Current climate change effects on the ground thermal regime in Central Yakutia

The evolution of ground thermal state has been studied to assess impacts of current climatic warming on permafrost in Central Yakutia. The analysis of long-term data of regional weather stations has revealed one of the highest increasing trends in mean annual air temperature in northern Russia. A forecast of surface air temperature fluctuations has been made by applying a frequency analysis method. Monitoring of ground thermal conditions allows us to identify inter-annual and long-term variability among a wide range of natural conditions. Experimental research has indicated a long-term dynamics of ground thermal state evolution： ground temperatures at the depth of zero annual amplitude and seasonally thawed layer depth. Long-term variability of thaw depth shows near-zero to weak positive trends in small valleys in contrast to weak negative trends on slopes. With significant climatic warming, the thermal state of near-surface layers of permafrost demonstrates steadiness. Anthropogenic impacts on ground thermal regime in various terrain types have been qualitatively evaluated. Clear-cutting, ground cover stripping, and post-fire deforestation in inter-alas type terrains result in a significant increase of temperature and seasonal ground thaw depth, as well as adverse cryogenic processes. The dynamics of mean annual ground temperature in slash and burn sites have been evaluated in reference to stages of successive vegetation recovery.     

A new finding of Cu-Au alloy in association with rodingite minerals in the Kaa-Khem ophiolitic belt,Tuva

A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks. Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite (2–18% grossular component) and diopside (X _Fe = 0.01–0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside (X _Fe = 0.14–0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07–13.67 wt % Cu, 0.48–1.56 wt % Zn, and 0.76–1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2–6.4 wt % Ag). They contain veinlet-shaped AuCu zones with 13.3–14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close to auricupride (AuCu₃). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation was estimated at 350–600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem ophiolitic belt.     

Au-Cu-Ag mineralization in rodingites and nephritoids of the Agardag ultramafic massif (southern Tuva,Russia)

Gold-bearing albite-amphibole-pyroxene rodingites of the Agardag ultramafic massif (southern Tuva, Russia) are confined to the E-W striking serpentinite crush zone. A zone of gold-bearing nephritoids is localized at the contact of rodingites with serpentinites. Optical and scanning electron microscopy, electron probe microanalysis, and fluorescent, chemical, ICP MS, and X-ray phase analyses were applied to study Au-Cu-Ag mineralization in the serpentinites, rodingites, and nephritoids. Copper sulfides, chalcocite and digenite, are present in the serpentinites, whereas gold and silver minerals are absent. Copper impurity is found in antigorite, Cr-spinel, and magnetite (up to 0.1-0.3 wt.%) as well as parkerite (up to 1.2 wt.%) and millerite (up to 7.9 wt.%). A wide variety of native gold and copper minerals has been identified in the rodingites: (1) cuproauride and tetra-auricupride free of or containing silver impurities (0.1 to 1.2 wt.%); (2) electrum of composition Ag0.50_-0.49Au0.50_-0.5₁ (650-660%c) intergrown with AuCu, sometimes as exsolution structures; (3) electrum of composition Ag₀._70-0.6₄Au₀._30-0.₃6 (440-510%c), with inclusions of AuCu and copper sulfides (geerite and yarrowite); (4) high-fineness gold (750-990%c) as veinlets in electrum; and (5) native copper. The composition of copper sulfides varies from chalcocite to covellite. Submicron inclusions of hessite Ag2Te were found in chalcocite. The amount of copper, gold, and silver minerals in the nephritoids is much less than that in the rodingites. The nephritoids contain chalcocite, electrum of composition Ag0.6_4-0.63Au0.36_-0.3₇ (530-540%c), cuproauride, and tetra-auricupride. The detected hypergene minerals are auricuzite, apachite, brochantite, high-fineness gold, native copper, and cuprite. The sequence of mineral formation in the Agardag ore occurrence has been established on the basis of mineral structures and mineral relations in the rodingites and nephritoids. It is proved that Au-Cu-Ag mineralization formed with the participation of Au- and Ag-bearing chloride-free low-sulfur carbon dioxide alkaline fluids in reducing conditions.     

Partial Melting of Plagioclase-Free Garnet–Two-Mica Metapelite As a Model of the Formation of Ultra-Potassic Felsic Magmas under Conditions of the Continental Crust

Doklady Earth Sciences - To study the formation of S-type ultra-potassic granitic melts under the conditions of the continental crust, we conducted experiments on partial melting of...     

Formation of a Ring-Shaped Region of Increased Electron Temperature in the Subauroral Ionosphere in Winter

Geomagnetism and Aeronomy - The results of numerical simulations of the electron temperature (Te) are compared with measurement data provided by the CHAMP satellite to show the possibility of the...     

Gold-Bearing Rodingites of the Agardag Ultramafic Massif (South Tuva,Russia) and Problems of Their Genesis

Geology of Ore Deposits - The limited literature data on gold-bearing albite–pyroxene rodingites are summarized for the Agardag ultramafic massif in southern Tuva. These data are supplemented...     

Mineralogy of Copper in Sulfide-Free Endogenic Pb–Zn–Sb Mineralization of the Pelagonian Massif,Republic of North Macedonia

Geology of Ore Deposits - Behavior of copper in sulfide-free metasomatic ores of the Pelagonian massif, Republic of North Macedonia has been studied. It is shown that the highest copper activity...     

Structural-metamorphic evolution of the Southern Yenisey Range of Eastern Siberia: implications for the emplacement of the Kanskiy granulite Complex

Summary ?The Southern Yenisey Range of Eastern Siberia consists of the granulite facies Kanskiy Complex bordered in the west by the lower-grade Yeniseyskiy and Yukseevskiy Complexes. Three deformational events were recognized in each of the three complexes along the Yenisey River cross-section: a D1 fabric forming event, a D2 shear and folding event, and a D3 shear event. Thrust kinematics across the Southern Yenisey Range suggest that during the D2 event the Kanskiy Complex was thrusted along a regional ductile shear zone onto the lower-grade complexes. This resulted in shearing and folding as well as the development of a dynamic metamorphic zonation. In the low-grade greenstone belt part of the cross section (Yukseevskiy complex) D2 shearing is associated with peak prograde (T ∼ 660 °C and P ∼ 5.8 kbar) metamorphism. The retrograde P-T path of the Yukseevskiy Complex coincides with minimum T of the near-isobaric cooling P-T paths for the adjacent granulites of the Kanskiy Complex (Perchuk et al., 1989). The metamorphism can therefore be attributed to deformation and heat transfer caused by exhumation of the Kanskiy Complex in the time period 2000–1800 Ma which also defines the most significant tectono-thermal event in the Southern Yenisey Range. The tectono-metamorphic pattern and evolution of the low- to high-grade metamorphic complexes of the Southern Yenisey Range is very similar to that described for the ∼ 2600 Ma Limpopo Complex of Southern Africa and the ∼ 1900 Ma Lapland Complex of the Kola Peninsula. Similar geodynamic processes were therefore possibly responsible for the formation of these high-grade terrains suggesting that their formation is linked to a general geodynamic model.

---

Zusammenfassung ?Strukturelle und metamorphe Entwicklung des südlichen Jenissei-Gebirges in Ost-Sibirien: Bedeutung für die Platznahme des Kanskiy Granulit-Komplexes Das südliche Jenissei-Gebirge in Ost-Sibirien besteht aus dem granulit-faziellen Kanskiy Komplex, der im Westen durch die niedrig-gradigen Jenisseiski und Jukseevski-Komplexe begrenzt wird. Drei Deformations-Phasen k?nnen in jedem der drei Komplexe l?ngs eines Profiles am Jenissei-Fluss beobachtet werden: Eine Phase, die zur Entwicklung des D1 Gefüges führte, eine Phase D2 mit Scher- und Faltvorg?ngen und eine D3 Scher-Phase. Die Kinematik von überschiebungen über das südliche Jenissei-Gebirge deuten an, dass w?hrend der D2-Phase der Kanskiy-Komplex l?ngs einer regionalen duktilen Scherzone auf die niedriggradigeren Komplexe überschoben wurde. Dies führte zu Scherung und Faltung, sowie zur Entwicklung einer dynamischen metamorphen Zonierung. In dem niedriggradigen Grünsteingürtel innerhalb des Profils (Jukseevski-Komplex) ist D2-Scherung mit dem H?hepunkt der prograden Metamorphose (T ≈ 660 °C und P ≈ 5,8 kbar) zusammengefallen. Der retrograde P-T-Pfad des Jukseevski-Komplexes f?llt mit der Minimum-Temperatur der fast isobaren Abkühlung der P-T-Pfade für die benachbarten Granulite des Kanskiy-Komplexes zusammen (Perchuk et al., 1989). Die Metamorphose kann deshalb auf Deformation und W?rmefluss zurückgeführt werden, die durch die Freilegung des Kanskiy-Komplexes zwischen 2.00 und 1.80 Ma verursacht wurde; letztere f?llt auch mit der wichtigsten tektono-thermalen Phase im südlichen Jenissei-Gebirge zusammen. Das tektono-metamorphe Muster und die Entwicklung von niedrig- zu hochgradigen metamorphen Komplexen des südlichen Jenissei-Gebirges ?hnelt in vielf?ltiger Weise dem ungef?hr 2.600 Ma alten Limpopo-Komplex im südlichen Afrika und dem 1.900 Ma alten Lappland-Komplex der Kola-Halbinsel. ?hnliche geodynamische Prozesse waren deshalb m?glicherweise für die Entstehung dieser hochgradig metamorphen Terrains verantwortlich; dies wiederum weist darauf hin, dass ihre Entstehung einem allgemeinen geodynamischen Modell entspricht.

---

---

Received April 27, 1999;/revised version accepted July 14, 1999