A. V. Kargin

Diamond resource potential of kimberlites from the Zimny Bereg field, Arkhangel’sk oblast

Kimberlites with different diamond grades from the Zolotitsa, Verkhotina, and Kepina occurrences of the Zimny Bereg field (Arkangel’sk oblast) have been compared in order to ascertain geochemical criteria of their diamond resource potential. A new collection of 21 core samples taken within a depth interval of 207–940 m from nine boreholes drilled in the central and western portions of the high-grade diamond-bearing Grib kimberlite pipe was subjected to comprehensive petrographic and geochemical examination, including Sr, Nd, and Pb isotopes and trace elements determined with ICP-MS. The compositional variations in kimberlites are controlled by the structural types of rocks. Porphyritic kimberlite (PK) distinctly differs from autolithic kimberlite breccia (AKB). Autoliths (Av) and PK are enriched in Th, U, Nb, Ta, La, Ce, Pr, P, Nd, Sm, Eu, Ti, LREE, and MREE, whereas HREE contents are rather uniform in all types of kimberlites. No lateral zoning was observed in pipes pertaining to the same structural type. The composition of kimberlites in the Zimny Bereg field and their diamond resource potential are variable. In the series of the Zolotitsa, Verkhotina, and Kepina occurrences, the Ti content increases, the La/Yb ratio grows from 18–44 to 70–130, and the diamond grade diminishes in the Kepina occurrence. The variations in kimberlite compositions are considered in terms of the degree of partial melting in the mantle, the role of volatiles, etc. As follows from the variation in the Ce/Y ratio, kimberlites from the Zolotitsa occurrence were formed at a lower degree of partial melting in comparison with the Kepina occurrence. Products of different degrees of partial melting are recognized within the Grib pipe; Av were likely formed at a somewhat higher degree of melting than AKB. An appreciable isotopic heterogeneity of the mantle is recorded in variable Nd and Sr isotopic compositions of kimberlites. The Kepina kimberlites were derived from a source slightly depleted relative to CHUR (ɛNd(t) reaches +4) and are close to kimberlites of group I in South Africa. Kimberlites from the Grib pipe with transitional Nd isotopic composition plotted near the Bulk Silicate Earth (BSE) value in the ɛNd(t)-ɛSr(t) diagram adjoin the first group. The source of kimberlites of the Zolotitsa occurrence falls in the field of enriched mantle and is considered to be a product of interaction of an asthenospheric plume with the ancient enriched lithospheric mantle. Kimberlites depleted in Ti, Zr, and Th are related to a source formed as a result of a multistage process that included mantle metasomatism with participation of fluids. Devonian kimberlites derived from sources that involve crustal material (a shift of 206Pb/204Pb, minimums of Th, U, Nb, and Ta contents) are diamond-bearing both in the East European Platform (the Zolotitsa and Verkhotina occurrences) and in the Siberian Craton (the Nakyn field).

Olivine from the Pionerskaya and V. Grib kimberlite pipes, Arkhangelsk diamond province, Russia: Types, composition, and origin

We report the first systematic study of different textural varieties of olivine (olivine from peridotite xenoliths, macrocryst-type Ol-I, and phenocryst-type zoned Ol-II) from two diamondiferous kimberlite pipes of the Arkhangelsk diamond province (V. Grib and Pionerskaya) differing in geologic setting, geochemical and isotopic characteristics, and diamond content. Approximately 550 olivine analyses were obtained by the EPMA technique using the precise method of Sobolev et al. (2007) adapted at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences (Kargin et al., 2014). Olivines from the V. Grib moderate-Ti kimberlites, which are similar to Group I kimberlites in geochemical and Sr-Nd isotopic characteristics and rich in diamond, are dominated by high-Mg# low-Ti Ol-I formed owing to the fractional crystallization of a carbonate-rich protokimberlite melt interacting with orthopyroxene-bearing peridotite material; the fraction of high-Ti Ol-I produced by the metasomatic alteration of peridotite under the influence of silicate aqueous fluids is significantly lower; and xenocrysts weakly affected by metasomatic agents (melts and fluids) occur in minor amounts. Olivines from the low-Ti Pionerskaya kimberlites, which are similar to Group II kimberlites in geochemical and Sr-Nd isotopic characteristics and show a moderate diamond content, are dominated by high-Ti Ol-I, and xenocrysts weakly affected by metasomatic agents are also abundant. In the kimberlites of both pipes, the cores of Ol-II crystals are usually composed of low-Ti olivine similar in composition to Ol-I; both high-Ti and low-Ti olivine cores occur in the Pionerskaya pipe; whereas cores corresponding to high-Ti Ol-I were never found in the V. Grib pipe. The outer zones of olivine and small olivine grains in the groundmass show considerable variations in minor element contents within a narrow Mg# range. It is suggested that the high-Ti rims of Ol-II from the V. Grib and Pionerskaya kimberlites were produced by the late crystallization of kimberlite melt, and the low-Ti rims on the outer zones of Ol-II in the Pionerskaya kimberlites were formed by late-stage equilibration with an aqueous fluid separated from the kimberlite melt and/or possible kinetic effects. Our study revealed the diversity of olivine origin in the kimberlites and showed that there is no single mechanism of olivine formation.

Mesoproterozoic orangeites (Kimberlites II) of West Karelia: Mineralogy, geochemistry, and Sr-Nd isotope composition

Mineralogical and petrological-geochemical features of the Mesoproterozoic (1.23–1.20 Ga) alkaline ultrabasic rocks from the Kostomuksha-Taloveis (Russia) and Lentiira-Kuhmo (Finland) areas, West Karelia, have been studied. In terms of mineralogy and geochemistry, these rocks more resemble group II kimberlites of South Africa (orangeites) than olivine lamproites or ultramafic lamprophyres. On the basis of phenocryst composition, the studied orangeites are divided into three types: Cpx-Phl-Ol, Phl-Ol, and Phl-Carb orangeites. The Cpx-Phl-Ol orangeites from the Kostomuksha cluster clearly differ from analogous rocks from the Lentiira cluster. The composition of Phl-Ol orangeites is indicative of derivation by intense fractional crystallization; Cpx-Phl-Ol orangeites from the Kostomuksha area display evidence of intense lithosphere assimilation. The Phl-Carb orangeites from the Taloveis cluster and Cpx-Ol orangeites from the Lentiira cluster most closely approximate primary melts. The Kostomuksha orangeites are characterized by lowto moderate-radiogenic (87Sr/86Sr)1220 ratio varying from 0.7038 to 0.7067. The Phl-Carb orangeites of Taloveis have less radiogenic Nd isotope composition (ɛNd from −11 to −12) as compared to the Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (ɛNd from −6.9 to −9.4). The Cpx-Phl-Ol orangeites from Lentiira contain fresh olivine. By morphology and composition, there are three olivine generations: (1) large rounded, usually zoned crystals with Fo92 core, 0.33–0.37 wt % NiO, and 0.03–0.04 wt% CaO, which are interpreted as xenocrysts from depleted peridotites; (2) anhedral rounded zoned olivines of intermediate size with Fo82–83 cores, 0.03–0.05 wt % CaO, 0.12–0.17 wt % NiO, and up to 0.40 wt % MnO. These olivines were entrapped by orangeite melt and presumably represent a cumulate of basaltic melts or were derived from metasomatized peridotites; (3) fine euhedral olivines and xenocryst rims corresponding to Fo88–89 with 0.10–0.42 wt % CaO, 0.14–0.35 wt % NiO, and up to 0.07–0.21 wt % MnO; their origin was presumably related to the crystallization from kimberlite melt. The calculation of

Polygenetic Sources of Kimberlites, Magma Composition, and Diamond Potential Exemplified by the East European and Siberian Cratons

f_{O_2 }

Kimberlite age in the Arkhangelsk Province, Russia: Isotopic geochronologic Rb–Sr and 40Ar/39Ar and mineralogical data on phlogopite

of kimberlite melt during crystallization of perovskites using Nb-Fe perovskite oxyba-rometer showed that Cpx-Phl-Ol orangeites of Kostomuksha and orangeites of Lentiira crystallized at similar oxygen fugacities corresponding to ΔNNO from −3.3 to −1.1 and from −3.3 to −0.9, respectively. The Sm-Nd and Rb-Sr isotope study provided evidence for the contribution from ancient enriched source in the genesis of the orangeites. It was proposed that their mantle source was formed in two stages: (1) metasomatic reworking of previously depleted lithospheric source at the Karelian Craton base during Paleoproterozoic orogenic events 2.1–2.0 Ga ago; (2) extension-related generation of orangeite melts 1.27–1.20 Ga ago.

Petrographic-geochemical types of Triassic alkaline ultramafic rocks in the Northern Anabar province, Yakutia, Russia

New petrogeochemical data on a collection of 138 samples taken from 101 kimberlite bodies of the Alakit region of Yakutia have been interpreted. It was concluded that all studied kimberlites are homogenous in geochemical composition and comparable with Group I kimberlites of South Africa. Based on cluster analysis, kimberlites of the region are subdivided into six clusters. From the first to sixth clusters, kimberlites show a decrease in carbonate material and increase in magnesian component. The spatial distribution of clusters allowed us to distinguish zoned areas with central parts consisting of kimberlites with elevated CaO, CO2, Rb, Sr, Ba, and lowered contents of SiO2, TiO2, Fe2O3, FeO, MgO, V, Cr, and Ni. From the center outward, the values of δNd and (87Sr/86Sr)i decrease, which indicate increasing contribution of the lithospheric source. The formation of magnesian kimberlites at the periphery was related to the intense interaction of protokimberlite melt with lithospheric mantle, which was accompanied by metasomatic reworking of mantle rocks with formation of minerals of megacryst assemblage and assimilation of mantle material. Economically viable diamondiferous kimberlites are confined to the peripheral parts of distinguished zones, i.e., to the kimberlites of 5–6 clusters.

Devonian ultramafic lamprophyre in the Irkineeva–Chadobets trough in the southwest of the Siberian Platform: Age, composition, and implications for diamond potential prediction

The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO2-K2O, TiO2-(Y, Zr, HREE), SiO2-MgO, SiO2-Al2O3, MgO-Ni, MgO-CO2, and MgO-H2O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (143Nd/144Nd)i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: 206Pb/204Pb of 16.19–19.14, 207Pb/204Pb of 15.44–15.61, and 208Pb/204Pb of 34.99–38.55. In the 207Pb/204Pb-206Pb/204Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO2). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.

Geochemistry and oxygen isotopic composition of olivine in kimberlites from the Arkhangelsk province: Contribution of mantle metasomatism

The geochemistry of mantle metasomatism related to the formation of kimberlites and allied rocks in the northern East European Platform (EEP) is considered with allowance for chemical systematics and geodynamic position. The Paleoproterozoic kimberlites of Kimozero, Mesoproterozoic orangeites of Karelia, Neoproterozoic kimberlites of Finland, and Devonian kimberlites of the Arkhangel’sk diamond province (ADP) are the objects of this research. Kimberlites from the EEP are characterized by wide variations in contents of rock-forming oxides and trace elements caused by secondary alteration, belonging to different lithofacies and specific sources. The diversity of kimberlites from the EEP is explained by the interaction of asthenospheric protokimberlitic melt with depleted or metasomatically enriched lithospheric mantle. A (Zr/Sm)n-Cr/Ni diagram has been proposed to identify specific attributes of mantle metasomatism. Comparison of the geodynamic setting of kimberlite formation in the EEP with the revealed geochemical features of mantle metasomatism shows that kimberlites having sources with the participation of MARID-type meta-somatic assemblages were formed under conditions of changing supercontinental cycles, when the breakdown of large supercontinent coincided in time with the initial stage of assembly of a new supercontinent. This is characteristic of the Kimozero kimberlites, orangeites of Karelia, and kimberlites from the ADP. Kimberlites of Finland were formed under geodynamic conditions characterized by supercontinent breakdown coeval with abundant within-plate mafic magmatism. These kimberlites bear geochemical features inherent to an asthenospheric source. The economic Devonian diamond-bearing kimberlites from the ADP display a subordinate role of the metasomatic lithospheric as a component of their source against the background of a significant interaction of melted asthenospheric material with the depleted lithospheric mantle.