Chemistry of cassiterite in rare metal granitoids and the associated rocks in the Eastern Desert, Egypt

Abstract

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 alternate 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 essentially dispersed in the mechanical aureoles), the pleochroism of any cassiterite present indicates the nature of the primary mineralizations (i.e., magmatic or metasomatic).