Abstract
Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.
Highlights
Porphyry Cu ore deposits are the products of large, longlived, trans-crustal arc magma systems that release and focus metal-charged fluids during the intrusion of porphyritic stocks and dykes (Dilles 1987; Seedorf et al 2005; Sillitoe 2010)
The formation of porphyry Cu deposits has been linked to distinctive changes in the trace element chemistry of igneous rocks that crystallised immediately prior to ore genesis, including increased whole-rock Sr/Y (> 50), La/Yb (> 20) and Eu/Eu* (> 0.3) (Lang and Titley 1998; Rohrlach and Loucks 2005; Richards and Kerrich 2007; Richards 2011; Richards et al 2012; Loucks 2014)
A total of 762 zircon analyses are reported from twelve samples with both U–Pb dates and trace element chemistry, acquired using LA-inductively-coupled plasma mass spectrometry (ICP-MS) (Supplementary Material 2)
Summary
Porphyry Cu ore deposits are the products of large, longlived, trans-crustal arc magma systems that release and focus metal-charged fluids during the intrusion of porphyritic stocks and dykes (Dilles 1987; Seedorf et al 2005; Sillitoe 2010). The origin of the characteristic trace element signatures of magmas parental to porphyry ore deposits has been attributed to a protracted evolution in the lower crust (e.g.,Rohrlach and Loucks 2005; Chiaradia et al 2009; Wilkinson 2013; Chelle-Michou et al 2015), linked to an inhibition of magma ascent. This is thought to be related to perturbations in regional geodynamics that generate intense but transient compression and crustal thickening (Cooke et al 2005; Rosenbaum et al 2005). This increased water content and pressure promotes the stability of amphibole ± garnet (in which Y and MREE/ HREEs are compatible) as early and abundant crystallising and fractionating phases, whereas crystallisation of plagioclase (in which Sr and Eu are compatible) is suppressed (Müntener et al 2001; Alonso-Perez et al 2008; Melekhova et al 2015)
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