Geology, tectonics and evolution of Pasha-Ladoga and East-Anabar uraniferous basins

Kuptsova A.V.

Saint-Petersburg State University, Saint-Petersburg, Russia

alina-kuptsova@yandex.ru

Proterozoic Pasha-Ladoga and East-Anabar basins are regions with high uranium potential for unconformity-type uranium deposits. Usually basins with this type of U deposits are compared with Athabasca basin (Canada) which hosts the largest and highest grade U deposits globally. Based on Athabasca Basin model, it was predicted that most of Proterozoic basins in Russia are not favorable for this type of uranium mineralization. However, in Pasha-Ladoga Basin has been discovered Karku deposit, and in East-Anabar Basin – several U occurrences.

Paleoproterozoic uraniferous basins of Canadian Shield could be divided into two types – rift grabens and flat depressions. The first type represented by Martin Basin (north from Athabasca Basin) with main hydrothermal event at 1,78 Ga. Similar basin (Baker Lake) is known at the base of Thelon Basin which is very close to the Athabasca Basin and developed like it in an interior position. Baker Lake Basin comprises two sedimentary sequences (1,85–1,75 Ga) that record rift and sin-rift stages of basin evolution. The overlying Thelon Basin (1,72–1,54 Ga) represents post-rift stage [Rainbird et al., 2003].

Athabasca Basin has the same tectonic setting and age as Thelon Basin. It is filled with flat-lying quartz-rich fluvial sandstones and conglomerates although the upper part of the basin consists of marine sandstones and carbonate units. Permeable basal sequence of Athabasca Basin creates favorable conditions for prolonged fluid circulation which is necessary for the formation of unconformity-related U deposits. Current genetic model for these types of deposits propose mixing of basement-derived reduced fluids with oxidized basin fluids which occurs at temperatures ~200ºC. Fluid migration pathways resulted in well-developed alteration profiles contacting clay minerals (illite, kaolinite, chlorite), sulfides, hematite, silica, rare calcite and adular [2].

Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Archean Karelian Craton and Paleoproterozoic Svekofennian Belt in the north-west part of Russia. It is a quite small, elongated intracratonic basin with evidences of rift nature. The architecture of the basin is influenced by bounding NW-SE fault system, reverse faults which formed horst and graben structures, and thrust faults.

Graben geometry controls sedimentation in the basin. Sedimentary cover composed of flat-lying unmetamorphosed terrigenous lithologies with total thickness up to 0.8–2 km. Basal sequences (immature sub-quartz to arcosic sandstones and conglomerates of Priozersk Formation) are common for the whole basin and suggest basin-wide fluvial deposits. Upward successions are different for eastern and western flanks of the Basin. On the western flank fluvial sediments overlain by lacustrine and shallow-marine successions, whereas on the eastern flank – by fluvial volcano-sedimentary units [8].

The maximum age of sedimentation in the basin is constrained by the timing of emplacement of the Salmi rapakivi-granite-anorthosite intrusion at 1530 Ma [1], whereas the minimum age comes from timing of Valaam Sill emplacement at 1457±2 and 1459±3 Ma [5]. Obtained in this study the age of the youngest detrital zircon (1477±8 Ma) from Priozersk Formation considered rapid sedimentation which took place less than 20 Ma and accompanied by volcanic activity.

Priozersk Formation represented by gravelstones, conglomerates and quartz-feldspar sandstones with rare thin beds of green siltstones a few tens to 450 m thick. It is overlained by basaltic lava flows up to 130 m thick. Sandstones with floating quartz pebbles and lens-shaped conglomerates are most typical. Sandstones have massive structures with abundant grade-beddings, cross-beddings and scour surfaces. Lithology, lenticular structures and thickness variety of Priozersk Formation do not allow to correlate beds even in adjacent wells that suggests deposition in braided fluvial systems.

Sandstones of Priozersk Formation are immature, may be classified as sub-quartz to arcosic, and consist of quartz (70–90%), feldspar (10–15%) and lithics. Detrital grains are angular, poorly sorted. Matrix constitutes 20–30% (up to 40%) of rocks and consists of rectorite (illite-smectite) – alteration product forming during low-temperature hydrothermal processes such as lava emplacement in the upper part of Priozersk Formation.

Reduced sediment thickness, immature lithology, wide facies variety, presence of regional aquitards (such as basaltic flows, sorptive property of illite-smectite), tectonic activity reduce permeability of sediments and prevent prolonged regional-scale fluid circulation which is critical for the formation of large-scale uranium deposits. Fluid movement possibly was realized through the paleoregolith and conglomerates of the basal part of Priozersk Formation. That is why Karku U deposit located in paleoregolith along unconformity, rarely some mineralization penetrates sandstones of Priozersk Formation and basement rocks.

Hydrothermal alteration produced kaolinite-dickite, calcite and chlorite which record temperatures near 200ºC. Calculated isotopic composition of H and O for fluids associated with alteration clays indicate that Karku deposit was formed from interaction of basement and basin fluids. Apparently faults were important for focusing mineralizing fluids.

The East-Anabar Basin (Anabar Shield, Central Siberian Plateau) overlay Archean-Paleoproterozoic terranes and Paleoproterozoic regional-scale shear zones of Siberian Platform [6]. Sedimentary successions consist of two main units: terrigenous Mukun Group (different-grained immature sandstones and conglomerates) and carbonate Billyakh Group (carbonates with rare shales and sandstones). Mafic sill that cuts upper contact of Mukun and Billyakh Group yields U-Pb age 1384±2 Ma [3]. The youngest detrital zircon grain in sandstone sample from the base of the Mukun Group yields concordant U-Pb age 1690±9 Ma [4]. Khudoley [4] reported in Mukun Group zircons group at probability peak ca. 1717 Ma. Morphology of these grains points to felsic volcanic provenance. Magmatic event of this age is very unusual for Siberian Craton and was reported only in Ulkan rift complex. Thus, there was a rift in this area that preceded sedimentation in East-Anabar Basin.

Mukun group overlies basement with sharp unconformity. It locally pinches out forming a number of sub-basins with the highest thickness about 200 m. Coarse-grained sandstones predominate, although in the lower part of the succession discontinuous conglomerate beds with thickness varying from several centimeters up to meters have been found as well. Pebbles are well rounded and consist of quartz, quartzite, and, rarely, feldspar. Sandstones of Mukun Group are interpreted as transition alluvial fan-shallow marine settings.

Sandstones are quartz to sub-arkose arenites with graded and cross-beddings. Detrital grains are well sorted, rounded to sub-rounded. Quartz grains are the most widespread and typically form more than 80% of framework grains. Field spar content varies from 0% to 20%, locally up to 45%, but generally decreases upward in the section. Locally thin beds are enriched in heavy minerals where zircon, magnetite and tourmaline are most widespread. Some beds are enriched with extremely altered glauconites. Quartz overgrowth and rim cementation are typical. Matrix constitutes 3–15% of rocks and composes of illite-muscovite, rarely calcite.

Abundant quartz cement reduces hydraulic conductivity of Mukun sandstones as a result some regional beds with quartz cementation become aquitards. Fluids migrated not through the whole sequence, but only through the horizons with illite and calcite cement. Thus alteration halos extended in a different part of Mukun Group which are not experienced quartz cementation.

The alteration mineralogy is characterized by transformation of 1M to 2M₁ illite , formation of kaolinite, sulfides (piryte, rutile etc.), hematite and limonite. Futhermore, the alteration process is followed by feldspar and quartz dissolution, which result new adular and quartz veins formation. Illite crystallinity indicates that sandstones underwent temperatures 150–200ºC. Fluids of the East-Anabar Basin are isotopically similar to basin-derived fluids which are responsible for the formation of sandstone-hosted deposits in Athabasca [2].

Main results of the study may be summarized as the follows:

1) Based on tectonic setting Russian basins which host unconformity-related uranium deposits and ore occurrences may be divided into two types: rift-related (Pasha-Ladoga) and post rift basins (East-Anabar).

2) Athabasca basin does not have potential analogs in Russia in terms of its age and sedimentology.

3) Basin-fill composition and character of diagenesis in Pasha-Ladoga and East-Anabar basins do not form favorable conditions for prolonged fluid circulation and formation of big uranium deposits.

4) Hydrothermal alteration in Pasha-Ladoga and East-Anabar basins corresponds to alteration types recognized in Athabasca and Thelon basins.

References:

1. Amelin, Yu. A., Larin, A. M., and Tucker, R. D. (1997): Chronology of Multiphase Emplacement of the Salmi Rapakivi Granite-Anorthosite Complex, Baltic Shield: Implications for Magmatic Evolution, Contrib. Mineral. Petrol., vol. 127, pp. 353-368.

2. Cuney, M., Kyser K., Editors (2008): Recent and not-so-recent developments in uranium deposits and implications for exploration. Mineralogical Association of Canada, Short Course Series, Vol. 39, 272 p.

3. Ernst R.E., Buchan K.L., Hamilton M.A., et al. (2000): Integrated paleomagnetism and U–Pb geochronology of mafic dikes of the eastern Anabar Shield region, Siberia: Implications for Mesoproterozoic paleoaltitude of Siberia and comparison with Laurentia // J. of Geol. V. 108. №3. P. 381–401

4. Khudoley A.K., Molchanov A.V., Okrugin A.V. North Siberia basement evolution according to U-Pb dating of detrital zircons from Mukun Group sandstones, Anabar Shield // XL Tectonics meeting. In Russian

5. Ramo, O. T., Mänttäri, I., Vaasjoki, M., et al. (2001): Age and Significance of Mesoproterozoic CFB Magmatism, Lake Ladoga Region, NW. Russia, in GSA Annual Meeting. Petrology I, Session 57, A-139, Boston: Massachusetts

6. Rosen, O.M., Condie, K.C., Natapov, L.M., Nozhkin, A.D. (1994): Archean and early Proterozoic evolution of the Siberian Craton: a preliminary assessment // Archean Crustal Evolution / Ed. Condie K.C. Elsevier. 1994. P. 411–459.

7. State Geological map of USSR, Scale 1:1000000, Sheet R-48-50. 1983. In Russian

8. State Geological map of Russia, Scale 1:1000000, Sheet P-35-37-37. 2000. In Russian