RATTLESNAKE HILLS ALKALINE PORPHYRY AU OF CENTRAL WYOMING: EVOLUTION OF AN IGNEOUS COMPLEX AND MINERALIZATION

Alkaline porphyry Au±Cu deposits are an important source of Au commonly found in diatreme complexes with multiple pulses of magmatism. At the Rattlesnake Hills, located ~100 km southwest of Casper, Wyoming, Au mineralization occurs in Precambrian schists and younger alkalic breccias, stocks, dikes and sills. The Rattlesnake Hills consist of mid-Tertiary magmatic intrusions that fall into three groups: the Central Alkalic, Eastern Felsic, and Western Felsic Groups based on their geochemistry and geographic locations. Previous work documented Au mineralization in the Rattlesnake Hills alkaline complex, whereas here we unravel the temporal evolution of the igneous complex and associated mineralization. We obtained U-Pb zircon ages for two magmatic phases: (1) Quartz latite from the Western Felsic Group is 45.4 Ma ± 1.2 Ma with rare inherited Archean (> 2.5 Ga) zircon cores, and (2) Quartz latite from the Eastern Felsic Group is 46.3 Ma ± 1.2 Ma. All analyzed zircons displayed oscillatory igneous growth textures suggesting limited alteration. Zircon U-Pb age uncertainties are reported at 2σ and based on internal and external error added in quadrature. Re-Os dating of pyrite (± trace molybdenite) from a mineralized sample of drill core provides a weighted mean age of 46.5 ± 2.2 Ma (n = 4), whereas Re and Os were too low for dating in three other mineralized samples. Whole rock geochemistry for sparse, discontinuous carbonatite dikes found throughout the Central Alkalic Group shows LREE enrichments, similar to REE trends exhibited by carbonatites globally.