Granitic boulders of the Dragoon Mountains exposed on I-10 west of Benson, Arizona. The knobby appearance of the rock results from resistance to chemical weathering by orthoclase (K-Spar) and quartz minerals.
These Early Tertiary to Late Cretaceous Granitic Rocks (50-82 Ma) are porphyritic to equigranular granite emplaced during the Laramide orogeny. Larger plutons are characteristically medium-grained, biotite +/- hornblende granodiorite to granite. Smaller, shallow-level intrusions are typically porphyritic. Most of the large copper deposits in Arizona are associated with porphyritic granitic rocks of this unit, and are thus named 'porphyry copper deposits'.
