Mineralogy of schists from the basement of the southwestern part of the Tazovsky peninsula of the West Siberian megabasin ( Lenzitskaya oil exploration area , YNAD )

Relevance of the work. The crystalline basement of Western Siberia is a promising site in the search for oil and gas, but not a sufficiently studied subject. Promising rocks are granitoids and partly their metamorphic margins. Core samples from wells, which uncovered rocks of the basement of Western Siberia, are unique because they are rare and extremely difficult to access, so it is necessary to conduct a comprehensive detailed core study for geodynamic reconstructions and to consider the geological evolution of the region. Purpose of the work. The determination of the material composition of quartz-chlorite-mica schists from the pre-Jurassic basement of the northern (Arctic) part of the West Siberian megabasin uncovered by the Lenzitskaya well No 77 (depth is 3516–3502 m) 265 km east of Salekhard within the Yamalo-Nenets Autonomous District (YNAD). Results. The studied quartz-chlorite-mica schists are fine-grained rocks of a greenish-gray color; they have an interdigitation of thin beds of rock enriched in mica-quartz-carbonate substance (up to 2 mm thick) with a small amount of chlorite, and layers of quartz-chlorite-mica composition (up to 3 mm) with the presence of carbonate boudin. Intimate crumpling is often observed in the rock. In the lower part of the section (depth is 3516 m), the rocks have a medium-grained structure and are composed of calcite (60%), quartz (25%), mica (10%) and chlorite (5%). The quartz-sericitic-chlorite-carbonate rock has a schistous form due to light layers of quartz-calcite composition with a thickness of up to 2 cm and thin layers of a chlorite-mica aggregate with a thickness of up to 2 mm. The following was established using X-ray electron probe microanalyzer CAMECA SX 100: muscovite, aluminoceladonite, quartz, chamosite, calcite, plagioclase, pumpellyite-(Fe ), rutile, fluorapatite, monazite, zircon, pyrite and chalcopyrite. Quartz-sericitic schists similar in composition were found by us in the basement rocks of the Priuralsky part of the West Siberian plate in the Shaimsk-Kuznetsovsk megaanticlinorium. Conclusion. The mineralogy of quartz-chlorite-mica schists from the pre-Jurassic basement of the northern part of the West Siberian megabasin (Lenzitskaya 77 well, depth is 3502–3516 m) was first described. It was established that the formation of quartz-chlorite-mica schists took place under the conditions of the upper prehnite-pumpellyite facie of metamorphism along the sedimentary substance. Later the rocks underwent changes in the process of propitilization.


I ntroduction
Th e study of the geology, tectonics and material composition of the pre-Jurassic basement of the West Siberian megabasin was carried out by many researchers [1-5 and others]. At the same time, studies of the geology of the Arctic and the Arctic part of Western Siberia are of great importance in connection with possible oil-and-gas content of this vast and still insufficiently studied territory and its possible upcoming division between countries. It is known that the most important criterion here is the results of the study of the basement of the sedimentary basins of the Arctic. Th e Yamal Peninsula and its nearest margins are the main gas provinces of our country and one of places where the crystalline basement is available for direct study (albeit with great diffi culty). Th e basements of oil and gas provinces are still one of few promising, but insuffi ciently studied objects. Interestingly, the granitoids of basements (in part, their metamorphic margins) are the most promising in the search for oil and gas [6-8 et al]. Core samples from wells, which uncovered rocks of the basement of Western Siberia, are unique because they are rare and extremely diffi cult to access, so it is necessary to conduct a comprehensive detailed core study for geodynamic reconstructions and to consider the geological evolution of the region. In this paper, the mineralogy of quartz-chlorite-mica schists from the pre-Jurassic basement of the northern (Arctic) part of the West Siberian megabasin (uncovered by the Lenzitskaya No 77 well at a depth of 3516-3502 m) is examined in detail for the fi rst time.
Geological setting of the Lenzitskaya area Th e Lenzitskaya oil and gas exploration area (on the territory of which the Lenzitsky oil fi eld was discovered during the drilling of the exploratory well No. 70 in 1985) is located in the south-western part of the Tazovsky peninsula at the southern coast of the Gulf of Ob 265 km east of Salekhard within the Yamalo-Nenets Autonomous District (Fig. 1).
Historical [9], the well of Lenzitskaya No 70 at a depth below 3500 m helped to reveal rocks of the pre-Jurassic basement represented by green schists. Th e explanatory note to the State Geological Map of the Russian Federation [9] indicates that Upper Proterozoic undivided metamorphic formations are established at the Lenzitskaya area. It is noted that in the Medvezhye oil and gas exploration area located southward, the schists similar in mineral composition were found by wells No 1001 (within the interval of 4458-4605 m), as well as Sosninskaya Research results and their discussion The studied core is represented by fine-grained, highly deformed quartz-chlorite-sericitic shales (samples from a depth of 3502-3509 m, Fig. 2) and quartz-chlorite-sericitic-carbonate rock (depth of 3515-3516 m, Fig. 3). Microscopically, rocks have a grano-lepidoblast and granoblast structure. The mineral assemblage of the rocks is as follows: muscovite, aluminoceladonite, quartz, chamosite, calcite, plagioclase, pumpellyite. Rutile, fluorapatite, monazite, zircon, pyrite and chalcopyrite are found among accessory and ore minerals.
The rocks are fine-grained, greenish-gray in color; they have an interdigitation of thin beds of rock enriched in mica-quartz-carbonate substance (up to 2 mm thick) with a small amount of chlorite, and layers of quartz-chlorite-mica composition (up to 3 mm) with the presence of carbonate boudin. Intimate crumpling is often observed in the rock. In the lower part of the section (depth is 3516 m), the rocks have a medium-grained structure and are composed of calcite (60%), quartz (25%), mica (10%) and chlorite (5%). The quartz-sericitic-chlorite-carbonate rock has a schistous form due to light layers of quartz-calcite composition with a thickness of up to 2 cm and thin layers of a chlorite-mica aggregate with a thickness of up to 2 mm.
In quartz-chlorite-sericitic schists, quartz is represented by polygonal grains with the inclusion of muscovite and apatite. Some of the grains have an undulatory extinction. Grains size is up to 0.5 mm. Micro-folding is shown in the beds enriched with quartz material (Fig. 4). In the superposed folds of the straticules, grains of quartz, mica and chlorite are perpendicular to stratification in the rock. Quartz contains fine grains of short prismatic apatite and zircon as inclusions.
Mica forms fine-grained banded and crimped aggregates usually deformed and folded; it is often observed together with the laths of chlorite. The size of individuals is up to 0.5 mm. Individuals of accessory minerals such as zircon, monazite, apatite and rutile can be found among the chlorite-mica aggregate. According to the modern classification of micas [11], the obtained compositions correspond to muscovite ( Chlorite in the rock tends to the mica aggregate, where it forms self-contained strips or lenses; the mineral also occurs in the form of elongated curved laths in the interstice of quartz and carbonate. Chlorite flake size does not exceed 0.7 mm. Mineral is pleochroic from yellow-green to bluish-green. In terms of composition, chlorite corresponds to magnesian high-alumina cha-   There is a small amount of tabular grains of plagioclase with polysynthetic twinning of up to 0.2 mm in size among the quartz-calcite aggregate. By chemical composition, the mineral corresponds to pure albite (An 0.2-0.6 ). The content of CaO in albite is up to 0.13 wt. % (Table 1, an. 9-12).
Pumpellyite-(Fe 2 + ) was found by us in thin sections in the form of a fine-grained aggregate of green color in association with calcite and chlorite. It has been established by microprobe analysis that the content of FeO in the mineral reaches 9.71 wt. %. It can be confidently identified as pumpellyite-(Fe 2 + ) by chemical composition with a high content of the pumpellyite-(Al) minal up to 34%. The crystallochemical formula of this pumpellyite is as follows: Ca 1  (OH) 2 · H 2 O (when converted to cationic method). The increased content of Al 2 O 3 (24.74 wt.%) is observed in the mineral; the content of FeO is slightly underestimated and there are almost no impurities (Table 1, an. 13). The presence of pumpellyite-(Fe 2 + ) may indicate that the rocks were formed under conditions of prehnite-pumpellyite facie of metamorphism. Rutile in the rock is the main accessory mineral; it is found both in the form of single individuals up to 0.5 mm in size and in clusters in the chlorite-mica aggregate. Almost all mineral individuals are oriented along schistosity. In thin sections, the mineral forms prismatic and acicular translucent grains of brown color with high relief. The content of TiO 2 in rutile varies from 98.54 to 99.91 wt. %. The composition of the mineral has an admixture of Fe 2 O 3 to 0.98 wt. %.
Apatite can be found in the interstice of quartz grains forming isometric and short prismatic grains, the maximum size of which does not exceed 70 μm by elongation. Moreover, the mineral is found in the form of clusters of grains of irregular shape in calcite. The chemical composition of phosphate corresponds to fluorapatite. There are FeO up to 0.33 wt. % and SiO 2 up to 0.40 wt. % among the impurities in the mineral. The fluorine content in the apatite is up to 4.21 wt. %; chlorine is not found.
Monazite in the rock is represented by single grains of irregular shape in the chlorite-mica aggregate. Grain size is up to 20 microns by elongation. According to microprobe analysis, the mineral corresponds to monazite-(Ce) with a Ce 2 O 3 content of 27.0 wt. % and La 2 O 3 11.9 wt. %. Zircon forms widely-spaced and small short prismatic grains up to 30 microns in size by elongation.
Sulfide mineralization of the studied rocks is represented by pyrite and chalcopyrite. The dominant among the rocks is pyrite. The mineral forms widely-spaced isometric grains and chains located along the schistosity of the rock (Fig. 5). The size of the individuals of pyrite does not exceed 250 microns. The chemical composition of the mineral contains Co impurities up to 0.79 wt. % and Pb up to 0.29 wt. % ( Table 2, an. [1][2][3][4]. Chalcopyrite occurs in the form of small elongated, irregularly shaped grains up to 40 microns in size in the interstice of pyrite. A small amount of Pb impurity is present in chalcopyrite (up to 0.18 wt.%, Table 2, an. 5-8).  The temperature of rock formation in the range of 350-360 ° C is calculated using a chlorite thermometer [12]. The obtained estimated data are close to the upper boundary of the prehnite-pumpellyite facie of metamorphism.
The mineralogy of metamorphic schists from the Lenzitskaya well No 77 differs from the previously studied quartz-plagioclase-mica-chlorite schists from the pre-Jurassic basement of the southern part of Yamal (the West-Yarotinskaya well No 300) by the presence of chamosite, calcite, pumpellyite-(Fe 2 + ), chalcopyrite and monazite. At the same time aluminous chlorite (donbassite), goyazite, dolomite, sphalerite, galena, cobaltite, native copper and silver were found in the schists from the Zapadno-Yarotinskaya well No. 300 [13]. Sulphide mineralization, as well as a large amount of calcite and rutile in quartz-carbonate-chlorite-mica schists, could have formed after the rocks were worked out by late low-temperature solutions with propylitic alteration along cracks at the boundary of two media, metamorphic rocks of the basement and the superincumbent Jurassic-Cenozoic sedimentary strata [14]. Such metamorphic schists are established in the basement of the Priuralsky part of the West Siberian plate, where they have margins for granite massifs and make up the major Shaimsk-Kuznetsovsk megaanticlinorium [15][16][17].
Conclusions Thus, we first described in detail the mineralogy of quartz-chlorite-mica schists from the pre-Jurassic basement of the northern part of the West Siberian megabasin (Lenzitskaya well No 77, depth is 3502-3516 m). The mineral assemblage of rocks is represented by muscovite, aluminoceladonite, quartz, chamosite, calcite, albite, pumpellyite-(Fe 2 + ), rutile, fluorapatite, monazite-(Ce), zircon, pyrite and chalcopyrite. Quartz-sericitic schists similar in composition were found by us in the basement rocks of the Priuralsky part of the West Siberian plate in the Shaimsk-Kuznetsovsk megaanticlinorium. The formation of rocks occurred under the conditions of the upper prehnite-pumpellyite facie of metamorphism along the sedimentary substance. Later the rocks underwent changes in the process of propitilization.