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

04BHA-0462 Granophyre, Kamiskotia Gabbroic Complex, Robb Township
(NAD83, Zone 17 UTM 451049E, 5381502N; #3 on Figure 2 and Table 1)
In central Robb Township, a granophyric phase from the Upper Zone (Barrie 1992) of the large Kamiskotia
Gabbroic Complex (KGC) crops out approximately 1.7 km northwest of Kamiskotia Lake. A representative
sample collected from this locality (04BHA-0462) consists of quartz, altered plagioclase, granophyric
intergrowth, and minor opaques, chlorite and epidote. Zircon recovery from the sample was sparse, but an
adequate number of good quality grains were found for analysis. These comprised colourless to pale pinkbrown
irregular grain fragments, parts of skeletal grains and rarer faceted crystals showing low-order faces.
Conventional isotope dilution U/Pb analyses of three abraded single zircons are concordant to slightly
discordant (0.2–0.6%) and yield 207 Pb/ 206 Pb ages between 2704 and 2706 Ma (Table A1). Weighting of all
three analyses yields a mean age of 2704.8±1.4 Ma (53% fit, MSWD = 0.64; Figure 3C), which is
considered a robust estimate for the age of crystallization of the Upper Zone of the KGC.
The 2704.8±1.4 Ma age for the KGC Upper Zone granophyre is slightly younger than, but within
error of, a 2707±2 Ma age determined for a stratigraphically lower, Middle Zone gabbro dated by Barrie
and Davis (1990). The granophyre age is also within error of a 2703.1±1.2 Ma age for one of the lowest
dated units (03BHA-0047; this study) of the Kamiskotia Volcanic Complex, which the gabbro and
granophyre appear to intrude.
03BHA-0382 Rhyolite, Halfmoon Lake, northeastern Robb Township
(NAD83, Zone 17 UTM 452485E, 5383020N; #4 on Figure 2 and Table 1)
This sample represents a section of rhyolite recovered from diamond drill hole collared west of the Kam-
Kotia Mine, at the Falconbridge Halfmoon Project, approximately 200 m south of Halfmoon Lake.
Material for U/Pb dating was collected from the 97–100.5 m section of hole R-44-14.
The general zircon population from this sample comprises poor to moderate quality, short, pale- to
medium-brown stubby, often ragged, prisms. Better quality grains show sharp, prismatic exteriors although
many grains are also broken. A selection of the highest quality, short (2:1 or less) and/or broken, clear and
crack-free prisms, essentially devoid of inclusions, were chosen for abrasion and analysis.
Three single grains yield completely overlapping data, straddling concordia and ranging between 0.2
and 0.4% reversely discordant (Figure 3D; Table A1). These results produce a weighted mean 207 Pb/ 206 Pb
age of 2700.0±1.1 Ma (40% probability of fit; MSWD = 0.9), which is interpreted as the primary
crystallization age of the rhyolite. The 2700 Ma age for the Halfmoon Lake rhyolite lies within error of
the age of felsic volcanic rocks from the Genex Mine (03BHA-0345) and confirms a Blake River
assemblage age for the volcanic stratigraphy immediately north and east of Kamiskotia Lake.
03BHA-0384 Felsic lapilli tuff, Kam-Kotia Mine, eastern Robb Township
(NAD83, Zone 17 UTM 455853E, 5381788N; #5 on Figure 2 and Table 1)
This sample of felsic lapilli tuff was collected from outcrop approximately 500 m southeast of the Kam-
Kotia Mine in easternmost Robb Township. Zircons recovered from this sample comprise fairly clear,
colourless to pale brown short and elongate prism populations. Apatite and fluid inclusions are relatively
common. Uranium and Pb isotopic data obtained for four single short prisms are all overlapping and
concordant and yield an average 207 Pb/ 206 Pb age of 2701.1±1.4 Ma (97% probability of fit, MSWD =
0.09; Figure 3E; Table A1). This age, interpreted to reflect the primary crystallization of the tuff, is
nearly identical to those obtained from other volcanic units between Halfmoon Lake and the Genex Mine
and appear to corroborate a Blake River assemblage age for this sequence.
112