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

03PJM-131 Quartz-feldspar porphyry, Pamour Mine area, Whitney Township
(NAD83, Zone 17 UTM 489978E, 5373473N; #19 on Figure 2 and Table 1)
As described by MacDonald and Piercey (2003) and MacDonald, Piercey and Hamilton (2005), the
recently identified Pamour body southwest of Pamour Mine is a weakly foliated, moderately fresh,
quartz- and feldspar-bearing porphyry carrying minor biotite alteration (disseminated fine flakes or 5–
20 mm clusters), and little pyrite. Phenocrysts within the porphyry average 1 to 2 mm, but reach up to
5 mm in maximum dimension. This unit sharply intrudes strongly altered (talc-carbonate) ultramafic
volcanic rocks of the Hersey Lake formation in the lower Tisdale assemblage. The Pamour porphyry
appears to truncate an earlier structural foliation developed in the host rocks, and is itself characterized
internally by only a weak fabric, which may have originated by magmatic flow (MacDonald and Piercey
2003; MacDonald, Piercey and Hamilton 2004; 2005; Bateman et al. 2004, 2005, this study). The
significance of this is discussed below.
A representative sample of the porphyry was collected for dating from the 340.8 to 392 m interval of
Porcupine Joint Venture drill hole 18986. Quartz-feldspar porphyry sample 03PJM-131 yielded a
relatively diverse population of poor-quality zircons, comprising small, generally square bipyramids, pale
to medium brown in colour, and often cracked, frosted or turbid, with some of the larger grains being
slightly rounded (resorbed xenocrysts?). The best-quality (clearest, inclusion- and crack-free) grains were
given strong air-abrasion before final selection and analysis, but the results are scattered: three fractions
lie on or near concordia between 2680 and 2703 Ma (2 of these fractions have large errors, due in part to
elevated common Pb; Table A1; Figure 5E). These data likely reflect varying degrees of zircon
inheritance from the Tisdale assemblage or underlying units within porphyry magmatic grains. Two
darker brown single grains are both highly discordant (A1b, A1c: Figure 5E). However, the datum for a
single brown square bipyramid (fraction A1f) is only slightly discordant (0.5%) and at present provides
the best estimate for the age of the sample, at 2677.5±2.0 Ma ( 207 Pb/ 206 Pb age). This interpretation is
based on the assumption that analysis A1f itself does not contain inheritance; further, because of its minor
discordance, the 2677.5 Ma age could be regarded as a minimum time of emplacement.
Preliminary U/Pb results have been obtained for minor, euhedral pale yellow titanite from this
sample. Two initial single-grain fractions are variably discordant, but suggest an upper intercept age of
~2673±11 Ma. Although imprecise, the titanite age is consistent with simple cooling through ca. 600°C
following igneous crystallization, as defined by the best zircon age estimate near 2677.5 Ma.
In summary, the best approximation of the age of emplacement of the Pamour porphyry is
provisionally estimated at 2677.5±2.0 Ma, though this conclusion is based upon a slightly discordant
single analysis. A more robust assessment of the true age of the body is hampered by inheritance and Pbloss
effects. Although younger than most other porphyry intrusions in the Timmins area (ca. 2687–
2692 Ma), the ca. 2678 Ma age is entirely consistent with the observation that the Pamour intrusion
carries less of a structural fabric than its counterparts and host rocks and may therefore independently
provide a minimum age for D2 deformation. This interpretation is supported independently by the age of
the Hoyle Pond porphyry (2684.4±1.9 Ma: see 03ED-096A above), which may be synchronous with D2
structures. An interesting additional observation is that the ca. 2678 Ma Pamour porphyry age correlates
with a distinct population of youngest detrital zircons found within Dome formation sediments of the
lowermost Timiskaming Group (2679 Ma: Ayer et al. 2003; Corfu, Jackson and Sutcliffe 1991).
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