The Ti, Mn, and Na oxide distribution in kimberlite pyropes of Angola as criterion of diamond grade

To determine diamond grade Ti, Mn, and Na in  pyropes from kimberlites of the Angola diamond-bearing sub-province, the triangular diagrams of their ratios is proposed. The JX-8230 microprobe, allows determining the composition of minerals by WDS and EDS spectrometers simultaneously.

The diagrams Fig.1 shows the compositions of these oxides in pyropes with their breakdown into cluster groups (CG) of Dawson J.B., Stephens W.E. classifications [3]. It complements the generally accepted diagrams [1,2,3] and creates an opportunity to determine the degree of diamond content of kimberlites and their belonging to the same field or cluster of kimberlite pipes. The diagrams shows the ratio of oxides of the main trace elements in pyropes of Angola kimberlites with diamonds and dots – Mn, Ti and Na in the diamondiferous kimberlites  (Luele, Chyuzu) and in empty ones (Shandongu, Lx 150).

The Na₂O content for the compositions of low-chromium pyropes is the main sign of their crystallization with diamonds, which is reflected in the Na₂O-TiO₂ diagram by J. Gurney [2].

The TiO₂ is undoubtedly an important and significant impurity oxide that determines diamond content and tthe CG of pyropes according to Dawson J.B., Stephens W.E. [3]: its content in G3, G10, G9 is low, <0,3; in G1 – medium, 0,3-0,6 and G2 – high,> 0,6 wt.%.

The MnO content in kimberlite pyropes, as a rule, does not exceed 0,6 wt%. Changes in the contents of this oxide can occur in the process of metasomatic transformations of pyropes, which affects the diamond content in kimberlites [4].

From the presented diagrams (Fig. 1) it can be seen that 97,5% of the compositions of pyrope grains from the highly productive kimberlites of the Luele pipe lie in the diamond-bearing contour, while high-magnesian-chromium pyropes CG G10, whose share is 46%, together with pyropes CG G9 – 21%, evenly distributed over this area and prevail over the rest of the CG. Medium-high titanium CG G1-G4 and G-11 are compactly concentrated in the lower area of ​​the diamond-bearing contour, next to the low-titanium G3.

In low diamondiferous kimberlites of the Chyuzu pipe, about 65% of pyrope grains fall into the diamondiferous contour, while the compositions of CG G10 and G9 are represented by less than 10% of grains, 90% of grains are high-medium titanium CG G1, G2 and G11, and the compositions of single pyropes CG G3 shifted to the upper region of the diamondiferous contour.

The non-diamond pipes Shandongu and Lx-150 are also characterized by the displacement of CG G10 (16% and 7%, respectively) and G9 to the upper part of the diagram, with a predominance of the proportion of pyropes G9, with an outflow of diamond content up to 30-50% of the grain compositions. The proportion of high-titanium CG G1, G2, and G11 (up to 25% in the Lx-150 pipe) is quite large here, most of the compositions of which go beyond the diamond-bearing contour of kimberlites.

Conclusions

New JX-8230 microprobe allows quantitative determination of trace elements in kimberlite pyropes. Diagram MnO, Na2O, TiO2 give additional criteria for kimberlite diamond grade