Assessment of kimberlite diamond grade by the indicator minerals chemical composition in Lunda region of Angola

To determine a diamond grade (ct/t) in the Lunda district kimberlites using the chemical composition of the KIM (indicator minerals) frequency of occurrence of their cluster groups (CG) we performed statistical analysis of the chemical composition of pyropes (3478 grains) of Cr-diopsides (714) and picroilmenites (1582) of the 6 kimberlite diamond deposits. Classification procedures of cluster and correlation – factor analysis were used (Ivanov, 2017). Significant correlation coefficients were determined between the variations of KIM compositions and diamond content in kimberlites. Figure 2 shows the distribution of diamond contents in 6 kimberlite pipes, correlated with the distribution of pyropes G10 (Dawson et al., 1975), chromium diopsides CG S6, as well as CG of picroilmenites – 12b and P12-16 in their frequency of occurrence, the interpretation of which is reduced to the following conclusions. The proportions of pyropes CG G10 in kimberlites of 5 pipes control the linear growth (R²=0.97) of the diamond content in pipes to the center of the Saurimo structure, excluding the CAT-E42 pipe. With a relatively high diamond grade, the proportion of  G10 in this pipe is low, which may be related to the extremely low quality of its diamonds. In kimberlites. This indicator is typical for the Catoca and Luele pipes, with the maximum proportions of low-ferrous picroilmenites (11.0% and 13.9%). In the NE direction, the conditions for the preservation of diamonds in kimberlites decrease, which affects their low diamond grade (0.2-0.4 ct / t), which decreases exponentially (R²=0.98) with an increase in the TiO₂ content in picroilmenites. The proportion of CG S6 Cr-diopsides belonging to the high-pressure variety of the deep mantle lithosphere (coesite facies) (Sobolev, 1971) increases in the kimberlites of the central part of the Saurimo structure to 15-32% and controls the high diamond content of the Catoca, CAT-E42 and Luele pipes (Fig. 1). The established regularities of changes in the frequency of occurrence of CG KIMs in the NE-SW direction in the Lunda kimberlite region confirm the regional pyrope trend of N. V. Sobolev's diamond content and other KIMs correlations with the diamond content of kimberlites in this region. They also meet the "rule of V. A. Milashev" on reducing the diamond content of kimberlites to the periphery of regional structural units of kimberlite provinces (Zinchenko et al., 2016).

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