be modern analogues. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Archean; Mafic-felsic volcanism; Subaqueous; Volcanic facies; Composite arc volcano; Geochemistry
1. Introduction
The majority of volcanogenic massive sulphide deposits VMS in the geologic record are associ-
ated with rhyolitic rocks Rona, 1988; Barrie and Hannington, 1997, whereas modern VMS coun-
terparts are observed mainly in mid-ocean ridge spreading centers which are generally basaltic in
composition Rona, 1988. Only recently has there been a shift in research emphasis to VMS mineral-
ization in modern arc-backarc systems Rona and Scott, 1993; Ishibashi and Urabe, 1995. Studies
on active maficfelsic and felsic submerged arc- backarc settings are restricted to submersible
dives, such as the Sumisu Rift Smith et al., 1990, the Myojin knoll Iizasa et al., 1999, the Lau
Fouquet et al., 1993 or Mariana backarc basins Lonsdale and Hawkins, 1985 as well as the
Kermadec arc volcanoes Wright et al., 1998. In contrast, ancient arc sequences Sato and Amano,
1991; Kano et al., 1993; Martin-Barajas et al., 1995 are important windows for the study of
modern subaqueous arc-forming processes. The Abitibi Subprovince, a well documented Archean
greenstone belt, is composed of numerous felsic or maficfelsic volcanic edifices Chown et al., 1992,
which favors in-depth volcanological analyses of subaqueous felsic volcanic complexes related to
VMS deposits. Understanding the physical vol- canology of submerged volcanoes is important for
establishing eruptive mechanisms, mode of vol- canic emplacement and geometry of volcanic
edifices, as well as defining proximal and distal volcanic facies with respect to VMS deposits.
Characteristics of felsic and mafic Archean lava flows have been addressed in detail Dimroth,
1977; Dimroth et al., 1978; de Rosen-Spence et al., 1980; Cousineau and Dimroth, 1982, but
little emphasis has been placed on the evolution of entire subaqueous volcanic complexes Gibson
and Watkinson, 1990; Morton et al., 1991. The principal aim of this study is to document the
volcanic construction of the mafic to felsic Norme´tal volcanic complex NVC in the Abitibi
greenstone belt. A two-fold approach based on regional and detailed mapping, sampling and
structural analysis was conducted emphasizing: 1 physical volcanology; and 2 geochemistry. In
addition, the Norme´tal volcanic complex is com- pared to other volcanic centers in the Abitibi belt
and modern arc volcanoes in order to constrain Archean arc models.
2. Geological setting
2
.
1
. Abitibi greenstone belt and regional geology The 300 × 700 km Abitibi greenstone belt, the
largest coherent Archean supracrustal sequence in the world, hosts numerous volcanic centers with
economically important VMS deposits Franklin et al., 1981; Chartrand and Cattalani, 1990.
Chown et al. 1992 divided the Abitibi belt into northern NVZ and southern volcanic zones
SVZ; Fig. 1 based on the work of Dimroth et al. 1982, U-Pb age determinations Mortensen,
1993a,b, stratigraphic and structural relation- ships Daigneault and Archambault, 1990, geo-
chemical signatures Ludden et al., 1984; Picard and Piboule, 1986a,b, and sedimentary basin
analysis Mueller and Donaldson, 1992. The NVZ is composed of volcanic cycle 1 2730 – 2720
Ma, representing an extensive, subaqueous primi- tive basalt plain with dispersed maficfelsic and
felsic complexes, which collectively define a dif- fuse and incipient arc. Volcanic cycle 2 2720 –
2705 Ma is consistent with a mature and dissected volcanic arc. Mueller et al. 1996 di-
vided the Abitibi greenstone belt into four distinct periods or tectonic stages: 1 arc formation and
construction 2730 – 2698 Ma; 2 arc – arc colli- sion 2696 – 2690 Ma; 3 arc fragmentation
2689 – 2670 Ma; and 4 arc exhumation 2660 – 2640 Ma. The Norme´tal volcanic complex
2727.7 + 2.6 − 2.1 Ma, Mortensen, 1993a, rep-
resenting the arc formation and construction pe- riod, is one of the least known maficfelsic centers
belonging to volcanic cycle 1 of the NVZ Fig. 1. The study area is divided into north and south
Norme´tal blocks Fig. 2, with the northern block composed of i basalt and iron formation of the
Gale Group Latulippe, 1976; and ii the Norme´tal volcanic complex NVC. Basalt, andes-
ite, iron formation and gabbro constitute the south Norme´tal block Pe´loquin, 1994. The con-
tact between the NVC and the south Norme´tal block is the Norme´tal deformation zone, charac-
terized by pure shear with local dextral reactiva- tion patten fault; Fig. 2; Lafrance et al., 1996.
The good rock exposure, principally in the west- ern part of the NVC, coupled with the limited
influence of deformation outside the regional fault corridors, enable good recognition of volcanic
textures and estimation of unit thickness and lat- eral extent. Elsewhere, industry diamond drilling
and geophysical surveys compensate for scarcity of outcrops. Greenschist facies metamorphism is
prominent in the NVC whereas upper greenschist facies biotite isograd and amphibolite facies
metamorphism
is observed
only near
the Rousseau and Patten plutons. A metamorphic
hydrothermal assemblage of quartz + sericite + chlorite + carbonate + chloritoid 9 tourmaline
is characteristic of the study area Teasdale, 1993;
Pe´loquin, 1994. A ubiquitous semi-concordant carbonate 9 chloritoid alteration zone is consis-
tent with a Mattabi-type VMS deposit Morton and Franklin, 1987. Precise plutonic U-Pb zircon
age determinations yielded 2716 + − 3 Ma for the
Val-St-Gilles pluton
Vaillancourt and
Machado, 1995, 2710 + −
2 Ma for the Norme´tal pluton Zhang, in Pe´loquin, 1994, and
2703 + − 2 Ma for the Rousseau pluton Davis et al., 1992.
Fig. 1. Location of the Norme´tal volcanic complex and other volcanic centers of the Abitibi greenstone belt. The supracrustal sequence is divided into distinct volcanic and sedimentary cycles. Study area is outlined. Modified from Chown et al. 1992 and
Mueller et al. 1996.
B .
Lafrance et
al .
Precambrian
Research
101 2000
277 –
311
Fig. 2. Regional geology of the Norme´tal volcanic complex NVC. The NVC is geographically divided into western Fig. 4, central Figs. 5 and 6 and eastern segments Fig. 7; see Fig. 1 for location.
Fig. 3. Schematic stratigraphic column of the Norme´tal vol- canic complex and adjacent units. The Norme´tal volcanic
complex includes the 4 km-thick sequence of mafic-felsic vol- canic rocks, the Norme´tal sedimentary rocks and the synvol-
canic Norme´tal and Val-St-Gilles plutons. Legend as in Fig. 2.
Norme´tal mine 11 Mt grading 5.12 Zn, 2.15 Cu, 0.549 gt Au and 45.25 gt Ag; Teasdale,
1993 and
the satellite
Normetmar deposit
160 000 t at 12.6 Zn, as well as associated volcanic rocks and intrusions of phase 5, are
herein referred to as the Mine Sequence Figs. 2 and 3.
3. Volcanology of the Norme´tal volcanic complex
