ISSN:
1420-9136
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
,
Physics
Description / Table of Contents:
Summary The present study is mainly concerned with the quantitative interpretation of aeromagnetic anomalies as an aid in the search of high grade magnetic taconites in the central portion of the Labrador Trough. A detailed survey of the volume ferromagnetic susceptibility as well as intensity and orientation of the NRM component allowed a much improved interpretation of these surveys. It is found that a major portion of the NRM component is of depositional nature and the remaining portion mainly of viscous nature and of chemical origin (partial chemical alteration of the iron formation). The orientation of the paleopoles is almost perpendicular to the present orientation of the earth's magnetic field. At least one reversal of the poles was observed within each member of the iron formation and the general trend indicates that with decreasing time (coming towards the younger members) the azimuth of the pole is moving towards the east and the plunge above the equator to below the equator. At the time of deposition in Precambrian time (early Proterozoic era), the north paleopole was oriented at 263° with respect to the astronomical north and plunged 9° above the equator. At that time, the earth's magnetic north pole was pointing in central Pacific ocean in the vicinity of the Christmas Islands and thus the iron formations were deposited in tropical regions. This study clearly indicates that the NRM component cannot be neglected in any kind of interpretation of magnetic surveys. An example shows that the estimated grade and attitude of an elongated magnetic mass are substantially altered by the presence of the NRM component whereas the volume and tonnage are not, but are mainly function of the geometric model used. It is shown that the grade and the volume can be estimated with a reasonable accuracy by proper interpretation of the aeromagnetic anomalies and a very selective choice of the magnetic masses of economic interest can then be made. Combined with an heliported magnetic and electromagnetic survey, a complete appraisal and estimate of the anomalies of economic interest is rendered possible and no expensive geophysical or geological ground follow-up is necessary; the anomalies selected are then consequently drilled.
Notes:
Résumé Le principal but de cette étude consiste à évaluer le potentiel des dépôts de taconites magnétiques rentables dans un rayon de 40 milles à partir de Schefferville, partie centrale de la fosse du Labrador. On s'est rendu compte que la composante du magnétisme rémanent naturel (MRN) est relativement importante dans les formations de fer de la région de Knob Lake. Dans la recherche de gisements de taconite magnétique rentable à partir de relevés aéromagnétiques, il faut tenir compte de la composante du magnétisme rémanent si l'on désire obtenir une image exacte des dimensions, de l'attitude, du volume (tonnage), du moment magnétique induit, car le moment magnétique rémanent est généralement indépendant de la concentration des minéraux magnétiques dans les roches. On discute de la collection et de l'orientation des échantillons, de l'interprétation des résultats corrigés des relevés magnétiques. Une étude détaillée de l'intensité et de l'orientation du MRN permet d'arriver à des conclusions importantes relatives au paléomagnétisme de la fosse du Labrador.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00875959
Permalink
