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First discovers of Pleistocene authigenic carbonate crusts (ACC) at the Mendeleev Rise, Arctic Ocean

Pakhalko A., Krylov A., Mirolyubova E., Taldenkova E., Rekant P.

Geology, Geophysics and Environment

2016

Vol. 42, no. 1

109--110

The Mendeleev Rise (or Mendeleev Ridge) is a part of Central Arctic Uplifts domain that extends from the Eastern Siberian Shelfto the central areas of the ocean, where it is adjacent to the Alpha Ridge bisecting the Amerasian Basin. The crust of Mendeleev Rise belongs to the continental type (Poselov et al. 2007). New geological, geophysical and tectonic data were obtained within Alpha-Mendeleev Rise after few expeditions to Arctic Ocean in year 2000, 2005 and 2012. Thousands of rock fragments were dredged: 50–65% – carbonate rocks (mainly dolomite and limestone); 20–25% – sandstones, siltstones, mudstones; 5–20% – igneous rocks (mainly granites, gabbro-dolerite and few types of basalt); ~ 10% – metamorphic rocks (mainly greenschist facies). Light dolomites with little flora and fauna represent about two-thirds of the total amount of carbonate rocks, the rest are limestones, often containing well-preserved faunal remains. Paleontological study of limestones show abundance of D-P 1 fauna remnants that give us an evidence of structural connection of Mendeleev Rise and Wrangel Island (Morozov et al. 2013). Carbonate crusts were dredged from steep slopes (25–29°) with neotectonic faults on two sites at water depth more than 2 km during expedition “Arktika-2012”. Primary study showed that crusts compose of strong matrix with rounded and angular debris of local (edaphogenic) material: dolomites, dolerites, granites, metasomatic and terrigenous rocks of different size (0.5 mm to 5 cm) (Morozov et al. 2013, Kremenetskii et al. 2015). Three samples of matrix and two of soft clay-carbonate crust’s cover were selected for detailed analysis. Petrographic features were studied using optical microscope, microprobe and X-ray analysis. Chemical elements analysis was performed with XRF and ICP-MS. All analyses were carried out in A.P. Karpinsky Russian Geological Research Institute (VSEGEI) in Saint-Petersburg. AAC’s Matrix studied with microprobe in details and consists offine-grained calcite with angular quartz grains from 1 μm to 300 μm. X-ray analysis shows calcite predominance in the matrix (>70%), rest content is presented with clasts of quartz, illite and albite – about 10%, dolomite, chlorite, montmorillonite, chamosite – 1–2%. Clasts of local debris are presented by two association: 1) large, mainly angular clasts with size from 0.5 mm to 5 cm; 2) small, mainly rounded clasts with size from 200 μm to 0.5 mm. Surface of matrix and debris is covered with soft rose clay-carbonate mass. Mineral content of clay-carbonate mass is: quartz and illite – 25–30%, calcite and albite 15–20%, chlorite, orthoclase, halite, dolomite, montmorillonite – 1–5%. Chemical composition (in percents) of matrix is close to clay-carbonate terrigenous rocks: SiO 2 – 18, Al 2 O 3 – 3.62, TiO 2 – 0.2, Fe 2 O 3 t – 1.4, MnO – 0.05 MgO – 2.35, CaO – 39.2, Na 2 O – 0.18, K 2 O – 0.47, P 2 O 5 – 0.12, L.O.I. – 34.3. Rose mass differs from matrix with silica – 46, CaO – 19, higher alkalis (Na and K) – 1.15 and 1.3. Difference in content of silica (18% vs 46%), CaO (39.2% vs 19%) says that AAC matrix and rose mass have various sources. In contrast to Paleozoic remnants in carbonates, the AAC contain planktonic and benthic foraminifera of Pleistocene age. In Arctic seas, these species are distributed in modern conditions mainly in places where the Gulf Stream arrives (Herman 1974). These data indicates local origin of ACC, main evidences includes distribution, good preservation of samples, local debris in matrix, paleontological age. However, carbonates are very limited in the Arctic Ocean (Emelyanov 2005, Chierici & Fransson 2009). In view of these parameters, AAC can’t form by itself so it may be due only to external factors. Bottom water doesn’t provide such factors. Neither necessary conditions nor material occur in these waters. So in our opinion AAC were formed with help of neotectonic fault which are supposed to be a possible path for hot fluids, which created the conditions for crusts forming and Paleozoic carbonate rocks was a source of CaCO 3.

Rocks and ores of the PGE-bearing Vuruchuaivench massif (Kola Peninsula, Russia)

Pakhalko A.

Geology, Geophysics and Environment

2015

Vol. 41, no. 1

118--119

The early Palaeoproterozoic mafic-ultramafic layered Monchegorsk pluton is located in the north-eastern Fennoscandian (Baltic) Shield, on the Kola Peninsula. The Monchegorsk pluton is the second largest layered pluton in Europe (ca. 60 km2), and consists of two branches: one N-E trending (the Nittis, Kumuzh’ya, and Travyanaya (NKT) massifs), and sublatitude trending (the Sopcha, Nud, and Poaz massifs). The Vuruchuaivench massif occurs at the marginal part of the Nyud-Poaz intrusions (Smolkin et al. 2003). The Vuruchuaivench massif entirely consists of hydrothermally altered gabbronorite that is exposed in separated outcrops (ranging from meters to tens meters) northeast-wards for a distance of 7–8 km, and disappears in Lake Imandra (Ivanchenko 2008). The width of the exposure is 1.5–2.0 km. The intrusive is known for its PGE mineralization (prognostic resources 100 tof PGE). Reef ore body is similar with Platinova (Skaergaard) and Sonju Lake (Duluth Complex) PGE reefs. Rock ores are presented by metasomatised taxitic leucogabbronorites and anortosites (Knauf et al. 2008; Smolkin et al. 2003). The reef ore body hosts low-sulphide Ni-Cu and PGE mineralization. Average content of PGE in rocks is 2.0–2.5 ppm, samples containing 19 ppm are also found (Knauf et al. 2008). The ratio of Pd/Pt varies from 2–8:1.The samples (13 samples) were selected from the detailed study area (0.03 km2). Petrographic features were studied using optical microscope and X-ray analysis. The trace elements and precious metals analysis was performed with ICP-MS. Isotopic research was provided with SHRIMP-II. All analyses were carried out in A.P. Karpinsky Russian Geological Research Institute (VSEGEI) in Saint-Petersburg. The presence of a propylite rock was established for the first time in the territory of massif. Two propylite groups that differ mineralogically were identified. PGE mineralization is associated with the quartz-chlorite-albite group, whereas mineralization is unrelated with another, the albite group. There is a difference in total REE content in gabbronorites and propylites. Propylites of first group differ from second group and gabbronorites by a noticeable spread of values in REE content. The similarities of REE patterns in all rocks may indicate a genetic relationship. Arsenides and sulphoarsenides are predominant among the platinum group minerals in the Vuruchuaivench massif. The age of the propylites (based on Rb-Sr isotopic system) is 2,470 ± 130 Ma. The Initial 87Sr/86Sr ratio (ISr= 0.703537) and δ34S = 1.4‰ (Grokhovskaya et al. 2009) indicate a slight part of a crustal component in formation of rocks and ores of the Vuruchuaivench massif.