TESTING ORE-FORMING MODELS OF BEHEMOTH PORPHYRY COPPER SYSTEMS USING HIGH-PRECISION GEOCHRONOLOGY (Invited Presentation)

Porphyry copper systems result from the focused discharge of magmatic fluids into upper crustal lithologies. Despite their formation shares similar chemical processes, the contained amount of metals in this class of deposits ranges over four orders of magnitude. This suggests that physical processes, and notably the mass and heat transfers, may play a critical and underexplored role in controlling the metal endowment of these deposits. In fossil systems, quantifying the timescale of porphyry formation may be one of the few method capable of informing us about such essential mass and heat fluxes.

In situ zircon U-Pb data (LA-ICPMS, SIMS) on magmatic rocks bracketing the mineralization have often yielded timescales on the order of 0.X–2 Myr. This has been initially interpreted as reflecting the superimposition of multiple magmatic-hydrothermal events as a key process for the formation of giant porphyry copper deposits. However, such timescales are of comparable magnitude as the accuracy of the analytical methods used. This suggests that conclusions not accounting for this external source of uncertainty might be fallacious.

Over the last decade, state-of-the-art zircon dating by CA-ID-TIMS has provided data both more accurate and precise by one order of magnitude. So far, only a handful of deposits have been dated at the permil precision level using this method. These data reveal that porphyry copper deposits would rather form in few kyr to 10s of kyr. Such timescales are consistent with numerical models that consider the emplacement and degassing of few tenth of km³ of magma in the upper crust. While this model is able to explain the formation of small to giant deposits (containing up to ~10 Mt of Cu), available low-precision geochronological data, observations, and numerical models on larger magmatic-hydrothermal systems suggest that behemoth deposits may form over more extended periods of time, potentially as a result of superimposed discrete magmatic-hydrothermal cycles. Yet, the link between duration, cyclicity and metal endowment of porphyry copper systems remains to be fully tested by careful high-precision and high-accuracy geochronology. When tamed, Time may show the way to some of the most precious mineral resources.