REVIEWS - 221
The diamonds in primary deposits and in placers range in weight from 0.1 mg to several metric carats. The largest diamond found weighed 32.5 metric carats. The diamonds are represented by crystals of octahedral, dodecahedral, or transitional habitus. The majority of diamonds are colorless; some yellowish crystals are encountered. Graphite inclusions along fractures are epigenetic. Graphite is also present in diamonds in a disseminated state. Microscopic inclusions of olivine, pyrope, and chrome spinel within diamonds have heen recorded.
The friable deposits of the Vilyuy River Basin contain diamonds almost everywhere. These sediments are pre-Quaternary and Quaternary in age. The most prolific placers contain, besides diamonds, pyrope, ilmenite, chrome diopside, and perovskite. These minerals serve as indicators of diamond. The decrease of average weight of diamonds downstream amounts to 3-4 mg per 100 km. Pyrope withstands transportation as far as 150 - 200 km from the source. The exploration includes panning, with the purpose of studying the concention of pyrope in placers, and to trace it back to the primary deposit. From placers with high concentration of pyrope, samples are taken for determination of diamond content. The magnetic survey, especially the airborne, has been successfully used for detection of kimberlite pipes.
The authors are cautious as to the problem of the genesis of the primary diamond deposits. They postulate that the basic reserves of diamonds in Vilyuy kimberlite Basin are connected with the kimberlite, and to a minor degree with the placers. Geologic parallels between the Siberian and African deposits have heen drawn in several cases. The Siberian diamonds are relatively of a low quality due to their small size and fragmentation, especially in placers. Diamonds of gem quality are rare. The deposits are considered by the authors to be of the same magnitude as those of South Africa. It is suggested that they can entirely satisfy the demand for industrial diamonds in Soviet Union.
EUGENE A. ALEXANDROV
DEPARTMENT OF GEOLOGY, COLUMBIA UNIVERSITY,
Dec. 27, 1957
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