2013

Genesis of molybdenite mineralization fluids: Significance from fluid inclusions studies in quartz veins and hosting  Gattarian granites,

 north Eastern Desert, Egypt.

Bataa H Ali and Abdel Aziz Abdel Warith

Nuclear Materials Authority

P.O. Box 530 El-Maadi, Cairo, Egypt

Fluid inclusion studies can contribute in the understanding of physico-chemical conditions controlling thegenesis of molybdenite mineralization, and to suggest the fluid evolution model of these mineralizations. Both quartz veins and their hosted granites in the Gabal Gattar area are choicen in the present study.

Taking all available information into consideration, the following model of fluid evolution is suggested. There are two stages of mineralizing fluids. The oldest recorded fluid is represented by remnants that are only preserved in granitic samples as a result of strong acidic hydrothermal fluids where oxidation of molybdenite to ferro-molybdenite (Fe₂(Mo₄)₃.H₂O) in granite is significance. These fluids are rich of water of pure NaCl system, low saline (1.73 to 11.70 wt% NaCl eq.) and with homogenization temperature (Th) values around 200°C. The tectonic history of the region and oxidation patterns confirm the fluid inclusion data that oxidation may have begun at high pressure 19.7 k.bar and reach to about 5.7 k.bar

However the pH can remain above the stability field of Mo when alkaline hydrothermal solution are affected the granites and alternatively it will be free to migrate and supergene enrichment of Mo in the form of molybdenite, will apply where there are only minor amounts of pyrite and increasing in H₂S fugacity then precipitate along the week fractured quartz vein surfaces. The detailed fluid inclusions study in the molybdenite bearing-quartz veins is in harmony with the previous modeling where it indicates that the last fluids are homogenized at lower temperature (ranging from 126.7 to 170°C) due to cooling with addition of some divalent salts (MgCl₂ ± CaCl₂) as a result of wall-rock interaction. These fluids are generated under lower pressure (from 4 to 5 k. bar) and characterized with high salinity (14.7 to 23.3 wt% NaCl eq.). Finally it can be concluded that subsequent cooling and change in the pH are considered the two factors that have triggered molybdenite precipitation.