The paper presents results of mineralogical and geochemical studies of PGE mineralization in the Paleoproterozoic (2449 +/- 1.1 Ma) Burakovsk layered complex (BLC). Several levels of PGE accumulation are distinguished in the BLC. They are confined to the rocks of pyroxenite and gabbronorite zones of a layered series and the rocks of the border series (up to 1-2 ppm Sigma PGE). The highest PGE concentrations were determined in sulfide-rich (1-5 vol %) pyroxene orthocumulates of pyroxenite and gabbronorite zones in the southeastern part of the Shalozersk block (up to 6.5 ppm Sigma PGE). Elevated PGE contents were also found in chromitites and overlying pyroxenites in the Shalozersk block (up to 3 ppm Sigma PGE). The PGE mineralization is generally composed of moncheite, kotulskite, merenskyite, sperrylite, and cooperite. Sobolevskite, froodite, sopcheite, minerals of the atokite-rustenburgite series, paolovite, tulameenite, zvyagintsevite, laurite, irarsite, Cu3Pt, Cu-Au-Pd, Pd-Pb-(S,Se), and Pd2Ge phases are rare. The PGE minerals are accompanied by native gold and silver, electrum, hessite, and minerals of the galenite-clausthalite series. Palladium also occurs as an admixture in cobaltite, pentlandite, and heazlewoodite. The low-sulfide PGE mineralization is distinctly controlled by rock stratigraphy and is confined to horizons of pyroxene and plagioclase-pyroxene orthocumulates. Fe-Ni-Cu sulfides occurs in interstices among plagioclase and orthopyroxene or form intergrowths with late- and postmagmatic minerals. PGE minerals are present in association with magmatic sulfides, in rock-forming minerals, and in intergrowths with hydroxyl-bearing silicates. Orthocumulates of ore zones are rich in biotite, phlogopite, amphiboles, calcite, and chlorite; plagioclase is replaced by albite and K-Na feldspar. Biotite is normally accompanied by apatite. Compositions of biotite, phlogopite, and apatite associating with PGE minerals in ore horizons indicate that PGE-Cu-Ni mineralization originated by crystallization from residual magma and coexisting high-temperature magmatic fluid. The replacement of primary PGE minerals and the formation of complex micrometer- and nanometer-sized inhomogeneous intergrowths of PGE minerals with silver and gold were related to fluid-hydrothermal processes and PGE redistribution from earlier to later assemblages.

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 Burakovka layered pluton of basic and mafic rocks is the largest intrusive massif in the Baltic Province composed of Si- and Mg-rich boninite-like rocks. The pluton consists of two individual bodies, each having its own internal structure, and contacting each other in their apical parts, known as the Aganozero and Shalozero-Burakovka bodies. Both bodies have a similar rock sequence including five dierentiated zones (upward): mafic rocks, pyroxenite, gabbro norite, pigeonite gabbro norite, and magnetite gabbro diorite (the latter found only in the Shalozero-Burakovka Body). Being generally similar to each other, these bodies dier notably in the styles of their cumulate stratigraphy and, to a lesser extent, in composition. The pluton is distinguished by the presence of markers - singular interlayers of high-temperature mafic cumulates emplaced in the sequence of lower-T formations. Their origin is believed to have been associated with the intrusion of fresh magma portions into the crystallizing magma chambers. The same mechanism is believed to have been responsible for a macrorhythmic pattern found in the southeastern portion of the Shalozero-Burakovka intrusive body. Using chemical and mineralogical data, it is shown that the bodies discussed were derived from similar high-Si and high-Mg magmas, except that the Aganozero Body was emplaced 50 million years later than the Shalozero-Burakovka intrusion: the former was dated (Sm-Nd isochron) 2372±22 Ma (Nd = 3.22 ± 0.13), and the latter, 2433±28 Ma (Nd = 3.14 ± 0.14). It is concluded that the Burakovka Pluton was a long-lived magma center which developed above a local mantle plume, the origin of which had been associated with the activity of a megaplume which had been responsible for the existence of the Baltic province throughout a period of 200 million years.

First data are presented on vein granophyric granites from Europe's largest Early Paleoproterozoic Burakovskii layered mafic-ultramafic pluton, Southern Karelia. The pluton consists of two individual bodies: Aganozero and Shalozero-Burakovskii. Granites were found only in the latter They are confined mainly to the Pigeonite Gabbronorite Zone and practically do not occur in other zones. The veins show abrupt contacts without chill zones. The granites range from leucogranodiorite to potassium leucogranite. Their texture is strongly variable from hypidiomorphic to granophyric, In terms of geologic and compositional features, these rocks are classified with transgressive granites of layered intrusions. The vein granites of the Burakovskii Massif belong to the calc-alkaline series and continue the trend of mafic rocks from the intrusion with a gap at SiO2 = 59-65 wt %. The epsilon(Nd) values of the granites vary slightly, from -2.4 to -3.7, and are very close to those of the pyroxenites and pigeonite gabbronorites of the Shalozero-Burakovskii intrusion (from -1.4 to -3.0), but differ sharply from those of the enclosing Archean rocks (from -5.7 to -10.7). These facts are inconsistent with granite generation through basement melting and suggest a single source for the granites and mafic rocks. Based on the negative epsilon(Nd) values, this source was contaminated by crustal material. The geologic, petrological, and isotope geochemical data indicate that the vein granites were coeval to grabbroids and formed in situ within the Pigeonite Gabbronorite Zone enriched in quartz-feldspar intercumulus material. It is suggested that the granites were formed from residual intergranular acid melt, which preserved in almost completely solidified cumulates of this zone. The contraction led to vacuum extraction of the melt into shrinkage fractures. In the largest fractures, the separated granite melt experienced subsequent differentiation up to hyperacid varieties. The independence of contraction phenomena and melt evolution in the intergranular space could be responsible for the compositional gap between mafic and acid rocks.

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).

The Early Proterozoic (2449 ± 1.1 Ma) Burakovsky layered intrusion of southern Karelia, Russia, is the largest (700 km2) mafic pluton in the Fennoscandian Shield. The layered series contains two main parts: (i) an ultramafic series (85% dunite), and (ii) a mafic series (mostly gabbros). Mineral and whole-rock chemical variations in the ultramafic zone indicate little variation in chemistry over the entire sequence (e.g., in olivine, variations are from Fo89 to Fo87 over the upper 1600 m). In contrast, the Mg# in pyroxenes from the gabbronorite zone varies from 85 to 58 over an interval of 500 m. On the basis of the current data, there are no apparent reversals in chemistry in the ultramafic zone, whereas there is evidence for several in the gabbronorite zone. Furthermore, there is a distinctive reversal to more primitive compositions in mineral and whole-rock chemistry at the top of the ultramafic zone. These differences indicate that either (i) the gabbronorite zone crystallized from the residue left from formation of the ultramafic zone and the subsequent "flushing" of the more evolved portions to form the Early Proterozoic Karelian volcanic fields, or (ii) there were two separate magmas. Plagioclase crystals from the gabbronorite zone are chemically zoned, providing evidence of the local processes affecting crystallization of this sequence. Plagioclase crystals from near the base commonly display relatively Na-rich cores that are believed to result from either supersaturation at the onset of nucleation or the prior crystallization of some other, Ca-rich phase (e.g., clinopyroxene). Many plagioclase grains also contain reversely zoned rims, indicative of either (i) exchange between trapped liquid and the main magma during times of replenishment; or (ii) an increase in the CIPW-normative plagioclase–liquid partition coefficient for Ca, due to an increase in the augite component of the melt. The lack of evidence for abundant magma influxes in the ultramafic zone contrasts with the chemical reversals observed in the gabbronorite zone, suggesting that replenishment was a prevailing process only in the upper half of the intrusion.