HYPERMAG; an interactive, 2- and 2 1/2-dimensional gravity and magnetic modeling program; version 3.5

The Hajar prospect is located in the Guemassa Paleozoic massif, about 30 km southwest of Marrakesh, Morocco. Visean volcano‐sedimentary formations are present in this massif and in the Jebilets massif north of Marrakesh. In these formations, syngenetic massive sulfides occur, and one of these bodies, Kettara in the Jebilets, has been partially mined. Using the Kettara magnetic anomaly to establish the magnetization parameters, we performed a preliminary interpretation of the Hajar anomaly. Our 2.5-D model determined the depth of the structure to be about 150 to 200 m below the surface. This relatively shallow depth was a decisive factor in siting a reconnaissance drillhole, which encountered massive sulfides between 158 and 276 m. After completing a systematic gravity and magnetic survey, 3-D magnetic modeling was attempted, constrained by the results of four drillholes. Now that more than 20 holes have been drilled, this magnetic model still conforms to the newly revealed geology. Unfortunately, it was not possible to extract useful information concerning the orebody from modeling and interpreting the gravity data. The gravity response is masked by disturbances such as faults and variations in depth and nature of the Visean basement. The discovery of the Hajar deposit shows that magnetic investigations, improved by pertinent modeling techniques, can be used at various stages of exploration to help recognize and define massive sulfide bodies.

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Gravity and Magnetic Modeling of Polyhedron and Iterative SVD Applied to Regional-Residual Gravity Data Separation

2nd Conference on Geophysics for Mineral Exploration and Mining ◽

10.3997/2214-4609.201802699 ◽

2018 ◽

Author(s):

E. Amarante ◽

E. Sampaio

Keyword(s):

Gravity Data ◽

Residual Gravity ◽

Magnetic Modeling ◽

Gravity And Magnetic ◽

Data Separation

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Earthquakes and Plutons in the Midcontinent - Evidence From the Bloomfield Pluton, New Madrid Rift Complex

Seismological Research Letters ◽

10.1785/gssrl.58.2.41 ◽

1987 ◽

Vol 58 (2) ◽

pp. 41-52 ◽

Cited By ~ 9

Author(s):

D. N. Ravat ◽

L. W. Braile ◽

W. J. Hinze

Keyword(s):

Country Rock ◽

Three Dimensional ◽

The Body ◽

Seismic Zone ◽

Gravity And Magnetic ◽

Magnetic Model ◽

Dimensional Gravity ◽

Constant Distance ◽

Gravity And Magnetic Anomaly ◽

New Madrid

Abstract A three-dimensional gravity and magnetic model of the Bloomfield Pluton near New Madrid, Missouri, has been generated that provides an excellent match to the observed gravity and magnetic anomaly maps. Although this modeling is inherently non-unique, the steep gradients on the flanks of the anomalies constrain the causative body to a relatively shallow depth (≈2 km) and define the outline of the steep-sided (near-vertical) pluton. Comparison of the configuration of the Bloomfield Pluton with concentrated microearthquake seismicity in part of the New Madrid Seismic Zone indicates that earthquakes in the northern part of the zone occur primarily on the southeastern side of the pluton at about 10 km from the edge of the body. Based on the location of hypocenters and earthquake focal mechanisms, there is no evidence that stress concentration associated with the mass excess of the pluton is the cause of earthquakes near the pluton. The seismicity may be along zones of weakness independent of the pluton. Among other explanations investigated to explain the constant distance of earthquakes from the pluton, we favor the possibility of a zone of rigidity contrast caused by the thermal metamorphism of the country rock associated with the pluton intrusion. We have estimated increased temperatures of the country rock using an instantaneous intrusion model. Maximum anomalous country rock temperatures of greater than 100°C dominated the region within 10 km of the pluton edge. This ‘thermally metamorphosed’ aureole may provide sufficient homogeneity in the upper crust to localize anomalous stresses away from the margin and along the most favorable zones of weakness, thus influencing the locations of earthquakes.

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