Browsing by Author "McFarlane, Chris"

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    Assembling an Archean magmatic plumbing system: Geochemical evolution, emplacement, and metallogenic implications of subvolcanic intrusions in the Yellowknife greenstone belt, N.W.T., Canada
    (University of New Brunswick, 2023-02) Speight, Sarah Catherine; McFarlane, Chris; Hanley, Jacob
    The Yellowknife greenstone belt (YGB), located in the Archean Slave Province in the Northwest Territories, Canada, consists of mafic and felsic metavolcanic sequences intruded by dikes and regional plutons. The locally known “#9” dikes comprise three generations of intrusions that were not geochemically differentiated or genetically linked to a parental source, including, feldspar-quartz porphyry (FQP), quartz porphyry (QZP), and aplite dikes. Despite the spatial association with mineralization and inferred relationship to overlying volcaniclastic units, the dikes remain poorly understood. Therefore, a multi-analytical approach was needed to characterize the evolution, emplacement, and metallogenic signature of YGB magmatism. The lithogeochemical signatures of the dikes, combined with U-Pb zircon geochronology, differentiated them into three groups: 1) the Ryan Lake Pluton (RLP), represented by tonalites (2662 Ma) and granodiorites (2647 Ma), and the FQP (2674 – 2662 Ma) and aplite-1(a) dikes (2663 Ma); 2) the Defeat Suite granite (2580 Ma) and aplite-1(b) dikes (2662 Ma); and 3) the Duckfish granite (2576 Ma) and the aplite-2 dikes (ca. 2611 Ma). The intrusions were also compared to the overlying felsic volcaniclastics of the Townsite Formation and Banting Group. When combined with field observations, this confirms that only the FQP dikes are subvolcanic feeders to these units. The ambiguous lithogeochemistry of the QZP dikes (2770 Ma) and the age gap with the main diking event (ca. 2662 Ma) did not allow for a plutonic connection. Rather, the QZP and one FQP dike proximal to the Duckfish granite may reflect magmatism related to the Central Slave Basement Complex. Additionally, macro- and micro-scale textural analysis of the aplite dikes provided evidence of emplacement during crystallization of the parental magmas. The three groups of aplite dikes (1(a), 1(b), and 2) commonly have quartz cores and discontinuous quartz pods at the outcrop scale that reflect crystallization from a volatile-saturated melt. Finally, analysis of zircon-hosted melt inclusions (MI) from a subset of YGB dikes and plutons showed evidence of early precious metal-enrichment. Careful data filtering to remove contaminated inclusions produced the first MI dataset from the Archean YGB which represents primary silicate melts. Zircon-hosted MI that contains accidentally trapped phases also provide valuable information, despite the semi-quantitative nature of the data. These mixed MI provide direct evidence of Ti- and H20-saturation through the presence of (Fe)-Ti-oxides and hydrous minerals (e.g., biotite, amphibole). This preliminary work establishes that critical information related to primary metal signatures in the YGB can be obtained from Archean MI.
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    Genesis of gold mineralization at the Cantung W-Cu skarn deposit, N.W.T.
    (University of New Brunswick, 2020) Lentz, Carlin Paul Edward; McFarlane, Chris
    This study was initiated to identify what phases host the gold mineralization, which skarn assemblages it is hosted within, the relationship between the tungsten and copper mineralization and the gold and bismuth mineralization, as well as the timing of these gold mineralizing events and their relationships to nearby intrusions at the Cantung W-Cu mine, Northwest Territories, Canada. Using detailed reflected light microscopy, the main gold-bearing phase was identified as electrum (Au-Ag alloy) with an assemblage that included native bismuth, several bismuth telluride and selenide minerals, as well as silver and lead sulfosalts. The textures exhibited by these Bi-Au-Te-Se -bearing phases suggest they precipitated from aqueous solutions as polymetallic melts which remained molten throughout the growth of other skarn and vein minerals and ultimately exsolved from these melts. After ascertaining the skarn and vein assemblages hosting gold, U-Pb geochronology of allanite and titanite was conducted to determine the timing of these events.
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    Petrology and geochronology of an inverted metamorphic sequence in the western Cape Breton Highlands, Nova Scotia
    (University of New Brunswick, 2020) McCarron, Travis John Angus; McFarlane, Chris
    In the western Cape Breton Highlands of Nova Scotia, medium- and high-grade metasedimentary rocks of the Jumping Brook Metamorphic Sequence and structurally overlying Pleasant Bay Complex comprise an inverted sequence that formed in the Late Silurian-Early Devonian in response to terrane accretion on Cape Breton Island. Using numerical models and core-rim major element zoning in garnet, tight clockwise pressure-temperature paths of metamorphism were determined for the garnet and staurolite zones of the inverted sequence. Systematic discrepancies between thermodynamic model predictions and geochemical observations led to the discovery of ilmenites that sequestered Mn during prograde metamorphism, causing garnet core isopleth intersections to be decoupled from garnet-in curves. A multi-chronometer dataset revealed a significant discontinuity between the medium- and high-grade rocks of the inverted sequence. Detrital zircon geochronology indicates that the Jumping Brook Metamorphic Suite was deposited in the Silurian and derived from local sources in Ganderia, whereas the Pleasant Bay Complex was deposited in the Cambrian-Ordovician and derived from sources in Ganderia and Amazonia. Garnet and monazite geochronology constrain prograde metamorphism in the Jumping Brook Metamorphic Suite and Pleasant Bay Complex to a 7 Myr period between 395 and 388 M and a 22 Myr period between 418 and 396 Ma respectively. Despite differences in provenance, depositional age, and prograde metamorphic histories, apatite cooling ages indicate that the Jumping Brook Metamorphic Suite and Pleasant Bay Complex cooled contemporaneously in the Late Devonian. The geochronological dataset and tight clockwise pressure-temperature paths are consistent with a tectonic model in which volcanic arc material was accreted, transposed, and rapidly exhumed in the western Cape Breton Highlands along west-directed ductile shear zones as outboard terranes were accreted. An alternative Lu-Hf garnet geochronology method was developed using a staurolite zone schist from the Jumping Brook Metamorphic Suite. A population of garnet porphyroblasts were liberated from the sample and sorted into eight size classes. Lutetium-Hf ages for the individual size classes decrease with garnet grain size, consistent with diachronous nucleation during heating. Ages for the largest and smallest size classes define a minimum timescale of 7 Myr for prograde metamorphism, identical to independent constraints.
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    The Nucleus deposit: superposed Au-Ag-Bi-Cu mineralization systems at Freegold Mountain, Yukon, Canada with constraint from titanite U-Pb dating
    Betsi, Thierry Bineli; Lentz, David, R.; McFarlane, Chris
    In this paper we present titanite U-Pb (both single crystal CA ID-TIMS and in situ LA ICP-MS) data, coupled with ore and gangue mineralogy and geochemical (both lithogeochemistry and microanalysis) data from the Nucleus Au-Ag-Bi-Cu deposit, in the Yukon (Canada) portion of the Tintina Au province. Arsenic-bearing Au-Ag-Bi-Cu mineralization at Nucleus consists of two distinct styles of mineralization including: (i) reduced Au skarn and sulfide replacement; and (ii) a relatively shallow-emplaced (as supported by textures and temperature of formation), vein- controlled mineralization occurring mainly as veins and veinlets of various shapes (sheeted, single, stockworks, and crustiform), breccias, and disseminated. Whereas Au, Bi, and Cu mineralization from skarn is associated with hydrous retrograde alteration phases (actinolite, ferro-actinolite, hastingsite, cannilloite, and hornblende), numerous alteration types are associated with the vein-controlled style of mineralization and these include: potassic (biotitization), phyllic, argillic, propylitic, carbonate, and quartz (silicification) alterations. The mineralization-alteration processes took place over a wide temperature range that is bracketed between 340-568 °C, as indicated by chlorite and arsenopyrite geothermometers. The Au-rich Nucleus deposit is characterized by anomalously high content of As and Bi (as much as 1 %), and whereas Au moderately correlates with Bi (r = 0.40) in the skarn mineralization style (where native Au is spatially associated with native Bi and Bi-bearing sulfides), the two elements poorly correlate (r = 0.14) in the vein-controlled type, in which native Bi- and Bi-sulfide-bearing veins are locally observed. Sphalerite from the vein-controlled mineralized type is Fe-rich (9.92-10.54 mol % FeS), indicative of low sulfidation conditions, as well as high temperature, with the latter further supported by arsenopyrite geothermometry (up to 491 °C), low Ag content (3-7 wt %) in Au, and the high gold fineness (926-964). Whereas molybdenite Re-Os ages from quartz-molybdenite veins range from 75.8 to 76.2 ± 0.3 Ma, titanite from the skarn type mineralization recorded CA ID-TIMS and LA ICP-MS U-Pb ages of 182.6 ± 2.4 Ma and 191.0 ± 1.5 Ma, respectively, thus precluding any genetic link between the two spatially associated styles of mineralization from the Nucleus deposit area. The Au-Ag-Bi-Cu Nucleus deposit is therefore regarded as a superposed system in which two mineralization types, without any petrogenetic relationship, overlapped, possibly with remobilization of early formed mineralization.