Nikolai Vladimirovich Sobolev

Mineralogical mapping of the North-East section of the Yakutian kimberlite province and its main results

“…It was shown that subcalcic Cr-pyropes and chromites with maximally enriched Cr contents (>62 wt.% C^Os) and low Ti and Fe3+, interpreted as originating from cpx-free harzburgite-dunite paragenesis (Sobolev et al 1969(Sobolev et al , 1973, are the main important indicators of potential diamond grade of Siberian kimberlites.On the basis of analyses of >500 Cr-pyropes and nearly 600 chromites included in diamonds, as well as of the compositions of representative numbers of pyropes and chromites (usually >200 grains of each mineral) from kimberlite concentrates of more than 100 pipes with wide variations in diamond grade, we have established the following compositional ranges of indicator minerals, which we use for estimation of potential diamond grade of kimberlites: 1) Pyropes: CaO<1.6 + 0.38 ^03 wt.%, with the condition that C^Os >5 wt.%.2) Chromites>71-1.637 AI2O3 wt.%, with the conditions that Cr203>62 wt.%; Al203<7.5 wt.%; TiO2<0.7 wt.%. Spesial significance was given to the P-T boundary of the graphite-diamond transition (Sobolev 1977), reflected in the composition of coexisting pyropes and chromites in the harzburgite paragenesis (Pokhilenko et al, 1991;1993). This factor was very important in increasing the reliability of the estimations of potential diamond grade of kimberlites.…”

“…We have found that there is a large possibility of very significant errors in such estimation where Cr-pyropes containing > 15-17% of the knorringite molecule are absent from a kimberlite pipe. This is because pyropes of such composition were only formed in the diamond stability field (Pokhilenko et al, 1991;1993).The work of the mineralogical mapping of the North-East part of the Yakutian kimbrlite province was undertaken by us in close cooperation with the Amakinskaya prospecting expedition (group of the late Yu.P.Belyk) since 1974. The territory studied covered approximately 60,000 km^, was delimited to the East by the Lena River and included the lower partof the Olenek River basin.…”

“…2) Chromites>71-1.637 AI2O3 wt.%, with the conditions that Cr203>62 wt.%; Al203<7.5 wt.%; TiO2<0.7 wt.%. Spesial significance was given to the P-T boundary of the graphite-diamond transition (Sobolev 1977), reflected in the composition of coexisting pyropes and chromites in the harzburgite paragenesis (Pokhilenko et al, 1991;1993). This factor was very important in increasing the reliability of the estimations of potential diamond grade of kimberlites.…”

“…We have found that there is a large possibility of very significant errors in such estimation where Cr-pyropes containing > 15-17% of the knorringite molecule are absent from a kimberlite pipe. This is because pyropes of such composition were only formed in the diamond stability field (Pokhilenko et al, 1991;1993).…”