Still holding out for a Rusty Ridge IOCG announcement...
In a typical vertically zoned IOCG system, magnetitedestructive, hematite-rich hematite-sericite-chlorite-carbonate (HSCC) alteration dominates upper levels, whereas magnetite-rich alteration (potassic at intermediate depths, grading into deeper sodic or sodic-calcic alteration) dominates at depth. Thus the current erosion level determines whether the exposed or near-surface portions of the system are hematite-rich or magnetite-rich. If the level of exposure is sufficiently deep, overprinting magnetite-rich potassic alteration tends to be more focussed around the deposits, enhancing the magnetic highs. On the other hand, if upper levels of the system have been retained, magnetite-destructive hematite-rich alteration hosts the mineralization. In this case the signature is a relatively smooth pattern, which can be a local magnetic low (depending on the host rock magnetization) within the overall broad high associated with deeper and/or more laterally extensive magnetite-dominant alteration. Tilting of a vertically zoned system, or upfaulting of the deeper magnetite-rich portion, may juxtapose the magnetite and hematite zones, producing juxtaposed, rather than ”coincident” gravity and magnetic anomalies. High grade regional or contact metamorphism of hematiterich zones can impart an intense thermoremanence to the hematite, which can cause large magnetic anomalies. This appears to explain the strong magnetic anomalies exhibited by massive hematite bodies of the Mount Woods Inlier that have been contact metamorphosed, such as the Peculiar Knob deposit (Schmidt et al. 2007), whereas similar unmetamorphosed massive hematite at Prominent Hill produces no discernible magnetic anomaly. At continent to province scale favourable tectonic settings for ancient deposits may be recognisable from regional potential field data sets, supplemented by seismic, magnetotelluric or other deeppenetrating methods. Ancient buried subduction zones are characterized by arc-parallel linear belts of magnetic highs, corresponding to magnetite-series granitoid provinces, and lows, corresponding to ilmenite-series granitoid provinces or sedimentary basins. Subduction-related magnetite-series belts are much more prospective for IOCG and porphyry copper-(gold) deposits. Belts of reduced, ilmenite-series granitoids are prospective for Sn-(W) and also for intrusive-related Au and reduced porphyry Au-(Cu) deposits. Within belts of magnetite-series granitoids, Cu-Au is associated with more magnetic magmatic-hydrothermal systems than Cu-Mo; W-Mo-Bi and Au in tin provinces is much less magnetic. In oxidized Au-bearing systems, Au mineralization is often associated with the felsic end of magmatic evolution and is then associated locally with a weaker magnetic character and higher radioelement contents. Continental rift settings, which are also prospective for IOCG deposits, may be associated with rift-parallel regional gravity and magnetic highs along the ancient continental margin, with a quiet magnetic zone outboard of the regional highs and relatively busy magnetic patterns inboard of the margin (Gunn 1997).