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. 103���� Takahashi,� page 318─ 326, 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 060-0810, 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 crystals. The characteristics of these lith- zones and are exclusively confined to fissures and frac- ium-enriched granitic rocks have been reported by Mo- tures (Fig. 1C). The deeper unaltered pink alkali feldspar hamed et al. (1999) and Abdalla et al. (2008). granite zone constitute the main mass (60-90% of the area The stocks commonly display a petrographical zonal covered) of the investigated apogranites. Textural charac- pattern in response to their magmatic evolution as exem- teristics are dominantly of subsolidus metasomatic replace­ plified by the Abu Dabbab granite, with lower medium- ments. grained albite granite and a roof zone of fine-grained, The mineral constituents of the deeper unaltered al- lithium white mica-albite granite (Fig. 1B). Moreover, the kali feldspar granite zone are microcline and microcline roof zone is capped by a carapace of banded pegmatoidal, perthite, plagioclase (An3–8), quartz and white mica with stockscheider, crust (0.5 to 4.5 m thick) with an upper traces of siderophyllite mica. Texturally, white mica oc- band consisting of gigantic quartz crystals and a lower curs as subhedral interstitial, fine- to medium-sized flakes band of amazonitic K-feldspar (see Helba et al., 1997). up to 3 mm, and appears to be of late-magmatic origin. The quartz crystals commonly grow inward from the The accessory minerals are zircon, fluorite, ilmenite, py- granite/country rock contact and show no evidence for rite, cassiterite and rare columbite. late emplacement of the pegmatite crust. Furthermore, a In addition to the development of Li-siderophyllite characteristic quenched taxitic, very fine-grained albite and zinnwaldite in the apical albitized and greisenized granite zone (~ 10 cm thick.) commonly shields the lithi- zones, the aforementioned accessory minerals of the um albite granite from the pegmatite crust. Textural char- deeper zone increase in abundance. Cassiterite mineral- acteristics are dominantly magmatic (e.g., the snow- ization, associated with apogranitic rocks, exhibits one or balled quartz) and indicate the co-precipitation of quartz more of the following mode of occurrences: i) greisens and albite from a progressively fractionating Na-rich melt (Fig. 2C); ii) beryl, cassiterite ± wolframite quartz veins (Abdalla et al., 1998; Abdalla and Mohamed, 1999 and and iii) placer deposits. A detailed contribution for the Mohamed et al., 1999). petrography of the albitized and greisenized granites is In these granitoids, cassiterite exhibits one or more given in Abdalla et al. (2008) and Abdalla (2008). of the following mode of occurrences: disseminated (Figs. 2A and 2B); stockwork greisen veins and bodies; beryl, CASSITERITE MINERALOGY cassiterite and wolframite quartz veins (Fig. 2D); and placer deposits (Figs. 2E and 2F). Some of cassiterite may The examined cassiterite crystals of the different mode of exhibit the wood tin pattern (Fig. 2J). Wood tin, a term occurrences, grains and pebbles of placer frequently ex- generally applied to concentrically-layered cassiterite. It hibit conspicuous symmetrical to asymmetrical color and typically exhibits a colloform, cuspate or botryoidal depo- pleochroic variations in the zones from core towards the sitional pattern. It generally contains patches of hematite rims in transmitted light. However, in reflected light these (Fig. 2J). Among cassiterite placers (stream tin), the rolled variations are hardly observed. In incident light, the dif- pebbles or individual nuggets, ~ 2 cm or less in diameter, ferent color bands exhibit a gray color with very faint ple- of one or twinned crystals are commonly present. ochroism. However, this color and pleochroic zonal varia- tion is confirmed by electron microprobe analyses and Metasomatized granites scanning microscope back-scattered images to be a reflec- tion of compositional variation. The metasomatized granites, most commonly referred to Cassiterite commonly displays three styles of zoning as apogranites, owe their features to late- to post magmat- namely; progressive, oscillatory, and sector. In progres- ic alteration processes (Beus et al., 1962; Sabet et al., sive zoning, the crystals show smooth color (and compo- 1976a; Abdalla, 1996; Abdalla et al., 1996). Apogranites sitional) variation towards the rim. The core is yellowish are small bodies, 1 to 10 km2 in area, and are spatially re- brown to black is grading outward into a faintly yellow to lated to the apical dome-shaped projections or apophyses colorless rim (Figs. 2A and 2B). of the high level, post orogenic, late intrusive phase. The In oscillatory zoned crystals, successively alternating tin-bearing apogranites commonly display a vertical pe- zones of colorless, white, yellow, orange, red and deep trographic zoning in response to post magmatic, metaso- reddish brown zones extend parallel to the c-axis (Figs. matic processes, with a deeper unaltered zone of alkali 2G-2I). For simplicity the colored zones are classified feldspar granites capped by bleached gray to whitish, al- into three shades namely: i) light (i.e., colorless, white, Chemistry of cassiterite in rare metal granitoids and the associated rocks 323 yellow); ii) intermediate (i.e., yellowish-orange); and (Hurlbut and Klein, 1979). Variable contents of Nb, Ta, dark (i.e., red, deep reddish brown to even opaque) as and generally appreciable amounts of ferrous or ferric shown in Table 1 and Figures 2-5. The zone boundaries iron are usually substituting for Sn in the cassiterite struc- are usually planar and parallel to simple crystallographic ture. growth planes (Figs. 2G-2H). The zones may be discon- The compositional differences between cassiterites tinuous due to a change in the conditions of the nucleation from different specialized host granitoid associations are of such planes (Fig. 2I). Oscillatory zones are usually nar- conspicuous (Table 1). Cassiterite of the lithium albite row, the width being 3-30 μm; exceptionally they are granite (e.g., Igla and Abu Dabbab) is enriched in Ta, Nb 60-100 μm. The wide zones often consist of many thin (with Nb/Ta ratio ranging between 0.33 and 1.50), Ti,

* 2+ - subzones only slightly different in composition. Darker FeO (total Fe as Fe ) and Ga2O3 0.01 0.04%. As men- and lighter zones alternate rhythmically and may form tioned before, this cassiterite is characterized by the de- more than 30 oscillations within a single crystal. Oscilla- velopment of a deep-orange to reddish brown pleochroic tory zoning was clearly visible from the core to rims and core or bands that are rimmed or oscillatory alternated by hence it can be considered a type of concentric zoning. lighter bands. The dark zones are conspicuously enriched However, in some crystals, oscillatory zoning seemed to in Ta, Nb, Ti, Fe and Ga relative to the lighter ones (Table be specific to distinct concentric growth crystallographic 1, and Figs. 3-5). direction. In such cases, weak oscillatory zoning appeared On the other hand, cassiterite of the metasomatized when the resolution of the BSE (back-scattered electron) apogranites (e.g., Mueilha and Homr Akarem) is charac- image was enhanced. terized by enhanced to moderate Nb, Ta, (with high Nb/Ta * It is noteworthy that the dark-colored zones or bands ratios, ranging between 5 and 20), Ti, FeO and lower

- in cassiterite of lithium albite granites are characteristi- Ga2O3 (<0.01 wt%.). The level of enrichment in the ele- cally deep-orange to reddish-brown whereas, the dark ments Nb, Ta, Ti and Fe in cassiterite of apogranites is zones in cassiterite of apogranites are typically brown to much less than that of the lithium albite granites. As, even black. mentioned before, this cassiterite is characterized by the Cassiterite crystals disseminated in stanniferous development of brown to even opaque pleochroic color granites (i.e. metasomatized and lithium albite granitoids) zones which are oscillatory alternating with lighter ones. commonly exhibit gentle progressive zoning character- The dark zones are characteristically enriched in Nb, Ta, ized by small-scale variations in Nb, Ta, and Fe (Figs. 2A Ti, and Fe relative to the lighter ones (Table 1, and Figs. and 2B). Meanwhile, cassiterite of quartz veins and grei- 3-5). sen bodies is distinguished by the development of oscilla- The collective trends exhibited by Ti, Fe*, Nb, and tory zoning. Ta are in contrast to those exhibited by Sn (Fig. 3). The Sector zoning is displayed as colored parts which are trends reveal that Sn is substituted by Ti, Fe*, Nb, and Ta confined to crystal’s acute termination. Such phenomenon in the cassiterite structure. This is reascertained in Fig- can be detected where the {100} growth sectors of cassit- ures. 4 and 5. It is clear that the reddish and dark color erite can incorporate greater concentrations of distinct zoning displayed by cassiterites is directly related to the trace elements (e.g., Nb, Ta, Ti and Fe) relative to {110} substitution of Nb5+, Ta5+, Fe2+ and Ti4+ for Sn4+ in the cas- sectors. siterite lattice structure. The color and pleochroism (and hence the chemis- try) of cassiterite vary among the different modes of oc- DISCUSSION currences (i.e., disseminated, greisens and quartz veins). The present study is focused upon the detection of color During magmatic fractionation, tin occurs as Sn4+ either variation and zoning styles of the cassiterite associated isomorphously substituting for Ti4+, Fe3+ and Mg2+ in their with the two distinguished types of stanniferous granit- minerals, particularly biotite (Groves and McCarthy, oids (i.e. metasomatized and lithium albite granitoids). 1978) or complexed by strong ligands such as F– and Cl–. Cassiterite precipitates under conditions of where by vola- CASSITERITE CHEMISTRY tiles are vaporized according to the reaction:

Cassiterite, SnO2, has a tetragonal lattice structure similar SnF4 + 2H2O → 4HF↑ + SnO2 (cassiterite)↓ (1). to that of rutile, belonging to the p42/mnm space group, in which a cation is in sixfold coordination with oxygen. Heinrich (1995) has proposed three mechanisms re-

The radius ratio, RMetal/Roxygen, in this isostructural group sponsible for cassiterite deposition. The first is vapor sep- of minerals lies between the limits of 0.732 and 0.414 aration due to boiling which effectively removes HCl and 324 H.M. Abdalla, H. Matsueda, M.A. Obeid and R. Takahashi

Figure 3. Sn versus Nb, Ta, Fe* (total Fe as Fe2+) and Ti variation diagrams for the investigated cassiterite in stanniferous granitoids, Egypt. The element contents are in atom/formula.

HF and hence causes the precipitation of cassiterite. The such a granitic association. Nb is usually more abundant second is the mixing of hot saline magmatic melt with than Ta in crustal rocks (the average crustal contents of non-magmatic fluid during the emplacement of granites Nb and Ta are 0.002% and 0.0025%, respectively; and subsequent metasomatic process. The third mecha- Smirnov et al., 1983). The Nb/Ta ratios in rare-metal nism is the acid neutralization by feldspar hydrolysis that granitoids showing alkaline affinity are also high (Raim- takes place during the greisenization process. bault et al., 1991). Abdalla et al. (1998) have termed the Rudorff and Luginsland (1964) have reported the lithium albite granitoids (e.g., Abu Dabbab and Igla) as

- presence of complete solid solutions between Nb2O and Ta granites as they are characterized by a high content of

TiO2 with an ordered rutile structure. Since rutile and cas- Ta and extremely low Nb/Ta ratios ranging between 0.30 siterite are isostructural, it may be reasonable to assure and 2.0.

- that TaO2 and NbO2 behave similarly in both the two lat- Figures 3 5 indicate that Nb and Ta in the cassiterite tices. structure are incorporated via the substitution scheme: The high Nb/Ta ratios in the host apogranites as well 2(Nb + Ta)5+ + (Fe + Mn)+2 → 3Sn4+ (Moller et al., 1988). as the associated columbite (major sink for Nb and Ta, see Besides, the scheme proposed by Cerny and Ercit (1985), Abdalla et al., 1998) and cassiterite (investigated in the i.e., Fe2+ + 2(Nb + Ta)5+ = 3(Ti + Sn)4+ can be also appro- present study) may suggest that the predominance of Nb priate. There is a reasonable correlation between [(Nb + over Ta is initially attributable to igneous fractionation Ta)/Sn]atom. and [(Fe + Mn)/Sn]atom. (Fig. 5), and (Nb + Ta)/ (i.e., crystal/melt or fluid partitioning) that characterizes (Fe + Mn) = 1.0-0.5. Further, there is a small deviation in Chemistry of cassiterite in rare metal granitoids and the associated rocks 325

Figure 4. Fe-(Nb + Ta)-(Sn + Ti), atomic ratios, ternary diagram for the investigated cassiterite, stanniferous granitoids, Egypt. Figure 5. [(Nb + Ta)/Sn]atom. versus [(Fe + Mn)/Sn]atom. in the inves- The shaded area in the inset triangle represents the portion of the tigated cassiterite, stanniferous granitoids, Egypt. Symbols simi- diagram displayed in detail. The dashed line denotes the ideal lar to those in Figure 3. trend defined by the substitution scheme: Fe2+ + 2(Nb + Ta)5+ = 4+ 3(Ti + Sn) for Nb, Ta-oxides (from Cerny and Ercit, 1985). The field boundaries of cassiterite and rutile are from Neiva (1996). pecting for primary cassiterite mineralization. In other Symbols similar to those in Figure 3. words, during the panning survey, which is largely ap- plied to rare metal mineralizations in general and cassiter- the trend exhibited by the investigated cassiterite from the ite deposits in particular, the pleochroism of any cassiter- ideal trend defined by the substitution scheme of Cerny ite present indicates the nature of the primary minera‑ and Ercit (1985), as shown in Figure 4. These characteris- lizations (i.e., magmatic or metasomatic). tics can be partly attributed to either the incorporation of another substitution scheme or to the oxidation state of CONCLUSIONS Fe. Moreover, Figure 5 shows that besides the presence of a solid solution between SnO2 and (Fe,Mn)(Nb,Ta)2O6 The present study revealed the validity of employing col- (columbite-tantalite phases), these elements are relatively or and chemical characteristics of cassiterite in monitor- homogeneously distributed in the cassiterite structure. ing the metallogenetic evolution of the host granitoids. It is obvious that the disseminated cassiterite (Figs. The cassiterite of the metasomatized alkali feldspar gran- 2A and 2B) is a magmatic phase which was crystallized ites (i.e., apogranites) is characterized by enhanced to * from residual interstitial melt (or melt-fluid system) be- moderate Nb, Ta, (with high Nb/Ta ratios), Ti, FeO and fore complete solidification of the granite cupola. On the lower Ga2O3 (<0.01 wt%). Also, it is characterized by the other hand, the narrow-spaced oscillatory zoning may development of deep brown to dark brown pleochroic arise from a kind of non-equilibrium partitioning of ele- color zones which are oscillatory alternating with lighter ments between the growing crystal faces and the confin- ones. ing medium as the result of the effect of the kinetic factors On the other hand, cassiterite of the lithium albite of crystal growth. One possible mechanism can be the re- granite is enriched in Ta, Nb (with low Nb/Ta ratio), Ti,

* - peated “swallowing” of adsorption layers due sporadic in- FeO , Ga2O3 0.01 0.04 wt%. This cassiterite is also char- crease crystal growth (Allegre et al., 1981). Also, abrupt acterized by development of a deep-orange to reddish changes in the diffusion rates of some elements (e.g., Nb, brown pleochroic core or bands which are rimmed or os- Ta, Ti and Fe) might have occurred, most probably due to cillatory alternate with lighter bands. rapid addition of new fluid to the system. The conspicuous variation in colors, pleochroism It is noteworthy, that the conspicuous variation in the and chemistry of cassiterite from metasomatized apogran- colors, pleochroism and chemistry of cassiterite from ites and lithium albite granites can be considered as a metasomatized apogranites and lithium albite granites can valuable exploration tool when prospecting for primary be considered as a valuable exploration tool when pros- cassiterite mineralization. Thus, during the panning sur- 326 H.M. 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