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Toronto, Canada

Mitchell R.,Lakehead University | Tappe S.,University of Alberta | Tappe S.,De Beers Canada Inc.
Lithos | Year: 2010

The origin of kimberlites and volatile-rich alkaline ultramafic magmas such as aillikites has been a matter of considerable scientific interest and the recent paper by Francis and Patterson (Francis and Patterson, 2009. Kimberlites and Aillikites as probes of the continental lithospheric mantle. Lithos 109, 72-80) attempts to summarize advances in this research field. They introduce new major element geochemical discrimination diagrams and present a petrogenetic model that allegedly has implications for diamond exploration. Contrary to the statements of Francis and Patterson we argue that kimberlite bulk chemical compositions are not determined by contamination with lithospheric mantle material alone and cannot be used to predict reliably the diamond grade of new discoveries. We dispute their petrogenetic model, specifically the proposal that calls for carbonate-rich aillikite melts being parental to kimberlite magmas. Rather, we promote the idea of carbonated ultramafic lamprophyre magmas, such as aillikites, being parental to many carbonatite intrusions in areas of rifted cratonic lithosphere, from which archetypal kimberlites appear to be absent. Furthermore, we believe that the geochemical discrimination diagrams (Si versus K and Fe, respectively) introduced by Francis and Patterson do not effectively distinguish between kimberlites and aillikites because there is significant data overlap and bias. Of particular concern is the omission of relevant compositional data for the ultramafic lamprophyre group. In this discussion we re-iterate that bulk chemical analyses of kimberlites and alkaline ultramafic rocks need to be accompanied by petrographic descriptions and mineralogical data. Only by the inclusion of such data can the petrogenesis of the rocks considered be discussed. © 2009 Elsevier B.V. All rights reserved.

Barnett W.P.,De Beers Group Services | Kurszlaukis S.,De Beers Canada Inc. | Tait M.,De Beers Group Services | Dirks P.,James Cook University
Bulletin of Volcanology | Year: 2011

Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe. © 2011 Springer-Verlag.

Eight water models were used to assess potential aquatic environmental effects of the proposed Gahcho Kué diamond mine on groundwater and surface water flow and quality in the Northwest Territories, Canada. This sequence of models was required to cover different spatial and temporal domains, as well as specific physico-chemical processes that could not be simulated by a single model. Where their domains overlapped, the models were interlinked. Feedback mechanisms amongst models were addressed through iterative simulations of linked models. The models were used to test and refine mitigation plans, and in the development of aquatic component monitoring programs. Key findings generated by each model are presented here as testable hypotheses that can be evaluated after the mine is operational. This paper therefore offers a record of assumptions and predictions that can be used as a basis for post-validation. © 2015 The Author(s)

Sader J.A.,University of Ottawa | Sader J.A.,Minerals and Metals Group | Hattori K.H.,University of Ottawa | Kong J.M.,De Beers Canada Inc. | And 2 more authors.
Geochemistry: Exploration, Environment, Analysis | Year: 2011

Peat groundwater compositions at depths of 0.4 and 1.1 m below ground surface in the Attawapiskat region of the James Bay Lowlands are evaluated for diamond exploration applications. Samples were collected along transects that typically extended at least 200 m beyond the margins of Yankee, Zulu, and Golf kimberlites. Locations of upwelling groundwater usually occur at or near kimberlite margins based on hydrogeological measurements and variations in peat groundwater geochemical parameters (pH and EC are high, and the Eh is low relative to ombrotrophic peat groundwaters). Concentrations of the kimberlite pathfinder metals Ni, Cr, light rare earth elements (LREEs), Ba, Mg/Ca, and alkalis are commonly elevated at sample sites at or near kimberlite margins and where groundwaters are upwelling. The presence of elevated kimberlite pathfinders at these sites suggests that fractures along the boundaries between kimberlites and limestone formed during kimberlite emplacement provide dilation for upward movement of groundwater with elevated kimberlite pathfinder metals. Typically, Ni, Cr, LREE, and Ba behave similarly and thus high concentrations of these metals are found at similar locations along transects. On the other hand, locations of elevated alkalis and Mg/Ca vary. The spatial variations among pathfinder metals in peat groundwaters are possibly due to geochemical processes in the peat, such as metal binding to dissolved organic material, adsorption to insoluble organics or Fe-oxyhydroxides, and incorporation into secondary mineral precipitates, which can act to increase or decrease metal solubility. The findings of this study are readily applicable in diamond exploration in wetlands elsewhere. © 2011 AAG/Geological Society of London.

Pittari A.,Monash University | Pittari A.,University of Waikato | Cas R.A.F.,Monash University | Lefebvre N.,De Beers Canada Inc. | And 2 more authors.
Economic Geology | Year: 2015

Diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes, which are generally determined from volcaniclastic facies characteristics. However, the effects and distribution of secondary alteration styles, which usually mask primary kimberlite facies, have been given little attention. At the Fort à la Corne kimberlite field, Saskatchewan, complex overprinting alteration textures and mineral phases are recognized. This study documents the characteristics, overprinting relationships, and spatial distribution of alteration styles within a crater-filling volcaniclastic deposit, Body 219 of the Orion Central volcanic complex. Data were acquired from macroscopic drill core observations, microscopic petrography, SEM imaging, EDS element mapping, and XRD analyses. Olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite. Late-stage serpentine has migrated into the interstitial medium. Bulk-rock alteration textures include (1) fine-grained crystal-rich domains, which are alteration pseudoclasts and/or relict juvenile pyroclasts; (2) a heterogeneous interstitial medium, varying from early-stage disseminated calcite-rich serpentine to amorphous serpentine and crystalline calcite; and (3) overprinting serpentine, carbonate and magnetite veins, and serpentine nodules. Alteration styles defined by unique alteration characteristics are spatially zoned through the Body 219 volcaniclastic deposit, and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit. Recognition of alteration processes, their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies, and by consequence the distribution of diamond grades. ©2015 Society of Economic Geologists, Inc.

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