THE MAJOR- AND TRACE-ELEMENT COMPOSITION OF GAHNITE AS A CHEMICAL DISCRIMINANT FOR GRANITIC PEGMATITES AND BROKEN HILL-TYPE PB-ZN-AG DEPOSITS

Gahnite (ZnAl₂O₄) is an accessory phase in a variety of rocks, including granitic pegmatites and metamorphosed massive sulfide deposits. Although variations in the major element content of gahnite in different geologic environments are known, those of trace-elements are still unknown for several settings. The major- and trace-element composition of gahnite in Broken Hill-type (BHT) deposits was recently investigated to distinguish prospective BHT deposits from non-prospective BHT occurrences. Here we present the first in-situ trace-element compositions of gahnite from various types of LCT (lithium-cesium-tantalum enriched) granitic pegmatites (25) worldwide obtained by LA-ICP-MS and compare them, along with major- and minor- element contents, with those from BHT deposits (12) of the southern Curnamona Province, Australia, to identify compositional differences indicative of each environment.

Major-element contents of gahnite in both environments partly overlap, but those in granitic pegmatites have higher Zn contents. In addition, gahnite in granitic pegmatites has higher Mn (1,300 – 8,800 ppm; avg. 3,850 ppm), Li (6 – 243 ppm; avg. 52 ppm), and Cu (8 – 68 ppm; avg. 25 ppm), and lower Mg (30-5,765 ppm; avg. 1,213 ppm), Fe (0.55 – 8.8 wt.% Fe; avg. 4.8 wt.%), Cr (1-11 ppm; avg. 3.5 ppm), Co (0-40 ppm; avg. 6 ppm), and Ni (1.8 – 18.7 ppm; avg. 6 ppm) contents than gahnite in BHT deposits. Gahnite with Mg < 5,800 ppm, Fe < 8.8 wt.%, Mn > 4,000 ppm, Li > 28 ppm, Cu > 40 ppm, Cr < 10 ppm, Co < 40 ppm, and Ni < 20 ppm is characteristic of granitic pegmatites, some of which contain economic rare-metal (Li, Ta) mineralization. In contrast, gahnite with Mg > ~6,000 ppm, Fe > 8.8 wt.%, Cr > 11 ppm, Ni > 20 ppm, and Co > 40 ppm characterizes BHT deposits. A Cu-Co-Cr ternary diagram clearly separates gahnite from both settings.

High Mg, Fe, Cr, Co, and Ni contents in gahnite from BHT deposits are consistent with a previously recognized genetic relationship with mafic magmatism. High Li and low Fe, Mg, Cr, Co, and Ni contents in gahnite from granitic pegmatites reflect the ultra-felsic composition of these rocks and extreme fractional crystallization. Combined major- and trace-element compositions of gahnite obtained from covered terranes can be used as chemical fingerprints to distinguish granitic pegmatites from BHT deposit sources.