SYNEXTENSIONAL MAGMATISM CONTROLLING THE LOCALIZATION OF EPITHERMAL MINERALIZATION IN THE GUAZAPARES MINING DISTRICT, NORTHERN SIERRA MADRE OCCIDENTAL, MEXICO

The Guazapares Mining District of western Chihuahua is located within the Sierra Madre Occidental Gold-Silver Belt of northwestern Mexico. Our recent field-based research in this region has determined that epithermal mineralization is closely related to extensional deformation and magmatism during the mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental silicic large igneous province, younger than inferred Laramide ages of mineralization from unpublished mining reports. Three Late Oligocene–Early Miocene synextensional rock units are mapped in the mining district: 1) ca. 27.5 Ma silicic outflow ignimbrite sheets (Parajes formation); 2) ca. 27–24.5 Ma locally erupted SCORBA mafic-intermediate lavas and interbedded alluvial deposits (Témoris formation); 3) ca. 24.5–23 Ma silicic plugs, lavas, proximal ignimbrites, and volcaniclastic deposits (Sierra Guazapares formation).

Epithermal mineralization appears to be associated with emplacement of Sierra Guazapares formation rhyolite plugs and is favored where pre-to-synvolcanic extensional structures are in close association with these hypabyssal intrusions. The Guazapares Fault Zone, a NNW-trending normal fault system, hosts most of the epithermal mineralization and resource areas in the mining district. The Monte Cristo resource area is a synvolcanic half-graben filled primarily with Sierra Guazapares formation rhyolitic volcaniclastic and fluvio-lacustrine rocks deposited concurrently with the growth of a rhyolite dome complex; mineralization is concentrated in the footwall rhyolite intrusions and along the normal fault and adjacent half-graben fill. The San Antonio resource area consists mainly of Témoris formation lavas that are faulted and tilted by two en echelon normal faults with opposing dip directions; mineralization is concentrated in an antithetic accommodation zone between the faults. There are no silicic intrusions at the surface within the San Antonio resource area, but they outcrop ~0.5 km E and are found in the subsurface at ~120 m-depth. The La Unión resource area is underlain by Témoris Formation andesite lavas and lapilli tuffs; mineralization is concentrated along a synthetic normal fault accommodation zone. Adjacent to the La Unión resource area (<1 km E) is one of the largest rhyolite plugs in the area.