E. V. Pushkarev

Ontogeny of ore Cr-spinel and composition of inclusions as indicators of the pneumatolytic–hydrothermal origin of PGM-bearing chromitites from Kondyor massif, the Aldan Shield

The thoroughly studied ontogeny and chemical composition of ore Cr-spinel from PGM-bearing chromitites of the Kondyor clinopyroxenite–dunite massif in the Aldan Shield show that this mineral crystallizes jointly with relatively low-temperature minerals: Cr-diopside, amphiboles, phlogopite, chlorite, serpentine, apatite, etc. Induction surfaces between Cr-spinel and silicates of the ore matrix are established. The ore Cr-spinel contains a great number of octahedral carbonic fluid and silicate inclusions, as well as octahedral gold and serpentine aggregates. These data, along with the relationships of Cr-spinel with low-temperature silicates, are evidence for the pneumatolytic–hydrothermal origin of Cr-spinel in Cr–PGE ore. Chromitite is a contrasting and nonequilibrium system with respect to host dunite in both chemical compositions of minerals and formation conditions in agreement with the concept of the epigenetic nature of chromitite stated more a century ago by A.N. Zavaritsky and A.G. Betekhtin.

Geology and ore deposits of the Urals

The Urals is a N-S trending mountain range forming thegeographic boundary between Europe and Asia. It repre-sents a 2000 km long Paleozoic orogen, extending from theIslands of Novaya Zemlya in the north to the Aral Sea in thesouth. For a long time the information about Uralian geol-ogy was very scarce in the international literature. A signif-icant breakthrough took place at the end of 20th centurywhen the EUROPROBE and GEODE international projectsstarted. In the frame of these projects, several seismic crosssections trough the Urals, together with detailed geologicalinvestigations, were carried out. The main results of thesestudies were published in a special Issue of Tectonophysics(1997) and in the Geophysical Monograph 132 “MountainBuilding in the Uralides: Pangea to the Present” publishedby American Geophysical Union (AGU) in 2002 and innumerous other papers. In the last decade a lot of newmineralogical, petrological and geochemical data, particu-larly in isotope geochemistry andgeochronology,weregath-ered. These new data represents the base for the need for afurther look onto Uralian geology. This special issue ofMineralogy & Petrology contains a profound collections ofnew data, interpretations and discussional aspects on Urali-an geology since 2002.The Urals belongs to the western flank of the hugetranscontinental Uralo-Mongolian fold belt and comprisesat least three billion years of geological history. Today wecan distinguish between five major structural levels andepochs of the Urals development (see Puchkov 2013). Theearliest Archean and later Meso-Neoproterozoic complexes,although related to the development of the Baltic craton andits eastern periphery, far before the Uralides existed, areincorporated in the Uralian orogen structure and sometimesthey display traces of young Paleozoic events of the foldbelt. The Archean rocks are represented by granulites of theTaratash complex, showing similarities to the granulites inthe basement of the East-European platform. The Meso-Neoproterozoic sedimentary and magmatic complexes wit-ness the geotectonic development along the eastern marginof the Baltic Shield. This time was very productive regard-ing the formation of some important mineral deposits, suchas siderite and magnesite within metasedimentary sequences(Prochaska and Krupenin 2012) and titano-magnetite oreassociated with rift-related layered gabbro intrusions andothers.The Paleozoic–Lower Jurassic period of time may beconsidered as the main stage of the development of theUralides, covering a complete Wilson cycle: starting fromthe rifting of the Baltic continent after the Timanian oroge-ny, through formation of an oceanic basin, a passive conti-nental margin and microcontinental blocks, to subduction,accompanied by a significant amount of volcanic eruptions,then arc-continent collision, continent–continent collisionwith granite magmatism, formation of a long chain of bath-oliths, and finally post-collisional extension with flood-basalt magmatism. The most important and famous volca-nic, intrusive, metamorphic and sedimentary complexes andrelated deposits in the Urals were formed during this periodof time. The world-class podiform chromite deposits relatedto the Kempirsay ophiolite complex in the Southern Uralswere formed during oceanic-island arc development. Thegiant Gay, Sibay and Uchaly volcanic massive sulfidedeposits and many numerous small ones are located withinthe Devonian island-arc volcanics of the Magnitogorsk zone

The age of late orogenic granitoids of the urals based on U-Pb isotope dating of zircons (Exemplified by the Shartash and Shabry massifs)

U-Pb dating of zircons (SHRIMP-II) has been performed for the first time for granites, granodiorites, and synplutonic granodiorite and melanodiorite bodies of the Shartash and Shabry massifs in the Middle Urals. The time of the formation of the massifs is 300–306 Ma, which is 25 Ma younger than the previous estimates. The age data obtained are in line with the time of the formation of the adamellite-granite series of the Verkhisetsk Batholite (the master sample of tonalite-granodiorite-granite magmatism in the Middle Urals), the series of which is petrochemically close to the rocks of the massifs.

New data on the composition and age of basement rocks of the Tagil paleo-island arc system

The Tagil paleo-island arc terrane is composed of Late Ordovician-Devonian intrusive, volcanic, and volcano-sedimentary complexes. The western margin of the terrane is comprised of dunite-clinopyroxenite-gabbro massifs of the Ural platinum-bearing belt, which are fringed by rock strata of widely different metamorphic grades. Work on isotope systematics of olivine gabbros enabled us to infer a Vendian age (550–540 Ma) of homogenization of the Sm-Nd isotopic system of the Kytlym and Knyaspa massifs within the Ural platinum-bearing belt. The Sm-Nd ages for metamorphic rocks of the Belaya Gora complex surrounding the studied massifs also agree with a Vendian age (573–574 Ma). Our results suggest that metamorphites of the Belaya Gora complex (amphibolites, plagiogneisses, two-mica and biotite gneisses, schists containing garnet, cordierite, staurolite, gedrite, and sillimanite) and dunite-clinopyroxenite-gabbro intrusions of the proto-platinum-bearing belt may have been formed in a convergence setting above a mantle plume, most likely in a back-arc (?) extension region. Reactivation of this plume during the late Precambrian resulted in the opening of the Ural paleo-ocean. The Late Ordovician-Early Devonian times were marked by metamorphic reworking and tectonic transport of pre-Paleozoic complexes into an accretionary prism setting of the Tagil paleo-arc that was accompanied by generation of gabbroid and granitoid magmas. Based on the obtained results, the Tagil terrane can be now considered as part of the Paleozoic paleo-island arc system developed on a heterogeneous Proterozoic basement.

Chemical composition and osmium-isotope systematics of primary and secondary PGM assemblages from high-Mg chromitite of the Nurali lherzolite massif, the South Urals, Russia

The isotopic and geochemical characteristics of PGE mineralization in high-Mg chromitite from the banded dunite–wehrlite–clinopyroxenite complex of the Nurali lherzolite massif, the South Urals, Russia is characterized for the first time. Electron microprobe analysis and LA MC-ICP-MS mass spectrometry are used for studying Cr-spinel and platinum-group minerals (PGM). Two processes synchronously develop in high-Mg chromitite subject to metamorphism: (1) the replacement of Mg–Al-rich Cr-spinel, orthopyroxene, and diopside by chromite, Cr-amphibole, chlorite, and garnet; (2) the formation of a secondary mineral assemblage consisting of finely dispersed ruthenium or Ru-hexaferrum aggregate and silicate–oxide or silicate matter on the location of primary Ru–Os-sulfides of the laurite–erlichmanite solid solution series. Similar variations of Os-isotopic composition in both primary and secondary PGM assemblages are evidence for the high stability of the Os isotope system in PGM and for the possibility of using model 187Os/188Os ages in geodynamic reconstructions.

Platinum-group minerals (PGM) nuggets from alluvial-eluvial placer deposits in the concentrically zoned mafic-ultramafic Uktus complex (Central Urals, Russia)

We report the results of a detailed mineralogical investigation of platinum-group minerals (PGM) and copper-gold nuggets from the Uktus Ural-Alaskan type complex in the Central Urals (Russia). The studied nuggets were sampled in alluvial-eluvial deposits from three small valleys, with temporary water flows, cutting across the Uktus massif. The volume of the washed samples varies from 0.03 to 0.08 m 3 and a few tens of PGM nuggets, ranging in size from about 100 μm to about 2 mm, were collected. According to their chemical composition, the most abundant PGM are native Ir-Os and alloys in the Pt-Fe-Cu-Ni system. The following less abundant PGM were also recognised: sulfarsenides of the irarsite-hollingworthite-platariste series, sulfides such as laurite, cuproiridsite, kashinite and the sulfantimonide tolovkite. One alloy corresponding to the formula Cu 3 Au 2 was found, and proved to be Cu-rich tetraauricupride. The nuggets of Uktus have, in some cases, a polygonal shape. However, most of them have an irregular morphology and are characterised by a porous rim and zoning. The investigated nuggets occur as single-phase crystals or as polyphase grains, composed of different PGM. One nugget displays a very complex texture, being composed of a Pt-Fe alloy associated with osmium and Cu-rich tetraauricupride. These minerals are in contact with quartz that contains minute inclusions of hollingworthite and platarsite. The mineralogical similarity with the PGM inclusions in the Uktus chromitites indicates these rocks as a possible source for the PGM nuggets. The presence of faceted morphology in some nuggets suggests that they were mechanically liberated and transported for a relatively short distance from their lode deposits. The nuggets characterized by a rounded shape and occurring in association with quartz and Cu-rich tetraauricupride have probably been reworked in the placer environment. Therefore, in the Uktus placers deposits, two types of PGM nuggets can coexist: (i) primary with a magmatic origin, i.e ., only mechanically liberated from their source rock, and (ii) secondary, i.e ., reworked and grown in the placers. The mineralogical assemblage of the Uktus PGM nuggets, the fact that the Uktus PGM placers have never been mined and the recent exponential increase in demand for noble metals make the placer deposits associated with the Uktus complex potentially important for the economic recovery of these rare metals, at least on a small scale.

Electron Microprobe and Raman Spectroscopy Investigation of an Oxygen-Bearing Pt-Fe-Pd-Ni-Cu Compound from Nurali Chromitite (Southern Urals, Russia).

One grain, about 100×80 μm in size, occurring in chromitite associated with the layered sequence of the Nurali mafic-ultramafic complex (Southern Urals, Russia) was investigated by electron-microprobe analyses and Raman spectroscopy. The grain is characterized by a spotty, rugged appearance and chemical zoning from which two compositions were calculated: (Pt(0.35)Pd(0.26)Fe(0.22)Cu(0.01)Ni(0.05))(0.98)O(1.02) and (Fe(0.90)Pt(0.58)Ni(0.28)Pd(0.13)Cu(0.08)Rh(0.01))(1.98)O(1.02). In the lack of X-ray data, Raman spectroscopy established the presence of a diffuse 500-700 band and a sharp peak at 657 cm⁻¹ of relative wavenumber that strongly resemble the Raman spectra of synthetic PtO and PdO (palladinite). It is concluded that the Nurali grain probably represents a platinum group element (PGE) oxide, and does not consist of a mixture of PGE alloys with Fe oxide or hydroxide as reported for other natural PGE-O compounds.

The upper geochronological limit for dunites of the East Khabarny gabbro-ultramafic complex (South Urals) by the data of U-Pb age of zircons extracted from vein hornblendites

This paper discusses the results of U-Pb isotopic dating of zircons from hornblendites that cut dunites of the East Khabarny dunite-clinopyroxenite-websterite-gabbro complex in the South Urals. This complex is included into the structure of the large Khabarny mafic-ultramafic allochthon, and the set of rocks and their petrogeochemical peculiarities allow this complex to be compared with the Ural-Alaskan-type complexes. Zircons from hornblendites have been dated on a SHRIMP-II ion microprobe and by the classical thermo-ionization mass spectrometric method by five fractions. The obtained results are comparable between each other within the analysis deviation interval and show that hornblendites were formed in the Early Devonian (408–402 Ma). This age is the upper geochronological limit in estimation of the time when dunites of the East Khabarny complex were formed. The synchronity of intrusive hornblendite manifestation in most of the Urals in the Early Devonian is discussed. The probable Early Devonian stage, in addition to the found Vendian and Ordovician-Silurian ones when dunite-clinopyroxenite-gabbro complexes formed in the Urals, is suggested.

Uranium-Bearing Srilankite from High-Pressure Garnetites of the Southern Urals: First Data

This work presents the results of studying srilankite, a rare zirconium titanate (ZrTi2O6), associated with ilmenite, rutile, zircon, uraninite, and other minerals discovered in high-pressure garnetites of the lherzolite Mindyak massif (Southern Urals). Srilankite occurs as inclusions in ilmenite and rutile of up to several tens of microns in size. It was established for the first time that srilankite contains a significant UO2 admixture (up to 20%). The negative correlation between Zr and U is evidence of isomorphism in the srilankite–brannerite system. The association of srilankite with high-Zr rutile indicates that formation of these minerals occurred at T > 850°С.

Ankaramite: a new type of high-magnesium and high-calcium primitive melt in the Magnitogorsk Island-arc zone (Southern Urals)

This work describes the geological position, mineral and chemical composition of high-Mg effusive ankaramites occurring as dykes and lava flows. They were found in the mélange zone of the western margin of the Magnitogorsk island arc zone in the Southern Urals. Data on the liquidus association of phenocrysts and on the composition of the matrix of effusives are given. According to the data obtained, the conclusion was drawn that the ankaramites studied can be attributed to the primary island arc melts, which were not subject to essential differentiation. This type of effusives has not been distinguished previously among island arc volcanogenic formations of the Urals. It is shown that ankaramites can be considered to be primary melts parental for dunite–clinopyroxenites–gabbro complexes of Ural–Alaskan type. The occurrence of ankaramites in the Paleozoic island arc formations of the Urals indicates the wehrlite composition of the mantle as the reason for the extremely wide development of wehrlites and clinopyroxenites in different mafic–ultramafic complexes of the Urals.