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

Three individual grains were analyzed for U and Pb and gave highly consistent results, all of which
are overlapping and only slightly discordant (0.4–0.6%: Figure 6F; Table A1). A calculated weighted
mean age for all three fractions is 2686.2±1.1 Ma (64% probability of fit; MSWD = 0.44), which is
interpreted as the primary age of crystallization of the porphyry. A 2686.2±1.1 Ma age for this intrusive
is identical to that of the adjacent, large, potassium feldspar porphyritic Adams stock immediately to the
west, dated at 2686±3 Ma (Frarey and Krogh 1986), as well as to a number of other porphyry bodies
occurring in the Timmins area and elsewhere in the Abitibi greenstone belt.
04PCT-0064 Quartz-phyric gabbro dike, Bartlett Township
(NAD83, Zone 17 UTM 482078E, 5331967N; #28 on Figure 2 and Table 1)
A sample of quartz-phyric gabbro was collected from a dike cutting Deloro assemblage rocks in Bartlett
Township. This dike is representative of numerous similar east to west striking dikes, which emanate
from the underlying Muskasenda gabbro body and have been interpreted to feed overlying Tisdale
assemblage mafic volcanic rocks occurring further to the east (Ayer et al. 2004).
Heavy mineral concentrates from this gabbro yielded very little zircon, except for extremely tiny,
clouded and stained grains of relatively low quality. Minor, small blade fragments of very pale brown
baddeleyite were also present, and a selection of these were chosen for U/Pb analysis. A dull lustre on all
grains suggests the possible presence of fine zircon coatings. Results for four fractions, each comprising
3 small, thin blades of baddeleyite are presented in Table A1 and Figure 7A. All of the data are highly
collinear and regress to yield an upper intercept age of 2705.7+4.4/–3.8 Ma (86% probability of fit;
MSWD = 0.15) and a lower intercept age of 810 Ma. The Th/U ratios for the analyzed grain fractions
range from 0.07 to 0.25. This is relatively high for baddeleyite, but is appropriate for a mixture of
baddeleyite and zircon, the latter of which may be present as (late magmatic?) coatings on early igneous
baddeleyite grains. This is also supported by the observation that the variation in Pb/U (i.e., Pb-loss)
appears to be broadly correlated with Th/U, suggesting that the grains with the greater proportion of
zircon rim development or growth also show the highest degree of discordance.
Direct dating of the large, sill-like Muskasenda gabbro has not been carried out previously, but the
intrusion is known to enclose an enclave of Deloro assemblage felsic metavolcanic rocks, which have
been dated at 2727.9±2.2 Ma (Ayer et al. 2003), thereby providing a maximum age for the younger host
gabbro. A primary igneous age of 2705.7+4.4/–3.8 Ma for the east-west gabbro dike presented here
establishes with reasonable confidence that the dikes (and by inference the Muskasenda gabbro sill)
intruded Deloro assemblage stratigraphy during a period contemporaneous with Tisdale assemblage
volcanism.
03JAA-0006 Feldspar porphyry, Discovery outcrop, Teck Township
(NAD83, Zone 17 UTM 571986E, 5333892N; #30 on Figure 2 and Table 1)
A sample of feldspar-porphyritic syenite was collected from an outcrop of Main Break mineralization on
the (former) Wright–Hargreaves property. At this locality, the syenite is recognized as one of the
youngest porphyries in the region, which cuts Timiskaming assemblage metasediments, but is itself
crosscut by Main Break mineralization. Thus, accurate and precise dating of the porphyry offers the
opportunity to establish a maximum age for the timing of mineralization.
Zircons recovered from this syenite are modest in number and relatively diverse in appearance. The
largest, clear and crack-free grains comprise colourless or pale brown irregular fragments, rarely with
prismatic crystal faces. However, the bulk of the sample is dominated by short, colourless to pale brown
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