Chapter 24: Muruntau, Uzbekistan: The World’s Largest Epigenetic Gold Deposit

2020 ◽  
pp. 497-521
Author(s):  
Reimar Seltmann ◽  
Richard J. Goldfarb ◽  
Bo Zu ◽  
Robert A. Creaser ◽  
Alla Dolgopolova ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Hydrothermal Activity ◽  
Tien Shan ◽  
The South ◽  
Thermal Event ◽  
Tectonic Transition ◽  
Fold And Thrust Belt ◽  
Order Of Magnitude ◽  
Metamorphic Terranes ◽  
East West

Abstract Muruntau in the Central Kyzylkum desert of the South Tien Shan, western Uzbekistan, with past production of ~3,000 metric tons (t) Au since 1967, present annual production of ~60 t Au, and large remaining resources, is the world’s largest epigenetic Au deposit. The host rocks are the mainly Cambrian-Ordovician siliciclastic flysch of the Besapan sequence. The rocks were deformed into a broadly east-west fold-and-thrust belt prior to ca. 300 Ma during ocean closure along the South Tien Shan suture. A subsequent tectonic transition was characterized by left-lateral motion on regional splays from the suture and by a massive thermal event documented by widespread 300 to 275 Ma magmatism. The Besapan rocks were subjected to middle to upper greenschist-facies regional metamorphism, an overprinting more local thermal metamorphism to produce a large hornfels aureole, and then Au-related hydrothermal activity all during early parts of the thermal event. The giant Muruntau Au deposit formed in the low-strain hornfels rocks at ca. 288 Ma at the intersection of one of the east-west splays, the Sangruntau-Tamdytau shear zone, with a NE-trending regional fault zone, the Muruntau-Daugyztau fault, which likely formed as a cross fault during the onset of left-lateral translation on the regional splays. Interaction between the two faults opened a large dilational zone along a plunging anticlinorium fold nose that served as a major site for hydrothermal fluid focusing. The Au ores are dominantly present as a series of moderately to steeply dipping quartz ± K-feldspar stockwork systems surrounding uncommon central veins and with widespread lower Au-grade metasomatites (i.e., disseminated ores). Pervasive alteration is biotite-K-feldspar, although locally albitization is dominant. Sulfides are mainly arsenopyrite, pyrite, and lesser pyrrhotite, and scheelite may be present both in preore ductile veins and in the more brittle auriferous stockwork systems. The low-salinity, aqueous-carbonic ore-forming fluids probably deposited the bulk of the ore at 400° ± 50°C and 6-to 10-km paleodepth. The genesis of the deposit remains controversial with metamorphic, thermal aureole gold (TAG), and models related to mantle upwelling all having been suggested in recent years. More importantly, the question as to why there was such a focusing of so much Au and fluid into this one location, forming an ore system an order of magnitude larger than other giant Au deposits in metamorphic terranes, remains unresolved.

Borges the Post-Orientalist

2017 ◽  
Author(s):  
Robin Fiddian
Keyword(s):  
South Atlantic ◽  
Conclusive Evidence ◽  
Binary Opposition ◽  
The South ◽  
Civilization And Barbarism ◽  
The Stranger ◽  
East West

The chapter examines several works including ‘The East’, ‘A Thousand and One Nights’, and ‘Buddhism’, which are on subjects relating to the East, and finds conclusive evidence of a post-Orientalist optic in Borges’s writing at this point in his life. Japan inspires ‘The Stranger’ and ‘Nihon’, both included in The Limit and outstanding examples of Borges’s wit and craftsmanship. A comparison between ‘Nihon’ and ‘Story of the Warrior and the Captive Woman’ from an earlier collection illustrates Borges’s evolved approach to the binary opposition between civilization and barbarism, across the East–West divide. Two milongas are also studied in the context of the war in the South Atlantic (1982) and point to a disillusioned conclusion about Argentina 170 years on from formal independence in 1810.

Structure and tectonic evolution of the South Patagonian fold and thrust belt: Coupling between subduction dynamics, climate and tectonic deformation

2019 ◽  
pp. 675-697
Author(s):  
Matías C. Ghiglione ◽  
Gonzalo Ronda ◽  
Rodrigo J. Suárez ◽  
Inés Aramendía ◽  
Vanesa Barberón ◽  
...  
Keyword(s):  
Tectonic Evolution ◽  
Tectonic Deformation ◽  
Thrust Belt ◽  
The South ◽  
Fold And Thrust Belt

Preliminary analysis of the spectral content of P and S waves from local earthquakes in the Garm, Tadjikistan region

1978 ◽  
Vol 68 (4) ◽  
pp. 949-971
Author(s):  
T. G. Rautian ◽  
V. I. Khalturin ◽  
V. G. Martynov ◽  
P. Molnar
Keyword(s):  
Seismic Moment ◽  
Tien Shan ◽  
Wave Spectra ◽  
The South ◽  
Spectral Content ◽  
Preliminary Results ◽  
Local Earthquakes ◽  
Calculated Stress ◽  
Stress Drops ◽  
Lower Frequencies

abstract This paper reports preliminary results of an analysis of the spectral content of seismic waves from over 1,000 local earthquakes in the Garm, Tadjikistan region. Very low values of Q (∼100) were obtained for the Mesozoic and Cenozoic sedimentary rocks of the Peter I Range compared with those in the crystalline rock of the South Tien Shan (Q ≳ 500). For events with the same low frequency spectra, earthquakes in the South Tien Shan are recorded with more energy at higher frequencies than earthquakes in the Peter I Range, at least in part, because of the greater attenuation of the sedimentary rock in the latter region. There is a wide variety of spectra radiated by earthquakes throughout the Garm region, but a marked difference between spectra radiated by earthquakes from these two regions is not apparent. Nevertheless although calculated stress drops of earthquakes appear to vary considerably throughout the region, there appears to be a dependence on the type of rock in which the earthquakes occurred. The variation in calculated stress drops, however, is greater within either region than between the two. For most regions and for most of the range of seismic moments studied the shape of the spectrum is relatively independent of seismic moment. Consequently, calculated stress drops increase with seismic moment, with the functional dependence varying from region to region. For the largest events, the spectrum shifts to lower frequencies with increasing moment and stress drops become essentially independent of moment. Preliminary results suggest that the stress drops are not detectably different for earthquakes with thrust or normal faulting fault-plane solutions. A limited amount of data are consistent with a change in spectral content and stress drop of earthquakes in the vicinity of and before stronger earthquakes but a clear, consistent pattern is not yet evident. Toward high frequencies, S-wave spectra both begin to decrease at lower frequencies and seem to decrease more rapidly than P-wave spectra, in contrast with predictions of published dislocation models of earthquakes.

The arc of the Western Alps : understanding its kinematics and formation mechanisms based on new structural and paleomagnetic datas, and thermo-mechanical modelling.

2021 ◽  
Author(s):  
Quentin Brunsmann ◽  
Claudio Rosenberg ◽  
Nicolas Bellahsen ◽  
Laetitia Le Pourhiet
Keyword(s):  
Western Alps ◽  
Strike Slip ◽  
Rheological Parameters ◽  
The South ◽  
Adriatic Plate ◽  
The North ◽  
Field Evidence ◽  
Internal Zone ◽  
The Alps ◽  
East West

<p>The Alps have an overall East-West orientation, which changes radically in their western termination, where they rotate southward into a N-S strike, and then eastward into an E-W strike, forming the arc of the Western Alps. This arc is commonly inferred to have formed during collision, due to indentation of the Adriatic plate into the European continental margin. Several models attempted to provide a kinematic explanation for the formation of this arched, lateral end of the Alps. Indeed, the radial nature of the transport directions observed along the arc of the Western Alps cannot be explained by a classic convergence model.<br>For more than 50 years the formation of this arc was been associated to westward-directed indentation of Adria, accommodated along East-West oriented strike-slip faults, a sinistral one in the South of the arc and a dextral one in the North. The dextral one correspond to the Insubric Fault. The sinistral strike-slip zone, inferred to be localized along the «Stura corridor» (Piedmont, Italy) would correspond to a displacement of 100 to 150 km according to palaeogeographical, and geometric analyses. However, field evidence is scarce and barely documented in the literature.<br>Vertical axis rotations of the Adriatic indenter also inferred to be syn-collisional could have influenced the acquisition of the morphology of the arc. Paleomagnetic analyses carried out in the Internal Zone and in the Po plain suggest a southward increading amount of counter-clockwise rotation of the Adriatic plate and the Internal Zone, varying from 20°-25° in the North to nearly 120° in the South.<br>Dextral shear zones possibly accommodating this rotation in some conceptual models is observed in several places below the Penninic Front and affect the Argentera massif to the south. However, the measured displacement quantities do not appear to be equivalent to those induced by such rotations.<br>The present study aims to constrain the kinematic evolution of the arc of the Western Alps through a multidisciplinary approach. The first aspect of this project is the structural analysis of the area (Stura corridor) inferred to accommodate large sinistral displacements allowing for the westward indentation of the Adriatic indenter. We discuss the general lack of field evidence supporting sinistral strike-slip movements, in contrast to large-scale compilation of structures suggesting the possible occurrence of such displacement. The second part consists of a palaeomagnetic study, in which new data are integred with a compilation of already existing data. This compilation shows that several parts of the arc in the External Zone did not suffer any Cenozoic rotations, hence suggesting that a proto-arc already axisted at the onset collision, as suggested by independent evidence of some paleogeographic reconstruction. Finally, 2D and 3D thermo-mechanical modeling in using the pTatin3D code is used to test which structural (geometrical), and rheological parameters affected the first-order morphology of the Western Alpin arc and its kinematics. The synthesis of these different approaches allows us to propose a new model explaining the kinematics and the mechanisms of formation of the Western Alps arc.</p>

Anisotropy of out-of-phase magnetic susceptibility due to eddy currents in graphite ore and its structural implications

2021 ◽  
Author(s):  
František Hrouda ◽  
Jan Franěk ◽  
Martin Chadima ◽  
Josef Ježek ◽  
Štěpánka Mrázová ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Eddy Currents ◽  
Regional Metamorphism ◽  
Ductile Deformation ◽  
Dynamic Susceptibility ◽  
Phase Component ◽  
Simultaneous Control ◽  
Order Of Magnitude ◽  
Moldanubian Unit ◽  
Sufficient Precision

<p>Magnetostatic susceptibility of single crystals of graphite is negative (the mineral is diamagnetic) and strongly anisotropic. The in-phase component of dynamic susceptibility (measured in alternating magnetic field) is also negative, but an order-of-magnitude stronger than the magnetostatic susceptibility. The out-of-phase component, which is no doubt due to electrical eddy currents, is positive and strong. Consequently, if the graphite crystals in graphite ore are oriented preferentially by crystal lattice (LPO), one would expect strong anisotropy of magnetic susceptibility (AMS) of graphite ore in both in-phase (ipAMS) and out-of-phase (opAMS) components. The ipAMS is controlled not only by the LPO of graphite, but also by the preferred orientation of paramagnetic and ferromagnetic minerals of the barren rock, while the opAMS indicates only the LPO of graphite. In graphite ores occurring in the Moldanubian Unit of Southern Bohemia, the in-phase susceptibility ranges from negative values in the order of 10<sup>-5</sup> [SI units] to positive values in the order of 10<sup>-4</sup>. This probably indicates simultaneous control by graphite and paramagnetic and/or ferromagnetic minerals. On the other hand, the out-of-phase susceptibility is much higher, in the order of 10<sup>-4</sup>, and no doubt indicates its graphite control. The degree of ipAMS is moderate, that of opAMS is truly high. The ipAMS foliation is roughly parallel to the metamorphic foliation in ores and wall rocks and the ipAMS lineation is parallel to the mesoscopic lineation. The opAMS is inverse to the ipAMS with the opAMS lineation being perpendicular to the metamorphic foliation. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation. The opAMS has therefore shown an effective tool for the investigation of the LPO of graphite in graphite ore or graphite-bearing rocks provided that the opAMS is strong enough to be determined with sufficient precision and graphite is the only conductive mineral in the samples investigated.</p>

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