-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

G10

DI C

PX

G10

D

G1

G3

DI C

R

G4

G12

Mg-

ILM

G4D

G3D

G5

G5D

CPX

G9

G0

CR Crs-

ILM

G11

PC3

PCA Scaled Vectors Showing Diamondiferous Kimberlite Variables

G10 DI CPX G10D G1 G3 DI CR G4 G12 Mg-ILM G4D G3D G5 G5D CPX G9 G0 CR Crs-ILM G11

Kimberlitic and Diamondiferous Crustal

Multivariate Analysis of Kimberlite Indicator Mineral Trends across CanadaBrett Ferguson1, Bruce Eglington1

University of Saskatchewan 1

Fig. 1.

Fig. 2. Fig. 3.

Fig. 4.

Fig. 5. Fig. 6.

Fig. 7.

Fig. 8.

TiO2-MgO Kimberlitic Ilmenite Discrimination Plot (Wyatt et al., 2004) showing Mg-rich ilmenites sourced from kimberlite pipes

Cr2O3-MgO Chromite Diamond Inclusion and Intergrowth Plot (Fipke et al., 1995) Cr2O3-CaO Clinopyroxene Diamond Inclusion Field Plot (Fipke et al., 1998)

Sample Locations of microprobed indicator minerals (n=122,913) shown in blue shown with Archean cratons

PCA scaled loadings showing negative (red) correlation between more favourable kimberlitic and diamondiferous minerals versus positive (blue) crustal correlation

PC3-PC1 analysis showing vectored trends. Positive and negative groupings can be seen along PC3 axis between kimberlitic and crustal components

High-Potential (High scoring) PC3 samples displayed with remainder of sample locations showing cluster following kimberlite trend

PCA Scaled Loadings Showing Diamondiferous Kimberlite Variables

The Grϋtter classification uses a variety of ranges of major oxides, as well as incorporating a Calcium Intercept (CA_INT) where; IF CaO ≤ 3.375 + 0.25*Cr2O3

THEN CA_INT= 13.5*CaO/ (Cr2O3 + 13.5)

IF NOT CA_INT= CaO – 0.25*Cr2O3

As well as a Magnesium number (MGNUM) where;

MGNUM= (MgO/40.3)/ (MgO/40.3 + FeOt/71.85)

Incorporating the ranges, an example of the classification of G10;

Classified as G10 where; Cr2O3 [wt.%]: ≥ 1.0 to < 22.0

CA_INT [wt.%]: 0 to < 3.375

MGNUM [wt.%]: ≥0.75 to < 0.95

The suffix “D” (Diamond-facies) may also be added to minerals that show a strong compositional and P-T association with diamond in the G10, G3, G4 and G5 classifications where;

G10D = Cr2O3 ≥ 5.0 + 0.94*CaO, or Cr2O3< 5.0 + 0.94*CaO and MnO< 0.36 (wt.%)

G3D, G4D, G5D = Na2O > 0.07 (wt.%)

(Grϋtter et al., 2004)

Garnet: Garnets were classified using the updated Grϋtter classification scheme (Grϋtter et al., 2004) which show compositional variations in Cr, Ca, Mg, Mn, Fe and Ti. These, in turn, reflect chemical, physical and lithological environments that can occur together with diamond (Grϋtter et al., 2004).

The microprobe analysis of major oxides (in wt.%) was used to categorize garnets as the following (Listed in order of classification, this being of significant importance due to slight overlap between classifications);

G1- Low-Cr megacrysts G11- High-TiO2 peridotitic G10- Harzburgitic G9- Lherzolitic G12- Wehrlitic G5- Pyroxenitic, websteritic G4- Pyroxenitic, websteritic G3- Eclogitic G0- Unclassified

All classification diagrams and multivariate analyses were performed using the ioGAS software package.

Principal Component Analysis: Principal component analysis (PCA) was used in this study to analyze correlations between different variables (classifications) of indicator minerals (i.e. G10, DI chromite, Mg-ilmenite, etc.) in the goal of finding a ‘kimberlite sourced and potentially diamondiferous’ trend unassociated to a more crustal sourced trend. PCA uses orthogonal transformation to convert correlated variables (clustered data) into a set of linearly uncorrelated values (principle components). In order to perform PCA at the per sample level, each sample location (n= 122,913, shown in Fig. 4.) was broken up into ratios of percent of mineral per sample (n-mineral class/total mineral) for each mineral analyzed (i.e. n-G10/Total Garnet = %G10).

Results: The ratios were analyzed in the PCA (per sample) resulting in two major trends (shown in Fig. 6.) along the principle component 3 (PC3) axis. The kimberlitic and diamondiferous is negatively loaded on PC3 while the crustal trend is positive (Fig. 5.). PC3 scores for samples provides a single variable (from multiple) which was used to discriminate ‘high-potential’ samples with high negative scores correlating to kimberlitic trends (Fig. 8.). Combining multiple variables into a single PC3 score decreased sample locations dramatically (n= 23,769), with clusters of high scoring samples becoming main areas of interest in terms of exploration potential.

Summary: Properly classifying indicator minerals into sub-categories of higher and lower potential for coinciding with diamond formation and transport is an essential part to any diamond exploration program. Using PCA on the ratios of these variables to create one new kimberlitic and diamondiferous variable dramatically decreases the number of samples to focus on as well as eliminates the need to layer multiple indicator minerals on one map, which often makes for more difficult interpretation. Even in areas where microprobe data is low in relation to total grains (i.e. NWT), by using PCA scores basic trends can be seen correlating to high-potential areas making this a powerful tool in any exploration program.

Introduction: The goal of this study was to reduce the dimensionality of a Kimberlite Indicator Mineral database in regard to microprobe data from grains across Canada, so as to identify prospective diamond districts. Principle component analysis (PCA) was applied for this purpose. Approximately 200,000+ indicator minerals were compiled and categorized into sub-classifications, some of which reflect their potential co-derivation and transport with diamonds via kimberlites. The study looked at ratios between these classifications (per sample) in the theory that the more ‘kimberlitic’ and ‘diamondiferous’ indicator minerals would occur together more frequently compared to the more crustal indicator minerals. PCA facilitated identification of a new variable based on a combination of mineral categories, creating an easier, more robust, way of selecting higher potential targets in lieu of looking at individual mineral categories. Indicator minerals assessed in this study were garnet, chromite, ilmenite and cr-diopside (clinopyroxene).

Methods: The kimberlite indicator minerals analyzed in this study include garnet, ilmenite, chromite and clinopyroxene. These are considered to be some of the more favorable mantle sourced minerals associated with diamond. Classifying these indicator minerals into various diamond potential sub-categories is key in aiding with any exploration program in the search for diamond-bearing kimberlite. The following methods were used in classifying the aforementioned indicator minerals of focus:

High-potential PC3 samples (n=23,769) displyed in pink, Archean cratons displayed in blue

Map Interpretation: Analyzing the high-scoring PC3 samples (Fig. 8.), several promising clusters (see attached figures below) are displayed with additional detail. Two trends in northern Alberta and one in northeastern Manitoba that cluster well in the PC3 variable show high abundances of diamond facies garnets, G10 Harzburgitic garnet, Mg-ilmenite (kimberlitic), DI chromite as well as DI clinopyroxenes.

This application can also be used in areas of high density clusters with many high-scoring samples. The NWT exhibits this, and using a larger scale map to look at the spatial distribution of high-scoring samples compared to the remainder (Fig. 7.) shows a definitive cluster of high-scores following the same trend as known kimberlites.

Note the correlation between the high-scoring samples with Archean craton. Typically diamond-bearing kimberlites are found on-craton, which is promising to see majority of high-potential samples plot on-craton or near craton margins.

References: Grütter, H.S., Gurney, J.J., Menzies, A.H. and Winter, F. (2004): An updated classification scheme for mantle-derived garnet, for use by diamond explorers; Lithos, 77, pages 841-857.

Fipke, C.E., Gurney, J.J. and Moore (1995): Diamond exploration techniques emphasising indicator mineral geochemistry and Canadian examples; Geological Survey of Canada, Bulletin 423, 86 pages.

Wyatt, B.A., Baumgartner, M., Ancar, E. and Grütter, H. (2004): Compositional classification of "kimberlitic" and "non-kimberlitic" ilmenite; Lithos, 77, pages 819- 840.

Simandl, G.J., Ferbey, T., Levson, V.M., Robinson, N.D., Lane, R., Smith, R., Demchuk, T.E., Raudsepp, I.M., and Hickin, A.S. (2005): Kimberlite and Diamond Indicator Minerals in Northeast British Columbia, Canada - A Reconnaissance Survey, British Columbia Ministry of Energy, Mines Petroleum Resources GeoFile 2005-25, 25 pages.

Harvey, S.E, Kjarsgaard, B.A., and Kelley, L.I. (2001): Kimberlites of central Saskatchewan: Compilation and significance of indicator mineral geochemistry with respect to diamond potential; in Summary of Investigations 2001, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.2.

Acknowledgements: This study would not have been possible without generous contributions of time and data from the following people: Barrett Elliot, Doug Irwin and Kelly Pierce (Northwest Territories Geological Survey), Colin Card (Saskatchewan Geological Survey), Shawn Harvey (Cameco) and Levi Kalinsky (Cameco).

Ilmenite: Ilmenite is an important indicator of kimberlite due to its Mg-rich species (picroilmenite) often being sourced from kimberlite pipes. The MgO-TiO2 Kimberlitic Ilmenite Discrimination Plot (Fig. 1.) pictured left was used to discriminate between ilmenites sourced from kimberlite (shown in red) versus those having formed in a crustal origin (shown in blue) (Wyatt et al., 2004). The ‘undefined’ region prevents any overlap between those of kimberlitic or crustal sources.

Chromite: 98% of worldwide chromite inclusions in diamond fall into the High-Cr, moderate to high MgO diamond inclusion and diamond intergrowth fields in Cr2O3-MgO plots (Harvey et al., 2001; Fipke et al., 1995). This is an important pathfinder to peridotite sourced diamonds. The same Cr2O3-MgO plot (Fig. 2.) was applied in classifying chromites for this study, with chromites that fall into the diamond inclusion and diamond intergrowth field being displayed in red on the Cr2O3 vs MgO in Chromites plot.

Clinopyroxene: Clinopyroxene (cr-diopside) found in association with other mantle-sourced minerals can be used as an indicator for kimberlite, and also provide proximity to source as clinopyroxenes typically are destroyed after relatively short transport. Cr2O3-CaO plots representing the compositions of CPX found as inclusions in diamonds (Simandl et al., 2005; Fipke et al., 1998) can also show potential for coinciding with diamond in the diamond stability field. The plot displayed below (Fig. 3.) shows CPX in this study that fall into the diamond inclusion field, highlighted in pink.

Identifying diamond districts, one can then apply geological interpretations, such as glacial transport, to find the source of indicator minerals.