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
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.