2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Sn-W-Mo-Be deposits potentiality of the Neoproterozoic Abu Diab highly fractionated calc-alkaline granite, Eastern Desert, Egypt

Abu El-Ela F. *, Lindh A.^**, Gahlan H. *, Abu El-Rus M.*, Mohamed M.*

* Department of Geology, Faculty of Science, Assiut University, Assiut 71516, Egypt

** Department of Geology, Solvegatan12, SE-223 62 Lund, Sweden

abuelelaf@yahoo.com

The Abu Diab granite has an oval shape covering an area of about 50 km­^2. It is discordant with the regional foliation and intruded into the calc-alkaline island arc metagabbro-diorite complex and the synorogenic calc-alkaline granitoid rocks of tonalite-granodiorite composition, as well as mafic layered intrusion. The Abu Diab granite is composed mainly of garnet-bearing muscovite granite and characterized by the presence of magmatic garnet and complete absence of magmatic cordierite. Garnet have higher spessartine content  (>66 mol%) and very low content of grossular (0.3-0.7 mol%)  suggesting the Abu Diab granite intrusion was emplaced at shallow crustal level. Geochemically, it is classified as alkali feldspar granite, highly fractionated high-K calc alkaline, weakly peraluminous (A/CNK= 1.04-1.12), I-type affinity and peraluminous leucocratic granites (Patino Douce, 1999). The Agpaitic Index [AI=  (Na+K)/Al mol%] range from 0.86 to 0.91, average 0.90, characterizing them as transitional from calc-alkaline to alkaline group. The garnet-bearing muscovite granite displays similarities with A-type-granites in terms of low CaO (0.23-0.40 wt%), MgO (0.10-0.15 wt%) and high SiO₂ (75.07-76.85 wt%), Na₂O+K₂O (8.42-9.18 wt%), Nb (19-46 ppm), Y (19-31 ppm), 10,000Ga/Al ratios are 3.06-4.04 with an average values of 3.44, which is similar to the global average of 3.75 for A-type granites and ZNCY (Zr+Nb+Ce+Y) indicating it is highly fractionated. The primitive mantle-normalized spidergrams show significant depletion in Ba, Nb, Sr, P, Eu and Ti. The chondrite-normalized REE patterns display unfractionated to slightly fractionated REE (La_N/Lu_N) = 0.65-2.33), slight enrichment of light REE  (La_N/Sm_N = 0.65-1.45), unfractionated heavy REE (Gd_N/Lu_N = 0.70-1.21) and pronounced negative Eu anomaly (Eu/Eu*= 0.14-0.33). The unfractionated flat heavy REE (HREE) patterns are indicative of melting at pressures below the garnet stability field. Lanthanide tetrad effect patterns are observed on chondrite-normalized REE patterns of the Abu Diab granite in terms of degrees of the REE tetrad effect (TE_1,3 values) versus the geochemically coherent element pairs (K/Rb, K/Ba, La/Nb, La/Ta, Nb/Ta, Zr/Hf, and Y/Ho ) which  can be attributed to the effect of magma-fluid interaction.

Metallogenic potential in the Abu Diab granite can be traced by using the Zr–Hf geochemical indicator, i.e., the Zr/Hf ratio (Zaraisky et al., 2009). Greisen Sn, W, Mo, and Be deposits are expected to accompany highly fractionated calc-alkaline granites exhibiting lanthanide tetrad effect with Zr/Hf < 25.