Cr-diopside (Clinopyroxene) as a Kimberlite Indicator Mineral for Diamond Exploration in Glaciated Terrains David Quirt 1 Quirt, D.H. (2004): Cr-diopside (clinopyroxene) as a kimberlite indicator mineral for diamond exploration in glaciated terrains; in Summary of Investigations 2004, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004- 4.2, CD-ROM, Paper A-10, 14p. Abstract For diamond exploration in glaciated terrains, Kimberlite Indicator Minerals (KIMs) are those minerals, contained in glacial deposits, that are characteristic of kimberlite, the dominant host for diamonds. As KIMs are many times more numerous in the kimberlite host rock than are diamonds, they are an important pathfinder for kimberlites. Typical KIMs include garnet, pyroxene, Cr-spinel, Mg-ilmenite, and olivine. Interpretation of the chemistry of KIM grains, such as clinopyroxene, allows evaluation of the kimberlitic affinities of the grains and delineation of possible glacial dispersion trains suggestive of potentially diamondiferous kimberlite sources. Pyroxenes from both peridotitic and eclogitic mantle sources can be KIMs, with clinopyroxene (cpx) being a common groundmass mineral in kimberlite. There have been two historical interpretative problems, however, with the use of cpx as a KIM. Firstly, the colour criteria used in picking KIM cpx (e.g., Cr-diopside) appear to vary widely between individual microscopists; and secondly, problems can arise from the presence of non-kimberlitic (alkalic gabbro, basalt, komatiite, syenite, carbonatite, ultrapotassic volcanic) Cr-diopsides in the glacial deposit sampled. Given the wide range of cpx host environments, determination of suitable cpx compositions for use in KIM evaluation and pyroxene thermobarometry is important. The chemical data should be constrained to compositions similar to those determined for kimberlitic cpx and diamond inclusion (DI) cpx worldwide. Kimberlitic cpx from peridotitic sources are typically diopsidic or, to a lesser extent, calcic/non-ferrous augitic on a Wo-En-Fs diagram; as are many cpx grains of crustal origin, most eclogitic (low-Cr) DI cpx, and many ‘eclogitic’ non-DI cpx. Consequently, the data must be screened for peridotitic cpx, permitting evaluation of only those grains having the selected criteria: diopsidic to calcic/non-ferrous augitic compositions (27.5%<Wo<55% and En/(En+Fs)>0.5) and low-Na content (J<0.5). Screening for high-Na DI eclogitic cpx uses the same Wo-En-Fs criteria as for peridotitic cpx, plus the selected criteria: Cr content <0.5% Cr2O3 and J>0.5 (i.e., cation Na>0.25). Several large kimberlite/diamond indicator mineral data sets were examined for pyroxene chemical trends. The entries of Fe, Al, Na, Ca, and Cr into the cpx structure are strongly affected by the P-T-X conditions during mineral crystallization, so multidimensional diagrams of cpx atomic cation proportions are used to illustrate the chemical variation of mantle-derived kimberlite cpx relative to the more Fe-rich non-kimberlitic (crustal) grains typically present in glacial deposits. Antipathic Fe-Cr, Fe-Na, Al-Cr, and Al-Na data trends present in four-dimensional data plots can be used to discriminate between ‘deeper mantle-derived’ grains and ‘more crustal’ grains. These interpretive guides provide useful discriminations between kimberlitic and non-kimberlitic peridotitic cpx in a suite of cpx KIM chemical data from samples of glacial deposits. Keywords: Diamond, kimberlite, drift prospecting, heavy minerals, indicator minerals, mineral chemistry, clinopyroxene, cpx, ternary discriminant diagrams, KIM. 1. Introduction Nearly all diamond mines recover diamonds from an ultramafic volcanic rock called kimberlite, typically containing many fragments of mantle peridotite and eclogite. The diamonds, which are xenocrysts, crystallized in the upper mantle and were brought to the surface by very deep-seated volcanic eruptions that formed the kimberlites. The peridotitic rock fragments, or xenoliths, are dominantly composed of garnet, olivine, and ortho- and/or clinopyroxene; eclogitic xenoliths consist of orange (Fe, Ti, Mg, Ca) almandine garnet, green clinopyroxene, and hornblende. Over the past 15 years, kimberlite pipes have been discovered in the Precambrian Shield of west-central 1 Saskatchewan Research Council, 15 Innovation Boulevard, Saskatoon, SK S7N 2X8; E-mail: [email protected]. Saskatchewan Geological Survey 1 Summary of Investigations 2004, Volume 2 and northern Canada – thus northern Saskatchewan and Alberta, Quebec, Ontario, Nunavut, and the Northwest Territories (NWT) have become prime diamond exploration areas. In glaciated areas, exploration for diamond-bearing kimberlite is undertaken mainly by two methods: airborne geophysics and drift prospecting. Drift prospecting involves sampling glacial deposits to identify economically significant components and trace them up ice to their bedrock source (DiLabio and Coker, 1989). It is routinely used in the Precambrian Shield areas of Canada because the dominant surficial material is glacial till and many ore deposits have been found using this technique, including those containing diamonds, gold, copper, zinc, uranium, and rare earth elements. An important component of a drift prospecting survey is extraction and identification of indicator minerals. An indicator mineral is a mineral characteristic or representative of a given host rock or mineral deposit. Examples of indicator minerals used in mineral exploration (cf. Averill, 2001) in Saskatchewan include clay minerals (uranium), gold grains (gold), and heavy minerals (base metals, gold, kimberlite/diamond). The heavy mineral (HM) component of sediment consists of all clastic grains with specific gravities greater than about 2.9. Many indicator minerals are part of the HM fraction, and as such this fraction is examined to determine the distribution and dispersion of HMs related to various types of mineralization, such as gold, base metals (volcanogenic massive sulphides, skarn, and magmatic Ni-Cu), and diamond. Kimberlite Indicator Minerals (KIMs), sometimes called Diamond Indicator Minerals (DIMs), are important in diamond exploration as diamonds are present in only trace amounts in only some kimberlites. Kimberlites contain other minerals, however, that are both specifically characteristic of kimberlite and much more abundant than diamond. These KIMs, which include garnet, Mg-ilmenite, chromite, clinopyroxene (cpx) (Cr-diopside), and olivine (Figure 1), are used to identify the presence of kimberlite and are thus useful in diamond exploration and diamond potential evaluations. In glaciated terrain, kimberlite exposed at surface has commonly been eroded and indicator minerals released. In a KIM drift prospecting survey, the sample material commonly obtained for analysis is the lowermost, or basal, till unit overlying bedrock. If KIMs are found in this material, their bedrock source usually lies in the up-ice direction. This exploration method was the primary tool used in the discovery of the Lac de Gras (NWT) kimberlites which are currently being exploited by Canada’s two operating diamond mines, Ekati and Diavik. 2. Kimberlite Indicator Minerals Heavy minerals, including KIMs, are separated from bulk till samples in support of kimberlite drift prospecting programs. Typically 10 to 25 kg till samples are processed for heavy mineral separation, KIM identification and extraction (picking), and diamond extraction and description. The sample size is reduced through non-destructive disaggregation, washing, and screening to obtain the <2 mm size-fraction (-10 mesh), which is further reduced by use of a shaker table to produce a rough heavy mineral concentrate. The concentrate then undergoes a heavy media separation procedure during which the HMs, including the KIMs, pass through a dense media. This separation procedure uses heavy liquids (MI: methylene iodide; TBE: tetrabromoethane) to separate the higher-density minerals of interest. Alternatively, a Magstream separator can be used to eliminate the need for toxic/dangerous heavy liquids. The KIMs and/or KIM intergrowths are variably magnetic, so the sample is further concentrated through the use of magnetic separation to separate the paramagnetic fractions from the non-magnetic fraction. The final HM concentrates are sieved to produce four weighed size-fractions (<0.25 mm, 0.25 to 0.5 mm, 0.5 to 1 mm, and 1 to 2 mm) within each magnetic fraction obtained. KIMs are identified, hand-picked (extracted), and sorted from selected HM concentrate size-fractions using optical binocular microscopy by highly trained mineral sorters. Quality control procedures commonly include spiking of samples with marker grains and/or the re-examination of some samples, depending on the background mineralogy and difficulty. Figure 1 - Kimberlite Indicator Minerals. From top left- clockwise: picroilmenite (Mg-rich ilmenite); eclogitic Fe- Following grain picking, further analysis includes Mg-Ca almandine G3 garnets; peridotitic chrome pyrope spreadsheet counting statistics and/or mineral chemical G9/G10 garnets; chromites; chrome diopsides; Ti-Cr-Mg pyrope G1/G2 garnets; and olivines in the centre. analysis. Selected grains are prepared for electron Saskatchewan Geological Survey 2 Summary of Investigations 2004, Volume 2 microprobe major-element chemical analysis. Interpretation of the KIM grain mineral chemistry is completed to evaluate kimberlitic affinities and to document and describe possible dispersion trains suggestive