In this paper, we present a preliminary assessment of the lithogeochemical characteristics of meta-volcanic rocks in the Finlayson Lake region. Unit 2 mafic meta-volcanic rocks are subdivided into three suites: 1) low Ti tholeiites and boninites (suite 2a); 2) transitional (oceanic island basalt, OIB?), Light Rare Earth Element (LREE) -enriched tholeiites (suite 2b); and 3) normal mid-ocean ridge basalts (suite 2c; N-MORB). Suite 2a has similarities to rocks formed in ancient suprasubduction zone ophiolites and in forearcs in modern intraoceanic arcs. Unit 3 felsic meta-volcanic rocks comprise two subdivisions: 1) a low Eu/Eu*, Zr/Y, and Ce/Yb N suite (3a); and 2) a higher Eu/Eu*, Zr/Y and Ce/Yb N suite (3b). All unit 3 felsic meta-volcanic rocks have calc-alkalic continental arc signatures. Meta-basaltic rocks of the Campbell Range belt (CRB) fall into three suites: 1) moderately LREE enriched E-MORB type rocks (CRB 1 ); 2) LREE depleted N-MORB t ype rocks (CRB 2 ); and 3) a high Mg#, High Field Strength Elements (HFSE) and LREE-enriched tholeiitic suite (CRB 3 ). All CRB meta-basaltic rocks have features consistent with generation in an ocean basin and/or back-arc/marginal basin setting. The most prospective suites for volcanogenic massive sulphide mineralization in the Finlayson Lake region are 2a, 3a, and CRB 1 and CRB 2 .

The Finlayson Lake district contains >30 Mt of volcanogenic massive sulphide (VMS) mineralization, but has not been the focus of field-based research since the mid-2000s. We present herein preliminary fieldwork on Yukon-Tanana terrane (YTT) host rocks that are the groundwork for future petrologic, isotopic, and geochronologic studies of the stratigraphy and crustal evolution of the VMS deposits and YTT rocks in the Finlayson Lake region and other peri-Laurentian terranes of the northern Cordillera. During the summer of 2017, we logged seven drill holes that intersected the stratigraphic hanging walls and footwalls of the mafic-hosted Fyre Lake and felsic-hosted Kudz Ze Kayah and GP4F VMS deposits. The stratigraphic results generally reveal finely laminated to bedded mafic or felsic volcaniclastic rocks that are interbedded with clastic rocks or cut by intrusive rocks and reflect changes in depositional environments and tectonomagmatic regimes in the Late Devonian to Early Mississippian.

The Yukon-Tanana Terrane (YTT) in the Finlayson Lake region (FLR), southeastern Yukon, Canada is host to five volcanic-hosted massive sulphide (VHMS) deposits (total -34 Mt) that have been discovered since the mid-1990's. In this thesis, field, lithogeochemical and Nd isotopic data are presented for felsic and mafic igneous rocks in the FLR to understand the tectonic setting, style of magmatism, and their relationships to VHMS mineralization. All rocks in the FLR were built upon a continental (or continent-derived) substrate of pre-Mississippian (>365 Ma) age. The Fire Lake unit (FLU) reflects Devonian-Mississippian (-365-360 Ma) arc and back-arc magmatism built upon a composite basement of oceanic and continental (or continent-derived) crust above an east-dipping subduction zone. Models proposed herein for the magmatic and tectonic evolution of FLU include: 1) arc magmatism punctuated by back-arc basin generation; 2) ridge propagation into an evolving arc with subsequent evolution to back-arc magmatism; and/or 3) ridge-subduction (slab-window) with eventual back-arc basin magmatism. The Kudz Ze Kayah (KZK) unit overlies the FLU and consists predominantly of crustally derived Devonian-Mississippian (-360-356 Ma) felsic volcanic and high-level subvolcanic rocks and variably carbonaceous sedimentary rocks; the latter are crosscut and overlain by alkalic mafic rocks. The high field strength element (HFSE)-enriched (A-type) felsic rocks and alkalic mafic rocks in the KZK unit are inferred to represent magmatism within an ensialic back-arc basin upon evolved crust. The Wolverine succession (WS) unconformably overlies the KZK unit and consists of a lower succession of felsic volcanic and subvolcanic rocks with carbonaceous sedimentary rocks; the upper portion of the succession, above the Wolverine VHMS deposit, consists predominantly of aphyric rhyolitic rocks that are overlain basalt flows. Felsic rocks ofthe WS are broadly similar to those in the KZK unit and represent ensialic back-arc basin magmatism. However, the succession is younger (-356-346 Ma), and post-dates a period of uplift, deformation, and erosion prior to commencement of back-arc magmatism. Back-arc spreading eventually evolved to true seafloor spreading within the WS. Massive sulphide deposits in the FLR are preferentially associated with rocks indicative of high temperature magmatism (e.g., boninites, A-type felsic rocks) and extensional tectonic activity (e.g., back-arc rifting and spreading).

Between the Cretaceous granitic rocks (Hake Batholith on the west; Cassiar Batholith to the east) are three belts of metamorphic rocks, collectively part of Yukon-Tanana terrane. These are remnants of oceanic and continental volcanic arcs, and marginal basin sediments of Early to mid-Paleozoic age. At the head of Borden Creek are thick carbonate and andesitic volcanic rocks correlated with Klinkit Group. The Ram Creek fault and Hidden Lake fault are not exposed but deduced to be steeply dipping brittle structures with northeastward thrust or transpressional offset, based upon more complete exposure to the southeast in 105B/3 map area. The former is likely of Cretaceous age; the latter was active between mid-Permian and Early Jurassic time.

Mafic volcanic and clastic strata of the Middle Triassic Joe Mountain Formation, east of Lake Laberge, Yukon, represent a juvenile volcanic arc sequence. Mafic volcanic rocks of the Upper Triassic Lewes River Group were formed in the spatial and temporal continuity of Joe Mountain volcanism. Carbonate sedimentation took place in shallow oceanic subbasins adjacent to the arc from the Carnian to Rhaetian; these subbasins were separated by physiographic boundaries inherent to the arc, resulting in lateral stratigraphic variations. Polymictic conglomerate and turbiditic sequences of the Lower-Middle Jurassic Laberge Group unconformably overlie Triassic rocks. Two north-northwest strike-slip faults, the Laurier Creek and the Goddard, control the distribution of units. Joe Mountain Formation rocks are characterized by an east-west structural trend, whereas the Upper Triassic and Jurassic sequences are characterized by north-northwest trending tight folds and thrust faults. At least five post-accretion igneous suites intrude or overlie older stratigraphy, including the Late Cretaceous Open Creek volcanic complex.

Relatively unstrained Devonian-Mississippian volcanic and volcano-sedimentary rocks have been documented in the Money Creek thrust sheet in Finlayson Lake map area. The succession comprises a five-unit volcanic stratigraphy containing subaerial and subaqueous mafic and felsic volcanic and volcaniclastic rocks and associated sedimentary rocks that are underlain, and locally crosscut by, sub-volcanic mafic intrusions and quartz porphyritic granite. Magma-mingling relationships between mafic dykes and quartz-porphyritic granite suggest that mafic and felsic volcanism was broadly coeval. A published 360.5 ± 1 Ma U-Pb date on a quartz porphyritic granitic intrusion establishes the age of volcanism. Biotite-hornblende granitic rocks of the Simpson Range Plutonic Suite (SRPS) intrude and metamorphose the volcanic sequence and related sub-volcanic intrusive rocks, and coupled with previously published U-Pb dates (345-350 Ma), this relationship implies that the SRPS is a distinctly younger pulse of magmatism. Mafic and ultramafic rocks of the Money Creek thrust sheet have previously been correlated with the Pennsylvanian-Permian Campbell Range belt and together both have been considered part of the Anvil Allochthon or Slide Mountain Terrane. Field characteristics, age, and geochemistry show that neither correlation is valid.

Upper Devonian and Lower Mississippian metavolcanic rocks of Yukon-Tanana Terrane in southern Finlayson Lake and Frances Lake map areas occur in three thrust sheets, locally modified by a Cretaceous normal fault. The lower thrust sheet, the Big Campbell sheet, comprises the Upper Devonian to Lower Mississippian metavolcanic stratigraphy that hosts the main volcanichosted massive sulphide (VHMS) deposits of the district. Metavolcanic rocks in the middle thrust sheet, the Money Creek sheet, include the Upper Devonian Waters Creek and Early Mississippian Tuchitua River formations. The former comprises primarily felsic metavolcanic rocks and carbonaceous phyllite and is extensively intruded by sheets of comagmatic porphyry. The latter comprises primarily intermediate metavolcanic, volcaniclastic and epiclastic rocks. The upper thrust sheet, the Cleaver Lake sheet, is in part made up of Late Devonian calc-alkaline basalt and rhyolite, the Cleaver Lake formation, and comagmatic felsic to ultramafic plutonic rocks. Of these, the Waters Creek formation and the formations in the Big Campbell sheet have the highest potential to host VHMS deposits.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

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The layered rocks in this area originated as continental shelf sediments overlain by volcanic arc successions. Now called Yukon-Tanana terrane, they tectonically over-rode the western edge of ancient North America beginning in Middle Jurassic time. Three elements are present in the map area. The west half comprises the Big Salmon Complex; the east half is a separate, in part contemporaneous succession composed of the Dorsey Complex and Swift River Group. Unconformably overlying both these elements are less metamorphosed Klinkit Group and Triassic sediments that are here interpreted as overlap assemblages. The unexposed contact between Big Salmon Complex and Swift River Group is inferred to be an east-side-down normal fault.