Magnetometer survey in the Jebel Idsas area, Saudi Arabia
A ground magnetometer investigation in: the southern part of the Jebel Idsas magnetite area, Kingdom of Saudi Arabia, was done in compliance to requests by D. R. Mabey and F. Keller to delineate the source of the aeromagnetic total-intensity anomaly that was interpreted as being the expression of a commercial magnetite deposit. Results of the investigation indicate that a zone of magnetite more than 1700 meters in length lies beneath southward protruding ridges and the adjoining margin of a wadi in the central part of the area; and that a small zone of less economic importance occurs beneath a low ridge to the east. Within these zones, the magnetite probably occurs as lenticular masses, stringers, and veins, which aggregate in substantial quantities that constitute the richer parts and are marked by magnetic anomalies. The main zone is inferred to be the source of the total -intensity anomaly and is several hundred meters longer than is indicated by the wide-spaced aeromagnetic profiles. This zone averages about 75 meters in width. It may contain as much as 50 percent magnetite, however an estimate of the tonnage represented cannot be made reliably without more complete subsurface information. A study of drilling sites in the area reveals that most of the holes are along the south edge of the main zone and were not properly located to test the richer parts as indicated by our magnetic data. Drill hole N 40, which was abandoned in November owing to broken drilling equipment, is properly located to test the richest part of the main magnetite-bearing zone. We recommended that this hole be completed and that the resulting subsurface information be used as a guide in selecting additional drilling sites along the trend of the anomaly.