The Geochemistry of the Snap Lake kimberlite dyke, NW Territories, Canada: Phlogopite and Spinel
The Snap Lake kimberlite is located in the southeastern part of the Slave Craton, 220 km northeast of Yellowknife, Northwest Territories. This kimberlite intrudes into Archaean granitoids and metavolcanics. Four different end-member lithofacies that exhibit different textural properties were identified within the Snap Lake kimberlite. The two most prominent end-members that occur within the dyke are: the olivine-rich kimberlite (ORK) and the olivine-poor kimberlite (OPK), which can be differentiated petrographically and geochemically. The ORK groundmass is composed of phlogopite (3 - 20%), apatite, Cr spinel with Fe-Ti rims and rutile. The OPK groundmass is composed of abundant amounts of phlogopite (30 - 60%), monazite and rutile and sparse Cr spinel cores and remnants of Fe-Ti rims. Both contain ubiquitous amounts of serpentine. OPK phlogopite occurs in two populations. These are large (100 - 600 μm), zoned phlogopites with Ba-poor cores (< 0.99 wt% BaO) and Ba-rich rims (1.08 - 4.07 wt% BaO) that contain occasional inclusions and smaller (< 90 μm), unzoned crystals with no inclusions. ORK phlogopites occur in small accumulations, mainly as large (100 - 800 μm) poikilitic crystals that have reverse zoning with Ba-rich cores (5.40 - 9.77 wt% BaO) and Ba-poorer rims (< 4.6 wt% BaO). The crystals have prominent 'opened' cleavage and 'localised' areas of Ba-enrichment within individual crystals. The Ba-enriched cores also exhibit an enrichment of F. The edges of the larger crystals and the smaller phlogopite crystals (< 20 μm) have been chloratized. The OPK phlogopites are more enriched in Ti (0.85 - 2.2 wt% TiO2) compared to ORK (0.12 - 0.84 wt% TiO2). The spinels in the ORK have good atoll textures with Cr cores (< 35 μm) and Fe-Ti rims (< 3 μm). The occasional Cr cores of the atoll spinels have been partially enclosed or replaced with rutile. Cr spinel also occurs in the groundmass and as inclusions as single-phase grains. The OPK has a much lower proportion of Cr spinel. The Cr cores (< 15 μm) of the spinel frequently have been replaced by either rutile or monazite. The Fe-Ti rims are only remnant and some have been replaced with rutile growing inwards towards the cores surrounding monazite. The lack of apatite and presence of monazite in OPK indicated a Ca-poor environment and vice versa for ORK. ORK and OPK have very different mineralogy and geochemistry indicating that they are contrasted magmas with different igneous histories, but which have been mixed together in the intrusion. The igneous events have been followed by multiple stages of metasomatism: 1) early metasomatism, 2) serpentinization and 3) alteration by acidic fluids derived from the surrounding granite.
Geological constraints on the emplacement of the Snap Lake kimberlite dyke, NW Territories, Arctic Canada
The Snap Lake kimberlite dyke, Northwest Territories, Canada, is a complex segmented ore-body. Detailed geological mapping was undertaken with a view of improving the productivity and efficiency of the mining operation. The relationship between the igneous geology of the kimberlite and complex structural features (e.g. ramps and steps) were investigated, to improve predictability of the dyke geometry. Geological mapping and analysis have established that the dyke is a multi-phase intrusion with four different magmatic lithofacies. In particular, olivine-rich and olivine-poor varieties of hypabyssal kimberlite have been identified. Key observations are that olivine-rich (phlogopite-poor) lithofacies has a strong tendency to be located where the dyke is thickest and that there is a good correlation between dyke thickness, olivine crystal size and crystal content. Accordingly the olivine-poor (phlogopite-rich) lithofacies tends to be most abundant where the dyke is thinnest. Empirical observations based on diamond size distributions recorded in three sets of closely spaced bulk samples, suggest that the diamond distributions might be controlled by the proportion of olivine-rich kimberlite in the sample. These relationships potentially have major economic implications for exploration and mining at other diamondiferous intrusions. Complex structural features are associated with changes in dyke thickness, and therefore parameters such as crystal and lithic size and content may allow better prediction of dyke complexity at a mining scale.