THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES

Varieties of spessartine-almandine solid solution garnet ([Mn,Fe]₃Al₂Si₃O₁₂) are common accessory phases in all kinds of granitic pegmatites. Previous studies suggest that the compositional variation of garnet in granitic pegmatites can provide information about the degree of pegmatite evolution and fractionation of the original magmatic melt. However, knowledge of the compositional variability of this mineral in granitic pegmatites, in particular in the important kind of rare-element (Li, Be, Ta, REEs) pegmatites, worldwide is still limited. Understanding the chemical variations of garnet in various pegmatites may help discriminate between Li-rich and Li-barren pegmatites, which can potentially be used as an exploration tool for this kind of deposit. In this study, we have done a comprehensive investigation of the major element composition of garnet in different kinds of pegmatites to determine the relationship between its chemical composition, the degree of pegmatite evolution, and the presence of Li mineralization. We have investigated Li-rich and Li-poor and LCT and NYF pegmatites, including pegmatites from the Borborema Pegmatite Province (Capoeira 2, Alto Mirador, Boqueirao, and Carrascao), northeastern Brazil, Eastern Brazilian Pegmatite Province (Golconda), Maryland (Ben Murphy), Maine (Lord Hill), Virginia (Rutherford # 2), New York (Baylis, Benson, Batchellerville, Greenfield), California (Little Three Mine), Pennsylvania (Avondale), New Mexico (San Miguel Co.), Poland (Szklary, Strzegom), and China (Guangdong). The new compositions obtained and those from previous studies indicate that garnet in most of the LCT pegmatites investigated has a higher spessartine component (<Spess₉₅) than that in the NYF pegmatites studied (<Spss₇₀). Of all the pegmatites, garnet from the LCT, Li-rich Alto Mirador pegmatite has the highest degree of fractionation with the highest amount of Mn, whereas that from the Li-poor, NYF Batchellerville pegmatite has the highest Fe content (<Alm₈₇). A new ternary diagram developed using garnet cation ratios is able to discriminate Li-rich from Li-poor pegmatites. The same diagram is useful for separating NYF and LCT pegmatites and will proven to be helpful for identifying pegmatites yet to be classified or those that seem to be hybrid between the two.