U-Pb GEOCHRONOLOGY AND MAJOR HYDROTHERMAL FEATURES OF LARAMIDE PORPHYRY Cu-(Mo) DEPOSITS OF SOUTHEASTERN ARIZONA AND SOUTHWESTERN NEW MEXICO, WITH A FOCUS ON MORENCI, SAFFORD, AND TYRONE

We report 49 new U-Pb zircon dates on igneous rocks from 10 mining districts conducted by laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) on igneous rocks from porphyry districts near the border of Arizona and New Mexico. Geochronologic results are interpreted in the context of relative ages from mapped crosscutting relations to mineralization.

The study area encompasses deposits with significant Cu metal endowments including the world-class Morenci cluster containing 43 M tonnes (Mt), plus Lone Star-Dos Pobres at 22 Mt and 14 Mt at Santa Rita. Virtually all the deposits contained variable-sized and locally huge tonnages of supergene mineralization as chalcocite enrichment and oxide copper deposits. The Morenci, Santa Rita, and Tyrone systems are related to multiple and independent clusters of multi-phased granodioritic-monzonitic stocks, some containing veinlet reversals. Cu-Mo deposits are in K-silicate veinlets localized within immense Cu and Mo shells situated along and within the outer stock margins. The major Safford deposits by contrast are hosted within Cretaceous andesite and associated with a monzonitic stock and coeval dikes at Lone Star and dioritic dikes at Dos Pobres. Hypogene mineralization consists of K-silicate veinlets that are Cu-rich, Mo-poor, and at Dos Pobres are Au-bearing. The form of the porphyry deposit is an inverted cup at Lone Star and single mass within and outward of the dike swarm as Dos Pobres.

The U-Pb ages of all the Eocene stocks in these deposits range from 59-54 Ma at Morenci, 59-54 Ma for the four deposits at Safford, and 58-54 Ma at Tyrone. The multi-phase composite stocks directly associated with the porphyry deposits have age ranges of ~1.5 to 2 m.y. and most have dating uncertainties that overlap between stocks likely related to the complex series of multiple intrusion and mineralization-alteration phases of each porphyry-related stock.

The regional space-time pattern of Laramide porphyry deposits of southwestern North America is complex in detail. The porphyry deposits we dated are younger than those in east-central Arizona and younger still than those in northwestern Arizona.