T-P-fO2 conditions of sulfide saturation in magmatic enclaves and their host lavas

Ariadni Georgatou,Massimo Chiaradia,Kalin Kouzmanov

doi:10.1016/j.lithos.2021.106313

Abstract

Physicochemical conditions under which early sulfide saturation occurs and the role of this process in the mineralising potential of magmas in different geodynamic settings have been the subject of interest in many recent studies. Here we present new temperature-pressure-fO2 data on sulfide-saturated magmatic enclaves and host lavas from volcanic systems in subduction, post-subduction and intraplate geodynamic settings, some of which are associated with porphyry and epithermal deposits. Petrographic investigations coupled with mineral chemistry of sulfide inclusions and their host minerals and with bulk chemistry of amphibole, plagioclase and pyroxene-rich enclaves and their host lavas indicate that sulfide occurrence, abundance, and composition depend on magma evolution. Sulfides are more abundant and have higher Ni/Cu bulk ratios when found in mafic enclaves compared to sulfide inclusions occurring in more evolved lavas. Thermo-oxibarometry estimates indicate that mafic hornblende-rich cumulates (SiO2 < 45 wt%) occurring at the studied intraplate setting evolve and reach sulfide saturation already in the mantle (61 ± 7 km) at high temperature and pressure conditions (17 kbar and 1135 ± 28 °C) and low ΔNNO (< −1). These cumulates are associated with a greater sulfide abundance (up to 0.23 area %) compared to other types of enclaves and have Cu-poor sulfides (mostly pyrrhotite, with Cu median = 0.16 wt%) with high Ni/Cu values (up to 65). In contrast, more evolved (SiO2 > 50 wt%) gabbroic plutonic enclaves found in arc settings saturate sulfides at shallower crustal levels (~26 ± 10 km, i.e. ≤ 7 kbar) and at lower temperature (1014 ± 28 °C) and higher ΔNNO (> +1), and are characterised by a lower sulfide abundance (down to 0.01 area %), dominated by Cu-rich sulfides (mostly chalcopyrite and bornite with Cu median = 50 wt%). By accounting for the sulfide volume, our results suggest that sulfides can retain significantly higher amounts of metals in the enclaves (Cu = 32–113 ppm) compared to sulfides in the host lavas (Cu < 8 ppm). Considered altogether, our results indicate that alkaline mafic, less oxidised, and barren magmas corresponding to intraplate and back-arc settings saturated in sulfides in the upper mantle. In contrast, calc-alkaline-felsic, oxidised and ore-related magmas corresponding to convergent margins started to be saturated in sulfides at the expected depth of the MASH zone and at shallower crustal levels. This seems to be a consequence of the investigated alkaline systems not forming shallow crustal magma chambers allowing magma differentiation, rather than the fact that late and shallow sulfide saturation is necessary for porphyry formation.

Highlights

Sulfide saturation occurring at depth may be responsible for the sequestration of significant amounts of chalcophile elements, con trolling the metal availability in the later exsolving volatile phase and affecting the mineralisation potential to produce porphyryepithermal deposits (Li and Audetat, 2015; Park et al, 2015)
We carried out detailed petrography combined with mineral and bulk rock chemistry and with thermo-oxibarometry of sulfide-hosting silicates in amphibole, plagioclase- and pyroxene-rich enclaves and in their host lavas in areas characterised by diverse geodynamic settings and magma compositions
In the upper mantle (61 ± 7 km) sulfides have been identified in mafic hornblende-rich cumulates (SiO2 < 45 wt%) from an intraplate alkaline volcanic system, recording high temperature and pressure conditions (17 kbar and 1160 ◦C) and low ΔNNO (< − 1)

Summary

Introduction

Sulfide saturation occurring at depth may be responsible for the sequestration of significant amounts of chalcophile elements, con trolling the metal availability in the later exsolving volatile phase and affecting the mineralisation potential to produce porphyryepithermal deposits (Li and Audetat, 2015; Park et al, 2015). Sulfide saturation does not seem to be detrimental for all ore forming systems This suggests either the possibility of later dissolution and mobilization of metals from the deep sulfide-rich zones (Halter et al, 2005) or that metal content is not the primary control on the formation of porphyry deposits and other parameters (magma volumes, amount of fluid) play a more important role (Chelle-Michou et al, 2017; Chiaradia and Caricchi, 2017; Du and Audetat, 2020; Lee and Tang, 2020). The latter Cu-rich sulfide type has been observed to occur only in the lavas and it appears to form at a late stage (Georgatou and Chiaradia, 2020)

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