Phillips, Joshua1,2,, Thompson, Jay1,2, Meffre, Sebastien1,2, Maas, Roland3, Danyushevsky, Leonid1,2, Cooke, David1,2
1Australian Research Council (ARC) Research Hub for Transforming the Mining Value Chain (TMVC), University of Tasmania, Hobart, Australia, 2Centre for Ore Deposits and Earth Sciences (CODES), University of Tasmania, Hobart, Australia, 3School of Earth Science, University of Melbourne, Melbourne, Australia
The Laramide aged Resolution porphyry Cu-Mo deposit, located within the Superior mining district, Arizona, has a resource of 1,787 Mt at 1.53% Cu and 0.035% Mo, making it one of the largest and highest grade porphyry Cu deposits in North America. Tertiary gravels and volcanic rocks related to Basin and Range extension buried all but the most distal epithermal veins and propylitic alteration under approximately 1.5 km of post-mineralisation cover. Identification and mapping of the distal propylitic alteration at surface is itself hampered by a diverse range and multiple generations of epidote-chlorite alteration assemblages observed within the district that could be related to multiple orogenic and/or hydrothermal events that have affected the area over its ~1,650 m.y. history.
Here we present the development of a LA-ICP-MS method for U-Pb geochronology applied to epidote to aid in resolving multiple epidote-forming events.
Our results demonstrate the presence of at least three spatially coincident but temporally distinct epidote-bearing alteration assemblages within the Superior district of Arizona. The first of these formed yielded a U-Pb LA-ICPMS age of 1,183 ± 23 Ma, broadly coeval with the emplacement of ca. 1,100 Ma dolerite sills temporally associated with the Midcontinent rift. The second event was related to the emplacement of a 74 Ma early Laramide weakly mineralized intermediate stock. The final phase of epidote alteration had insufficient U for high-precision dating but is temporally constrained through cross cutting field relationships and relates to the 65 Ma distal propylitic halo surrounding the Resolution porphyry Cu-Mo deposit.
By constraining the relative ages of epidote-bearing alteration, It is possible to isolate the Laramide signal using LA-ICPMS mineral chemistry trace element data. Multivariate statistical classification demonstrates that the Laramide epidote and chlorite are chemically distinct from, but in some cases overgrow the Proterozoic epidote throughout the Resolution propylitic alteration halo. LA-ICPMS mapping of epidotes reveals complex growth and sector zoning within Proterozoic epidotes (enriched in As, Bi, P, REE), overgrown by a much later Laramide epidote strongly enriched in Pb and Sr.
This development in understanding between local background epidote compositions and Laramide hydrothermal epidote gives explorers new tools to more fully understand the alteration geochemistry observed within Proterozoic rocks in the SW US porphyry province and, ultimately, better target undiscovered porphyry systems.
Josh started working as a exploration geologist in 2011 in WA and NSW before completing a PhD at CODES in 2018, investigating the geochemical vectors to mineralisation at the Resolution porphyry deposit in Arizona. Since then he has worked in porphyry exploration teams for Freeport McMoRan and Fortescue Metals Group