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Chemistry of Cassiterite in Rare Metal Granitoids and the Associated Rocks in the Eastern Desert, Egypt

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Chemistry of Cassiterite in Rare Metal Granitoids and the Associated Rocks in the Eastern Desert, Egypt 318 Journal of MineralogicalH.M. Abdalla, and H. Petrological Matsueda, M.A.Sciences, Obeid Volume and R. ���,��� Takahashi, page ��8─ �26, 2008 Chemistry of cassiterite in rare metal granitoids and the associated rocks in the Eastern Desert, Egypt * ** *** Hamdy M. ABDALLA , Hiroharu MATSUEDA , Mohamed A. OBEID ** and Ryohei TAKAHASHI *Nuclear Materials Authority, P.O. Box 259, C.N. 21111, Alexandria, Egypt ** Natural History Museum, Hokkaido University, Sapporo �6�-�810, Japan ***Faculty of science, El Fayoum University, Egypt The study of chemical zoning, color and pleochroism of cassiterites aids in understanding of the metallogeny and exploration of the primary Sn source. These characteristics assist in discriminating the host granitoids into two associations: metasomatized granites and lithium albite granites. The cassiterite of the metasomatized alkali feldspar granites (i.e., apogranites) is characterized by enhanced * to moderate Nb, Ta, (with high Nb/Ta ratios), Ti, FeO and lower Ga2O3 (<0.01 wt%). Also, it is characterized by the development of deep brown to dark brown pleochroic color zones which oscillate or progressively alter- nate with lighter color zones. On the other hand, the cassiterite in the lithium albite granite is enriched in Ta, Nb * - - (with low Nb/Ta ratio), Ti, FeO , and Ga2O3 0.01 0.04%. It is also characterized by deep orange to reddish brown pleochroic core or bands which are alternate with lighter color bands. It is noteworthy that the conspicuous variation in the colors, pleochroism, and chemistry of cassiterite from metasomatized apogranites and lithium albite granites can be considered as a valuable exploration tool when prospecting for primary cassiterite mineralization. In other words, during the panning survey, which is largely applied to rare metal mineralizations in general and cassiterite deposits in particular (where cassiterite is essen- tially dispersed in the mechanical aureoles), the pleochroism of any cassiterite present indicates the nature of the primary mineralizations (i.e., magmatic or metasomatic). Keywords: Cassiterite, Color, Pleochroism, Zoning, Apogranite, Lithium albite granite INTRODUCTION rin et al., 1971; Sabet et al., 1976a, 1976b). These geolo- gists have attributed the formation of rare metal mineral- Stanniferous or tin-bearing granitoids are those genetical- ization to the metasomatic alteration processes affecting ly and spatially associated with cassiterite mineralization. the host granites. In the Pan-African shield that is mainly These granitoids represent the high-level, post orogenic exposed in southern Sinai and Eastern Desert (Fig. 1A), and the last intrusive phases of the comagmatic granitic Sn-granitoids or stanniferous granitoids (as referred to in rocks. They are both petrographically and geochemically the present study) can be classified into two types of asso- specialized, particularly their anomalous concentrations ciations; i) metasomatized granite and ii) lithium albite of elements such as F, Li, B, Sn, and Rb (Tischendorf, granite. 1977). Formation of rare-metal granitoids in general and In the first case, Sn mineralization is associated with stanniferous ones in particular can be attributed to mag- the alkali feldspar granites which that have suffered post- matic or post-magmatic, metasomatic processes (Schwartz, magmatic, metasomatic alteration (i.e., the so-called apo- 1992; Abdalla et al., 1998; Abdalla and Mohamed, 1999). granites in the sense of Beus et al., 1962; Abdalla, 1996; One of the most important discoveries in 1970 was Abdalla et al., 1998). Meanwhile, cassiterite belong to the the detection of rare metal mineralization in apogranites second types of association is related to lithium-albite of the albitite type in the Central Eastern Desert of Egypt granites which are formed as ultimate differentiates from by a group of Soviet and Egyptian geologists (e.g., Babu- haplogranitic melt by simultaneous crystallization of min- doi:10.2465/jmps.070528a erals from melt and fluid under conditions of high F,a Li ; H.M. Abdalla, [email protected] Corresponding author the activities of lithium and fluorine; (Pollard, 1983; Chemistry of cassiterite in rare metal granitoids and the associated rocks 319 Figure 1. (A) Location and detailed geological maps for some of the investigated Cassiterite-bearing granitoids, Eastern Desert, Egypt. (B) Geological Map of Abu Dabbab lithium-albite granite (modified from Sabet et al., 1976b). (C) Geological map of Mueilha apogranite (modi- fied from Soliman, 1984). Kovalenko and Kovalenko, 1984; Schwartz, 1992). (i.e., chemical, morphological, color and pleochroic char- The present study is concerned with the cassiterite acteristics that typically develop under specific physico- mineral chemistry of the aforementioned lithium albite chemical conditions). Four tin-bearing stocks were se- granite and metasomatised granitoid associations. This lected to achieve the goals of the present study. They are: study seeks to clarify the physico-chemical conditions re- Homr Akarem and Mueilha for metasomatized granitoids sponsible for the evolution of these granitoids through de- and Igla and Abu Dabbab for lithium albite granite asso- ducing the typomorphic characteristics of their cassiterite ciation (Fig. 1). 320 H.M. Abdalla, H. Matsueda, M.A. Obeid and R. Takahashi Table 1. Selected EPMA analysis of the investigated cassiterite of stanniferous granitoids, Egypt * Analysis Nos. 1, 2, 5, 8 and 10 refer to light-colored zones; 3, yellowish-orange (intermediate colored) zone; 4, 6, 7, 9 and 11 refer to dark- colored zones in the investigated cassiterite. ** Total iron as FeO. - , below detection limit. The metasomatic nature of the investigated metaso- Hokkaido University, Japan) has been used for analyzing matized granitoids and the magmatic characters of the polished thin sections and polished thin grain mounts ob- lithium albite granite stocks were based on the criteria cit- tained from placer samples (Fig. 2). Standards used for ed by Pollard (1983), Schwartz (1992), Abdalla (1996), EPMA analysis were synthesized pure oxides and natural Abdalla et al. (1996) and Abdalla et al. (1998). Detailed minerals. The operating conditions were an accelerating geochemical characteristics of the investigated tin-bearing voltage of 15 kV, probe current of 5 nA, a beam diameter granitoids can be found elsewhere (e.g., Abdalla et al., of about 1 μm and counting time of 20 seconds. The ob- 2008; Abdalla, 2008). tained total weight percents were fairly reasonable within the acceptable range of 100% ± 2.5%. Some analyses SAMPLING AND ANALYTICAL TECHNIQUES were selected (Table 1) to show the variations among the composition of the investigated cassiterite. An electron probe microanalyzer (JEOL JXA-50A in Figure 2. Textural characteristics of cassiterite in stanniferous granitoids, Eastern Desert of Egypt. (A) and (B) Photomicrographs showing dis- seminated cassiterite crystal which is included within lithium muscovite in Abu Dabbab Li-albite granite. Notice that the cassiterite crystal is zoned with deep-brown core and pale yellow rim. (C) Polished slab showing cassiterite-bearing exo-greisen vein cutting across the coun- try metasediments, Mueilha apogranite. (D) Polished slab showing cassiterite veinlet cutting through Igla lithium-albite granite. Notice that cassiterite constitutes more than 95% of the veinlet. (E) Rolled pebbles of placer cassiterite, stream tin, Igla tin field. Notice that some nug- gets exhibit the characteristic elbow-shaped twins. (F) Thin polished grain mount for the Igla placer cassiterite. Notice the development of color zoning (with different styles, refer to the text) which grades from colorless to nearly opaque. (G) Oscillatory zoning in Igla cassiterite. (H) Cssiterite crystal exhibiting the elbow-shaped twinning and the characteristic reddish-brown color and pleochroism, Igla cassiterite. (I) Rolled and twinned cassiterite pebble that exhibit crystallographic boundaries consisting of many close planes indicating repeated twinning. The planar growth color zones parallel to natural face are seen to have been contoured with some zones become discontinuous at the twin- ning planes. (J) Wood tin with the characteristic colloform texture. Notice the development of hematite inclusions. Abbreviations: Ct, cas- siterite; Lm, lithium muscovite; Ms, muscovite; Q, quartz; Ab, albite; Met, metasediment country rock; Gr, granite; Ec, Elbow-shaped twin; Dis, discontinuous color zoning; Cl, colloform-textured cassiterite; He, Hematite inclusions. Chemistry of cassiterite in rare metal granitoids and the associated rocks 321 GEOLOGIC SETTING AND PETROGRAPHY Lithium albite granites On the basis of field and petrographical studies, the inves- These granites occur as small stocks, 0.2 to 4 km2 in area, tigated tin-bearing granitoids are classified into two types with circular to dike-like outcrops. They are characterized of associations: by the presence of snowball quartz (euhedral, phenocrys- tic quartz with albite inclusions arranged concentrically along its growth zones) set in a white matrix of fine- 322 H.M. Abdalla, H. Matsueda, M.A. Obeid and R. Takahashi grained and randomly oriented albite, K-feldspar, Li bitized granite (zone of Na-metasomatism). Moreover, a + mica, topaz and quartz. The accessory minerals include smaller volume of greisenized (H -metasomatized) or columbite-tantalite, cassiterite, wolframite, beryl and traces muscovitized granites are superimposed on the albitized of minute zircon
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