macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía 205

New Insights into the Concept of Ilmenite as an Indicator for Diamond Exploration, Based on Kimberlite Petrographic Analysis / SANDRA ROBLES CRUZ (1*), MANUEL WATANGUA (2), JOAN CARLES MELGAREJO (1), SALVADOR GALI (1)

(1) Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals. Facultat de Geologia. Universitat de Barcelona. Martí i Franquès s/n. 08028, Barcelona (España) (2) ENDIAMA, Major Kanhangulo 100, Luanda (Angola)

INTRODUCTION. diamond potential is currently being studied. The Catoca kimberlite is the This study presents results of the initial most important primary diamond phase of the research project, deposit in Angola, hosted by “Kimberlites associated to the Lucapa Precambrian rocks and covered by structure, Angola (Africa)”, within the Mesozoic-Cenozoic sedimentary deposits framework of a multilateral agreement (Janse et al, 1995). between the Faculty of Geology- Universitat de Barcelona, the Empresa PETROGRAPHY. Nacional de Diamantes de Angola and the Agostinho Neto University (Luanda- There are some minerals which are Angola). frequently associated to diamond inside kimberlites and they are used as fig 1. Intercumular ilmenite in peridotitic xenolith. The research is based on two sets of indicator minerals for the diamond core sampling down to 600 m deep. The exploration. The main indicator minerals first set comes from Catoca pipe and are: magnesian ilmenite (Pell, 1998), allowed us to identify complete crater garnet and chromite (Wyatt et al, 2004). and diatreme facies. The second one (18 However, for this instance we will focus kimberlites) comes from Cucumbi, on ilmenite since it is the first mineral Cacuilo, Tchiuzo, Alto Cuilo, Camitongo analyzed in 2007. and Kambundu, whose samples were gathered during fall 2008. Currently, we Diverse xenoliths, comprising lherzolite, are working on these sets of samples. eclogite, harzburgite, carbonatite, gneiss and amphibolite are distributed through The kimberlites are ultrabasic rocks with the Catoca and Cucumbi kimberlites. high content of volatiles mainly CO2, and Some shales and sandstones can be a typical inequigranular texture present in the upper part of this fig 2. Nodular xenocrysts of ilmenite with characterized by the presence of macro- Kimberlite. Accessory minerals and replacement. megacrysts which can be xenoliths or xenocrysts comprise garnet, zircon, Cr- xenocrysts embedded in a fine-grained rich diopside, amphibole, phlogopite, matrix (Mitchell, 1995; Benvie, 2007). chromite and several generations of These special rocks have a great ilmenite. Secondary minerals include importance, not only in scientific terms serpentine-group minerals being the since they add valuable information most abundant, calcite, barite, about lithospheric mantle but also barytocalcite, witherite and strontianite. because they can contain diamond. Based on optical petrographic studies REGIONAL SETTING. and BSE images from SEM-ESEM with EDS microanalysis, we have been able

The area of interest is localized in to discriminate up to six textural types of fig 3. Euhedral crystals of ilmenite in matrix. northeastern Angola (Africa), being ilmenite in Catoca and Cucumbi tectonically controlled by the Lucapa kimberlite: a) intercumular ilmenite in Zircon xenocrysts are partially replaced structure, a former rift (Guiraud et al., peridotitic xenoliths (Fig 1); b) anhedral by fine-grained baddeleyite, and at least 2005) of early Cretaceous that extends ilmenite in carbonatite xenoliths; c) two populations exist according to the NE-SW across Angola. Associated to this ilmenite unaltered megacrysts; d) trace element distribution. All of these structure there is a magmatic belt, nodular xenocryst of ilmenite with crystals are enriched in HREE, but with a which is composed by kimberlites different grades of replacement, some noticeable positive Ce anomaly, similar toward NE and carbonatites toward SW. of them with symplectitic textures (Fig. to that reported in zircon in a MARID At present, over 2000 kimberlites have 2); e) skeletal ilmenite; and f) euhedral xenolith from a southern African been identified in this structure and their crystals of ilmenite in matrix (Fig. 3). kimberlite (Dawson et al., 2001). The

palabras clave: kimberlita, diamante, ilmenita, manto, Angola. key words: kimberlite, diamond, ilmenite, mantle, Angola.

resumen SEM/SEA 2008 * corresponding author: [email protected]

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crystals are not optically zoned, but environment. Magnesian ilmenite is also ilmenite in a higher or lower grade. there is a slight depletion in REE from enriched in Cr and Ni. More advanced These fluids are reducing, especially the core to the rim. replacement produces a symplectitic those rich in Mn. Picroilmenite has replacement of ilmenite II by ilmenite III. traditionally been interpreted as an MINERAL CHEMISTRY OF ILMENITE. indicator of kimberlite associations, as well as an indicator of low fO2, which is Every texture has been systematically necessary for the preservation of analyzed with EPMA which allowed us to diamond. Although Catoca and Cucumbi identify three compositional types of are diamondiferous kimberlites, they ilmenite (I, II and III). This combined show that Mg ilmenite is clearly a late technique –texture and composition replacement product, and the grade of analysis- has been suitable for analyzing replacement of the primary grains is zircon and garnet as well. very variable. Therefore the absence of magnesian ilmenite in a kimberlite does The primary ilmenite (type I) in not appear to be a convincing argument megacrysts (xenocrysts) is generally rich to exclude the presence of diamonds. in Cr and Fe3+. Its composition is similar Accordingly, this work proposes a new to the intercumulus crystals in peridotite insight into the concept of ilmenite in xenoliths. This ilmenite is replaced, in diamond exploration. the first instance, by magnesian ilmenite (type II). This process takes place along ACKNOWLEDGMENTS. microdiscontinuities (cleavage, border grains, contour subgranes, kink band This research is supported by the project planes, etc.), producing diffusive CGL2006-12973 of Ministerio de replacements. In a more advanced Educación y Ciencia (Spain), the AGAUR stage, symplectitic replacements occur, fig 4. MgTiO3-FeTiO3-Fe2O3 ratio for the different SGR 589 of Generalitat de Catalunya involving an early generation of types of ilmenite (I, II and III) discriminated for each and a FI grant sponsored by the texture. magnesian ilmenite (type II) at the Departament d’Educació i Universitats expense of Fe3+-rich primary ilmenite A late generation of ilmenite III (Mn-rich de la Generalitat de Catalunya i del Fons (from texture a to d ). A late generation ilmenite) is found rimming all the above Social Europeu. The authors of Mn-Nb-Zr-rich ilmenite (type III) cuts mentioned generations, and is strongly acknowledge the Serveis the previous ones. enriched in Nb, Ta, Zr, W, Hf, Th and U, Cientificotècnics de la Universitat de and poor in Mg and Fe3+. The Barcelona. Contrastingly, the late euhedral Mn-Nb- composition of this ilmenite is similar to Zr ilmenite crystals found in the that of the fine-grained euhedral REFERENCES. kimberlite matrix do not present any ilmenite crystals found in the kimberlitic evidence of replacement. This ilmenite matrix and also to that of the ilmenite Benvie, B.(2007): Mineralogical imaging of is poor in Mg and Fe3+. Their kimberlites using SEM-based techniques. crystals found in the carbonatitic Minerals Engineering, 20, 435-443. compositions are identical with the Mn- xenoliths. Crystals of Mn-rich ilmenite rich ilmenite produced during late Dawson, J.B., Hill, P.G., Kinny, P.D. (2001): (ilmenite type III) are not replaced or Mineral chemistry of a zircon-bearing, replacement stages of ilmenite zoned, and seem to have crystallized in composite, veined and metasomatised megacrysts. Compositions of Mn-rich equilibrium with the kimberlitic magma. upper-mantle peridotite xenolith from ilmenite are similar to those found in Both the late generations of ilmenite kimberlite. Contrib. Mineral Petrol., 140, carbonatite xenoliths. and the baddeleyite replacing zircon can 720-733. be produced by interaction of a Guiraud, R., Bosworth, W., Thierry, J., DISCUSSION AND CONCLUSIONS. Delplanque, A. (2005): Phanerozoic carbonate-bearing kimberlitic magma geological evolution of Northern and enriched in Mn and HFSE. The Textural evidences indicate a different Central Africa: An overview. Journal of replacement of Fe3+-rich ilmenite by Mg- African Earth Sciences, 43, 83-143. complex history of growth in the and Mn-rich ilmenite implies that the Janse, A.J.A. & Sheahan, P.A. (1995): xenocrysts. Unaltered megacryst early ilmenite was formed under Catalogue of world wide diamond and 3+ ilmenite (ilmenite type I) rich in Fe (fig. oxidizing conditions in the mantle, and kimberlite occurrences: a selective and 4), indicates crystallization under high the lastest compositions of ilmenite annotative approach. Journal of fO2 conditions; this ilmenite contains Nb, were produced by reaction with the Geochemical Exploration, 53, 73-111. Cr, Ni and Ta in low contents. Its Mitchell, R.H. (1995): Kimberlites, Orangeites, kimberlitic magma. and related rocks. New York, Plenum composition is similar to those ilmenite Intercumular megacrysts that occur in Press, 410 pp. Megacrysts of ilmenite are frequently Pell, J. (1998): Kimberlite-hosted Diamonds, peridotite xenoliths. Hence, most of the present in diamondiferous kimberlites, in Geological Fieldwork 1997. British ilmenite xenocrysts seem to have been contrasting with ilmenite observed in Columbia Ministry of Employment and produced by disaggregation of mantle barren kimberlites. This might become a Investment 1998-1, 24L1-24L4. xenoliths. Ilmenite I is replaced along new guide in diamond exploration. Wyatt, B.A., Mike, B., Anckar, E., Grutter, H. discontinuities by magnesian ilmenite In conclusion, the composition of this (2004): Compositional classification of “kimberlitic” and “non-kimberlitic” (ilmenite type II); the elemental ilmenite is the result of a set of ilmenite. Lithos, 77, 819-840. distribution of Mg in these grains points replacement processes with rich fluids in to processes of replacement through Mg and Mn affecting an oxidized primary solid-state diffusion in a typical reducing