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Targeting Archean orogenic mineralization using physical properties and integrated geophysical methods Mitchinson, Dianne Edith


Although Archean orogenic gold mineralization is not readily detected using geophysical methods, due to a lack of petrophysical contrast between typical low volumes of gold and hosting rocks, it is possible to use geophysics to detect other petrophysically distinct gold indicators. Geophysical inversion methods, in particular, make it possible to not only detect important gold-related rocks in the subsurface, but to map their distribution in three dimensions. The research presented examines the effectiveness of geophysical inversion as an exploration tool in the Archean orogenic gold environment through extensive physical property analysis, synthetic modeling, and inversion of various geophysical data over the Hislop gold deposit, Ontario. As understanding rock properties is imperative to interpreting geophysical data, it was necessary to establish the physical property ranges of typical host rock types, hydrothermally-altered, and mineralized rocks in this deposit setting. Felsic dikes, known to be associated with gold at Hislop, have low magnetic susceptibility and density ranges that allow them to be distinguished from mafic and ultramafic rocks. Additionally, many potentially mineralized, carbonate-altered mafic and ultramafic rocks can be isolated from their least-altered equivalents using susceptibility. Synthetic modeling showed that narrow, near-vertical felsic dikes, and sulfiderich zones hosted by mafic and ultramafic volcanic rocks can be imaged up to 35O m in the subsurface using inversion methods. It is necessary however, to focus on small areas, to have closely spaced measurements, and small inversion cell sizes. It was demonstrated that constraining inversions through addition of basic prior geologic and physical property information, yields models with improved physical property distribution, and estimates. Applying knowledge gained from physical property, and synthetic modeling work lent confidence to interpretations of inversion results for the Hislop area. At regional scales, susceptibility and density models reveal a steep southward dip for the gold-related Porcupine-Destor Deformation Zone, and a greenstone depth of approximately 7000 m. Fe-rich mafic rocks directly hosting the Hislop deposit are complexly faulted and extend to 3000 m depth. At deposit-scales, model cells with combined low susceptibilities and high chargeabilities, occurring proximal to faults, felsic intrusions, and Fe-rich mafic rocks, highlight prospective areas for further investigation.

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