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 differentiated 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 differ 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 (ε Nd = −3.22 ± 0.13), and the latter, 2433±28 Ma (ε 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.
New data on the composition, assemblages, and formation conditions of platinum-group minerals (PGM) identified in platinum-group element (PGE) occurrences of the Monchetundra intrusion (2495 ± 13 to 2435 ± 11 Ma) are described. This intrusion is a part of the Paleoproterozoic pluton of the Monche-ChunaVolch'i and Losevy tundras located in the Pechenga-Imandra-Varzuga Rift System. The rhythmically layered host rocks comprise multiple megarhythms juxtaposed to mylonite zones and magmatic breccia and injected by younger intrusive rocks in the process of intense and long magmatic and fluid activity in the Monchetundra Fault Zone. The primary PGM and later assemblages that formed as a result of replacement of the former have been identified in low-sulfide PGE occurrences. More than 50 minerals and unnamed PGE phases including alloys, Pt and Pd sulfides and bismuthotellurides, PGE sulfarsenides, and minerals of the Pd-As-Sb, Pd-Ni-As, and PdAg-Te systems have been established. The unnamed PGE phases-Ni 6 Pd 2 As 3 , Pd 6 AgTe 4 , Cu 3 Pt , Pd 2 NiTe 2 , and ( Pd , Cu ) 9 Pb ( Te , S ) 4 -are described. The primary PGM were altered due to the effect of several mineral-forming processes that resulted in the formation of micro-and nanograins of Pt and Pd alloys, sulfides, and oxides, as well as in the complex distribution of PGE, Au, and Ag mineral assemblages. New types of complex Pt and Pd oxides with variable Cu and Fe contents were identified in the altered ores. Pt and Pd oxides as products of replacement of secondary Pt-Pd-Cu-Fe alloys occur as zonal and fibrous nanoscale Pt -Pd -Cu -Fe -( ± S )-O aggregates.
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