New morphogenetic type of Cr-spinellide in diamondiferous areas in connection with the problem of false kimberlite indicators

A specific variety of Cr-spinellide was found in the 1970s during the diamo nd exploration near Mirnyi (Yakutia). On composition diagram its field over laps with that of kimberlite varieties but differs considerably in morpholo gy. This type of Cr-spinellide was named, after the geographic place, "kuru ng" and was initially regarded as an indicator of highly diamondiferous kim berlite bodies, because, according to the contents of Cr2O3, Al2O3, and TiO 2, more than 30% of their grains fall in the field of diamond association. However, searches for their primary sources have failed. Studies of the kur ung Cr-spinellide has shown (1) their wide occurrence not only in Yakutia b ut in all diamondiferous and potentially diamondiferous regions of the worl d, (2) independence of their distribution from the distribution of indicato r kimberlite minerals and diamonds (kurung Cr-spinellide might associate wi th kimberlite minerals in sediments, but more often they occur independentl y), and (3) their absence in the world-wide known kimberlite bodies. Theref ore, we have distinguished the kurung Cr-spinellide as an individual morpho genetic type and tried to determine the type of their primary sources. This permitted us to widen the range of the compositions of this morphogenetic type of Cr-spinellide and show their analogy in morphology and in compositi on to Cr-spinellide of other ultrabasic massifs, such as Kempirsai, Chad, K onder, etc. Therefore, we suggest that Cr-spinellide of the kurung type wer e supplied into sediments of diamondiferous and potentially diamondiferous regions from ultrabasic massifs. In addition to the analogy of the morpholo gy and composition, the evidence comes from studies of inclusions in these Cr-spinellide (olivine, pyroxene, and amphibole). The temperatures of forma tion of the kurung-type Cr-spinellide, determined from the compositions of inclusions, are 650-850 degreesC, and the lack of pyrope as inclusions indi cates a low crystallization pressure. This differs them from Cr-spinellide from kimberlites and their xenoliths. We distinguished two morphogenetic ty pes of Cr-spinellide - mantle, typical of kimberlites, diamondiferous lampr oites, and their xenoliths, - and crustal, common to ultrabasic intrusions. They have similar compositions but differ in morphology, which is due to d ifferent formation conditions. The compositional similarity of crustal Cr-s pinellide to mantle ones brings geologists ignoring their morphology to the wrong conclusion that crustal Cr-spinellide are indicators of kimberlites and lamproites and to unsuccessful exploration for diamond deposits, i. e., crustal Cr-spinellide might be false indicators of kimberlites and lamproi tes.