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

Timmins Area
In the Timmins area, the Timiskaming assemblage is confined to a narrow east-trending unit of
conglomerates and sandstone overlying the Tisdale and Porcupine assemblages. An angular unconformity
occurs along the north contact of the Timiskaming assemblage and it is truncated by the PDDZ and/or the
Dome fault to the south (see “Timmins Subproject”). Born (1995) subdivided the Timiskaming
assemblage in this area into the lowermost Dome formation, consisting of alluvial sediments grading
upwards into proximal and mid-fan turbidites, in turn overlain by the Three Nations formation grading
upwards from alluvial to fluvial to shelf-facies sandstones. Two samples from the Dome formation at the
Dome Mine were analyzed using the TIMS methods (Ayer, Barr et al. 2003): wacke from the basal part of
the Dome formation in the “Greenstone Nose” contains detrital zircons with ages ranging from 2710 Ma
to 2679±4 Ma, whereas conglomerate in the “sedimentary trough” yields a broader spectrum of detrital
zircon ages ranging from 2814 Ma to 2674±2 Ma (#11 and #12: Figure 2). Less precise SHRIMP results
from these 2 samples reveal bimodal age distributions with a significant pre-Abitibi population ranging in
age from 2820 to 2780 Ma and an Abitibi-age population ranging from 2730 Ma to approximately 2670
Ma (see Figures 12A and 12B). Detrital zircons from a pebbly sandstone unit with the Three Nations
formation in German Township about 25 km to the northeast indicate a considerably younger maximum
depositional age of 2669±1 Ma. In addition, older detrital zircons ranging from 2700 to 2816 Ma are also
present in this sample (Bleeker 1999).
Newly recognized portions of the Timiskaming assemblage consisting of conglomerate, sandstone,
siltstone and heterolithic volcanic breccia have been identified along the PDDZ west of the Dome Mine
(see Figure 2). A sample of heterolithic conglomerate from the Buffalo Ankerite Mine in Deloro
Township yielded ages of ca. 2682±4 Ma and 2669±7 Ma and a fine-grained epiclastic unit at the Nabob
Mine in Ogden Township yielded ages ranging from ca. 2673 Ma to 2722 Ma (Ayer, Barr et al. 2003).
Further to the west in Thorneloe Township, the youngest detrital zircon age of 2684.4±1.7 Ma from a new
geochronology sample of conglomerate closely associated with the PDDZ, in addition to older detrital
zircons with ages of 2705 Ma, 2713 to 2716 Ma, 2726 to 2728 Ma (#9: Table 1, Figure 4C), suggest this
unit may also be part of the Timiskaming assemblage, although the age of the youngest detrital zircon
also allows for the possibility that it may alternatively be part of the Porcupine assemblage.
East of Matheson Area
Timiskaming assemblage clastic sedimentary rocks composed of conglomerate, wacke-sandstone,
siltstone, argillite and schist are closely associated with the PDDZ from the Quebec border to Hislop
Township. These rocks were originally lumped together with structurally interleaved older volcanic
assemblages in a unit identified as the Destor–Porcupine Complex by Jensen and Langford (1985). Berger
(2002) reported banded magnetite-hematite iron formation is complexly interbedded and structurally
interleaved with clastic sedimentary rocks in Michaud Township, and sedimentary and alkalic volcanic
rocks correlative with the Timiskaming assemblage underlie the southeast part of Hislop Township, south
of the PDDZ, and display a close spatial relationship with the Hislop and Ross faults. Ropchan et al.
(2002) reported TIMS geochronology results from Timiskaming assemblage units in Holloway
Township. The youngest concordant detrital zircons from sandstone interbedded with conglomerate
yielded an age of 2684±1.3 Ma, a second discordant zircon (1.8%) yielded an age of 2676.5±1.6 Ma, in
addition to older detrital zircons with ages ranging from 2695 to 2724 Ma. The youngest detrital zircon
from a heterolithic volcanic breccia closely associated with the Lighting Zone mineralization at the
Holloway Mine (sample 3D-CG, #37: Figures 2 and 10A) yielded an age of 2686.6±1.8 Ma with older
zircons ranging from 2695 to 2717 Ma. SHRIMP analysis of this sample has yielded less precise zircon
ages ranging from 2682 to 2796 Ma (Figure 16).
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