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

608 OLIVO ET AL.
Kirkland Lake, Dome, Kerr Addison, Sigma-Lamaque, and
Pamour mines (Jenkins et al., 1997). These deposits formed
through complex and multistage processes, allowing for concentration
of gold at average grades typically around 10 g/t
and locally attaining more than 100 g/t.
Most studies of the processes related to the gold concentration
in these world-class deposits have focused on the ore
zones with less attention on the related veins and alteration
systems that are barren. The youngest vein system in the
Sigma deposit in the Abitibi greenstone belt, the North Dipper
system (Gaboury et al., 2001), provides an opportunity to
evaluate the processes related to ore formation because this
system is relatively young and less deformed than early systems,
and auriferous and barren veins are well exposed in the
Sigma pit and in the North zone, respectively, allowing for detailed
mapping and sampling.
In this study, detailed field observations reveal that the
geometry of the plumbing system and its position relative to
the major fluid conduit are fundamental for formation of the
auriferous zones. Petrographic descriptions, microthermometry,
crush-leach chemical analyses, and time-of-flight laser ablation
induced coupled plasma-mass spectrometry (TOF-LA-
ICPMS) of fluid inclusions, and stable isotope data also show
that the mechanisms for gold precipitation in the North Dipper
and related veins changed during the evolution of the system.
Comparisons of the elements enriched in the auriferous
fluids versus the barren fluids suggest various sources for the
metals associated with the gold, and the isotopic composition
of the fluids that carried the gold indicates low fluid/rock
ratios.
Regional Geologic and Tectonic Setting
The Val d’Or district is located in the eastern segment of
the southern Volcanic zone (Daigneault et al., 2002) of the
Archean Abitibi subprovince at the boundary with the Pontiac
subprovince (Fig. 1). This boundary is characterized by
an extensive deformation zone, referred to as the Larder
Lake-Cadillac break, which is interpreted as a suture zone between
these two subprovinces (Robert et al., 1995). The
Larder Lake-Cadillac break major fault zone dips to the north
and is spatially associated with numerous gold deposits and
occurrences (Robert, 1989, 1994). The geology of the Val
d’Or district has been described by Gunning and Ambrose
(1940), Norman 1947), Latulippe (1966), Dimroth et al.
(1982, 1983a, b), Imreh (1984), Desrochers et al. (1993),
Desrochers and Hubert (1996), and recently reviewed by Pilote
et al. (1997, 1998, 1999, 2000) and Scott et al. (2002). Information
from these sources, as well as from specific studies
in the area, is briefly summarized below.
The Val d’Or district comprises a complex sequence of volcanic-sedimentary
and intrusive rocks that evolved from
2714 to 2611 Ma. The volcanic-sedimentary sequences are
part of the Malartic Group (i.e., La Motte-Vasson, Dubuisson,
Lac Caste, and Jacola Formations) and the Louvicourt
Group (Val d’Or and Héva Formations: Scott et al., 2002).
The Malartic Group comprises mainly ocean floor komatiite
and tholeiitic basalt flows and sills, with minor sedimentary
rocks, which are interpreted to be formed in an extensional
environment related to mantle plumes. The Louvicourt
Group is composed mainly of mafic to felsic volcanic rocks
that formed in a subduction-related deep marine volcanic
arc. The studied barren and auriferous veins are hosted in the
transition zone between the Jacola and Val d’Or Formations,
which are described below.
The Jacola Formation extends for more than 5 km and consists
of a 1- to 2-km-thick sequence of pillowed, brecciated,
and massive tholeiitic basalts, intercalated with komatiite and
massive to pillowed komatiite flows (100–200 m thick). Discontinuous
interflow volcaniclastic deposits occur locally and
have been dated at 2703.08 ± 1.3 Ma (Pilote et al., 1999). This
sequence is interpreted to represent fissure-style eruptions
formed in a proto-arc environment. The contact between the
Jacola and Val d’Or Formations to the south is gradational and
interefingers laterally (Dimroth et al., 1982), being characterized
by a significant increase in the volume of volcaniclastic
deposits and a change to intermediate and felsic composition
at the base of the Val d’Or Formation (Scott et al., 2002).
The overlying Val d’Or Formation is composed of a 3- to 5-
km-thick sequence of discontinuous, interstratified, massive to
pillowed lavas and volcaniclastic deposits of andesite (50%),
dacite (30%), and rhyolite (20%). The lower units are tholeiitic
to intermediate, and the upper units are intermediate to
calc-alkaline in composition. The dacitic and rhyolitic units
have an age of 2704 ± 2 Ma (Wong et al., 1991; Machado et al.,
1992; Pilote et al., 1998). The distribution and composition of
the Val d’Or Formation rocks suggest that they formed as numerous
small volcanic vents analogous to a modern arc setting
(Scott et al., 2002). Primary volcanic features in both formations
indicate younging directions toward the south.
These volcanic and sedimentary sequences are intruded by
felsic to mafic bodies that have been grouped into three
classes (Pilote et al., 2000): (1) 2700 Ma synvolcanic (Wong et
al., 1991) stocks and batholiths, (2) 2680 Ma syn- to late-tectonic
intrusions (Jemielita et al., 1990; Pilote et al., 1998), and
(3) late- to post-tectonic, undeformed and unmetamorphosed
stocks and dikes intruded between 2675 and 2611 Ma (Feng
and Kerrich, 1991; Ducharme et al., 1997).
All these units, with the exception of the late stocks and
dikes, have been metamorphosed to greenschist facies and
multiply deformed in three major phases (Robert, 1990a, b;
Desrochers and Hubert, 1996). The first phase of deformation
(D 1 ) is recognized only locally; the second phase (D 2 ) is
the dominant structural trend in the area and is characterized
by tight and isoclinal folds, a regionally penetrative subvertical
east-west foliation, faults, and shear zones. Gold deposits
in the Val d’Or disctrict are hosted or spatially associated with
these faults and shear zones. The D 2 deformation event is estimated
to have occurred after 2672 Ma, the age of the detrital
zircons in sedimentary units folded by D 2 (Robert, 2001,
and references therein). The D 3 phase of deformation is characterized
by steep, open folds and associated east-northeast
crenulation cleavage and also involved the reactivation of the
D 1 and D 2 faults and shear zones.
Sigma Mine Geology: Host Rocks and Vein Systems
The geology and auriferous vein systems of the Sigma mine
have been described by Robert (1983), Robert and Brown
(1986a, b), Robert et al. (1995), Garofalo (2000), and Gaboury
et al. (2001). The relevant features observed by the authors,
integrated with previous studies, are presented below.
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