Soutenance mémoire
SULPHIDE MINERALIZATION
General Problem
Dykes are described by Pollard (1987) as vertical, or nearly so, sheet intrusions (tabular bodies) that cut discordantly the host rocks and are used as conduits for molten rock involved in heat and mass transport throughout the crust .
Dyke swarms are defined as numerous coeval dykes emplaced during a single event and the number can vary from tens to hundreds. Dispersion patterns vary according to the environment in which the dykes are emplaced in, and are controlled by different factors. These intrusions can occur in a wide variety of geologic and tectonic settings (i.e. divergent and convergent plate boundaries and intraplate environments) as in figure 1.2. Geometry and dispersion of the swarms, at the regional scale are controlled by the regional stress fields (Nakamura, 1977; Chadwick and Howard, 1991: figure 1.3) and gravitational stress in volcanic edifices (Fiske and Jackson, 1972).
Swarms can be irregular or they can follow a preferential orientation, where controlled by a horizontal stress field in extensional regimes, or as ring and radial dykes swarms where associated with sub-volcanic magma chambers related to volcanic edifices or central intrusive complexes (Ernst et al., 1995). Where related to volcanic edifices, some dykes are feeders to lava flows. They can be classified in three main patterns according to their topology, tectonic setting, and their composition: 1) regional or linear (figure 1.3A), 2) circumferential (figure 1.3B), and 3) radial (Acocella and Neri, 2009: figure 1.3C). The circumferential (concentric) and radial types of dykes swarms are documented in several studies, it occurs in the Slieve Gullion district in Ireland (Anderson, 1937), in the Fernandina volcano in Galapagos Islands (Acocella and Neri, 2009) and associated with an intrusive centre related to the Spanish Peaks, on Iceland (Gudmundsson, 1983). Even if the swarm is not presented as a radial and/or concentric pattern, they have an orientation associated with the structure of the intrusive centre (Ernst et al., 1995).
Dykes may also form as a late magmatic stage commonly associated with mafic to granitic intrusions. This type of dyke is known as aplite, which are finegrained to phaneritic leucocratic tabular intrusions associated with plutons. They are residual melts drawn into self-generated extensional fractures in the cooling and contraction within the intrusions (Best, 2002). Dyke morphology is controlled by 1) the elastic deformation of the country rock, and 2) the viscous resistance to flow which is also related to the fracture resistance of the host rock (Turcotte et al., 1987).
Textures, structures and the petrology of the dykes result of different factors. They are initially affected by the composition, the process of emplacement, characteristics of the host rock, and the tectonic environment. After emplacement, other contributing factors such as cooling rates, hydration, devitrification, hydrothermal alteration, and metamorphism may affect dykes. Detailed description of these features is used to determine the various processes involved during emplacement. The table 1.1 contains cited examples where distinct processes are interpreted from different types of textures and structures. It is also important to note that some of the features associated with dyke emplacement are analogous to textures observed during cooling of volcanic rocks.
Dykes are commonly associated with economic concentrations of metals. Dykes can play different roles in the mineralization process for distribution such as conduits, barriers, or as receptacles for hydrothermal fluids. According to the hydrothermal fluid source and the role of dykes for the mineralization, resulting deposits can be classified in three main types: 1) magmatic, 2) volcanogenic and 3) late. In deposits with complex geologic evolution, it can be difficult to attribute a main mineralization event due to superimposed processes. However, more important than the division itself, it is important to precise the role of the dykes in the mineralization processes. The main features for each type of deposit are summarized in table 1.2, with subsequent discussion below.
Magmatic deposits
This class constitutes a type of deposit where the metal source is magmatic as the intrusion-related deposits and the porphyries. The dyke swarms are usually associated with a central intrusion and represent conduits for hydrothermal fluids originating from the magma chamber. Mineralization usually occurs in quartz and carbonate veins within dykes and country rock, but also occurs as disseminated sulfide mineralization. Gold, silver, and copper are some metals that are commonly associated with this process. Lang and Baker (2001) classified this type as intrusion-related gold systems. In this classification, there is a distinct subgroup represented by the Au-Cu-Mo-W-Bi-(As-Te) Kidston mine in Australia, where the deposit is hosted in breccias associated with dykes and sills (Baker and Andrew, 1991). Another example is the Canadian Malartic mine (Sansfaçon et al., 1987) in Québec, Canada .
Volcanogenic deposits
Volcanogenic massive sulfides deposits (VMS) are the most studied type of this group, but it is not the only type. Another important type of mineralization is associated with dyke swarms related to volcanic centers. Mineralization is directly related to the dyke systems, which are conduits connected to the sub-volcanic magma chamber, serving as feeders for lava extrusion. Hydrothermal fluids rising from the chamber are commonly composed of mixed fluid sources that have both magmatic and seawater signatures. Metals associated with these processes are generally gold, silver, copper, zinc, and lead. They usually occur hosted in quartz and/or carbonate veins and stringers around the dykes, but are also disseminated in the dykes and volcanic rocks. Two classical examples of this type are Géant Dormant mine and Chevrier deposit, both located in the Abitibi greenstone belt, Québec, Canada .
Late deposits
Late deposits represent all deposits that cannot be classified as magmatic or volcanogenic. In this class, dykes can exercise a different genetic role in the mineralization process. As example, the Sigma gold mine in the Abitibi greenstone belt (also classifies as G-dyke style) have a system of flat veins, where dykes are the host rock. The hydrothermal fluids are commonly associated with late metamorphic events that remobilize metals. The competency contrasts between dykes and the country rock act as the control for fluid flow.
Another role attributed to late dykes is the barrier to contain and focus hydrothermal fluids in a preferential corridor. This was proposed at the Troilus gold mine , where the hydrothermal fluids associated with a later metamorphic one percolated through the more permeable host rocks and stopped at the boundary with the less permeable dykes. As a result, dykes are both the footwall and the hanging wall for the mineralization.
In all these examples, the compositions of the dykes are clearly not a criterion for establishing mineralization origin. The most important components are the fluids, their source and geometry, as well as the texture and structure of the dykes. Additionally, the dyke components are linked with the environment in which these sheet intrusions are emplaced.
Specific Problems
Monsabrais is a volcanic centre located in the Blake River Group of the Abitibi greenstone belt in the Superior Province of Canada, where the regional metamorphism reaches subgreenschist fades (Powell et al., 1993). The volcanic sequences are dominated by submarine mafic to intermediate lavas, with massive to pillowed lavas, as well as volcaniclastic rocks (Ross et al., 2008), and intruded by the Monsabrais pluton. The units are crosscut by fine to medium grained gabbroic, monzogabbroic and dioritic ring dykes. These rocks are considered as the magmatic roots of a subaqueous summit caldera (Mueller et al., 2009).
Fieldwork and geochemistry indicates that the Monsabrais pluton is a multiphase intrusion ranging in composition from tonalité to quartz diorite with U-Pb zircon age of 2696.2 + 0.9 Ma (Ross et al., 2008) and 2696.3 + 1.3 Ma (Mueller et al., 2007). Geochemical analyses of the volcanic units indicate basaltic to andesitic composition with transitional to calc-alkaline characteristics, but there is no clear field relationship between the volcanic faciès and the intrusive units (Ross et al., 2008).
The geometry and cross-cutting relationships of the dykes and the pluton are thought to represent the feeder system and sub-volcanic magma chamber, respectively. No detailed studies have been done in the area to determine the relative chronology between the pluton, the dykes, and the extrusive rocks, as well as the role played by the dykes for the mineralization observed in the region. Similarities between Monsabrais and other mineralization sites support the possibility of mineralization related to the intrusive rocks, and more specifically to the dykes.
Determination of different families of dykes
The first approach used in determining distinct families of dykes is field work. Important characteristics to observe are the mineralogy, where possible, primary textures and structures usually at the borders, the dyke geometry, and the crosscutting relationships between them and the other units.
Subsequent pétrographie observations with a microscope are important in classifying the different families according to primary mineral and hydrothermal alteration phases. Microscopic textures and structures can also reveal parameters about the emplacement and deformational history. Lithogeochemical analyses of major and trace elements analyzed by XRF and ICP-MS were performed by ALSChemex. Resulting data are used to determine the magma composition, affinity, and signatures of trace and REE elements in relation to tectonic setting. The lithogeochemistry and the mineralogical classification are complementary and permit a better division of dykes based on more than one characteristic.
Variety and characteristics of the mineralization and alteration assemblages
Geological mapping permits the determination of the characteristics and distribution pattern of the hydrothermal alteration and mineralization in the area. Field work is one of the most important components because observations can be used to determine which units are associated with mineralization. These observations are important criteria to determine if the mineralization is syn or post dyke emplacement. However, if the terrain is geologically complex, it is commonly necessary to use other techniques to establish these relationships.
Detailed description of the petrography and mineralogy is important to characterize the mineralization, such as 1) the sulfide composition, 2) alteration phases, and to establish, which metamorphic mineral assemblages are in equilibrium with the sulfides. Lithogeochemistry will be used to determine field anomalies, distribution patterns, and to characterize the composition of alteration and mineralization phases. Through these chemical analyses, it is possible to compare the least and most altered samples and determine what elements are defining the hydrothermal alteration and how this is reflected in the mineralogy.
Metallogenic context of the mineralization and the dykes
The last topic brings together all the other objectives, and constitutes a data compilation from the dykes, the other lithologies, the crosscutting relationships, the hydrothermal alterations, and the mineralizations resulting from observations made in the field, petrological descriptions, geochemical interpretations, and the established relative chronologies.
CONCLUSION
The Monsabrais Volcanic Complex is located in the important mining camp of Noranda. The new insights about the Blake River Group, interpreted as a megacaldera complex (Pearson and Daigneault, 2009), opened new perspectives for exploration. The gap of information about the relationship between lithologies, alteration and mineralization turned the Monsabrais Complex as an interesting area of research.
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Table des matières
CHAPTER 1 -INTRODUCTION
1.1 PROBLEM
1.1.1 General Problem
1.1.1.1. Magmatic deposits
1.1.1.2 Volcanogenic deposits
1.1.1.3 Late deposits
1.1.2 Specific Problems
1.2 OBJECTIVE
1.3 METHODOLOGY
1.3.1. Determination of different families of dykes
1.3.2. Variety and characteristics of the mineralization and alteration assemblages
1.3.3. Relative chronology for the emplacement and mineralization episodes
1.3.4. Metallogenic context of the mineralization and the dykes
1.4 REGIONAL GEOLOGY
1.4.1 Sub-Province of the Abitbi
1.4.2 Blake River Group
1.4.3 Monsabrais Volcanic Complex
CHAPTER 2 – LOCAL GEOLOGY
2.1 INTRODUCTION
2.2 VOLCANIC ROCKS
2.2.1 Massive flows
2.2.2 Pillow lavas
2.2.3 Volcaniclastic rocks
2.3 MONSABRAIS PLUTON
2.4 DYKES
2.4.1 Major dykes
2.4.1.1 West major dyke
2.4.1.2 East major dyke
2.4.2 Equigranular dykes
2.4.2.1 Aphanitic equigranular dykes
2.4.2.2 Phaneritic equigranular dykes
2.4.3 Porphyritic dykes
2.4.4 Aplitic dykes
2.5 VEINS
2.6 HYDROTHERMAL ACTIVITY
2.6.1 Quartz
2.6.2 Epidote
2.6.3 Chlorite
2.6.4 White mica
2.6.5 Carbonate
2.6.6 Other secondary minerals
2.7 DISCUSSION
CHAPTHER 3 – LITHOGEOCHEMISTRY
3.1 INTRODUCTION
3.2 ROCK CLASSIFICATION
3.3 MAGMATIC AFFINITY
3.4 SPIDERGRAMS
3.5 DISCRIMINANT DIAGRAMS
3.6 HYDROTHERMAL ALTERATION
3.7 DISCUSSION
CHAPTER4-SULPHIDE MINERALIZATION
4.1 INTRODUCTION
4.2 DISTRIBUTION
4.3 PETROGRAPHY
4.4 LA-ICP-MS
4.4.1 Methodology
4.4.2 Trace elements signature
4.5 DISCUSSION
CHAPTER 5 – DISCUSSION
5.1 SYNTHESIS OF RESULTS
5.1.1 U-Pbages
5.1.2 Field observations
5.1.3 Petrography
5.1.4 Lithogeochemistry
5.1.5 Sulfides
5.2 EVOLUTION MODEL
CHAPTER 6 – CONCLUSION
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