The 2.68 Ga Zn-Pb-Cu-Ag Hackett River Main Zone (HRMZ) volcanogenic massive sulfide (VMS) deposit, within the Hackett River Greenstone Belt of the Archean Slave Craton is highly enriched in Ag (and Pb) compared to other VMS deposits of a similar age and type. The mineralization has been sub-divided into five categories based on mineralogy, textures and stratigraphic location: 1) disseminated footwall sulfides, 2) copper-rich stringer sulfides, 3) pyrite-poor sphalerite-pyrrhotite-chalcopyrite mineralization located at the top of the stringer zone, 4) mineralization in calc-silicate altered units and 5) sphalerite-pyrite massive sulfide mineralization. Using a mass-balance for Ag calculated from electron microprobe analyses, pyrrhotite and chalcopyrite in type 1 mineralization contain negligible Ag and in type 2, Bi-Ag-(Pb) sulfides, Ag-Bi-Se enriched galena and chalcopyrite are the dominant Ag hosts. Within type 3, freibergite and galena are the main silver hosts. In type 4, Ag is hosted in disseminated electrum and freibergite while freibergite in type 5 hosts 99% of the Ag. Overall, Ag-rich freibergite contains 79.4% of the total Ag, chalcopyrite hosts 6.3% and galena contains 1.8% of the Ag. Trace minerals such as electrum, stephanite, acanthite and Bi-bearing sulfides host the remainder of the Ag (12.5%) and have a restricted spatial distribution. Mineral assemblages have undergone pervasive recrystallization and annealing during amphibolite grade metamorphism with localized redistribution of base and precious metals from metamorphism at a grain scale only. Within freibergite and chalcopyrite, Ag directly substitutes for Cu within the mineral lattice and replaces Pb in galena by coupled substitution with Bi and to a lesser extent, Sb. The principal controls on Ag residence in the HRMZ are temperature and redox conditions (which varies with distance to the hydrothermal vent) and the ratio of Bi and Sb available for coupled substitution with silver within galena. Subsequent deposit-scale zone refining is the principal factor influencing the distribution of Ag. Lower temperatures and more oxidizing conditions favour partitioning of Ag into freibergite and less oxidizing conditions favour galena. At higher temperatures, the most reducing conditions favour incorporation of Ag in Ag-Bi rich galena (plus Se) and Bi-bearing sulfides or Ag-rich chalcopyrite under lesser reducing conditions. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2009-03-12 10:46:49.993

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The oxidation state, reflected in the oxygen fugacity (fO2), of the lithospheric mantle is both laterally and vertically heterogeneous. Depth-fO2 profiles from kimberlite-borne peridotitic mantle xenoliths from the Bultfontein kimberlite, Kimberley, South Africa and the A154-N and A154-S kimberlites of the Diavik Mine, NWT, Canada were constructed by measuring ferric iron concentrations in garnets using the flank method. These data demonstrate that mantle metasomatic re-enrichment processes had a significant effect on fO2. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO2 (FMQ) of -2 at 110 km to -4 at 210 km. The lithospheric mantle beneath Diavik is vertically layered with respect to its bulk and trace-element composition. The shallow ‘ultradepleted’ layer is oxidized, to the point that carbonate rather than graphite is the anticipated carbon host. The deeper layer is more fertile and has fO2 conditions extending down to Δlog fO2 (FMQ) -3.8. Deviations from predicted depth-fO2 trends in both xenolith localities result from metasomatic re-enrichment caused by transient fluids and melts. Diamond formation in the Kaapvaal lithospheric mantle may have occurred through the infiltration of reduced fluids into relatively more oxidized mantle. Trace-element concentrations in garnets preserve evidence of two distinct melt metasomatic enrichment events. One was a craton-wide event that is commonly observed in garnet peridotite xenoliths and xenocrysts worldwide; the other was melt infiltration event, preserved as MARID xenoliths, related to the eruption of the Group 2 kimberlites in the western portion of the Kaapvaal craton. The effect of the former melt metasomatism on fO2 is unclear ambiguous whereas the MARID event was clearly oxidizing. Diavik xenoliths preserve evidence for events similar to the fluid and ‘common’ melt metasomatism seen in the Bultfontein samples. Fluid metasomatism affected the entire depth range of xenoliths sampled from Diavik and was oxidizing. A stage of melt metasomatism affected only the deeper (>140 km) portion of the lithospheric mantle and had an overall reducing effect. The observation of sharp-edged octahedral diamonds in microxenoliths affected by the fluid metasomatic event may indicate that this was a major diamond-forming event in the mantle beneath Diavik.

This thesis presents the results of a study on diamonds from the Artemisia kimberlite, Nunavut, Canada. This study integrates the isotopic, chemical and physical characteristics of the diamonds in an attempt to provide insight into the nature of the micro-/macro-diamond genetic relationship, and its possible implication pertaining to the use of size frequency distributions in modern kimberlite exploration programs. Despite geochemical commonalities of the Artemisia micro- and macro-diamonds, distinct signatures are observed, particularly in regards to enriched and depleted carbon isotopic compositions, the abundance of nitrogen and hydrogen impurities and different resorption histories. This implies that, despite an observed lognormal size frequency distribution for Artemisia diamonds, micro- and macro-diamonds do not represent a single population at this locality.

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