TECTONIC SETTING OF THE IGNIMBRITE FLARE-UP AND EPITHERMAL MINERALIZATION IN THE NORTHERN SIERRA MADRE OCCIDENTAL (MEXICO): PRELIMINARY EVIDENCE FROM THE GUAZAPARES MINING DISTRICT, WESTERN CHIHUAHUA

The Sierra Madre Occidental (SMO) of western Mexico is the world's largest Tertiary silicic large igneous province and consists largely of undeformed Oligocene to Miocene ignimbrites surrounded by the highly extended southern Basin and Range and Gulf Extensional Provinces. The stratigraphy of the northern SMO consists of intermediate-composition arc volcanic rocks of the late Cretaceous to Eocene Lower Volcanic Complex (LVC), intruded and overlain by silicic ignimbrites, lava flows, and intrusions of the late Eocene to early Miocene Upper Volcanic Complex (UVC). With support from Paramount Gold, we have mapped a ~260 km2 area on the western (paleo-trenchward) flank of the northern SMO, in the Guazapares Mining District of the Temoris-Palmarejo region. Our new geologic mapping shows that the LVC can be subdivided into a welded silicic ignimbrite lower member, an andesitic-dominated middle member, and a mixed intermediate-silicic volcanic upper member, while the UVC consists of silicic lava flows, hypabyssal intrusions, ignimbrites and reworked tuffs. The stratigraphy of this area thus makes it ideal for studying the transition from “normal” andesite arc magmatism to the “flare-up” style of a silicic large igneous province.

We show for the first time that significant extension preceded ignimbrite flare-up in this region, rather than beginning during or after the flare-up as proposed by numerous previous workers. The Guazapares Mining District lies along the NW-trending normal fault system of the Guazapares fault zone, which forms the western boundary of a series of NW-trending half-graben basins that were active during deposition of the LVC. The fill of these half graben basins coarsens and thickens toward the syndepositional normal faults and onlaps half graben shoulders. These faults show offsets of up to 300 m prior to eruption or intrusion of UVC rocks. These faults controlled the location of silicic plugs and fissure-like intrusions of the UVC, and vent-proximal assemblages that fringe the intrusions, including co-ignimbrite breccias, surge-like ignimbrites, and thick silicic lava flows. The silicic intrusions are important because they appear to be spatially associated with epithermal precious metal deposits.