The results of studying the mineralogy of metamorphic schists from the Pre-Jurassic base of the Arctic part of the West Siberian plate are presented. The accessory and ore mineralization of schists from the Zapadno-Yarotinsky license area located in the southern part of the Yamal Peninsula is studied. The schists was uncovered by the Zapadno-Yarotinskaya No. 300 well at a depth of 2762 m. Above the section, the metamorphic rocks are overlain by a young Meso-Cenozoic cover. The schists are mainly composed of quartz, plagioclase (albite), carbonates (dolomite and siderite), mica (muscovite) and chlorite (donbassite). The discovered accessory and ore minerals in the metamorphic schists of the Zapadno-Yarotinsky area can be divided into two groups. The first group includes minerals that were formed during the metamorphism of schists, or were preserved as detrital matter. These minerals include zircon, fluorapatite, and rutile as the most stable compounds. The remaining mineralization (pyrite, sphalerite, chalcopyrite, cubanite, galena, cobaltite, barite, xenotime-(Y), goyazite, synchysite-(Nd), native silver and copper) is clearly secondary and was formed as a result of superimposed metasomatic processes. Judging from the described mineralogy, the schists underwent changes as a result of superimposed propyllitization. The temperature range of this process is determined by the formation of cubanite in association with chalcopyrite at a temperature of 200-210 оС.
The paper presents the results of a study of the mineralogy of chromitites from an ancient mine located on the slope of Mount Kron near the village of Raskuikha in the Middle Urals. Geologically, the studied chromitites form small lens-shaped bodies in apodunite and apoharzburgite serpentinites of the Early and Middle Ordovician. Ultrabasites are linear bodies confined to the melange zone developed in the northwestern exocontact of the Sysert metamorphic complex. It has been established that chromitites are represented by primary aluminous magnesiochromite, which, under the conditions of the greenschist facies of metamorphism, was transformed into magnesian and ferruginous chromite. Microprobe analysis determined that the green garnet that makes up the cement in chromitites and the brushes of crystals on it is uvarovite with a content of 53-63 % uvarovite end-member, up to 28 % grossular end-member and up to 22 % andradite end-member. Nine sulfides have been identified in chromitites: godlevskite, nickel pentlandite, heazlewoodite, millerite, chalcopyrite, pyrite, bornite, galena and geerite. The formation of such rich mineralization is associated with the metamorphism of chromitites and their subsequent hydrothermal development.
Relevance of the work. The Western Siberian megabasin is the main source of oil and gas in Russia. Therefore, the study of geological structure and evolution of the development of sedimentary cover rock complexes and the basement of the region is important for estimating the oil and gas potential of this vast territory. The object of this paperis the mineralogical and petro-geochemical study of volcanites from the pre-Jurassic basement of the Lakyuganskaya area (well no. 101) of the Longyugansk exploration acreage within the territory of the Nadym megadepression of the Western Siberian megabasin. Scope of work. This work can be useful when constructing geological maps of the basement of the northern part of the Western Siberian Plate. Results and conclusions. We have studied and described the mineralogy of strongly altered volcanites from the pre-Jurassic basement of the Lakyugansk area (well no. 101) of the Western Siberian Plate. According to chemical composition, the studied volcanites are classified as moderate and high potassic basalts, andesite-basalts and andesites. The rocks were subjected to significant secondary changes in the mineral composition, such as greenschist metamorphism and propylitization. Only titanomagnetite has been preserved from primary minerals in volcanites; all other minerals were subjected to changes. In general, we have determined the following minerals – albite, clinochlore, titanite, calcite, goethite, titanomagnetite (magnetite, ulvospinel), fluorapatite, and rutile. For the first time, ferroaluminoceladonite (dioctahedral mica) and three relatively rare secondary copper sulfides – spionkopite, yarrowite, and geerite – were identified and described in basalts from the basement of Western Siberia. Sulfide coppery mineralization in the studied basalts was due to overlapped propylization processes. The rocks have features of volcanites of island arcs, as well as evidence of calc-alkali and intraplate basalts. The petrological and geochemical characteristics of the studied volcanites are similar to basalts composing the Koltogorsko-Urengoysky rift of the Western Siberian Plate.
It is commonly supposed that a very substantial volume of early basalt magmatism effused synchronously on Siberia platform and West Siberia in a very short time interval at 249.4 ± 0.5 Ma (Reichow et al., 2002, etc.). This magmatism and induced climate change are considered as a main reason of the most catastrophic in the Earth history extinction at the border of Permian and Triassic time. But these conclusions were based on incomplete and unrepresentative data on West Siberia. We have obtained by analysis of pyroxenes monofraction from kainotype basalts of Guslinskaya P-430 well Ar-Ar age 268.4 ± 7.5 Ma. In Taurovskaya 503 well this age is 268.1 ± 7.5 Ma. Hence, volcanism in axial rift zones of the basement of West Siberia plate began earlier than that considered before and significantly earlier than on Siberia platform.
Relevance of the work. The Arctic part of the West Siberian megabasin is the main source of oil and gas in Russia, therefore, the study of the geological structure of this region is extremely important. Recently, Russia has lodged an application to extend its territory in the Arctic Ocean along the ridges that stretch from the continental shelf. Unfortunately, at the same time, we know little about the geological structure of the Arctic in the region of Western Siberia, where the thickness of the sedimentary cover is very high (about 3–4 km), therefore, the study of the basement of the Yamal Peninsula seems to be extremely urgent. The purpose of this work is mineralogical, petrological and geochemical study of dolerites from the pre-Jurassic basement of the Bovanenkovskaya area (well No. 114) within the territory of the gas condensate field of the same name, located in the western part of the Yamal Peninsula. Scope of the work. This work can be useful in constructing geological maps of the pre-Jurassic basement of the Yamal Peninsula. Results and conclusions. We have studied the mineralogical and geochemical features of dolerites from the pre-Jurassic basement of the Bovanenkovskaya area (well No. 114, sampling depth – 3210 m) of the West Siberian megabasin. The mineralogy of the rocks is represented by augite, diopside, albite, magnesian chamosite, ferrous hornblende, calcite, siderite, dolomite, anorthoclase, grossular, zeolite (gmelenite-K), pyrite, chalcopyrite, and rare lead chloride – cotunnite. The rocks underwent minor transformations in the conditions of the lower greenschist metamorphism, as well as secondary alterations in the form of superimposed propylitization. As a result of this low-temperature metasomatic process, zeolite, carbonate (calcite, dolomite, and siderite) and sulfide mineralization composed of pyrite and chalcopyrite, as well as cotunnite, which apparently replaced the dissemination of galena, were formed in the rock. Judging by geological position of the region, these dolerites are most likely formed at shallow depths during continental rifting. Remelting of the Paleozoic island arc substrate during the Early Triassic rifting and volcanism provided some closeness to the island arc trend in the geochemical features of these rift volcanics.
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