Behaviour of Ore-Forming Elements in the Sub-Continental Lithospheric Mantle Below the Slave Craton

Abstract

The transport and depositional controls of ore-forming elements such as Ag, Au, Bi, Cu, Mo, Pb, Pt, and Zn has been extensively studied in crustal environments, but the behaviour of these elements within Earth’s mantle require further study. This thesis presents new petrographic and geochemical data on the abundance of these and other trace and major elements for a suite of mantle peridotites from the subcontinental lithospheric mantle beneath northern Canada. Better constraining the behaviour and residence of base- and precious-metals in the lithospheric mantle will help develop new models for the transport of these ore-forming components from the mantle to the crust. The samples studied were collected from drill-core from the Jericho, Muskox, and Voyageur kimberlites that erupted through the northern Slave Craton in Nunavut, Canada. The mid-Jurassic Jericho kimberlite cluster was used due to the relatively low level of metasomatism of the kimberlites and their entrained xenoliths, the abundance of mantle xenoliths, and the relatively close proximity (∼30 km) to the former Lupin gold mine and three VMS deposits (30–50 km). The selected sample suite is comprised of peridotites with varying modal mineralogy and mantle xenocrysts (i.e., lherzolite, harzburgite, and wehrlite). Major-, minor- and trace-elements were analyzed by electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. Single-grain Al-in-olivine thermometry was used to estimate temperatures of formation, which were then projected onto a well-constrained xenolith-based mantle geotherm to estimate pressure and equilibration depth. The trace element composition of olivine from different depths was then used to reconstruct the “mantle stratigraphy” in this part of the northern Slave Craton. The equilibration conditions experienced by the peridotites, prior to kimberlite sampling, range from ∼100–200 km deep and ∼810–1210 ◦C, i.e., a large fraction of the garnet-bearing lithospheric mantle was sampled. Depth profiles were constructed for the analyzed elements to reveal the vertical distribution of trace elements within the lithosphere mantle. QEMScan analysis was used to quantify peridotite modal mineral abundances, which combined with mineral geochemical data to perform mass balance calculations, to estimate whole-rock concentrations. Notable features in the trace element data include high field strength element (i.e., Nb, Ta) enrichment at ∼130 km depth, a systematic enrichment in Cu with depth towards the base of the lithosphere, and a variably depleted mantle composition. Trace element trends found in the northern Slave Craton are consistent with those documented previously in the sub-continental lithospheric mantle below the Superior Craton, suggesting consistency in geochemical behaviour in the depth-related variability of some trace elements (i.e., Cu). The results from the mass balance calculations demonstrate the mantle sample suite yield depleted (e.g., high modal olivine Mg#’s) and/or enriched trace element concentrations (e.g., Cu, Zn, Nb, Ta) compared to depleted mantle estimates. The variations found are most likely the product of the complex, multi-stage metasomatic history, where enrichment is produced through protokimberlitic and carbonatite melt metasomatism re-fertilizing rocks that were previously depleted through melt extraction. Depending on the composition of the metasomatic agent different element groups were re-fertilized. HFSE were determined to be enriched in the mantle through carbonatitic metasomatism, resulting in a substantial reservoir hosted in the lithosphere. As shown by the systematic variation with depth, elements of economic interest (e.g., V, Cr, Cu) were re-enriched in the lithosphere by the metasomatic fluids and/or melts and incorporated into silicate minerals through substitution mechanisms. Inconsistent enrichment trends in precious metals and ore-forming elements make determining the metasomatic source, if any, problematic. Elements enriched in the northern Slave peridotites were found to be incorporated into the host minerals through multiple mechanisms. Those elements with moderate compatibility into mantle silicates, were found to be included though substitution (e.g., Cu2+ and Mg2+) based mechanisms. Whereas incompatible elements, such as Au, are incorporated as micro-inclusions, that were not observed during microscopy. The mode of incorporation is reflected in the time-resolved mass spectrometer signal intensity. Substitution mechanisms produce smooth spectra even at low signal intensities; whereas inclusions produce irregular and spikey spectra that are orders of magnitude above background. Trace element systematics in kimberlite derived mantle xenoliths and xenocrysts demonstrate that silicate minerals can host significant concentrations of ore-forming elements and contain a plethora of information about the metasomatic history of the mantle source region.