A VIEW OF THE ROOTS OF A PRODUCTIVE LARAMIDE PORPHYRY COPPER SYSTEM EXPOSED IN THE GLOBE-MIAMI DISTRICT, ARIZONA

Several porphyry copper deposits and prospects occur in and around the Schultze Granite, a composite, felsic Laramide pluton that crops out over ~50 km² in between Miami and Superior in south-central Arizona. A new interpretation of the mid-Tertiary extension based on new and published mapping implies that most of the deposits and prospects (n>25) are dismembered, tilted portions of perhaps as few as four large magmatic-hydrothermal systems. Extension telescoped the upper crust such that levels from <2 to >=10 km paleodepth have been exposed at or near the present-day land surface. Our new mapping, combined with a new structural interpretation, provides insights into the deepest levels of known, productive systems.

Several piercing points establish displacements of faults across the Globe-Miami district. We recognize one area, exposed along Pinto Creek near US Highway 60 in the west-central portion of the district, to be the deep levels or “roots” of the well-mineralized Miami-Inspiration porphyry copper system. Hornblende geobarometry on a nearby sample yielded a paleodepth of 10 km. In these deep exposures, the features in the center of the system contrast markedly with those from shallower levels. We observe dominantly phaneritic rocks rather than porphyries; alternating ductile and brittle deformation rather than mainly brittle deformation; variable vein and dike orientation rather than consistently oriented, sheeted veins; alteration dominated by K-feldspar and relatively coarse-grained muscovite rather than biotite and sericite; and sparse rather than abundant Cu-Fe sulfides.

The alternating ductile and brittle character exhibited by the dikes and veins at deep levels likely reflects transient changes in the evolving magma chamber and may correlate with episodic hydrothermal fluid release and/or introduction of new magma. These deep-level rocks should allow us to better understand magma chamber dynamics, to recognize structures that were active during Laramide arc magmatism, and to explore from deep exposures (paleo-) upwards in the crust for economic mineralization in this structurally complex region.