Distribution and chemistry of kimberlite indicator minerals in the southern Slave Province, NWT

Abstract

As part of the greater Slave Province geophysical, surficial materials and permafrost study, a Northwest Territories Geological Survey (NTGS) led government-academic-industry research program, this study is intended to identify and interpret indicator mineral glacial dispersal trains using publicly available mineral chemistry data and discuss the uses of the NTGS kimberlite indicator mineral chemistry database (KIMC) for diamond exploration in the southern Slave Province. In addition to the database, 21 till samples were collected from the southern Slave Province national topographic system map sheets 075M and 075N during the 2017 field season (17-DECS sample suite). Kimberlite indicator minerals (KIM) were recovered from the till samples and selected grains were subsequently analyzed using scanning electron microscopy and laser ablation techniques to identify mineral chemistry that is representative of KIMS in surficial sediment samples in the KIMC for the southern Slave. Mineral chemistry data collected during this study were evaluated and compared to those published in a database of the Slave Province by the NTGS. The database was created as a collaborative effort between the NTGS and exploration companies in order to compile an all-encompassing kimberlite indicator mineral database with raw mineral chemistry data. Mineral chemistry data retrieved from the NTGS database and from analysis of the 17-DECS sample suite were used to interpret kimberlite potential of the region. Ilmenite, chromite, Cr-diopside, olivine, and garnet grains in surficial sediment samples were assessed in terms of their chemistry and areal distribution in the Slave Province. The raw mineral chemistry data for garnets were classified according to their Gnumbers and chromite, ilmenite, olivine, and Cr-diopside were classified as kimberlitic or nonkimberlitic. Data from indicator minerals in till samples collected during this study were classified using the same criteria. Indicator minerals distribution patterns were mapped based on the classification of individual mineral grains. These maps show disparity in the amount of data contained in the database and the variation in kimberlite indicator mineral dispersal train direction, length, and composition between the north and south Slave. Kimberlite indicator mineral dispersal trains in the southern Slave Province are disjointed with highly variable indicator concentrations per sample location, and trend approximately westward. These trains are near monomineralic, often exclusively consisting of garnet. Trains in the northern Slave Province are more consistent in concentration (concentration increases with increasing distance down ice) and trend northwest and west. These trains have greater variety of kimberlitic mineral species. The direction, length, and composition of the trains reflects glacial processes (erosion, entrainment, transportation, deposition), permafrost conditions, and the nature of the source kimberlites. Of the kimberlite indicator minerals identified, garnet was the most abundant and informative mineral recovered from surficial sediment samples in the southern Slave Province. Overall, there was little variation in abundance of garnet G-number classes which could not be contributed to variations in sample density. However, garnet grains recovered from till samples in the southern Slave Province have lower sodium concentrations than till samples in the northern Slave. It has been proposed that the concentration of sodium in certain garnets is indicative of kimberlite diamond potential. Sodium concentrations of the southern Slave Province are below the diamond indicator threshold (Na2O>0.07%). Although this may be indicative of a lower diamond potential, it may also be a result of differing geochemical compositions of the subcontinental lithospheric mantle.