TERTIARY TILTING AND DISMEMBERMENT OF A LARAMIDE MAGMATIC-HYDROTHERMAL SYSTEM, PIMA MINING DISTRICT, SOUTHERN ARIZONA

Faulting and rotation of the crust can yield greater three-dimensional exposures. In turn, this can lead to a better understanding of the dynamics of igneous and hydrothermal systems, with applications to mineral exploration. In the Pima Mining District, the composite Ruby Star Granodiorite (units range from 60-64 Ma) is widely recognized as being responsible for porphyry copper mineralization at the Sierrita-Esperanza deposit. Unpublished mapping within the granodiorite shows a wide range of alteration suggesting two sources of fluid: magmatic and external brines. In order to understand the distribution of alteration, the amount and character of dismemberment and tilting of rocks in the district needs to be quantified. This study uses geologic relationships (including results of new geologic mapping of rock types and hydrothermal alteration), paleomagnetism, and hornblende geobarometry to quantify the amount and direction of tilting.

Several lines of evidence indicate that the district has been tilted 50-60° to the south. First, the Cretaceous Demetrie Volcanics and the Tertiary Helmet Fanglomerate (32-28 Ma) dip 60° to the south, although younger units dip less steeply. Second, the ages of strata become younger to the south. Third, paleomagnetic studies (11 sites) from tuffs, granites, and dikes (~60-70 Ma) indicate a rotation of ~48° along an axis of N50E. Fourth, samples of the Ruby Star Granodiorite distributed throughout the district have been analyzed for Al-in-hornblende barometry, and results represent paleodepths of 4 to 12 km. Repetition of geologic features and the patterns of the geobarometric results indicate that the pluton has been dismembered into at least three major structural blocks, and the paleodepths increase to the north in each block. We conclude that the present surface represents an oblique section through the magmatic-hydrothermal system. Alteration around the Sierrita-Esperanza deposit exhibits Na(Ca) alteration deeper in the system to the east, potassic alteration in the center of the system, and sericitic alteration shallow in the system. Na(Ca) alteration is only found on the eastern side of the system near Paleozoic sedimentary units and may have been created by an influx of external brines from the sedimentary rocks.