Evidence for sulfur saturation from zircon-hosted melt inclusions in felsic plutons and dikes of the Yellowknife greenstone belt, NWTTuesday, November 19, 2019 - 9:40am to 10:00am Theatre One
The Yellowknife greenstone belt (YGB) is located in the Archean Slave province and consists of mafic and felsic metavolcanic sequences deposited between 2722-2701 Ma. The YGB is crosscut by at least three generations of porphyritic, feldspar-quartz and quartz-feldspar dikes (FQP and QFP). The belt was also intruded by several plutonic bodies that were emplaced episodically between 2675-2608 Ma and range from tonalite to granite in composition. These include the Ryan Lake pluton (2675 Ma), Defeat Suite (2630 Ma), and the Duckfish Granite (2608 Ma). The YGB hosts two historic high-grade, shear zone-hosted gold mines, the Giant and Con mines which produced a total of 14.2 Moz gold. Throughout the YGB, gold mineralization is spatially associated with FQP and QFP dikes and has been documented in shear-zones that crosscut other plutonic bodies. However, it is poorly understood whether this association is purely spatial or if a genetic connection exists between the anomalous precious metal concentrations and felsic intrusions. Qualitative melt inclusion data was collected to evaluate the mineralization potential of these intrusions.
Melt inclusions represent a sample of magma trapped in a growing crystal during fractionation and crystallization of a rock. They can provide a snapshot of information related to magma sources, pre-existing metal endowment, and pressure and temperature conditions during entrapment. Due to their robust nature and prevalence in the lithologies of interest, zircons were the prime candidate for analysis. Zircons from mineral separates were obtained, mounted in epoxy and polished until the melt inclusions were exposed or close to the surface. LA-ICP-MS analysis was conducted for major, minor, and trace element data for zircon-hosted melt inclusions. Ablations targeted spherical and ovoid, opaque inclusions while attempting to avoid any obvious cracks or mineral inclusions. Although the data obtained is qualitative, many ablations of dark brown-black inclusions were found to be sulfur-rich, likely due to the presence of sulfide melts. These sulfide inclusions were also enriched in Cu, As, Sb, and Pb. This data suggests that sulfur saturation was achieved at the time the inclusions were trapped. Therefore, gold mineralization may be genetically associated with the felsic plutons and dikes as intrusion-related gold mineralization.