Developing magnetite chemistry as an exploration tool for porphyry copper deposits

Abstract

There is increasing interest in the use of magnetite as a geochemical proxy to interpret ore forming processes. Using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS), a large range of trace elements in magnetite are detectable. However, in order to utilise magnetite as a petrogenetic indicator, a comprehensive understanding of the controls of its chemistry is required. Experiments were conducted, both at 1-atm and at high-pressure, investigating magnetite-melt partitioning as a function of oxygen fugacity (fO2), temperature, bulk composition, pressure and water content. Magnetite-melt partitioning can be explained in terms of thermodynamic equilibria between magnetite and melt. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. This relationship can be used to derive expressions which explain magnetite-melt partitioning as a function of aFe3O3(spinel), XFe2+O(melt), XFe3+O1.5(melt) and fO2. As a result of this, there is an increase in magnetite-melt partitioning of divalent cations and Cu with increasing fO2 in silicic, but less so in mafic bulk compositions. There are also strong decreases in magnetite-melt partitioning of 4+ and higher valence cations with increasing fO2. A study was conducted to develop a method which optimises the use of LA-ICP-MS to analyse natural magnetite. Using this method, over 2000 magnetite grains, both hydrothermal and igneous, were analysed in rocks associated with porphyry copper systems. Discriminant projection analysis using elements frequently detected using LA-ICP-MS can be used to effectively discriminate various populations of hydrothermal magnetite. Using these discrimination methods, it is possible to distinguish well-mineralised from less-mineralised porphyry ore-forming environments. Igneous magnetite is susceptible to having its chemistry modified by hydrothermal fluids and commonly undergoes oxy-exsolution, which alters its chemistry. Consequently, it is difficult to petrogenetically study the magmatic environment using magnetite associated with porphyry copper systems.