Recent advances in the characterization of metasomatic iron and alkali-calcic (MIAC) systems with associated iron-oxide apatite (IOA) prospects and iron-oxide–copper–gold (IOCG) and metasomatic cobalt deposits of the Great Bear magmatic zone were used to determine if the geochemistry of glacial sediments can unveil pathfinder elements indicative of mineralization and associated alteration. Analysis of variance within bedrock lithogeochemical (n = 707 samples) and till geochemical datasets (n = 92 samples) are compared. Results show that Fe, Co, Ni, Cu, As, Mo, Bi, La, Th, U, and W were identified as potential vectoring elements in different fractions of till due to their anomalous concentrations down-ice of various mineralized outcrops within the study area. For instance, Fe, Co, Cu, and Mo were established as the most useful vectoring elements in the locally derived till (<2 km down-ice) near the Sue Dianne IOCG deposit, and Fe, Co, Ni, Cu, Mo, W, Bi, and U near the Fab IOCG prospect. At the Sue Dianne deposit, the ratios of near-total (4-acid digestion) versus partial (modified aqua regia digestion) concentrations in the silt + clay-sized till fraction (<0.063 mm) for both La and Th reflect the mineralization alteration signature and define a more consistent dispersal train from mineralization compared to element concentrations mapped alone. Additional testing in an area of continuous till cover near an isolated point source is recommended to further develop the elemental ratio method for exploration of MIAC systems.
Apatite is the main REE-bearing phase in 19 thin sections from two mineralized systems and their host rocks, the Cu-Ag-(Au) Sue-Dianne IOCG deposit and the nearby Brooke Zone, located within the southern Great Bear magmatic zone, Northwest Territories, Canada. The thin sections show a range of IOCG-related alterations and igneous protoliths. Apatite occurs in all thin sections, in amounts ranging between traces to 5-10%. It can be divided into four populations on the basis of size, cathodoluminescence (CL) response and petrographic association: 1) small euhedral apatite (20-40 µm), occurring in clusters of 5 to 20 grains, generally free of inclusions, found in the volcaniclastic samples; 2) single apatite crystals (60-80 µm) with pitted edges, numerous fluid inclusions and sometimes monazite and hematite inclusions; 3) large single apatite crystals (100-300 µm) within epidote veins, sometimes associated with scheelite; and 4) large fractured apatite crystals (0.5 to 4 mm) with heavy hematite staining associated with hydrothermal breccias. The first 3 populations have a green to yellow CL response and the third population shows some irregular zoning. The fourth population shows a blue CL tint, with irregular green zones throughout grains or near the crystal rims. In that fourth population, mineral inclusions with an orange CL response are likely calcite. Group 3 and 4 also show an heterogeneous REE distribution within crystals under EDX. Given the overprinting of multiple alteration types in many of the specimens, additional work is needed to relate these apatite populations to specific alteration stages associated with IOCG-type mineralization. However, the presence of apatite coarse enough to be picked from glacial sediment samples and showing distinctive CL signatures in rocks related to IOCG mineralization suggests potential as an indicator mineral method.
<p>The Laurentide Ice Sheet (LIS) during the Pleistocene-Holocene transition provides a useful natural laboratory for examining the behavior of a mid- to high-latitude ice sheet during a period of climatically driven ice sheet thinning and retreat. While the timing and pattern of Pleistocene recession of the LIS are well-constrained along the southern and eastern margins, there is limited chronology constraining the ice margin retreat along the northwestern margin. Here we present new cosmogenic <sup>10</sup>Be exposure ages retreat of the western margin of the LIS during the Pleistocene-Holocene transition. Sampling was performed along three transects located between the northern shore of Great Slave Lake and Lac de Gras. Each of the transects is oriented parallel to the inferred ice retreat direction in an attempt to capture a regional rate of retreat. Our new <sup>10</sup>Be cosmogenic exposure ages from the southeastern Northwest Territories demonstrate that regional deglaciation occurred around 11,000 years ago. The population of ages broadly overlaps, indicating that either the retreat occurred within the resolution of our chronology or that the ice sheet experienced widespread stagnation and rapid down-wasting. These ages, not corrected for changes in atmospheric depth due to isostatic rebound, are older than minimum limiting radiocarbon constraints by ~1000 years, indicating that existing LIS reconstructions may underestimate the timing and pace of ice margin recession for this sector. Constraining the timing of the recession of the northwest sector of the LIS has the potential to inform our understanding about the damming of large proglacial lakes, such as Glacial Lake McConnell. The ages from our southern transect, collected from elevated bedrock hills, indicate LIS retreat from through the McConnell basin occurred after 12,000 years ago, and thus constitute maximum limiting constraints on the expansion of Glacial Lake McConnell southeastward into the present-day Great Slave Lake basin. Our chronology, combined with other emerging cosmogenic exposure ages constraining LIS deglaciation indicates retreat of the ice margin over 100s of kilometres during the Pleistocene-Holocene transition, exhibiting no evidence of a significant readvance during the Younger Dryas stadial.</p>
Abstract Significant uncertainty surrounds the crystallization conditions and the composition of kimberlite melts, including the role of volatiles (H2O and CO2) due to their hybrid nature, intense alteration, and volatile loss during emplacement. In this study, we address these uncertainties by investigating the interaction between oxides (ilmenite and chromite) and kimberlite magma. During kimberlite ascent, mantle minerals react with the magma and develop dissolution textures, compositional zoning, and rims of secondary mineral phases in response to crystallization conditions and the composition of kimberlite magma. We examined oxides from several lithologies within the BK1 and AK15 kimberlites of the Orapa cluster in Botswana, where diamonds demonstrate distinct dissolution styles in each lithological unit owing to differences in magma saturation with volatiles. Here we discovered a strong correlation of the reaction products on ilmenite and chromite with the dissolution style of diamonds in the same kimberlite unit. Diamonds with glossy, low-relief surface features indicative of fluid-rich magma occur in the kimberlite units where ilmenite and chromite develop reaction rims of Ti-bearing phases. Diamonds with corrosion sculptures implying a volatile-undersaturated magma occur in kimberlite units with heavily resorbed chromite and ilmenite completely replaced by a MUM (magnesio-ulvöspinel-magnetite)–perovskite symplectite. Furthermore, the composition of ilmenite reaction rims depends on kimberlite lithology, where MUM co-exists with perovskite or its break-down product anatase in the two coherent kimberlite units, or with perovskite and titanite in the massive volcaniclastic unit. We examine how decompression, cooling, degassing, or assimilation of crustal rocks by kimberlite magma could have shifted conditions from perovskite to titanite stability in the volcaniclastic kimberlite unit. We propose perovskite replacement by anatase-calcite pseudomorphs in the top coherent unit, from which diamonds exhibit an overprint of fluid resorption with a melt resorption. Composition of ilmenite reaction rims provides estimates of kimberlite crystallization temperatures of 730–1275 °C and oxygen fugacities of +0.5 to −3.5 relative to the nickel-nickel oxide buffer, which are validated through controlled experiments. Our study shows that preservation of ilmenite, the type of Ti-phase in its reaction rim, the relative rate of chromite dissolution, and compositional re-equilibration with kimberlite can help model the eruption process as well as the style and rate of diamond dissolution.
