Magmatic to Hydrothermal Evolution: Insights from Savage River Magnetite Deposit, Tasmania, Australia

Savage River is one of the largest iron ore deposits in Australia, yet its genetic classification remains debated. It occurs in the Proterozoic Arthur Metamorphic Complex in northwest Tasmania. Mineralisation dominantly consists of magnetite – an important petrogenetic indicator used for a wide array of applications. This research presents the first study of the paragenesis and composition of magnetite from Savage River, integrating field observations, core logging, petrography, micro-texture and geochemistry. Results indicate four distinct generations of magnetite that give insights into the ore-forming history. Analysis of the results and comparison to published data suggest an iron oxide-apatite (IOA) type genetic affinity for Savage River. Testing was carried out on 40 samples collected from two drill holes in the North Pit of the deposit. Graphic core logging, hyperspectral logging, magnetic susceptibility, backscattered electron imaging (BSE) and automated mineralogy data were used to: (1) identify different magnetite generations based on texture and cross-cutting relationships; (2) differentiate lithologies and host rocks; and (3) understand associated alteration minerals. Trace elements of the four identified magnetite generations were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Magnetite I is massive to semi-massive, with well-developed ulvöspinel/ilmenite exsolution lamellae and it is highly fractured and inclusion poor, with highest Ti (< 1 wt%), V (up to 0.8 wt%) and lowest Ni (up to 378 ppm) concentrations. These characteristics record high-temperature magmatic crystallisation and rapid cooling and represent the earliest iron enrichment in the system. Magnetite II forms euhedral to subhedral grains and show moderately reduced Ti (up to 670 ppm), V (up to 4000 ppm), and increased Ni (up to 637 ppm) contents. Magnetite II partially overgrows Magnetite I, indicating precipitation during an early hydrothermal overprint, marking a transition from magmatic to magmatic–hydrothermal fluid regimes. Magnetite III occurs as subhedral to anhedral grains that are inclusion rich, highly porous, and mostly have hematite replacement on the rims. Its low Ti (up to 670 ppm) and V (up to 234 ppm), elevated Ni (727 ppm), and strongly depleted Cr (up to 0.6 ppm) trace-element signature indicate extensive re-equilibration with evolving lower-temperature fluids. This generation is interpreted to correspond to a major hydrothermal alteration phase involving fluid rock reaction with mafic host rocks. Magnetite IV occurs as fine disseminated subhedral to anhedral grains that are comparatively pristine, and inclusion poor. It exhibits the lowest Ti (up to 426 ppm), V (up to 3000 ppm), and the highest Ni (up to 899 ppm) concentrations, consistent with its precipitation from a highly evolved, oxidised hydrothermal fluid. In addition to Ti and V, other discriminatory trace element (like Mn, Ga, Cr) systematics define a clear vector from high temperature magmatic to low temperature hydrothermal conditions accompanied by increasing oxygen fugacity. Comparisons of Savage River magnetite with magnetite from other deposit types shows the most similarities in texture and magnetite chemistry to those of IOA-type deposits. Collectively, these findings suggest that the four magnetite generations at Savage River deposit record a complete magmatic to hydrothermal continuum.