Life cycle of a giant A-type batholith in the Proterozoic and its link to mineral resources: The Pikes Peak system, Colorado

Abstract Constraints on the time scales of magmatic-hydrothermal processes are key factors in understanding the formation of mineral resources of economic importance that are associated with silicic magmatic systems, including porphyry Cu-Mo-Au systems, Sn-W greisens, and pegmatites. Zircon petrochronology is a widely used tool for determining crystallization ages and temperatures of magmatic systems. In this study, we used the Proterozoic A-type Pikes Peak batholith as a case study to discuss the time scales and chemistry of giant silicic systems, and their relationship with magmatic-hydrothermal mineralized zones. Such mineralizations occur as Nb-Y-F-pegmatites and as mineralized zones in the Redskin granite, a Nb-rich subunit of the Pikes Peak batholith, Colorado (USA). We used zircon petrochronology, feldspar chemistry, and thermodynamic modeling coupled with trace-element modeling via Rayleigh fractionation to understand how the Pikes Peak melt evolved and generated the mineralized zones in the pegmatites and Redskin granite. Our results show that zircon from the main granite is typically magmatic, characterized by bright cathodoluminescence (CL), oscillatory zoning, and low U contents. Pegmatitic zircon grains, in contrast, show high degrees of metamictization, with dark CL and high U contents, indicating crystallization from a highly differentiated magmatic hydrous fluid. Zircon grains from the Redskin granites show bimodal compositions with both low- and high-U zircon grains, marking a transition from magmatic to hydrothermal crystallization. Through isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dating of the Pikes Peak main granite and the late-stage Redskin granite, we obtained high-precision dates that allowed us to constrain the period from the beginning of the magmatic activity in the Pikes Peak batholith to the crystallization of late exsolved fluid phases contributing to pegmatite formation, spanning a temporal interval of at least 15 m.y. for the full lifetime of the system. Although extensive, the observed 15 m.y. duration for this large-sized batholith (>3000 km2) is comparable to other long-lived magmatic systems of similar size.