Paper No. 4
Presentation Time: 9:05 AM


VON QUADT, Albrecht, GALLHOFER, Daniela and HEINRICH, Christoph, ETH Zurich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, Zurich, 8092, Switzerland,

Magmatic-hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion, vein opening, and hydrothermal ore deposition. It is driven by larger subjacent magma reservoirs, acting as the source of fluid and ore-forming components. Porphyry Cu ± Mo ± Au deposits form by hydrothermal metal enrichment from fluids that immediately follow the emplacement of porphyritic stocks and dikes at 2–8 km depth. Ore-generating fluids originate from water-rich magma chambers underlying the pervasively fractured and altered porphyry stocks. Large magma chambers in the hot lower crust may grow and retain partial melt over millions of years, whereas upper crustal magma chambers without open magma throughput have a maximum lifetime of <1 m.y. Small porphyry stocks emplaced at shallow depth crystallize within 0.01 m.y. or less1. They are difficult to keep at the temperature of hydrothermal mineralization (>350 °C) for more than a few tens of thousands of years, even with massive heat advection by magmatic fluids.

To better define these contrasting time scales, high-precision U-Pb zircon age data from two world-class porphyry Cu-Au deposits are selected, where geological mapping has documented an intrusion sequence of several porphyry phases that truncate, and therefore bracket in time, distinct pulses of hydrothermal veining and Cu-Au mineralization. High-precision U-Pb zircon ages of individual zircon crystals from porphyries immediately predating and postdating Cu-Au mineralization at Bingham Canyon (Utah, United States) and Bajo de la Alumbrera (northwestern Argentina) show a significant spread of reliably concordant ages2. This demonstrates zircon crystal formation over a protracted period of ~1 m.y., which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted. The youngest zircons in all pre-ore and post-ore intrusions overlap within a much shorter time interval of 0.32 m.y. at Bingham Canyon and 0.090 m.y. at Alumbrera; these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods, consistent with thermal constraints.

1: Cathles, L.M., 1977, Econ. Geol., 72, 804–826.

2: von Quadt, A. et al., 2011: Geology, v.39, no.8, p.731-734.