Diamonds from semi-industrial kimberlites

Abstract

Introduction: On the Siberian platform (SP), in addition to a group of industrial and successfully exploited diatremes, some kimberlite pipes with semi-commercial diamond content have also been discovered. Changes in the diamond market over the past 20–25 years and additional exploration and appraisal activities on individual pipes (Krasnopresnenskaya, Zapolyarnaya, and Poiskovaya) allowed some of them to be categorised as being industrial in nature. Therefore, in this study, we focused on the characterization of individual SP diatremes classified as pipes with a semi-industrial diamond content. Methods: The article presents the results of a comprehensive study of a number of kimberlite bodies (Dalnyaya, Irelyakhskaya, Radiovolnovaya, and Malokuonapskaya pipes) with semi-industrial diamond content, which made it possible to establish the typomorphic peculiarities of diamond. Results and discussion: Dalnyaya pipe is part of the southernmost group of bodies of the Daldyn kimberlite field (DKF). Diatreme has a complex geological structure and is represented by two varieties corresponding to two stages of its formation. In the first (intrusive) one, a stock-like body appeared, composed by dense porphyry kimberlite (PK). In the second (explosive) stage, a diatreme, filled with autolithic kimberlite breccia (AKB) of the second phase was formed. The Dalnyaya pipe is a classic example of kimberlite bodies, in which petrographic varieties differ by the typomorphic features of diamonds. The Irelyakhskaya kimberlite pipe is located on the south western flank of the DKF. This pipe has a rather complex internal structure due to the presence of at least two interstitial phases. With a sharp (up to 90%) predominance of diamonds of variety I in kimberlites, a distinctive feature of the diamond association is the increased role (compared to other DKF pipes) of uniformly coloured yellow octahedrons of variety II (IIa) and milky white, less often gray and yellow coloured coated diamonds of variety IV. The Radiovolnovaya kimberlite pipe based on diamond size, belongs to diatremes with a slightly increased size of gem stones with an average crystal weight of 3.44 mg. The vast majority of diamonds in the pipe are colourless, less often epigenetically coloured in smoky-brown colours of variety I (over 90%) with a noticeable (up to 5%) amount of polycrystalline aggregates of variety VIII and IX and somewhat less - yellow-green cuboids of variety II and grey cubes of variety III. The Malokuonapskaya kimberlite pipe located in the Kuranakh field in the south of the Anabar diamond-bearing region, it consists of two ore shafts (northern and southern) conjugated with each other, corresponding to two stages of diatreme formation and composed of various types of kimberlites. The pipe is the only deposit in the joint venture with a semi-commercial diamond content of the Middle Mesozoic age with two different phases of kimberlite magmatism. According to their typomorphic features, the diamonds of the northern and southern shafts differ significantly. Conclusions: Diamonds from semi-industrial diatremes of the studied diamond-bearing regions of the SP are characterized by the predominance of rhombododecahedral crystal habit with a high content of typical rounded diamonds, which are an unfavourable factor for diamond content. The difference in the features of diamonds in individual kimberlite bodies is lower than within diamond-bearing regions among themselves, which confirms the heterogeneity of the structure of the upper mantle in different parts of the plat- form. The predominance of laminar crystals of octahedral, rhombododecahedral, and transitional crystal habit in the pipes of the peripheral part of the SP (the Malokuonapskaya pipe) indicates the absence of horizontal zoning in the change in typomorphic features within the study area.