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Article

Fertility of Gabbroic Intrusions in the Paleoproterozoic Lynn Lake Greenstone Belt, Manitoba, Canada: Insights from Field Relationships, Geochemical and Metallogenic Characteristics

by
Xue-Ming Yang
Manitoba Geological Survey, 360-1395 Ellice Avenue, Winnipeg, MB R3G 3P2, Canada
Minerals 2025, 15(5), 448; https://doi.org/10.3390/min15050448 (registering DOI)
Submission received: 27 March 2025 / Revised: 21 April 2025 / Accepted: 25 April 2025 / Published: 26 April 2025
(This article belongs to the Special Issue Novel Methods and Applications for Mineral Exploration, Volume III)
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Abstract

:
Magmatic nickel–copper–platinum group element (PGE) deposits hosted in mafic–ultramafic intrusions within volcanic arc systems are highly attractive targets for mineral exploration, yet their genesis remains poorly understood. This study investigates metagabbroic intrusions in the Paleoproterozoic Lynn Lake greenstone belt of the Trans-Hudson Orogen to identify the key factors, in the original gabbros, that control the formation of magmatic Ni-Cu-PGE deposits in volcanic arc systems. By examining the field relationships, geochemical and sulfur and oxygen stable isotope compositions, mineralogy, and structural fabrics, this study aims to explain why some intrusions host mineralization (e.g., Lynn Lake and Fraser Lake intrusions), whereas others remain barren (e.g., Ralph Lake, Cartwright Lake, and Snake Lake intrusions). Although both the fertile and barren gabbroic, likewise original, intrusions exhibit metaluminous, tholeiitic to calc-alkaline affinity with volcanic arc geochemical signatures, they differ significantly in shape, ranging from vertical and tube-like to tabular forms, reflecting distinct geological settings and magma dynamics. The gabbroic rocks of fertile intrusions exhibit erratic trace element profiles, lower (Nb/Th)N and higher (Cu/Zr)N ratios, as well as a larger range of δ34S values than those in barren intrusions. Key factors influencing Ni-Cu-PGE mineralization include the degree of partial melting of the mantle, early sulfide segregation, and crustal contamination, particularly from volcanogenic massive sulfide deposits. These processes likely triggered sulfide saturation in the mafic magmas. Geochemical proxies, such as PGE concentrations and sulfur and oxygen stable isotopes, provide critical insights into these controlling factors. The results of this study enhance our understanding of the metallogenic processes responsible for the formation of magmatic Ni-Cu-PGE deposits in the gabbroic intrusions emplaced in an extensional setting due to slab rollback, during the geological evolution of the Lynn Lake greenstone belt, offering valuable guidance for mineral exploration efforts.

1. Introduction

Major-to-giant nickel–copper–platinum group element (Ni-Cu-PGE) ore deposits are genetically associated with mafic–ultramafic mineral (magmatic) systems derived from the mantle and emplaced into extensional settings at continental margins [1,2,3,4,5,6,7,8]. However, the nature of magmatic Ni-Cu-PGE deposits, with relatively small tonnage and a high grade in orogenic belts, is less well-understood. Although these deposits are enigmatic in origin, they have recently become more important and attractive exploration targets because of the high demand for critical metals and a foreseeable supply shortage [9,10,11,12]. The Lynn Lake magmatic Ni-Cu deposit [13] is hosted in the Lynn Lake gabbroic, likewise original, intrusion. It occurs together with many other mafic–ultramafic intrusions intruding the ca. 1892 to 1986 Ma Wasekwan group supracrustal rocks of the Lynn Lake greenstone belt in the Paleoproterozoic Trans-Hudson Orogen [14,15,16,17,18,19,20,21,22,23] (Figure 1), one of the largest orogenic belts in Earth history [24,25,26,27,28]. This provides an excellent opportunity to investigate the metallogeny of magmatic Ni-Cu mineral deposits in orogenic belts.
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Figure 1. Schematic map of the Paleoproterozoic Trans-Hudson Orogen in Manitoba and Saskatchewan (Canada), showing major lithotectonic domains of the Reindeer zone of the Trans-Hudson Orogen, Archean cratons (Superior, Sask, and Hearne) and Paleoproterozoic cover (modified from [26]). Dashed box shows Figure 2 location.
Figure 1. Schematic map of the Paleoproterozoic Trans-Hudson Orogen in Manitoba and Saskatchewan (Canada), showing major lithotectonic domains of the Reindeer zone of the Trans-Hudson Orogen, Archean cratons (Superior, Sask, and Hearne) and Paleoproterozoic cover (modified from [26]). Dashed box shows Figure 2 location.
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This study focuses on a detailed investigation of original gabbroic intrusions in the Lynn Lake greenstone belt to evaluate the critical controls in regard to magmatic Ni-Cu-PGE mineralization via detailed bedrock geology mapping, structural and magnetic susceptibility (MS) measurements of outcrops, and geochemical analysis of representative bulk rock samples, together with a review of geochemical, sulfur, and oxygen stable isotope data previously released by the Manitoba Geological Survey [29,30,31]. The results from this study indicate that metaluminous, tholeiitic to calc-alkaline mantle-derived mafic magmas may have been contaminated largely by crustal materials, which may have triggered their sulfide saturation and segregation in the magmas, as well as Ni-Cu mineralization, mostly at the base of the mafic–ultramafic intrusions. Structural analysis of the deformed gabbroic intrusions can help target Ni-Cu mineralization through the reconstruction of their original configuration.

2. Regional Geology

The Lynn Lake greenstone belt (LLGB), an important element of the Paleoproterozoic Trans-Hudson Orogen [24,25,26,27,32,33], is endowed with several styles of mineralization, such as volcanogenic massive sulfide (VMS), zinc–copper (Zn-Cu), magmatic nickel–copper–cobalt (Ni-Cu-Co), and orogenic gold (Au; Figure 2). The LLGB is bounded to the north by the southern Indian domain and flanked to the south by the Kisseynew domain [14,26,34]. It is composed primarily of two east- to northeast-trending, steeply dipping belts (Figure 2), consisting of various supracrustal rocks of the Wasekwan group that were intruded by diverse granitoid plutons from the 1.892–1.870 Ga Pool Lake intrusive suite [35,36,37,38]. Younger, molasse-type sedimentary rocks of the Sickle group (>1.836 Ga; [21]) unconformably overlie the Wasekwan group and the Pool Lake intrusive suite. Based on their crosscutting relationships with the Sickle group, the granitoid intrusions in the LLGB are divided into pre- and post-Sickle intrusions. The supracrustal rocks and the granitoid rocks experienced peak amphibolite facies metamorphism at ca. 1.81–1.80 Ga [21,22]. Gabbroic rocks in the LLGB are mostly pre-Sickle intrusions and were metamorphosed to amphibolites or metagabbros in terms of their mineral assemblage, such as the original pyroxene that was replaced by metamorphic green hornblende. The prefix ‘meta’ in the rock names, for e.g., metagabbro and metabasalt, as described in this study, is omitted for brevity, following [14,15,17,18].
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Figure 2. Regional geology, with U-Pb zircon ages and neodymium isotopic composition of the Lynn Lake greenstone belt (modified and compiled from [14,16,17,18,21,23,31,36,37,38,39,40,41,42,43,44]). The localities of gabbroic intrusions sampled in this study are shown. The Ni-Cu deposit is hosted in the Lynn Lake gabbroic intrusion that comprises of the A and EL plugs. The location of Figure 3 is indicated by the red box. Abbreviations: CLI, Cartwright Lake intrusion; FLGC, Fraser Lake gabbro complex; GLI, Gemmell Lake intrusion; MLP, Motriuk Lake plug; MORB, mid-ocean ridge basalt; RLI, Ralph Lake intrusion; SCLI, Southern Cockeram Lake intrusion; SLGI, Snake Lake gabbro intrusion; WOLI, White Owl Lake intrusion.
Figure 2. Regional geology, with U-Pb zircon ages and neodymium isotopic composition of the Lynn Lake greenstone belt (modified and compiled from [14,16,17,18,21,23,31,36,37,38,39,40,41,42,43,44]). The localities of gabbroic intrusions sampled in this study are shown. The Ni-Cu deposit is hosted in the Lynn Lake gabbroic intrusion that comprises of the A and EL plugs. The location of Figure 3 is indicated by the red box. Abbreviations: CLI, Cartwright Lake intrusion; FLGC, Fraser Lake gabbro complex; GLI, Gemmell Lake intrusion; MLP, Motriuk Lake plug; MORB, mid-ocean ridge basalt; RLI, Ralph Lake intrusion; SCLI, Southern Cockeram Lake intrusion; SLGI, Snake Lake gabbro intrusion; WOLI, White Owl Lake intrusion.
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Significant differences in the geology and geochemistry of the northern and southern belts of the LLGB may reflect regional differences in tectonic settings that were obscured by the structural transposition of multiple deformation events (D1 to D6; [19,36,37,38,42,45]). The northern and southern belts contain disparate volcanic assemblages that were later structurally juxtaposed, likely representing a tectonic collage [17] formed by northward subduction, followed by contraction and underthrusting of the Kisseynew domain beneath the LLGB during terminal collision [46]. Field observations and historical drilling records indicate the presence of disseminated massive sulfide minerals (e.g., sphalerite, chalcopyrite, pyrrhotite) in Wasekwan supracrustal rocks, including the Fox VMS Zn-Cu ore deposit (Figure 2) [14,18,23,47,48,49].

3. Geological Settings

Bedrock geological mapping reveals that various gabbroic intrusions occur in both the northern and southern belts of the LLGB, including the Lynn Lake (A plug, EL plug), Fraser Lake, Ralph Lake, Cartwright Lake, Motriuk Lake, White Owl Lake, Gemmell Lake, Southern Cockeram Lake, and Snake Lake areas (Figure 2). It is noted that the Motriuk Lake plug comprises very coarse-grained pyroxenite [50], which displays a cumulative texture and contains abnormally high Cr values (up to 2180 ppm; Supplementary Material Table S1), and that the Cartwright Lake intrusion has the highest MS values (80 × 10−3 to 150 × 10−3 SI) registered in the gabbroic rocks of the LLGB, although there is no notable Ni-Cu mineralization evident in its exposures. These gabbroic intrusions are all ascribed to the pre-Sickle intrusive suite, and detailed descriptions of the gabbroic rocks can be found in [18,23,43,44,50,51,52,53]. Bulk rock geochemical data of representative samples for these intrusions are tabulated in Table S1 (for details, see Section 4 Methods, below).
The Lynn Lake and Fraser Lake gabbroic intrusions are the focus of the following sections as they host magmatic Ni-Cu ore deposits (Figure 3). The other intrusions are barren, although some of their outcrops show evidence of trace sulfide disseminations.
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Figure 3. Geology of gabbroic intrusions in the Lynn Lake area (modified from [14,18,51,52]), showing geological relationships between the Lynn Lake A plug and EL plug, and the Fraser Lake gabbro complex, associated with magmatic nickel–copper–cobalt mineralization. Abbreviations: FLGC, Fraser Lake gabbro complex; RLI, Ralph Lake intrusion.
Figure 3. Geology of gabbroic intrusions in the Lynn Lake area (modified from [14,18,51,52]), showing geological relationships between the Lynn Lake A plug and EL plug, and the Fraser Lake gabbro complex, associated with magmatic nickel–copper–cobalt mineralization. Abbreviations: FLGC, Fraser Lake gabbro complex; RLI, Ralph Lake intrusion.
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3.1. Lynn Lake Gabbroic Intrusion

The Lynn Lake gabbroic intrusion [14,18,53,54] comprises two separate plugs exposed on the surface: A plug (~4 km2) and EL plug (~0.12 km2; Figure 3). The A plug contains approximately 28.4 Mt of ore grading 0.91% Ni and 0.49% Cu, with an average Ni/Cu ratio of 1.86, and the EL plug has 1.9 Mt of ore grading 2.07% Ni and 0.76% Cu, with a Ni/Cu ratio of 2.72 [13]. About 16.3 Mt of ore grading 0.77% Ni and 0.33% Cu was produced during the period from 1953 to 1976; a significant amount of Ni-Cu ore remains unmined below ground, where high concentrations of Co were also intercepted during recent diamond drilling by Corazon Mining Limited (https://corazon.com.au) (accessed on 26 May 2023).

3.1.1. A Plug

The Lynn Lake A plug is emplaced into the Wasekwan group volcanic to volcaniclastic rocks and appears to be a vertical tube-like intrusion, with fabrics generally parallel to the regional structural trend [13,18,53,55]. The inclusions of the country rocks are variable in regard to their lithologic composition, size and shape; some of the xenoliths display diffusive edges, indicative of assimilation by intruding mafic magmas (Figure 4a). Contact metamorphism aureoles are not well-developed, likely due to a relatively smaller difference in temperature between the supracrustal succession and the intrusion. The A plug is composed primarily of medium-grained gabbro (that was metamorphosed to amphibolite) associated with minor peridotite, norite, mottled gabbro, and diorite to quartz diorite. The gabbro is composed of 40% to 45% plagioclase (An60–75) laths, from 2 to 5 mm long, 55% to 60% greenish amphibole (pseudomorphs after clinopyroxene), minor iron-oxide minerals, and trace-disseminated pyrrhotite and chalcopyrite. The gabbro displays weak to strong foliation manifested by the preferred orientation of amphibole and plagioclase. The foliation is generally subvertical, in parallel to the regionally dominant D2 structures [35,36,37,38]. The A plug gabbros display a consistent range of magnetic susceptibility (MS) values (0.23 × 10−3–0.954 × 10−3 SI); mineralized zones are characterized by slightly lower MS values. Magmatic fractionation led to the formation of very coarse-grained to pegmatitic leucogabbro to anorthositic gabbro as in situ patches and/or pods in the intrusion (Figure 4b).
Minor peridotite and norite were reported as cumulate phases [13] and were not exposed on the surface. More ultramafic phases (e.g., peridotite) are noted to occur mostly in the western section of the A plug, as shown in underground working, wherein they are associated with most of the Ni-Cu sulfide mineralization [13,55].
Late diabase dikes of about 10 to 50 cm in width, trending north–northeast and west–northwest, cut the A plug gabbro intrusion. The diabase dikes have higher MS values (1.38 × 10−3 to 1.808 × 10−3 SI) than the A plug gabbros. This confirms that both magnetic high and low domains are present in the A plug, as demonstrated by detailed magnetic survey data collected by Corazon Mining Limited (https://corazon.com.au) (accessed on 26 May 2023).
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Figure 4. Field photographs of gabbroic rocks in the Lynn Lake area, showing: (a) subrounded to angular xenoliths of basaltic to felsic rocks and sedimentary rocks trapped in the A plug (375972E, 6302850N; note that coordinates of locations were recorded using a GPS device), with some xenoliths showing diffusive edges, which is a solid piece of evidence for assimilation by intruding mafic magmas; (b) very coarse-grained to pegmatitic leucogabbro to anorthositic gabbro pod and/or patch in the A plug (375959E, 6302859N); (c) primary layering in the EL plug (375836E, 6299203N); (d) fine-grained mafic (diabase) dike cutting the EL plug gabbro (375836E, 6299203N); (e) angular aphanitic basalt xenolith in the Fraser Lake gabbro complex (372429E, 6297114N); and (f) medium-grained granite dike cutting the Fraser Lake gabbro complex (372461E, 6297269N). All coordinates for sample locations are in the system of the UTM NAD83 Zone 14.
Figure 4. Field photographs of gabbroic rocks in the Lynn Lake area, showing: (a) subrounded to angular xenoliths of basaltic to felsic rocks and sedimentary rocks trapped in the A plug (375972E, 6302850N; note that coordinates of locations were recorded using a GPS device), with some xenoliths showing diffusive edges, which is a solid piece of evidence for assimilation by intruding mafic magmas; (b) very coarse-grained to pegmatitic leucogabbro to anorthositic gabbro pod and/or patch in the A plug (375959E, 6302859N); (c) primary layering in the EL plug (375836E, 6299203N); (d) fine-grained mafic (diabase) dike cutting the EL plug gabbro (375836E, 6299203N); (e) angular aphanitic basalt xenolith in the Fraser Lake gabbro complex (372429E, 6297114N); and (f) medium-grained granite dike cutting the Fraser Lake gabbro complex (372461E, 6297269N). All coordinates for sample locations are in the system of the UTM NAD83 Zone 14.
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3.1.2. EL Plug

The EL plug, dated at 1871.3 ± 2.4 Ma [40], was emplaced into the ca. 1892–1886 Ma Lynn Lake rhyolite of the Wasekwan group [13,14,36,55]. There is no sign of a chilled margin on the contact of the EL plug with the Wasekwan rhyolitic volcanic to volcaniclastic rocks, although xenoliths of felsic to mafic volcanic rocks are evident. This plug shows a cored pipe structure, narrowing from a surface diameter of about 500 m to a diameter of 200 m at depth [13].
The EL plug consists mainly of medium-grained gabbro with minor diorite and peridotite, with an outer margin of contact diorite and gabbro, and an inner core of gabbro (and/or amphibolite) and peridotite [13,55]. The EL gabbro consists of 40 to 45% plagioclase (An65–80) laths, from 3 to 5 mm long, 60 to 75% greenish amphibole (pseudomorphs after clinopyroxene), minor iron-oxide minerals, and trace-disseminated pyrrhotite, locally with a pentlandite rim, and chalcopyrite. Locally, primary layering is well-preserved, manifested by alternating plagioclase- and amphibole-rich layering (Figure 4c). As with the A plug, late mafic, west–northwest-trending (diabase) dikes cut the EL plug (Figure 4d); these are likely post-mineralization dikes [13]. Notably, there is no remarkable difference in the MS values between the dikes (0.613 × 10−3 to 0.725 × 10−3 SI) and EL gabbros (0.418 × 10−3 to 0.704 × 10−3 SI).
High-grade mineralization consists mainly of sulfide breccia-type ore in the core of the EL plug, from the surface to about 300 m in depth; however, low-grade disseminated ore extends to about 925 m in depth from the surface [13]. The primary sulfide minerals are pyrrhotite, pentlandite, and chalcopyrite, with lesser to trace amounts of pyrite and sphalerite.

3.2. Fraser Lake Gabbro Complex

The Fraser Lake gabbro complex (FLGC; [56]), dated at 1870 ± 6.2 Ma [40], is a tabular intrusion of about 9.2 km2 in area [14,54]. The FLGC intrudes the Wasekwan group supracrustal rocks and has been cut by large granitoid plutons (Figure 3). Field observations revealed the presence of diverse volcaniclastic to basaltic xenoliths in the FLGC (Figure 4e), which is cut by granite dikes (Figure 4f). At contact zones, both volcanic and gabbroic inclusions are evident in the granitoid plutons southeast of Frances Lake and southwest of Flag Lake (Figure 3).
The FLGC is composed mostly of medium-grained gabbro with minor peridotite and amphibole (pseudomorphs of clinopyroxene)-phyric fine-grained gabbro. The gabbro appears homogeneous in terms of its mineralogy, ~55 to 70% amphibole and ~30 to 45% plagioclase (An55–70), and its textures in most outcrops. It is metamorphosed to amphibolite facies, resulting in amphibole replacing primary pyroxene, despite the preservation of its magmatic texture. Green fibrous amphibole (actinolite–tremolite) occurs mostly along foliation planes and is likely attributed to retrograde metamorphism in greenschist facies conditions. Minor amounts of finely disseminated sulfide minerals (e.g., pyrrhotite, chalcopyrite) and magnetite occur in gabbroic outcrops, but no major accumulation of sulfide is evident in most exposures (e.g., [54,55,56]; this study). Recently, Corazon Mining Limited in Perth, Australia (https://corazon.com.au) (accessed on 26 May 2023) reported an intersection, measuring 55.4 m, of sulfide (pyrrhotite–pentlandite–chalcopyrite) mineralization, including meter-scale intervals of massive sulfide intermixed with semi-massive to disseminated sulfides, within the FLGC.
The magnetic susceptibility values of the FLGC gabbros range from 0.547 × 10−3 to 8.22 × 10−3 SI, despite the presence of magnetic low and high domains within the intrusion, which are higher than most of the surrounding granitoid rocks (mostly <0.5 × 10−3 SI), as shown in an aeromagnetic survey image produced by Corazon Mining Limited (https://corazon.com.au) (accessed on 26 May 2023).

4. Methods

In the field, gabbro rock exposures were mapped and sampled to document the field relationships, textures (fabrics), mineral assemblages, magnetic susceptibility (MS), and the presence of mineralization and/or alteration. A Terraplus KT-10 MS meter (maximum sensitivity 0.001 × 10−3 SI units) was used with a pin to measure the MS values of natural outcrops or exposures. Each outcrop was measured at least five times, and the average of these measurements was recorded as the outcrop’s MS value. Outcrops were sampled using a rock saw or sledgehammer for subsequent petrographic and geochemical laboratory studies. Sample locations were recorded using a GPS device (Garmin, Taibei, China) (see Supplementary Materials Table S1). The MS values were used to assess the relative oxidation and reduction state of the rocks.
The major and trace element contents of the samples were analyzed using the 4-lithoresearch package at Activation Laboratories Ltd. (Actlabs) in Ancaster, Ontario, Canada. The major elements (i.e., SiO2, TiO2, Al2O3, Fe2O3T, MnO, MgO, CaO, Na2O, K2O, P2O5) and some trace elements (i.e., Sc, Be, V, Ba, Sr) were analyzed using ICP-OES (inductively coupled plasma–optical emission spectrometry). The loss on ignition (LOI) values were determined by the gravitational difference. The other elements (i.e., Cr, Co, Ni, Cu, Zn, Ga, As, Mo, Ag, Sn, Pb, Ge, Rb, Y, Zr, Nb, In, Sb, Cs, Hf, Ta, W, Tl, Bi, Th, U, Se, Te) and rare earth elements were analyzed using ICP-MS (inductively coupled plasma–mass spectrometry). Precious metals (i.e., Pd, Pt, Au) were measured using the fire assay (FA) technique, plus ICP-MS. The detailed analytical procedures and detection limits of the methods, such as ICP-OES, ICP-MS, and FA + ICP-MS, are available on the Actlabs website (www.actlabs.com) (accessed on 30 September 2023). An in-house diabase standard was analyzed multiple times at Actlabs to monitor data quality. The reproducibility of the major element analyses was generally better than 5%, and that of trace element analyses was generally better than 10% of the measured amount.

5. Geochemical Characteristics

The whole-rock geochemical data of the gabbroic rocks acquired in this study are presented in Table S1, together with previously published geochemical, sulfur (S), and oxygen (O) stable isotope data of mafic–ultramafic rocks in the LLGB [17,29,30,31], which are reviewed and investigated to extract some critical information about the metallogeny of the gabbroic intrusions. This section uses a set of diagrams (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11) to illustrate discernable features that can be used for fingerprinting magmatic sources and processes, as well as recognizing Ni-Cu mineralization in mafic–ultramafic mineral systems.

5.1. Classification

On the total alkalis versus silica (TAS) diagram [57], gabbroic rocks display a range of lithologies, mostly from gabbroic diorite to diorite, even to monzonite and granodiorite, plotted on an anhydrous basis, indicative of more evolved features (Figure 5a). The Lynn Lake gabbro intrusion shows a relatively smaller range in the TAS space than the Fraser Lake gabbro complex, although both display a positive correlation between the alkalis (Na2O + K2O) and SiO2 contents, suggesting that the magmatic fractionation of the intra-intrusions played a role in their formation. Geochemically, the gabbroic samples from both the fertile and barren intrusions are dominantly tholeiitic (or calcic), although some are calc-alkalic (calc-alkaline) (Figure 5a,b), based on the Rittmann Serial Index (σ) [58] and AFM diagram (Figure 5b), according to [59].
They are exclusively metaluminous, based on the Shand index [60,61], manifested by A/NK > 1.0 and A/CNK < 1.0 [A/NK = molar Al2O3/(Na2O + K2O); A/CNK = molar Al2O3/(CaO + Na–O + K–O)] (see Table S1). The Lynn Lake intrusion (A plug and EL plug) hosts a Ni-Cu deposit showing a differentiation trend, which consists mainly of clinopyroxene gabbronorite, whereas the Fraser Lake gabbro complex, with its significant Ni-Cu-Co mineralization, is composed mainly of clinopyroxene leucogabbronorite (Figure 5c), based on the classification in [62,63], using cation normative compositions of plagioclase, orthopyroxene, and clinopyroxene.
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Figure 5. Geochemical classification of gabbroic rocks (Table S1) from the Lynn Lake greenstone belt: (a) total alkalis (Na2O + K2O) versus SiO2 diagram (after [57]), plotted on an anhydrous basis, the red dashed curve is the division between alkaline and subalkaline volcanic series as defined empirically by [59], the black dashed lines labeled σ values indicate calcic (σ ≤ 1.2), calc-alkalic (1.2 < σ < 3.5), alkali-calcic (3.5 < σ < 8.8), and alkalic (8.8 ≤ σ) series, based on [58], σ denotes the Rittmann Serial index, and σ = (Na2O + K2O)2/(SiO2–43), units in wt% [58]; (b) AFM diagram (after [59]), A = (Na2O + K2O), F = FeO*, and M = MgO, units in wt%; (c) cation normative Cpx−Opx−Pl diagram (after [63]); and (d) Zr/Ti versus Nb/Y diagram (after [64]). Abbreviations: Cpx, clinopyroxene; Opx, orthopyroxene; Pl, plagioclase.
Figure 5. Geochemical classification of gabbroic rocks (Table S1) from the Lynn Lake greenstone belt: (a) total alkalis (Na2O + K2O) versus SiO2 diagram (after [57]), plotted on an anhydrous basis, the red dashed curve is the division between alkaline and subalkaline volcanic series as defined empirically by [59], the black dashed lines labeled σ values indicate calcic (σ ≤ 1.2), calc-alkalic (1.2 < σ < 3.5), alkali-calcic (3.5 < σ < 8.8), and alkalic (8.8 ≤ σ) series, based on [58], σ denotes the Rittmann Serial index, and σ = (Na2O + K2O)2/(SiO2–43), units in wt% [58]; (b) AFM diagram (after [59]), A = (Na2O + K2O), F = FeO*, and M = MgO, units in wt%; (c) cation normative Cpx−Opx−Pl diagram (after [63]); and (d) Zr/Ti versus Nb/Y diagram (after [64]). Abbreviations: Cpx, clinopyroxene; Opx, orthopyroxene; Pl, plagioclase.
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On the Zr/Ti versus Nb/Y classification diagram (Figure 5d), based on [64], the gabbroic rocks fall mostly into the gabbro field, although a few samples from the Lynn Lake intrusion and the FLGC shift into the diorite field. The gabbroic rocks, regardless of their respective fertility, have Nb/Y ratios less than 0.8, typical of subalkaline affinity [64,65].

5.2. Trace and Rare Earth Element Patterns

The gabbroic samples show typical volcanic arc signatures, as manifested by enrichment in large-ion lithophile elements (e.g., Cs, Rb, Ba, K) relative to high-field-strength elements (e.g., Nb, Ta) in primitive mantle normalized trace element profiles (Figure 6a), similar to the Wasekwan group volcanic rocks and pre-Sickle granitoid rocks [65,66] in regard to the trace element patterns, but lower than the trace element abundances. Most samples from fertile intrusions (e.g., Lynn Lake, Fraser Lake) display pronounced positive Pb anomalies, although most barren intrusions (e.g., Gemmell Lake, White Owl Lake) lack such features. Such positive Pb anomalies are typical of arc rocks, strongly reflecting the presence of an arc component in the mantle source. Alternatively, they are likely related to crustal contamination [2,53].
Chondrite-normalized rare earth element (REE) patterns show that fertile intrusions (e.g., Lynn Lake) display relatively low REE abundances and a flat pattern with slightly light REE enrichment, whereas barren intrusions (e.g., Ralph Lake) have relatively higher REE concentrations and more LREE-enriched patterns (Figure 6b). The gabbroic rocks in the Lynn Lake intrusion have chondrite-normalized (La/Yb)N ratios of 1.2 to 4.7 (except pegmatitic leucogabbro having a ratio of 8.8; Table S1), and (Gd/Yb)N ratios of 0.9 to 2.2, without notable Eu anomalies indicated by Eu/Eu* rations of 0.8 to 1.2. (it is noted that three gabbroic samples exhibit Eu/Eu* ratios > 1.3, consistent with the presence of cumulative plagioclase). In contrast, the barren gabbroic rocks in the Ralph Lake intrusion show higher (La/Yb)N ratios of 2.1 to 12.2 and (Gd/Yb)N ratios of 1.4 to 2.4, without Eu anomalies suggested by Eu/Eu* ratios of 0.9 to 1.1. The FLGC gabbroic rocks display fractionation of LREE and HREE between the Lynn Lake intrusion and the Ralph Lake intrusion.
The niobium/thorium (Nb/Th) ratios of mafic–ultramafic rocks are sensitive to crustal contamination, whereas copper/zirconium (Cu/Zr) ratios are sensitive to the state of sulfur saturation in the mafic melts [2,53]. Figure 7 shows variation in the primitive mantle-normalized (Cu/Zr)PM and (Nb/Th)PM ratios of the studied gabbroic intrusions, suggesting that crustal contamination may have played an important role in triggering sulfide saturation in the mafic melts, regardless of whether it is mineralized or barren (Figure 7a,b). It is noted that sample 111-15-129A01 has an unusually high (Nb/Th)PM ratio of 5.7, likely reflecting the accumulation of Nb-bearing phases, such as ilmenite and/or magnetite (Figure 7a). A mineralized gabbro (sample 111-15-134A01) has a (Cu/Zr)PM ratio > 437 and a (Nb/Th)PM ratio of 0.75, simply because its Cu content is higher than 10,000 ppm (Table S1), and is plotted on the diagram using the analytical maximum value to illustrate the remarkable effect of sulfide accumulation (Figure 7a). Furthermore, the presence of high Zn contents in the Lynn Lake Ni-Cu deposits [29,30] suggests that VMS deposits associated with the Wasekwan group volcanic and sedimentary succession assimilated by intruding mafic–ultramafic magmas may have played an important role in providing the external sulfur required for their sulfide saturation, segregation, and mineralization [2,5].
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Figure 6. Primitive mantle-normalized extended trace element patterns (a) and chondrite-normalized rare earth element patterns (b) of gabbroic samples selected in terms of the lowest SiO2 content from each intrusion (Table S1) in the Lynn Lake greenstone belt. Normalizing values from [67].
Figure 6. Primitive mantle-normalized extended trace element patterns (a) and chondrite-normalized rare earth element patterns (b) of gabbroic samples selected in terms of the lowest SiO2 content from each intrusion (Table S1) in the Lynn Lake greenstone belt. Normalizing values from [67].
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Figure 7. Plots of (Cu/Zr)PM versus (Nb/Th)PM in: (a) barren and fertile gabbroic rocks; and (b) sulfide-bearing gabbros in the Lynn Lake greenstone belt. Data plotted in (a) from Table S1, and data in (b) from [29]. Primitive mantle-normalized (PM) values from [68].
Figure 7. Plots of (Cu/Zr)PM versus (Nb/Th)PM in: (a) barren and fertile gabbroic rocks; and (b) sulfide-bearing gabbros in the Lynn Lake greenstone belt. Data plotted in (a) from Table S1, and data in (b) from [29]. Primitive mantle-normalized (PM) values from [68].
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The Lynn Lake intrusion and the Fraser Lake gabbro complex samples (n = 78; [29]) display high sulfur/selenium (S/Se) ratios, which are mostly higher than the mantle value (3000 ± 200; see [69]). This further confirms the effect of crustal contamination on these intrusions.
Metal tenors (i.e., metal contents in 100% sulfide) are calculated for 262 samples in the dataset in [30], with the assumption that the metals are hosted in pyrrhotite, pentlandite, and chalcopyrite [5]. The calculated metal tenors display a positive correlation between platinum, palladium, and gold (Pt + Pd + Au) in ppb versus (Pt + Pd) in ppm/(Cu + Ni) in terms of wt% ratios, with a correlation coefficient (r) of 0.58 (n = 262). They show that barren gabbro intrusions (e.g., Cartwright Lake) appear to have higher (Pt + Pd)/(Cu + Ni) ratios and metal tenors (Figure 8). This indicates that the sulfide (melt) in the Cartwright Lake intrusion may have been in equilibrium with a larger volume of mafic melts (i.e., higher R-factor), possibly without extensive early sulfide segregation. In contrast, mineralized gabbro intrusions (e.g., the Lynn Lake A plug and EL plug) show relatively lower metal tenors and (Pt + Pd)/(Cu + Ni) ratios, likely reflecting either sulfide segregation from a smaller volume of mafic (silicate) melts (e.g., [4]) or earlier sulfide segregation involving a dynamic magmatic system.
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Figure 8. Diagram of metal tenors (Pt + Pd + Au) in ppb versus (Pt + Pd) in ppm/(Cu + Ni) in wt% on the basis of 100% sulfide for gabbro intrusions in the Lynn Lake greenstone belt. Data from [30]. Calculation of metal tenors based on the formula in [5]. Abbreviation: n, number.
Figure 8. Diagram of metal tenors (Pt + Pd + Au) in ppb versus (Pt + Pd) in ppm/(Cu + Ni) in wt% on the basis of 100% sulfide for gabbro intrusions in the Lynn Lake greenstone belt. Data from [30]. Calculation of metal tenors based on the formula in [5]. Abbreviation: n, number.
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Most of the Lynn Lake intrusion samples display (Pt + Pd)/(Cu + Ni) ratios < 0.1, whereas the FLGC complex and the Cartwright Lake intrusion samples have ratios > 0.1 (Figure 7) and higher metal tenors of (Pt + Pd), which suggests that there might not have been a significant segregation of sulfide melt from the mafic melts. On the other hand, a substantial amount of sulfide melts may have separated from the Lynn Lake mafic melts and concentrated at the base of the intrusion, or the exit of the conduit/pathway connected to the emplacement sites.

5.3. Highly Siderophile Elements (HSEs)

Highly siderophile elements (HSEs) include PGEs (Os, Ir, Ru, Pt, Rh, Pd), as well as Re and Au [69]. PGEs can be divided further into two subgroups: the iridium subgroup (IPGEs: Os, Ir, Ru) and the palladium subgroup (PPGEs: Pt, Rh, Pd). Not only do HSEs show strong affinity for a metallic iron phase, but they are also strongly chalcophile and, thus, concentrate in a sulfide phase, when lacking the metallic iron phase in mafic–ultramafic systems [70]. Therefore, HSEs together with chalcophile element (Ni, Cu) whole-rock data are commonly used to investigate the fractionation and the state of sulfide saturation of mafic–ultramafic magmas and associated ore systems [4,5,69,70,71,72].
Seventy-eight bulk rock and/or ore samples [29] were analyzed for HSEs, which are evaluated and used to characterize the PGE geochemistry of the Lynn Lake and Fraser Lake gabbroic intrusions. Based on the S contents, these samples can be subdivided into two subgroups: unmineralized (S ≤ 1 wt%) and mineralized (S > 1 wt%). Supplementary material Table S2 presents a summary of the data, revealing that the mineralized samples are relatively depleted in regard to their total ΣPGE concentrations (ranging from 27.2 to 222.1 ppb) and enriched in terms of their Co contents (i.e., averaging 855 ppm in the A plug and 410 ppm in the EL plug), when comparing them with most major and giant magmatic Ni-Cu-PGE mineral deposits elsewhere [4]. However, the unmineralized samples have abnormal ΣPGE contents from 9.4 to 53 ppb.
The HSEs with Ni and Cu data for the gabbroic rocks and associated ores [29] in the LLGB are plotted in order of their compatibility during partial melting of the mantle in Figure 9. The diagram shows that the contents of less HSE-compatible elements (i.e., Au, Re, Pd) are higher and decrease in the direction of more compatible elements (i.e., Ru, Ir, Os), which suggests that the HSEs are controlled strongly by the partial melting of the mantle. The most compatible element, Ni, and least compatible element, Cu, both of which are attributed to the chalcophile affinity, show much higher concentrations than the HSEs that had previously partitioned into the iron–nickel (Fe-Ni) core during differentiation from the mantle [5] and that were subsequently controlled by sulfide–silicate melt separation. The Ni-Cu deposits formed by mantle-derived magmas tend to be depleted in regard to PGEs relative to Ni and Cu, thus demonstrating a trough-shaped pattern in the primitive mantle-normalized profile, with typical positive steep HSE patterns of the gabbroic hosts (Figure 9).
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Figure 9. Diagram of primitive mantle-normalized highly siderophile elements, together with contents of strongly chalcophile elements, Ni, Re, and Cu, for mineralized and unmineralized gabbroic rocks in the Lynn Lake Ni-Cu deposits and the Fraser Lake gabbro complex. From left to right, the elements are arranged in order of decreasing mantle compatibility (after [70]). Data summarized in Table S2 from [29]. Normalized values of primitive mantle from [73].
Figure 9. Diagram of primitive mantle-normalized highly siderophile elements, together with contents of strongly chalcophile elements, Ni, Re, and Cu, for mineralized and unmineralized gabbroic rocks in the Lynn Lake Ni-Cu deposits and the Fraser Lake gabbro complex. From left to right, the elements are arranged in order of decreasing mantle compatibility (after [70]). Data summarized in Table S2 from [29]. Normalized values of primitive mantle from [73].
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According to [70], the primitive mantle-normalized ratio of (Pd/Ir)PM in mafic–ultramafic rock(s) can be used as a measure of the degree of fractionation during the partial melting of the mantle, reflecting the fractionation between Pd (more incompatible) and Ir (more compatible) in PGEs. Most samples from the Lynn Lake intrusion and FLGC display enrichment of PPGEs over IPGEs indicated by (Pd/Ir)PM > 1.0, which points to the lack of a monosulfide solid solution in the residues during partial melting of the mantle. In this context, a general positive correlation between Re/Os and (Pd/Ir)PM ratios, with a large range from 1 to 200, evident in the Lynn Lake intrusion (A and EL plugs), indicates that the separation of the monosulfide solid solution plays a key role [69] in fractionating PPGEs from IPGEs in residual sulfide melts that formed Ni-Cu mineralization.

5.4. Platinum Group Elements (PGEs)

Most of the samples with low S contents from the database [29] were reanalyzed in regard to a suite of elements (such as Pd, Pt, Au, S, Ni, Cu, Zn, Se, Te) by [30]. This review of the dataset confirms that the PGE content of the Lynn Lake samples is much lower than that of major-to-giant magmatic Ni-Cu-PGE deposits elsewhere [4,5]. For instance, the average ΣPGE content of 22 samples in the Lynn Lake A plug deposit is 222.1 ppb (Table S2), which is equivalent to 7.2 times the content of primitive mantle [73]. As a comparison, the average ΣPGE contents of those major-to-giant deposits, for e.g., as detailed in [4], are 100 to 1000 times greater than that of primitive mantle.
Figure 10a shows that the Lynn Lake intrusion (A and EL plugs) is compositionally different from the FLGC, based on Σ(PGE + Au) concentrations and (PPGE/IPGE) ratios. The Lynn Lake intrusion displays relatively high Σ(PGE + Au) values and less fractionation of PPGEs from IPGEs, which suggests that it may have been derived from a higher degree of partial melting of the mantle or formed by earlier phases of fractionating mantle-derived magmas. On the other hand, the FLGC contains lower Σ(PGE + Au) contents (<80 ppb; Figure 10b), with higher fractionation of PPGEs from IPGEs, likely reflecting an origin based on a lower degree of partial melting of the mantle or derived from late phases of fractionating mantle-derived magmas. To determine which process was involved requires more precise age data, because of the lack of field relationships for these two intrusions. Because their ages are identical within analytical uncertainties, the difference in timing between emplacement of the Lynn Lake intrusion (EL plug 1871 ± 2.4 Ma) and that of the Fraser Lake (1870 ± 6.2 Ma) intrusion could not be distinguished given the available age data [40].
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Figure 10. Discrimination diagram of geochemical data from the Lynn Lake and Fraser Lake gabbroic intrusions in the Lynn Lake greenstone belt, showing: (a) Σ(PGE + Au) (ppb) contents versus (IPGE/PPGE) ratios and three dashed lines labelling a slope equal to 1.0, 0.2, and 0.01, from left to right, indicating that fractionation between PPGE and IPGE decreases with increasing Σ(PGE + Au) concentrations; (b) Σ(PGE + Au) (ppb) versus MgO (wt. %). Data from [29]. Values on primitive mantle from [73], with a (IPGE/PPGE) ratio of 1.1, Σ(PGE + Au) content of 32.4 ppb, and MgO of 36.77 wt%. Abbreviations: PGE, platinum group element; IPGE, iridium–platinum group element subgroup (Os, Ir, Ru); PPGE, palladium–platinum group element subgroup (Pt, Rh, Pd).
Figure 10. Discrimination diagram of geochemical data from the Lynn Lake and Fraser Lake gabbroic intrusions in the Lynn Lake greenstone belt, showing: (a) Σ(PGE + Au) (ppb) contents versus (IPGE/PPGE) ratios and three dashed lines labelling a slope equal to 1.0, 0.2, and 0.01, from left to right, indicating that fractionation between PPGE and IPGE decreases with increasing Σ(PGE + Au) concentrations; (b) Σ(PGE + Au) (ppb) versus MgO (wt. %). Data from [29]. Values on primitive mantle from [73], with a (IPGE/PPGE) ratio of 1.1, Σ(PGE + Au) content of 32.4 ppb, and MgO of 36.77 wt%. Abbreviations: PGE, platinum group element; IPGE, iridium–platinum group element subgroup (Os, Ir, Ru); PPGE, palladium–platinum group element subgroup (Pt, Rh, Pd).
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Using MgO (wt%) as a proxy for the fractionation degree of mafic magmas versus the Σ(PGE + Au) values reveals that the gabbroic samples of the Lynn Lake intrusion (A plug and EL plug) plot along a different trajectory from those of the FLGC, which are characterized by relatively lower concentrations of precious metals (<80 ppb) and lower MgO contents (Figure 10b). With an increasing degree of fractionation, Σ(PGE + Au) values appear to elevate in the Lynn Lake intrusion, whereas such values decline in the FLGC. This strongly suggests that different petrogenetic processes may have formed these two intrusions. Based on magnesium oxide (MgO) and Al2O3 contents, the FLGC is more evolved in regard to its chemical composition than the Lynn Lake intrusion (A and EL plugs; Figure 10b). This observation is also supported by the fact that the FLGC contains relatively lower Ni and Co contents than the Lynn Lake intrusion (Table S1).

5.5. Sulfur and Oxygen Stable Isotope Compositions

The sulfur stable isotope composition (δ34S) of 53 bulk sulfide samples reported in [29] are plotted in Figure 11a to show the isotopic variation in three gabbroic intrusions of the LLGB, i.e., the Lynn Lake (A and EL plugs), Cartwright Lake, and Fraser Lake intrusions. The distribution of the δ34S ratios in these intrusions are skewed toward relatively lighter sulfur isotope values, with a mean value of 2.53 ± 2.65‰. Most samples from the Lynn Lake intrusion (i.e., A and EL plugs) and the Cartwright Lake gabbro intrusion display a very narrow range of δ34S ratios, from 0.0 to 3.0‰, similar to typical mantle-derived mafic rocks, for e.g., as reported in [70,71]. The Fraser Lake gabbro complex (FLGC) shows a larger range of δ34S values, from 1.0 to 13.2‰, although mainly falling between 1.0 and 6.0‰. The δ34S values indicate that the gabbros in the FLGC may have been affected by marine sedimentary rocks rich in heavy sulfur isotopes (up to 20‰) or contaminated by felsic rocks, e.g., as detailed in [70,71,74,75,76,77]. High Zn contents in mineralized gabbroic samples and massive sulfide ores (Tables S1 and S2; [29,30]) from the FLGC imply that the complex may have been contaminated (through assimilation) by VMS in the Wasekwan group.
The oxygen stable isotope composition (δ18O) of 41 bulk rock samples [29] also show a skewed distribution of the δ18O values (Figure 11b), with a mean value of 6.9 ± 1.2‰. Interestingly, the FLGC demonstrates a much smaller range of δ18O ratios (5.5 to 7.0‰) than those of the Lynn Lake intrusion (A plug and EL plug), mostly within the mantle value (5.7 ± 0.2‰; [70,71,76]), which is consistent with a mantle-derived origin. In contrast, samples from the Lynn Lake A plug and EL plug show a large range of δ18O values (5.5 to 10.0‰), which suggests that they may have interacted with low-temperature fluids, given that their δ34S remains unaffected. These observations are consistent with the conclusion arrived at by [13], whereby hydrothermal activity may have locally remobilized sulfide ore and formed secondary sulfide veinlets present in faults and/or fractures.
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Figure 11. Histogram of (a) sulfur isotope composition (‰) and (b) oxygen isotope composition (‰) of bulk sulfide samples from three gabbro intrusions in the Lynn Lake greenstone belt (data from [29]). Abbreviations: n, number of samples; SD, standard deviation.
Figure 11. Histogram of (a) sulfur isotope composition (‰) and (b) oxygen isotope composition (‰) of bulk sulfide samples from three gabbro intrusions in the Lynn Lake greenstone belt (data from [29]). Abbreviations: n, number of samples; SD, standard deviation.
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6. Discussion

Gabbroic intrusions in the Lynn Lake greenstone belt are evaluated for their fertility in terms of their geological setting, field relationships, variation in lithologies and fabrics, and geochemical, as well as S and O stable isotope, features. Most gabbro intrusions display magmatic arc signatures, but only some host magmatic Ni-Cu deposits. The major controls in regard to magmatic Ni-Cu mineralization are discussed below, including the magma source, magmatic fractionation, crustal contamination and assimilation, and tectonics.

6.1. Mantle Source

Constraints in terms of mineral assemblages, geochemical compositions (Tables S1 and S2, Figure 7, Figure 8, Figure 9 and Figure 10), and sulfur and oxygen stable isotopes (Figure 11) suggest that the gabbroic magmas in the Lynn Lake greenstone belt were derived from partial melting of a mantle source. Although the degree of melting may have varied, subsequent magma fractionation is evident across different intrusions. For instance, the fertile Lake Lynn intrusion exhibits a relatively lower degree of partial melting and less fractionation compared to the Fraser Lake gabbroic complex (FLGC), as indicated by their HSE profiles (Figure 9) and PPGE/IPGE ratios (Figure 10a). Furthermore, these intrusions display distinctly different fractionation trends, strongly suggesting that they may have formed as a result of separate petrogenetic processes.
Additionally, three gabbroic samples exhibit high positive initial εNd(t) values ranging from +3.5 to +3.8 (X.M. Yang, unpublished data), indicating that the mafic magmas were derived from a depleted mantle source [65,66,70,71].

6.2. Magmatic Fractionation

Major element compositions indicate that the gabbroic rocks in the Lynn Lake intrusion, which host a Ni-Cu deposit, are predominantly tholeiitic (or calcic) in terms of their magmatic affinity (Figure 5a,b). These rocks evolve along a trend of FeO enrichment (Figure 5b), a pattern characteristic of fertile mafic intrusions [2,3,4,5,78]. In contrast, barren mafic intrusions, such as the Ralph Lake intrusion, are calc-alkaline to tholeiitic and do not exhibit FeO enrichment. The FLGC appears to be intermediate between these two, spanning a broader range on the TAS diagram (Figure 5a) and evolving toward more leucogabbroic and felsic compositions (Figure 5c). With decreasing MgO contents, precious metals (ΣPGE + Au) in the Lynn Lake intrusion and the FLGC exhibit distinct trajectories of fractionation. This suggests that accumulative phases (e.g., olivine) are not likely involved before sulfide segregation from silicate melts in the Lynn Lake intrusion under dynamic environment conditions. In contrast, mafic-phase crystallization occurs simultaneously with sulfide separation from the magmas in the FLGC.
Assuming other physicochemical conditions (e.g., pressure and temperature) remain constant, sulfur solubility in felsic magmas is significantly lower than in mafic melts [e.g., 1,5,78]. Consequently, magmatic fractionation is one of the key factors that can trigger sulfide saturation in mafic systems [1,2,5]. This process may operate for the FLGC, but does not seem to work for the Lynn intrusion, indicated by its more restricted compositions in terms of silica and alkalis. A few gabbroic samples from the Lynn Lake intrusion and FLGC exhibit higher Zr/Ti ratios, but similar Nb/Y ratios, compared to the barren intrusions (Figure 5c). The high Zr/Ti ratio suggests that the mafic magmas forming the fertile intrusions were relatively hotter [64,65,66] and capable of dissolving more sulfur [5,78].

6.3. Crustal Contamination

The geochemical characteristics of trace elements in the gabbroic rocks, such as the low (Nb/Th)N ratios (Figure 7), high Zn concentrations of the ores (Tables S1 and S2; [29,30]), and erratic distributions of trace elements with pronounced positive Pb anomalies (Figure 6), point to the possibility that fertile gabbroic intrusions may have been strongly contaminated by crustal rocks. A large variation in the δ34S values of the Fraser Lake gabbro complex, in particular, points to the effect of crustal contamination. Such crustal contamination of the mantle-derived mafic magmas appears to have triggered sulfide saturation and segregation in the magmas and mineralization systems. Sulfide-bearing Wasekwan sediments and VMS deposits, in particular, are potential candidates for providing external sulfur to the gabbroic intrusions. This process is critical for magmatic Ni-Cu mineralization, as sulfide solubility in magma decreases with increasing pressure [78,79]. In other words, as magma ascends from its deep mantle source to the emplacement site, its sulfide solubility increases. Consequently, an external source of sulfur is required for the magma to achieve sulfur saturation and form a sulfide melt [2,3,4,5,8].
However, assimilation from the Wasekwan felsic volcanic to volcaniclastic rocks (e.g., Lynn Lake rhyolite) likely facilitates sulfide saturation in the intruding magmas. Cumulative mafic phases at contact zones seem to have higher potential to host Ni-Cu sulfide mineralization. The PGE discrimination diagram (Figure 10) presented in this study demonstrates the difference between the Lynn Lake intrusion (A plug and EL plug) and the Fraser Lake gabbro complex, probably indicative of two distinct mineralizing systems.

6.4. Tectonic and Structural Controls

Gabbroic rocks in the Lynn Lake intrusion host a Ni-Cu deposit and the Fraser Lake gabbro complex, with significant Ni-Cu-Co mineralization, are synchronous with the 1872.6 ± 2.5 Ma A-type granites in the northern belt of the Lynn Lake greenstone belt [65], which suggests that these mafic and granite intrusions may have been emplaced into an intra-oceanic arc extensional setting, although part of the Wasekwan mafic volcanic rocks may have formed in a rifted continental margin [19]. Such an arc association at the Lynn Lake Ni-Cu deposit is underreported in the literature. There is likely a broader implication in regard to how granitoid geochemistry [65,66] can track geodynamic transition(s) linked to the origin of VMS Cu-Zn, magmatic Ni-Cu, and orogenic Au deposits in the Lynn Lake greenstone belt of the Paleoproterozoic Trans-Hudson Orogen and elsewhere in other orogenic belts.
The Wasekwan group and the intruded gabbro intrusions underwent six phases of deformation (D1 to D6), with D2 being the most dominant [14,15,18,36,37,42,44]. This phase led to the development of upright folds, subvertical foliations, and related structures, such as dextral shear zones (Figure 2 and Figure 3; [36,37]). The tube- and funnel-shaped orebodies within the Lynn Lake intrusion (e.g., A Plug and EL Plug) were significantly influenced by D2 deformation, which reoriented their originally horizontal geometries into subvertical orientations. As a result, structural analysis is critical for reconstructing the three-dimensional geometry of these mafic–ultramafic intrusions and their associated orebodies. Additionally, localized remobilization and redistribution of sulfide ores due to deformation and metamorphism are also likely (e.g., the EL plug; see [13]).

7. Conclusions

The fertility of gabbroic intrusions in the Paleoproterozoic Lynn Lake greenstone belt is primarily controlled by the degree of mantle partial melting, early sulfide segregation, and crustal contamination, particularly from VMS (volcanogenic massive sulfide) deposits, which likely promoted sulfide saturation and Ni-Cu mineralization. More primitive, mantle-derived mafic magmas appear to have a higher potential for magmatic Ni-Cu mineralization. Intra-arc extension due to slab rollback during the geological evolution of the Lynn Lake greenstone belt may have triggered mantle partial melting, generating tholeiitic mafic magmas that intruded through conduits (deep faults) into the Wasekwan group volcano-sedimentary sequence. Subsequent assimilation and contamination led to sulfide saturation, segregation, and accumulation at the base of the intrusions. Given the strong influence of structural deformation on the gabbroic intrusions, a structural analysis has significant implications for magmatic Ni-Cu exploration in orogenic belts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15050448/s1, Table S1: Major (wt.%), trace element (ppm) and precious metal (ppb) geochemical compositions of gabbroic rocks from the Lynn Lake greenstone belt; Table S2: Average bulk rock and ore geochemical compositions in the Lynn Lake and Fraser Lake gabbro intrusions (data from [29]).

Funding

This research was funded by Manitoba Geological Survey.

Data Availability Statement

Data reported in this study are presented in Supplementary Materials Tables S1 and S2, which are available on the website of Minerals https://www.mdpi.com/article/10.3390/min15050448/s1.

Acknowledgments

I would like to express my gratitude to Paul Alexandre, Guest Editor, for inviting this study to be part of the Special Issue of Minerals, entitled “Novel Methods and Applications for Mineral Exploration, Volume III”. The author thanks W. Ezeana for providing enthusiastic field and laboratory assistance; C. Epp and P. Belanger for their thorough logistical support and processing of the samples; and H. Adediran and G. Keller for their technical support. Thanks go to C.J.M. Lawley and M.G. Houlé of the Geological Survey of Canada for their collaboration and engaging discussions. An earlier version of the manuscript benefited greatly from constructive reviews by M. Rinne, T. Kennedy, T, Martins, M.-F. Dufour, and C. Steffano. Corazon Mining Limited is acknowledged for allowing field crew access to its property and drillcore; L. Hulbert and K. Wells are thanked in particular for guidance in regard to core sampling; and Alamos Gold Inc. for providing their support. The author is grateful to three anonymous journal reviewers for their constructive comments and suggestions, which have significantly helped to improve the presentation of the manuscript.

Conflicts of Interest

The author declares that there are no conflicts of interest.

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Yang, X.-M. Fertility of Gabbroic Intrusions in the Paleoproterozoic Lynn Lake Greenstone Belt, Manitoba, Canada: Insights from Field Relationships, Geochemical and Metallogenic Characteristics. Minerals 2025, 15, 448. https://doi.org/10.3390/min15050448

AMA Style

Yang X-M. Fertility of Gabbroic Intrusions in the Paleoproterozoic Lynn Lake Greenstone Belt, Manitoba, Canada: Insights from Field Relationships, Geochemical and Metallogenic Characteristics. Minerals. 2025; 15(5):448. https://doi.org/10.3390/min15050448

Chicago/Turabian Style

Yang, Xue-Ming. 2025. "Fertility of Gabbroic Intrusions in the Paleoproterozoic Lynn Lake Greenstone Belt, Manitoba, Canada: Insights from Field Relationships, Geochemical and Metallogenic Characteristics" Minerals 15, no. 5: 448. https://doi.org/10.3390/min15050448

APA Style

Yang, X.-M. (2025). Fertility of Gabbroic Intrusions in the Paleoproterozoic Lynn Lake Greenstone Belt, Manitoba, Canada: Insights from Field Relationships, Geochemical and Metallogenic Characteristics. Minerals, 15(5), 448. https://doi.org/10.3390/min15050448

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