Three-Dimensional Evolution of Magmatic Hydrothermal Systems, Schultze Granite and Ruby Star Granodiorite, Arizona

Abstract

The biotite bearing Schultze Granite (Globe-Miami district) and the biotite-hornblende bearing Ruby Star Granodiorite (Pima district) compose two intrusive centers that produced multiple porphyry copper deposits during the Laramide orogeny. Both magmatic-hydrothermal systems were dismembered and tilted by Tertiary extension, as indicated by tilted Tertiary sedimentary rocks, paleomagnetic data, and geobarometry, thereby producing extraordinary exposures of these magmatic-hydrothermal systems: ~ 1 to ~10 km (Globe-Miami district) and <1 to>12 km (Pima district). Ages of emplacement range from 68 to 61 Ma for the Schultze Granite and 64 to 58 Ma for the Ruby Star Granodiorite. The plutons were formed by rapid accumulation of magma within short periods of time (~1 m.y.). The Schultze Granite is a high-silica granite and did not evolve chemically with time, except during formation of late porphyry and aplite dikes. Phases of the Ruby Star pluton range from granodiorite to granite, but appear to be distinct intrusive events separated in time by several million years. Each pluton is chemically homogenous with depth, probably due to convection. The low iron contents of biotites suggest that magmas related to porphyry copper deposits have higher oxidation states than typical granitic bodies. Hydrothermal alteration was associated with most phases of each pluton, with multiple alteration types overlapping to create complex centers. Veins persist to >10 km beneath porphyry copper deposits. Deep styles of alteration differ in the two plutons. The Schultze Granite contains biotite veins and greisen veins (coarse-grained muscovite) (~10 km). The Ruby Star Granodiorite contains sodic-calcic alteration (4-8 km) and greisen veins (4-12 km). The sodic-calcic alteration is asymmetrically distributed on the eastern side of the Sierrita deposit and is interpreted to have been created by influx of external sedimentary brines from Paleozoic sedimentary rocks that only are present on the eastern side of the pluton. Greisen alteration occurs late in the hydrothermal history and may be the last fluids that were exsolved from the magma as the magma chamber completely crystallized. These deep alteration styles can be used to predict where porphyry copper deposition may have occurred, which can lead to discoveries in extended terranes.