Mineralogical-geochemical criteria of diamond potential of kimberlites in the Yubileinaya multiphase Pipe (Yakutia)

Abstract

Pursuance and refinement of the available criteria for the assessment of diamond potential remain an urgent issue in diamond geology. The universality of mineralogical criteria, which determine the interrelation between the content of high-Cr low-Ca pyropes and high-Cr spinels (xenocrystals) with the diamond potential of kimberlitic breccia, has been confirmed for various diamondiferous provinces of the world [1‐3]. Other compositional criteria of the diamond potential of kimberlites are developed insufficiently as yet. Their application for assessment of the productivity of separate phases of kimberlites, pipes, or fields of kimberlite provinces has certain limitations [4‐6 and others]. Our paper shows for the first time that the complex of mineralogical and petrogeochemical criteria of diamond potential can be used for some kimberlite phases with the Yubileinaya Pipe as an example. The lithological composition of mineralogical‐petrographic varieties of kimberlites in autonomous emplacement phases of the Yubileinaya Pipe was investigated systematically during the mining of various levels of the deposit in the years 1986‐2006. In order to analyze the possible correlation between the productivity and compositional features of separate kimberlite varieties, we used the exploitation and geological exploration data on the diamond content (in conventional units) in kimberlite ores from different horizons of the pipe. Mapping of the southwestern and northeastern flanks of the Yubileinaya Pipe made it possible to outline three autonomous phases of kimberlite emplacement with different textural-structural, mineralogical, and petrogeochemical parameters and diamond potentials: coarse-porphyric micaceous kimberlite, kimberlitic breccia, and autolithic kimberlitic breccia. The central part of the pipe is characterized by a greater diversity of kimberlites. Based on the texturalstructural features and mineral compositions, they are divided into kimberlitic breccias, autolithic kimberlitic breccias, and kimberlites with various (schlieren‐ taxitic, aphyric, and sporadophyric) textures. This zone also includes some blocks of the porphyric kimberlite with the mineral composition similar to that of the coarse-porphyric kimberlite at flanks of the pipe. All these varieties are commercial kimberlite ores with a variable content of the useful component. The upper horizons of the central part are mainly composed of sheeted carbonate breccia with a small amount of the kimberlitic material. Their thickness varies from several meters at the margins of the pipe to 100‐130 m in the central part. The carbonate breccias grade downsection into kimberlite rocks of the “xenolithic belt” characterized by the presence of a great amount (up to 60‐70 vol %) of large xenoliths and “floating reefs” of sedimentary rocks. The large sedimentary xenoliths are cemented by the sporadophyric and schlieren‐taxitic kimberlites and the less common kimberlitic breccias and autolithic kimberlitic breccias. The xenolithic belt encloses the autolithic kimberlitic breccias that make up lower horizons of the pipe. The carbonate breccias (with sedimentary rock fragments up to 90%), sporadophyric and aphyric varieties, and some schlieren‐taxitic kimberlites are noncommercial ores.