Characterizing the supra- and subsolidus processes that generated the Current PGE-Cu-Ni deposit, Thunder Bay North Intrusive Complex, Canada: insights from trace elements and multiple S isotopes of sulfides

Abstract

The Current deposit is hosted by serpentinized peridotite that intruded rocks of the Quetico Subprovince in the Midcontinent Rift, and is subdivided into three morphologically distinct regions - the shallow and thin Current-Bridge Zone in the northwest, the deep and thick 437-Southeast Anomaly (SEA) Zone in the southeast, and the thick Beaver-Cloud Zone in the middle. The magma parental to the Current deposit became saturated in sulfide as a result of the addition of external S from at least two sources - a deep source characterized by high Δ³³S (< 3‰) values, and a shallow source, potentially the Archean metasedimentary country rocks, characterized by low Δ³³S (< 0.3‰). Variations in Δ³³S-S/Se-Cu/Pd values indicate that the contamination signatures were largely destroyed by interaction of the sulfide liquid with large volumes of uncontaminated silicate melt. The intrusion crystallized sequentially, with the Current-Bridge Zone crystallizing first, followed by the Beaver-Cloud Zone, and lastly by the 437-SEA Zone. This, along with the elevated Cu/Pd ratios in the 437-SEA Zone, which formed as a result of sulfide segregation during an earlier saturation event, and development of igneous layering in this zone, suggests that it represents the feeder channel to the Current deposit. After the intrusion crystallized, the base-metal sulfide mineralogy was modified by circulation of late-stage hydrothermal fluids, with pyrrhotite and pentlandite being replaced by pyrite and millerite, respectively. This fluid activity mobilized metals and semi-metals, including Fe, Ni, S, Se, Co, Cu, Ag, and As, but did not affect the PGE. This contribution highlights the importance of the interplay between magma dynamics and magmatic-hydrothermal processes in the formation of Ni-Cu-PGE-mineralized deposits.

Supplementary information: The online version contains supplementary material available at 10.1007/s00126-023-01193-9.

Keywords: Base-metal sulfide chemistry; Current deposit; Ni–Cu–PGE; S isotopes.

Fig. 1

(A) Simplified geologic map of the North American Midcontinent Rift illustrating the distribution of rock types and highlighting the location of several Ni–Cu–PGE-mineralized intrusions and complexes, including the Thunder Bay North Intrusive Complex (modified from Good et al. 2015). (B) Simplified geologic map showing the locations of mafic–ultramafic intrusions of the Thunder Bay North Intrusive Complex, including the Ni–Cu–PGE-mineralized Current and Escape intrusions (modified from Thomas et al. 2011)

Fig. 2

(A) Simplified geologic map illustrating the morphology of the Current intrusion (grey) crosscutting the Archean metasedimentary rocks of the Quetico Subprovince (south of Quetico Fault Zone) and Archean granite (north of the Quetico Fault Zone) in plan view, highlighting the relative locations of the five mineralized zones and the Southeast Anomaly (modified from Chaffee 2015). The yellow circles represent the locations of drill holes from which samples were characterized in this study. The grey dashed lines are the UTM locations where the intrusion has been subdivided into the Current–Bridge, Beaver–Cloud, and 437–SEA zones. (B) Schematic cross section of the Current intrusion illustrating the change in morphology of the conduit with depth and the relative location of sulfide mineralization in the five zones (modified from Thomas et al. 2011)

Fig. 3

Schematic cross sections of the (A) Current Zone, (B) Bridge Zone, (C) Beaver Zone, and (D) Southeast Anomaly illustrating the relationships of the main rock units in the conduit, the change in conduit morphology from northwest to southeast along the intrusion, and the change in location of mineralization that accompanies this change in morphology (modified from Thomas et al. ; Bleeker et al. 2020)

Fig. 4

Binary diagrams illustrating the variation in bulk-rock (A) Cu–S, (B) Pd–S, (C) Pd–Pt, and (D) Pd–Ir. The red arrow highlights data that exhibits a positive trend on these diagrams, whereas the green field highlights data that falls off this trend. Dashed lines represent constant Pd/Pt and Pd/Ir ratios. DL = detection limits

Fig. 5

Images of drill core samples (A–D) and reflected–light photomicrographs (E–N) illustrating representative examples of base-metal sulfides and textures in the Current deposit. (A) Mingling between the mafic Current magma and a felsic melt. Note the occurrence of base-metal sulfides where the two magmas mingled. (B) Disseminated, (C) net-textured, and (D) blebby base-metal sulfide mineralization. (E) An equilibrium (magmatic) assemblage comprising pyrrhotite–pentlandite–chalcopyrite–cubanite. (F) Cross-polarized, reflected-light photomicrograph illustrating pyrrhotite occurring as a single crystal and as an aggregate of multiple crystals. Note the 120° dihedral angles between crystals in the latter. (G) Pyrite partially replaced by pyrrhotite. (H) Pyrrhotite partially replaced by pyrite. (I) An assemblage comprising chalcopyrite–pentlandite–pyrite. (J) Chalcopyrite restricted to an alteration patch and physically associated with pyrite. (K) An assemblage of chalcopyrite–pentlandite–pyrite in which the pyrite was partially replaced by pentlandite. (L) An assemblage of chalcopyrite–millerite–pyrite. (M) An assemblage comprising largely chalcopyrite–pentlandite, with violarite occurring along fractures in pentlandite. (N) An assemblage of chalcopyrite–pyrite–gersdorffite. Po = pyrrhotite, Pn = pentlandite, Ccp = chalcopyrite, Cbn = cubanite, Py = pyrite, Mill = millerite, Viol = violarite, Gdf = gersdorffite

Fig. 6

Binary diagrams illustrating the variation in abundances of (A) chalcopyrite–pyrrhotite, (B) pentlandite–pyrrhotite, (C) chalcopyrite–pyrite, (D) pyrrhotite–pyrite, (E) pentlandite–millerite obtained by mineral liberation analysis. Note the strong negative, non-linear correlation between the abundances of pyrrhotite–pyrite and pentlandite–millerite. The grey squares are data from an unpublished metallurgical study by Clean Air Metals Inc

Fig. 7

Binary diagrams illustrating the variation in metal (A–H) and semi-metal (I–L) concentrations in base-metal sulfides as a function of Co concentration. All concentrations are in ppm

Fig. 8

Binary diagrams illustrating the variation in bulk-rock Cu/Pd and sulfide S/Se in (A) the primary chalcopyrite–cubanite–pyrrhotite–pentlandite assemblage and (B) the secondary pyrite–millerite assemblage. The red, dashed field outlines the range of bulk-rock Cu/Pd and S/Se values. The purple and pink dashed fields represents the bulk-rock Cu/Pd ratio of Archean granitic and metasedimentary country rock and S/Se of its pyrite (n_Py = 2 and n_Py = 4, respectively); data for metasedimentary pyrite is from Caglioti (2023). The mantle ranges for Cu/Pd (1,000–10,000) and S/Se (2,632–4,350) are from Barnes et al. (1993, 2015b), and Eckstrand and Hulbert (1987) and Palme and O’Neil (2014), respectively

Fig. 9

Binary diagrams illustrating the variation in (A) Δ³³S–δ³⁴S and (B) Δ³³S–Δ³⁶S of base-metal sulfides. The mantle range for Δ³³S is from Farquhar (2002) and Bekker et al. (2009), and for δ³⁴S is from Lesher and Burnham (2001) and Ripley and Li (2003). The mantle ranges for Cu/Pd (1,000–10,000) and S/Se (2,632–4,350) are the same as in Fig. 8. The pink, dashed field highlights the composition of pyrite from Quetico metasedimentary rocks from Caglioti (2023). Error bars for S isotopes are 2σ

Fig. 10

Binary diagrams illustrating the modeled variations in (A) S/Se–bulk-rock Cu/Pd, (B) Δ³³S–bulk-rock Cu/Pd, and (C) Δ³³S–S/Se as a function of variable R factor and contamination by rocks with different Δ³³S values (colored, solid lines). The green, dashed line represents modeled compositional variations as a function of sulfide liquid removal. The colored fields represent the S/Se ratio of primary chalcopyrite–cubanite–pyrrhotite–pentlandite in the different mineralized zones and bulk-rock Cu/Pd. References for the mantle ranges are the same as those in Fig. 9. The purple and pink dashed fields represents the bulk-rock Cu/Pd ratio of Archean granitic and metasedimentary country rock and S/Se of its pyrite; data for metasedimentary pyrite is from Caglioti (2023). The numbers on the model curves represent R factor and the degree of sulfide liquid removal

Fig. 11

Binary diagram illustrating the variation in bulk-rock Cu/Pd and Pd in the Current deposit. The colored fields represent the distribution of data for the Current–Bridge and Beaver–Cloud zones, whereas the data points represent the 437–SEA Zone. The solid curves represent modeled variations as a function of R factor and sulfide abundance. The dashed curve represents modeled variations as a function of sulfide liquid removal. The Cu–Pd contents of the starting melt and the sulfide liquid–silicate melt partition coefficients are provided in Table 1. The numbers on the curve represent sulfide percent, either as the amount present in the rock (in the case of the R factor models) or the amount of sulfide liquid removed (in the case of the sulfide segregation model)

Fig. 12

Two-stage schematic model illustrating the processes that led to the formation of sulfide liquid and its subsequent enrichment in metals in the Current deposit — (A) high-energy stage and (B) waning stage of magmatic activity