Geology of the San Isidro iron ore deposit,Venezuela

The iron ore deposits of Cuadrilatero Ferrifero de San Isidro represent the largest iron ore reserves in Venezuela. The district is a part of the iron metallogenic province of northern Guayana, one of the richest iron-bearing regions of the world. All presently known iron ore deposits of Venezuela are situated within this province: Cerro Bolivar, Altamira, Rondon, San Isidro, María Luisa, El Pao and others. Their total ore reserves amount to 2,000 million tons (disregarding the unenriched or slightly enriched iron-formation). The Imataca belt to which the iron ore deposits are confined consists of metamorphosed sedimentary and igneous rocks of Early Precambrian age, the oldest rocks presently known in South America. This belt extends some 450 km from the Orinoco delta southwesterly to the Cauro River. Iron ore is formed from banded iron-formation, a member of the Imataca complex, by removal of silica. The process of supergene enrichment is controlled to a certain degree by structural elements. There are five ore bodies in the San Isidro district, extremely varied in shape and size. Single bodies extend up to 3–4 km in length, approximately parallel to the regional structure pattern, and a few hundred meters in width. The morphology of the bottom of the ore bodies is rather irregular, particularly in transversal sections. Contacts between ore and the unaltered iron-formation beneath are gradational. Maximum vertical section through ore is 260 m; the average is 60 m approximately. The stratigraphic thickness of iron formation has been magnified by structural deformations. The primary stratigraphic thickness is estimated to be some 50–150 m. The iron ore is classified into two main types: a) hard, crustal ore, b) soft, friable ore. Hematite grains which remained after the leaching of silica, and goethite (as cement) are the two main constituents of crustal ore. Hematite and magnetite and a minor amount of quartz are almost the only constituents of friable ore. The crustal ore forms a 15–60 m thick mantle covering friable ore. The overall volume ratio between the friable and the crustal ore is about 2:1. However, it varies in different zones. The mean composition of iron ore on the basis of 10,800 chemical analyses is 64.41% Fe, 2.62% SiO₂, 0.6% Al₂O₃. The ore contains a minor amount of Mn, P, Ti (no S, As, Ba). The ore reserves amount to 750 million tons; in addition, 180–300 million tons of possible ore reserves are estimated.

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Zusammenfassung Die Eisenerzlagerstätten der Cuadrilatero Ferrifero de San Isidro beinhalten die größten Eisenerzreserven in Venezuela. Der Erzbezirk ist ein Teil der reichsten Eisenerzregionen der Welt. Alle bekannten Eisenerzvorkommen Venezuelas befinden sich in dieser Provinz (Cerro Bolivar, Altamira, Rondon, San Isidro, Maria Luisa, El Pao und andere). Die Gesamtvorräte werden auf etwa zwei Milliarden Tonnen geschätzt (ohne die nichtangereicherten oder nur wenig angereicherten Eisenquarzite). Die Imataca-Zone, an die die Eisenerzvorkommen angrenzen, besteht aus metamorphosierten sedimentären und magmatischen Gesteine des Archaikums, die ältesten bisher in Südamerika bekannten Gesteine. Die Imataca-Zone erstreckt sich ungefähr 450 km vom Delta des Orinoco in südwestlicher Richtung bis Rio Cauro. Die Eisenerze entstanden aus feingeschichteten (gebänderten) Eisenquarziten (Itabirite). Die Prozesse der deszendenten Anreicherung werden teilweise durch strukturelle Elemente bedingt. Fünf Erzkörper des San Isidro-Bezirks sind bekannt. Die Lagerstätten sind 3 bis 4 km lang und einige Hunderte Meter breit. Sie sind den regionalen Strukturen vorwiegend parallel gelagert. Die Morphologie der Erzkörperunterlage ist ziemlich unregelmäßig, besonders senkrecht zum Streichen. Der Kontakt zwischen dem Erz und den unterliegenden unveränderten Eisenquarziten ist stufenförmig. Das Erz ist durchschnittlich etwa 60 m mächtig, mit maximalen vertikalen Mächtigkeiten von 260 m. Die primäre stratigraphische Mächtigkeit des Eisenquarzites wurde durch strukturelle Deformationen vergrößert. Man kann die primäre Mächtigkeit auf 50–150 m schätzen. Das Eisenerz wird in zwei Typen klassifiziert: a) hartes Krustenerz, b) weiches, bröckeliges Erz. Die Hämatitkörner, die nach der Entfernung der Kieselsäure übrig blieben nebst Goethit (als Zement), sind die zwei wichtigsten Komponenten des Krustenerzes. Das weiche Erz enthält Hämatit, Magnetit und etwas Quarz. Das Krustenerz bildet eine 15–60 m mächtige Decke über dem bröckeligen, weichen Erz. Das Gesamtvolumenverhältnis zwischen dem weichen und harten Erz ist ungefähr 2:1. In anderen Zonen ist es jedoch unterschiedlich. Die durchschnittliche Zusammensetzung des Eisenerzes ist: Fe 64,41%, SiO₂ 2,62%, Al₂O₃ 0,6%; das Erz enthält auch etwas Mn, P, Ti (kein S, As, Ba). Die Eisenerzvorräte wurden auf 750 Millionen Tonnen berechnet, wozu wahrscheinlich weitere 180–300 Millionen Tonnen kommen.

     

Zur Kenntnis der Kalkinkrustation und Kalkkorrosion im Seelitoral

Ohne Zusammenfassung     

Equatorial flattening and principal moments of inertia of the earth

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Deflections of the vertical on the territory of Europe from satellite data

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The influence of raw domestic sewage on density and distribution of meiofauna

The influence of raw domestic sewage on density and distribution of meiofauna in the sea bottom sediments in the Northern Adriatic Sea, in the area of Rovinj (Yugoslavia), was investigated, comparing meiofaunal density at the stations which are under direct influence of sewage and the stations which are far from the source of pollution. It is concluded that raw domestic sewage does not have a negative influence on the density and distribution of meiofauna, but Parker's index of pollution indicates that meiofauna at these stations were under stress.     

Laboratory investigations on fault plane induced tensile cracks

Summary Tensile cracks induced by stress concentration around a fault plane in physical models under uniaxial load were analyzed. The results characterizing the decisive role of the fault plane inclination to the the stress direction are presented. Two stages of tensile crack life were determined, i.e. the first, seismoactive stage, accompanied by seismic energy release and the second aseismic one characterized by low velocity of crack propagation. Results obtained seem to be relevant to a better understanding of the character and regime of actual seismoactive faults.     

Anomalous behaviour of seismic waves at an interface between anisotropic media

Summary An account of possible anomalous effects in reflection and refraction of elastic waves at an interface between anisotropic media is presented. These effects are due to anisotropy and they cannot occur at an interface between isotropic media. The shape of the slowness surface (its local deviations from spherical symmetry) is the decisive factor for appearance of these effects. A numerical example of such anomalous behaviour of elastic waves at a free boundary of the crystal of spinel is presented.     

Coronal interconnection of two active regions observed in 3.5–8.0 keV X-rays

Using HXIS data, we have studied the further development of the coronal arch extending towards the SE above active region No. 17255 in November 1980. The arch, studied originally by vestka (1984) disappeared on 10 November (after pronounced revival), but since 9 November HXIS revealed another arch-like structure towards the SW. We have studied the development of this new structure which appeared to be most likely an arch interconnecting AR 17255 with AR 17251, located nearly 30° to the west. This interconnection revived many times during the following days with intensity varying with the activity in both active regions. We have estimated the physical characteristics in this coronal structure and compared them with observations of interconnecting loops made at lower energies by Skylab in 1973. The temperature (maximum values 7.5–14 × 10⁶ K) and the density (1.1–5.0 × 10⁹ cm^–3) are found to be higher than in the Skylab loops (a result that could be expected because the HXIS energy range was harder than that of Skylab) and similar to the values deduced for the earlier arch system extending to the SE. However, much shorter decay times of the brightness variations indicate the presence of conduction in contrast to the SE arch in which conduction was clearly inhibited. This supports the assumption that the SE and SW coronal structures were two different phenomena.Presently at Delft Institute of Technology, Landbergstraat 3, 2628 CE Delft, The Netherlands.