Unraveling histories of hydrothermal systems via U-Pb laser ablation dating of skarn garnet

Skarns are one of the few direct genetic links between magma and base metals in ore deposits, yet temporal relationships between skarns and their assumed causative plutons are rarely confirmed with radiometric formation ages. Skarns have been notoriously difficult to date directly due to a limited primary mineral assemblage, generally garnet and pyroxene. Discrepancies in timescales of skarn formation versus pluton emplacement emphasize our tenuous understanding of temporal relationships between plutons and hydrothermal systems including the duration of hydrothermal activity and skarn mineral growth rates. Here we apply a new method for U-Pb dating grossular-andradite (grandite) garnet via LA-ICP-MS and present the first comprehensive study to focus on U-Pb dates of primary skarn garnet. We present U-Pb age constraints on garnets from multiple skarns within the Jurassic and Cretaceous arcs of the North American Cordillera. Two case studies (Darwin and Empire Mountain skarns) illustrate the effectiveness of the U-Pb grandite method by demonstrating that garnet U-Pb ages agree within error of the pluton- and skarn-hosted U-Pb zircon ages. Two additional study localities (Black Rock and Mojave skarns) demonstrate the ability of grandite U-Pb ages to: 1) directly identify a causative igneous event, and 2) record timing of major tectonic events, specifically extensional to trans-tensional shifts in the Jurassic Mojave of California.