Northwest Territories Geological Survey

The Yellowknife greenstone belt (YGB) consists of a homoclinal sequence of mafic and felsic metavolcanic rocks that young to the southeast and are cross-cut by at least three generations of felsic to intermediate dykes. Early mappers identified these dykes as porphyritic quartz-feldspar and feldspar-quartz intrusions and grouped together as the #9 dykes, despite their relative timing and textural differences. Narrow aplitic and granitic dykes were also observed and appear to be the youngest dykes in the belt. There has been limited work regarding the whole-rock and trace-element geochemistry of the YGB, and no studies have focused on the cross-cutting porphyritic dykes. This study aims to shed light on the genesis of these dykes by analyzing a larger data set from across the YGB and using immobile elements to characterize each lithology, complimenting isotope work, age dating, and field relationships.

Representative samples from the lithologies of interest were pulverized and made in to pressed pellets for analysis by micro-X-Ray Fluorescence (µXRF) and fused in to glass beads for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Quantitative µXRF data was gathered to use for internal standardization during LA-ICP-MS. This combination of analytical techniques has allowed for the complete geochemical characterization of the intrusions. Previous geochemistry studies of the Yellowknife region have focused on the Kam and Banting metavolcanic groups. However, this work used classification diagrams that compared elements, such as Fe, Mg, Na, Ca, and K, which are potentially mobile during even low degrees of hydrothermal alteration. This type of weak to intense alteration is nearly pervasive throughout the YGB, therefore any classifications done using these elements could give misleading results and warrants verification using more appropriate immobile high field strength elements, such as those involving alteration indices and immobile trace elements and applicable discrimination diagrams. By evaluating data in this manner, we can assess mass elemental losses and gains, and which trends may relate to mineralization.