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

Therefore, on geochronological grounds, there is no compelling reason to doubt the validity of
assigning this unit to the Deloro assemblage. Rather, an alternate rationale such as structural repetition
may be a likely explanation for the occurrence of this unit within the younger (Tisdale assemblage)
stratigraphy.
03LAH-0627 Mt. Logano quartz-feldspar porphyry, Shaw Township
(NAD83, Zone 17 UTM 492156E, 5365021N; #23 on Figure 2 and Table 1)
The Mt. Logano intrusion represents one of the largest single bodies of quartz-feldspar (albite) porphyry
in the Timmins area, exposed south of the Porcupine–Destor deformation zone, on the northern flank of
the Shaw Dome in northeastern Shaw Township (Hall and Houlé 2003). This elongate (roughly 1x5 km)
unit was sampled in order to establish an age constraint for the southern trend porphyries (terminology of
MacDonald and Piercey 2003).
Sample 03LAH-0627 yielded an abundance of high-quality, slender, pale brown to colourless wellterminated
prismatic zircons, as well as rarer needle or acicular forms, and flat prisms. The highest
quality, air abraded, elongate prisms yielded a very tight agreement of 207 Pb/ 206 Pb ages with two singlegrain
fractions overlapping one another and concordia, while a third is somewhat reversely concordant
(Figure 6C; Table A1). A fourth single-grain fraction is slightly discordant, has a slightly older
207 Pb/ 206 Pb age, and probably contains a small amount of inheritance (A1d: Figure 6C). The three
concordant and near-concordant fractions provide a well-defined weighted mean 207 Pb/ 206 Pb age of
2689.0±1.4 Ma, which is interpreted as the primary crystallization age of the Mt. Logano porphyry.
Given the similarity in chemistry and field relationships of the intrusions on the northern margin of the
Shaw Dome, it is likely that this age is representative of all or most southern trend intrusions.
The 2689 Ma age for the Mt. Logano porphyry is indistinguishable from the majority of ages
determined for other porphyritic intrusions from the central, northern, and western trend bodies (2688–
2691 Ma (n=7): Corfu et al. 1989; Gray and Hutchinson 2001; Ayer et al. 2003). The Mt. Logano
intrusion was also emplaced within error of a number of other regional granitoid plutons, including the
potassium feldspar porphyritic Adams stock (2686.4±2.8 Ma: Frarey and Krogh 1986), the Clifford stock
and associated felsic dikes (2686.9±1.2 Ma, 2688.5±2.3 Ma: this report), the Clarice Lake pluton
(2689±1 Ma: Corfu and Noble 1992), and several plutons in the eastern Abitibi region of Quebec (ca.
2690 Ma: Mortensen 1993b).
03LAH-0161 Felsic to intermediate flow, Shaw Township
(NAD83, Zone 17 UTM 493476E, 5362615N; #24 on Figure 2 and Table 1)
During the 2003 mapping season, a unit of intermediate to felsic metavolcanic rock was sampled by
L.A.F. Hall and M.G. Houlé from an outcrop on the northeastern flank of the core of the Shaw Dome,
eastern Shaw Township. This aphyric and amygdaloidal flow is interpreted to represent a member of the
“Supracrustal Package A” (Hall and Houlé 2003), and a geochronological study of this unit was attempted
in order to test whether or not the volcanic package is of probable Deloro assemblage affinity.
Zircon recovery from this sample was remarkably low. Only a few scattered grains were found and
most were of very poor quality. One single, clear, brown irregular prism fragment was considered
appropriate for analysis and was given a strong air-abrasion treatment. Uranium–lead results for this
fraction are provided in Table A1 and Figure 6D. The data are slightly discordant (1.2%), have elevated
common-Pb, and yield a measured 207 Pb/ 206 Pb age of 2727.3±11.9 Ma. If a simple (modern-day) Pb-loss
history is assumed for the fraction, then the model 207 Pb/ 206 Pb age of 2727 Ma confirms that the central part
of the Shaw Dome structure contains rocks of the Deloro assemblage (2724–2730 Ma: Ayer et al. 2003).
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