Sulfide assemblages and metamorphic episodes at Mahd Adh Dhahab gold mine, kingdom of Saudi Arabia

Abstract EN

The Mahd Adh Dhahab gold mine is loeated in the west central part of the Arabian Shield and has been the most productive gold mine in Saudi Arabia in both ancient and recent times.

Textural and rnineralogical features observed in the ore deposits at Mahd Adh Dhahab indicate that the deposits have been subjected to metamorphism.

Three main effccts were recognized.

Recrystallization of the ores during regional mctamorphism resulted in changes mainly in the fabrics, but also in the mineralogy, of the ores.

With increasing metamorphism; a) there is a general increase in grain size, b) growth of pyrite as porphyroblasts, and c) presence of triple junction point texture.

Deformational effects present varied from none, through brittle cataclasis to ductile deformation.

The features observed include: a) fracturing, brecciation and mylonitization of fine-grained sulfides and porphyroblasts, b) deformation twinning, c) folding and disruption of clcavage traces, d) rotation of pyrite crystals, and e) stretching and elongation of soft sulfides.

Remobilization produced irregular bodies of vein quartz and ore minerals, either within the deposits or in their immediate country rocks.

The remobilization distances are limited, either within parent body or in restricted halo surrounding it (up to some tens of meters).

The remobilization is selective and proceeded by creep or fluid-phase transport.

The apparent order of decreasing mobility is: galena, chalcopyrite, sphalerite, pyrite.

Certain replacement phenomena observed between the euhedral-subhedral pyrite grains and the base metal sulfides could be ascribed to metamorphism.

At Mahd Adh Dhahab area, successive episodes of low grade metamorphism prevailed during a long lime span.

Intermittent with such pervasive regional metamorphism, the ores were subjected to short periods of essentially dynamic and thermal metamorphism.