# Sulfide and Fe-Ti-P liquid immiscibility in the Ni-Cu-Co ovoid deposit of the Voisey's Bay complex, Labrador, Canada

Stefano Caruso, Margaux Le Vaillant, Stephen J Barnes, Ville Juhani Virtanen, Giada Iacono-Marziano

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Stefano Caruso, Margaux Le Vaillant, Stephen J Barnes, Ville Juhani Virtanen, Giada Iacono-Marziano. Sulfide and Fe-Ti-P liquid immiscibility in the Ni-Cu-Co ovoid deposit of the Voisey's Bay complex, Labrador, Canada. Mineralium Deposita, 2024, 60 (2-3), pp.503-531. 10.1007/s00126-024-01319-7. insu-05344871

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# Sulfide and Fe-Ti-P liquid immiscibility in the Ni-Cu-Co ovoid deposit of the Voisey's Bay complex, Labrador, Canada

Stefano Caruso<fnref n="1" />,<fnref n="2" /> ID · Margaux Le Vaillant<fnref n="1" /> ID · Stephen J. Barnes<fnref n="1" /> ID · Ville J. Virtanen<fnref n="3" />,<fnref n="4" /> ID · Giada Iacono-Marziano<fnref n="3" /> ID

Received: 6 March 2024 / Accepted: 18 September 2024 / Published online: 9 October 2024
© Crown 2024

## Abstract

In the Voisey's Bay complex, sulfide-matrix breccias developed through the percolation of dense sulfide melt leading to the displacement of the silicate melt within partially molten silicate-matrix breccias. In these sulfide matrix-breccias, hydrous silicate rims are commonly present at the interface between the sulfide matrix and the silicate framework. Multiple lines of evidence support a magmatic origin of these hornblende-biotite rims, which was largely coeval with the emplacement of the sulfide melt in the magmatic breccias. The formation of the hornblende-biotite rims required the addition of alkalis and water that could not have entirely been sourced from either the sulfide melt or the silicate framework. Through the integration of compositional maps with major and trace element analyses of the main accessory minerals, we propose that the critical components required for the development of the hydrous silicate rims in sulfide-matrix breccias originated from an immiscible Fe-Ti-P melt. Distinct textural and compositional features of apatite, hercynite, ilmenite and magnetite support the presence of small amounts of Fe-Ti-P melt in the sulfide melt. This Fe-Ti-P melt likely formed through melt immiscibility in the early stages of the development of the Voisey's Bay complex, and was transported in the magma conduits together with the sulfide melt.

**Keywords** Nickel · Magmatic sulfide · Immiscibility · Fe-Ti-P melt · Magmatic breccia · Volatiles

## Introduction

The Voisey's Bay complex is a mafic-ultramafic sill-dike system hosting multiple Ni-Cu-Co sulfide accumulations in Labrador, Canada (Li and Naldrett 1999; Naldrett et al. 2000). Mining operations are currently owned by Vale S.A., which reported in 2018 over 32 Mt of reserves with a Ni grade of 2.13% and a Co grade of 0.13%, contributing to a

total of over 140 Mt of available and mined reserves. The mineralization occurs in different portions of the magmatic system with a wide range of textures. Disseminated sulfides are hosted in the upper portions of the Western Deeps intrusion, while disseminated to net-textured sulfides occur in the lower portions of the Eastern Deeps intrusion within or closely associated with vari-textured troctolite that commonly contains xenoliths (Evans-Lamswood et al. 2000; Barnes et al. 2017a). Vari-textures are a distinctive feature of mineralized mafic-ultramafic intrusions, and comprise large irregular short-range variations in grain size, which are attributed either to the migration of fluids through mafic magmas, or to processes of mingling between magmas with different compositions (Lightfoot 2007). Semi-massive to massive sulfides occur near the base of the Eastern Deeps intrusion at the entry point of the feeder dikes, and as lenses within magma conduits (Li and Naldrett 1999; Naldrett et al. 2000). Massive sulfide accumulations are typically overlain by leopard-textured troctolite, which consists of cumulus plagioclase and olivine with augite and olivine oikocrysts in a sulfide matrix, the oikocrysts appearing as black spots in a yellow background (Naldrett et al. 2000; Barnes et al.

Editorial handling: D. A. Holwell

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Stefano Caruso
stefano.caruso@csiro.au

<footnote n="1">¹ CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, WA 6151, Australia</footnote>

<footnote n="2">² Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Perth, WA 6009, Australia</footnote>

<footnote n="3">³ Centre National de la Recherche Scientifique, Orleans 45071, France</footnote>

<footnote n="4">⁴ Department of Geosciences and Geography, University of Helsinki, Helsinki 00560, Finland</footnote>
2017b). Sulfide ores also occur in magmatic breccias at the basal contact of the intrusions underlying massive sulfide accumulations and in multiple sections of the magma conduits (Li and Naldrett 1999; Evans-Lamswood et al. 2000). Magmatic breccias are polymict, including endogenous mafic-ultramafic fragments and enderbitic gneiss country rock, with a matrix consisting of variable proportions of sulfides and magmatic silicates (Li and Naldrett 2000; Mariga et al. 2006a; Barnes et al. 2017a). Sulfide-matrix breccias are interpreted to develop through gravity-driven percolation of a sulfide melt displacing the inter-clast silicate melt in partially molten magmatic breccia (Barnes et al. 2017a). In sulfide-matrix breccias, primary magmatic silicates and xenoliths are commonly surrounded by biotite-hornblende rims (Barnes et al. 2017a). While the volatiles required to develop the hydrous silicate rims may have been partially sourced from the sulfide melt, the origin of the alkalis has a less clear explanation (Barnes et al. 2017a).

In this contribution, we look at the accessory mineral assemblages in barren and mineralized lithologies to investigate the origin of the hydrous silicate rims in sulfide-matrix breccia and sulfide-rich troctolite in the Voisey's Bay complex. Building on the petrographic and textural study by Barnes et al. (2017a), we present additional micro-beam X-ray fluorescence (μXRF) maps and high-resolution X-ray computed tomographs (HRXCT), along with new major and trace element analyses of the main accessory minerals. We propose that the alkalis and at least part of the volatiles necessary for the development of the hydrous silicate rims originated from an immiscible Fe-Ti-P melt coexisting with the sulfide melt. This interpretation aligns with the genetic model established by previous studies on the Voisey's Bay complex, because the processes that promoted the formation of the Fe-Ti-P melt was the same that facilitated the development of the sulfide melt (Li and Naldrett 1999; Lightfoot et al. 2012; Barnes et al. 2017a).

## The Voisey's Bay complex

The Voisey's Bay complex and its mineralized occurrences have been the subject of multiple geological, structural and geochemical studies (Li and Naldrett 1999; Ripley et al. 1999; McKenzie 2000 special volume; Mariga et al. 2006a, b; Lightfoot et al. 2012; Hiebert et al. 2013; Saumur et al. 2015). The geologic setting and genetic model presented in these studies are summarized below.

The 1330–1334 Myo Voisey's Bay complex is a mafic sill-dike system belonging to the Nain Plutonic Suite, northern Canada (Fig. 1a; Amelin et al. 1999; Ryan 2000). The mafic complex comprises a network of dikes connecting two weakly layered gabbroic and troctolitic intrusions, the

Eastern and Western Deeps, situated between the Archean orthogneiss of the Nain Province to the east and the Paleoproterozoic Tasiuyak paragneiss and enderbitic orthogneiss of the Churchill Province to the west (Figs. 1b and 2; Evans-Lamswood et al. 2000; Ryan 2000). Structural and geobarometric estimations (Berg 1979) suggest that the Voisey's Bay complex was emplaced in the upper-middle crust at a depth of 9–11 km (Saumur et al. 2015). After the emplacement, the mafic complex was not affected by any significant deformation or metasomatic events, thus preserving its magmatic mineralogy, textures and structures (Li and Naldrett 1999; Ryan 2000; Saumur et al. 2015).

Magmatic sulfide mineralization occurs both in the intrusions and in multiple segments of the dikes (Evans-Lamswood et al. 2000). In contact with the Tasiuyak paragneiss on the southern side of the Western Deeps intrusion is the lowest known portion of the Ovoid conduit, one of the main dikes of the complex (Evans-Lamswood et al. 2000). The Ovoid conduit extends further to the east in the enderbitic orthogneiss and connects with other dikes, which ultimately join with the Eastern Deeps intrusion (Li and Naldrett 1999).

Dikes commonly consist of ferrogabbro and olivine gabbro at the margins, vari-textured troctolite in the inner portions, with magmatic breccias heterogeneously distributed throughout the dikes (Li and Naldrett 1999; Evans-Lamswood et al. 2000). These lithologies form a diverse array of textures that are collectively referred to as the feeder breccia sequence (Li and Naldrett 1999). Within the dikes, magmatic breccias have variable amounts of clasts (both endogenous and exogenous) and sulfides within a matrix of olivine gabbro and vari-textured troctolite (Li and Naldrett 1999; Evans-Lamswood et al. 2000). Increasing concentrations of sulfides result in the formation of sulfide-matrix breccias, sulfide-rich troctolite and massive accumulations (Evans-Lamswood et al. 2000). Discontinuous domains of sulfides occur in low-flux sections of the dikes, such as those in the proximity to inflection points, embayments, extensional step-over structures, or at the intersections with the larger sill-like intrusions (Evans-Lamswood et al. 2000; Lightfoot et al. 2012). From west to east, mineralized domains include the Reid Brook, Discovery Hill, Mini-Ovoid, Ovoid, South-eastern Extension Zone and North of Eastern Deeps deposits (Fig. 2; Evans-Lamswood et al. 2000; Lightfoot et al. 2012; Saumur et al. 2015).

The Eastern Deeps intrusion is considered the upper chamber of the Voisey's Bay complex (Li and Naldrett 1999; Lightfoot et al. 2012). It consists of fine- to medium-grained ferrogabbro, olivine gabbro, troctolite and olivine norite that were emplaced as a series of stacked sill-like bodies, locally intruded by syenite and monzonite (Lightfoot et al. 2012; Saumur and Cruden 2016). A magmatic breccia, termed the basal breccia sequence, forms the basal
<figure><img src="image_3.png" /><figcaption>**Fig. 1** a) Geological map of the Voisey's Bay area, northern Labrador, Canada; b) Geological map of the Voisey's Bay complex with the main orebodies projected to the surface (modified after Lightfoot et al. 2012)</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Sketch of long section illustrating the genetic model for the Voisey’s Bay complex that includes the main components of the magmatic system. The Western Deeps and Eastern Deeps intrusions, interpreted as a magma chamber, are connected by a network of feeder dikes controlling the upward and downward movements of multiple magma pulses. The formation of abundant sulfide melt is promoted</figcaption></figure>

by the partial melting of the sulfidic Tasiuyak paragneiss. The sulfide orebodies occur in multiple sections of the conduits and at the base of the intrusions, particularly at the entry point of the feeder dikes, in association with thick magmatic breccias (modified after Lightfoot et al. 2012)

contact of the magma chamber and contains more than 25 vol% of fragments (Li and Naldrett 1999). These fragments comprise gneiss (mostly Tasiuyak gneiss), ultramafic rocks (wehrlite and dunite) and endogenous troctolite that are hosted in a matrix of vari-textured troctolite and olivine gabbro with variable amounts of magmatic sulfides (Li and Naldrett 1999). Magmatic sulfides are concentrated in the lower portions of the Eastern Deeps intrusion, forming thick lenses of massive to semi-massive sulfides in the basal breccia sequence, particularly near the feeding conduit (Li and Naldrett 1999; Naldrett et al. 2000). They also occur in

trace amounts or less frequently as blocky or semi-massive mound-shaped accumulations in the vari-textured troctolite that overlays the basal breccia sequence (Li and Naldrett 1999; Naldrett et al. 2000).

Geological and geochemical evidence indicates at least two distinct generations of magmas were emplaced to form the Voisey’s Bay complex (e.g., Li and Naldrett 1999; Lightfoot et al. 2012). During emplacement, a favorable combination of protracted magma flow and crustal assimilation promoted the development of a dynamic sill-dike system with numerous Ni-Cu-Co magmatic sulfide deposits

<figure><img src="image_5.png" /></figure>
(Evans-Lamswood et al. 2000; Li and Naldrett 2000; Barnes et al. 2016).

Low whole-rock Cu/Zr ratios in ferrogabbro and the olivine gabbro in the Eastern Deeps intrusion and in the marginal portions of the dikes indicate that the first magma generation was depleted in chalcophile elements, because it reached sulfide saturation at an early stage of its emplacement history (Li and Naldrett 1999). Consistent with many other magmatic Ni-Cu-Co deposits, sulfide saturation was attained through a combination of crustal assimilation and crystal fractionation (Li and Naldrett 1999; Ripley et al. 1999). In a hypothetical, so-far undiscovered deep magma chamber, fragments of wall-rock gneiss, analogous to the Nain orthogneiss, were assimilated by the magma, whereas crystal fractionation developed ultramafic/mafic cumulates that were subsequently remobilized upwards in the magma conduits (Li and Naldrett 1999; Ripley et al. 1999; Naldrett 2010). During the first generation emplacement through the magma conduits to upper crustal levels, the magma reacted with the sulfide-bearing Tasiuyak paragneiss, further increasing the amount of sulfide melt and the degree of crustal contamination (Li et al. 2000). The protracted magma flow led to variable degrees of partial melting of the wall-rock gneiss, as documented by multiple mineralogical, isotopic and geochemical studies (Lambert et al. 2000; Li and Naldrett 2000; Li et al. 2000; Ripley et al. 2002; Mariga et al. 2006b; Hiebert et al. 2013). As a result of solid-state diffusion and partial melting, the wall-rock gneiss developed a pervasive refractory assemblage, consisting of different proportions of hercynite, corundum and anorthite, through the incorporation of FeO and MgO from the magma and the release of SiO₂, alkali and volatiles to the magma (Li and Naldrett 2000). This process likely occurred within a thermal aureole rather than after incorporation of wall-rock fragments into the mafic magma, because the equilibration times required for the solid-state diffusion reactions are not compatible with common magma transport rates (Mariga et al. 2006a; Robertson et al. 2015; Barnes and Robertson 2019).

A second generation of more primitive and chalcophile-undepleted magma was subsequently injected through the conduits up to the Eastern Deeps intrusion (Li and Naldrett 1999). The normal troctolite in the upper portion of the Eastern Deeps intrusion is the most pristine product of this new magma generation, which is characterized by high whole-rock Cu/Zr ratios coupled with Ni enrichments and relatively high forsterite contents of olivine (Li and Naldrett 1999). Injections of second-generation magmas locally mixed with first-generation magmas and remobilized the sulfide melt that was previously accumulated in slow-flow sections of the conduits (i.e., ‘sulfide trap’; Li and Naldrett 1999; Evans-Lamswood et al. 2000). Saumur

and Cruden (2016) demonstrated that at least some of the sulfide emplacement took place by downward injection into fractures, whereas Barnes et al. (2016, 2017a) proposed that much of the ore emplacement could have occurred by downward flow during drain-back of magma and entrained sulfide through the sill-dike network.

## Origin of sulfide-matrix breccias and leopard textures

Magmatic breccias in the basal breccia and feeder breccia sequences are commonly polymict and clast supported, with textures varying considerably depending on the relative proportions of sulfides and silicate in the matrix, and the type and relative proportion of xenoliths (Li and Naldrett 2000; Barnes et al. 2017a). Sulfide-rich endmembers of the spectrum are termed sulfide-matrix breccias in the terminology of Barnes et al. (2018). Breccia clasts comprise fragments of gneiss (mostly Tasiuyak paragneiss), ultramafic cumulates (wehrlite and dunite), endogenous olivine gabbro and troctolite (Li and Naldrett 1999; Evans-Lamswood et al. 2000; Barnes et al. 2017a). Gneiss xenoliths are typically elongated, a few centimeters in size, and consist of a resistitic mineral assemblage with variable proportions of anorthite, hercynite and corundum commonly defining zoning patterns (Li and Naldrett 2000). The restitic mineral assemblage is largely uniform throughout the entire Voisey’s Bay complex, suggesting that both orthogneiss and paragneiss protholiths converged to the same mineral assemblage while preserving distinct Th, La and Yb ratios (Li and Naldrett 2000; Mariga et al. 2006a).

The relative amount of sulfides and ‘primary’ silicates in magmatic breccias is used to discriminate between silicate-matrix and sulfide-matrix breccias (Barnes et al. 2018). Silicate-matrix breccias have low contents of sulfides (<10%), but still form fine interconnected networks (Fig. 3; Barnes et al. 2017a). The silicate matrix has troctolitic and olivine gabbro compositions, consisting of olivine, randomly oriented plagioclase laths and pyroxene oikocrysts and clinopyroxene with diminishing modal abundances with increasing sulfide contents (Barnes et al. 2017a). Sulfide-matrix breccias are commonly clast-supported and characterized by a continuous network of sulfides surrounding ‘primary’ magmatic silicates and breccia fragments (Fig. 3; Barnes et al. 2017a). A typical feature of sulfide-matrix breccias is the presence of idiomorphic plagioclase laths radiating from both endogenous and exogenous clasts, particularly from paragneiss xenoliths (Barnes et al. 2017a).

Barnes et al. (2017a) interpreted the sulfide-matrix breccia textures as the result of the displacement of the silicate melt by the sulfide melt, a process that is analogous to that responsible for the development of interspinifex ore in
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Micro-beam X-ray fluorescence (μXRF) map and high-resolution X-ray computed tomography (HRXCT) of sulfide-matrix breccia (VB-OV-B) representing the spatial relationships between the silicate</figcaption></figure>

framework and the sulfide matrix. In the HRXCT the sulfide matrix is segmented as the combination of all sulfides and oxides

komatiites (Barnes et al. 2017b). According to this model, the dense and low-viscosity sulfide melt would percolate downward into silicate-matrix breccias during early stages of solidification. The infiltration of sulfide melt occurred after (i) the development of the restitic assemblage in gneiss clasts; (ii) the formation of plagioclase rims around silicate fragments caused by the interaction with the mafic melt; and (iii) the partial crystallization of the mafic melt, which involved the growth of olivine, plagioclase laths and clinopyroxene oikocrysts. The absence of clinopyroxene in sulfide-matrix breccia can be attributed either to sulfide infiltration preceding clinopyroxene crystallization or, more likely, to the re-melting of clinopyroxene induced by the heat carried into the rock by percolating sulfide melt. In xenolith-poor portions of the magmatic breccia, the complete re-melting of clinopyroxene would develop sulfide-rich troctolite textures, whereas the preservation of clinopyroxene oikocrysts in a sulfide-dominate matrix would form the characteristic leopard texture.

## Samples and analytical methods

In this study, we investigate the texture, modal mineralogy, and composition of biotite and accessory minerals of thirteen samples representing the main lithotypes of the Voisey's Bay complex, with a particular focus on the breccia sequence of the Ovoid and mini-Ovoid orebodies (Table 1). The modal mineralogy of selected samples was determined through image analysis of µXRF maps acquired with an iXRF Atlas M. The mineral composition of oxides, apatite and biotite was defined through wavelength dispersive X-ray (WDX) spot analyses using a JEOL 8530 F field-emission gun electron micro probe (EMP). Additional the trace element compositions of apatite and ilmenite were determined by laser ablation inductively coupled mass spectrometry (LA-ICP-MS). For both WDX-EMP and LA-ICP-MS, the composition of biotite, ilmenite, and apatite in each sample was determined, whenever possible, by analyzing multiple spots on multiple grains. Representative values within the text are provided as median, whereas ranges are reported as first and third quartiles. The spatial distribution of the main phases within sulfide-matrix breccias was investigated
<table><caption>**Table 1** Description of samples and type of analyses performed</caption><thead><tr><th>Sample ID</th><th>Strati-graphic group</th><th>Location</th><th>Lithology</th><th>Description</th><th>Origin</th><th>Sample type</th><th>EMP analyses</th><th>LA-ICP-MS analyses</th><th>Other analyses</th></tr></thead><tbody><tr><td>BH165</td><td>Normal troctolite</td><td>Eastern Deeps</td><td>Troctolite</td><td>Uniform-textured medium-grained troctolite with randomly oriented plagioclase laths and interstitial clinopyroxene</td><td>VB95-165 @ 56 m</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td>Apatite<br/>– ilmenite</td><td></td></tr><tr><td>VB95-165</td><td>Normal troctolite</td><td>Eastern Deeps</td><td>Troctolite</td><td></td><td>VB95-165 @ 48 m</td><td>Slab</td><td></td><td></td><td>μXRF map</td></tr><tr><td>VB14-1</td><td>Feeder dike</td><td></td><td>Olivine gabbro</td><td>Vari-textured olivine gabbro with clinopyroxene oikocrysts and bands of biotite around sulfide-oxide patches</td><td>VB95-205 @ 215–216 m</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td>Apatite<br/>– ilmenite</td><td></td></tr><tr><td>VB14-2</td><td>Feeder dike</td><td></td><td>Olivine gabbro</td><td></td><td>VB95-205 @ 220–221 m</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td>μXRF map</td></tr><tr><td>OV-FOG-VB95-205-1</td><td>Feeder dike</td><td></td><td>Olivine gabbro</td><td></td><td>VB95-205 @ 233–237 m</td><td>Slab</td><td></td><td></td><td>μXRF map<br/>(from Barnes et al. 2017a)</td></tr><tr><td>PL-OV-BX-1 A</td><td>Basal breccia sequence</td><td>Ovoid</td><td>Silicate-matrix breccia</td><td>Clast-supported magmatic breccia in a continuous network of vari-textured olivine gabbro with abundant paragneiss xenoliths and minor sulfides</td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td>Apatite<br/>– ilmenite</td><td></td></tr><tr><td>PL-OV-BX-1B</td><td>Basal breccia sequence</td><td>Ovoid</td><td>Silicate-matrix breccia</td><td></td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td>μXRF map</td></tr><tr><td>1858-A</td><td>Basal breccia sequence</td><td>Ovoid</td><td>Sulfide-rich troctolite</td><td>Troctolite with clinopyroxene oikocrysts and net-textured sulfides</td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td>Apatite<br/>– ilmenite</td><td>μXRF map</td></tr><tr><td>VB95-39</td><td>Basal breccia sequence</td><td>Southeastern Extension Zone</td><td>Sulfide-rich troctolite</td><td></td><td>VB95-39 @ 98 m</td><td>Slab</td><td></td><td></td><td>μXRF map<br/>(from Barnes et al. 2017b)</td></tr><tr><td>SB-1 A</td><td>Basal breccia sequence</td><td>Mini-Ovoid</td><td>Sulfide-matrix breccia</td><td>Clast-supported magmatic breccia in a continuous network of sulfides with abundant paragneiss and olivine gabbro xenoliths intergrown with large plagioclase laths</td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td></td></tr><tr><td>SB-1B</td><td>Basal breccia sequence</td><td>Mini-Ovoid</td><td>Sulfide-matrix breccia</td><td></td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td></td></tr><tr><td>VB-OV-A</td><td>Basal breccia sequence</td><td>Ovoid</td><td>Sulfide-matrix breccia</td><td>Clast-supported magmatic breccia with abundant xenoliths of olivine orthocumulate paragneiss large interconnected domains of sulfides intergrown with large plagioclase laths</td><td>Grab sample</td><td>Thin section | Off-cut</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td>μXRF map</td></tr><tr><td>VB-OV-B</td><td>Basal breccia sequence</td><td>Ovoid</td><td>Sulfide-matrix breccia</td><td></td><td>Grab sample</td><td>Thin section</td><td>Biotite<br/>– apatite<br/>– ilmenite</td><td></td><td>μXRF map<br/>– HRXCT</td></tr></tbody></table>

<footnote>[1] Fluid phases tend to concentrate Cl relative to F, so that (Cl/F) volatile > (Cl/F) melt > (Cl/F) apatite (Webster et al. 1989; Piccoli and Candelà 2002)</footnote>
through HRXCT. The detailed description of each analytical method is included as supplementary material (ESM 1). The complete mineral chemistry datasets are included as supplementary materials (ESM 2).

# Results

## Mineralogy and textures

The observed and described mineralogy and textures of mineralized and barren samples builds upon observations from previous studies on the Voisey's Bay complex (e.g., Li and Naldrett 1999, 2000; Naldrett et al. 2000; Lightfoot et al. 2012; Barnes et al. 2017a). Particular attention is given to the distribution and textures of hydrous silicates, sulfides, oxides and apatite.

## Silicate-matrix breccia

Silicate-matrix breccias are sulfide-poor and have troctolitic and olivine gabbro matrices enclosing a large variety of endogenous and exogenous fragments (Fig. 4a; Barnes et al. 2017a). Paragneiss xenoliths are the most common fragments and consist of hercynite-anorthite aggregates with plagioclase rims and sometimes preserving corundum-plagioclase cores (Li and Naldrett 2000; Barnes et al. 2017a). Hercynite has acicular and prismatic habits and is transparent under transmitted light (Fig. 4a). In the paragneiss xenoliths, interstitial spaces are occupied by ilmenite and less frequently by chalcopyrite and pyrrhotite, whereas interstitial spaces in the silicate matrix are occupied by a variety of assemblages including (i) ilmenite and minor sulfides rimmed by biotite, which grades into hornblende near augite and olivine (Fig. 4b); (ii) fine olivine and sulfide intergrowths; and (iii) plagioclase-orthopyroxene symplectites which transitions into hornblende and biotite (Barnes et al. 2017a). Euhedral apatite grains are commonly included in augite, or intergrown with interstitial minerals, such as sulfides, biotite and ilmenite.

## Sulfide-matrix breccia

Sulfide-matrix breccias have homogeneous sulfide assemblages consisting of pyrrhotite (± troilite), pentlandite, chalcopyrite, with accessory oxides and apatite (Naldrett et al. 2000; Lightfoot et al. 2012; Barnes et al. 2017a). 'Primary' magmatic silicates comprise (i) thin domains of troctolite and olivine gabbro; (ii) complex ortho-clinopyroxene intergrowths and hornblende-olivine symplectites associated with biotite, both of which are located in the interstitial

**Fig. 4** Optical photomicrographs of the main lithotypes in the Voisey's Bay complex. **a)** silicate-matrix breccia (PL-OV-BX-1 A) in plane- and cross-polarized transmitted light; **b)** detail of picture a, interstitial minerals of silicate-matrix breccia in reflected and cross-polarized transmitted light; **c)** sulfide-matrix breccia (SB-1B) in reflected and cross-polarized transmitted light; **d)** detail of picture c, hydrous silicate rim and oxide minerals of sulfide-matrix breccia in cross-polarized transmitted light and reflected light; **e)** sulfide-rich troctolite (1858-A) in reflected light and cross-polarized transmitted light; **f)** detail of picture e, hydrous silicate rim and oxide minerals of sulfide-rich troctolite in reflected light and cross-polarized transmitted light; **g)** normal troctolite (BH165) in plane- and cross-polarized transmitted light; **h)** detail of picture g, interstitial minerals of normal troctolite in cross-polarized transmitted light and reflected light; **i)** feeder olivine gabbro (VB14-1) in cross- and plane-polarized transmitted light; **j)** detail of picture i, accessory minerals of feeder olivine gabbro in plane-polarized transmitted light and reflected light. Abbreviations: ap=apatite, bt=biotite, ccp=chalcopyrite, cpx=clinopyroxene, her=hercynite, ilm=ilmenite, mag=magnetite, ol=olivine, pl=plagioclase, po=pyrrhotite, tro=troilite

spaces of the primary magmatic silicate domains; and (iii) plagioclase overgrowths associated with hornblende-biotite rims on clasts and 'primary' magmatic silicates (Fig. 4c; Barnes et al. 2017a). The plagioclase overgrowths consist of a combination of (i) fine-grained granular plagioclase rimming all types of clasts and growing within paragneiss clasts; and (ii) idiomorphic coarse plagioclase laths radiating from granular plagioclase rims and extending into the sulfide-rich matrix (Barnes et al. 2017a). Biotite-hornblende rims are a typical feature of sulfide-matrix breccias, predominantly replacing plagioclase at the interface with sulfides (Fig. 4d; Barnes et al. 2017a). Hornblende, typically pargasite, forms thicker rims on olivine, whereas biotite preferentially develops at the margins of paragneiss clasts and idiomorphic plagioclase laths (Barnes et al. 2017a). Another characteristic feature is the presence of zoned rims consisting of hornblende near 'primary' silicates, transitioning to biotite at the contact with sulfides (Fig. 5a-d). Both mono-mineral and zoned rims commonly include (10–50 µm) sulfide blebs near or at the contact with plagioclase laths and olivine (Fig. 5a-d). Euhedral apatite occurs as isolated grains or associated with oxides and biotite. Apatite grains are distributed in the interstitial spaces of troctolite and olivine gabbro domains, within dunite xenoliths and within the sulfide matrix. Apatite in the sulfide matrix is commonly located at the contact with plagioclase laths, locally intergrown with biotite-hornblende reaction rims and fully enclosed within sulfides (Fig. 5a-d). Apatite commonly hosts round sulfide and hercynite inclusions (Fig. 6a). Oxides in the sulfide matrix form sinuous aggregates consisting of ilmenite, magnetite and hercynite, sometimes including small (10–100 µm) sulfide blebs (Fig. 4c-d). Hercynite is opaque under transmitted-light and occurs as large anhedral grains and within magnetite as inclusions and exsolutions (Fig. 6c-f). Exsolutions in magnetite occur in three forms: (i) as thick
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<figure><img src="image_18.png" /><figcaption>**Fig. 5** a to d) Back-scattered electron (BSE) images combined with energy-dispersive X-ray spectroscopy (EDS) maps of hydrous silicate rims in sulfide-matrix breccia (SB-1 A) and sulfide-rich troctolite (1858-A). Abbreviations: ap = apatite, bt = biotite, ccp = chalcopyrite,</figcaption></figure>

(5–30 µm) ilmenite-hercynite trellis-like laminae; (ii) as thin disks (1–3 µm) of either ilmenite and hercynite along {100}, locally associated with smaller (<1 µm) ilmenite-only thin-disk exsolutions; and (iii) as dendritic rims protruding from the margins of magnetite (Fig. 6c-f).

## Sulfide-rich troctolite

Sulfide-rich troctolite consists of a xenolith-free olivine-plagioclase(+/-clinopyroxene) framework in a sulfide matrix (Fig. 7a). Plagioclase occurs as laths, which are locally bent at the impingement point between silicate grains (Barnes et al. 2017a). The sulfide mineralogy, the apatite and hydrous silicates distributions, as well as the textures of the oxide aggregates are identical of those of sulfide-matrix breccias (Fig. 4e-f).

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<figure><img src="image_20.png" /><figcaption>cpx = clinopyroxene, her = hercynite, ilm = ilmenite, mag = magnetite, ol = olivine, pl = plagioclase, po = pyrrhotite, prg = pargasite, tro = troilite</figcaption></figure>

## Normal troctolite

Normal troctolite is a typical olivine-plagioclase cumulate (Figs. 4g and 7b). Plagioclase laths are randomly oriented and occasionally partially or fully enclosed in olivine. Minor augite occurs as reaction rims around olivine in association with hornblende epitaxial overgrowths. Interstitial minerals include biotite, ilmenite, apatite and rare sulfides. Biotite commonly surrounds ilmenite and sulfide clusters, whereas apatite is only occasionally associated with the other interstitial phases (Fig. 4h).

## Feeder olivine gabbro

Feeder olivine gabbro mainly consists of olivine-plagioclase cumulate with higher amounts of ilmenite, sulfides and hydrous minerals than normal troctolite (Fig. 7c).
<figure><img src="image_21.png" /><figcaption>**Fig. 6** Back-scattered electron (BSE) images of apatite inclusions and magnetite-ilmenite exsolutions. **a)** pyrrhotite and hercynite inclusions in apatite from sulfide-rich troctolite; **b)** pyrrhotite inclusions in apatite from feeder olivine gabbro; **c)** oxides and hydrous silicate rims in sulfide-matrix breccia; **d)** hercynite-ilmenite exsolutions; **e)** oxides in sulfide-matrix breccia; **f)** hercynite-ilmenite exsolutions in magnetite</figcaption></figure>

with dendritic hercynite rims protruding from the margins of magnetite in ilmenite. Abbreviations: ap=apatite, bt=biotite, ccp=chalcopyrite, cpx=clinopyroxene, her=hercynite, ilm=ilmenite, mag=magnetite, ol=olivine, pl=plagioclase, po=pyrrhotite, prg=pargasite, tro=troilite
<figure><img src="image_22.png" /><figcaption>**Fig. 7** Micro-beam X-ray fluorescence (μXRF) maps and modal mineralogy of a) sulfide-rich troctolite (VB95-39), b) feeder olivine gabbro (OV-FOG-VB95-205-1), c) normal troctolite (VB95-165)</figcaption></figure>

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Plagioclase has polygonal and less frequently platy habits with a bimodal size distribution (0.2–0.4 vs. 0.8–1.6 mm) forming the characteristic vari-texture (Fig. 4i; Li and Naldrett 1999). Olivine and to a minor extent augite oikocrysts enclose plagioclase laths. Ilmenite and sulfides occur in clusters commonly spatially associated with biotite, hornblende and apatite (Fig. 4i-j). Apatite locally hosts spherical sulfide inclusions (Fig. 6b).

## Mineral compositions

### Biotite

In all samples, biotite has a phlogopitic composition with a Mg-number (Mg#, molar MgO/[MgO + FeO]) commonly between 0.65 and 0.70, with higher values between 0.80 and 0.82 in normal troctolite (Fig. 8a). Aluminum per formula unit (Al<sub>F</sub>) is commonly between 1.34 and 1.37, with higher values between 1.53 and 1.57 in sulfide-matrix breccia and sulfide-rich vari-textured troctolite (Fig. 8a). Aluminum in octahedral coordination (Al<sup>VI</sup>), is commonly low between 0.04 and 0.08, with higher values between 0.17 and 0.20 in sulfide-matrix breccia and sulfide-rich vari-textured troctolite (Fig. 8b). Titanium in formula (Ti<sub>F</sub>) varies between 0.16 and 0.25 and is negatively correlated with the content of Al in octahedral coordination (R<sup>2</sup> = 0.84; Fig. 8b). Sodium in biotite substitutes for K and has variable formula contents that decrease from sulfide-matrix breccia (0.22–0.25) to silicate-matrix breccia (0.11–0.13), with the lowest contents in normal troctolite and olivine gabbro (0.05–0.07; Fig. 8c).

The total content of halogens in biotite is generally low (0.01–0.12 wt%), with higher concentrations only in olivine gabbro (0.44–0.52 wt%). Chlorine is commonly below 0.02 wt%, with higher concentrations in olivine gabbro (0.25–0.30 wt%; Fig. 8d). Fluorine content ranges between 0.16 and 0.24 wt% in normal troctolite and olivine gabbro, whereas is less than 0.09 wt% in silicate-matrix breccia and below the detection limit of 0.03 wt% in sulfide-matrix breccia and sulfide-rich vari-textured troctolite (Fig. 8e).

### Apatite

Halogens in apatite formula are calculated from WDX-EMP analyses using the method defined by Ketcham (2015). Apatite in olivine gabbro is a chloro-fluoro-apatite. The composition changes to fluoro- hydroxy-apatite in normal troctolite and silicate-matrix breccia and to hydroxy- fluoro-apatite in sulfide-matrix breccia, and sulfide-rich troctolite (Fig. 9a). Chloro-fluoro-apatite in feeder olivine gabbro and hydroxy-apatite in sulfide-rich troctolite have high chondrite-normalized Ce/Yb ratios (Ce/Yb<sub>cn</sub>; 26–30 and 35–44, respectively; Fig. 9b), high Sr/Y ratios (2.89–4.18

and 3.15–4.19, respectively; Fig. 9c), high Th/Lu ratios (20–27 and 13–22, respectively; Fig. 9d), low V contents (2–3 ppm for both; Fig. 9d), low REE contents (0.26–0.28 wt% and 0.21–0.24 respectively; Fig. 9b), small chondrite-normalized Eu anomalies (Eu/Eu<sub>cn</sub> *; 0.77–0.98 and 0.81–1.06, respectively; Fig. 9c). Conversely, fluoro-apatite in normal troctolite and silicate-matrix breccia have low Ce/Yb<sub>cn</sub> (8–9 and 8–10, respectively; Fig. 9b) and low Sr/Y ratios (0.43–0.49 and 0.26–0.34, respectively; Fig. 9c), low Th/Lu ratios (2–4 and 3–7, respectively; Fig. 9d), variable V contents (44–55 and 3–8 ppm, respectively; Fig. 9d), moderate REE contents (0.37–0.40 wt% and 0.60–0.68 wt%, respectively; Fig. 9b) and negative Eu anomalies (0.33–0.36 and 0.18–0.36 Eu/Eu<sub>cn</sub> *, respectively; Fig. 9c).

### Ilmenite

Ilmenite from silicate-matrix and sulfide-matrix breccia samples has similar MgO contents (1.06–1.16 wt% and 0.77–1.11 wt%), while the MgO contents are slightly higher in olivine gabbro (1.33–1.85 wt%), slightly lower in sulfide-rich troctolite (0.77–1.11 wt%) and are relatively high in sulfide-rich troctolite (3.11–3.40 wt%; Fig. 10a). The content of Cr<sub>2</sub>O<sub>3</sub> is generally low in all lithotypes (0.02–0.06 wt%), with relatively higher contents restricted to exsolution lamellae and ilmenite hosted in the interstitial spaces of xenolith olivine cumulate, compared to blebby ilmenite associated with sulfides or hosted in troctolite (e.g., 0.01 vs. 0.06 wt% in sample 1858 A; Fig. 10a).

### Hercynite

Hercynite occurs either as an optically transparent phase in gneiss xenoliths, or as an optically opaque phase forming blebby aggregates with magnetite and ilmenite in the sulfide matrix of mineralized breccia and troctolite. Opaque hercynite is commonly associated with ilmenite and sulfides and has a different composition from hercynite in gneiss xenoliths. Compared to green-transparent hercynite, opaque hercynite has lower MgO contents (6.66–7.31 vs. 8.18–11.07 wt%, respectively), higher ZnO contents (4.43–4.99 vs. 0.19–0.53 wt%, respectively) and higher Cr<sub>2</sub>O<sub>3</sub> contents (0.60–1.08 vs. 0.09–0.16 wt% respectively; Fig. 10b-c).

### Magnetite

Magnetite occurs in the sulfide matrix of mineralized breccia and troctolite as well as in the barren feeder olivine gabbro. In all the samples analyzed, magnetite is significantly enriched in Cr (0.33–0.95 wt%), V (0.43–0.83 wt%) and Ti (0.82–1.13 wt%; Fig. 10d-e). Magnetite in sulfide-matrix breccia has the highest contents of Ti (0.91–2.00 wt%) and
<figure><img src="image_24.png" /><figcaption>**Fig. 8** Biotite composition from quantitative wavelength dispersive X-ray (WDX) electron micro probe (EMP) analyses. **a)** Al in octahedral coordination vs. Mg# (Mg/(Mg + Fe) mol); **b)** Al in octahedral coordination vs. Ti; **c)** K vs. Na; **d)** Cl wt% vs. Mg#; **e)** F wt% vs. Mg#</figcaption></figure>
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<figure><img src="image_29.png" /><figcaption>**Fig. 9** Apatite composition from quantitative wavelength dispersive X-ray (WDX) electron micro probe (EMP) analyses and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses. **a)** F</figcaption></figure>

vs. Cl vs. OH in atom per formula unit; **b)** chondrite-normalized Ce/Yb ratio vs. total REE (La to Lu) in wt%; **c)** Sr/Y ratio vs. chondrite-normalized Eu anomaly; **d)** Th/Lu vs. V ppm
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**Fig. 10** Oxide composition from quantitative wavelength dispersive X-ray (WDX) electron micro probe (EMP) analyses and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses. **a)** MgO wt% vs. Cr₂O₃ wt% in ilmenite; **b)** ZnO wt% vs. MgO wt% in hercynite; **c)** Cr₂O₃ wt% vs. MgO wt% in hercynite; **d)** V ppm vs. Cr ppm in magnetite compared with data from Boutroy et al. (2014); **e)** Ti ppm vs. Cr ppm in magnetite compared with data from Boutroy et al. (2014)

V (0.82–1.03 wt%), whereas magnetite from feeder olivine gabbro has the highest contents of Cr (1.14–1.41 wt%; Fig. 10d-e).

## Discussion

### Magmatic volatiles in the Voisey's Bay complex

Previous studies have demonstrated that mafic melts at Voisey's Bay contained water throughout different pulses and magma generations (Li and Naldrett 1999; Mariga et al. 2006b; Glasgow 2014). This is reflected in variable modal amounts of magmatic hydrous silicates among lithologies, ranging from 2 to 5% in sulfide-matrix breccias, 8–13% in normal troctolite and up to 30% in feeder olivine gabbro (Li and Naldrett 1999). These contents are likely influenced by the relative amount of interstitial melt relative to cumulate silicates, yet they closely align with the modal proportions of biotite and hornblende in chilled margins (3–30%; Glasgow 2014). Furthermore, biotite has oxygen isotope signatures that increase from the chilled margins (+4.2 δ¹⁸O) to the mafic-ultramafic cumulates (+5.2 – +6.0 δ¹⁸O), with noticeable crustal anomalies (+7.3 δ¹⁸O) in lithologies that recorded substantial interactions with paragneiss xenoliths (+8.6 δ¹⁸O; Mariga et al. 2006b; Glasgow 2014). These patterns suggest that the parental magma(s) of the Voisey's Bay intrusion (i) were already hydrous at depth; and (ii) increased their water contents through the interaction with the Tasiuyak paragneiss (Mariga et al. 2006b; Glasgow 2014).

In the samples investigated in this study, variations of major elements in biotite reflect distinct parental melt compositions and growth mechanisms. These patterns are in agreement with previous interpretations of a magmatic origin of the hydrous phases (Glasgow 2014; Barnes et al. 2017a). At Voisey's Bay, biotite has a phlogopitic composition typical of igneous mafic micas (Fig. 8a; Fleet 2003). In normal troctolite from the Eastern Deep intrusion, high Mg-numbers in biotite reflect the primitive nature of the parental melt, whereas lower Mg-numbers in biotite from feeder olivine gabbro and magmatic breccias from the Ovoid and mini-Ovoid are indicative of more evolved melt compositions (Fig. 8a; Abdel-Rahman 1994; Fleet 2003). These Mg-numbers are consistent with the origin of normal troctolite

in the Eastern Deep intrusion and the feeder olivine gabbro from the second and first magma generation, respectively (Li and Naldrett 1999). The evolved melt compositions reflected in the lower Mg-numbers in biotite from magmatic breccias likely resulted from a combination of mixing between pulses of different magma generations and assimilation of gneiss xenoliths (Li and Naldrett 1999; Ripley et al. 1999).

The abundance of TiF₃ relative to AlVI in biotite is commonly used as a geothermometer (Henry et al. 2005; Wu and Chen 2015). Although the absence of a specifically calibrated formulation for magmatic environments precludes the development of reliable estimates (Rezaei and Zarasvandi 2020), variations of TiF₃ and AlVI of biotite coexisting with Ti-saturating minerals can still be used to evaluate relative temperature variations among rocks with similar compositions crystallized at the same depth (Douce 1993). At Voisey's Bay, high TiF₃ relative to AlVI in normal troctolite and feeder olivine gabbro are suggestive of higher crystallization temperatures than silicate-matrix breccias, sulfide-matrix breccias and sulfide-rich troctolite (Fig. 8b). Interestingly, these relative differences in temperature are associated with distinct biotite textures and alkali contents.

In normal troctolite and feeder olivine gabbro, biotite has high KF/NaF ratios and occurs in the interstitial spaces closely associated with ilmenite (Figs. 4b and 8c). These features are common to many mafic intrusions and are indicative of the residual nature of biotite. In the normal troctolite from the Eastern Deep intrusion, biotite likely precipitated after extensive closed-system crystallization (> 89%), similarly to the biotite in the troctolite of the 1306 Myo Kiglapait intrusion, one of the closest non-mineralized analogues of the Voisey's Bay complex (Morse 1969, 1979a), whereas the clusters of biotite in the feeder olivine gabbro likely crystallized earlier (Glasgow 2014).

Conversely, biotite in sulfide-matrix breccia and sulfide-rich troctolite has textural and compositional features that provide evidence for a late magmatic origin related to the percolation of the sulfide melt (Barnes et al. 2017a). In mineralized lithologies, biotite has lower KF/NaF ratios and is closely associated with pargasitic hornblende forming reaction rims on magmatic silicates (Figs. 5 and 8c). These reaction rims are preferentially developed on plagioclase, with hornblende typically occurring near silicates and biotite at the interface with sulfides. Such distribution and composition of the hydrous silicates confirm their origin as replacive overgrowths. Furthermore, it suggests that K was a limiting component during the formation of the reaction rims, as the replacive overgrowth progressed with the precipitation of pargasitic hornblende after the available K had been consumed through the earlier precipitation of biotite.
Previous interpretations on the origin of hydrous minerals in sulfide-matrix breccia and sulfide-rich troctolite

In their study on the development of sulfide-matrix breccias at Voisey's Bay, Barnes et al. (2017a) explored two hypotheses for the origin of hydrous phases at the interface between the sulfide-matrix and the silicate framework.

The first hypothesis considers the biotite-hornblende replacement rims as the remnants of a residual volatile phase that derived from the crystallization of silicate melt (i.e., ‘residual melt hypothesis’). During the last stages of solidification, volatile over-pressure would expel a residual fraction of the silicate melt in the molten sulfide matrix (Barnes et al. 2017a). At these conditions, the volatile-rich residual silicate melt would concentrate in the peripheral portions of the sulfide melts, due to the tendency of the silicate melt to wet the surface of the silicate minerals (Mungall and Su 2005; Barnes et al. 2017b). This model explains the development of the hydrous minerals between the sulfide matrix and the magmatic silicate framework. However, it does not account for the distribution of apatite and ilmenite, which would be expected to be spatially associated with biotite if they had a residual origin. Although apatite is sometimes intergrown in biotite-hornblende reaction rims at the contact with plagioclase laths, it also occurs within the sulfide matrix and in the interstitial spaces of troctolite and olivine gabbro domains. Ilmenite has a similar distribution pattern to apatite, but it mostly occurs in the sulfide-matrix. Overall, biotite, apatite and ilmenite lack a close spatial correspondence.

According to the ‘residual melt hypothesis’, apatite and ilmenite should have crystallized prior to the displacement of the silicate melt by the sulfide melt (Barnes et al. 2017a). However, a restitic origin of apatite and ilmenite seems implausible, as the silicate melt should have crystallized almost completely in order to reach saturation of ilmenite and apatite. For comparison, in the Kiglapait intrusion, ilmenite and apatite reached saturation after 86 and 94% of melt crystallization, respectively (Morse 1969, 1979a; Scoates and Mitchell 2000). As a result, the silicate-matrix would have needed to extensively re-melt in order to incorporate apatite and ilmenite in the sulfide matrix. Barnes et al. (2017a; their Fig. 17) envisaged that remelting of the silicate proceeded by initiating pseudo-eutectic minimum melts at the multi-phase boundaries of olivine, pyroxene and plagioclase; as melting progressed, pyroxene progressively consumed leaving first olivine plus plagioclase, then finally plagioclase alone, preserved as idiomorphic laths radiating from clasts. In this scenario, near solidus components such as ilmenite and apatite would be expected to have melted out first, before any of the major silicates, and

not to be preserved. Additionally, upon liberation, restitic apatite would be displaced rather than included in the sulfide melt, due to its lower density (3.2 g/cm³) relative to the sulfide melt (4.5 g/cm³).

The second hypothesis contemplates the origin of the volatile component from the sulfide melt (i.e., ‘hydrous sulfide-melt hypothesis’). While petrological experiments have demonstrated that halogens can dissolve in sulfide melts, the ability of sulfide melts to dissolve water has not yet been experimentally determined (Mungall and Brenan 2003). Nevertheless, the existence of hydrous sulfide melts has been suggested by multiple empirical observations, such as the presence of magmatic hydrous haloes at the base of the Sudbury deposit and lower solidus in hydrous sulfide melt experiments (Hanley et al. 2004; Wykes and Mavrogenes 2005). In this scenario, biotite-hornblende rims would develop through a hydration reaction similar to the one proposed by Barnes et al. (2017a):

$$2\text{KAlSi}_3\text{O}_8 + 6\text{FeS} + 8\text{H}_2\text{O} = 2\text{KFe}_3\text{AlSi}_3\text{O}_{10}(\text{OH})_2 + 6\text{H}_2\text{S}$$

The ‘pure’ hydration of plagioclase requires the addition of Fe, Mg and K to form biotite. Iron would be provided by the sulfide melt, whereas K (and Mg) would originate from late-stage partial melting of the paragneiss xenoliths, as suggested by the preferential development of biotite-hornblende rims around paragneiss xenoliths. The ‘hydrous sulfide-melt hypothesis’ is in good agreement with the development of hornblende rims on plagioclase laths being deformed during the percolation of the sulfide melt. However, similarly to the ‘residual melt hypothesis’, the ‘hydrous sulfide-melt hypothesis’ does not offer a simple explanation for the distribution of apatite intergrown in biotite-hornblende rims and apatite and ilmenite included in the sulfide matrix. It is questionable that partial melting of the paragneiss xenoliths would concentrate enough P and Ti to reach apatite and ilmenite saturation, as the Tasiuyak paragneiss is depleted in P relatively to all mafic rocks in the Voisey’s Bay complex, and mass-balance calculations suggest that Al₂O₃, P₂O₅ and TiO₂ were not significantly gained or lost from the paragneiss xenoliths (Li and Naldrett 2000; Li et al. 2000). Even the derivation of Mg and K from late-stage partial melting of the paragneiss xenoliths has some problematic aspects. The xenoliths do not preserve evidence of late partial melting in the form of ‘channels’ or interstitial melt pockets, and are typically rimmed by a continuous 1 to 3 mm-thick calcic plagioclase overgrowth with an outer biotite band (Mariga et al. 2006a, b; see Fig. 9 in Barnes et al. 2017a). The study of oxygen isotope signatures of gneiss xenoliths and their surrounding silicate matrices has demonstrated that (i) the biotite-anorthite rim developed during the early-stage interactions between xenolith-derived partial melt and mafic
magma in the immediate vicinity to the xenoliths; and (ii) the outer hydrous-silicate rim insulated the inner anorthite rim and the xenolith from late-stage interactions with the magma (Mariga et al. 2006b). Furthermore, the composition of the hydrous-silicate rim consisting of pargasitic hornblende in contact with plagioclase and biotite in contact with the sulfide matrix exclude that K was sourced from the gneiss xenoliths (Fig. 5d). Collectively, these observations confirm that Mg, K, P and Ti remained locked within the gneiss xenoliths during late-stage interactions with the mafic melt.

Although the two hypotheses examined by Barnes et al. (2017a) offer plausible mechanisms for the formation of biotite-hornblende reaction rims on the silicate framework of sulfide-matrix breccia and sulfide-rich troctolite, they cannot reconcile the distribution of apatite and ilmenite in the sulfide matrix with the availability of alkalis necessary for the formation of biotite.

## The Fe-Ti-P melt hypothesis

### The existence of an immiscible Fe-Ti-P melt

Textural observations in conjunction with mineral chemical analyses offer a novel perspective on the origin of the hydrous reaction rims within sulfide-matrix breccias and sulfide-rich troctolite of the Voisey's Bay complex. In particular, distinctive trace element compositions of apatite and hercynite, together with the abundant presence of ilmenite in the sulfide matrix, leads us to propose that the sulfide melt was accompanied by an immiscible Fe-Ti-P melt. In this scenario, the alkalis and at least part of the volatiles necessary for the formation of the biotite-hornblende reaction rims in sulfide-matrix breccias likely originated from an Fe-Ti-P melt. This mechanism parallels the process responsible for the development of biotite-hornblende bands commonly surrounding massif-related Fe-Ti oxide ores (e.g., Korosten Pluton; Duchesne et al. 2006).

Apatite in sulfide-rich troctolite has trace element signatures that are suggestive of an origin as a cumulus phase either from a primitive melt or from an immiscible Fe-Ti-P melt. This is supported by enrichments in light REE (LREE; 35–44 Ce/Yb$_{cn}$), high Sr/Y ratios (3.15–4.19), low REE contents (0.21–0.24 wt%) and small Eu anomalies (0.81–1.06 Eu/Eu$_{cn}$*) indicating that apatite precipitated or separated from a mafic melt prior to the crystallization of plagioclase (Fig. 9b-c; Belousova et al. 2002; O’Sullivan et al. 2020; Kieffer et al. 2024). Conversely, apatite in normal troctolite has trace element signatures that are consistent with a residual origin after extensive plagioclase crystallization (Piccoli and Candela 2002; Belousova et al. 2002). In this instance, low LREE enrichments (8–9 Ce/Yb$_{cn}$), low

Sr/Y ratios (0.43–0.49), moderate REE (0.37–0.40 wt%) and negative Eu anomalies (0.33–0.36 Eu/Eu$_{cn}$*) reflect the accumulation of incompatible elements, such as REE and Y, in the residual silicate melt, and the preferential partition of Sr, Eu and LREE into plagioclase (Fig. 9b-c; Belousova et al. 2002; O’Sullivan et al. 2020). Although these trace element signatures may have been influenced by the subsolidus re-equilibration of apatite with various proportions residual melt (Cawthorn 2013; Pedersen et al. 2021), the composition of apatite in olivine gabbro suggests that subsolidus re-equilibration had a minimal impact at Voisey’s Bay. The composition of apatite in feeder olivine gabbro closely resembles that of apatite in sulfide-matrix breccia, having high LREE enrichments (26–30 Ce/Yb$_{cn}$), high Sr/Y ratios (2.89–4.18), relatively low REE (0.26–0.28 wt%) and small Eu anomalies (0.77–0.98 Eu/Eu$_{cn}$*), despite the presence of larger amounts of residual silicate melt (Fig. 9b-c). This similarity implies that (i) apatite in feeder olivine gabbro formed before the onset of extensive plagioclase crystallization; and (ii) re-equilibration of apatite with residual silicate melt did not lead to significant modifications of the original trace element signatures. These findings support (i) the hypothesis that apatite in sulfide-rich troctolite and normal troctolite had different origins; and (ii) confirm the minimal influence of the ‘trapped silicate melt shift’ at Voisey’s Bay, as previously documented for the olivine of the basal breccia sequence in the Eastern Deeps intrusion (Bulle and Layne 2015). Furthermore, considering that sulfide-rich troctolite and sulfide-matrix breccias developed through the displacement and partial re-melting of a silicate melt in its early stages of crystallization, it is unlikely that apatite in sulfide-rich troctolite could represent a resistitic phase that originated from the displaced silicate melt. This is supported by trace element signatures of apatite in sulfide-matrix breccia, which are more similar to those of apatite in normal troctolite, having low LREE enrichments (8–10 Ce/Yb$_{cn}$) and Sr/Y ratios (0.26–0.34), moderate REE (0.60–0.68 wt%) and negative Eu anomalies (0.18–0.36 Eu/Eu$_{cn}$*), than to those of apatite in sulfide-rich troctolite (Fig. 9b-c).

The substantial amount of ilmenite within the sulfide matrix of magmatic breccia and mineralized troctolite support the presence of an Fe-Ti-P melt (Fig. 3). As previously discussed for both apatite and ilmenite, textural features exclude a restitic origin of ilmenite in sulfide-matrix breccia and sulfide-rich troctolite. Furthermore, it is unlikely that ilmenite in sulfide-rich troctolite and sulfide-matrix breccia originated directly from the sulfide melt, owing to the low partition coefficient of Ti between silicate and sulfide melt ($D_{Ti}^{sul/sil}$ 0.05; Naldrett 1969; Dare et al. 2012 and references therein). In these lithologies, ilmenite occurs as ‘blebs’, forming sinuous aggregates with magnetite and
hercynite and as trellis-like laminae and thin-disks in magnetite (Fig. 6c-f). Most trellis-like laminae and thin-disks ilmenite formed as ‘oxy-exsolutions’ through the oxidation of ulvöspinel exsolutions from titanomagnetite at temperatures above 600°C (Buddington and Lindsley 1964). Whereas a small proportion of ilmenite exsolutions, those forming (<1 µm) thin disks, are likely to have exsolved directly as ilmenite from a cation-deficient titanomagnetite (Sauerzapf et al. 2008; Tan et al. 2016). The presence of hercynite-magnetite fringes at the contact between magnetite and ilmenite implies a process of expulsion and coalescence of the exsolutions, which may have contributed to the formation of ilmenite and hercynite blebs at the margins of the oxide aggregates (Fig. 6e-f; e.g., Morisset et al. 2013). However, it is unlikely that ilmenite blebs in sulfide matrix formed exclusively through this process, as ilmenite blebs have similar or larger volumes to the associated magnetite blebs, which are still densely packed with ilmenite-hercynite exsolutions (Buddington and Lindsley 1964; Dare et al. 2012; Duran et al. 2016). Additionally, the negligible contribution from the coalescence of ilmenite exsolutions to the formation of larger ilmenite blebs is supported by distinct Cr contents among magnetite, ilmenite exsolutions and ilmenite blebs. Magnetite in sulfide-matrix breccia and sulfide-rich troctolite is enriched in lithophile elements such as Cr₂O₃ and V₂O₃ (0.49–0.97 and 0.62–1.23 wt%, respectively), which are typical of early forming magnetite from sulfide melts (Fig. 10d; Boutroy et al. 2014; Dare et al. 2014; Duran et al. 2020). Similarly, ilmenite laminae and thin-disks in magnetite have relatively high Cr₂O₃ contents (0.06–0.13 wt%), reflecting their formation as exsolution from titanomagnetite (Fig. 10a; He et al. 2016). Conversely, ilmenite blebs have low Cr₂O₃ contents (0.01–0.02 wt%), suggesting that their formation did not depend significantly on the expulsion and coalescence of ilmenite exsolutions (Fig. 10a; He et al. 2016). These mineralogical and compositional features suggest that only a small fraction of ilmenite in sulfide-matrix breccia and sulfide-rich troctolite may have derived from the sulfide melt, while the majority of ilmenite likely originated from an Fe-Ti-P melt.

In the basal breccia and feeder breccia sequences, hercynite was previously documented exclusively as part of the restitic assemblage of gneiss xenoliths (Li and Naldrett 1999, 2000; Mariga et al. 2006a). Textural and mineralogical observations from this study reveal a new ‘type’ of hercynite occurring exclusively within the sulfide matrix of magmatic breccia and mineralized troctolite. In contrast to the transparent hercynite in gneiss xenoliths, hercynite in sulfide matrix is opaque, forming sinuous aggregates with ilmenite and magnetite, as well as trellis-like laminae in magnetite (Figs. 4b-c and 7c). Opaque hercynite has high ZnO and Cr₂O₃ contents (4.43–4.99 and 0.60–1.08 wt%,

respectively), likely resulting from equilibration with sulfide melt and Cr-rich magnetite (Figs. 4b-c and 10b-c). These features imply a ‘primary’ magmatic origin of hercynite within sulfide matrix, in contrast to the restitic nature of hercynite in gneiss xenoliths. At Voisey’s Bay, opaque hercynite is restricted to sulfide-matrix breccia and sulfide-rich troctolite, although ilmenite is the prevalent oxides in all lithologies and the sole oxide mineral in normal troctolite and silicate-matrix breccia. This observation implies a close association between the sulfide mineralization and the presence of magmatic hercynite in the oxide assemblage of the basal breccia and feeder breccia sequences. Interestingly, the hercynite-ilmenite-magnetite assemblage is not common in ‘barren’ mafic intrusions, whereas hercynite is a typical accessory phase in Fe-Ti deposits (e.g., Wang and Zhou 2013; Pinto et al. 2020). For example, hercynite is absent in the nearby Kiglapait intrusion, even though it has a similar parental melt composition to the Voisey’s Bay complex (Morse 1979b; Scoates and Mitchell 2000). Whereas Zn-rich hercynite is a common accessory mineral in large Fe-Ti deposits that are thought to originate from Fe-Ti-P-rich immiscible melts, such as those in the Panxi region, China (Wang and Zhou 2013; Zhou et al. 2013). These deposits are also characterized by distinctive hornblendebiotite rims at the interface between the oxide matrix and magmatic silicate framework, which closely resemble the hydrous reaction rims in the magmatic breccias at Voisey’s Bay (see Fig. 4a in Wang et al. 2018).

Altogether, these textural and mineral chemistry characteristics are best explained by the presence of an immiscible Fe-Ti-P melt associated with the sulfide melt. The trace element signatures of apatite in sulfide-matrix breccias and sulfide-rich troctolite suggest an early separation from the silicate melt prior to plagioclase crystallization, which is typical of apatite in nelsonite and Fe-Ti oxide deposits originated from immiscible Fe-Ti-P melts (e.g., He et al. 2016; Kieffer et al. 2022, 2024). Similarly, the presence of ilmenite and hercynite in sulfide matrix deviates from the ‘typical’ mineral assemblage crystallized from a sulfide melt, while they are typical products of immiscible Fe-Ti-P melts (e.g., Wang and Zhou 2013; Pinto et al. 2020). The occurrence of ilmenite as the major oxide in the sulfide matrix can hardly be ascribed to the partition of Ti between silicate and sulfide melts (Dare et al. 2012). Likewise, the occurrence of hercynite in the oxide assemblage of mineralized samples is unlikely to have originated from the sulfide melts, or from late-stage interactions with gneiss xenoliths (Naldrett 1969; Mariga et al. 2006a, b).
## The formation of a Fe-Ti-P melt

The presence of an immiscible Fe-Ti-P melt has been invoked in the genetic models of multiple mineralized systems, including the contentious Kiruna-type magnetite-apatite deposits (e.g., El Laco, northern Chile; Bain et al. 2021), Fe-Ti oxide accumulations in mafic layered intrusions (e.g., Skaergaard, eastern Greenland; Holness et al. 2011; Humphreys 2011), and Fe-Ti-P deposits associated with massif-related anorthosites (e.g., Damiao, North China; Chen et al. 2013). Recent petrological experiments have successfully documented the separation of Si-poor Fe-Ca-P-rich melts from intermediate magmas, thereby affirming the relevance of melt immiscibility in the genesis of Kiruna-type deposits (Lester et al. 2013; Hou et al. 2018). The precise mechanisms responsible for formation of Fe-Ti-P melts are still a subject of debate, with petrological studies yet to reproduce the immiscibility of Ti-rich oxide-phosphate melts from tholeiitic magmas (Lindsley and Epler 2017; Brenan 2020). Despite ongoing debates, experimental results still provide valuable insights into the conditions promoting the immiscibility of Fe-Ti-P melts, because robust geological evidence supports the existence of these melts in several intrusive systems, such as massif-related anorthosite and mafic layered intrusions. The weight of evidence lies with melt inclusions with distinct compositions reflecting the coexistence of distinct Si-rich and Fe-rich melts (e.g., Jakobsen et al. 2005; Namur et al. 2012; Wang and Zhou 2013), as well as unequivocal cross-cutting relationships between Fe-Ti oxide accumulations and their host rocks (e.g., Duchesne 1999; Chen et al. 2013). Multiple processes have been invoked for the formation of Fe-Ti-P melts in mafic intrusive complexes, such as the immiscibility of conjugate Si-rich and Fe-rich melts, filter-press compaction and plagioclase buoyancy (Veksler et al. 2007; Holness et al. 2011; Charlier et al. 2015).

At Voisey's Bay, low modal proportions of ilmenite, apatite and hercynite in sulfide-matrix breccia and sulfide-rich troctolite point to a limited presence of Fe-Ti-P melt in the magmatic system. Thus, it is likely that the formation of an Fe-Ti-P melt had a minimal influence on the processes governing the formation of the magmatic sulfide orebodies. Small amounts of Fe-Ti-P melt likely developed in the first magma generation during the early stages of the development of the Voisey's Bay complex. This is suggested by trace element signatures of apatite in feeder olivine gabbro resembling those of apatite in sulfide-rich troctolite, thereby indicating the 'separation' of apatite from the silicate melt prior to the crystallization of plagioclase. We suggest that the 'separation' occurred through the segregation of an immiscible Fe-Ti-P melt.

An Fe-Ti-P melt could have formed from residual liquids developed in a deep magma chamber. Previous studies established that the first magma generation reached sulfide saturation at depth due to a combination of fractional crystallization and assimilation of wall-rock gneiss (Li and Naldrett 1999; Ripley et al. 1999). Within the feeder breccia sequence, a diverse range of endogenous fragments, including ultramafic cumulates, troctolite and olivine gabbro, provides evidence for significant fractionation of high-Al olivine-normative basaltic melts (Li and Naldrett 1999; Lightfoot et al. 2012). The related residual liquids, particularly those originating from the crystallization of troctolite, may have attained substantial Fe-Ti enrichment and subsequent segregation of an immiscible Fe-Ti-P melt during situ fractionation as observed in the Skaergaard intrusion (Holness et al. 2011; Humphreys 2011) and elsewhere (e.g., Vander Auwera et al. 2006). Additionally, interactions with graphitic layers of the Tasiuyak paragneiss during upward and downward magma movements may have facilitated the development of Fe and Ti enrichments, as reducing conditions suppress the stability of magnetite (e.g., Kilinc et al. 1983; Toplis and Carroll 1995).

Silicate melt immiscibility is known to be promoted by the presence of water, F and alkalis, which expand the temperature and compositional range of the two-melt immiscibility field (Bogaerts and Schmidt 2006; Charlier and Grove 2012; Lester et al. 2013). The presence of F has been observed to increase the Fe activity in mafic melts through complexation with Mg (Filiberto et al. 2012; Hou et al. 2017). Upon the formation of conjugate Si-rich and Fe-rich melts, an immiscible Fe-rich melt would evolve into an Fe-Ti-P melt through the crystallization of small proportions of mafic minerals accompanied by nucleation of new silica-rich immiscible droplets (Veksler et al. 2008). In this scenario, the presence of significant contents of water and alkalis in the first magma generation are supported by high proportions of pargasite and biotite in the chilled margins of feeder olivine gabbro (Glasgow 2014), whereas the presence of F and Cl is indicated by chloro-fluoro-apatite in feeder olivine gabbro. The origin of significant amounts of halogens in the first magma generation can be ascribed to the extensive interaction with wall-rock gneiss, similarly to the origin of K (Li and Naldrett 2000; Mariga et al. 2006a). This is suggested by the high Th/Lu ratios of chloro-fluoro-apatite in feeder olivine gabbro (20–27), indicating extensive crustal contamination (Belousova et al. 2002; Kieffer et al. 2022, 2024). The contribution of significant amounts of halogens from the assimilation of crustal material is further confirmed by the distribution of Cl among hydroxy-apatite grains in sulfide-matrix breccia. Apatite grains in the gneiss xenoliths are enriched in Cl (0.96–1.05 wt%), whereas those in the sulfide matrix are commonly Cl-poor (0.14–0.20
wt%). Elevated Cl contents of apatite in gneiss xenoliths not only indicate the source of halogens of the first magma generation, but also confirm the insulation of the xenoliths from the silicate magma after the formation of the plagio-clase rims (Mariga et al. 2006b).

## Coupled Fe-Ti-P and sulfide melts

In addition to the previously discussed mineralogical and compositional features, inclusions in apatite as well as the composition of magnetite in both sulfide-matrix breccia and olivine gabbro provide further evidence supporting the coexistence of sulfide and Fe-Ti-P melts in the first magma generation, and their subsequent remobilization by second-generation magmas. Apatite inclusions, comprising magmatic hercynite and sulfides, are restricted to mineralized breccia and troctolite as well as to feeder olivine gabbro. Round sulfide inclusions indicate that the crystallization of apatite was coeval with that of the sulfide melt, while inclusions of magmatic hercynite in apatite are consistent with the presence of an Fe-Ti-P melt accompanying a sulfide melt (Liu et al. 2021). Similarly to apatite inclusions, the occurrence of magnetite in the basal breccia and feeder breccia sequences is restricted to mineralized breccia and troctolite as well as to feeder olivine gabbro. In these rocks, magnetite has high concentrations of lithophile elements (i.e., Al, Cr, Ti and V; Fig. 10d-e) and low Ni contents, mirroring those of magnetite from the marginal portions of the massive sulfide accumulations of the Ovoid deposit (Boutroy et al. 2014). Although high Al and Ti contents can also occur in magnetite derived from a fractionated silicate melt or from the interaction between sulfide and residual silicate melts, low Mg and Zn contents suggest minimal involvement of the silicate melt in the genesis of magnetite (Dare et al. 2012; Wang et al. 2023). Furthermore, the distribution of magnetite within the sulfide matrix of mineralized breccia and troctolite further dismisses a residual or reactive origin (Naldrett 1969; Dare et al. 2012). A plausible explanation for the enrichments of Al and Ti in magnetite is an origin from an Fe-Ti-P melt, particularly when considering the distinctive ilmenite-hercynite-magnetite assemblage (Dare et al. 2014). Overall, the presence of sulfide and hercynite inclusions in apatite, along with the trace element signatures of magnetite in sulfide-matrix breccia and olivine gabbro conform with the coexistence of an Fe-Ti-P melt and the sulfide melt.

The presence of sulfide and Fe-Ti-P melts in the first magma generation is consistent with the segregation of Si-rich and Fe-rich melts, as various factors promoting the immiscibility of conjugate melts coincide with those favoring the development of a sulfide melt (e.g., fractional crystallization and crustal assimilation). Experimental results

confirm that modest amounts of S (2–3 wt%) are sufficient to induce three-liquid immiscibility in mafic and intermediate magmas (Lester et al. 2013; Hou et al. 2018). Once formed, sulfide and Fe-Ti-P melts would be transported together through the conduit system due to their similar density and viscosity (4.5 g/cm³ and 0.02 Pa·s, 4.0 g/cm³ > 0.03 Pa·s, for sulfide and Fe-Ti-P melts, respectively; Kolker 1982; Dobson et al. 2000; Robertson et al. 2015). In line with previous studies, immiscible sulfide and Fe-Ti-P melts, accumulated in slow-flow sections of the conduits by first-generation magmas, would be remobilized by multiple second-generation magma pulses (Li and Naldrett 1999; Evans-Lamswood et al. 2000). The two immiscible melts would then be emplaced in the breccia sequence through the percolation and displacement of the silicate-melt forming the characteristic sulfide-matrix breccia (Fig. 11; Barnes et al. 2017a).

## Halogens in apatite as a tracer of the emplacement history

The content of halogens in apatite reflects the mobilization of sulfide and Fe-Ti-P melts and is compatible with the complex emplacement history of the Voisey's Bay complex. As mentioned earlier, the presence of chloro-fluoro-apatite in feeder olivine gabbro implies significant contents of halogens in first-generation magmas. Increased halogen contents were likely acquired through extensive crustal assimilation, as indicated by high Th/Lu ratios of apatite in feeder olivine gabbro (20–27) and high Cl contents of apatite grains within gneiss xenoliths (0.96–1.05 wt%).

In contrast, fluoro-apatite in normal troctolite suggests that second-generation magmas had lower halogen contents, particularly in terms of Cl, owing to reduced interactions with wall-rock gneiss. While Cl-poor apatite may result from the exsolution and loss of a fluid phase from the magma, according to the preferential partitioning of Cl relative to F into fluid phases[1] (Webster et al. 1989; Piccoli and Candela 2002), the low-Cl contents of fluoro-apatite in normal troctolite likely represent a primary feature of second-generation magmas. This is evident by the combination of (i) high V contents (44–55 ppm; Fig. 9d), which are typical of apatite from primitive mafic melts; and (ii) low Th/Lu ratios in apatite (2–4), reflecting minimal crustal contamination (Fig. 9d; Belousova et al. 2002; Kieffer et al. 2022, 2024).

Hydroxy-apatite in sulfide-matrix breccia and sulfide-rich troctolite suggest that the sulfide and Fe-Ti-P melt pair lost some fluids during its prolonged entrainment in the conduit system. While hydroxy-apatite has the lowest halogen contents among the samples analyzed in this study, it is likely that the Fe-Ti-P melt from which apatite precipitated had higher contents of halogens that were at least comparable
<figure><img src="image_36.png" /><figcaption>**Fig. 11** Genetic model of sulfide-matrix breccia (modified after Barnes et al. 2017a). **a)** Molten silicate-matrix breccia consisting of polymict clasts in an olivine gabbro melt. The continuous fine-grained plagioclase overgrowths on the gneiss xenoliths developed through early-stage interactions with the mafic melt. **b)** Crystallization of the mafic melt forming randomly oriented plagioclase laths radiating from the clasts, along with olivine and clinopyroxene oikocrysts. **c)** Small amounts of Fe-Ti-P melt within the sulfide melt percolated in the silicate-matrix breccia displacing the residual mafic melt. The heat</figcaption></figure>

advected into the silicate matrix breccia by the sulfide melt (partially re-melted the silicate matrix, eliminating clinopyroxene. **d)** The crystallization of the sulfide melt paired with small amounts of Fe-Ti-P melt allowed for the development of the biotite-hornblende rims, the formation of apatite within the sulfide matrix and within the hydrous silicate rims, along with the formation of the characteristic oxide assemblage comprising hercynite, magnetite and significant amounts of ilmenite
to those of first-generation magmas. This is suggested by high Th/Lu ratios of apatite (13–22), which are indicative of extensive crustal assimilation and are comparable to those of apatite in feeder olivine gabbro (Belousova et al. 2002; Kieffer et al. 2022, 2024). The extended interactions between the silicate and the immiscible melts are evident in the significant Ni contents of the sulfide accumulations in the basal breccia sequence, as residence time is one of the key factors promoting the metal upgrading of sulfide melts (Naldrett et al. 2000; Yao and Mungall 2021). As the sulfide and Fe-Ti-P melt pair went through multiple cycles of transport, deposition and re-entrainment, the sill-dike system continued to propagate and enlarge (Naldrett et al. 2000; Barnes et al. 2017a). During the development of the conduits, fluid phases may have separated from the magma because of decrease in magma overpressure, similarly to the vapor exsolutions that accompanied the emplacement of the Norilsk-Talnakh Ni-Cu-PGE deposit (Barnes et al. 2023). The presence of abundant magmatic breccias within the Voisey’s Bay complex, particularly at the entry point into the magma chambers, suggest that intense fracturing was aided by the formation of fluids due to rapid pressure drops (Rivalta et al. 2015; Kavanagh et al. 2015). Significant decreases in magma overpressure likely occurred in connection with sharp changes in dike orientation, especially at the intersections between steeply and shallowly dipping wall-rock anisotropies where thin melt injections protrude away from the main conduits (Saumur et al. 2015). These geometric configurations resemble those of incipient sills, which have been linked by laboratory experiments to significant pressure drops of approximately 150 MPa at crustal depths between 1 and 10 km (Kavanagh et al. 2015). At the depth of emplacement of the Voisey’s Bay complex (9–11 km), an analogous pressure drop would exsolve ~2 wt% of H₂O from a water-saturated mafic melt, which would still retain ~3 wt% of H₂O (Moore et al. 1998; Lesne et al. 2011b; Iacono-Marziano et al. 2012). While it is unlikely that a significant amount of Cl and F would exsolve from the melt due to relatively high pressure conditions (Alletti et al. 2007; Lesne et al. 2011a; Witham et al. 2012), both halogens are likely to concentrate into the fluid phase due to generally positive, albeit variable, fluid/melt partition coefficients (Webster et al. 2018 and references therein).

## Analogue systems with three immiscible melts

Magmatic systems with three immiscible melts are relatively uncommon, possibly due to a combination of natural scarcity and previous oversights. Nonetheless, in recent years, researchers have identified a growing number of deposits believed to have originated from the interaction of three immiscible melts. When collectively examined, these

deposits constitute a continuum in which immiscible sulfide and Fe-Ti-P melts coexist in variable proportions with a silicate melt.

According to our hypothesis, the mineralized portions of the basal breccia and feeder breccia sequences of the Voisey’s Bay complex represent the sulfide end-member of a three-immiscible-melt system where a sulfide melt coexists with small amounts of Fe-Ti-P melt. In this scenario, the Fe-Ti-P melt was transported by the sulfide melt without significantly influencing the main ore-forming processes, effectively behaving as an accessory phase.

Moving along the spectrum of compositions, the Babbitt deposit is an example of a mineralized systems dominated by sulfide melt with increasing amount of Fe-Ti-P melt. The Babbitt deposit has been the first magmatic sulfide mineralization to be interpreted having a significant fraction of apatite and oxides derived from an Fe-Ti-P melt (Ripley et al. 1998). Situated within the troctolitic intrusive of the layered series of the 1109–1086 Myo Duluth complex, Minnesota, the Babbitt deposit hosts cross-cutting vein-like massive sulfide accumulations which comprise significant amounts of euhedral apatite (up to 10 vol%; Ripley et al. 1998; Miller et al. 2002). The origin of apatite from an Fe-Ti-P melt is deduced by the presence of an accessory assemblage consisting of ilmenite, magnetite, Zn-rich hercynite, biotite, zircon and baddeleyite (Ripley et al. 1998). This accessory assemblage closely resembles that of nelsonites (magnetite-apatite rocks) in the Boulder Lake area within Duluth complex (Ripley et al. 1998). Furthermore, the presence of round sulfide inclusions in apatite and oxides provides additional evidence for the coexistence of an Fe-Ti-P melt with the sulfide melt (Ripley et al. 1998).

The Lanjanina series in the Itsindro gabbro complex hosts magmatic sulfide-oxide accumulations that originated from equal proportions of sulfide and Fe-Ti-P melt (Augé et al. 2017). Located in the south-central Madagascar, the 790 Myo Itsindro gabbro complex is an anorogenic intrusion that emplaced in Archean migmatitic gneisses and Mesoproterozoic meta-sediments (Augé et al. 2017). The mineralization occurs in the pyroxenite of the Lanjanina series as disseminated to massive accumulations of apatite-Fe-Ti oxides and Fe-Cu-Ni sulfides (Augé et al. 2017). The distinctive magmatic textures of both apatite-Fe-Ti oxides and Fe-Cu-Ni sulfides suggests that the mineralization originated from the coexistence of a sulfide melt and an Fe-Ti-P melt (Augé et al. 2017). Mineralogical and whole-rock data suggest that the Fe-Ti-P melt segregated from an evolved mafic magma, while the sulfide melt likely developed through crustal assimilation (Augé et al. 2017).

At the end of the compositional spectrum, near the Fe-Ti-P end-member, mineralized systems with three immiscible melts are characterized by small amounts of sulfide melt
within abundant Fe-Ti-P melt. The Hongge and the Abu Ghalaga and intrusions are two representative examples of this type of magmatic systems.

The Hongge intrusion is part of the 263–256 Myo Emeishan Large Igneous Province, south west China (Wang and Zhou 2013). It has emplaced in the upper crust between Neo-proterozoic granitic gneiss and dolomitic limestones (Wang et al. 2021). Within its middle zone, consisting of olivine and plagioclase-bearing clinopyroxenite, the Hongge intrusion hosts massive and net-textured Fe-Ti oxide accumulations associated with minor lenticular sulfide aggregates (Chen et al. 2023). Several lines of evidence point towards the genesis of the Fe-Ti oxide ore as an immiscible Fe-Ti-P melt. These include (i) the intimate association of apatite with Fe-Ti oxide accumulations; (ii) the presence of Fe-rich melt inclusions in plagioclase; and (iii) the occurrence of massive Fe-Ti oxide in syenite dikes crosscutting the mafic intrusives (Wang et al. 2021; Chen et al. 2023). Furthermore, the identification of biotite, Fe-Ti oxide and sulfides as inclusions in apatite in massive and net-textured Fe-Ti oxide accumulations support the presence of minor amounts of sulfide melt coexisting with the main Fe-Ti-P melt (Chen et al. 2023).

Located in Egypt, the Neoproterozoic Abu Ghalaga gabbroic intrusion hosts massive to semi-massive Fe-Ti oxide accumulations as lenses, layers and veins (Khedr et al. 2022). The mineralization consists of hemo-ilmenite (ilmenite with titanohematite lamellae) and ilmenite with accessory apatite and minor sulfides (3–5 vol%; Khedr et al. 2022). Apatite and pyrrhotite occur as inclusions in Fe-Ti oxides, whereas pyrite, chalcopyrite and sphalerite are distributed in the interstices among Fe-Ti oxides (Khedr et al. 2022). The pyrrhotite inclusions are interpreted to represent the small fraction of sulfide melt associated with the large amounts of Fe-Ti-P melt formed through a combination of crystal fractionation and plagioclase buoyancy (Khedr et al. 2022).

## Conclusions

Textural observations and mineral chemical analyses of barren and mineralized samples from the basal breccia sequence of the Ovoid and mini-Ovoid deposits suggest that the sulfide melt was accompanied by an immiscible Fe-Ti-P melt. In these portions of the Voisey's Bay complex, the mineralization developed as a three-immiscible-melt system, in which silicate and sulfide melts coexisted with small amounts of Fe-Ti-P melt.

The existence of an Fe-Ti-P melt is inferred from the accessory mineral assemblage of sulfide-matrix breccia and sulfide-rich troctolite. In these lithotypes, distinct textural

and compositional features suggest that accessory apatite, hercynite and magnetite have a primary magmatic origin, which is further supported by the presence of hercynite and sulfide inclusions in apatite. Magmatic hercynite hosted in sulfides is typically opaque and Zn-rich, similarly to hercynite in Fe-Ti deposits formed through melt immiscibility. Abundant ilmenite in the sulfide matrix supports the presence of an Fe-Ti-P melt, as it could not have directly originated from the sulfide melt.

Despite the limited influence of Fe-Ti-P melt on the processes governing the formation of magmatic sulfide mineralization in the basal breccia sequence of the Ovoid and mini-Ovoid deposits, the presence of small amounts of Fe-Ti-P melt played a crucial role in the formation of the characteristic biotite-hornblende rims at the interface between the sulfide-matrix and the silicate framework of mineralized breccia and troctolite. The development of these replacement biotite-hornblende rims required the addition of water, Fe, Mg and K to the magmatic silicate framework. Whereas Fe and possibly water may have originated from the sulfide melt, the alkalis and at least part of the volatiles, including halogens, were sourced from the Fe-Ti-P melt, mirroring the formation of biotite-hornblende bands around massif-related Fe-Ti oxide ores.

The trace element signature and the halogen contents of apatite provide insights into the formation processes of the Fe-Ti-P melt. The combination of high LREE enrichments, high Sr/Y ratios, low REE contents and small Eu anomalies in sulfide-rich troctolite and feeder olivine gabbro suggest that the immiscible Fe-Ti-P melt formed in a deep-seated magma chamber during the early stages of development of the Voisey's Bay complex. The segregation of the Fe-Ti-P melt was likely coeval with that of the sulfide melt, as fractional crystallization and crustal assimilation played critical roles in promoting the development of both immiscible melts. The similar density and viscosity of the sulfide and Fe-Ti-P melts facilitated their joint transportation, leading to their accumulation in slow-flow sections of the conduit system. Multiple cycles of transport, deposition and re-entrainment were associated with periodic pressure drops that likely caused the release of a fluid phase which depleted the halogens contents of the Fe-Ti-P melt paired with the sulfide melt. While magmatic systems with three immiscible melts are considered relatively uncommon, recent studies have recognized a growing number of deposits with variable proportions of sulfide and Fe-Ti-P melts coexisting with a silicate melt.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00126-024-01319-7.
**Acknowledgements** We thank Steve Beresford and Peter Lightfoot for providing the samples, Louise Schoneveld for the support during LA-ICP-MS analyses, Belinda Godel for the acquisition of high-resolution X-ray computed tomography, and Michael Verrall for the assistance during the acquisition of µXRF maps. We thank Karen Kelley for their editorial handling of the manuscript, David Holwell and Eduardo Mansur for their helpful comments and feedback.

**Author contributions** Stefano Caruso (SC), Margaux Le Vaillant (MLV) and Stephen J. Barnes (SJB) conceptualized this study. SC was responsible for data acquisition and elaboration. The initial draft of the manuscript, as well the creation of figures, tables, and supplementary materials was conducted by SC. All authors contributed to the review and editing of the final version of the manuscript.

**Funding** Open access funding provided by CSIRO Library Services.

## Declarations

**Conflict of interest** The authors have no conflicts of interest to declare that are relevant to the content of this article.

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