The Archean Abitibi and Pontiac Subprovinces (Figure 1-1) are located in the southern part of the Superior Craton. The Abitibi Subprovince is renowned as one of the richest gold and base metal producing region of the world. This plutonic and supracrustal belt is mainly composed of volcanic rocks ranging from komatiites to rhyolite, sediments intruded by many plutonic bodies with ages varying in a range of 2670 to 2759 Ma (Goutier and Melançon, 2007 and references therein). The Abitibi Subprovince has been defined as a collage of two arcs, the older Northern and younger Southern volcanic zones (Chown et al., 1992). However, recent studies indicate that the Abitibi Subprovince could be much more complex than a simple collage of younger volcanic episodes onto old ones (Thurston, 2008). In many cases, geochronological data show a dismembering of older volcanic episodes with rifting during extrusion of younger volcanics (Goutier et al., 2008). Two main faults cross cut the Abitibi Green stone belt, Cadillac Larder Lake Fault (CLLF) and the Porcupine Destor Fault (PDF). The Pontiac Subprovince is (2,685 Ma to 2,672 Ma; Davis, 2002) located South of the Abitibi Subprovince is mainly composed of highly folded and deformed turbiditic sediments with rare horizons of mafic to ultramafic volcanics.
In the Province of Quebec, the limit between these two Subprovinces is marked by one of the most important crustal scale fault zones in the area, the Cadillac Larder Lake Fault (Fig. 1-1). This major structure has been interpreted as a suture zone separating these terrains of different affinity (Daigneault & Mueller, 2004, Kerrich & Feng, 1992). The CLLF has a lateral extent of more than 200 km and can be identified from the city of Matachewan in Ontario to the city of Val-d’Or in Quebec. More than 2000 Mt of gold have been extracted from its vicinity from many world class mining camps ( >100 Mt) such as Val-d’Or, Malartic, Cadillac, Larder Lake and Kirkland Lake (Poulsen et al., 2000).
Recent studies (Benn and Plesher 2005) based on analogue modeling, gravity models, seismic data and recent field work have interpreted the western segment of the CLLF and Porcupine Destor Fault located 200 km North as shallowly rooted fold-related structures. A consensus has been established on the fact that the eastern part of the CLLF actually represents a transcrustal structure marking the Boundary between the Pontiac Abitibi and the Pontiac Subprovinces (Dimroth et al., 1982) which lie respectively north and south of the CLLF.
Distribution of orogenic gold deposits along the CLLF
Spatial distribution of mineral deposits has been of great interest for the exploration and the research community even considering that mine or deposit distribution can only give an evaluation of the true metallic distribution of an area considering undiscovered deposits and economic factors. A systematic geographical distribution of mineralizations can be exploited as a valuable regional exploration tool and analogues to evaluate natural resource potentiality in under explored regions presenting similar geological context (McCammon, 1993). The mathematical function that determines the spatial distribution of orogenic gold mineralization can be used to determine fault segment which are the most favorable for undiscovered deposits along important Archean structures. Furthermore, the distribution of mineral deposits can provide insights on the formation of gold deposits.
A fractal approach was considered to explain the spatial distribution of precious metals within the Sierra Nevada (Carlson, 1991) and gold deposits in the Zimbabwe craton (Blenkinsop and Sanderson, 1999). The distribution of Archean orogenic gold deposits in the vicinity of crustal scale fault zone has also been tested along the Boulder-Lefroy Fault zone in Australia by Weinberg et al (2004) which noted a link with the presence of a gold mine and geological complexity. The approach proposed in this study is focussed on the distance separating each deposit from its neighbors and the possible relation with the distribution of mineralizing fluids pathways in structural dilation sites.
Available Data and Methodology
The spatial analysis of the distribution orogenic gold deposits along the CLLF was undertaken using information contained in the public database of the Ministry of Natural Ressources and Fauna of Quebec. The study focuses on the segment where the CLLF marks the boundary between the Abitibi and Pontiac. The length of this segment of the CLLF represents about 160 km and almost 80% of its total length.
The CLLF is often represented as a line on the 2D map even though the associated damage zone of this major structure represents a much larger area, corresponding to many hundred meters. To account for the thickness of the deformation zone and the multiple splays directly associated to the CLLF, a one kilometer buffer zone was established around the map trace of the CLLF in the sector of interest. All deposits located within this zone were used to conduct the spatial analysis. Mineralized occurrences defined by a single sample or a drill hole interception were not taken into account. The final deposit database is presented in table 1.
The deposit database was filtered to conduct the analysis only on deposits belonging to the orogenic gold type, and avoid incorporating other mineralization types with distinct characteristics, source, timing, and formation mechanism. Other types of gold mineralizations encountered in the study area are mainly volcanic massive sulfides or porphyry type deposits. Individual studies of deposits were used to determine the type of mineralization (Couture, 1996, Legault & Rabeau, 2006; Pilote et al., 2000; Lafrance et al., 2003; , Poulsen et al., 2000; Robert et al. 2005; Dubé and Gosselin, 2007). Certain known characteristics of orogenic gold deposits were also used to discriminate orogenic gold deposits from other types of gold mineralization. The metal content was the main discriminating factor on the basis that f the mineralizing fluids involved in the formation of orogenic gold have very low salinity deposits implying low base metal content (Groves et al., 2003).
Regional potential mapping using structural failure location
The location of known deposit should correspond to the localization of a structural failure du to near lithostatic fluid pressure. The cumulative frequency distribution FD [d] of the curvilinear distances D between two neighbor deposits is the proportion of occurrences which were found within a distance d of a known deposit. By considering the number of deposits in the study, this relation can be interpreted as the conditional probability of finding a mineralized occurrence at a distance less than d from a known mineralized occurrence along a fault zone. The theoretical fitted distribution FD [d] was used to evaluate the probability of potentially unrecognized mineralized occurrences within the close vicinity of the CLLF. Deposits presenting inter-distance greater than predicted by the theoretical model (Fig. 1-4) were interpreted as indicating potential zones with possible unrecognized mineralized occurrences. To determine the probability distribution, the trace of the CLLF was regularly sampled every 10m in order to calculate the curvilinear distance to the nearest known projected deposit. This curvilinear distance was interpolated laterally on a regular grid band of 2km in extension and centered on the CLLF, giving the distance d to the nearest known deposit of any locations situated in this zone (Fig. 1-6A). The log(d) uniform distribution model was then used to derive a probability map along the full length of the Eastern segment of the CLLF. Figure 1-6b presents those results for various levels of confidence.
The results presented in Figure 1-6 have to be interpreted with caution. Since the methodology used inter distance calculation between deposits, the distance measured for deposits located on the extremities of the faults are biased. Consequently, the part of the CLLF located near the Grenville front presents a higher occurrences inter distance compared to the rest of the fault, and therefore a highest derived probability, because no deposits are present passed the Grenville Front. The Eastern part of the studied portion of the CLLF presents the same bias but it is important to note that the Kerr Addison deposits is located only a few kilometers from the Ontarian border and that there is truly a lack of deposits in this area since the CLLF is covered by the Proterozoic sediments of the Gowganda Formation (Rabeau et al., 2009) which renders classical exploration techniques difficult.
Regional Gold Metallogeny
Although no mines are currently active in the sector, 50 t Au have been extracted along the CLLF in the study area (Couture, 1991). Additionally, a world class deposit, the Kerr-Chesterville (336 t Au; 1930-1996 (Guindon et al., 2007)), occurs along the western extension of this segment of the CLLF in Ontario. Four different types of gold mineralizations have been recognized in the study area (Legault and Rabeau, 2007): (1) Gold-rich quartz-carbonate ± tourmaline ± sulfides constitute the most represented type (70 %). Typical examples are the Stadacona, Astoria and Senator-Rouyn mines (Couture, 1996). (2) Replacement type deposits (~20 %) is associated with a strong pervasive albite/sericite, carbonate alteration and disseminated sulphides. The Francoeur and Wasamac deposits (Couture et Pilote, 1993) are representative examples. (3) Mineralization spatially associated with syenitic intrusions (~5%) includes two subtypes: quartz-carbonate veins such as the Granada Mine (Couture and Willoughby, 1996) and in a lesser manner disseminated sulphides enriched in Au-Cu- (Couture and Marquis, 1996; Legault and Lalonde, in prep.). (4) Volcanogenic massive sulphide (VMS) deposits related polymetallic mineralization (~5%) are also encountered in the study area (Legault and Rabeau, 2007). Additionally, some occurrences of epithermal-type mineralization have been observed north of the study area near the transcrustal Porcupine-Destor Fault (Legault et al., 2005) but have not been described near the CLLF.
More than one type of mineralization can coexist in a single deposit as observed in the Kerr-Chesterville deposit located within the CLLF west of the study area. This deposit (the second largest along the CLLF) is composed of a quartz-carbonate vein type (carbonate ore) and a replacement type (flow ore) (Kishida and Kerrich, 1987; Smith et al., 1990).
Most of the gold mineralisation considered in this study belongs to a broad class of orogenic gold deposit types. These deposits are characterized by epigenetic, structurally controlled mineralization located in accretionary orogens that formed over a large crustal-depth range from deep seated fluids (Groves et al., 1998). The quartz-carbonates vein, replacement and veins associated to syenite types fit with this definition.
Orogenic gold mineralizations post-date regional metamorphism, plutonism and early phases of orogenic deformation (Groves et al., 2000; Goldfarb et al. 2001) combined with their location within a craton imply that the present geometry of the host terrains can be considered as very similar to the one present during ore formation. At local scale, displacement may be observed on ore but at regional scale the displacement should be minor.
Geological modeling enhances the geological understanding of a specific region by allowing interpretations using all available geoscientific data in a 3D environment. A thorough comprehension of the geological setting and an abundance of quality 3D data are prerequisites for the construction of such models. As opposed to a 2D environment, the 3D approach offers the benefit of avoiding bias generated by projection of data on the surface and allows the calculation of true distances between objects taking depth and 3D geometry into account (Fig. 2-3). The geology and structural context of the study area are well defined and its proximity to the prolific Rouyn-Noranda central camp provides access to a multitude of high quality data for the sector (Table 2-1). These data were first compiled with standardized units and referenced using the UTM NAD83 spatial projection.
A 3D geological model (50 x 9 x 1.5 km3) was built based on 23 vertical cross-sections (Fig. 2-4a) at a mean spacing of 2 km. The sections were interpreted by recent geological mapping in the study area (Legault and Rabeau, 2006, 2007), and compiled structural and drill hole data (2-1). These sections then served as geometrical constraints for building triangulated surfaces representing the boundaries of geological units (Fig. 2-4b). The creation of triangulated surfaces respecting the range of constraints of available data was carried out using the Discrete Smooth Interpolation method (Mallet, 1992; Fig. 2-4c). Finally, a volumetric model was derived from the partition of space by the surface model (Fig. 2-4c) composed of the ribbons representing geological contacts and faults.
Table of contents :
CHAPITRE 1: SPATIAL DISTRIBUTION OF OROGENIC GOLD DEPOSITS ALONG MAJOR ARCHEAN FAULTS
1.1 Introduction générale
1.4 Geological setting
1.5 Distribution of orogenic gold deposits along the CLLF
1.6 Available Data and Methodology
1.6.1 Validation of the spatial distribution
1.7.1 Regional potential mapping using structural failure location
1.11 Annexe 1
CHAPITRE 2: GOLD POTENTIAL OF A HIDDEN ARCHEAN FAULT ZONE: THE CASE OF THE CADILLAC-LARDER LAKE FAULT
2.1 Introduction générale
2.4 Geological Setting
2.5 Geological Modeling
2.5.1 Modeling the Archean-Proterozoic Unconformity and the Hidden Archean Units
2.5.2 Geophysical Inversion
2.6 Spatial Analysis of Mineralized Occurrences
2.6.2 Faults and Fault Intersections
2.6.3 Distance to Intrusive Rocks
2.7 3D Mineral Potential Mapping under the Proterozoic Cover
2.7.1 Cross-Validation of the 3D potential map
CHAPITRE 3: 3D STRAIN MODELING DRIVEN BY FIELD DATA IN THE VICINITY OF THE CADILLAC LARDER LAKE FAULT ZONE: INSIGHTS ON ARCHEAN OROGENIC GOLD DISTRIBUTION
3.1 Introduction générale
3.4 Geological Setting
3.5 Tectonic evolution and timing of gold mineralizations
3.6 Geomechanical modeling
3.7 3D Geological Model and Strain Modeling applied to the CLLF
3.7.1 Modeling parameters
3.7.2 Boundary conditions
3.8.1 Spatial association between mineralized sites and dilatant zones