U-Pb Geochronology of Laramide Magmatism Related to Cu-, Zn-, and Fe- Mineralized Systems, Central Mining District, New Mexico

摘要

Eleven new zircon U-Pb dates show that most igneous rocks, and perhaps all mineralization, formed between 57 and 61 Ma in the Central Mining District (CMD), New Mexico. The Central Mining District contains multiple igneous rock types in two major and multiple minor intrusive centers, many with distinctive hypogene mineralization (Santa Rita: porphyry, skarn, and breccia Cu-Mo; Hanover: skarn Fe ± Zn, skarn and porphyry Cu; Copper Flat: breccia and skarn Cu ± Fe; diorite and quartz diorite porphyry dikes: skarn Zn ± Cu ± Pb; North Star: unmineralized). Key samples were collected for U-Pb dating; 11 yielded zircons that were dated by LA-ICP-MS at the Arizona LaserChron Center. All samples were imaged by cathodoluminescence to examine textures and identify new igneous zircons and inherited cores. All zircons had concordant lower intercept dates (reported as~(238)U/~(206)Pb ages) and most showed inheritance with a mixed group of discordant and older concordant cores. The new dates are compatible with observed crosscutting relations as established by Jones and others (1967, USGS PP 555) and field-checked in this study. All but one of the new dates fall within a range from 57.5 to 60.5 Ma. Hornblende-biotite granodiorite and quartz monzodiorite sills formed first at 60.5±1.3 Ma. The composite Santa Rita stock (59.5±1.5 Ma) is a biotite-hornblende granite and granodiorite associated with mineralization at the Chino mine, the principal porphyry Cu deposit in the CMD. The Hanover-Fierro (58.5±0.9 Ma) stock is a biotite-hornblende granodiorite/ quartz monzodiorite associated with porphyry Cu enrichment at Hanover Mountain and Fe ±Cu (magnetite) skarn and replacement deposits, principally at the Continental mine and the Jim Fair pit. Felsic dikes, including hornblende-biotite quartz diorite porphyry (58.5±0.7 Ma), biotite granite porphyry (58.3±0.8 Ma), and late biotite-hornblende granodiorite porphyry (59.0±0.7 Ma) overlap in age but cut the earlier intrusions; they are associated with Zn ± Cu skarn and replacement mineralization at the Empire, Pewabic, Groundhog, and Bullfrog mines. The biotite-hornblende granodiorite porphyry stock at Copper Flat formed at 57.5±0.7 Ma and has breccia and skarn Cu ± Fe. An age of 46.6±0.9 Ma on the North Star biotite-hornblende monzonite plug northwest of Hanover Mountain suggests an independent, later phase of magmatism. Several samples contained inherited cores with ages of 1.4 and 1.6 Ga, similar to results found elsewhere in the Laramide province. In the CMD, magmatism and multiple associated mineralized centers formed in no more than about 3 m.y. This is relatively quick compared with many Laramide-age porphyry copper districts in southwestern North America, where magmatism evolves from early mafic and intermediate compositions to more felsic centers over 5 m.y. or more (e.g., Patagonia Mountains, Vikre et al., 2014; Pima district, Stavast, 2006; Globe-Miami to Superior, Seedorff et al., 2005, Barton et al., 2007). These new U-Pb ages are consistent with an evolving composite magmatic system (sourced in the mid or lower crust) rather than construction by the superposition of unrelated batches of magma.