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

subhedral prisms, slightly flat to square in cross-section, but variably cracked and clouded with
inclusions. Higher quality representative zircons were selected from each of the main populations and
subjected to strong air abrasion treatment. Results for five single-grain fractions are presented in Table
A1 and Figure 7B. The U/Pb isotopic data for these zircons is compromised in some cases by a
combination of very small grain size and higher-than-average levels of common-Pb. Most analyses are
concordant within error, and have 207 Pb/ 206 Pb ages that fall between ca. 2690 and 2700 Ma (Table A1).
Specifically, data from two grains overlap on or near concordia at approximately 2700 Ma
(2700.2±2.9 Ma, 2700.6±4.4 Ma: Figure 7B). One analysis sits at 2694.4±7.4 Ma, while two younger
analyses reflect an age at about 2690 Ma (2689.6±4.7 Ma, 2689.7±41.3 Ma).
Field relationships at this locality show unequivocally that the syenite porphyry intrudes
Timiskaming Group sediments. Where constrained in the Kirkland Lake basin (and elsewhere), the age of
deposition of the Timiskaming detritus is considered to be younger than approximately 2679 Ma (e.g.,
Corfu, Jackson and Sutcliffe 1991); moreover, results presented in this study further constrain the age of
Timiskaming basin development to be younger than 2677.7±3.1 Ma or even 2669.6±1.4 Ma (samples
03VOI-0570-1, 03VOI-0422-1, below, respectively). Therefore, by virtue of being consistently older than
the unit it intrudes, all of the single-grain zircon analyzed from the crosscutting syenite must be regarded
as inherited, xenocrystic grains. Consequently, no absolute age information can be retrieved from the
TIMS analyses carried out to date on this sample.
A reconnaissance SEM imaging and SHRIMP study were carried out on representative
subpopulations of zircons from this syenite, and are reported elsewhere in detail (Appendix 2). In
summary, however, backscatter electron (BSE) imaging of sectioned and polished zircons from sample
03JAA-0006 (see Figure 18) confirmed the presence of broad, subhedral to slightly rounded and embayed
cores, locally, but not always rimmed by a very thin shell of bright (higher U, Th) exterior zircon growth
that is often sharply magmatic. Ion microprobe U/Pb analysis of approximately 20 core domains revealed
ages as old as ca. 2729 Ma, but the complete spectrum of ages form a broad cluster near concordia
between this age and approximately 2675 Ma. Insufficient data were acquired on the very thin
magmatic(?) overgrowths to permit a precise assessment of the timing of the igneous mantling of the
xenocrystic cores. The ages determined on the zircon cores are consistent with the established
provenance age modes known from Timiskaming detrital grains.
The BSE imaging and in situ U/Pb evidence, together with the TIMS results presented above,
provide strong support for the interpretation of the bulk of the zircon age data to reflect an inherited,
xenocrystic origin, presumably from the host Timiskaming assemblage sediments. The magmatic age of
the syenite porphyry remains unclear, although independent new regional age constraints on host
Timiskaming supracrustal units suggest that it may be younger than 2678 Ma or even 2670 Ma.
04VOI-4529 Intermineral dike, Teck Township
Sample 04VOI-4529 was collected from an intermineral dike associated with quartz vein mineralization
at Macassa. Unfortunately, no accessory phases were found in the heavy mineral concentrate from this
sample that were amenable to U/Pb geochronology.
04VOI-1518-4 Crosscutting dike, Teck Township
Sample 04VOI-1518-4 represents a dike which crosscuts a folded contact of syenite porphyries and tuffs,
and constrains a maximum age for D4 deformation. Although a heavy mineral separate was recovered
from this sample, no datable minerals were observed.
125