Lithology and Mineral Resources最新文献

The formation of pyrophyllite is attributed to various geological phenomena. This mineral is found in various regions around the world and used in many industries. In the Ougarta. region (south western Algeria), pyrophyllite clays of Silurian age are widely exposed. This study to examine the petrographic, mineralogical, and geochemical characteristics of these clays to better understand their formation conditions. Petrographic analysis identified three distinct facies: gray to reddish clays, black clays and white level containing alunite. These results indicate that the clay is mainly composed of pyrophyllite, kaolinite, and illite, orthoclase, quartz, and iron oxides. The presence of alunite, suggests a hydrothermal origin. Rare earth elements (REE) concentrations are relatively low, ranging from 306.95 to 445.05 ppm. Light rare earth element (LREE) to heavy rare earth element (HREE) ratios indicate enrichment in LREE, with values ranging from 2.81 to 6.47. Most clay samples present low levels of total organic carbon (TOC) and thermally mature, indicating organic matter depletion and reflecting oxic rather than anoxic conditions. On the other hand, the black facies are considered to be thermally immature source rocks, with very high TOC (16%). Based on geochemical data, the formation conditions of these clays are defined as continental deposits in a semi-arid climate. These conditions were initially influenced by diagenesis and later by hydrothermal activity, with the sediments source rock being felsic to intermediate in composition.

The metamorphic framing of granite-gneiss domes in the Кochkar anticlinorium (Southern Urals) includes the Lower Carboniferous terrigenous-carbonate sequence that underwent zonal metamorphism of the epidote–amphibolite to greenschist facies. Numerous soft-sediment deformation structures were found in marbles of this sequence uncovered by the quarry at the Svetlinsk gold deposit and in the bedrock of the Eleninsk placer. The main structure types are represented by landslides associated with the laminar sediment flows and less often by seismites of various morphologies. The formation mechanism of these structures is associated with the underwater gravitational landslide of nonlithified sediments due to change in the seabed slope and the impact of earthquakes. The seismic activity was triggered by both the anticlinorium-bounding regional faults and, probably, the formation of granite-gneiss domes. The discovery of landslides and seismites in the terrigenous–carbonate sequences of the anticlinorium provides insight into its paleotectonic formation environment during the Late Paleozoic Ural collision. The structural position of carbonate rocks indicates the formation of granite-gneiss domes under shallow-water conditions in the anticlinorium.

Results of the study of Lower Devonian rocks in the Chernyshev Ridge to determine their hydrocarbon (HC) generation potential are presented. Assessment of the organic matter (OM) content in Lower Devonian rocks showed that most of the studied section is characterized by low C_org concentrations (<0.3%). Elevated OM contents were determined only in Unit III rocks of the Ovinparma regional stage, which are considered as potential petroleum source rocks: C_org 0.13–0.35 and 0.54–1.44%, respectively, in the carbonate and clayey–carbonate rocks. Based on the Rock-Eval pyrolysis data, S₁ + S₂ (0.04–6.20 mg HC/g rock) and hydrogen index (HI) value (19–430 mg HC/g C_org), they are characterized as poor and moderate petroleum source rocks. These source rocks were deposited under open-marine outer ramp conditions during the marine transgression. The OM maturity of the petroleum source rocks, determined based on the bituminite reflectance value, RV_eq (0.75–0.81%), maximum HC yield temperature, T_max (437–449°C), and conodont color alteration index, CAI (1.5), corresponds to the middle of the main oil generation zone (catagenesis grade MC₂). Thus, the presence of rocks with the HC generation capacity and sufficient OM maturity for oil generation indicates that unit III source rocks of the Ovinparma regional stage represent an effective element of the Lower Devonian petroleum system.

New data on the lithochemistry of clayey–carbonate rocks of the Solikamsk Formation (Ufimian stage of the Lower Permian) in the southwestern part of the Solikamsk depression of the Ural foredeep are reported. The cyclicity of sedimentation was determined by the subsequent introduction of aluminosiliciclastic materials into the evaporite and marine basins from adjacent land. Based on variations of the terrigenous material, the Solikamsk Formation consists of nine cyclites, in which the content of clay minerals in the rock subsequently gradually increases and then relatively decreases. Some sulfate layers are not related to the distinguished cyclicity, but were formed through the diagenetic growth of anhydrite nodules on the groundwater level during periodical drying off the basin. Analysis of the chemical composition of the terrigenous component of the Solikamsk rocks made it possible to show that the basin at the beginning of Solikamsk time gained the destruction products of mafic rocks (data points are confined to the field “chlorite, smectite, and illite” in the IM–NAM diagram), while the end of this time was characterized by the influx of intermediate and acid material (decrease of HM and AM upsection, shift of data points of marls to the field of essentially illite clays with significant amount of K-feldspars in the IM and NAM diagrams). The different lithochemical maturity of delivered terrigenous material follows from the variations of CIA, CIW, and ICV indices.

The lithochemical features of clastic rocks at several stratigraphic levels of the Riphean stratotype (Ai, Bakal, Zigalga, Zil’merdak, and Uk formations), as well as the distribution of detrital zircon populations in the sandstones therein, are analyzed. The clastics in the Riphean stratotype section show two main formation trends—entry into the cycle 1 sedimentation area (petrogenic) and recycled (redeposited, lithogenic) material. It is suggested that the detrital zircon, delivered from protosources into sandstones of the Ai stratigraphic level, was repeatedly subjected to redeposition: crystals with close maximums on the probability density curve are present in sandstones of younger stratons of the Riphean-type section, up to the Uk Formation. The most striking renewal of detrital zircon protosources occurred at the beginning of the Late Riphean. The arkosic and subarkosic sandstones of the Bir’yan Subformation of the Zilmerdak Formation contain both detrital zircon with ages similar to those in the sandstones of the Lower and Middle Riphean, as well as new zircon characterized by Paleo- and Mesoproterozoic maximums on the probability density curve. A small part of such new zircon is repeated in the oolitic limestones of the Uk Formation, but some renewal of protosources is also recorded here. Thus, the Middle/Late Riphean boundary was marked by a radical change in age populations of the detrital zircon, probably, related to the formation of a fundamentally new system of the clastic material transport from sources to sink.

The article provides detailed structural and crystal-chemical characteristics of mixed-layer chlorite-corrensite minerals in <1-μm fractions extracted from Upper Pleistocene sediment samples from ODP Hole 1036A, which was drilled at the Dead Dog hydrothermal field in the northern Middle Valley, Juan de Fuca Ridge, in the northeastern Pacific Ocean. Using the XRD pattern modeling method, it was shown for the first time that the studied mixed-layer minerals should be considered as chlorite-corrensites rather than chlorite-smectites. In the structure of the studied mixed-layer minerals, trioctahedral chlorite and corrensite layers occur in a ratio of 50 : 50–60 : 40, and ~90 : 10, alternating with a tendency to segregation at a short-range order factor R = 1. Averaged crystal-chemical formulas were obtained for the mixed-layer chlorite-corrensites and their individual chlorite and corrensite layers. It is assumed that the model with an asymmetric distribution of charges in tetrahedral sheets of 2 : 1 layers is realized in corrensite packages of natural chlorite-corrensite. All samples belong to Mg–Fe varieties of chlorite-corrensites. The most probable mechanism for the formation of chlorite-corrensites of different compositions is the dissolution of original terrigenous clay minerals during the interaction of hydrothermal fluid with terrigenous sediments and the synthesis of these mixed-layer minerals from solution.

Lithogeochemical characteristics of the pelitic and silty–pelitic bottom sediment samples, collected during Cruise 89 of the R/V Akademik Mstislav Keldysh (September 2022), from several areas in the southwestern Kara Sea (Baidaratsk Bay, Pukhuchansk Depression, West Kara Step, and Novaya Zemlya Depression) are discussied. It has been established that the muds contain a significant proportion of the lithogenic component. This conclusion agrees well with the localization of sediment data points on various discriminant diagrams near the reference points of average Paleozoic graywackes and PAAS. Such component could be sourced either from the fine-grained suspended particulates of the Ob River or from the sedimentary rocks and friable sediments of the Yamal and Yugra Peninsulas, Vaigach, and Novaya Zemlya. Although the upper layer of bottom sediments in the southwestern Kara Sea contains a noticeable amount of the mafic igneous rock fragments from Novaya Zemlya, which should make a significant contribution to their erosion products in the mud composition, parameters of the chondrite-normalized REE distribution in the sediments are inconsistent with such assumption.

Modern data on the formation of the mineral composition, geochemical and isotope-geochemical features of sediments from large rivers in Africa (Zambezi, Nile, Congo), as well as U–Pb isotope ages of detrital zircon populations in sands, obtained during complex long-term studies by Prof. E. Garzanti (Universita’ Di Milano-Bicocca, Italy) and his colleagues are considered. Ideas about the influence of chemical weathering on the composition of both silts and sands of the named large rivers are briefly summarized. The influence of river segmentation on the composition of the transported aluminosiliciclastics is discussed. The key points of the research results presented in the overview are outlined for specialists studying ancient sedimentary sequences.

A comparative analysis of the carbonate sedimentation conditions in two Paleoproterozoic basins (Onega and Pana-Kuolajarvi), located in the southeastern and northern areas of the Late Archean Karelian Craton is presented. The carbonate accumulation began in both basins during the Late Jatulian. Carbonate rocks in the Onega succession are predominantly dolostones, including stromatolite varieties; in the Pana-Kuolajarvi succession, they comprise both dolostones and limestones. During the Late Jatulian, cyanobacteria thrived in coastal marine settings of the Onega Basin. Some areas of the basin lost connection with the open sea at this time, facilitating the development of evaporation processes. The Pana-Kuolajarvi Basin lacked such diversity of cyanobacterial communities and any evaporitization. The geological and lithological data suggest that shallow, lagoon, playa lake, and sabkha environments existed in the Onega Basin in the Late Jatulian. The Pana-Kuolajarvi Basin was characterized by shallow (with occasional increased water input from the continent) and open marine settings. The geochemical characteristics of carbonate rocks obtained in our work lead to the same facies conclusions. Stromatolites in the Onega Basin were formed mainly in the intertidal zone. Occasionally, the connection of basin with the open sea was reduced and the conditions approached the lagoonal type. The oxic conditions existed for a limited time during the Jatulian only in the Onega Basin. Basically, the oxygen content in both sedimentation basins was close to the disoxic/oxic transition boundary. Fluctuations in the Ce anomaly magnitude in stromatolite laminas reflect changes in the oxygen content in water at immediate contact with the stromatolite buildup, suggesting the existence of oxygen “oases” in the basin with disoxic and oxygen-deficient conditions. Conclusions about redox conditions existing in the basin, based only on geochemical markers, are not sufficient, whereas conclusions about the carbonate sedimentation conditions prevailing in the basin agree maximally with the geological and lithological data based on several geochemical characteristics, such as REE spectra, Ce and Eu anomaly values, and ratios of individual REEs supported by the pair correlations between redox-sensitive elements.