Overview of Results from the Greenstone ... - Geology Ontario

of the inversion product. (Recent additions to the WinDisp software permits converting the inversion
model directly to negative components; other imaging systems may handle this differently.)
The 3D inversions have been imaged using viewer software (WinDisp 3DViewer, Scientific
Computing and Applications) and presented as freeze-frame images (bitmaps) displaying various aspects
of the view. The viewer can see the selected product images, without loading the viewing software,
provided with the report of this work (Reed 2005a). The image shown here is from the Currie Township
area (Figure 31, Chart A, back pocket). The magnetic model (orange to red) extends to a depth of 3.75 km
(as seen by the upper wire frame). The positive gravity model in green extends to a depth of 5 km (lower
wire frame). The image also contains amplitude-enhanced magnetic data on and above the surface of the
model (an additional product developed in WinDisp, but not a part of the 3D process). The view looks
west from slightly below the surface. The 3D magnetic image suggest a syncline in mafic volcanic rocks
on the south (left) side. The gravity image suggests an overall south dip. The conflict between the
northerly dipping magnetic images at the south edge and the southerly dipping gravity is not resolved. It
has been recognized that different mafic volcanic rocks north of the magnetic unit have bimodal densities
but low magnetic response. The more dense unit to the north (right) appears to dip steeply to the north.
The amount of point data used to construct the primary images determines the resolution achieved in
the inversion. The magnetic data have the highest resolution (survey lines spaced 50 to 150 m apart), and
show dips, plunges and extensions to depth in the inversions (a few hundred metres to several
kilometres). The gravity inversions are less detailed as the primary data represents data points spaced an
average of 1 km apart. These inversions show broad features commonly with substantial depth extent
(several kilometres in the Currie Township example), which may complement or, in places, contrast with
the magnetic inversions. A byproduct of this subproject has been the development of magnetic, gravity
and geologic maps which have been brought together in various viewing relationships. These provide the
ability to refine the surface geologic maps and distinguish among rock units not easily separated in the
existing mapping. Geological interpretation, structure and stratigraphy in the third dimension are
suggested by these results.
Three-dimensional inversion images may be presented together. Figure 32 (Chart A, back pocket) is
an example from the Tisdale–Deloro townships area—the core zone of Timmins camp gold production—
containing the Hollinger–McIntyre gold mines in the west-central area and the Dome Mine towards the
southeast corner. The 3D magnetic inversions are represented in apple green, with yellow and red cores
(succeeding isosurfaces). The 3D gravity inversion is represented by aquamarine (positive shell) and light
green (negative shell). The positive gravity expression, to the north and south, and associated magnetic
highs correlate with mafic to ultramafic volcanic rocks of the Tisdale assemblage. This view from above
helps visualize some of the features extracted from the surface maps by the 3D process. Near the centre of
the image, a magnetic feature extends into a trough within a large, low density feature. Some caution is
needed because of the differences in resolution of the magnetic and gravity data, but there is an
appearance of magnetic and perhaps more dense volcanic rocks overlying lower density felsic volcanic
rocks and/or sedimentary rocks of the Porcupine assemblage.
The WinDisp software permits the presentation of geology on top of the 3D inversion. Products from
the Clifford–Ben Nevis–Arnold–Katrine townships area are displayed in a view looking north (Figure 33,
Chart A, back pocket). The geology has been made transparent to better view the 3D inversion of the
negative or low density component of the gravity underneath. The geology used is from Ayer, Berger and
Trowell (1999). The core of the Clifford stock is identified by the red feature over the larger 3D gravity low
event. Surrounding the stock are felsic volcanic units that are the likely source of the low gravity field.
The Clifford stock sits in a depression in the density low, or higher density event, as can be seen in
Figure 34 (Chart A, back pocket) showing a north-south slice through the stock and surrounding volcanic
82