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

An internal contact within the lower Tisdale assemblage in Hutt Township (triangle 4 on Figure 1)
displays heterolithic felsic volcanic debris flows and arkoses intercalated with finely laminated chert,
oxide- and sulphides-facies iron formation. This intercalation of mass-flow units and iron formation
would suggest that internal contacts within the lower unit of the Tisdale assemblage may represent a
localized submarine unconformity.
The lower Tisdale assemblage also occurs in the western part of the study area, north of the
Porcupine–Destor fault (PDF). Here, unlike the more extensive lower Tisdale unit south of the PDF
(described above), the lowermost portion of the lower Tisdale and the upper Tisdale assemblage (and the
Blake River assemblage) are absent. Instead, here, the Tisdale stratigraphy established by Ferguson et al.
(1968) consists of komatiites and tholeiites of the Hershey Lake formation, in turn succeeded by the
tholeiites of the Central formation, the variolitic tholeiites of Vipond formation (2706.9±3.1 Ma: Ayer,
Amelin et al. 2002) and capped by the tholeiitic Gold Centre formation. Here, the Tisdale formations are
unconformably overlain by, and complexly infolded with, the Porcupine assemblage (see “Timmins
Subproject”).
Upper Part
The upper part of the Tisdale assemblage ranges in age from 2706 to 2704 Ma. It is dominated by calcalkaline
felsic to intermediate volcanic rocks including amygdaloidal flows, heterolithic debris flows and
volcaniclastic sedimentary rocks. This part of the Tisdale assemblage conformably overlies the lower
Tisdale assemblage in the central part of the map area and includes the Marker Horizon (Corfu and Noble
1992) extending from Hislop to Michie townships, which is, in turn, conformably succeeded by the lower
Blake River assemblage (see “Currie Township”). Other upper Tisdale assemblage units occur further to
the west and south within the cores of easterly trending synclinorial folds. One such unit, centred in
Argyle and Baden townships, was previously identified as part of the Blake River assemblage in Ayer,
Amelin et al. (2002). Despite several unsuccessful attempts to directly obtain an age for this unit, it is
herein reassigned to the upper Tisdale assemblage because of its lithological and geochemical
characteristics and the fact that it is intruded by a synvolcanic granitic body with an age of 2699±3 Ma in
northeastern Baden Township (see Figure 2). A synvolcanic intrusion of this age is considered to be more
likely within the Tisdale assemblage than the Blake River assemblage.
A felsic volcanic unit with an age of 2702±3 Ma from Cleaver Township (#29: Figure 2) previously
indicated the presence of Deloro-age inheritance in this part of the upper Tisdale assemblage (Ayer,
Amelin et al. 2002). SHRIMP II analyses of additional zircons from this sample yielded a number of
inherited zircons with ages of approximately 2735 to 2720 Ma as well as magmatic zircons with ages of
approximately 2704 to 2698 Ma (Figure 15).
A unit of the upper Tisdale assemblage also occurs in the northern part of the map area. Its northern
contact with the Stoughton–Roquemaure assemblage is marked by the North Branch of the Porcupine–
Destor deformation zone (see “Structural Framework”) and an age gap of about 15 my. However, the
presence of the 2704±5 Ma mafic to ultramafic intrusive complex in Mann and McCart townships (Barrie
1999) intruding the contact between the 2 assemblages suggests an originally unconformable contact
modified by faulting. The unit youngs southward where it is overlain in the east by tholeiitic basalts that
also face south and are assumed to be part of the lower Blake River assemblage (see “Blake River
Assemblage”). Further to the west, the southern margin is in fault contact with the Deloro assemblage
across to the Buskegau River deformation zone.
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