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

PACAUD ASSEMBLAGE
The 2750 to 2735 Ma Pacaud assemblage is the oldest outcropping supracrustal unit identified to date in
the southern Abitibi greenstone belt (Ayer, Amelin et al. 2002). It consists predominantly of tholeiitic
volcanic rocks with calc-alkaline intermediate to felsic volcanic rocks and minor komatiites. The
assemblage crops out on the margins of the major batholiths such as the Round Lake and Kenogamissi
batholiths. The base of the assemblage is cut by batholiths and, thus, its relationship to any older
assemblage, such as the 2790 Ma unit in the northern Abitibi Subprovince within Quebec, is unknown
(Rhéaume et al. 2004). The recently discovered presence of 2860 to 2800 Ma zircon xenocryst in isolated
volcanic units within the study area (e.g., samples 96JAA-0094, 04BHA-0333 and 3D-CG) in conjunction
with similar-age xenocrysts in isolated synvolcanic batholith samples (Ketchum et al., in press) indicated
that older crust was locally sampled by the Abitibi-age magmas, but the precise nature of the relationship
to the older basement remains elusive as other radiogenic isotopic systems, such as Nd and Hf, display
signatures that typically indicate these magmas were derived from juvenile-depleted mantle, but also with
some isolated samples indicating minor contamination by ancient crust (e.g., Corfu and Noble 1992;
Ayer, Amelin et al. 2002; Ketchum et al., in press).
Our new SHRIMP data confirm speculation by Ayer, Amelin et al. (2002) that the Pacaud assemblage
was more widespread than its present known outcrop distribution indicates (see Figures 1 and 2). This is
indicated by xenocrystic zircon cores found within samples scattered across the southern Abitibi greenstone
belt, including within a Deloro assemblage sample (97JAA-0106) overlying the Pacaud assemblage in the
Shining Tree area; a Tisdale assemblage sample (96TB-079) overlying the Deloro assemblage in Langmuir
Township, south of Timmins; and within Timiskaming assemblage volcanic units within the Swayze area
(99JAA-0057) and Holloway Township (3D-CG) (see Figures 9 and 10).
DELORO ASSEMBLAGE
The 2730 to 2724 Ma Deloro assemblage (Ayer, Amelin et al. 2002) is composed primarily of calc-alkaline
volcanic rocks. In the study area, it is exposed on the eastern flanks of the Kenogamissi batholith, the east
flank of the Nat River batholith, in the core of the Shaw dome and north of Timmins (see Figure 2). It is also
found on the north flank of the Ramsey–Algoma granitoid complex in the Shining Tree and Swayze areas
and the western flank of the Kenogamissi batholith in the Swayze belt (see Figure 1). A similar age range is
displayed by rocks of the Hunter Mine Group (Mueller and Mortensen 2002) to the east in Quebec, which
we equate with the Deloro assemblage. In most instances, the assemblage is underlain by rocks of the
slightly older Pacaud assemblage. The age range of the 2 assemblages suggests a gap of about 5 my. The
Deloro assemblage is underlain by chert breccia in the Shining Tree area (Johns and Amelin 1998; Johns
1999). This chert breccia may be indicative of a basal unconformity for the Deloro assemblage. The
assemblage is capped by a regionally extensive unit of oxide facies iron formation in the Woman River and
Hanrahan antiforms of the Swayze greenstone belt and in the localities listed above.
New U/Pb TIMS analyses of zircons from felsic tuff intercalated with iron formation and ultramafic
volcanic rocks in the northern part of the Shaw Dome yielded an age of 2724.1±3.7 Ma (#10: Table 1,
Figure 4D). Based on this new Deloro-age sample from the northern margin of the Shaw Dome (#10:
Figure 2), we have revised the interpretation of a Krist formation unit occurring immediately south of the
Porcupine–Destor deformation zone (PDDZ) in Deloro and Ogden townships, which was based on the
2685.1±1.3 Ma age for a foliated quartz-feldspar porphyritic felsic rock in Ogden Township (Ayer, Barr
et al. 2003). We now believe this nondescript quartz feldspar porphyritic rock is a porphyry intrusion
cutting a package of structurally interleaved Deloro, Tisdale and Timiskaming assemblage units proximal
to the PDDZ in Deloro and Ogden townships. This porphyry age fits quite well with the age of the
slightly younger porphyry intrusion found at the Hoyle Pond Mine (see “Intrusion Framework”).
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