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

comprising a variety of essentially colourless, euhedral and subhedral grains, occasionally elongate (up to
~4:1), and frequently with numerous cracks or inclusions. Equant morphologies and irregular fragments
of zircon are also present, as is a distinct population of finer grained, rounded grains suggestive of
sedimentary reworking.
Highest quality, clear, colourless, short or slightly elongate prisms with sharp facets were selected
for air-abrasion and analysis. Results of U/Pb geochronology carried out on five fractions from this tuff
are presented in Figure 4D and Table A1. The analytical data are variably discordant, with two fractions
(Z1, Z5) overlapping concordia. Results from all fractions are collinear, and a free regression of all data
results in a calculated upper intercept age of 2724.1±3.7 Ma (65% probability of fit; MSWD = 0.55), and
a lower intercept age of 299 Ma. Choosing to exclude the relatively imprecise but concordant analysis for
Z5 makes little difference to the upper intercept age or errors, and so the preferred interpretation of the
primary age of this feldspar-phyric lapilli tuff is 2724.1±3.7 Ma.
SM85-60 Albitite dike, McIntyre Mine, Tisdale Township
(NAD83, Zone 17 UTM 477860E, 5369470N; #13 on Figure 2 and Table 1)
At the McIntyre Mine in Tisdale Township, albitite dikes transect tholeiitic basalts of the Tisdale
assemblage as well as the local, 2689±1 Ma Pearl Lake granitic porphyry. On the basis of geochemical
arguments, these dikes were interpreted by Burrows and Spooner (1986) as altered equivalents of
lamprophyres, and have been considered petrogenetically unrelated to most regional porphyries (Wood et
al. 1986). Moreover, the dikes appear to have intruded late in the tectonic history of the area, are
overprinted by hydrothermal alteration, and are themselves crosscut by gold-bearing veins (Burrows and
Spooner 1986; Bateman et al. 2004).
Corfu et al. (1989) presented U/Pb isotopic data for five zircon fractions from this dike (sample “Ab”
in their study). These authors concluded that the best constraint on the age of the albitite dike was
established by the precise multigrain analysis of equant prisms (their analysis #41), as well as two other
less precise age determinations on equant single grains (#40, 42). When combined with a discordant
analysis of long-prismatic brownish prisms, they obtained an age of 2673+6/–2 Ma (37% probability of
fit; Corfu et al. 1989). Inheritance was also noted in one multigrain fraction (#39) comprising subrounded
long-prisms, suggesting incorporation of xenocrystic zircons with a minimum age of 2689 Ma. Although
slightly imprecise, the 2673 Ma age confirmed that the albitite dike is distinctly younger than, and
unrelated to, the nearby Pearl Lake porphyry (2689±1 Ma: Corfu et al. 1989).
Additional, new analyses of single-grain zircon fractions from this sample were sought in order to
improve the error on the primary age of dike emplacement, as well as to test a more refined age
correlation with another precisely dated albitite dike from the Timmins area (e.g., 04JAA-0010, this
study). For this purpose, three more single-grain fractions were analyzed—two abraded equant grains and
one subrounded elongate prism—and their results are presented in Table A1 and Figure 4E. The two
equant grains (Z1, Z2) are completely overlapping, concordant, and yield an average 207 Pb/ 206 Pb age of
2672.7±1.3 Ma, while the third fraction (Z4) is 1.4% discordant and has a 207 Pb/ 206 Pb age of 2694±4 Ma.
The new data in Figure 4E (white ellipses) are plotted together with the most concordant data of Corfu et
al. (1989; analyses 39–42: shaded ellipses in Figure 4E). Although a total of five analyses cluster on
concordia between 2670 and 2676 Ma, a weighted mean calculated for the most precise three analyses
(Z1, Z2, 41) yields an age of 2672.8±1.1 Ma, which is interpreted here as the best estimate of the primary
age of crystallization of the albitite dike. The older age for the subrounded long prism (Z4) is consistent
with the earlier results of Corfu et al. (1989) for this morphology suggesting possible inheritance from
units such as the porphyry suite in the McIntyre Mine area (ca. 2688–2691 Ma).
115