On Some Regularities in the Relationship Between the Structure of Magnetic Fields and the Geological Structure of Vein-Type Gold Ore Deposits Localized in Intrusive Massifs
Abstract Generated abstract
This paper examines how magnetic field structures relate to the geology of vein-type gold deposits hosted in intrusive massifs, addressing the limited basis for using magnetic surveys in detailed ore-field studies. Drawing on special investigations in several gold ore provinces, it argues that these deposits show anisotropic magnetic fields whose patterns are controlled chiefly by structural features such as blocks, tectonic disturbances, fractures, dikes, and vein concentrations, often requiring statistical analysis of observed or transformed fields to detect. The study identifies several orders of magnetic variations and concludes that ore bodies are commonly expressed as relatively reduced, linearly elongated anomalies or complex anomalous zones, shaped more by near-contact enclosing rocks than by vein material itself.
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On Some Regularities in the Relationship Between the Structure of Magnetic Fields and the Geological Structure of Vein-Type Gold Ore Deposits Localized in Intrusive Massifs
L. Ya. Erofeev (TPI)
Studies in recent years in the field of methods for investigating vein-type gold ore deposits have shown that in a number of cases magnetic surveying can be used to solve a wide range of practically important problems connected with the study of such deposits.
However, its broad application for the detailed study of ore fields is restrained by a number of reasons. The chief of these is that at present there is very scant information on the magnetic fields of these deposits and on the forms of relationship between the structure of magnetic fields and their geological structure.
As a result of special investigations carried out to study these questions in a number of gold ore provinces, we established the following:
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The magnetic fields of the deposits are anisotropic. The character and degree of anisotropy of the field are entirely determined by the structural features of the ore field. This regularity in the relationship of the field to their structure is most often not obvious; it is veiled. It can be established only by statistical analysis of the observed fields, and in a number of cases only of transformed fields. This relationship is due to the fact that tectonic disturbances of all orders, down to the fracturing of rocks, find a noticeable reflection in the magnetic field of deposits.
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The magnetic fields of these deposits are characterized by a noticeable predominance of several types of variations in intensity over all other variations of the vertical component of the intensity of the magnetic field.
The largest of them have dimensions of several hundred meters and an intensity of several thousand gammas. Spatially, they correspond well to the relief of the terrain and are probably caused by the block structure of the ore fields.
Anomalies of the second order accompany tectonic disturbances, which are often filled by dikes of various rocks or bear commercial mineralization. The change in intensity here occurs within an interval from several tens to several ...
thousands of gammas; their linear dimensions (in cross section) range from several meters to the first hundreds of meters.
Changes in the intensity of the vertical component of higher orders, which are observed in places of the greatest concentration of ore veins, reflect the character of the development of the supporting elements. This same class of anomalies includes changes in intensity caused by irregularities of the buried microrelief, and anomalies reflecting the nonuniformity of the magnetic properties of the rocks. The latter play a substantial role in the structure of the magnetic fields of deposits composed of strongly magnetic rocks.
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Ore bodies in the magnetic field of gold deposits, as a rule, are marked by relatively reduced, linearly elongated anomalies or by anomalous zones of complex shape. The change in intensity along the strike of the veins does not remain constant. These parameters are closely related to the morphology of the vein formation. At the same time, it has been established that both the vein material itself and part of the near-contact rocks enclosing the vein participate in the formation of the magnetic anomaly. Moreover, the decisive role in the formation of the anomalies is played by the latter factor.
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In general, the structure of the magnetic fields of gold deposits of this type is determined mainly by the structural makeup of the deposit. The features of the petrographic composition of the enclosing and vein rocks, as well as of the ores, affect only the magnitude of the anomalies.