Radon over Kimberlite Pipes: Surface Field Experiments and Calculations of Vertical Diffusion (Arkhangelsk Diamondiferous Province, NW Russia)

We present the results of field and experimental studies to assess the influence on the formation of the radon field over the kimberlite pipes of the Arkhangelsk diamondiferous province. Measurements were made in the field to establish the radon radiation in the soil air and the gas permeability of soils in the area of the Chidvinskaya pipe. Experimental work was aimed at determining the radiation and physical parameters of the rocks occurring within the kimberlite field. Based on a set of field and experimental data, a model of the diffusion transfer of radon in the area of the Chidvinskaya pipe was calculated for three profiles, represented by the rocks of the pipe, sedimentary rocks of the exocontacts of the pipe, and host sandy and clay sedimentary rocks. The results of the calculations show that the rocks of the exocontacts of the pipe have the greatest potential for increased radon radiation. The calculated values of the radon radiation produced by these rocks exceeded 9000 Bq·m−3. The diatreme kimberlites produced the lowest radon radiation. We showed that the source of the increased values of radon radiation is the rocks of the pipe’s exocontacts. This fact will make it possible to use the emanation method as an additional one for the search for kimberlite pipes.

Ellinaite, CaCr₂O₄, a new natural post-spinel oxide from Hatrurim Basin, Israel, and Juína kimberlite field, Brazil

Ellinaite, a natural analog of the post-spinel phase β-CaCr2O4, was discovered at the Hatrurim Basin, Hatrurim pyrometamorphic formation (the Mottled Zone), Israel, and in an inclusion within the super-deep diamond collected at the placer of the Sorriso River, Juína kimberlite field, Brazil. Ellinaite at the Hatrurim Basin is confined to a reduced rankinite–gehlenite paralava, where it occurs as subhedral grains up to 30 µm in association with gehlenite, rankinite and pyrrhotite or forms the rims overgrowing zoned chromite–magnesiochromite. The empirical formula of the Hatrurim sample is (Ca0.960Fe0.0162+Na0.012Mg0.003)0.992(Cr1.731V0.1833+Ti0.0683+Al0.023Ti0.0034+)2.008O4. The mineral crystallizes in the orthorhombic system, space group Pnma, unit-cell parameters refined from X-ray single-crystal data: a 8.868(9), b 2.885(3), c 10.355(11) Å, V 264.9(5) Å3 and Z=4. The crystal structure of ellinaite from the Hatrurim Basin has been solved and refined to R1=0.0588 based on 388 independent observed reflections. Ellinaite in the Juína diamond occurs within the micron-sized polyphase inclusion in association with ferropericlase, magnesioferrite, orthorhombic MgCr2O4, unidentified iron carbide and graphite. Its empirical formula is Ca1.07(Cr1.71Fe0.063+V0.06Ti0.03Al0.03Mg0.02Mn0.02)Σ1.93O4. The unit-cell parameters obtained from HRTEM data are as follows: space group Pnma, a 9.017, b 2.874 Å, c 10.170 Å, V 263.55 Å3, Z=4. Ellinaite belongs to a group of natural tunnel-structured oxides of the general formula AB2O4, the so-called post-spinel minerals: marokite CaMn2O4, xieite FeCr2O4, harmunite CaFe2O4, wernerkrauseite CaFe23+Mn4+O6, chenmingite FeCr2O4, maohokite MgFe2O4 and tschaunerite Fe(FeTi)O4. The mineral from both occurrences seems to be crystallized under highly reduced conditions at high temperatures (>1000 ∘C), but under different pressure: near-surface (Hatrurim Basin) and lower mantle (Juína diamond).

Tectonic setting of kimberlites in the Vilyui-Markhinsky fault zone according to modern data

The lateral heterogeneity of the Vilyui-Markha fault zone was determined, the central and western subzones were identified. The high-grade diamondiferous Mir and Nakyn kimberlite fields are confined to the central subzone. The low-grade diamondiferous Syuldyukar kimberlite field is confined to the western subzone of the VilyuiMarkha zone. The analysis of the fault network density in the research area was carried out. It was found that the fault network density increases within the subzones, which characterizes them as increased permeability areas favorable for kimberlite melts uprising. This fact can be another tectonic criterion for setting up diamond prospecting operations.

The role of plicative structures as forecast criteria in the Alakit-Мarkhinsky field (western Yakutia)

This paper describes the tectonic features of the Alakit-Markha kimberlite field, regional factors of kimberlite magmatism control in this area, structural and tectonic preconditions for kimberlite pipe prospecting. The paper highlights kimberlite pipe formation features and the role of tectonics in this process. The most promising areas are those related to low-amplitude negative structures (e.g. depressions), especially transverse low-amplitude complications of the opposite sign for the main plicative structure: for antiforms (elevations), these are saddle-shaped depressions, and antiform elevations are for synforms (depressions).

Prospects for diamond potential of the Charo-Sinskaya fault zone

New data on the geological structure of the CharoSinskaya zone of deep faults located on the southern side of the Vilyui syneclise are presented. Based on the processing of the seismic survey results, the deep structure of the territory has been analyzed, and graben-like structures similar to those found near the known kimberlite fi lds of Yakutia have been identifi d. Taking into account the results on the mineralogy of the indicator minerals of kimberlite, a new kimberlite field location is predicted.

Groundmass Chromospinellides from Kimberlites of Khompu-May Kimberlite Field

The current paper presents the results of studying chromites of kimberlite mesostasis forming the Manchary, Aprelskaya, Erel, Turakhskaya, and Artemova pipes within the Khompu-May kimberlite field (central Yakutia). Despite shared texture and structural characteristics and mineral composition of the kimberlites, chromospinellide composition is distinctive in each pipe. Groundmass chromium spinel of the Aprelskaya and Erel kimberlite pipes is characterized by the highest aluminum oxide content (>10 wt. %). Chromites from the Erel and Turakhskaya pipes as well as a fraction of grains from the Manchary pipe with titanium oxide (<4 wt. %) form a field of common composition by Cr2O3 and TiO2 content. The Aprelskaya and Artemova pipes show up to 17 wt. % TiO2 in chromites. Such a difference in titanium content correlates with perovskite content in kimberlite groundmass of the Khompu-May field. The results of the study revealed two trends in evolution of chromospinellide microcrystals (R. Mitchell, 1986) – ulvöspinel associated with typical kimberlites and titanomagnetite characteristic of micaceous kimberlites. Chromospinellides of the Aprelskaya pipe demonstrate the ulvöspinel trend only, suggesting earlier spinel crystallization relative to groundmass mica. Spinellides from the Erel and Artemova pipes follow the titanomagnetite trend only, being crystallized after formation of mesostasis mica. Spinellides from the Manchary and Turakhskaya pipes meet the ulvöspinel and titanomagnetite trend, indicating two stages of mineral crystallization relative to phlogopite.

Carbonatite-Like Rock in a Dike of the Aikhal Kimberlite Pipe: Comparison with Carbonatites of the Nomokhtookh Site (Anabar Area)

––A dike of rock similar in composition to carbonatites has been found in the Aikhal diamondiferous pipe of the Alakit–Markha field of the Yakutian kimberlite province (YaKP). The fine-grained rock of essentially carbonate composition (dolomite and calcite) rich in thin-platy phlogopite contains minerals typical of carbonatites: monazite, baddeleyite, and pyrochlore. In the high contents and distribution of incompatible elements the rock differs significantly from kimberlites and is transitional from kimberlites to carbonatites. The content of incompatible elements in this rock is 3–5 times lower than that in carbonatite breccias of the pipes in the Staraya Rechka kimberlite field of the YaKP (Nomokhtookh site). The compositions of accessory trace element minerals from the Aikhal dike rock and the Nomokhtookh carbonatite breccias are compared. An assumption is made that the high contents of incompatible elements in the carbonatite-like rock, which caused the crystallization of accessory minerals, are due to the differentiation of kimberlite melt/fluid. The high Sr isotope ratios indicate that the rock altered during hydrothermal and metasomatic processes. The obtained data on the composition of the carbonatite-like rock cannot serve as an argument for the genetic relationship between the Aikhal kimberlites and typical carbonatites. The genetic relationship between kimberlites and carbonatites in the northern fields of the YaKP remains an open issue.