COMPOSITIONS OF MAGNETITE AND SULFIDES FROM BARREN AND MINERALIZED IOCG DEPOSITS IN THE EASTERN SUCCESSION OF THE MT ISA INLIER, AUSTRALIA

We have analysed magnetite and sulfides from Starra, Mt Elliot, Osborne, and Ernest Henry IOCG deposits and compare the compositions with magnetite and sulfides from regional unmineralized hydrothermal magnetite matrix breccias. Variations in trace element compositions of magnetite may record specific geologic processes such as fluid temperature, fluid origin, redox conditions, or pH. Similarly, the partitioning of elements between magnetite and coexisting minerals such as carbonate, sulfides, titanite, or quartz may be a useful indicator of geologic process. In addition, we evaluate whether the composition of magnetite and sulfides may be useful as a chemical fingerprint to predict the proximity to nearby IOCG-style mineralization.

Numerous trace elements are detectable in magnetite from both IOCG deposits and non-ore bearing magnetite-matrix breccias. Concentrations of many of these elements vary over several orders of magnitude among all samples, but within any single generation of magnetite in a single sample, variation is typically small. Typical ranges of the most commonly detected trace elements are (all values are in ppm): Mg (100-1000), Al (100-7000), Ti (10-2000), V (10-2000), Cr (1-1000), Mn (100-10000), Co (1-100), Ni (20-100), Ga (5-50), and Sn (1-5). Pyrite exhibits far more variation of trace element concentrations within a sample and among samples, however fewer elements are commonly detected in pyrite: Ti (3-10), Co (200-20000), Ni (50-1000), As (20-10000), Sn (1-5) and Au (<30 ppb). Common trace elements in chalcopyrite include: Ti (2-10), Co (1-50), Zn (2-200), Ag (0.5-70), In (5-20), Sn (1-300), Au (<50 ppb-2 ppm), and Bi (1-25).

So far, we have not recognized a clear difference in sulfide or magnetite trace element concentrations between IOCG deposits and unmineralized magnetite-matrix hydrothermal breccias. Compositions of trace elements in these minerals likely result from a combination of factors, including fluid source, fluid composition, host rock composition, and pressure and temperature conditions. Further work is aimed at attempting to decipher the role of these variables on the compositions of trace elements in these minerals.