Paleomagnetic investigations at Mahd adh Dhahab, Kingdom of Saudi Arabia

Open-File Report 85-15




Paleomagnetic studies of 25 oriented bedrock specimens from Jabal Mahd adh Dhahab, located 160 km southeast of Al Madinah, Kingdom of Saudi Arabia, have yielded important structural information relating to the geologic history of this base and precious metal deposit. Samples were collected along one traverse away from the mineralized zones, across the northeast-plunging antiform that constitutes the dominant regional structure of the area, and along another traverse down the axis of the mineralized zones. Lithologies range from andesite flows through andesitic to rhyodacitic tuffs, lapilli tuffs, and volcaniclastic sediments. Measurements of remanent magnetization direction and intensity before and after stepwise alternating-field demagnetization ranging from 25 Oersted (Oe) to 1000 Oe were carried out on all samples. Three classes of demagnetization behavior were observed: A very stable class with little change in direction and intensity of magnetization; a class in which rapid changes were observed at first but which then settled on a high coercivity stable component of magnetization; and a class in which the magnetization was composed of a spectrum of low coervicity components, and continuous variation of direction and intensity of magnetization occurred.

A structural correction restoring the bedding to horizontal resulted in a significant reduction in dispersion of magnetization directions, and implies that the magnetization was acquired before the northeast-trending deformation. At least one geomagnetic reversal is recorded and, together with drillhole electric logs, suggests structural repetition of some parts of the area.

The determined paleopole location, after correction for Tertiary rotation of Arabia from Africa, falls on the apparent polar wander path for Gondwanaland at an age of 600-590 Ma, which is younger than the mineralization age of about 680 Ma and the age of the Mahd group of rocks (772 Ma) determined from geochronologic studies, and implies that the rocks have been remagnetized during the middle Pan African times.

The results of this work suggest that more extensive paleomagnetic investigations of the felsic volcanic and volcanic-derived rocks of the northern Arabian shield should provide greater insight into the structural evolution of the shield, and these investigations are recommended.

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USGS Numbered Series
Paleomagnetic investigations at Mahd adh Dhahab, Kingdom of Saudi Arabia
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Open-File Report
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U.S. Geological Survey
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Reston, VA
i, 43 p., ill., map: 28 cm.
Saudi Arabia
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