Mineralogical and petrochemical features of Maiskoe kimberlite deposit.
ALROSA Co., Ltd., Botuobiyan Geological Prospecting Expedition, Mirny, Yakutia (Sakha Republic), Russia
At the present moment within the Nakinskoe field (Middle-Markhinskyi area) two kimberlite pipes (Botuobinskaya and Nyurbinskaya) and the series of dike bodies are known.
In May 2006 as a result of detailed searching at the net 100x100 m by core drilling boreholes a new dike body of submeridional course was revealed, and it was named Maiskoe. As a result of exploration by deviated boreholes it was revealed that the Maiskoe body has an unequal thickness within from 8 to 40 metres. It is formed by typical variety of kimberlites that are typical of pipes of the Nakinskoe field.
As a whole kimberlites of the Nakinskoe field are characterised by high diamondiferousness with low content of diamond's accessory minerals, mainly pyropes and chrome-spinellids, and absence of picroilmenite that is typical of kimberlites in Yakutia.
As a result of the research of texture-srtuctural features of the kimberlites' collection of the Maiskoe deposit three rock types were singled out - a porphyric kimberlite (PK), an autolithic kimberlite breccia (AKB) and a xenotuphobreccia (XTB). These varieties are localized within the ore body and they are notable for the content of indicator minerals and diamondiferousness.
Porphyric kimberlites are localized in the north-east part of ore occurance among autolithic kimberlite breccias in the form of a column ore body that is traced to the depth. Macroscopically porphyric kimberlites are the rock of dark grey colour with diffirent tints. The rock texture is breccial, the texture of the kimberlite, cementing clastic material, is massive, the rock structure is porphyric with phenocrysts of olivine and carbonate, and with the size to 15mm. The groundmass is cryptocrystal, consists of the serpentine-chlorite-carbonate aggregate.
Autolithic kimberlite breccias are localized in the north-east part of ore occurance, and form the body that gradually extents to south-west direction and north-east direction. On the contact with tuff breccias autolithic kimberlites are enriched by xenogenic material and have illegible texture-srtuctural features.
In AKB there are noted massive, freak, taxitic and spheric textures. The kimberlite material, cementing xenoliths and autoliths, consists of porphyric phenocrysts that are buried in cryptocrystal serpentine-chlorite-carbonate basis. The amount of phenocrysts of olivine and calcite in connecting serpentine-carbonate mass makes up 25-35%.
Xenotuphobreccias make the body of mold form with supposed sizes 210x180 mm in the south-west part of ore occurance. Macroscopically XTBs are the subcemeting rock of dirt-green-grey, brown-grey colour. The rock structure is lithocrystalloclastic. The content of clastic material practically always exeeds 60-70% of the rock volume. There is from 5%, rarely 10%, to disappearing paucity of the kimberlite material.
In the analised kimberlites chrome-spinellids and garnets were founded. Among garnets dominate crimson garnets, and in auxiliary amount garnets with the alexandrite effect are found. As a result of the reseach the important difference of structures of crimson garnets in kimberlites, enriched by calcium, from kimberlites, enriched by magnesium, is found out. Garnets with the alexandrite effect are characterised by the structure consistency in kimberlites with different content of CaO and MgO. Thus, it can be considered that crimson garnets paragenetically connect with protokimberlitic fluxes, and garnets with the alexandrite effect are xenocrysts of rocks of other structure.
The pecularities of concentrations allocation of mineralopoietic oxides in the analisis of chrome-spinellids allow to subdivide them into chromic and aluminuos. The conducted researches allow to relate chromic chrome-spinellids to primery igneous rocks, and aluminuos – to rocks of other igneous and metasomatic systems.
The research of pecularities of mentioned chemical structures of kimberlites of the Maiskaya pipe were carried out on the basis of populational petrochemical model. The comparison of populational models of kimberlite bodies of the Nakinskoe field allows to disclose that the Maiskaya pipe is a logical continuation of populational series of kimberlites of other bodies of this field.
The Maiskoe kimberlite body
consists of low-titanous, carbonate-magnesia, high-diamondiferous
kimberlites. Similar rocks are found in Botuobinskaya, Nyurbinskaya,
Internationalnaya and Aichal pipes. At the same time the combination
of kimberlite structures and garnet structures says about the
unicity of the Maiskoe body.