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.

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.