MAJOR, MINOR, AND TRACE ELEMENT CONTENTS OF GAHNITE (ZINCIAN SPINEL) AS GUIDES IN THE SEARCH FOR BROKEN HILL-TYPE MINERALIZATION NEAR BROKEN HILL, AUSTRALIA: A PRELIMINARY STUDY

Gahnite (ZnAl₂O₄) occurs in a wide variety of geological settings, including metamorphosed massive sulfide deposits (MMSD), and has been used as a guide in the exploration for ores of this type. The major element composition of gahnite depends on host rock composition, T, P, fS₂, and fO₂. Major (Zn, Fe, and Mg) and minor (Mn) element contents of gahnite have been used to fingerprint potential host rock lithologies, effectively defining a distinct compositional range for gahnite spatially associated with MMSD. However, while useful in distinguishing between various geological settings, major element compositions of gahnite fail to discriminate between sulfide-rich rocks and sulfide-poor uneconomic prospects. In an attempt to overcome this deficiency, we combine the major and minor element compositions of gahnite with trace element data. To date, no trace element studies of gahnite have been published.

LA-ICP-MS and electron microprobe studies of gahnite from 12 Broken Hill-type (BHT) deposits, including the supergiant (200 Mt) Broken Hill Pb-Zn-Ag deposit, were undertaken to evaluate whether or not a geochemical fingerprint can be used to distinguish prospective BHT deposits from non-prospective BHT occurrences. LA-ICP-MS analyses (n = 220) of gahnite, show compositions of up to 4000 ppm Mn, 240 ppm Co, 6400 ppm Cr, 880 ppm V, 520 ppm Ti, 140 ppm Ni, 2 ppm Sn, 570 ppm Ga, 26 ppm Cd, 16 ppm Li, 21 ppm Be, and 4 ppm Sc, with rare earth elements being generally below detection limits (< ~1 ppb). Gahnite from the main Broken Hill deposit and smaller BHT deposits north of Broken Hill (Potosi, Globe, Round Hill, Flying Doctor), can generally be distinguished from those south of Broken Hill (Ten Two, Henry George, 11:30, Stirling Hill) in various discrimination diagrams (e.g., V vs Cr, Co vs Cr, Ni vs V, Zn/Fe vs V, Zn/Fe vs Cr), and statistically through a principal component analysis. A dendrogram plot of our data suggests that it can be used to identify gahnite in smaller deposits with compositions similar to gahnite from the main Broken Hill lode. Before a geochemical fingerprint can be established for exploration purposes, further studies will need to consider the dependency of ionic radii, and the partition coefficients for each element between gahnite and coexisting minerals (e.g., garnet, biotite, muscovite, and staurolite).