Overview of Results from the Greenstone ... - Geology Ontario
Overview of Results from the Greenstone ... - Geology Ontario
Overview of Results from the Greenstone ... - Geology Ontario
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04JAA-0010 Albitite dike cutting Tisdale assemblage ultramafic volcanic rocks,<br />
Whitney Township<br />
(NAD83, Zone 17 UTM 491529E, 5372949N; #20 on Figure 2 and Table 1)<br />
South <strong>of</strong> <strong>the</strong> Pamour Mine, near <strong>the</strong> Porcupine–Destor deformation zone (PDDZ) in Whitney Township,<br />
an 8 m thick, white albitite dike cutting ultramafic volcanic rocks <strong>of</strong> <strong>the</strong> Tisdale assemblage was<br />
intersected in drill core (DDH 18979; E. Barr, Placer Dome (CLA) Ltd., Porcupine Joint Venture,<br />
personal communication, 2004). Similar, though much thinner dikes were also observed intruding<br />
Timiskaming assemblage conglomerates at this locality, but were too small to investigate. Sample<br />
04JAA-0010 was collected <strong>from</strong> <strong>the</strong> thick albitite dike and consists <strong>of</strong> a fine-grained and sugary textured<br />
albititic feldspar with 1 to 2% disseminated pyrite and rare, fine-grained quartz phenocrysts. This dike<br />
yielded a small but adequate amount <strong>of</strong> good quality zircon. The best quality grains comprised essentially<br />
clear, nearly colourless prismatic forms characterized by (4-5:1) needles ranging up to 300 µm in length,<br />
or by short, stubby doubly terminated crystals. Most grains show a small amount <strong>of</strong> subrounding. Five<br />
single, clear and colourless to pale brown single zircons were analyzed for U and Pb; results are presented<br />
in Figure 5F and Table A1. The data range <strong>from</strong> being nearly concordant (0.6–0.7% discordant: A1d,<br />
A1e) to moderately discordant (5.5%: A1c). However, data for four <strong>of</strong> <strong>the</strong> five fractions are strongly<br />
collinear, and regression <strong>of</strong> <strong>the</strong>se four (A1b to A1e) through <strong>the</strong> origin yields a tightly constrained upper<br />
intercept age <strong>of</strong> 2676.5±1.6 Ma (95% fit; MSWD = 0.12). Fraction A1a contained elevated total<br />
common-Pb levels; <strong>the</strong> datum for this grain is resultingly imprecise, is almost 2% discordant and has a<br />
207 Pb/ 206 Pb age <strong>of</strong> 2665.2±13.9 Ma, falling to <strong>the</strong> left <strong>of</strong> <strong>the</strong> discordia. For <strong>the</strong>se reasons, <strong>the</strong> datum for<br />
fraction A1a is considered spurious and is not included in <strong>the</strong> age regression.<br />
The age result for <strong>the</strong> albitite dike in Whitney Township is similar to <strong>the</strong> provisional age <strong>of</strong> ca.<br />
2677.5 Ma for <strong>the</strong> nearby Pamour porphyry (03PJM-131) and <strong>the</strong> possibility <strong>of</strong> a genetic relationship<br />
between <strong>the</strong> two is raised. Burrows and Spooner (1986) and Wood et al. (1986), however, have argued,<br />
on <strong>the</strong> basis <strong>of</strong> geochemical comparisons with o<strong>the</strong>r porphyry bodies, that a petrogenetic linkage is<br />
doubtful. The 2676.5±1.6 Ma age is almost within error <strong>of</strong> <strong>the</strong> revised age for <strong>the</strong> albitite dike <strong>from</strong> <strong>the</strong><br />
McIntyre Mine (SM85-60: 2672.8±1.1 Ma; Corfu et al., 1989; this study). At <strong>the</strong> McIntyre Mine, <strong>the</strong><br />
albitite dikes are crosscut by an auriferous vein and alteration system and <strong>the</strong> albitite dikes, <strong>the</strong>refore,<br />
provide an important maximum age constraint on <strong>the</strong> timing <strong>of</strong> gold mineralization.<br />
03RJB-18972-6 Sandstone, Porcupine assemblage, Whitney Township<br />
(NAD83, Zone 17 UTM 491548E, 5372704N; #21 on Figure 2 and Table 1)<br />
This sample was recovered <strong>from</strong> <strong>the</strong> 140 to 148 m interval <strong>of</strong> Porcupine Joint Venture drill hole 18972,<br />
and comprised fine sands within a broader section <strong>of</strong> turbiditic beds between 124 and 152 m. This unit<br />
occurs south <strong>of</strong> <strong>the</strong> main unit <strong>of</strong> Timiskaming assemblage sandstone and south <strong>of</strong> <strong>the</strong> Porcupine–Destor<br />
deformation zone, in rocks previously mapped as <strong>the</strong> Whitney formation member <strong>of</strong> <strong>the</strong> Porcupine<br />
assemblage (after Born 1995).<br />
Zircon populations <strong>from</strong> this sample comprised a variety <strong>of</strong> clear and colourless, short-prismatic and<br />
slightly rounded grains, to pale and darker brown-red varieties as well as a distinct group <strong>of</strong> well-rounded<br />
and pitted grains. Analysis <strong>of</strong> five strongly air-abraded single grains by TIMS yielded an essentially<br />
bimodal clustering <strong>of</strong> concordant data having 207 Pb/ 206 Pb ages <strong>of</strong> 2726.2±1.6 Ma and 2722.9±3.7 Ma, and<br />
2691.6±8.0 Ma, 2689.9±2.4 Ma and 2689.5±1.7 Ma; <strong>the</strong> datum for a sixth grain (B4) is moderately<br />
discordant (3.2%) and has a younger 207 Pb/ 206 Pb age <strong>of</strong> 2658.8±1.5 Ma (Figure 6A; Table A1). The older<br />
detrital zircon populations likely reflect provenance <strong>from</strong> sources such as <strong>the</strong> Deloro assemblage (ca.<br />
2724–2730 Ma), whose present-day exposures are in relatively close proximity. This interpretation is<br />
supported, in part, by <strong>the</strong> euhedral nature <strong>of</strong> <strong>the</strong> two measured older grains. The younger ages, at ca.<br />
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