The paper presents geochronological data and results of petrological modeling of the Monchegorsk and Monchetundra layered massifs. The U-Pb ID-TIMS zircon dating of norite from the marginal zone of Mt. Nittis yielded 2506 ± 7 Ma, which coincides with previous data on similar rocks from other areas. Based on the comparison of sections of the Monchegorsk and Kivakka massifs, the pre-erosion thickness of the Monchegorsk massif is estimated as 3700 m. Calculations using rocks of the marginal zone of the Monchegorsk intrusion at a pressure of 6.5–5.5 kbar showed that the melt in equilibrium with Ol contained SiO 2 ≤ 55 wt %, TiO 2 ≤ 0.50 wt %, and MgO ≤14 wt %. The temperature of the primary magma could be equal to 1390°C. At a lower pressure (below 6.5 kbar), the obtained melt becomes more siliceous. In the Monchegorsk intrusion, the composition of intratelluric olivine according to this model could vary within 88–92 mol % Fo . The content of intratelluric olivine in the melt, depending on pressure, could vary from 11 to 24 vol %. Magmas that formed the Monchegorsk intrusion and the layered series of the Monchetundra intrusion (hole 742) were derived from different sources.

Made an attempt of reconstruction structure of the Monchegorsk layered intrusion, assessing the composition of the original magma and the conditions of its kristallization. The problem was solved by simply comparing the structure of the Monchegorsk Intrusive with a similar analog (for this purpose, the Kivakka layered Intrusive was chosen), as well as using model constructions using the COMAGMAT software package.

We have made an attempt to reconstruct a pre-erosion structure of the Monchegorsk layered intrusion and to assess the parent magma (melt) composition and crystallization conditions. The problem has been solved by simple comparison of the Monchegorsk intrusion structure with a similar analog, that is, the Kivakka layered intrusion, and by modeling using the COMAGMAT software package.

Предложен новый метод изотопной геохронологии для датирования самородных минералов платины, основанный на -радиоактивности природного изотопа платины 190Pt. Рассмотрение механизма термодесорбции гелия в кристаллической решетке самородных металлов, в том числе и платины, позволяет прогнозировать очень высокую термическую устойчивость (сохранность) радиогенного 4 в самородных минералах платины, вплоть до температуры их плавления. Для подтверждения эффективности предложенного 190Pt-4He метода проведено прямое изотопное датирование изоферроплатины Гальмоэнанского дунит-клинопироксенитого и Кондерского щелочно-ультраосновного массивов. Результаты определения возраста для коренной рудной платины Гальмоэнанского массива (70 ± 5 млн. лет), полученные данным методом, сопоставимы с геологическими наблюдениями и средними значениями Sm-Nd и Rb-Sr изотопных датировок. Установленная для массива Кондер 190Pt-4He датировка (112 ± 7 млн. лет) россыпной изоферроплатины также хорошо согласуется с геологическими данными и близка к значениям возраста косвитов (флогопит-магнетитовым пироксенитам, габбро, нефелиновых сиенитов и метасоматитов по дунитам), оцененным K-Ar и Rb-Sr методами. Экспериментальные данные подтвердили, что 190Pt-4He метод изотопной геохронологии может быть успешно применен для датирования самородных минералов платины.

A new method of isotope geochronology was proposed for dating native platinum minerals on the basis of the α-decay of the natural isotope 190 Pt. The analysis of the thermal desorption of helium in the crystal lattice of native metals, including platinum, allowed us to predict a very high thermal stability (retentivity) of radiogenic 4 He in native platinum minerals up to their melting temperatures. In order to validate the proposed 190 Pt- 4 He method, direct isotopic dating was performed for isoferroplatinum from the Galmoenan dunite-clinopyroxenite and Kondyor alkaline ultramafic massifs. The results of dating obtained by this method for primary ore platinum from the Galmoenan Massif (70 ± 5 Ma) are consistent with geological observations and mean Sm-Nd and Rb-Sr isotopic age estimates. The 190 Pt- 4 He age obtained for placer isoferroplatinum from the Kondyor Massif (112 ± 7 Ma) also agrees with geological observations and is close to the K-Ar and Rb-Sr ages of koswites (phlogopite-magnetite pyroxenites, gabbros, nepheline syenites, and metasomatic rocks after dunites). Our experimental data demonstrated that the 190 Pt- 4 He method is a promising tool for dating native platinum minerals.

A new method of determining the age of native platinum based on alpha-radioactivity of one of its natural isotopes 190Pt is proposed. Due to a special form of occurrence of radiogenic helium in the crystal lattice of native metals as a helium cluster-bubbles, the stability of 190Pt-4He of the isotope system is extremely high in the natural environment. To check the efficiency of the proposed 190Pt-4He method of isotope geochronology, six independent mineral aggregates of native platinum from chromite-bearing dunites of the southern part of platinum-bearing zonal Galmoenan dunite-cllinopyroxenite-gabbro plutonic complex (Koryak-Kamchatka belt, Russia) were analyzed. The age calculated by the tangent of the 190Pt-4He isochrone slope angle equals 69.5 ± 4.9 mln years. The obtained age value coincides with the results of isotope datings, which were made previously by different methods of isotope geochronology.

The Nadezhda ore-controlling structure and related rocks of the Lukkulaisvaara layered intrusion from the Oulanka plutonic group have been studied. Tensile deformation arising at the cooling stage at interfaces of rocks dissimilar in composition with different compressibility coefficients controlled the intensity of secondary processes and the high concentration of sulfide and platinum group minerals (PGM) along the perimeter of the fine-grained gabbronorite body in contact zones with country rocks. These zones were specific geochemical barriers. Magmas of supplementary injections could have been one of the sources of fluid affecting the rocks. Fe-Ni-Cu sulfides and PGM crystallized within the temperature range T = 800-350°C. Regional metamorphism did not exceed conditions of greenschist facies. Results of modeling of metasomatic processes indicate that decompression played the leading role in formation of the major secondary mineral assemblages: Czo-An 10 -Chl-Tr(Act) and Qtz-An 70 -Amph-Bt. The decreasing pressure under isothermal conditions resulted in a rise in solubility of metals in chloride solution and their transport into dilatable zones. The cooling model leading to the enrichment of rocks in potassium and then to its leaching is realized more seldom, resulting in a Qtz-Ms-Chl-Act-Czo mineral assemblage and eventually in Qtz-Chl veins. The change of P and T during pluton cooling resulted in changing of the direction of metasomatic processes and telescoping of different superimposed mineral assemblages: Amph(±Act)-Bt ± Chl 1 ± Qtz as an early one and Qtz-Czo-Chl 2 -Ab-Ms as a late one. The model mineral assemblages that match natural counterparts are as follows: (1) Qtz-Pl-Amph-Bt and Qtz-Pl-Chl-Bt(Ms) and (2) Tr(Act)-Chl-An 10 -Czo, Qtz-An 10 -Chl-Ms(Bt), and Qtz-Czo-Chl-An 10 -Ms. At the same time, the An 40–50 -Amph-Bt, Qtz-An 40–50 -Amph-Bt, Qtz-An 10–20 -Chl-Bt(Ms), An 30–40 -Chl-Tr(Act)-Bt, An 30 -Chl-Tr(Act), and An 30 -Tr (Act)-Bt assemblages are suggested to be intermediate and reflect the direction of metasomatic processes.

This study addressed several issues related to the ore potential of the intrusion. Both accessory and low-sulfide (>1-2 vol %) mineralization was investigated. Our study focused on the distribution of sulfide minerals in the section of the layered complex; their characteristics (chemical composition, mineral assemblages, and structures); the petrography and geochemistry of the ore-bearing rocks; and the genesis of ore mineralization. The sulfides were grouped into three paragenetic assemblages: (1) Py-Pn-Ccp, (2) Pn-Ccp-Po, and (3) Bn-Mil-Pn-Ccp.(1) The appearance of Po, Mil, and Bn in the mineral assemblages was observed at contacts between zones or in the areas of interlayering of rocks of contrasting compositions. The same zones host the highest sulfide concentrations. Low-sulfur assemblage (2) forms the highest grade mineralization. The quantitative relationships of sulfides in the assemblage are highly variable. This assemblage usually shows sideronitic or micro-schlieren (ovoid) and coronal textures. Cubanite, troilite, talnakhite, sphalerite, and magnetite were noted in this assemblage. The intercalation of low- and high-sulfur sulfide assemblages in the rocks of the Aganozerskii block coincides with petrographic rhythms. Such rhythmic patterns were not found in the Shalozerskii block, which is probably related to the paucity of currently available data. The formation of relatively high-grade (more than 5 vol %) sulfide mineralization was related to the processes of late magmatic redeposition of ore matter under conditions of extensional deformations generated at the boundaries between rocks of contrasting compositions. These deformations are controlled by the physical properties of rocks, in particular, thermal expansion and compressibility during cooling and decompression, which are functions of rock composition. In our opinion, extensional deformations increase rock porosity and may lead to the formation of detachment fractures (contraction fractures), if the stress is sufficient to overcome tensile strength. Such zones of relative extensions pump residual melt or fluid. In the case considered, the latter may be either a solution or a sulfide liquid, which can migrate until complete crystallization. The schlieren textural type has a primary magmatic (liquid immiscibility) nature. The formation of sulfides in amygdaloidal segregations could be related to the postmagmatic interaction of inclusion material with the host mineral. This suggestion is in agreement with the localization of sulfides along the contacts of amygdules with their host minerals.

Исследование объединяет круг задач, связанных с проблемой рудоносности интрузива. Изучалась как акцессорная, так и малосульфидная (>1-2 об. %) минерализация. Главным предметом исследования было распределение в разрезе расслоенного комплекса сульфидных минералов, их характеристика (химический состав, минеральные парагенезисы, текстуры), петрографические и геохимические особенности рудосодержащих пород, происхождение рудной минерализации. Сульфиды объединяются в три парагенетические ассоциации: (1) Py-Pn-Ccp 1, (2) Pn-Ccp-Po, (3) Bn-Mil-Pn-Ccp. Появление в ассоциациях Po, Mil, Bn отмечается на контактах зон или в участках контрастного переслаивания пород, там же фиксируются наиболее высокие концентрации сульфидов. Низкосернистая минеральная ассоциация (2) образует наиболее значимые концентрации. Количественные соотношения сульфидов в парагенезисе широко варьируют. Для него типичны сидеронитовая или микрошлировая-овоидная и коронарные типы структур. В этой ассоциации встречаются кубанит, троилит, талнахит, сфалерит и магнетит. Отмеченное чередование низко- и высокосернистых ассоциаций сульфидов в породах Аганозерского блока совпадает с петрографической ритмичностью. В пределах Шалозерского блока подобная ритмичность не выявлена, что, скорее всего, связано с недостаточной детальностью исследований. Формирование относительно богатой (больше 5 об. %) сульфидной минерализации связывается с процессами позднемагматического переотложения рудного вещества на фоне деформаций растяжения, возникающих на границах неоднородных по составу пород. Эти деформации определяются физическими свойствами пород, и в частности коэффициентами теплового расширения и сжимаемости при остывании и декомпрессии, которые являются функцией состава пород. По мнению авторов, деформации растяжения ведут к увеличению их пористости, а в случае превышения предела прочности на растяжение могут привести к образованию трещин отрыва (трещины контракции). Такие зоны относительного растяжения являются своеобразным “насосом” для остаточного расплава или флюида, который в данном случае может быть как раствором, так и сульфидной жидкостью, способной мигрировать вплоть до полной кристаллизации. Шлировый структурный тип имеет первично-магматическую (ликвационную) природу. Формирование сульфидов в миндалевидных обособлениях может быть связано с постмагматическим взаимодействием вещества включения с минералом-хозяином. Это предположение согласуется с локализацией сульфидов по контактам обособлений с вмещающими минералами.

The paper considers the structure of the Burakovskii-Aganozero layered complex, which was formed at 2449 Ma. Petrographic and geochemical characteristics of rocks making up the layered series of the Aganozero and Shalozero blocks were studied in detail. New isotopic and geochemical data are presented and previous results on isotopic characteristics are generalized. It was concluded that the Burakovskii-Aganozero layered intrusion was a polyphase complex. The main phases correspond to the emplacement of "Aganozero" (epsilon (Nd) = -0.4 to +0.9 and Sr-87/Sr-86(T) = 0.7021-0.7035) and "Shalozero" (epsilon (Nd) = -3.31 and Sr-87/Sr-86(T) = 0.7020) magmas, which formed the intrusion. There is evidence for the presence of other small-volume intrusion phases. The Aganozero and Burakovskii-Shalozero blocks are two subchambers. According to geophysical data, each of them has its magma conduit. The Aganozero portion of the layered complex was formed earlier as the first intrusion phase. The Aganozero magma of this phase differed from the magma of the second intrusion phase, which produced the Burakovskii-Shalozero portion of the layered complex, both in isotopic characteristics and chemical composition (lower silica and iron contents). This is suggested by the petrography of the layered series rocks, compositions of rock-forming minerals, and geochemical characteristics of rocks composing blocks of the layered complex. This allows us to suppose that the initial magmas of these blocks were genetically different and represented successive intrusions of evolving magmas from a single magma chamber or from two independent chambers. Our data provide a new insight into the problem of ore potential of the layered complex, in particular, its platinum-bearing capacity (Korneev et at, 1999a, 1999b).