Mineral-exploration aspects of gravity and aeromagnetic surveys in the Sudbury-Cobalt area, Ontario
Published:January 01, 1985
V. K. Gupta, F. S. Grant, 1985. "Mineral-exploration aspects of gravity and aeromagnetic surveys in the Sudbury-Cobalt area, Ontario", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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A detailed gravity survey comprising approximately 11 800 gravity stations has been carried out in a 33 000-km2 region of the southern Canadian shield between the Sudbury and Kirkland Lake mining camps in east-central Ontario. The purpose of the survey was to shed some light on the economic mineral potential of the less well-explored parts of this very important mining district of Canada.
This article describes an interpretation of gravity and aeromagnetic data in the Sudbury-Cobalt area, the aim of which has been to derive information of exploration interest for base and precious metals. In addition to using standard procedures such as vertical gradient mapping to increase the resolution of the Bouguer gravity data, a new analytical method called the “apparent-density map” has been introduced as a lithologic-mapping tool. The apparent-density algorithm is used to define rock-unit boundaries and to assign average densities to these units for the purpose of preparing a basement-lithology map. The results suggest that the mineral-rich Archean Abitibi greenstone belt, consisting mainly of mafic rocks, extends beneath the Lower Proterozoic sedimentary cover much farther than had previously been realized. The volumes, depth extent, and major stratigraphic subdivisions of Archean greenstones, together with a lithologic map of the Archean basement, all derived from the gravity and magnetic data, are of material benefit in outlining prospective regions for mineral exploration. Three-dimensional gravity models of the Round Lake Batholith and the adjacent steeply dipping mafic and ultramafic metavol-canics suggest a favorable environment for gold exploration. The gravity and magnetic maps have also been helpful in locating possible diabase feeder zones and in determining their volumes and depths—information that is important for outlining potential silver-cobalt prospects.
It is concluded that gravity and aeromagnetic surveys provide an informative, cost-effective approach to regional exploration for mineral-prospective zones in the Sudbury-Cobalt area, and that these surveys should be followed up by direct-search exploration strategies.
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The Utility of Regional Gravity and Magnetic Anomaly Maps
The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features.
The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits.
The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.