Earth Sciences Report 2001-05

Earth Sciences Report 2001-05

ESR 2001-05

Major- and Trace- Element Geochemistry of Kimberlitic Rocks in Northern Alberta

Author(s) Eccles, D.R. Luth, R.W. Date 2003-05-01

During the 1990s, kimberlitic rocks were discovered in three separate areas of northern Alberta. These pipes comprise mainly lapilli-bearing olivine crystal tuff and volcaniclastic rocks that are interpreted to be pyroclastic crater facies. Eighty-three bulk rock samples were collected from the Mountain Lake cluster, and Buffalo Head Hills and Birch Mountains fields for analysis of major and trace elements. To place these data in context, the compositions are compared to kimberlite and ultramafic occurrences worldwide.

This report provides data that allow for geochemical distinction between kimberlitic fields in northern Alberta. Samples from the Mountain Lake pipes and the Buffalo Head Hills and Birch Mountains fields contain low SiO2 and Na2O/K2O, with Cr, Ni, MgO, rare-earth elements (REE), Zr and Nb concentrations elevated relative to their abundance in common crustal rocks.

Samples from the Buffalo Head Hills and Birch Mountains have close geochemical affinities with Group I South African kimberlites. Of the two areas, the Buffalo Head Hills kimberlites contain the highest MgO, Cr and Ni, the lowest Al 2O3, SiO2, V, Y, Pb, Sr and Ga values, and have chemistry similar to that of primitive kimberlite in the Northwest Territories. In addition, a high proportion of the Buffalo Head Hills kimberlites are diamondiferous. The Buffalo Head Hills field may therefore be classified as the more primitive kimberlite magma in this dataset. In contrast, the Birch Mountains kimberlites are more evolved, with lower SiO2, Ni and Mg#, and higher Fe2O3, TiO2, Nb, V, Sc, Zr, Hf, Y, Ba, Rb, light rare-earth elements (LREE), Ga and Pb.

A linear trend is found for northern Alberta kimberlite, which reflects compositional evolution from primitive to evolved kimberlite magmas. Northern Alberta pipes with elevated contents of diamonds generally have similar chemistry and reside on the primitive end of this trend line. This study will therefore enable exploration companies to compare future discoveries of kimberlite with these results for a quick and economical evaluation of a prospective target.

The Mountain Lake pipes are not kimberlite, but rather a hybrid rock with geochemical affinities to basanite (olivine potassic basalt), the Sweet Grass olivine minette and Montana alnoite. Compared to northern Alberta kimberlite (Buffalo Head Hills and Birch Mountains), the Mountain Lake pipes contain higher SiO2, Al2O3, Na2O, K2O, Na2O/K2O, Ga, Rb and peralkalinity index, and lower MgO, Nb, LREE and Sr.

Eccles, D.R. and Luth, R.W. (2003): Major- and trace-element geochemistry of kimberlitic rocks in northern Alberta; Alberta Energy and Utilities Board, EUB/AGS Earth Sciences Report 2001-05, 63 p.