Abstract
Porphyry copper deposits are formed by aqueous fluids exsolved by differentiated, mantle-derived magmas variably mixed with crustal melts. Water is essential to form porphyry Cu mineralization, and this explains why these deposits are found only at convergent margin settings, where subduction has enriched the mantle source of magmas with slab-derived H2O. Intuitively, the more water occurs in the parental magmas of porphyry deposits, the more fertile the latter should be. Indeed, several studies have proposed that anomalously high H2O contents in the source basalt, resulting, for instance, from subduction of large-scale serpentinized fracture zones of the oceanic slab, could increase the fertility of magmas. However, no studies have ever quantified the effects of variable H2O contents on the fertility of parental basalts to form porphyry deposits. Here, using petrological modeling with a Monte Carlo approach, I show that the optimum amount of slab-derived H2O in fertile parental basalts is ∼2– 6 wt%, which coincides with the measured range of H2O content in arc basalts. Lower and higher amounts of H2O in the parental basalt lead to less porphyry-fertile magmas. The lower fertility of H2O-poor parent basalt (i.e., 6 wt%) predicted by the model is counterintuitive. The reason for the decreased fertility of parental basalts with >6 wt% H2O is that such H2O-rich magmas undergo fluid saturation, losing their fluid and metal cargo at deep crustal levels. Additionally, water saturation-induced crystallization of amphibole at these deep levels prevents such H2O-rich magmas from ascending to shallower crustal levels, where they can form porphyry deposits. The conclusion that arc basalts with normal H2O contents (∼2–6 wt%) are the most porphyry-fertile adds evidence to the hypothesis that intermediate-felsic magmas associated with porphyry Cu deposits are parented by arc basalts formed through normal subduction-related processes and that intracrustal and tectonic processes play the most relevant role in the modulation of Cu endowments of these deposits.
Highlights
Water is one of the essential ingredients to make porphyry copper deposits, together with Cu, Cu ligands (e.g., Cl− and HS−), and sulfur combining with Cu to form ore minerals (Burnham, 1979; Richards, 2011; Seward et al, 2014; Chiaradia and Caricchi, 2017)
Primary basaltic melts, from which intermediate-felsic magmas associated with porphyry copper deposits derive, are considered to form by partial melting of a mantle metasomatized by H2O-rich fluids of the subducted slab
The aim of this study is to provide, through petrologic modeling, constraints on the optimal H2O contents of primary arc basalts that are parental to intermediate-felsic magmas associated with porphyry copper deposits
Summary
Water is one of the essential ingredients to make porphyry copper deposits, together with Cu, Cu ligands (e.g., Cl− and HS−), and sulfur combining with Cu to form ore minerals (Burnham, 1979; Richards, 2011; Seward et al, 2014; Chiaradia and Caricchi, 2017). Porphyry copper deposits are typically associated with intermediate to felsic magmas with calc-alkaline to variably alkaline affinity in convergent margin settings (e.g., Richards, 2009; Sillitoe, 2010; Chiaradia, 2020) In this geodynamic context, porphyry copper deposits occur in both syn-subduction (typical Andean- or Cordilleran-type porphyry deposits) and post-subduction (syn- or post-collisional, extensional) settings (Richards, 2009; Chiaradia, 2020). Porphyry copper deposits occur in both syn-subduction (typical Andean- or Cordilleran-type porphyry deposits) and post-subduction (syn- or post-collisional, extensional) settings (Richards, 2009; Chiaradia, 2020) In both situations, primary basaltic melts, from which intermediate-felsic magmas associated with porphyry copper deposits derive, are considered to form by partial melting of a mantle metasomatized by H2O-rich fluids (supercritical or melts) of the subducted slab. The modalities of the melting of the metasomatized mantle may change in these two geodynamic settings (e.g., mostly flux melting of asthenospheric mantle in the syn-subduction case and influx of hot asthenosphere inducing partial melting of metasomatized lithospheric mantle in the case of post-subduction; Richards, 2009), the ultimate source of water in the primary basaltic melts formed in both situations above is the slab-derived H2O
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