Geochemistry of deep Tunguska Basin sills, Siberian Traps: correlations and potential implications for the end-Permian environmental crisis

A vast portion of the plumbing system of the Siberian Traps Large Igneous Province (STLIP) is emplaced in the Tunguska Basin, where borehole data reveal ubiquitous and abundant sills with great lateral extension. These intrusions intersect Cambrian–Ordovician evaporite, carbonate and siliciclastic series, and locally coal-bearing Permian host rocks, with a high potential for thermogenic gas generation. Here we present new geochemical data from 71 magmatic and 4 sedimentary rock samples from the Tunguska Basin center and periphery, recovered from 15 deep sills intercepted by boreholes. The studied samples are all low-Ti basalt and basaltic andesites, confirming absence of high-Ti and alkaline STLIP magmatism in the Tunguska Basin. The sills derive from picritic parental melts produced by extensive melting of a mantle source with recycled crustal components below a thinned lithosphere (50–60 km), within the spinel stability field. The mantle source was dominantly peridotitic, with enriched pyroxenitic domains formed by recycled lower crust, in agreement with previous models for the main tholeiitic STLIP phase. Limited amounts (up to 5%) of highly radiogenic granitoids or moderately radiogenic metapelites were assimilated in upper crustal magma reservoirs. After emplacement, sills intruded in Cambrian evaporites assimilated marlstones and interacted with the evaporitic host rocks, probably via fluids and brines. This is the first time that such process is described in subvolcanic rocks from all across the volcanic basin. The sills are correlated geochemically with the established chemostratigraphy for the on-craton STLIP lava piles and intrusions (Norilsk region). Sills correlated with the Morongovsky–Mokulaevsky Fm. and the Norilsk-type intrusions are the most voluminous, present all across the central Tunguska Basin, and bear the strongest evidence of interaction with evaporites. Massive discharge of thermogenic volatiles is suggested by explosive pipes and hydrothermal vent structures throughout the Tunguska Basin. We propose that this voluminous pulse of magmatism is a good candidate for the hitherto unidentified early intrusive phase of the STLIP, and may link the deep Tunguska basin sills to the end-Permian environmental crisis.

BIOGEOCHEMICAL INDICATORS, ANOMALIES AND THEIR OIL-GEOLOGICAL SIGNIFICANCE (TUNGUSKA BASIN)

Hydro-biogeochemical studies were carried out in the west of the Siberian platform in the basin of the river Podkamennaya Tunguska, where 1204 groundwater sources and watercourses were studied in terms of biogeochemical indicators. The most informative indicators have been established - bacteria that oxidize propane, butane, pentane, benzene, toluene. According to these parameters and total biogenicity, 42 anomalies with areas of 10-200 km2 were identified. In structural and tectonic terms, 20 biogeochemical anomalies are confined to local positive structures, which can be considered promising for oil and gas exploration.

Sulfide Minerals as Potential Tracers of Isochemical Processes in Contact Metamorphism: Case Study of the Kochumdek Aureole, East Siberia

Marly limestones from the Lower Silurian sedimentary units of the Tunguska basin (East Siberia, Russia) underwent metamorphism along the contact with the Early Triassic Kochumdek trap intrusion. At ≤ 2.5 m from the contact, the limestones were converted into ultrahigh-temperature marbles composed of pure calcite and sulfide-bearing calcsilicate layers. The sulfide assemblages in the gabbro and marbles were studied as potential tracers of spurrite-merwinite facies alteration. The gabbro-hosted sulfides show Fe-Ni-Cu-Co speciation (pyrrhotite and lesser amounts of chalcopyrite, pentlandite, and cobaltite) and positive δ34S values (+2.7 to +13.1‰). Both matrix and inclusion sulfide assemblages of prograde melilite, spurrite, and merwinite marbles consist dominantly of pyrrhotite and minor amounts of troilite, sphalerite, wurtzite, alabandite, acanthite, and galena. In contrast to its magmatic counterpart, metamorphic pyrrhotite is depleted in Cu (3–2000 times), Ni (7–800 times), Se (20–40 times), Co (12 times), and is isotopically light (about –25‰ δ34S). Broad solid solution series of (Zn,Fe,Mn)Scub, (Zn,Mn,Fe)Shex, and (Mn,Fe)Scub indicate that the temperature of contact metamorphism exceeded 850–900 °C. No metasomatism or S isotope resetting signatures were detected in the prograde mineral assemblages, but small-scale penetration of magma-derived K- and Cl-rich fluids through more permeable calcsilicate layers was documented based on the distribution of crack-filling Fe-K sulfides (rasvumite, djerfisherite, and bartonite).

Hyaloclastite formation during the effusion of the first lava flows of Siberian traps into a shallow freshwater basin

<p>Hyaloclastites have long been described within numerous volcaniclastic sequences in the Siberian Traps Large Igneous Province. They are typical for the southern and central parts of the Tunguska basin, and we inspected them in 2004-2010. In recent years, we have focused our attention on the northwestern region of the Tunguska basin (Norilsk area) with a volumetric manifestation of basaltic lava flows. We have completed fieldwork in this region from 2006 to 2019, with a recent focus on the understanding of the emplacement environments for the lowermost lava flow erupted directly on the end-Permian boggy surface. We studied pillow basalt at the basal part of the lowermost lava flow in the Norilsk region (Ivakinskaya Formation). In the upper part of this pillow basalt horizon, hyaloclastite is very common, and at the basal part, several tree trunks occur. The hyaloclastite includes black equant angular clasts and rusty red matrix and easily recognize at any outcrops. We studied hyaloclastite with optical microscopy and SEM-EDS. Black clasts composed of sideromelane cracked and altered to palagonite. Sideromelane fragments include crystals of olivine (Fo70), plagioclase (An63-70), and likely OPx altered to chlorite. Sideromelane glass has a basalt composition with elevated P2O5, CaO, and decreased amount of MgO and minor halogens (F, Cl). Some sideromelane clasts bear round inclusions (blobs) entirely infill with dolomite, siderite, and calcite. Every single carbonate inclusion has a specific structure and minerals infill.</p><p>We interpret these hyaloclastite rocks formations with carbonate inclusions as a result of lava flow effusion onto the shallow freshwater basin or boggy surface. Water and organic-rich sediments transferred with an explosion to steam and carbon dioxide gas, and this gas mixture was formed a hyaloclastite horizon at the basal part of a lava flow. We suppose that these sideromelane clasts with carbonate blobs are additional evidence of greenhouse gas generation during the early stage of the Siberian Traps lavas eruption.</p>

Reservoir quality assessing and petroleum potential of Nepa horizon in Lena-Tunguska basin

Reservoir layers were located within the Nepa horizon according to the log data interpretation of 26 wells. Porosity and gas saturation were calculated within these layers. The research provides an opportunity to determine reservoir potential of sediments and to identify further research lines.

Sills and gas generation in the Siberian Traps

On its way to the surface, the Siberian Traps magma created a complex sub-volcanic plumbing system. This resulted in a large-scale sill emplacement within the Tunguska Basin and subsequent release of sediment-derived volatiles during contact metamorphism. The distribution of sills and the released sediment-stored gas volume is, however, poorly constrained. In this paper, results from a study of nearly 300 deep boreholes intersecting sills are presented. The results show that sills with thicknesses above 100 m are abundant throughout the upper part of the sedimentary succession. A high proportion of the sills was emplaced within the Cambrian evaporites with average thicknesses in the 115–130 m range and a maximum thickness of 428 m. Thermal modelling of the cooling of the sills shows that the contact metamorphic aureoles are capable of generating 52–80 tonnes of CO 2 m −2 with contributions from both marine and terrestrial carbon. When up-scaling these borehole results, an area of 12–19 000 km 2 is required to generate 1000 Gt CO 2 . This represents only 0.7–1.2% of the total area in the Tunguska Basin affected by sills, emphasizing the importance of metamorphic gas generation in the Siberian Traps. These results strengthen the hypothesis of a sub-volcanic trigger and driver for the environmental perturbations during the End-Permian crisis. This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.