| Summary: | 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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