A volatile situation: zircon and apatite insights into the plutonic-volcanic transitions in Yerington, NV

The formation of porphyry copper deposits (PCDs) has been proposed to be incompatible with the occurrence of coeval explosive volcanism. In fact, these deposits need a substantial volume of magma in the upper crust that degasses large quantities of metal-bearing hydrothermal fluids, which would be lost in the case of a volcanic eruption. We examine the Jurassic batholith in Yerington, NV, where a tilted crustal section has exposed a sequence of cogenetic volcanic rocks, mineralising porphyry dikes, and an underlying composite batholith.

We study the petrochronology of zircon from the Yerington volcanics by pairing in-situ LA-ICP-MS and high-precision CA-ID-TIMS U-Pb methods. Geochronological data show that eruptions occurred both pre- and post-mineralisation, and reveal a <1 Myr gap in volcanism that corresponds to the emplacement of porphyry dikes associated with the copper mineralisation. Whole-rock and zircon trace element compositions show that the pre-porphyry volcanic rocks are compositionally distinct (e.g. higher Ti, lower Yb/Dy, Eu/Eu* in zircon) from later units, and are more closely related to older (i.e. precursor) intrusive phases of the composite Yerington batholith. Conversely, the porphyry dikes and post-porphyry volcanic rocks show similar whole-rock and zircon trace element compositions, indicating a similar, more evolved magma source.

To investigate potential mechanisms behind the renewed volcanic activity in the latter stages of the Yerington system, we characterized the volatile content (Cl, F, OH) of apatite inclusions in zircon from the porphyry dikes and post-porphyry volcanics using EMPA, applying numerical modelling to those results to reconstruct melt volatile contents. Results indicate a sharp increase in apatite Cl and OH between porphyry dikes and immediately post-porphyry volcanic units that indicates a reinjection of volatiles into the system prior to eruption. We conclude that recharge by a deeper, volatile-loaded melt into the existing magmatic system, or fluid recharge from a more deeply emplaced magma likely triggered explosive volcanism late in the lifespan of the Yerington magmatic system, terminating copper mineralisation potential, and thus limiting the total copper endowment at Yerington.