrop inside the curve of the ring dike. Continued movement along NW-SE normal faults during eruption resulted in minor unconformities within the Calderones Formation, and larger post-volcanic offsets tilted the entire Tertiary volcanic section and the underlying redbeds towards the NE. Eruption of high-silica rhyolite domes, flows and pyroclastic materials, largely peripheral to the central part of the District, followed earlier eruptions after a hiatus of unknown duration; these were followed by ascent of mineralizing solutions along fractures at 28 to 27 Ma.