- 273 views
- 203 downloads
Behaviour of Ore-Forming Elements in the Sub-Continental Lithospheric Mantle Below the Slave Craton
-
- Author / Creator
- Veglio, Christian
-
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. -
- Graduation date
- Fall 2020
-
- Type of Item
- Thesis
-
- Degree
- Master of Science
-
- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.