Abstract

Hyper-enriched black shale (HEBS) Ni-Mo-Zn-Pt-Pd-Au-Re mineralization is geographically widespread across the Richardson trough in northern Yukon (Canada), where it discontinuously outcrops at the regional contact between the Road River Group and overlying Canol Formation. Stratigraphic relationships indicate that the contact is Middle Devonian, but there are no precise age constraints for the HEBS. We apply Re-Os geochronology to HEBS mineralization from two localities that are 130 km apart, the Nick prospect and the Peel River showing, to date directly the age of sulfide mineralization. The Nick prospect yields an isochron age of 390.7 ± 5.1 (2σ) Ma, whereas the Peel River showing yields an isochron age of 387.5 ± 4.4 (2σ) Ma. Within error, these ages are identical and overlap with the biostratigraphically constrained age of the sedimentary host rocks, indicating that mineralization and sedimentation were coeval. Significantly, the ages of the HEBS overlap those of Middle Devonian Kačák, pumilio, and Taghanic global-scale biotic events which are characterized by eustatic sea-level rise and black shale deposition. Linkage of the Yukon HEBS to one (or more) of these bio-events indicates that sea-level rise may have been requisite to formation of basin-scale HEBS mineralization in northwestern Canada during latest Eifelian and Givetian time.

INTRODUCTION

Hyper-enriched black shales (HEBS), or polymetallic shales, are an important global resource for Zn, Ni, Cu, Mo, Se, U, V, ± Cr, Co, Ag, Au, platinum group elements (PGEs), and rare earth elements (REEs) (Jowitt and Keays, 2011; Johnson et al., 2017). HEBS deposits are thin (<3–10 cm) and laterally extensive (Lehmann et al., 2007; Goodfellow et al., 2010; Johnson et al., 2017). The Nick prospect, one of at least four HEBS localities in Yukon, Canada (Fig. 1; Gadd and Peter, 2018), is the best-known HEBS in Canada and contains an average of 5.3 wt% Ni, 0.72 wt% Zn, 2400 ppm Mo, 670 ppb PGEs, 90 ppb Au, and 24.5 ppm Re (Hulbert et al., 1992).

The HEBS at each of the two localities in this study is conspicuously situated at a regional stratigraphic contact between the Road River Group and the Canol Formation (Fig. 2A). Recent work has shown that the ambient paleoenvironment in which HEBS formed was redox stratified (Gadd and Peter, 2018; Crawford et al., 2019; Gadd et al., 2019a) and that pyrite is the mineralogical host of economically important metals (Gadd et al., 2019b); however, the age of mineralization with respect to host rock deposition is not known. Precise age constraints are required for mineralization to test the favored model that elemental enrichments in HEBS originate from ambient seawater (Gadd and Peter, 2018; Crawford et al., 2019; Gadd et al., 2019a, 2019b), and they also can provide a clearer understanding of the broader environmental parameters that may have facilitated mineralization. An early Re-Os geochronologic study on HEBS mineralization at the Nick prospect (Horan et al., 1994) determined a Devonian age for the HEBS, which only broadly agreed with established stratigraphic relationships. Since then, sulfide Re-Os geochronology has been refined and is suited for dating HEBS mineralization (Mao et al., 2002; Xu et al., 2011; Fu et al., 2016) and other sediment-hosted base-metal sulfide deposits (Pašava et al., 2010; Hnatyshin et al., 2015; Kelley et al., 2017).

Our study aims to resolve the age of the HEBS mineralizing event(s) in northern Yukon by applying Re-Os geochronology to HEBS from the Nick prospect and the Peel River showing (Fig. 1). These data are supplemented with conodont biostratigraphic ages that further develop the chronological framework in the context of global episodic changes in the Devonian marine environment. The combined age data allow us to assess the possible role of global-scale eustatic and biotic events that punctuated the Devonian, and may have been climacteric in forming HEBS mineralization along the western Laurentian continental margin.

REGIONAL GEOLOGY

The northern Yukon HEBS showings are located within Paleozoic basinal strata of the Richardson trough (Fig. 1). This tectonic depression has been interpreted as a failed rift (Pugh, 1983; Norris, 1985). It is bounded to the east by the Mackenzie platform and to the west by the Yukon Stable Block; the latter a stable, offshore shelf during the pre-Carboniferous Paleozoic (Fig. 1; Morrow, 1999). Sedimentary strata within the Richardson trough are characterized by >1000 m of fine-grained, carbonaceous siliciclastic rocks of the Upper Cambrian to Middle Devonian Road River Group (Figs. 1 and 2A; Morrow 1999). The Canol Formation overlies the Road River Group and consists of as much as 220 m of Middle Devonian to lower Upper Devonian cherty, carbonaceous shale (Fraser and Hutchison, 2017). It is at this contact that a thin (1–10-cm-thick) HEBS layer is documented in several localities throughout northern Yukon. These HEBS layers consist of ∼60 vol% sulfides and 40 vol% non-sulfides (Hulbert et al., 1992; Gadd et al., 2019b), and sulfides are interbedded with black, siliceous shales. Samples typically contain a number of sedimentary features that include laminar bedding disrupted by soft-sediment deformation (Figs. DR1A and DR2A in the GSA Data Repository1). The upper and lower contacts with the enclosing host rocks are typically sharp, but minor slumping into the underlying siliceous shales may be locally present (Fig. DR2A). There is minor to abundant biogenic debris, which includes conodont elements and pyrite-permineralized plant matter (cf. Gadd and Peter, 2018), and a relatively low abundance of terrigenous clastic detritus (Gadd et al., 2019a).

The stratigraphic sections for the Peel River showing (Fig. 2B) and the Nick prospect (Fig. 2C) are similar lithologically, with local variations in the thicknesses of units. The lithologies comprise (1) black shale with 0.5–1.5-m-diameter calcareous concretions; (2) siliceous black shale with minor calcareous intercalations; (3) a 1–10-cm-thick stratiform semi-massive Ni-Zn-Fe-sulfide HEBS layer; and (4) siliceous to cherty black shale that is in sharp contact with the underlying HEBS mineralization. The significant difference between the stratigraphic sections of the two locations is that three distinct HEBS layers have been documented at Peel River (Fig. 2B; Gadd et al., 2019b), whereas only one is identified at Nick prospect (Fig. 2C). Our study focuses on the HEBS layer that is common to both localities (i.e., at the regional stratigraphic contact).

The contact between the Road River Group and the overlying Canol Formation is conformable. The terminal Road River Group (i.e., the HEBS) is a regionally extensive condensed section (Fraser and Hutchison, 2017; Gadd and Peter, 2018). Condensed sections form in response to shoreline transgressions that form during late transgressive to early highstand systems tracts (Loutit et al., 1988). They are clastic sediment-starved, such that thin (centimeter-scale) beds represent relatively long durations (Schutter 1996). Several criteria are used to identify condensed sections (cf. Schutter, 1996), and relevant ones present within the HEBS are high abundances of authigenic minerals (e.g., sedimentary sulfides), abundant biogenic debris, high abundances of metals, and widespread geographic distribution. Moreover, HEBS are starved of clastic sediments, especially in comparison to the immediately underlying Road River Group (Gadd et al., 2019a).

MINERALOGY

Pyrite is the most abundant sulfide in the Yukon HEBS, and there are several textural varieties (Figs. DR1B and DR2B), including framboidal, microcrystalline, and nodular (Gadd et al., 2019b). A key mineralogical difference between the localities is that at Nick, the predominant Ni sulfide is vaesite (NiS2) (Fig. DR2B), whereas millerite (NiS) predominates at Peel River (Fig. DR1B). Another difference is that vaesite and pyrite host PGEs and Re at Nick (Gadd et al., 2017), but pyrite is the primary host for these elements at Peel River (Gadd et al., 2019b). The sulfide paragenetic sequence is broadly the same at both places, wherein framboidal pyrite is the earliest sulfide. The large range in framboid diameters (0.003–0.1 mm; Figs. DR1B and DR2B) suggests that precipitation was initiated within reducing pore waters rather than in the water column (Wilkin et al., 1996; Bond and Wignall, 2010). Subsequent pyrite growth overprints framboids or consists of aggregated microcrystals within nodules (Figs. DR1B and DR2B). Significant Ni-sulfide postdates pyrite, as evidenced by Ni-sulfides encrusting (Fig. DR1B) or replacing (Fig. DR2B) pyrite.

Re-Os GEOCHRONOLOGY

Samples of the HEBS mineralization were collected from the regional stratigraphic contact at both Peel River (65.893°N, 135.931°W) and Nick (64.728°N, 135.235°W) and were processed at the University of Alberta (Edmonton, Alberta, Canada) using the methods of Hnatyshin et al. (2016). Mineral separates (n = 9) from Peel River sample 17-POA-049c have Re and Os abundances of 30.1–44.2 ppm and 179–325 ppb, respectively. Mineral separates (n = 6) from the Nick prospect sample 18-POA-077 have Re and Os abundances of 8.8–42.2 ppm and 46.3–311 ppb, respectively. The host shale at Peel River contains 1.12 ppm Re and 5.82 ppb Os, whereas the host shale at Nick contains 1.42 ppm Re and 8.87 ppb Os. The approximate modal mineralogy of the mineral separates and the Re-Os data are presented in Tables DR1 and DR2, respectively (in the Data Repository).

Isochron ages were calculated using IsoplotR software (https://www.ucl.ac.uk/∼ucfbpve/isoplotr/) with the Model 3 regression, which accounts for potential variability in initial 187Os/188Os ratios (Osi; Vermeesch, 2018). Sulfide mineralization from the Peel River sample yields an isochron age of 387.5 ± 4.4 Ma (Fig. 3A) that overlaps within error with the isochron age of sulfide mineralization from the Nick prospect (390.7 ± 5.1 Ma; Fig. 3B). Unmineralized host shale isochrons coincide with the sulfide isochrons at Peel River (Fig. 3A) and Nick (Fig. 3B). Including the latter data modifies the isochron ages to 386.6 ± 3.1 Ma (Fig. 3A) and 390.6 ± 4.8 Ma (Fig. 3B) for Peel River and Nick, respectively. The Osi is 0.23 ± 0.12 and 0.31 ± 0.23 for Peel River and Nick, respectively.

Excess scatter beyond the calculated analytical uncertainties, indicated by mean square weighted deviation (MSWD) >1, is evident in the Nick isochron (MSWD = 15). Variability in Osi during mineralization can explain this scatter if individual points incorporated material with slightly different Osi signatures. The Model 3 solution estimates this variability to be 0.05–0.23. Alternatively, the higher MSWD may be explained by very limited open-system behavior, potentially arising from minor oxidative alteration.

AGE OF MINERALIZATION

Isochron ages for the Peel River (Fig. 3A) and Nick (Fig. 3B) localities, which are ∼130 km apart, reveal that mineralization is coeval within error. Ages of conodonts extracted from shales 0.9 m above and 1.1 m below (Fig. 2B) the HEBS at Peel River indicate that the age of sedimentation is within the (Polygnathus) eiflius (388.6–388.2 Ma; Becker et al., 2012) to the ansatus zones (386.3–385.4 Ma; Becker et al., 2012) (Table DR3). These ages bracket the Peel River isochron age. Carbonate concretions 1.2–1.9 m below the HEBS at the Nick prospect yield conodonts from the australis (389.2–389.0 Ma; Becker et al., 2012) to the ensensis zones (388.2–387.7 Ma; Becker et al., 2012) (Table DR3). The Nick isochron age is within this range, but must be younger than the age of the underlying sediments from which the conodont elements were extracted. The close agreement between Peel River and Nick isochron ages for the semi-massive sulfides and the conodont ages (Fig. 4) signifies that mineralization was synchronous with sedimentation.

The radiogenic nature of the HEBS samples limits the precision of the projected Osi and precludes unequivocal determination of the origin of the Os-bearing fluids. However, the Osi of the HEBS at Peel River (0.22 ± 0.13) and Nick (0.31 ± 0.23) are comparable to Middle Devonian seawater values. Temporally, the best estimate for the Os isotope composition of seawater comes from Middle Devonian organic matter that indicates an Osi of 0.262 ± 0.034 (Miller, 2004). The Osi for Peel River and Nick overlap with this value, which would be expected if Os was scavenged from seawater. Furthermore, the host black shales, which derived Os from ambient seawater, produce a statistically indistinguishable isochron when added to the sulfide isochron (Figs. 3A and 3B), indicating that the host black shales and sulfides share an Os source (i.e., seawater). A seawater source for metals is also supported by REE distributions (Crawford et al., 2019; Gadd et al., 2019a), Tl and Mo isotopes (Crawford et al., 2019), S isotopes (Hulbert et al., 1992; Gadd et al., 2019a), and mineral chemical compositions (Gadd et al., 2019b). Due to imprecision of the Osi data, a magmatic-hydrothermal Os source cannot be definitively discounted; however, the geochemical data presented in this and previous studies (Gadd and Peter, 2018; Crawford et al., 2019; Gadd et al. 2019a, 2019b) indicate a seawater source for the metals is the most probable.

GEOGRAPHIC DISTRIBUTION OF HEBS AND IMPLICATIONS

HEBS at the Peel River showing and Nick prospect occurs at the Road River Group–Canol Formation stratigraphic contact (Figs. 2A–2C), as does HEBS at the Moss (Gadd et al., 2019a) and Monster River showings (Fig. 1) (Goodfellow et al., 2010). Based on the synsedimentary mineralization ages and shared stratigraphic relationships between Peel River and Nick, we speculate that HEBS at Moss and Monster River also formed during deposition at ca. 390–386 Ma. Metal enrichments in black shales from this time period are documented elsewhere in western Canada and globally. At Trail River (Fig. 1), shales at the stratigraphic contact between the Road River Group and Canol Formation are enriched in a similar suite of elements (e.g., Ni, Mo, Se, etc.) to those in the HEBS (Fraser and Hutchison, 2017), albeit at lower abundances. Recently discovered Ni-Zn HEBS occurs at the Akie property in northeastern British Columbia in the Kechika trough (1100 km southeast of the Yukon HEBS; Peter et al., 2018), a southeast-striking, finger-like extension of the Selwyn Basin (Goodfellow, 2007). Here, crinoid-bearing bioclastic wackestone collected 6 m above the HEBS yields conodont elements from the australis to the ansatus zones (DR3), suggesting that the pre-ansatus zone age for HEBS in the Kechika trough correlates with that of HEBS in the Richardson trough. Interestingly, the Eifelian-Givetian stage boundary at Jebel Mech Irdane, Morocco, contains pyritic beds (Walliser et al., 1995) with anomalously high (albeit lower than HEBS in Yukon and British Columbia) abundances of redox-sensitive trace elements (e.g., Ni, V, Mo, As; Ellwood et al., 2003) and PGEs (Schmitz et al., 2006).

Sea-level instability is a well-established aspect of the Middle Devonian (Haq and Schutter, 2008), and the radiometric and biostratigraphic age constraints of the HEBS overlap with the ages of the Kačák, pumilio, and Taghanic biotic events (Fig. 4; Becker et al., 2012; McGhee et al., 2013; Narkiewicz et al., 2016; Brett et al., 2018). These events were of global extent, and are defined by abrupt faunal changes, eustatic sea-level rise, continental shelf flooding, and black shale deposition (House, 1996, 2002; Becker et al., 2012; Brett et al., 2018). Some of the salient controls on HEBS formation are: nutrient-rich upwelling, effective trapping of these nutrients due to basin architecture, highly efficient organic matter remineralization, and condensed sedimentation (Lehmann et al., 2016; Johnson et al., 2017; Gadd and Peter, 2018). Additionally, we suggest that metal enrichment may be a hallmark feature of Middle Devonian eustatic-biotic events. Eustatic sea-level rise associated with the Middle Devonian event(s) may have facilitated basin-scale HEBS mineralization by providing steady-state input of nutrient and other trace elements in seawater over long durations in a setting with little clastic input in favorable basinal positions (e.g., semi-restricted sub-basins) on the western Laurentian continental margin.

ACKNOWLEDGMENTS

Funding for this research came from the Lands and Minerals Sector of Natural Resources Canada through its Targeted Geoscience Initiative program. Additional funding and logistical support came from the Polar Continental Shelf Program (projects 00717 and 01018). Pedro Acosta-Góngora, Carlton E. Brett, Maurice Colpron, Sean Johnson, and two anonymous reviewers provided valuable feedback that improved the quality of this manuscript. Finally, we express our gratitude to the Gwich’in Tribal Council for permitting us to perform research on their traditional lands. This is Natural Resources Canada contribution 20190272 and Yukon Geological Survey contribution 043.

1GSA Data Repository item 2020063, Figures DR1 and DR2 (supplemental photographs and photomicrographs of HEBS from the Peel River showing and the Nick prospect, respectively), Table DR1 (estimated modal mineralogy of mineral separates produced for this study), Table DR2 (Re-Os data), and Table DR3 (supplemental information regarding specific conodont taxonomic data and metadata), is available online at http://www.geosociety.org/datarepository/2020/, or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY license.