The highly irregular and localized distribution of tungsten deposits worldwide constitutes a supply challenge for basic industries such as steel and carbides. Over Earth's history, tungsten has preferentially accumulated at paleocontinental margins formed during the breakup of supercontinents. Later crustal thickening of these paleogeographic regions and the magmas they produce are associated with large tungsten districts. However, all of the largest tungsten deposits in the modern North American Cordillera, which preserves over 3 b.y. of geologic record in a paleocontinental margin with abundant crustal magmatism, are limited to the narrow Canadian Tungsten Belt in northwestern Canada. We use neodymium isotopic compositions of scheelite (CaWO4) from the Canadian Tungsten Belt and the paleogeographic distribution of tungsten deposits in the North American Cordillera to constrain the factors that control tungsten distribution. We document that tungsten is specifically associated with materials that, on average, were derived from the mantle during the Mesoarchean to Paleoproterozoic. Weathering and erosion of the supercontinents Columbia and Rodinia favored pre-enrichment of tungsten in sediments. The orogenic heating of pre-enriched sediments produced reduced melts that were capable of efficiently scavenging tungsten and formed the largest deposits in North America.

Tungsten is a key strategic metal for modern society, used in alloys due to its hardness, density, and high temperature resistance; its production is highly localized, and there are currently no substitutes (European Commission, 2020). Recent disruptions in supply chains have drawn attention to the strategic need to secure local supplies of critical elements (Guan et al., 2020). In this context, identifying prospective locations for critical elements such as tungsten is a global priority (European Commission, 2020; Simandl et al., 2021). The tungsten cycle over Earth's history has been affected by supercontinent assembly and breakup (Romer and Kroner, 2016), the evolution of early life (Kletzin and Adams, 1996), and crustal melting and differentiation (Candela and Bouton, 1990). Therefore, determining how/if current tungsten distribution relates to paleogeography, the evolution of early life, and source rock characteristics is of high scientific and economic interest.

The factors that control tungsten distribution can be best identified in a province with diverse paleogeography and geology that also hosts various degrees of tungsten enrichment. The North American Cordillera has long been a tectonically active region beneath which the age of the crustal lithosphere extends back to the Archean–Paleoproterozoic (Whitmeyer and Karlstrom, 2007). Despite a favorable tectonic setting across the Cordillera, the economically most significant tungsten mineralization is focused along the narrow Canadian Tungsten Belt (CTB), which is defined by peraluminous intrusions, and hosts two of the largest tungsten deposits in the world (Figs. 1 and 2). The Cordillera therefore provides an ideal region for evaluating which factors control tungsten distribution.

Current continents, including North America, are a mosaic of multiple paleogeographic provinces that have migrated, assembled, and disassembled over geologic time. The regional neodymium isotopic composition (εNd) can be a proxy for either the age (depleted mantle model age, TDM) at which the crust differentiated from the mantle or the average age (TDM) of a mixture of materials derived from the mantle at different times (Arndt and Goldstein, 1987). Therefore, εNd and TDM are used in this study to fingerprint paleogeographic provinces, but not to define absolute ages of crustal formation. Crustal materials derived from the same paleogeographic provinces or from the same mixture of materials from different provinces follow a comparable evolution of εNd versus time (e.g., provinces with Grenvillian, Paleoproterozoic, and Archean TDM; Fig. 3). The εNd and TDM of tungstate minerals in tungsten deposits can therefore provide an indirect proxy for characterizing the source(s) of tungsten in mineralized systems.

The initial values of εNd in intrusive rocks at the time of formation (εNdt) reach a minimum landward of subduction trenches that reflects the homogenized isotopic signature of the underlying crustal basement (Chapman et al., 2017). The εNdt of landward Cretaceous intrusive rocks in Yukon (Fig. 1, a−a′) reaches a minimum of −17 (Morris and Creaser, 2008), which is consistent with sources derived dominantly from materials with Archean to Paleoproterozoic TDM. In Arizona (USA) (Fig. 1, b−b′), εNdt of Mesozoic to Cenozoic intrusive rocks reaches a landward minimum of −10 with a natural variability of ~2 epsilon units (Chapman et al., 2017), reflecting materials with an average Proterozoic TDM.

The CTB is the most important tungsten metallogenic province within the North American Cordillera (Figs. 1 and 2). The CTB consists of mid-Cretaceous peraluminous intrusions (Rasmussen et al., 2011) hosted in Neoproterozoic to Devonian pelitic, carbonaceous, and calcareous sedimentary units that were deposited along the ancestral western margin of North America in the Selwyn Basin (Gordey and Anderson, 1993; Hart et al., 2004). Farther east, the oldest shallow shelf equivalents of the Selwyn Basin include the Paleoproterozoic Wernecke, the Mesoproterozoic/Neoproterozoic Mackenzie Mountains, and the Neoproterozoic Windermere Supergroups in the Mackenzie Platform, but their extent beneath the Selwyn Basin is unknown (Gordey and Anderson, 1993). The CTB hosts deposits of comparable size and grade to other world-class tungsten deposits (Werner et al., 2014). Most notable, the CTB includes one of the top 10 historic producers of tungsten worldwide (the Cantung deposit; Fig. 2A) and currently the greatest reserves in North America (the Mactung deposit; Fig. 2B), as well as numerous additional tungsten deposits (Werner et al., 2014; Karl et al., 2020). Tungsten mineralization in the CTB occurs dominantly as scheelite (CaWO4) in skarns, which are hydrothermal mineral deposits hosted in calc-silicate rocks (see Appendix S1 in the Supplemental Material1).

Depleted mantle model ages (TDM) dominantly between 3.1 Ga and 1.5 Ga (see the Supplemental Material and Dataset S1 therein) and strongly negative εNdt mainly between −20 to −15 (Fig. 3) in scheelites from both the Mactung and Cantung deposits demonstrate that mineralization in the CTB is sourced from crustal materials with a Mesoarchean to mid-Paleoproterozoic average age. Neoproterozoic to Cambrian sedimentary units and Cretaceous intrusive rocks analyzed in this study have isotopic signatures that are similar to those of scheelite at the time of their formation (Fig. 3). In this context, the scheelite Nd isotopic signatures reflect a mixture of the source and the Neoproterozoic to Cambrian host rocks to the mineralization (e.g., Scanlan et al., 2018), with the Nd signature of scheelite closer to that of its main Nd contributor.

The compiled isotopic and geographic evidence shows that the largest tungsten deposits in the North American Cordillera have a source of Mesoarchean to Paleoproterozoic TDM (Figs. 1 and 3). In the North American Cordillera, tungsten tonnages approaching those of the CTB are only found at the Andrew deposit in the Tungsten Hills District of southern California and Nevada, USA (Karl et al., 2020). The Tungsten Hills District consists dominantly of Upper Jurassic granite and quartz diorite intruded in Paleozoic sandstone, shale, and limestone that are variably metamorphosed (Lemmon, 1941). The TDM of the crust in the Tungsten Hills District is 2.3–2 Ga, and mineralization is associated with peraluminous granites that have εNdt < −16 (Bennett and DePaolo, 1987). This early Paleoproterozoic signature likely reflects a dominant Nd contribution from the underlying Archean Mojave Block (Fig. 1) or materials derived from it. Similarly, the richest tungsten deposit in Idaho (the CuMo deposit) is related to the Atlanta lobe of the Idaho Batholith, a metaluminous granite with Mesoarchean to Paleoproterozoic TDM, which is unlike other Sierran equivalents (Gaschnig et al., 2011).

The new and compiled data presented here demonstrate that large tonnage tungsten deposits throughout the North American Cordillera are sourced from materials that have a Mesoarchean to Paleoproterozoic average age. Additional geological parameters required to form tungsten deposits include the association with peraluminous and reduced intrusions and their exposure at the Earth's surface (Candela and Bouton, 1990; Barton, 1996).

Peraluminous melts can result from low melt/rock ratios in the crust that optimize the ability of a melt to either scavenge or accumulate incompatible elements such as tungsten (Cerny et al., 2005). The lowest melt/rock ratios occur when a melt is extracted at low temperature (<700 °C) because minerals that accumulate tungsten in the source rocks melt at relatively low temperatures (Yuan et al., 2019). As tungsten is more compatible with minerals present in oxidized lithologies than in reduced lithologies, reduced melts are more effective than oxidized melts at scavenging tungsten (Candela and Bouton, 1990), limiting the ability of oxidized melts to extract tungsten.

In the North American Cordillera, peraluminous plutons were emplaced as an extensive semi-continuous belt (Chapman et al., 2017). The most tungsten-fertile regions of the peraluminous belt have temperatures of melt extraction that are consistently above 800 °C based on zircon saturation temperatures (Fig. 4). Therefore, at the Cordilleran scale, the temperature of melt extraction is not the primary limiting factor for tungsten mineralization. Reduced intrusive rocks in the CTB, as indicated by the ratio of reduced to oxidized iron, are associated with the largest tungsten deposits (Fig. 4), and smaller deposits are associated with less reduced magmas in the Mojave Block (Fig. 4). Finally, tungsten occurrences are most commonly exposed where the amount of exhumation ranges between 4 km and 8 km (mineralization depth; Barton, 1996), but within this range large tungsten systems are only found in the CTB. In summary, the major tungsten deposits in the North American Cordillera are specifically associated with reduced peraluminous magmas derived from sediments with Mesoarchean to Paleoproterozoic TDM.

The association between tungsten mineralization and sources of Mesoarchean to Paleoproterozoic average TDM in western North America has important implications for tungsten exploration and links paleogeographic observations with the geochemical cycling of redox-sensitive elements in ancient Earth. Stable supercontinents lead to extensive periods of chemical weathering that particularly favor tungsten enrichment in continental crust (Romer and Kroner, 2016). The reason for this enrichment is that tungsten is dominantly hosted in minerals with low solubility in surface water (e.g., rutile; Cave et al., 2017). Leaching of other elements through chemical weathering and minimal mechanical transport increases the tungsten concentration in the residual sediment (Romer and Kroner, 2016). Continental breakup then leads to the deposition of tungsten-rich refractory minerals along the passive margin by erosion of the weathered interior of the continent (e.g., Fig. 1; Romer and Kroner, 2015). Tungsten enrichment through this process in Atlantic North America and Europe was likely associated with the breakup of the supercontinent Gondwana (Romer and Kroner, 2016). In the North American Cordillera—including the CTB— the isotopic results and paleogeographic context instead suggest a more likely association with the breakup of the supercontinents Columbia and Rodinia. Specifically, between 2.3 Ga and 2.2 Ga, continental evolution went through a quiescent period in terms of magmatism, orogeny, and passive margin sedimentation that was followed by the assembly of the supercontinent Columbia between 2.1 Ga and 1.8 Ga (Spencer et al., 2018). The period between 1.8 Ga to 0.8 Ga marks a period of tectonic stability, during which Columbia did not completely disassemble and when the paleogeographic relationships within western North America did not change significantly (Tang et al., 2021). The stratigraphic record of this period within the Canadian Cordillera is represented by the Paleo/Mesoproterozoic Wernecke and Mackenzie Mountains Supergroups, both of which are interpreted to have been deposited in epicratonic basins (Gordey and Anderson, 1993). Deposition of the Windermere Supergroup marked the end of this period of tectonic stability (Tang et al., 2021) and the onset of the breakup of Rodinia at the end of the Neoproterozoic (e.g., Moynihan et al., 2019). Paleogeographically, the western edge of North America remained part of a supercontinent from 2.2 Ga to 0.8 Ga, which provided an extended quiescent period for the enrichment of tungsten in weathered Archean to Proterozoic crust. Subsequent erosion of these materials during the breakup of Rodinia (Neoproterozoic to early Paleozoic) would have allowed their transport and redeposition as the passive margin sediments that are spatially and isotopically associated with tungsten mineralization in the CTB. Additionally, some paleogeographic reconstructions place the Cathaysia Block, the top tungsten-producing province worldwide and host to the tungsten-enriched Neoproterozoic Shuangqiaoshan Group in China (Huang and Jiang, 2014), contiguous to the western edge of Laurentia prior to the breakup of Rodinia (Li et al., 2008). The Mesoarchean to Paleoproterozoic was further a dynamic period in Earth's history; it was characterized by the rise of oxygenic photosynthesis and attendant changes in redox cycling in surface environments (Lyons et al., 2014) as well as continental emergence (Tang et al., 2021). The role of such fundamental changes in pre-enriching tungsten in sediments remains a topic in need of further study.

Our data combined with this synthesis suggest that the distribution of much of the tungsten ore deposits in the North American Cordillera was controlled by the Columbian-Rodinian supercontinent cycle and its effects on the concentration of tungsten-enriched, reduced source rocks—with a Mesoarchean to Paleoproterozoic average age—along the continental margin.

We thank Kevin Ansdell and an anonymous reviewer for thoughtful comments that helped improve this manuscript, and Ehsan Salmabadi for donating some of the samples used in this study. Krystle Moore, Andy DuFrane, Hélène Legros, Adrien Vezinet, and Nathan Gerein from the University of Alberta (Canada) assisted with sample preparation and/or analysis. The Polar Continental Shelf Program and North American Tungsten are thanked for logistical and in-kind support. Funding was provided by Northwest Territories Geological Survey contribution agreements (to P. Lecumberri-Sanchez), the Targeted Geosciences Initiative from Natural Resources Canada GC-130028S (to P. Lecumberri-Sanchez), a Natural Sciences and Engineering Resources Canada Discovery grant (to P. Lecumberri-Sanchez), and the Polar Continental Shelf Program.

1Supplemental Material. Appendix S1 (detailed geology of Cantung and Mactung, Canada); Appendix S2 (materials) Appendix S3 (methods); Appendix S4 (supplemental Figure S1 and details for data presented in figures); Dataset S1 (data presented in Figure 3 and Figure S1); and Dataset S2 (data presented in Figure 4). Please visit https://doi.org/10.1130/GEOL.S.19400945 to access the supplemental material, and contact editing@geosociety.org with any questions.
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