The geology of kimberlite pipes of the Ekati property, Northwest Territories, Canada

This paper reviews key characteristics of kimberlites on the Ekati property, NWT, Canada. To date 150 kimberlites have been discovered on the property, five of which are mined for diamonds. The kimberlites intrude Archean basement of the central Slave craton. Numerous Proterozoic diabase dykes intrude the area. The Precambrian rocks are overlain by Quaternary glacial sediments. No Phanerozoic rocks are present. However, mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late-Cretaceous and Tertiary cover (likely <200 m) was present at the time of emplacement. The Ekati kimberlites range in age from 45 to 75 Ma. They are mostly small pipe-like bodies (surface area mostly <3 ha but up to 20 ha) that typically extend to projected depths of 400–600 m below current surface. Pipe morphologies are strongly controlled by joints and faults. The kimberlites consist primarily of variably bedded volcaniclastic kimberlite (VK). This is dominated by juvenile constituents (olivine and lesser kimberlitic ash) and variable amounts of exotic sediment (primarily mud), with minor amounts of xenolithic wall-rock material (generally <5%). Kimberlite types include: mud-rich resedimented VK (mRVK); olivine-rich VK (oVK); sedimentary kimberlite; primary VK (PVK); tuffisitic kimberlite (TK) and magmatic kimberlite (MK). The presence and arrangement of these rock types varies widely. The majority of bodies are dominated by oVK and mRVK, but PVK is prominent in the lower portions of certain kimberlites. TK is rare. MK occurs primarily as precursor dykes but, in a few cases, forms pipe-filling intrusions. The internal geology of the kimberlites ranges from simple single-phase pipes (RVK or MK), to complex bodies with multiple, distinct units of VK. The latter include pipes infilled with steep, irregular VK blocks/wedges and at least one case in which the pipe is occupied by well-defined sub-horizontal VK phases, including a unique, 100-m-thick graded sequence. The whole-rock compositions of VK samples suggest significant loss of kimberlitic fines during eruption followed by variable dilution by surface sediment and concurrent incorporation of kimberlitic ash. Diamond distribution within the kimberlites reflects the amount and nature of mantle material sampled by individual kimberlite phases, but is modified considerably by eruption and depositional processes. The characteristics of the Ekati kimberlites are consistent with a two-stage emplacement process: (1) explosive eruption/s causing vent clearing followed by formation of a significant tephra rim/cone of highly fragmented, olivine-enriched juvenile material with varying amounts of kimberlitic ash and surface sediments (predominantly mud); and (2) infilling of the vent by direct deposition from the eruption column and/or resedimentation of crater rim materials. The presence of less fragmented, juvenile-rich PVK in the lower portions of certain pipes and the intrusion of large volumes of MK to shallow levels in some bodies suggest emplacement of relatively volatile-depleted, less explosive kimberlite in the later stages of pipe formation and/or filling. Explosive devolatilisation of CO₂-rich kimberlite magma is interpreted to have been the dominant eruption mechanism, but phreatomagmatism is thought to have played a role and, in certain cases, may have been dominant.     

Zoned sodic amphibole: Petrologic indicator of changing pressure and temperature during tectonism in the Bathurst Area,New Brunswick,Canada

Amphibole, zoned from an actinolite core through a barroisite layer to a crossite or ferroglaucophane rim, coexists with epidote, chlorite, albite, quartz, phengite, sphene, magnetite, and calcite in metabasites of the Tetagouche Group near Bathurst, New Brunswick. Stratigraphic, structural and regional-metamorphic evidence suggests that these rocks were situated in the hangingwall of a SE-dipping subduction zone during the Taconian Orogeny. Using microprobe analyses of the coexisting minerals, a dehydration reaction and a subsequent hydration reaction have been balanced in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-(Fe, Mg, Mn)O-CaO-Na₂O-K₂O-H₂O, so as to describe the development of the zoned amphiboles. Relatively large coefficients for magnetite in these reactions accord with the absence of sodic amphibole in associated magnetite-free metabasites, indicating that ferric iron has a significant effect on the transition from greenschist to epidote-blueschist fades. Relatively small residuals of FeO, MgO and MnO show that these components are only weakly partitioned between the reactant and product assemblages. The procedure of Holland and Richardson (1979) has been used to calculate apparentP-T paths that trace the growth histories of the zoned amphiboles from a low-P environment to a high-P environment. Although the apparent pressure-change is implausibly large and the final temperature implausibly low, it is clear that the Bathurst metabasites followed a very differentP-T path than those of the Austrian Alps, despite their identical mineral assemblage.