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

Worming Geophysical Data Treatment and Display Technique
“Worming”, which tracks the displacement of edge features in potential field data (gravity or magnetic
data) by use of upward continuation, is a relatively new interpretive tool that was applied experimentally
in the study area. The technique highlights contacts between units with contrasting physical properties
and can give an indication of dip. Figure 19 (back pocket) combines computer-generated gravity worms
(provided by P. Keating, Geological Survey of Canada) with the assemblage and structural map in an
attempt to visualize the nature of contacts between regional-scale units of contrasting density at depth. In
this figure, the grey-scale shaded image in the background is of the Bouguer gravity and dipping contacts
are shown by gravity worms as progressive colour changes from dark blue (near surface) to red (deep). As
one might expect given their differences in density, gravity contrast anomalies commonly conform to the
surface contacts of many of the larger felsic intrusions within the supracrustal rocks. In many of these
intrusions, the worms suggest the surrounding greenstones dip beneath the plutons, a phenomenon which
is also evident on some of the Discover Abitibi reflection seismic profiles south of Timmins. In other
cases, the worms only partially conform to the surface contacts, suggesting these intrusions may be larger
at depth, or plunge in the direction of the offsets (see discussion of Clifford stock in “Geophysics
Subproject”).
The regional deformation zones are also commonly coincident with pronounced gravity worms. For
example, both the Pipestone and north branch Porcupine–Destor deformation zones have worms which
indicate moderate dips towards each other at depth (see Figure 19), thus suggesting a graben-like structure
bounds the eastern part of the Kidd–Munro assemblage. Worms indicating a northerly dip also occur at
the contact between the Porcupine and Kidd–Munro assemblages in Wark and Tully townships and likely
represent the western extension of the Pipestone deformation zone. This feature is also confirmed by a
seismic reflector dipping north beneath the Kidd–Munro assemblage from this general locality on the
South Timmins regional seismic line. The PDDZ is associated with near-surface gravity worms which
more or less conform to its surface location in Hislop Township, but which become progressively
deflected to the southwest by up to 5 km in Macklem Township. On the other hand, near-surface gravity
worms more or less conform to the LLCDZ west of Kirkland Lake, are progressively deflected to the
northeast, following the trace of the Kirkland Lake fault and change to an easterly trend following the
contact between the upper Tisdale and the lower Blake River assemblages in Gauthier Township (see
Figure 2). It is possible these gravity worms are related to splays of the main gold-bearing deformation
zones.
REGIONAL STRATIGRAPHIC AND STRUCTURAL INTERPRETATION
At a regional scale, supracrustal units in the SAGB are dominated by east-west striking volcanic and
sedimentary assemblages (see Figure 1). The structural grain is also dominated by east-trending Archean
deformation zones and folds (see Figure 2). The regional deformation zones commonly occur at
assemblage boundaries and are spatially closely associated with long linear belts representing the
sedimentary assemblages (i.e., Porcupine and Timiskaming). Ayer, Amelin et al. (2002) hypothesized that
the regional association of the PDDZ and LLCDZ and major assemblage boundaries, in conjunction with
the regional distribution of stratigraphic gaps between certain assemblages, indicates that the current
locations of these regional deformation zone are proximal to the locus of early synvolcanic extensional
faults dating as far back as the time of deposition of the upper part of the Deloro assemblage (i.e.,
2724 Ma).
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