Geology of the Selene perlite deposit in the northern Sierra Madre Occidental, northeastern Sonora, Mexico

Abstract

The Selene perlite ore deposit of northeastern Sonora, Mexico, lies at the southern edge of an extensive (85 × 35 km) assemblage of Early Oligocene-Miocene volcanic rocks. This fault-bounded and east-tilted volcanic assemblage is of variable composition and was erupted onto Cretaceous non-volcanic rocks. The volcanic ridge is located within the tectonically extended region of the northern Sierra Madre Occidental (SMO) silicic large igneous province. Geologic mapping reveals a faulted and uplifted bimodal volcanic sequence, dated as Late Oligocene, that contains more perlite outcrops than previously recognized and two new coplanar normal faults of Early Miocene age or younger that promoted the development of an adjacent, small half-graben filled by a volcaniclastic unit. The bimodal volcanic sequence comprises a basal Early Eocene rhyolitic lava and rhyolitic tuff overlain by a rhyolitic ignimbrite sheet, a middle flow-banded rhyolitic lava dome that hosts the Selene perlite flow, and the upper basalt on an erosional unconformity. The volcaniclastic unit rests in angular unconformity with the bimodal volcanic sequence, and was likely deposited in the Early Miocene. It comprises a lower fault breccia containing clasts of local eruptive products, a previously unidentified thin quartz arenite bed, and an upper, thick polymictic conglomerate containing clasts of perlite, rhyolite, andesite, and basalt. Potential sources for the bimodal (silicic and mafic) volcanic facies are distal and proximal-distal volcanic fissure vents, the locations of which were controlled by preexisting NW-SE and N-S normal faults and related fractures linked to the Mexican Basin and Range Extensional Province. This implies the bimodal volcanism to be synchronous with crustal extension, as recently shown for other areas of the SMO silicic large igneous province. Normal faulting in the area also influenced the formation, preservation, and exposure (uplift) of the Selene perlite flow. The structural discontinuities created a permeable uppermost crust that channeled meteoric water into the subsurface and promoted both the percolation and underground circulation of meteoric water and gases rising into the subsurface. The water-lava interactions led to the hydration of the flow-banded rhyolitic lava flow and the formation of huge amounts of perlite. The Selene perlite was partly preserved by the overlying basalt, but continued local normal faulting and subsequent erosion exposed the perlite ore at its current location.