TRACE ELEMENTS IN SULFIDES AND MAGNETITE FROM THE ERNEST HENRY IRON OXIDE-COPPER-GOLD DEPOSIT, AUSTRALIA

The Mesoproterozoic Ernest Henry iron oxide-copper-gold (IOCG) deposit is located in the Cloncurry district, Queensland, Australia. The deposit is hosted by K-feldspar-altered brecciated metavolcanics within the Mary Kathleen Group. Ore minerals including chalcopyrite and native Au comprise the breccia matrix along with magnetite, carbonate, and minor quartz. We used LA-ICPMS to analyse trace elements in magnetite, pyrite, and chalcopyrite in ore-bearing and non-ore-bearing samples from within and around the deposit. The concentrations of trace elements in ore and non-ore assemblages may be a proxy for geologic conditions such as temperature, redox, or fluid origin or composition, and may also aid in exploration for IOCG deposits.

The most abundant trace elements in magnetite at Ernest Henry are Al, Mg, V, Ti, Cr, Mn, Co, and Ni; the most abundant trace elements in pyrite are Co, Ni, Ge, and As; the most abundant elements in chalcopyrite are Ti, Zn, Ge, As, and Ag. Within the breccia orebody only, no obvious correlations exist between ore grade and magnetite or sulphide composition; however the compositions of these minerals are consistent within individual samples, but vary between samples, suggesting some geologic control on incorporation of trace elements in these minerals.

However, the concentration of trace elements in magnetite does vary depending on the geologic origin and the paragenetic stage of magnetite precipitation. Magnetite in the hanging wall shear zone has higher V, Cr, Co, Ni than magnetite within the ore body, with ~ 200 ppm Ti, 1270 ppm V, 150 ppm Cr, 420 ppm Mn, 40 ppm Co, and 670 ppm Ni. Ore and non-ore bearing hydrothermal breccia-infill magnetite contains about 330 ppm Ti, 600 ppm V, 18 ppm Cr, 300 ppm Mn, 7 ppm Co, and 160 ppm Ni. Paragenetically late, second generation magnetite has lower Ti (~190 ppm), V (~480 ppm), Ni (~23 ppm), moderate Cr (~18 ppm) and high Mn (~870 ppm), and Co (~92 ppm). Chalcopyrite and pyrite also show variations between hanging-wall shear zone and the orebody. The progressive decrease in V and Ni in magnetite as the orebody is approached (and with paragenetic time) is a potential exploration vector but these elements do not allow distinction of high grade and low grade parts of the main orebody.