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Abstract: A detailed study of apatite and biotite compositions in multiple intrusive phases from five composite plutons in the northern Canadian Cordillera was undertaken with the aim of determining the composition of magmatic fluids relative to F:Cl:OH for several plutons―both barren and mineralizing―and for specific intrusive phases from each pluton that may be related to nearby tungsten skarns. Magmatic apatite and biotite compositions are consistent with a crustal source of magma, either derived from predominantly supracrustal rocks, and (or) derived from predominantly infracrustal rocks and fractionated to felsic compositions. Increasing MnO (±FeO and XF:XCl) with decreasing CaO in apatite broadly correlates with an increasing degree of magmatic differentiation, although Fe# vs. total aluminum in biotite is a better indicator of inter- and intra-plutonic differentiation. Anomalously iron-rich biotites occur in highly fractionated and (or) wallrock-contaminated phases of plutons associated with tungsten skarns. Estimates of magmatic fluid composition―calculated as the activity ratios log[a HCl / a HF] and log[a HOH / a HF] from apatite and biotite compositions―show several trends with respect to magmatic differentiation. Two barren plutons demonstrate that fluids in silicic magmas become HF enriched relative to HCl with increasing differentiation, although re-equilibration with late-stage sub-solidus or hydrothermal fluids may obscure this trend. The three intrusions with associated tungsten skarn mineralization, including the world-class Cantung deposit, also become HF-enriched with magmatic evolution. However, magmatic apatite and biotite in individual intrusive phases that are the most closely associated with mineralization have equilibrated with compositionally distinct fluids. In these particular intrusive phases, apatite appears to have equilibrated with an earlier HCl- and H2O-rich magmatic fluid, and biotite appears to have equilibrated with a later HCl- and (a particularly) H2O-rich magmatic fluid. The fluid in these magmas apparently evolved to a H2O-rich (or less saline) composition as the temperature of the magma decreased. None of the other intrusive phases, from either the barren or tungsten-associated plutons, have apatite and biotite activity ratios that are suggestive of equilibration with such an HCl/HF- and H2O/HF-rich fluid. Instead, the activity ratios calculated from apatite and biotite in intrusive phases that are not as closely associated with mineralization are well-coupled and unremarkable (although small variations are common), which would suggest that both minerals equilibrated with similar fluids. These intrusive phases do not appear to have produced a large quantity of saline hydrous fluid capable of seggregating and transporting tungsten. The identification of intrusive phases that did produce magmatic fluids that were anomalously enriched in HCl and H2O could, therefore, be a predictor of nearby tungsten skarn mineralization. [Copyright &y& Elsevier]

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8-25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10-40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05-0.17%) Cu in the gold alloy, together with low (5-9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration. [ABSTRACT FROM AUTHOR]

Presents a study that examined neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the Yukon-Tanana terrane (YTT) in the Finlayson Lake region, Yukon, Canada. Geological setting of the YTT in the Finlayson Lake region; Geochemical attributes of the intrusive rocks; Systematics of the neodymium isotopes.

Calc-alkaline batholiths of the Archaean Minto block, northeastern Superior Province, Canada, have pyroxene- and hornblende-bearing mineral assemblages inferred to have crystallized from hot, water-undersaturated magmas at 2·729–2·724 Ga. A regional amphibolite- to granulite-facies tectonothermal event at 2·70 Ga resulted in mild to negligible metamorphic effects on the dominantly granodioritic units. Geochemical, textural and thermobarometric studies define the crystallization history in compositions ranging from cumulate pyroxenite through quartz diorite, granodiorite, granite, and syn-magmatic gabbroic dykes. Early magmatic assemblages include orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti oxides and ternary feldspar, indicating crystallization from magmas containing <2 wt % H2O at 1100–900°C. Water enrichment in the residual melt induced hornblende crystallization at 5 ± 1 kbar, 800–600°C. Characterized by a continuum of large ion lithophile element (LILE)-enriched, high field strength element (HFSE)-depleted compositions, the I-type suite resembles modern continental arc batholiths in composition and size but not primary mineralogy. Magmatic arcs produced between 2·75 and 1·85 Ga commonly have charnockitic components, possibly because slab-derived fluids interacted with mantle wedges at ambient temperatures higher by ∼100°C than at present, producing large volumes of water-deficient magma. [ABSTRACT FROM PUBLISHER]

Northwestern Laurentia, after cratonization at about 1.85 Ga, underwent a series of tectonic and magmatic events during the Proterozoic that were followed by separation of Laurentia from another landmass, probably Australia. The oldest magmatic event produced the Bonnet Plume River Intrusions (BPRI), which intruded the Wernecke Supergroup as short dikes and small stocks. The BPRI are hydrothermally altered tholeiitic diorites, gabbros, and subordinate anorthositic and syenitic rocks, with trace element signatures consistent with a rift origin. Depleted mantle model ages range from 2.29 to 2.57 Ga and ℇ[sub Nd] values range from +0.7 to –1.7. An increasing crustal component is apparent in rocks with more evolved compositions. Four U–Pb zircon ages (1705.9 ± 0.7, 1709.4 ± 1.4, 1711.1 ± 5.1, and 1713.6 ± 12.7 Ma) indicate a Paleoproterozoic age for the BPRI. These dates constrain the age of the Wernecke Supergroup to ca. 1710 Ma, making it the oldest supracrustal succession in western Laurentia, e.g., >240 Ma older than the Belt Supergroup of southeastern British Columbia and the northwestern United States. The Wernecke Supergroup was deposited in the first rift basin to open along the western margin of Laurentia, but was later inverted by the pre-1.6 Ga Racklan Orogeny, an event possibly influenced by transmission of compression from the Yavapai and Mazatzal orogenies in southern Laurentia. The Neoproterozoic southwestern United States – east Antarctica (SWEAT) reconstruction, which places Australia next to northwestern Laurentia, is supported by linkages between Paleoproterozoic and Mesoproterozoic geological features in northwestern Canada and Australia.Après une cratonisation vers 1,85 Ga, le nord-ouest du continent laurentien (Laurentia) a subi une série d'événements tectoniques et magmatiques au cours du Protérozoïque; ces événements ont été suivis par la séparation de Laurentia d'une autre masse continentale, probablement l'Australie. L'événement magmatique le plus ancien a produit les intrusions de la rivière Bonnet Plume (BPRI) qui ont pénétré le Supergroupe de Wernacke en tant que de courts dykes et de petits massifs intrusifs. Les BPRI consistent en des diorites tholéiitiques, des gabbros et des roches subordonnées anorthositiques et syénitiques qui ont subi une altération hydrothermale, avec des signatures d'éléments traces qui concordent avec une origine de fosse. La plage pour les modèles de manteau appauvri varie de 2,29 à 2,57 Ga et les valeurs ℇ[sub Nd] varient de +0,7 à –1,7. Une plus grande composante crustale est apparente dans les roches avec des compositions plus évoluées. Quatre déterminations d'âge U–Pb sur des zircons donnent un âge paléoprotérozoïque pour les BPRI : 1705,9 ± 0,7 Ma, 1709,4 ± 1,4 Ma , 1711,1 ± 5,1 Ma et 1713,6 ± 12,7 Ma. Ces dates limitent l'âge du Supergroupe de Wernecke à environ 1710 Ma, faisant de la séquence supracrustale la plus ancienne du Laurentia occidental, par ex., > 240 Ma plus ancien que le Supergroupe de Belt du sud-est de la Colombie-Britannique et du nord-ouest des États-Unis. Le Supergroupe de Wernecke a été déposé dans le premier bassin de distension qui s'est ouvert le long de la limite ouest de Laurentia mais il a été par la suite inversé par l'orogène de Racklan pré-1,6 Ga, un événement possiblement influencé par la transmission de la compression des orogènes Yavapai et Mazatzal dans le sud du Laurentia. La reconstruction du transect sud-ouest américain – est de l'Antarctique (SWEAT), au Néoprotérozoïque, qui place l'Australie à côté du nord-ouest de Laurentia, est appuyé par des liens entre les caractéristiques géologiques Paléo- et Méso- protérozoïques au nord-ouest du Canada et en Australie.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Determines the age of peak metamorphism and the postmetamorphic cooling history of the metamorphic-plutonic belt in the southwestern Yukon. Geologic framework; Isotopic dating of metamorphic rocks; Tectonic implications.

This paper provides a comprehensive synthesis of virtually all units and events of Early and Middle Proterozoic age in the Yukon, spanning ~1 Ga. Early and Middle Proterozoic time was dominated by a series of extensional-basin-forming events punctuated by orogenesis, magmatism, and hydrothermal activity. Basinal deposits include the Wernecke Supergroup (>1.71 Ga), Pinguicula Group (~1.38 Ga), and Mackenzie Mountains Supergroup (1.00–0.78 Ga). Igneous rocks include the Bonnet Plume River Intrusions (1.71 Ga), Slab volcanics (≥1.6 Ga), Hart River sills and volcanics (1.38 Ga), and Bear River (Mackenzie) dykes (1.27 Ga). A voluminous hydrothermal event generated the widespread Wernecke breccias at 1.60 Ga. The Racklan orogeny deformed the Wernecke Supergroup prior to emplacement of the Wernecke Breccia. The Corn Creek orogeny deformed Mackenzie Mountains Supergroup and older rocks prior to deposition of the Windermere Supergroup (<0.78 Ga). Long intervals with scanty rock records extended for as much as 300 Ma and appear to represent periods of crustal stability and subaerial conditions. By the time of Windermere rifting (<0.78 Ga), the supracrust of northwestern Laurentia was a mature, largely denuded orogenic belt with a composite sedimentary–metamorphic–igneous character. New isotopic data include Nd depleted mantle model ages for the Wernecke Supergroup (2.28–2.69 Ga) and Wernecke Breccia (2.36–2.96 Ga), a U–Pb zircon age for a Hart River sill 1381.9+5.3-3.7 (Ma), detrital U–Pb zircon ages from the basal Pinguicula Group (1841–3078 Ma), detrital muscovite ages from the Mackenzie Mountains Supergroup (1037–2473 Ma), and muscovite 40Ar/39Ar cooling ages from the Wernecke Supergroup (788 ± 8 and 980 ± 4 Ma). [ABSTRACT FROM AUTHOR]

The Fire Lake formation of the Yukon-Tanana terrane in the Finlayson Lake region, Yukon, Canada, consists primarily of Late Devonian (ca. 365–360 Ma) mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions. In this paper, field, geochemical, and Nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting, the magmatic processes involved in their formation, and their roles in Cordilleran crustal growth. The mafic rocks of the Fire Lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites: (1) backarc-basin basalt, (2) enriched mid-oceanic-ridge basalt (E-MORB), (3) oceanic-island basalt (OIB), (4) Th-rich OIB, (5) boninite, (6) island-arc tholeiite, and (7) fight rare earth element (LREE)—enriched island-arc tholeiite. The diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric (MORB-type) mantle, subarc mantle wedge, and lithospheric (OIB-type) mantle with or without elemental contributions from the subducted slab and/or continental crust. These suites of rocks are also associated with fine-grained basinal sedimentary facies, variations in metavolcanic and metasedimentary unit thickness, extensional synvolcanic faults, and apparent extensional-fault—controlled emplacement of mafic intrusive rocks and hydrothermal volcanic-hosted massive sulfide mineralization. The suites also exhibit a broad spatial distribution; those with “arc” signatures (Nb/Thmn. < 1; mn—normalized to primitive mantle values) are located primarily in the western parts of the formation, and suites with “nonarc” signatures (Sb/Thmn ≥ 1) are located primarily within the eastern parts of the formation. Collectively, these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east-dipping subduction zone. The initiation of backarc-basin magmatism recorded in the Fire Lake formation was part of a much larger Late Devonian backarc basinal system forming along the western edge of the margin of North America. The Fire Lake formation is interpreted to represent (1) the commencement of Yukon-Tanana arc rifting and separation from the North American cratonic margin, and (2) the initiation of a marginal (backarc) basin (now the Slide Mountain terrane) inboard of the Yukon-Tanana arc system. This tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for Yukon-Tanana arc activity or as a result of the propagation of the Slide Mountain backarc-basin spreading ridges into the Yukon-Tanana arc system. This Yukon-Tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the North American craton. The geochemical and isotopic signatures of magmatic rocks in the Fire Lake formation have some features similar to intraoceanie arc rocks (e.g., boninites, island-arc tholeiites), and many have juvenile Nd isotope signatures (i.e., εNd(t) > 0; most have εNd(t) > +5), suggesting that the pericratonic terranes of the northern Cordillera have a significant juvenile component. If this is the case throughout the Yukon-Tanana terrane, then the pericratonic terranes may have contributed much more juvenile material to Cordilleran crustal growth in the Phanerozoic than has previously been considered. [ABSTRACT FROM AUTHOR]

The 1.27 Ga Mackenzie dyke swarm, the largest on Earth, radiates from a point in the western Canadian Arctic and extends across much of the Canadian Shield. Possible western extensions of the swarm are largely obscured by younger sedimentary cover. Paleoproterozoic inliers in northern Yukon host the Bear River dykes (BRD), herein dated by U–Pb baddeleyite and zircon methods at 1268.5 ± 1.5 Ma and 1264.6 ± 1.2 Ma. The BRD share similar geochemical and Nd-isotopic characteristics with the Mackenzie dykes and the coeval Coppermine River basalts, and are regarded as products of the same plume. Hydrothermal activity at ∼1270 Ma in breccia of the Nor mineral occurrence, northwest of the BRD, was probably generated by BRD at depth. The BRD generally strike northwest, approximately 90° to the orientation predicted by the radiating dyke model. This difference may be explained by an anomalous local stress field at the time of BRD emplacement, or reorientation of BRD during subsequent events of Cordilleran deformation. Using the Nor mineral occurrence as the westernmost locale of Mackenzie dyking, and assuming a model of uniform dyke radiation, the arc of the Mackenzie swarm is at least 50° greater than previously recognized, bringing the total arc of dyke radiation to >150°. [Copyright &y& Elsevier]

Jurassic sedimentary rocks of southern to central Quesnellia record the history of the Quesnellian magmatic arc and reflect increasing continental influence throughout the Jurassic history of the terrane. Standard petrographic point counts, geochemistry, Sm–Nd isotopes and detrital zircon geochronology, were employed to study provenance of rocks obtained from three areas of the terrane. Lower Jurassic sedimentary rocks, classified by inferred proximity to their source areas as proximal or proximal basin are derived from an arc source area. Sandstones of this age are immature. The rocks are geochemically and isotopically primitive. Detrital zircon populations, based on a limited number of analyses, have homogeneous Late Triassic or Early Jurassic ages, reflecting local derivation from Quesnellian arc sources. Middle Jurassic proximal and proximal basin sedimentary rocks show a trend toward more evolved mature sediments and evolved geochemical characteristics. The sandstones show a change to more mature grain components when compared with Lower Jurassic sedimentary rocks. There is a decrease in εNdT values of the sedimentary rocks and Proterozoic detrital zircon grains are present. This change is probably due to a combination of two factors: (1) pre-Middle Jurassic erosion of the Late Triassic – Early Jurassic arc of Quesnellia, making it a less dominant source, and (2) the increase in importance of the eastern parts of Quesnellia and the pericratonic terranes, such as Kootenay Terrane, both with characteristically more evolved isotopic values. Basin shale environments throughout the Jurassic show continental influence that is reflected in the evolved geochemistry and Sm–Nd isotopes of the sedimentary rocks. The data suggest southern Quesnellia received material from the North American continent throughout the Jurassic but that this continental influence was diluted by proximal arc sources in the rocks of proximal derivation. The presence of continent-derived material in the distal sedimentary rocks of this study suggests that southern Quesnellia is comparable to known pericratonic terranes. [ABSTRACT FROM AUTHOR]

Documents boninitic magmatism within the continental Yukon-Tanana terrane in Canada. Geologic setting and stratigraphy; Geochemical characteristics of the boninitic rocks; Evidence for boninitic magmatism linked to continental crust; Petrological models explaining the thermal and petrological requirements of the Fire Lake boninites.