TRACING CRUSTAL-SCALE FLUID PATHWAYS UNDER COVER WITH MAGNETOTELLURIC IMAGING

Magnetotelluric (MT) imaging studies in multiple Australian mineral districts and in the Missouri Iron Province in the United States have shown a spatial correlation between major iron oxide-copper-gold/iron oxide-apatite (IOCG-IOA) deposits and zones of elevated bulk electrical conductivity that transect much of the crust. These conductive anomalies have generally been loosely interpreted as zones of fluid flow and crustal metasomatism; however, the exact cause of elevated electrical conductivity has been uncertain, particularly because these anomalies are found within stable Precambrian-aged crust. Here, we interpret these conductors specifically to be the result of hydrothermal graphite precipitation associated with magmatically driven CO₂±H₂O fluid flow through the crustal column. We propose that graphite precipitated during cooling through ~800-500°C along crustal-scale, tectonically controlled fluid pathways via carbon saturation in a C-O-H fluid with oxygen fugacity near the fayalite-magnetite-quartz buffer. The fluids involved in this process would have exsolved from mafic to intermediate magmas at mid- to lower-crustal depths and been concentrated along deep-rooted, tectonically controlled pathways; saline magmatic fluids that could facilitate mineralization via mixing with basinal or lacustrine brines were likely derived from related magmatic intrusions at shallower crustal levels. In this interpretation, graphite is then a marker of magmatic fluid movement along localized crustal-scale conduits that could underlie near-surface magmatic, magmatic-hydrothermal, or hydrothermal mineralization. Electrical conductivity anomalies that are associated with such graphite precipitation would demarcate zones of localized, deep-rooted magmatic activity. Although they do not directly mark the locations of mineral deposits, these anomalies consequently do indicate regions that once hosted an appropriate lithotectonic environment for mineral system formation. Our results provide a scientific basis for and thereby validate the inference that lithosphere-scale MT imaging is a critical component of mineral-resource exploration within the mineral-systems paradigm.