A structural and geochemical study of the Sierrita porphyry copper system, Pima County, Arizona

Abstract

Detailed mapping of the Sierrita-Esperanza orebody has led to a revised interpretation of the Paleocene intrusive units, and a structural model for the emplacement of those units. A left-lateral wrench fault, expressed through Riedel shears, arranged in a flower structure geometry, is proposed to localize the porphyry intrusions, and control mineralization trends. Miocene extensional deformation has not tilted the orebody more than 10°. The San Xavier Thrust is interpreted to be a low angle detachment, upon which the upper plate has moved to the north-northwest approximately 10 km. When the San Xavier "Thrust" ceased movement, a set of low angle normal faults, striking nearly north-south dissected the Sierrita Range. Molybdenite dating, using the Re-Os system, shows that mineralization of the orebody began at 63.5 Ma (±300 ka, 2σ), and ended sometime after 60.0 Ma (±300 ka, 2σ). An isochron of chalcopyrite and pyrite samples with an age of 59.7 Ma (±17.1 Ma, 2σ), and an initial 187Os/188Os of.152 (±0.33, 2σ) suggests that the lower crust beneath the Sierrita Range has a low 187Os/188Os ratio.