Abstract
The temporal and metallogenic relationships between BIFs and IOCGs in the Mt Isa Block are controversial and difficult to assess quantitatively. In this study we examine new magnetic data, anisotropy of magnetic susceptibility (AMS) and remanent magnetisation to define potential structural controls on mineralisation, and try to determine temporal relationships between a barren hematite-magnetite BIF and a carbonate-hosted IOCG, Monakoff. The results showed that the BIF was magnetically isotropic, but that it had a high remanent component (Q=13). The remanent magnetisation has Dec= 238.4°, and Inc= -38.6°, which is offset from the present field and previous data from Cloncurry, so it likely formed during deposition, or early in the deformation history e.g., D1, D2. Conversely, the ore has very low remanent magnetisation (Q<0.2) but significant AMS, oriented Azimuth=225°, Plunge=75°, which is consistent with NE-SW (D3) shortening. 3-D magnetic modelling, constrained by magnetic property data and geophysical enhancements, showed that the BIF horizon formed a tight synform. However, modelling of the ore body showed it to be sub vertical. Based on the recognition that the ore body formed during D3 shortening, we infer that the ore formed in a zone of dilation sub-perpendicular to the shortening direction, rather than in a layer sub-parallel jog. The results suggest that the intersection of NW-oriented, sub-vertical faults with stratiform BIF horizons, adjacent to mafic volcanics are prospective for Monakoff-style IOCGs.
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