Clinopyroxenite Dunite Massifs Research Articles

Sources of Ore Material in the Platinum-Group Element Deposits of Polar Siberia and the Middle Urals Based on the Data from Radiogenic (Re–Os, Pt–Os) and Stable (Cu, S) Isotopes

Abstract —Understanding the main events of platinum-group element (PGE) ore formation is impossible without analysis of the sources and behavior of major ore-forming components, namely, platinum, osmium, sulfur, and copper, which are important indicators of magmatic and hydrothermal processes. In contrast to the Re–Os isotope system, the radiogenic Pt–Os isotope system, as well as stable isotopes of Cu and S in PGE deposits, are still relatively understudied. Our comprehensive research is aimed at filling this gap. The paper presents data for the Guli massif of ultramafic and alkaline rocks and carbonatites in Polar Siberia and on the zonal Nizhny Tagil and Svetly Bor clinopyroxenite–dunite massifs in the Middle Urals, which include: (1) the contents of the highly siderophile elements (HSE) in whole rocks and platinum-group minerals (PGM), (2) the Re–Os and Pt–Os isotope systematics of chromitite, Os–Ir alloys, and Ru–Os sulfides, (3) the sulfur isotope composition in Ru–Os and Ir–Rh sulfides in primary and secondary PGM assemblages, and (4) the copper isotope composition in Pt–Fe minerals from chromitites and placers. The research was performed using scanning electron microscopy, electron probe microanalysis, and high-precision isotope-geochemical analysis. The high-precision Re–Os and Pt–Os isotope data show that the HSE contents in chromitites and PGM of the Guli massif were controlled by the composition of the mantle source that evolved with near-chondritic time-integrated Re/Os and Pt/Os ratios, which are also typical of the sources of most komatiites and abyssal peridotites. The δ65Cu values of the studied samples of ferroan platinum and isoferroplatinum are identical within the analytical uncertainty and are close to 0‰, which is typical of high-temperature Cu-containing minerals. The sulfur isotope compositions of the Ir–Rh sulfides of the kashinite–bowieite series and of the Ru–Os sulfides of the laurite–erlichmanite series in the primary PGM assemblages indicate that the source of sulfur has a chondritic isotope composition, which is in agreement with the osmium isotope composition of the Ru–Os sulfides and Os–Ir alloys. The heavy sulfur isotope composition (δ34S = 5.6 ± 1.5‰) of As-containing erlichmanite is consistent with its secondary origin. The new data on the isotope compositions of osmium, copper, and sulfur can be used as new important parameters that characterize the sources of PGE mineralization.

First Finds of Native Iridium in Chromite–Platinum Ledge Ore and Placers of Zonal Clinopyroxenite–Dunite Massifs of the Urals

First Finds of Native Iridium in Chromite–Platinum Ledge Ore and Placers of Zonal Clinopyroxenite–Dunite Massifs of the Urals

Morphology, Composition, and Ontogenesis of Platinum-Group Minerals in Chromitites of Zoned Clinopyroxenite–Dunite Massifs of the Middle Urals

Abstract —We present results of analysis of the morphology and chemical composition of platinum-group minerals from chromite– platinum orebodies of zoned clinopyroxenite–dunite massifs in the Middle Urals (Nizhnii Tagil, Svetlyi Bor, Veresovyi Bor, and Kamenushenskii). Study of more than 500 grains has given an insight into the sequence of formation of platinum-group minerals in chromitites of the studied massifs. Three assemblages of platinum-group minerals have been revealed: magmatic (Os–Ir–Ru intermetallic compounds, isoferroplatinum, ferroplatinum, sulfides of the isomorphous series erlichmanite–laurite and kashinite–bowieite, and thiospinels of the series cuproiridsite–cuprorhodsite–malanite); postmagmatic (with a predominance of tulameenite, tetraferroplatinum, and ferronickelplatinum, resulted from serpentinization of dunites); and latest secondary (minerals with a predominance of PGE sulfides, arsenides, sulfoantimonides, sulfoarsenides, plumbides, and amalgams).

Platinum Group Minerals from Veresovka River Deluvial Placer, Veresovoborsky Dunite–Clinopyroxenite Massif (Middle Urals)

For the first time data on platinum group minerals (PGM) from the deluvial placer of the Veresovka River are given; their root source are dunites of the northern part of the Veresovoborsky massif. Among minerals typical of this type of placers, Pt–Fe minerals, Os–Ir–(Ru) intermetallic compounds, kashinite, bowieite, laurite, erlichmanite, and Ir–Rh thiospinels have been identified. Irarsite, hollingworthite, zvyagintsevite, potarite, cooperite, ferhodsite, and some unnamed Pb–Те minerals were also revealed, which rare for platinum placers associated with zonal clinopyroxenite–dunite massifs, but are widely distributed in the studied placer. Xingzhongite was detected there for the first time in placers of the Ural Platinum Belt. The morphological features of all these minerals are characterized in detail, together with their chemical composition, determined by electron microprobe analysis. Their genetic relationships are characterized taking into account the aggregate data obtained. This comprehensive study of PGM individuals and aggregates is used as a basis to distinguish the parageneses in their association.

Онтогенические ориентиры для выбора модели формирования платинового оруденения в зональных клинопироксенит-дунитовых массивах Урала

В статье обсуждаются результаты онтогенических исследований раннего парагенезиса рудных минералов в зональных клинопироксенит-дунитовых массивах Среднего Урала. Показано, что образование этих минералов происходило в следующей последовательности: иридистый осмий → хромшпинель → лаурит-эрликманит → осмистый иридий → изоферроплатина. Выявленные онтогенические признаки рудных фаз хорошо согласуются с моделями, предполагающими концентрирование металлов платиновой группы на магматическом этапе эволюции клинопироксенит-дунитовых массивов из расплава, до проявления наложенных процессов высокотемпературной пластической деформации.

The Distribution and Speciation of PGEs in Chromitite from the Svetloborsky, Veresovoborsky, and Kamenushensky Clinopyroxenite–Dunite Massifs (Middle Urals)

The mineralogical and geochemical patterns of platinum-bearing vein–impregnated and massive chromitite from dunite of the Svetloborsky, Veresovoborsky, and Kamenushensky clinopyroxenite–dunite massifs (Middle Urals) are reported. Platinum is characterized by an extremely uneven distribution in chromitite and is concentrated in the form of Pt–Fe intermetallic compounds. The structural and textural patterns of chromitite from these massifs, the PGE distribution in it, and the composition of chrome spinels are similar to those in well-studied chromitite from the zoned clinopyroxenite–dunite massifs of the Urals (Nizhny Tagil) and Koryakia (Gal’moenansky).

PGE Mineralization of Massive Chromitites of the Iov Dunite Body (Northern Urals)

This work presents the results of the first comprehensive study of PGE mineralization from massive chromitites of the Iov dunite body (Northern Urals). The chromitites are composed of chromespinelides with a higher content of Cr2O3 with respect to those from other zonal clinopyroxenite–dunite massifs of the Urals. However, the composition of chromespinelides fits the trend that is characteristic of the dunite–clinopyroxenite–gabbro formation. PGE minerals, in particular Pt–Fe solid solutions, were identified in chromitites and in chromespinelides in the form of crystals and aggregates of a complex non-crystallographic habit and less often of an idiomorphic cubic habit. In terms of stoichiometry, Pt–Fe minerals correspond to ferroplatinum (Pt2Fe) and isoferroplatinum (Pt3Fe). The minerals of the isomorphic tetraferroplatinum–tulameenite–nickelferroplatinum series are widely distributed. Thus, the PGE mineralization of the Iov dunite body has features that are characteristic of clinopyroxenite–dunite massifs of the Urals.

PGE mineralization of massif chromitites of Iov dunite massif (Northern Urals)

In the current work, the platinum group minerals of massif chromitites of Iovsky dunite massif are firstly fully characterized. Chromitites are composed of chromoshpinelide with increased in comparison with chromoshpinelides from other zoned clinopyroxenite-dunite massifs contents of Cr, but their composition keeps within trend of dunite-clinopyroxenite-gabbro magmatic formation. PGE minerals, and in particular Pt-Fe solid solutions, are found in chromitites or directly in chromoshpinelide as distinct crystals or aggregates of irregular non- crystallographic habit and rarely with euhedral cubic habit. Composition of Pt-Fe minerals corresponds to stoichiometry of ferroplatinum (Pt 2 Fe) and isoferroplatinum Pt 3 Fe. Minerals of isomorphous system tetraferroplatinum-tulameenite-nickelferroplatinum are widely developed. Thus, platinum mineralization of Iovsky dunite massid has typical features of clinopyroxenitedunite complexes of Urals.

Bedrock platinum-group element mineralization of zonal clinopyroxenite–dunite massifs of the Middle Urals

Features of the chemical composition of platinum-group mineral assemblages from chromitites of the zonal Svetly Bor, Veresovy Bor, and Nizhny Tagil clinopyroxenite–dunite massifs of the Middle Urals are compared for the first time.

Results of U–Pb dating of zircons from wehrlite of the platinum-bearing Feklistov massif (Shantar Archipelago, Russia)

The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.

Platinum group element mineralization of the Svetly Bor and Veresovy Bor clinopyroxenite–dunite massifs, Middle Urals, Russia

The new data for the geology and mineralogy of the platinum group element (PGE) mineralization related to the chromite–platinum ore zones within the dunite of the Svetly Bor and Veresovy Bor massifs in the Middle Urals are discussed. The geological setting of the chromite–platinum ore zones, their platinum content, compositional and morphological features of the platinum group minerals (PGM) are compared to those within the Nizhny Tagil massif, the world standard of the zonal complexes in the Platinum Ural belt. The chromite–platinum orebodies are spatially related to the contacts between differently granular dunites. Majority of PGM are formed by Pt–Fe alloys that are close in terms of stoichiometry to isoferroplatinum (Pt3Fe), and associated with Os–Ir alloys, Ru–Os and Ir–Rh sulfides, and Ir–Rh thiospinels of the cuproiridsite–cuprorhodsite–ferrorhodsite solid solution. The tetraferroplatinum (PtFe)–tulameenite (PtFe0.5Cu0.5) solid solution and Pt–Cu alloys belong to the later PGM assemblage. The established features of the chromite–platinum ore zones testify to the highly probable identification of the PGE mineralization within the dunite of the Svetly Bor and Vesesovy Bor massifs and could be used in prospecting and exploration for platinum.

Os-rich nuggets from Au-PGE placers of the Maimecha-Kotui Province, Russia: a multi-disciplinary study

¶Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first time. The detrital platinum-group minerals (PGM) are dominated by Os–Ir–(Ru) species, which is typical for ophiolites or Alpine-type complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite–dunite massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are (1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly osmium. Dominant chromite, olivine and clinopyroxene inclusions recorded in Os–Ir–(Ru) alloys imply that they were derived from ultramafic sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation - multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (− 2.39 to − 2.07). These values are indicative of a common chondritic or subchondritic mantle source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from − 2.67 to − 1.30). The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing parent bedrock sources.

Pt Fe NUGGETS DERIVED FROM CLINOPYROXENITE DUNITE MASSIFS, RUSSIA: A STRUCTURAL, COMPOSITIONAL AND OSMIUM-ISOTOPE STUDY

Combined structural, compositional and osmium-isotope data on selected Pt–Fe nuggets from economically important placer deposits closely linked to clinopyroxenite–dunite massifs of the Siberian Platform (Kondyor, Inagli, Guli) and the Middle Urals (Nizhny Tagil), Russia, are presented for the first time. Pt–Fe alloys investigated are ferroan platinum (space group Fm 3 m ) with a composition close to Pt3Fe. This emphasizes the necessity of an X-ray study in identifying the particular Pt–Fe alloy species. Less common are compositions such as Pt2Fe and an intimate intergrowth of Pt3Fe2 and PtFe. Other platinum-group minerals (PGM) observed in ferroan platinum include a diversity of Os–Ir–Ru alloys, PGE sulfides [laurite, malanite, cuproiridsite, cooperite, and an unnamed base metal – (Ir,Pt) monosulfide], PGE sulfarsenides (hollingworthite, irarsite), Pt–Pd tellurides (moncheite, telluropalladinite) and stibiopalladinite. This suite of PGM is consistent with those from other zoned or Uralian–Alaskan-type massifs. However, unusually Ru-rich alloys included in ferroan platinum of Guli are characteristic of PGM derived from an ophiolite source and underline the transitional signature of the Guli massif between zoned- and ophiolite-type complexes. Pd-rich ferroan platinum nuggets indicate a derivation from clinopyroxenite source-rock, whereas Ir-rich Pt–Fe alloys suggest a chromitite source. The presence of numerous Os–Ir–Ru exsolution lamellae in ferroan platinum are indicative of a high-temperature origin of the PGM. The first Os-isotope data from Os-rich minerals from chromitites and placers closely associated to the Kondyor and Inagli massifs reveal low 187Os/188Os values with a very narrow range, indicative of a common mantle source of the PGE, implying that the PGM are of primary origin. Disintegration of parent ultramafic source-rocks and short-range mechanical transport of liberated PGM formed the placers. Os-isotope model ages in the range of 340 to 355 Ma constrain the formation age of the Kondyor and Inagli massifs of the Aldan Province at the southeastern part of the Siberian Craton, and closely match those from the Guli massif (370 Ma) of the Maimecha–Kotui Province at the northern part of the Siberian Craton.