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

A new metamorphic framework has provided additional constraints on the setting of gold deposits
and a new tool for gold exploration.
▪ The metamorphic pattern in the study area is the result of superposition of regional metamorphism
on narrow higher grade contact metamorphic aureoles that formed at different times immediately
adjacent to granitic intrusions, indicating that most of the granitoids are older than the regional
metamorphic event.
▪ Pre-Timiskaming phases of deformation were less penetrative and occurred at shallower depths in
the crust and at lower temperatures than post-Timiskaming deformation.
▪ Post-Timiskaming deformation, when peak regional metamorphic conditions prevailed, was most
conducive to formation of large syn-metamorphic (orogenic) gold deposits. There is a striking spatial
relationship of the boundary between the lower and upper greenschist metamorphic zones and a
significant number of gold mines. Newly identified high priority targets are defined by the
coincidence of metamorphic anomalies with major structural features, specific rock compositions,
and moderate to intense deformation.
▪ The pre-regional metamorphic ages inferred for most of the intrusive plutonic rocks in between the
Porcupine–Destor and Larder Lake–Cadillac deformation zones suggests targeting for granite- and
porphyry-related gold deposits in this area.
Regional structural patterns are now better understood, in part based upon improved knowledge of
the distribution of the stratigraphy and intrusions in conjunction with detailed and regional-scale
geophysical surveys including magnetic, gravity and reflection seismic surveys. Major external intrusive
units, such as Round Lake and Kenogamissi batholiths, include predominantly synvolcanic phases that
occupy anticlinal culminations in common with the situation in Quebec (Daigneault, Mueller and Chown
2004), whereas the late syntectonic intrusions had a relatively minor localized structural effect on the
surrounding supracrustal rocks. Regional deformation zones are the loci of major faults, which have been
reactivated repeatedly and have exerted control on the distribution of early volcanic (“Keewatin”) and late
sedimentary assemblages. All assemblages were constructed in an autochthonous fashion and have been
locally juxtaposed along regional structures during major ductile deformation events that involved
predominantly north-south transpressional shortening. Our geophysical interpretations and conclusions
and preliminary interpretations from the Discover Abitibi seismic reflection surveys (Reed, Snyder and
Salisbury 2005)
▪ demonstrate the sense of dip on major structures and lithological units with inversions of magnetic
and gravity data;
▪ demonstrate the sense of dip on major lithological units and major strike-slip faults using reflection
seismic data;
▪ demonstrate a southerly dip for the Porcupine–Destor fault in the Matheson to Nighthawk Lake
segment, with a change to steep northerly dip in the Timmins camp;
▪ verify that the Shaw structure is domal and may be cored by a granitic body;
▪ indicate higher impedance units (Tisdale assemblage volcanic rocks?) occur in anticlinal structures
(D2?) beneath the Porcupine assemblage sedimentary rocks north of Timmins.
This report represents the first comprehensive multidisciplinary synthesis at greenstone belt scale
since the widespread acceptance of autochthonous models for greenstone belt evolution (Ayer, Amelin et
al. 2002; Thurston 2002). As such, it considerably advances understanding of Abitibi greenstone belt
architecture and metallogeny with specific emphasis on copper-zinc, nickel-copper-PGE and gold
mineralization and with numerous insights and recommendations that are directly applicable to
exploration for these commodities.
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