Fluid Dynamics and Fluid-Structural Relationships in the Red Lake Mine Trend, Red Lake Greenstone Belt, Ontario
The Red Lake mine trend in the eastern Red Lake greenstone belt is a SE-trending deformation zone hosting the world-class Campbell-Red Lake gold deposit and a number of smaller gold deposits and occurrences. Multistage deformation events have been identified in the trend, with the major structures being SE-trending folds, brittle-ductile fault zones and a pervasive SE-striking S2 foliation. These structures formed during D2 deformation with NW-SW shortening and they controlled major hydrothermal alteration and gold mineralization. The Red Lake mine trend is characterized by numerous banded colloform-crustifrom, carbonate-quartz veins and cockade breccias. The majority of these carbonate-quartz veins parallel the regional S2 foliation in the host rocks, however the colloform-crustiform textures with carbonated fibers at a high angle to vein walls indicate open-space filling. Measurement of the orientations of fluid-inclusion bearing microfractures reveals that there are two sets of microfractures in the quartz-carbonate veins. One set is parallel to the S2 foliation, whereas the other is at a high angle to this foliation. The two sets of microfractures crosscut each other, and no relative timing relationship can be established, suggesting that they may have been formed repeatedly in the same period of time, although not contemporaneously at any given time. Microthermometric studies have been carried out on secondary and pseudosecondary fluid inclusions along these microfractures. Gold-bearing inclusions along the same fractures indicate that the inclusions formed synchronous with gold mineralization. The similarity in fluid inclusion types and microthermometric data between these secondary-pseudosecondary inclusions and primary inclusions suggest that microfracturing and gold mineralization took place repeatedly during the multiple and incremental formation of the veins. This study confirms that the gold mineralization in the Red Lake mine trend was controlled by a CO2-dominated fluid system, and the variation of homogenization temperatures (Th) of carbonic fluid inclusions reflects a significant pressure fluctuation during the formation of quartz-carbonate veins and deformation processes. Maximum fluid pressures correspond to a formation depth of 7 to 14 km, which is compatible with an orogenic origin of the gold deposits. The alternating development of veins-microfractures parallel to the S2 foliation and microfractures at a high angle to this foliation may be related to periodic fluctuation of fluid pressures. At very high fluid pressures, fractures and microfractures parallel to foliation would be forced to open even though they are perpendicular to the regional compressional stress, whereas at low fluid pressures, microfractures at high angles to the foliation would form under the regional stress field. Alternatively, the orientations of the veins and microfractures may be explained by a change of the stress field, possibly related to doming associated with nearby pluton emplacement. However, such a hypothesis does not seem compatible with the repeated change in orientation of the microfractures. In conclusion, our microstructural and fluid inclusion studies suggest that gold mineralization in the Red Lake mine trend resulted from the focusing of fluid flow along deformation zones and were accompanied by repeated paleo-seismic events.
The Development of Shear Band Cleavages & C'-type Shear Bands in a General Shear Zone & Their Implications for its Kinematic History
Conjugate shear band cleavages (SBC) and well-developed synthetic and antithetic C' shear bands in an extending shear zone suggest changing deformation parameters during increments of the same progressive deformation. The structures are developed in Archean talc-chlorite schist in the Marbanite shear zone in the Abitibi greenstone belt, Val d'Or, Quebec, a major NW-SE striking, steeply-north-dipping regional dextral non- coaxial shear zone associated with gold mineralization. The shear zone is characterized by a complex distribution of well-developed classic dextral S-C foliation, bands of intense C-foliation, ~E-W to NE-SW- trending Z-folds, and areas of complex folds. Synthetic and antithetic SBC is developed in the bands of intense C-foliation. New field data from the Marbanite shear zone suggests that shear band cleavages (SBC) do originate sub- parallel to the bisectors of the angles between the eigenvectors (AB and OB) in natural shear zones undergoing general flow. After initiation, both the synthetic and less-well-developed antithetic SBC undergo backward or forward rotation toward the stable eigenvector (the shear zone boundary) as material lines. Once they rotated out of favourable positions for the accommodation of extension in the flow, isolated, through-going C'-type shear bands developed. Extension along the dominantly synthetic C' shear bands deformed and/or reactivated some SBC in an antithetic sense. Late, antithetic C'-shear bands occur at smaller angles to the shear zone boundary than the antithetic SBC, and cut all structures in the shear zone. If SBC are precursors to the development of C' -shear bands, and both initiate sub-parallel to AB and OB, then the data suggest that dextral non-coaxial strain was approximately homogenous during the main phase of progressive deformation in the Marbanite shear zone, and was followed by a late increment of more-strongly-coaxial strain. These observations suggest the possibility of tracking changes in the vorticity of flow during progressive deformation using the orientations of SBC and C' populations, and a long and complex history for strain accommodation along a major regional shear zone in the Val d'Or mining camp.
Yttrium Zoning in Spiral Garnet Indicating Isotropic Garnet Growth
Grain shape and major-element zoning patterns have been used to determine whether individual garnets with spiral shapes grew in rotational or non-rotational environments. In some studies manganese zoning patterns have been used to delineate crystal shape during growth, but Mn zoning can be ambiguous because of rounding of the corners of crystals and fast diffusion rates at high metamorphic grade. We examine the relationship between Y-zoning patterns and microstructure in spiral garnet porphyroblasts from the Paleoproterozoic Knob Lake Group in the Gagnon terrane of the Grenville Province of western Labrador. Spiral garnet porphyroblasts from garnet to kyanite zone calc-pelitic rocks show single or double euhedral Y-annuli of various widths formed by the breakdown of Y-rich accessory phases. The shapes of Y-annuli suggest that garnet porphyroblasts maintain their euhedral shapes during the growth of spiral garnet. Y-annuli intersecting inclusion trails indicate isotropic garnet growth rather than preferential growth along the spiral arms. Constant widths of Y-annuli are also consistent with isotropic growth in spiral garnets. Double Y-annuli in a sigmoidal garnet porphyroblast show no sign of rotation relative to each other. These Y zoning patterns can be formed by isotropic growth of garnet without rotation or with rotation if the entire garnet grain rotates as one body. Therefore care must be taken using chemical zoning of garnet to distinguish rotation versus non-rotation growth models and emphasis should be placed on slow-diffusing elements.
Structural Evidence for Synchronous Vertical and Horizontal Tectonism at the Late Stages of Archean Cratonization in the Superior Craton and Implications for Gold Mineralization
"Vertical tectonism" is due to density inversion and is characterized by buoyant rising of granitoids (diapirism) and sinking of greenstones (sagduction). Horizontal tectonism is similar to the present-day plate tectonics and is characterized by regional scale horizontal motion (drift) of "plates" or "microplates" and the resulting interactions (e.g. collision) among them. In the Superior craton, there is evidence that the two processes occurred synchronously (and potentially interactively) at the late stages of Archean cratonization. The Superior craton consists of a series of east-west trending "subprovinces" or terranes. Many geologists have described evidence that the subprovinces represent a series of microcontinents, remnant arcs, oceanic terranes and accretionary prisms and that the Superior craton grew by lateral accretion of these elements. The present study is based on detailed work in the northwestern Superior craton, and the general conclusions are believed to be applicable to the entire Superior craton and possibly other Archean cratons as well. The northwestern Superior craton is characterized by narrow greenstone belts surrounded and intruded by voluminous granitoid plutons. The plutons mostly occur in open domes, whereas greenstones generally occur in narrow synclinal keels. Regional scale shear zones, with a dextral strike-slip movement component, are spatially coincident with greenstone belts. Two discrete episodes of deformation have been recognized in the northwestern Superior craton: an earlier recumbent folding and thrusting event and a later upright folding and shearing event. The former was possibly related to terrane accretion and collision, and the latter to the formation of the dome-and-keel structure and the regional scale shear zones. The dome-and-keel structure formed as a result of diapirism and sagduction (vertical tectonism), and the regional scale shear zones have regionally consistent kinematics and were a result of regional horizontal shearing (horizontal tectonism). Results of detailed structural analysis show that diapirism/sagduction and regional horizontal shearing occurred synchronously. The dome-and-keel structures and shear zones overprint and help to obscure earlier accretional/collisonal structures, and the shear zones do not necessarily coincide with terrane boundaries. The synclinal keel-shear zone association provided a link between the upper crust and the lower crust or mantle, and might have served as a conduit for magma and mineralizing fluids that were generated in the crust and/or mantle during the process. Such a process at the late stages of Archean cratonization can readily explain the common association of gold deposits with greenstone belts in synclinal keels, shear zones, late felsic to intermediate intrusions and Timiskaming-type sedimentary rocks. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and represents a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic. It is further suggested that the process and the associated gold mineralization were both related to a range of tectonometamorphic and magmatic processes that arise in response to slab break-off and associated extensional orogenic collapse following terrane accretion.