Olivo, G.R., Chang, F., and Kyser, T.K., 2006. Formation of the ...

610 OLIVO ET AL.
Host rocks
The auriferous veins of the Sigma mine are hosted exclusively
in the Val d’Or Formation and their intrusive rocks
(Fig. 2). The Val d’Or Formation in the mine site comprises
mainly calc-alkaline, andesitic, massive to pillow lavas intercalated
with volcaniclastic horizons and porphyritic phases
(Robert, 1983). The host rocks of the studied auriferous veins
in the Sigma open pit are mainly lapilli tuffs and minor lapilli
tuff breccias (West zone) and crystal tuffs (Central zone; Fig.
3A-B), described in detail by Chang (2002).
The studied barren North Dipper vein system crops out in
the North zone property (Fig. 4), located about 500 m north
of the Sigma mine open pit (Fig. 2). The rocks in this property
comprise mainly pillowed basaltic andesite with minor
massive flows and lapilli tuff. Preserved primary volcanic
structures indicate south-facing directions. The pillows are
deformed, showing an elongated shape, averaging 0.3 m in
width and 1.5 m in length, with weak foliation striking eastwest
and dipping steeply to the north. Based on their chemical
composition, these pillowed units are interpreted as part
of the Jacola Formation and are classified as tholeiitic,
basaltic andesite to andesite (Chang, 2002), according to the
criteria of Winchester and Floyd (1977) and Barrett and
MacLean (1999), respectively.
Three types of feldspar porphyritic intrusive rocks were
identified in the study areas. Type 1 feldspar porphyry is
found in the Central pit and North zones, where it forms irregular
bodies (3–5 m thick), cutting the volcanic and volcaniclastic
rocks. These dikes are correlated with C porphyry
(mine nomenclature, Robert, 1983), also referred to as porphyritic
diorite (Gaboury et al., 2001). Feldspar porphyry
type 2 was observed only east of the North zone, where it cuts
feldspar porphyry 1, strikes east-west, dips 84º to the south,
FIG. 2. Geology of the Sigma-Lamaque deposit, showing the various host rocks, major shear zones, and quartz tourmaline
veins. The zones mapped in detail are indicated by “W” and “C” (auriferous West and Central zones, respectively) and
“NZ” (North zone which hosts the barren North Dipper vein). Modified after Gaboury et al. (2001).
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