Evolution of copper isotopes in arc systems: Insights from lavas and molten sulfur in Niuatahi volcano, Tonga rear arc

Abstract

Metal transfer from mantle wedge to primitive arc magmas and subsequent enrichment by magmatic fractionation and volatile exsolution are critical processes for mineralization in arc systems. Copper is one of the most important oreforming elements whose behavior is sensitive to oxygen fugacity. Copper isotope composition (delta Cu-65) may provide valuable insights into Cu transfer and enrichment in hydrous oxidized arc magmas. However, the extent of Cu isotopic variation in arc systems and its link to Cu transfer and enrichment for ore mineralization have been poorly explored. Here we report the Cu isotopes in basalts, dacites and molten sulfur in Nivatahi volcano, Tonga rear arc to address the issue. These samples, as well as associated black smoker chimneys, represent products of magmatic fractionation and degassing of hydrous oxidized arc magmas with ore mineralization. Sulfide-undersaturated differentiation of basalts in the Nivatahi and their high water content and oxygen fugacity suggest complete exhaustion of sulfides in the mantle source during fluxed melting and transfer of nearly all Cu, Ag and other chalcophile metals to the primary magmas. The delta Cu-65 of Nivatahi basalts thus reflect that of the mantle source. The basalts display delta Cu-65 of 0.01 parts per thousand to 0.17 parts per thousand (n = 3; external uncertainty of 0.05 parts per thousand, 2sd), similar to mid-ocean ridge basalts (MORBs), komatiites and the depleted mantle (0.06 +/- 0.20 parts per thousand, 2sd). These results, together with their Cu contents indistinguishable from MORBs, suggest that oxidized slab components are very likely to have limited influence on the Cu budget and mean delta Cu-65 of the mantle wedge. The Nivatahi magma became sulfide saturated after magnetite crystallization during magma differentiation from basalt to dacite. Constant Cu/Ag in the basalts and dacites suggests segregation of immiscible sulfide melts instead of crystalline sulfides. The sulfide segregation significantly decreased contents of Cu and other chalcophile metals but hardly changed delta Cu-65 in dacites (-0.01 parts per thousand to 0.35 parts per thousand, n = 11 with a mean of 0.21 +/- 0.24 parts per thousand, 2sd), implying restricted fractionation of delta Cu-65 during magnetite fractionation and sulfide melt segregation. Molten sulfurs, which are formed by intensive magmatic degassing of arc lavas and characterized by substantial enrichment of Cu and other metals, show delta Cu-65 of 0.30 parts per thousand to 0.37 parts per thousand. These values are indistinguishable from those of comagmatic dacites (0.34 parts per thousand). Although the published delta Cu-65 of sulfide chimneys in the Niuatahi appears slightly lighter (0.00 parts per thousand to 0.29 parts per thousand +/- 0.18 parts per thousand, 2sd), the overall limited range of delta Cu-65 in molten sulfur and sulfide chimneys indicates that discharging magmatic volatiles and hydrothermal venting with significant removal of Cu hardly fractionates delta Cu-65. The delta Cu-65 data from arc lavas, molten sulfur and sulfide chimneys thus reveal limited variations in delta Cu-65 (within 0.35 parts per thousand) during fluxed melting, magmatic fractionation, magma degassing and mineralization in arc systems. If these results represent general processes, they imply that the heavier or lighter delta Cu-65 in other sulfide chimneys and associated deposits should result from the complex hydrothermal processes and/or low-temperature secondary reworking. (C) 2019 Elsevier Ltd. All rights reserved.