July 2012 LIP of the Month

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Geochemical Assessment of the Metallogenic Potential of Proterozoic LIPs of Canada

Authors:
Simon M. Jowitt1* and Richard E. Ernst2

Affiliations:

  1. School of Geosciences, Monash University, Melbourne, Australia, VIC 3800
  2. Dept. of Geology, Carleton University and Ernst Geosciences, 43 Margrave Avenue, Ottawa, Ontario, Canada K1T 3Y2

 

* Corresponding Author, e-mail: simon.jowitt@monash.edu

Summarised from Jowitt, S. M., and Ernst, R. E., Geochemical assessment of the metallogenic potential of Proterozoic LIPs of Canada, Lithos, In Press, http://dx.doi.org/10.1016/j.lithos.2012.03.026.

Introduction

The association between mafic–ultramafic Large Igneous Provinces (LIPs) and magmatic Ni–Cu–PGE mineralisation is well established (e.g., Begg et al., 2010; Borisenko et al., 2006; Eckstrand and Hulbert, 2007; Naldrett, 1997, 1999, 2010; Pirajno, 2000; Schissel and Smail, 2001). However, in comparison to the Phanerozoic LIP record, the Proterozoic LIP record is poorly understood in terms of the processes that operated during LIP formation, the potential for LIPs to host hitherto unknown Ni–Cu–PGE mineralisation, and the lithogeochemical techniques and signatures that can differentiate between barren and prospective LIPs (e.g., Ernst, 2007; Zhang et al. 2008).

A number of important Ni–Cu–PGE camps and deposits are associated with LIPs, the most important of which is the world-class magmatic Ni–Cu–PGE sulphide mineralisation at Noril’sk–Talnakh, associated with the end-Permian Siberian Trap LIP (Arndt et al., 2003; Hawkesworth et al., 1995; Lightfoot and Keays, 2005; Naldrett et al., 1992). Numerous other LIP-related magmatic Ni–Cu–PGE sulphide deposits have also been discovered, including Ni–Cu–PGE mineralisation associated with the Permian Emeishan LIP (e.g., Borisenko et al., 2006) and mineralisation of the Duluth Complex associated with the 1115–1085 Ma Keweenawan LIP (e.g., Miller and Ripley, 1996). The evidence from these LIPs indicate that a number of key questions need to be determined before the Ni–Cu–PGE prospectivity of a LIP can be identified: (1) are the magmas that formed the LIPs fertile (cf. Zhang et al., 2008); (2) what processes control the formation of Ni–Cu–PGE mineralisation in LIPs and what geochemical signatures do these processes have; (3) when and where does sulphur saturation occur in an individual LIP and how can this be identified and (4) what techniques are effective in discriminating between barren and prospective LIPs and between different segments of a LIP. To address these points, here we present initial interpretations of whole-rock geochemistry from a number of LIPs within Canada, using the database of Ernst and Buchan (2010): the 1.14 Ga Abitibi, 1.87 Ga Chukotat, 0.72 Ga Franklin, 0.59 Ga Grenville, 1.27 Ga Mackenzie, 2.49–2.45 Ga Matachewan, ~1.25 Ga Seal Lake and 1.24 Ga Sudbury LIPs (Figure 1).


Figure 1 : Temporal distribution of LIP events discussed in this study; thickness of lines denotes relative duration of LIP events. Adapted from Ernst and Buchan (2010).

Geology of the Canadian LIPs Studied

The LIPs being considered here formed in differing tectonic settings and times during the geological evolution of Canada; it is important to note that the Abitibi, Grenville and Sudbury LIPs discussed here are not the same as the ca. 2.7 Ga Abitibi greenstone belt, ca. 1.0 Ga magmatism related to the Grenville orogen, nor 1.85 Ga magmatism of the Sudbury impact event.

Geological setting of Proterozoic LIPs of Canada

The oldest event considered here, the ~2445–2490 Ma Matachewan (East Bull Lake) event (Figure 2A), consists most prominently of a radiating dolerite dyke swarm covering an area of about 2.5 x 105 km2 in the southern and central Superior province, and converging to a focal point on the southeastern margin of the Superior craton (Bates and Halls,1991; Buchan and Ernst 2004; Ernst and Bleeker 2010; Ernst and Buchan, 1997; Evans and Halls, 2010; Fahrig, 1987; Phinney and Halls, 2001; West and Ernst, 1991). Associated volcanic rocks and intrusives of the East Bull Lake Intrusive Suite are distributed near this focal point (e.g., Easton, 2005; James et al., 2002; Vogel et al., 1998) that represents the mantle plume centre responsible for the event.

The 1.87 Ga Chukotat LIP event (Figure 2B) is part of the wider Circum-Superior LIP (Eckstrand and Hulbert, 2007; Heaman et al., 2009; Ernst and Bell, 2010) and is found within the Cape Smith Fold Belt in northern Quebec (Lamothe, 2007). The Cape Smith belt is dominated by three lithological units, the Chukotat and Povungnituk groups and the Lac Esker intrusive suite. The Chukotat Group is a sequence of tholeiitic–picritic basalt to komatiitic basalt flows, has a MORB-like trace element geochemistry (Modeland et al., 2003) and is thought to be the extrusive equivalent of the Lac Esker suite, a series of mafic to ultramafic sills, flows and feeder dykes emplaced within the older Povungnituk Group and along the Povungnituk–Chukotat boundary that hosts all known magmatic sulphide mineralisation in the Cape Smith belt (Jowitt et al., 2010; Lamothe, 2007; Lesher, 2007; Mungall, 2007).

The most dramatic component of the 1.27 Ga Mackenzie LIP is the giant radiating Mackenzie dyke swarm that extends away from a focal plume-related point on western Victoria Island (Baragar et al., 1996; Buchan and Ernst, 2004; Ernst and Baragar 1992; Fahrig, 1987; Fahrig and Jones, 1969; Gibson et al., 1987; LeCheminant and Heaman 1989). Comagmatic volcanic, sill and layered intrusions are distributed throughout northern Canada, the most prominent of which are the Coppermine River flood basalts and the Muskox intrusion (Baragar et al., 1996; Barnes and Francis, 1995; Day et al., 2008; Francis, 1994; Irvine, 1988; LeCheminant and Heaman, 1989; Figure 2C).

The 1.24 Ga Sudbury and 1.25 Ga Seal Lake LIP events are both located within a back-arc setting of the Grenville Orogen (e.g., Percival et al., 2004; Rivers and Corrigan, 2000; Figure 2D and Figure 3A) although a mantle plume-related origin for these events cannot be ruled out (cf. Fahrig, 1987; Ernst and Bleeker, 2010). The Seal Lake volcanics are thought to be analogous to flood basalts (e.g., Baragar, 1977) whereas the Sudbury dykes have an aulacogen-type geometry that suggests they originated from a plume centre to the southeast of the swarm (Ernst and Buchan, 1997; Fahrig, 1987).


Figure 2 : Distribution of geochemical sample sites (dots) in LIPs studied here. (A) ~2490–2450 Ma Matachewan dykes; associated mafic–ultramafic intrusives (East Bull Lake intrusive suite) and bimodal volcanics are located along the southern end of the swarm near the inferred plume centre (marked by a star). The Matachewan dykes which are superimposed on the Palaeozoic sediments of the Hudson Bay lowlands are not exposed at surface, but are imaged geophysically. Also shown is the ca. 2500 Ma Mistassini radiating swarm centred about 800 km to the northeast. (B) 1880 Ma Chukotat volcanics of the Cape Smith Belt. Base map modified after Lamothe (2007). (C) 1270 Ma Mackenzie dyke swarm. Associated volcanics are in black, and sills are in grey. Patterned areas near the plume centre (star) locate presumed mafic–ultramafic intrusions recognized on the basis of gravity anomalies. Muskox layered intrusion is in white. (D) 1240 Ma Sudbury dykes. Patterned area to the east of the Grenville Front (solid grey line) marks Sudbury dykes that were deformed during the Grenville orogeny. Solid black line at top of diagram indicates Paleozoic cover of Hudson Bay lowlands. Dashed black line in southern part marks limit of southern province. Base map in C modified after Fig. 5 in Buchan et al. (2010), and in D extracted and modified after Buchan and Ernst (2004).

The 1.14 Ga Abitibi event is widely but sparsely distributed within the central Superior craton (Ernst and Bell, 1992; Ernst and Buchan, 2004; Fahrig, 1987; Figure 3B) and is thought to be an early phase of the 1115–1085 Ma Keweenawan LIP (e.g., Heaman et al., 2007).

The 0.72 Ga Franklin event (e.g., Bédard et al., 2011, 2012; Buchan and Ernst 2004; Buchan et al., 2010; Denyszyn et al., 2009; Ernst and Buchan, 1997; Fahrig, 1987; Heaman et al., 1992; Macdonald et al., 2010; Shellnutt et al., 2004) consists of a radiating dyke swarm that is best developed on Baffin Island and other units including the Coronation sills and associated sills and Natkusiak volcanics in the Minto Inlier of Victoria Island (Figure 3C).

The middle pulse of magmatism during Iapetus Ocean formation generated the 700 km long linear 590 Ma Grenville dyke swarm; the swarm may be slightly converging to the east suggesting a potential plume centre (Davidson et al., 2009; Ernst and Buchan, 1997; Ernst and Bleeker, 2010; Kamo et al., 1995; Kumarapelli et al., 1990; McCausland et al., 2007; Puffer, 2002; Figure 3D).


Figure 3 : (A) Distribution of geochemical sample sites (dots) in LIPs studied here. (A) ca. 1250 Ma Seal Lake volcanics and sills with other units of similar ages and uncertain relationship. (B) 1140 Ma Abitibi dykes with uncertain relationship to 1115–1085 Ma Keweenawan volcanics (grey) of the Lake Superior area. (C) 725 Ma Franklin event; samples from Baffin Island (BI), Prince of Wales Island (PW) and the mainland are dykes and those from Victoria Island (VI) are sills. Associated volcanics are in black, and sills are in green. Star marks plume centre inferred from the convergence of the dykes. Greenland moved to its interpreted position prior to the opening of the Labrador Sea. (D) 590 Ma Grenville swarm which is spatially associated with the Paleozoic Ottawa–Bonnechere rift system (e.g., Bleeker et al., 2011). The Grenville swarm trends toward the region of the Tibbet Hill volcanics. Rideau River and Lac Pommeroy dykes may also be related to the Grenville LIP. Base maps A and B extracted and modified after Buchan and Ernst (2004), C after Fig. 7 in Buchan et al. (2010), and D after version provided by K. Buchan.

Ni–Cu–PGE Mineralisation associated with some of these LIPs

Several of the LIPs studied here are associated with known Ni–Cu–PGE mineralisation, the most important of which is the Chukotat event that hosts several economic Ni–Cu–PGE sulphide deposits, primarily at the Raglan mining camp (e.g., Lesher, 2007). The Matachewan LIP hosts a number of currently subeconomic mineralised bodies, including the River Valley, Falconbridge, Wisner, Drury, Agnew Lake, May and East Bull Lake intrusions, grouped as the East Bull Lake Intrusive Suite (James et al., 2002; Peck et al., 2001). The Muskox intrusion of the Mackenzie LIP hosts magmatic Ni–Cu–PGE sulphide mineralisation, although to date no economic sulphide concentrations have been discovered (Mackie et al., 2009; Naldrett, 2004). The other LIPs studied here currently have no known magmatic Ni–Cu–PGE mineralisation associated with them.

Sampling and Analysis

All geochemical data used in this paper are from Ernst and Buchan (2010) and represent a homogeneous reanalysis of samples provided from a variety of sources; details of the analytical techniques can be found in Ernst and Buchan (2010) and Jowitt and Ernst (2012). With respect to the suites of interest in the present study: the samples for the Matachewan event were previously analyzed and discussed in Phinney and Halls (2001), the Chukotat samples are from traverses of Baragar (1974) with the original geochemistry reported in Baragar (1977), and Mackenzie swarm samples were previously analyzed and reported in Baragar et al., (1996). The Sudbury samples are from Ernst (1994), Seal Lake samples were reported in Baragar (1981), and the Abitibi samples are from Ernst and Buchan (1993), with Great Abitibi dyke samples originally analyzed and reported in Ernst and Bell (1992). Franklin samples are from the collection of W.F. Fahrig at the Geological Survey of Canada (cf. Fahrig, 1987), and from A. LeCheminant (c.f., LeCheminant et al., 1996; Rainbird et al., 1997; Rainbird and LeCheminant, 2002), and Grenville samples were provided by A. Davidson (c.f. Davidson et al., 2009).

Geochemistry of the Canadian LIPs studied

General Geochemistry of Proterozoic LIPs of Canada

The majority of samples are classified as basalts on a Nb/Y vs. Zr/Ti diagram (Pearce, 1996) barring samples from the Abitibi LIP which plot within the alkali basalt field (Figure 4) and have Ti/V ratios >50 (Figure 5A), plotting close to and within the ocean island and alkali basalt field of Shervais (1982). A significant number of samples from the Sudbury and Seal Lake LIPs also plot within this higher Ti/V ratio field. Samples from the Mackenzie, Franklin and Grenville LIPs have Ti/V ratios between 20 and >50, recording a transition between the continental and alkali basalt fields of Shervais (1982), whereas the majority of samples from the Matachewan and Chukotat LIPs have Ti/V ratios <50, corresponding to the continental flood basalt field of Shervais (1982). The REE patterns of the differing LIP events also vary significantly (Figure 5B), with Abitibi samples again distinct from the other LIPs with consistently high Gd/Yb ratios and Chukotat and Matachewan samples having (Gd/Yb)PM values just slightly higher than 1, where PM denotes normalisation to primitive mantle values of McDonough and Sun (1995). Grenville, Seal Lake and Sudbury LIP samples have Gd/Yb ratios consistently higher than Chukotat and Matachewan LIP samples, whereas Franklin LIP Gd/Yb ratios vary significantly. Significant variations in (La/Sm)PM ratios are also present, from the majority of the Chukotat and Franklin samples which have La/Sm ratios close to that of the primitive mantle (McDonough and Sun, 1995) to samples within the Matachewan and Abitibi LIPs which have (La/Sm)PM values around 3–3.5.


Figure 4 : Basalt Nb/Y vs. Zr/Ti classification diagram (Pearce, 1996) showing the differing types of basalt associated with the eight Proterozoic LIPs discussed here.


Figure 5 : (A) Diagram showing variation in Ti/V ratios of samples from the eight LIPs discussed here (after Shervais, 1982); note the split between alkaline-type basalts of the Abitibi, Seal Lake and majority of the Sudbury LIPs compared to the Continental Flood Basalt (CFB)-type samples from the majority of the other LIPs. (B) Variations in REE systematics as shown by (Gd/Yb)PM and (La/Sm)PM ratios: elevated Gd/Yb ratios correspond to deeper sourcing of melt and variations in La/Sm ratios relate to either melting of an enriched source or variable crustal contamination.

A number of trends are apparent in a (Nb/Th)PM vs. (Th/Yb)PM diagram (Figure 6), corresponding with mixing lines between primitive melts and typical crustal compositions. Consistent mixing trends are apparent in the Chukotat, Grenville, Franklin, Seal Lake, Grenville and Matachewan LIPs, with weaker trends in the Abitibi and Sudbury LIP datasets, and a split into lower and higher (Th/Yb)PM value datasets within the Franklin LIP (Figure 6).


Figure 6 : Diagram showing variations in (Nb/Th)PM and (Th/Yb)PM ratios in samples from the Abitibi, Chukotat, Franklin, Grenville, Mackenzie, Matachewan, Seal Lake and Sudbury LIPs; ratios are normalised to Primitive Mantle values of McDonough and Sun (1995), N-MORB composition is from Hofmann (1988) and average Upper Continental Crust (UCC) composition is from Taylor and McLennan (1985).

On a Ni vs. MgO diagram, a significant number of samples plot along a within plate basalt trend (Keays and Lightfoot, 2007). Copper concentrations correlate with decreasing MgO within most Mackenzie and Franklin LIP samples and all Grenville samples, with little or no relationship observed for other LIPs (Figure 7B). Mackenzie, Franklin and Grenville LIP samples have the highest concentrations of Cu, whereas the Abitibi, Sudbury and Seal Lake LIPs have generally low Cu, and Chukotat and Matachewan LIPs having variable Cu concentrations. A weak correlation between decreasing MgO and increasing Pd is present within Grenville LIP samples but no other significant correlations are present (Figure 7C, D), although the Abitibi, Seal Lake and Sudbury LIPs have uniformly low Pd and Pt concentrations. In comparison, the Grenville and the majority of the Mackenzie and Franklin LIP samples have uniformly high Pd and Pt concentrations, with Matachewan and Chukotat samples again highly variable.


Figure 7 : Chalcophile element variations within the Abitibi, Chukotat, Franklin, Grenville, Mackenzie, Matachewan, Seal Lake and Sudbury LIPs showing Ni (A), Cu (B), Pt (C) and Pd (D) variations compared to MgO. Dashed line in (A) is an array of within plate basalts after Keays and Lightfoot (2007); small symbols denote samples with Pt or Pd concentrations below DL and are plotted at concentrations half of the DL values of 0.1 ppb for Pd and 0.5 ppb for Pt.

The relative abundance of chalcophile elements compared to incompatible elements also varies significantly from LIP to LIP and even within individual LIPs, with all Abitibi, Seal Lake and Sudbury samples uniformly chalcophile depleted (Figure 8A, B). Samples from the Grenville LIP are uniformly undepleted and have (Cu/Zr)PM and (Pd/Yb)PM values close to Primitive Mantle values, whereas the Matachewan, Chukotat, Mackenzie and Franklin LIPs have both chalcophile depleted and undepleted samples. The relative abundance of Cu and Pd also correlates well with increasing (Th/Yb)PM ratios in a number of the LIPs studied here (Figure 8), with some samples with the highest (Th/Yb)PM ratios having Pd concentrations below the limit of detection.


Figure 8 : Variations in (Cu/Zr)PM (A, C) and (Pd/Yb)PM (B, D) ratios with changing MgO concentration (A, B) and with increasing (Th/Yb)PM ratios (C, D) as a proxy for increasing crustal contamination. Ratios are normalised to Primitive Mantle values of McDonough and Sun (1995) and small symbols denote samples with Pt or Pd concentrations below DL and plotted at concentrations half of the DL values of 0.1 ppb for Pd and 0.5 ppb for Pt.

Discussion

Sourcing of magmas

The LIPs studied here show significant variations in terms of magma sourcing. For example, a clear split is evident in Ti/V ratios, with the Chukotat, Matachewan, Franklin, Mackenzie and Grenville LIPs having Ti/V ratios suggestive of continental flood basalt-type magmatism (Figure 5A; Shervais, 1982), whereas samples from the Abitibi, Seal Lake and Sudbury LIPs have higher Ti/V ratios indicative of OIB or alkali basalt magmatism (Figure 5A; Shervais, 1982).

The elevated (Gd/Yb)PM and Ti/V ratios of the more alkaline LIPs, in particular the Abitibi LIP, indicates derivation from deep, low degree partial melting of the mantle (Figure 5B). In comparison, samples with lower Ti/V ratios have lower (Gd/Yb)PM ratios, suggesting shallower, larger degree partial melting. A significant number of the LIPs studied here have both low and high (Gd/Yb)PM ratio samples, with Mackenzie LIP samples recording a gradual transition in (Gd/Yb)PM ratios from around 1.5 to 3, suggesting that the source depth of magmas within these LIPs varies over time, as discussed for other LIPs by White and McKenzie (1995).

The generally high (La/Sm)PM ratios of the more alkaline Abitibi and Sudbury LIPs are probably indicative of an enriched mantle source, as addition of material with higher La/Sm ratios or metasomatism of the mantle by the addition of small-degree partial melts can enrich the mantle in La relative to Sm (e.g., Sun and McDonough, 1989; Figure 5B). Assimilation of crustal material with La/Sm ratios higher than the primitive mantle can also increase (La/Sm)PM values, which is probably the case for variations in (La/Sm)PM ratio independent of (Gd/Yb)PM variations in the Chukotat and Matachewan LIPs.

Crustal Contamination

A (Nb/Th)PM vs. (Th/Yb)PM diagram (Figure 6) can be used to investigate whether magmas assimilated significant amounts of crustal material prior to emplacement and eruption. Matachewan samples lie on a mixing trend between mantle-derived melt and crustal compositions, indicating that Matachewan magmas assimilated significant amounts of crustal material prior to emplacement and eruption. Clear trends are also present within the Chukotat, Grenville and Franklin LIP data, although the amount of crustal material that these magmas assimilated was significantly less than that assimilated by Matachewan magmas. Several Chukotat and Franklin samples have higher (Th/Yb)PM ratios and do not plot along the mixing trend defined by other Matachewan, Chukotat, Grenville and Franklin samples, with all Abitibi samples and significant numbers of Mackenzie, Sudbury and Seal Lake samples plotting at higher Nb/Th ratios than the mixing line between typical mantle melt and crustal compositions. This suggests that all Abitibi, Mackenzie, Sudbury and Seal Lake magmas, and some Chukotat and Franklin magmas, may have originated from melting of mantle with higher Nb/Th ratios than the majority of the Chukotat, Grenville, Matachewan and Franklin LIPS, consistent with differences in Ti/V and (Gd/Yb)PM ratios.

A (Th/Yb)PM vs. (Nb/Yb)PM diagram can effectively discriminate between crustal contamination and variations in magma sourcing (Figure 9A), splitting LIPs with compositions controlled by crustal contamination (e.g., Matachewan) from LIPs with compositions controlled by source region differences (e.g., Abitibi, Seal Lake and Grenville), and from LIPS with compositions controlled by both effects (e.g., Mackenzie). LIPs with compositions indicative of mantle source variations (Figure 5) are more alkaline (Figure 4, Figure 5A) and have uniformly low chalcophile element concentrations (Figure 7B, C, D).


Figure 9 : A. Diagram showing variations in (Th/Yb)PM and (Nb/Yb)PM ratios; ratios are normalised to Primitive Mantle values of McDonough and Sun (1995). N-MORB composition is from Hofmann (1988), Upper Continental Crust (UCC) composition is from Taylor and McLennan (1985), and enriched mantle (EMI, EMII) and HIMU values are from Condie, 2001. B. Diagram showing variation in Cu/Pd ratio compared to Pd concentration (after Barnes et al., 1993); dashed line corresponds to a modelled magma undergoing equilibrium fractionation and removal of immiscible sulphides from a primary melt containing ~10 ppb Pd and ~100 ppm Cu. The solid lines to the right of the diagram correspond to R-factors (Campbell and Naldrett, 1979) of 100, 1,000 and 10,000 respectively; samples that plot on these lines contain sulphides that formed under these R-factor conditions. The vertically elongated rectangle at 0.25 ppb Pd includes data from 33 Abitibi, 5 Chukotat, 6 Franklin, 15 Mackenzie, 34 Matachewan, 12 Seal Lake, and 26 Sudbury LIP samples in which the Pd values were below the detection limit of 0.5 ppb Pd, but for which a value of half of this was assigned for the purposes of illustrating the significant number of samples with very low Pd concentrations. No Grenville samples had Pd concentrations below the limits of detection.

Chalcophile element ratios, enrichment and depletion

The generally low sulphide content of the samples considered here indicates that their chalcophile element chemistry is likely to be controlled by the fertility and S-saturation status of the magma they formed from. The presence of samples that plot below the continental flood basalt array in Figure 7A suggests that some of these LIPs formed from S-saturated magmas, and have lost Ni to immiscible sulphide melts, a model that is corroborated by Cu, Pd and Pt concentrations (Figure 7) and Cu/Zr and Pd/Yb ratios (Figure 8). The uniformly low chalcophile element concentrations of the more alkaline Abitibi, Seal Lake and Sudbury LIPs, and the alkaline parts of other LIPs studied here, indicate that these LIPs formed from unfertile magmas that did not sequester chalcophile elements during partial melting. The Grenville LIP is uniformly undepleted in Cu, Pd and Pt, and has Pt and Cu concentrations that correlate well with MgO (Figure 8), suggesting that Grenville magmas, although fertile, did not undergo S-saturation, and may be unprospective for Ni–Cu–PGE mineralisation.

The Matachewan, Chukotat, Mackenzie and Franklin LIP events have both chalcophile depleted and undepleted samples, suggesting that they formed from originally chalcophile-undepleted fertile magmas that lost Cu and the PGE to immiscible magmatic sulphides, and therefore implies that these LIPs may be prospective for Ni–Cu–PGE exploration. This is unsurprising, given the known mineralisation associated with the Matachewan, Chukotat and Mackenzie LIPs, but does confirm the efficacy of the approach being utilised here.

Chalcophile element depletion can also be monitored using a Cu/Pd vs Pd diagram (Figure 9B, after Barnes et al., 1993). On this diagram, a large number of samples have not lost or gained Pd preferentially to Cu and cluster around and just above the Primitive Mantle Cu/Pd ratio, indicative of S-undersaturated magmas. However, a number of samples, for example from the Chukotat and Matachewan LIPs, have lost Pd, suggesting the magmas that formed these samples underwent S-saturation and segregation of magmatic sulphides (Figure 9B). These samples plot along a equilibrium fractionation trend, indicating removal of immiscible sulphides from a primary magma containing ~10 ppb Pd and ~100 ppm Cu.

The uniformly high Cu/Pd ratios and low Pd concentrations of the Abitibi, Seal Lake and Sudbury LIP samples is indicative of unfertile magmas, whereas clustering of Grenville, Franklin and Mackenzie LIP samples at low Cu/Pd ratios and Pd concentrations between 10–40 ppb suggests that the entirety of the Grenville LIP, and parts of the Franklin and Mackenzie LIPs, formed from fertile magmas but did not undergo S-saturation. In addition, Pd depleted samples with elevated Cu/Pd ratios indicate that parts of the Mackenzie and Franklin LIPs did undergo S-saturation, suggesting that these LIPs are at least in part prospective.

Crustal contamination and assimilation of crustal sulphur are thought to be important mechanisms during magmatic sulphide deposit formation (e.g., Keays and Lightfoot, 2010; Lesher and Keays 2002; Lightfoot and Keays 2005). The data presented here suggests that prospective LIPs formed from initially S-undersaturated magmas that also assimilated significant amounts of crustal material, as shown in Figure 8C and D where samples with the lowest (Cu/Zr)PM and (Pd/Yb)PM ratios are the most crustally contaminated. This relationship between crustal contamination and chalcophile element depletion suggests that S-saturation was caused by assimilation of sulphur-rich crustal material (cf. Keays and Lightfoot, 2010).

Ni–Cu–PGE prospectivity of the Chukotat, Mackenzie, Grenville, Abitibi, Matachewan, Franklin, Sudbury and Seal Lake LIPs

Of the eight LIPs studied here, the Chukotat, Mackenzie and Matachewan LIPs have known Ni–Cu–PGE mineralisation, with the potential equivalent of the Franklin event in Siberia, the Dovyren intrusion, hosting known Ni–Cu–PGE mineralisation. These LIPs are generally characterised by basalts with Ti/V ratios <50, uniformly near-primitive mantle value Gd/Yb ratios and a distinct trend in La/Sm ratios (Figure 5B), suggesting assimilation of crustal material. These LIPs contained both chalcophile depleted and undepleted magmas (Figure 8), suggesting that fertile magmas underwent S-saturation, forming immiscible magmatic sulphides that were then presumably deposited within these LIPs. The close relationship between chalcophile element depletion and crustal contamination suggests that S-saturation was caused by assimilation of crustal material.

Of the other LIPs, only the Grenville and part of the Franklin LIPs have similar magma source characteristics to LIPs with known Ni–Cu–PGE mineralisation. Magmas from these LIPs also assimilated crustal material, although probably within less dynamic, lower energy systems. In addition, although both Grenville and Franklin LIPs contain chalcophile element undepleted samples, only a small number of Franklin and none of the Grenville LIP samples are chalcophile depleted. This suggests that although both LIPs formed from fertile S-undersaturated magmas, the Grenville LIP, at least in the areas studied here, did not undergo S-saturation prior to emplacement. In comparison, the Franklin LIP may have undergone S-saturation, suggesting that, at least in part, the Franklin LIP should be considered prospective for Ni–Cu–PGE sulphide mineralisation, as indicated by Jefferson et al., (1994).

The alkaline LIPs considered here, namely the Abitibi, Sudbury, Seal Lake and parts of the other LIPs discussed here, are characterised by high Gd/Yb ratios, wide ranges in La/Sm ratios and Ti/V ratios >50 (Figure 5) and formed from magmas generated by deep melting of variably enriched regions of the mantle. All samples from the Abitibi, Seal Lake and Sudbury LIPs are chalcophile element depleted (Figure 7, Figure 8), indicating that the magmas that formed the majority of these LIPs were low degree partial melts that left residual sulphide within the mantle during melting, suggesting that these LIPs are most likely unprospective.

 

Implications for mineral exploration

Our study has shown that lithogeochemical analysis of dyke swarms can also be an effective tool for mineral exploration. Although the volcanics and intrusives studied here are themselves unlikely to host Ni–Cu–PGE sulphide mineralisation, they are useful in terms of mineral exploration and tracking spatial and temporal LIP evolution. Even considering that the Mackenzie, Chukotat and Matachewan LIPs host known Ni–Cu–PGE mineralisation, the research presented here suggests that only certain parts of the Mackenzie LIP may be prospective for Ni–Cu–PGE exploration. In addition, further research is warranted to determine if the Grenville LIP magmas ever reached S-saturation, to identify whether fertile magmas were present at any stage of the Abitibi, Seal Lake and Sudbury LIP events, and to spatially and temporally discriminate between the fertile and unfertile, less prospective areas of the Franklin LIP.

Conclusions

Three key factors, namely magma fertility, crustal contamination and chalcophile element segregation, determine the Ni–Cu–PGE prospectivity of a LIP; these factors can be assessed using lithogeochemistry. This has been demonstrated here by comparing the lithogeochemistry of eight LIPs within Canada, three of which host Ni–Cu–PGE magmatic sulphide mineralisation, one of which is considered prospective for Ni–Cu–PGE mineralisation and four others which are mineralisation free and may be unprospective. The three prospective LIPs, the Chukotat, Mackenzie and Matachewan events, are characterised by basalts with Ti/V ratios <50, Gd/Yb ratios close to primitive mantle values and variable La/Sm ratios. These magmas assimilated crustal material in high energy and dynamic settings and both chalcophile depleted and undepleted magmas were present during LIP formation, indicating the presence of fertile magmas that assimilated crustal material and became S-saturated S-saturation, forming immiscible magmatic sulphides. The magmatic sulphides produced during this event were presumably deposited within known and currently unknown mineral deposits associated with these LIPs.

Of the other LIPs studied here, only the Grenville and part of the Franklin LIPs have similar magma source characteristics to those with known Ni–Cu–PGE mineralisation. Although magmas from both LIPs were fertile and assimilated crustal material, the Grenville LIP, at least in the areas studied here, did not undergo S-saturation before emplacement, whereas the Franklin LIP may have, indicating that determining the timing and location of S-saturation may aid Ni–Cu–PGE exploration within this LIP.

The other LIPs considered here, the Abitibi, Sudbury and Seal Lake events, and parts of the other LIPs discussed here, are alkaline and are characterised by high Gd/Yb ratios, wide ranges in La/Sm ratios and Ti/V ratios >50, suggesting that the magmas associated with these LIPs formed during partial melting at depths >90 km, potentially of regions of enriched mantle. All Abitibi, Seal Lake and Sudbury LIP samples are chalcophile element depleted, suggesting that the magmas that formed these LIPs were low degree partial melts that left residual sulphide within the mantle during melting, and implying that these LIPs may be unprospective for Ni–Cu–PGE sulphide exploration.

Although the Mackenzie, Chukotat and Matachewan LIPs have known Ni–Cu–PGE mineralisation, only part, or only a temporally constrained period, of the Mackenzie LIP event may be prospective for Ni–Cu–PGE mineralisation. Further research is needed to determine the S-saturation status of all of the Grenville LIP magmas, to determine whether any fertile magmas were involved during the formation of the Abitibi, Seal Lake and Sudbury LIPs, and to determine which areas of the Franklin LIP are prospective and which are unprospective for Ni–Cu–PGE mineralisation.

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