MAJOR AND TRACE ELEMENT CHEMICAL COMPOSITION OF GAHNITE FROM GRANITIC PEGMATITES AND A METAMORPHOSED MASSIVE SULFIDE DEPOSIT : SIGNIFICANCE FOR PEGMATITE FRACTIONATION AND DISCRIMINATION BETWEEN Li-RICH AND Li-POOR PEGMATITES

Abstract

Rare-element granitic pegmatites are common hosts to economic deposits of rare metals, including Li, Ta, and rare-earth elements, which are increasing in economic importance due to advancing technology. Gahnite (ZnAl₂O₄) occurs as an accessory mineral in metamorphosed massive sulfide deposits (MMSDs) and some rare-element granitic pegmatites, including those with rare-metal mineralization, but detailed chemical studies of gahnite in these rocks are very scarce. In this study, gahnite from twenty-four granitic pegmatites and the Broken Hill-type Nine Mile MMSD, Australia, was analyzed for major and trace element chemical compositions to determine the relative degree of evolution of the pegmatites and identify chemical differences between Li-rich and Li-poor pegmatites and between these and MMSDs. In the spinel ternary diagram in terms of mol % gahnite (Ghn), hercynite (Hc), and spinel (Spl) end-members, the compositions of gahnite from the pegmatites fall within the previously defined pegmatite field and are given by: Ghn₇₀.₆₃₋₉₈.₄₈Hc₀.₉₅₋₂₈.₆₁Spl₀.₀₀₋₄.₅₂. Gahnite from the Nine Mile deposit (Ghn₅₅.₆₂₋₇₆.₀₆Hc₁₇.₄₇₋₃₇.₃₄Spl₃.₈₁₋₁₀.₇₃) falls within the MMSD field characterized by compositions that reach higher Mg and lower Zn contents compared to gahnite from granitic pegmatites. Gahnite from the LCT (lithium, cesium, tantalum) family, rare-element class, beryl-columbite-phosphate subtype and LCT family, muscovite-rare element class, Li subclass granitic pegmatites of the Comechingones (Blanca Dora, Juan Román, Magdalena, La Ona, and Sin Nombre pegmatites) and Conlara (Nancy pegmatite) pegmatite districts, Pampean Pegmatite Province, Argentina, was analyzed in detail. The chemical composition of gahnite from these pegmatites is defined by the ranges Ghn₇₈.₄₉₋₉₀.₃₅Hc₉.₀₇₋₂₀.₅₂Spl₀.₂₅₋₃.₃₇. Gahnite from the Nancy pegmatite has higher Mg and Mn contents than gahnite from the Comechingones pegmatites. Chemical zoning within gahnite crystals is characterized by an increase in Zn (~2.8 wt.% ZnO) and a decrease in Fe (~2.6 wt.% FeO) from core to rim, reflecting the evolution of the pegmatite melt via simple fractional crystallization. Plots of molecular Fe vs. Zn and Fe+Mg vs. Zn+Mn and Zn/Fe ratios in gahnite display the diadochy and the relative degree of fractionation of the pegmatites. The Zn/Fe ratios range from 3.82 to 9.96. Based on all these parameters, the relative degree of evolution of the pegmatites increases in the order: Sin Nombre [arrow to] Magdalena [arrow to] Juan Román [arrow to] Blanca Dora [arrow to] Nancy [arrow to] La Ona. This order is consistent with mineralogical evidence and indicates that the composition of gahnite in granitic pegmatites can effectively be used to determine the relative degree of evolution of pegmatite melts. The trace elements present in gahnite are first-series transition metals (Ti, V, Cr, Co, Ni, and Cu), as well as Mn, Li, Ga, Cd, Sn, and Pb. Gahnite in the Nine Mile deposit reaches higher Ti, V, Cr, Co, Ni, and Pb and lower Mn, Li, Ga, Sn, and Cu contents than gahnite from granitic pegmatites. Gahnite from highly evolved granitic pegmatites of the Borborema Pegmatite Province, Brazil, has the highest Cu (up to 68 ppm), Mn (up to 8,819 ppm), Li (up to 376 ppm) and Zn (up to 43 wt.% ZnO) contents and these compositions may be good indicators of Li-rich pegmatites. This study shows that the major and trace element chemistry of gahnite in granitic pegmatites may be used to understand relative pegmatite evolution and to distinguish pegmatites that contain Li-mineralization from barren pegmatites.