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Diamonds have been discovered in a variety of potassic ultramafic rocks including group-I and group-II kimberlites, olivine lamproites and aillikites, all of which are macroscopically similar and can be difficult to differentiate when viewed under the microscope. However, group-I kimberlites, and to a much lesser extent group-II kimberlites and olivine lamproites, are known to contain economic concentrations of diamonds. This study addresses the problem of distinguishing among different types of kimberlitic and related rocks by developing a geochemically-based method for classifying them. / Geochemical methods have been largely ignored in the classification of kimberlites and related rock types due to high concentrations of xenoliths. However, this problem can be largely overcome by only selecting matrix material for analysis. An evolving kimberlitic magma will become enriched or improvished in Si due to the fractionation of olivine and phlogopite, depending on the initial Si concentration of the magma. As they have low Si concentrations, group-I kimberlites and aillikites can be separated from group-II kimberlites and meimechites, which have higher Si concentrations for any Mg content. Furthermore, since aillikites and meimechites are relatively rich in Fe compared to group-I and group-II kimberlites, these rock types form four separate fields on a Si vs. Fe discrimination diagram. Similar rock-type separation is observed when the ratio of La to Yb is plotted against the ratio of Sm to Yb. Kimberlite and other potassic ultramafic rocks were sampled from nine areas in Quebec: the Otish Mountains, Wemindji, Torngat Mountains, Desmaraisville, Temiscamingue, Ile Bizard, Lac Leclair, Baie James and Ayer's Cliff regions. Major and selected trace element concentrations were determined by XRF analysis for all samples, while a subset of representative samples was selected for trace element analysis by ICP-MS. Electron microprobe analyses of unaltered olivine and phlogopite were also conducted. / Of the 37 samples that were classified both mineralogically and chemically, 23 or 62% were correctly classified using Fe and Si. This number increases to 84%, if the REE are used in conjunction with Si and Fe. The Si vs. Fe discrimination diagram separates group-I kimberlite from most aillikite and meimechite rocks and group-II kimberlite/olivine lamproite rocks from most aillikite and meimechite rocks. Therefore, major and trace element geochemistry offers an important tool for the classification of kimberlitic rocks. / Vasilenko et al. (2002) and Francis (2003) both suggested that diamond grades can be correlated with the major element compositions of the kimberlites. The data collected in this study confirm the inverse relationship between TiO2 concentration and diamond grade. The lowest TiO 2 values were obtained on samples from the Otish Mountains and Renard samples in particular. Other areas of Quebec are characterized by higher TiO2 contents with most samples containing greater than 2 wt% TiO 2. Therefore, the kimberlitic rocks from the Renard locality have the greatest potential for an economic diamond deposit. The origin of this correlation needs to be explored, however, because it is unclear whether this is a feature of the mantle source, or reflects the survivability of diamonds within the kimberlites.

Kimberlite whole rock geochemistry has been under-appreciated in deciphering Group I kimberlite petrogenesis. Although there is considerable debate on the definition of Group I kimberlite, there is agreement on certain characteristics. These include that Group I kimberlites are ultramafic rocks (MgO >15 wt%), have near primitive mantle nickel (∼ 1400 ppm) and chromium (∼1600 ppm) contents, have close to bulk silicate earth radiogenic isotopic signatures, are volatile rich (predominantly CO2) and have stable isotopic signatures typical of mantle sources. The debate, however, centers on petrogenetic models and parental magmatic compositions of Group I kimberlites. Petrogenetic models include both low degree (<1 %) partial melting of metasomatized asthenospheric mantle (Dalton and Presnall, 1998a; 1998b; Becker and Le Roex, 2006) and, conversely, high degree partial melting (10<20 %) of metasomatized veined asthenospheric mantle (Mitchell, 1995, 2004). Inferred kimberlite parental magma compositions have been suggested to be low silica (<5 wt%) and high CO2 (<44 wt%) melts, as suggested by high-pressure melt experiments conducted on synthetic carbonatized mantle (Dalton & Presnall, 1998a, 1998b), or much higher silica (<30 wt%) and lower CO2 (<12 wt%) melts as suggested by geochemical analysis of aphanitic kimberlite (Price et al., 2000; Kopylova et al., 2007). In an effort to contribute to the resolution of this debate on petrogenic models and parental magma compositions of Group I kimberlites, I have completed a systematic investigation of the whole rock geochemistry and petrology of three kimberlite occurrences in the Foxtrot Kimberlite Field of Northern Quebec. A clear correlation exists between whole rock chemistry, mantle source, and diamond grade of the three Group I kimberlites in the Foxtrot Kimberlite Field. Two phases, olivine and carbonate, dominant theserocks and thus control the kimberlites geochemistry. Electron microprobe analysis of over 800 olivines in the Foxtrot kimberlite indicate that it is predominantly xenocrystic (Mg# (Mg/(Mg+Fe)) of 91.5), largely derived from harzburgite mantle, but the absence of harzburgitic orthopyroxene in the Foxtrot kimberlites suggests that it has been assimilated. Whole rock geochemistry (major and trace elements) of the Foxtrot hypabyssal kimberlite dykes indicate they have chemical characteristics similar to Group I kimberlite from the Slave Province. The olivine content of the Foxtrot kimberlites, and thus their Mg#'s, are positively correlated with diamond abundance indicating that diamond grade is a reflection of the incorporation of harzburgite mantle. These results indicate that Group I kimberlite petrogenesis in the Foxtrot example is best modeled as a mixture of harzburgitic mantle xenocrysts and a carbonate rich fluid, and that the parental magma is silica poor "5 wt %) and CO 2 rich (<40 wt%).

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Le craton Archéen canadien, en partie situé au Québec, possède plusieurs occurrences de roches mantelliques reliées aux kimberlites, dont les intrusions Le Tac et Lac Shortt situées dans la région de Chibougamau. Les phases minérales dominantes sont l'olivine, la phlogopite et la serpentine. Les minéraux matriciels sont la serpentine, l'apatite, la pérovskite, le diopside et la calcite. Deux générations d'olivine sont observées dans les deux intrusions. La phlogopite possède une composition variable suggérant une interaction chimique avec un magma de type kimberlitique. La composition des intrusions Le Tac et Lac Shortt est similaire aux kimberlites de faciès hypabyssal des Groupes I et II. Les diagrammes des profils des multi-éléments corrèlent avec ceux des kimberlites avec des anomalies négatives en Rb, Sr et K et positives en Nd et La. Le potentiel diamantifère des deux intrusions étudiées est faible, mais cette région peut tout de même être intéressante pour l'exploration diamantifère.