Metals in subduction related magmatism: Insights from melt inclusions and associated glassy groundmass from the Southern Kermadec Arc, New Zealand
Authors:   , Leath Jacob 
Published: December 2021

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited.  This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing.  Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components.  Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources.  All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing.  Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>