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Acid-sulphate alteration comprising of argillic and advanced argillic alteration is common in South Um Monqul area, North Eastern Desert, Egypt. The alteration is exclusively hosted by NE-SW and NW-SE sheared volcanic rocks (dacite and rhyodacite porphyry) as well as intrusions (sub-porphyritic biotite granite and granite porphyry). Argillic alteration is dominated by clay minerals that are usually associated with phyllic alteration. Advanced argillic alteration, on the other hand, comprises quartz, alunite, clay minerals, pyrophyllite and specularite. Mineralization linked to advanced argillic alteration is represented by gold with the high sulphidation minerals enargite, calcocite and covellite. The high sulphur content of acid sulfate alteration is further emphasized by the common presence of barite veins in the examined area.

It is envisaged that volatiles rich in SO2 degassed from the nearsurface intrusions, which were unable to re-equilibrate with the country rocks along their relatively short path of ascent. As a result, the SO2 produced sulphuric acid in conjunction with meteoric water. The interaction of the sulphuric acid solution with the wall rock resulted in the advanced argillic alteration.

The recognition of such alteration in the Eastern Desert is of prime importance where it can be used, as a shallow level indicator of a possible deeper porphyry system, in any exploration program.