Reevaluating the Schultze Granite: A New Temporal Model for the Globe-Miami Magmatic-Hydrothermal System and Its Implications for Porphyry Exploration

Abstract

Porphyry copper deposits form from magmatic-hydrothermal fluids generated by large and long-lived magmatic systems, typically in subduction-related settings. Although this subject has been extensively studied, our understanding of it is fragmentary because of a lack of exposed magmatic root zones. Key questions remain as to how sufficient volumes of mineralizing fluids emanate from parent magmas and are focused on the deposit-forming environment and over what timescales. To address this, we have integrated the results of field, microtextural, geochemical, and geochronological studies into a self-consistent 4-D framework for the Globe-Miami district, Arizona, United States, where extensive post-ore extension and ~NE tilting offers exposures down into ~7- to 10-km-paleodepth porphyry root zones in the Schultze Granite. We show that mineralization within related deposits (Resolution, Pinto Valley, Diamond H, and Copper Cities) occurred over a period of ~1.3 m.y. (~65.1–63.8 Ma). This period coincides with the age of mineralized porphyry dikes at Pinto Valley (~64.5 Ma) and Copper Cities (~64.1 Ma) but significantly predates (by ~1–2 m.y.) emplacement of the Schultze Granite, which occurred over ~400 to 900 k.y., from ~62.6 Ma, from an estimated depth of ~20 km. The Schultze Granite, therefore, did not contribute to the formation of these deposits. Instead, the magma that formed it evolved and upwardly injected aplitic groundmass porphyry dikes with an aplitic groundmass that underwent syn-emplacement first-type boiling and fluid exsolution to produce quartz unidirectional solidification textures; miarolitic cavities; and early, high-temperature deep-quartz-type veins. Copper-molybdenum mineralization occurred as these exsolved fluids cooled below ~600°C, with ~1.1 m.y. (~62.6–61.5 Ma) of hydrothermal mineralization now strewn across the structurally dismembered Miami deposit and younger Ocelot and Old Dominion mineralized system. These findings demonstrate the need to integrate field; microtextural; geochemical; and multifaceted, high-precision geochronological studies to refine our understanding of fluid exsolution and flow through the upper crust in porphyry systems.