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Mineral Transport - Indicators

KIMBERLITE INDICATOR MINERALS

Several minerals, when found in glacial sediments, are useful indicators of the presence of kimberlite, and to a certain extent, in evaluation of the diamond potential of kimberlite. These minerals are far more abundant in kimberlite than diamond, survive glacial transport, and are visually and chemically distinct. Cr-pyrope (purple colour, kelyphite rims), eclogitic garnet (orange-red), Cr-diopside (pale to emerald green), Mg-ilmenite (black, conchoidal fracture), chromite (reddish-black, irregular to octahedral crystal shape), and olivine (pale yellow-green) are the most commonly used kimberlite indicator minerals in drift prospecting, although in rare cases, diamond is abundant enough to be its own indicator. Kimberlite indicator minerals are recovered from the medium to very coarse sand-sized fraction of glacial sediments, and analyzed by electron microprobe to confirm their identification. For this study, indicator minerals were picked from the 0.25-0.5 mm, 0.5-1.0 mm, and 1.0 to 2.0 mm non-ferromagnetic heavy mineral concentrates of 10 to 20 kg glacial sediment samples and analyzed at the GSC using a four spectrometer Cameca SX50 electron microprobe.

Mg-ilmenite

Mg-ilmenite Ilmenite occurs in many of the Archean rocks in the Kirkland Lake area as well as in kimberlite. Kimberlitic ilmenites can be distinguished from other ilmenites by their high MgO content, typically containing >4 wt.%. Ilmenites from the A4, B30, C14 and Diamond Lake kimberlites (red dots) contain 4 to 15 wt.% MgO, and are low in Cr₂O₃, most grains contain <1 wt.% Cr₂O₃. Glacial sediments (blue dots) contain kimberlitic ilmenite as well as ilmenite form other sources. Each kimberlite has a distinct Cr₂O₃ versus MgO signature that is mimicked by the glacial sediments and kimberlite boulders collected down-ice, as shown in the Cr₂O₃ versus MgO plots. For example, one 25 kg kimberlite boulder found 3 km southeast of the Diamond Lake pipe, has a similar Cr₂O₃ versus MgO signature indicating it was derived from the Diamond Lake pipe.

Garnet

Cr-pyrope Subcalcic harzburgitic garnets are associated with diamondiferous kimberlites. They can be differentiated from other lherzolitic, harzburgitic or dunitic garnets by plotting CaO versus Cr₂O₃. The diagonal line separating lherzolitic and harzburgitic garnets is the 85% line defined by Gurney (1984). Garnets that fall below the 85% line, i.e. low-Ca Cr-pyropes, are "subcalcic" or G10 garnets derived from harzburgite. The high chrome content gives these garnets a distinct lilac purple colour. The vertical line separates Cr-poor, orange-red garnets having <2 wt. % Cr₂O₃ from the purple peridotitic garnets. Garnets from the A4, B30, C14 and Diamond Lake kimberlites (red dots) are mostly G9 (lherzolitic), and only a few are subcalcic-G10 garnets. Garnets in the glacial sediments (blue dots), in general, have similar compositions to those in the kimberlites. The low abundance of G10 garnets in the kimberlites and glacial sediments is consistent with the low diamond grades of these pipes.

Chromite

Chromite associated with diamonds has a high Cr₂O₃ content (>60 wt. %) and moderate to high level (12-16 wt.%) of MgO. The compositions of chromites from the A4, B30, C14 and Diamond Lake kimberlites represent a "poor" chromite population with only a few chromite xenocrysts approaching the diamond inclusion field. This is consistent with the trace quantities of diamonds found in these pipes. Chromite grains in glacial sediments show similar compositions to those from the kimberlites, suggesting many of the grains are from kimberlite.

Cr-diopside

Cr-diopside Cr-rich (>0.5 wt.% Cr₂O₃) diopside is easily identified by its distinctive green colour. It indicates the presence of kimberlite but provides little information on the presence of diamonds in the kimberlite. Cr-diopsides in the Kirkland Lake area occur in ultramafic rocks as well as kimberlites, although only kimberlites contain very Cr-rich (>1.5 wt.% Cr₂O₃) diopsides. The B30, A4, C14 and Diamond Lake kimberlites contain pale green (0.5 wt.% Cr₂O₃) to bright green (1.0 to 4.0 wt.% Cr₂O₃) Cr-diopside. The presence of Cr-diopside is not a useful kimberlite indicator on its own, because of the ubiquitous distribution of Cr-diopside grains in glacial sediments across the Kirkland Lake region and the difficulty in distinguishing between Cr-diopsides from kimberlite and those from other rocks.

olivine

Mg-ilmenite

Pyrope

Distribution of Mg-ilmenite grains in overburden drill holes south of the Diamond Lake kimberlite.

Source:

Mg-ilmenite
Mg-ilmenite	Ilmenite occurs in many of the Archean rocks in the Kirkland Lake area as well as in kimberlite. Kimberlitic ilmenites can be distinguished from other ilmenites by their high MgO content, typically containing >4 wt.%. Ilmenites from the A4, B30, C14 and Diamond Lake kimberlites (red dots) contain 4 to 15 wt.% MgO, and are low in Cr₂O₃, most grains contain <1 wt.% Cr₂O₃. Glacial sediments (blue dots) contain kimberlitic ilmenite as well as ilmenite form other sources. Each kimberlite has a distinct Cr₂O₃ versus MgO signature that is mimicked by the glacial sediments and kimberlite boulders collected down-ice, as shown in the Cr₂O₃ versus MgO plots. For example, one 25 kg kimberlite boulder found 3 km southeast of the Diamond Lake pipe, has a similar Cr₂O₃ versus MgO signature indicating it was derived from the Diamond Lake pipe.
Garnet
Cr-pyrope	Subcalcic harzburgitic garnets are associated with diamondiferous kimberlites. They can be differentiated from other lherzolitic, harzburgitic or dunitic garnets by plotting CaO versus Cr₂O₃. The diagonal line separating lherzolitic and harzburgitic garnets is the 85% line defined by Gurney (1984). Garnets that fall below the 85% line, i.e. low-Ca Cr-pyropes, are "subcalcic" or G10 garnets derived from harzburgite. The high chrome content gives these garnets a distinct lilac purple colour. The vertical line separates Cr-poor, orange-red garnets having <2 wt. % Cr₂O₃ from the purple peridotitic garnets. Garnets from the A4, B30, C14 and Diamond Lake kimberlites (red dots) are mostly G9 (lherzolitic), and only a few are subcalcic-G10 garnets. Garnets in the glacial sediments (blue dots), in general, have similar compositions to those in the kimberlites. The low abundance of G10 garnets in the kimberlites and glacial sediments is consistent with the low diamond grades of these pipes.
Chromite
Chromite associated with diamonds has a high Cr₂O₃ content (>60 wt. %) and moderate to high level (12-16 wt.%) of MgO. The compositions of chromites from the A4, B30, C14 and Diamond Lake kimberlites represent a "poor" chromite population with only a few chromite xenocrysts approaching the diamond inclusion field. This is consistent with the trace quantities of diamonds found in these pipes. Chromite grains in glacial sediments show similar compositions to those from the kimberlites, suggesting many of the grains are from kimberlite.
Cr-diopside
Cr-diopside	Cr-rich (>0.5 wt.% Cr₂O₃) diopside is easily identified by its distinctive green colour. It indicates the presence of kimberlite but provides little information on the presence of diamonds in the kimberlite. Cr-diopsides in the Kirkland Lake area occur in ultramafic rocks as well as kimberlites, although only kimberlites contain very Cr-rich (>1.5 wt.% Cr₂O₃) diopsides. The B30, A4, C14 and Diamond Lake kimberlites contain pale green (0.5 wt.% Cr₂O₃) to bright green (1.0 to 4.0 wt.% Cr₂O₃) Cr-diopside. The presence of Cr-diopside is not a useful kimberlite indicator on its own, because of the ubiquitous distribution of Cr-diopside grains in glacial sediments across the Kirkland Lake region and the difficulty in distinguishing between Cr-diopsides from kimberlite and those from other rocks.
olivine