Misztela, Monika1, Campbell, Prof. Ian1
1Research School of Earth Sciences, Australian National University, Canberra, Australia
Sulfide saturation is believed to play an important role in porphyry systems fertility. It can also determine the type of ore in an economic deposit. During the early stages of magma evolution Cu, Au and Pd behave incompatibly, and they are concentrated in the melt by fractional crystallisation. If sulfide saturation occurs early, the chalcophile elements are trapped in sulfide phases and locked in an underlying magma chamber, where they are able to enter the fluid phase, which results in a barren system. However, if sulfide saturation occurs later, after or shortly before volatile saturation, the metals are able to enter the fluid phase and form an economic deposit.
Platinum Group Elements are sensitive indicators of sulfide saturation due to their high partitioning into immiscible sulfide melts. Cu and Au also partition into sulfide melts but with lower partition coefficient, so that PGE are more sensitive to any changes in the system. Furthermore, their solubilities in hydrothermal fluids are lower; they are less mobile than Cu or Au, so their concentration in rocks reflect magmatic rather than hydrothermal processes.
QEMSCAN (Quantitative Evaluation of Materials by Scanning Electron Microscopy) can provide valuable information when studying magmatic suites. It provides high-resolution maps and images of mineral and elemental distributions, porosity structure maps, the density of samples and most importantly, quantitative mineral and elemental analyses.
Mount Hagen is an arc system that could potentially be related to a porphyry deposit. Its favourable location and proximity to other deposits was the initial motivation for undertaking this project. Eighteen samples, covering a compositional range from 2 to 11.5 wt.% MgO, were analysed for the PGE. Thirteen rocks were selected for detailed petrological description and mineral quantification by QEMSCAN. Quantitative reports were used to plot the cumulative abundance of major mineral phases as a function of whole-rock MgO. The results show that significant changes occur at ca. 6 wt.% MgO. Hornblende appears as a primary phase, which is attributed to major input of wet magma into the system at that time. Analysis of olivine cores revealed a reversal in their compositional trend at 6 wt.% MgO, which confirmed this hypothesis. Pd, Pt and Au analysis showed that concentrations of these elements initially increase, up to about 8.5 wt.% MgO, followed by a decrease. The change at 8.5 wt.% Mg is interpreted to mark the start of sulfide saturation, and it correlates well with the similar change in the Cu trend. At about 6 wt.% MgO there is a reversal of the trend, followed by a second decline, which can also be explained by a new magma influx to the system.
Mount Hagen was an open system, with at least one major magma recharge into the chamber. The magma experienced early sulfide saturation, which makes it unlikely that this system produced economic mineralisation.
Monika Misztela is a PhD student at the Research School of Earth Sciences, Australian National University interested in petrological and geochemical studies. Monika comes from Poland, where she did her Bachelor`s and Master`s degree in Economic Geology at the AGH University of Science and Technology in Krakow.