U-PB GEOCHRONOLOGY OF LARAMIDE MAGMATISM RELATED TO CU-, ZN-, AND FE- MINERALIZED SYSTEMS, CENTRAL MINING DISTRICT, NEW MEXICO
Regionally distributed quartz monzodioritic sills formed first at 60.5 ± 1.3 Ma (3 samples). The granodioritic to granitic Santa Rita (59.5 ± 1.5 Ma) and Hanover-Fierro (58.5 ± 0.9 Ma) stocks are associated with the principal porphyry Cu and Fe skarn and replacement deposits. Felsic dikes, including quartz diorite porphyry (58.5 ± 0.7 Ma), granite porphyry (58.3 ± 0.8 Ma), and late granodiorite porphyry (59.0 ± 0.7 Ma) overlap in age but cut earlier intrusions; they are associated with Zn ± Cu skarn and replacement mineralization. The Copper Flat stock formed at 57.5 ± 0.7 Ma and is associated with breccia and skarn Cu ± Fe. An age of 46.6 ± 0.9 Ma on the monzonite plug in the North Star basin suggests an independent, later phase of magmatism. These ages, when combined with relative ages based on crosscutting relationships, constrains the timing of mineralization of porphyry copper in the CMD associated with the Santa Rita stock to between 61.0 and 59.5 Ma. Available K-Ar and Ar-Ar dates are mostly concordant with these results, indicating rapid cooling from magmatic to Ar closure temperatures.
The ages determined here are similar to many other Laramide porphyry copper-related plutons in the Southwest (~55-65 Ma). Unlike some of the other districts, there is no evidence for older mafic to intermediate magmatism that predates mineralized plutons by 5-10 m.y. Indeed, the <2 m.y. time span is considerably less than that observed between mineralized centers in the same area (e.g., Globe-Miami, Ray, and Pima districts). Nevertheless, the span in ages and geologic relationships indicate that the several centers reflect separate magma batches which, if linked at all, would share only a deep crustal origin.