An Integrated Textural and Fluid Inclusion Study of Crack-Seal Textured Quartz Veins in the Meguma Group, Nova Scotia, Canada
Crack-seal, also ribbon-textured or laminated, is a common microstructure in quartz veins where excessive fluid pressure has been an important process in vein formation, such as in regionally deformed terranes and some mineralized granites. Although widely referred to in the literature and used to imply conditions of fluid overpressure accompanying vein formation, few studies have focused on this microtexture from a textural, mineral chemical or fluid pressure perspective. Exceptional examples of such veins occur in the regionally deformed (410 Ma), greenschist facies, Lower Paleozoic flysch sequence of the Meguma Group, Nova Scotia. In this area, the bedding-concordant quartz veins, commonly auriferous, occur at sandstone-shale contacts and are frequently characterized by a laminated or crack-seal texture; rarer examples are also seen in temporally-related discordant veins. The bedding-concordant veins are considered to have formed during flexural-slip folding in concert with high fluid pressure. The quartz veins are composite with two distinct stages of growth; an early laminated quartz and paragenetically later, clear, massive quartz. The laminated quartz veins are up to 10s cm to 1 m thick and may contain several thousand laminae of 10 to 100 micron thickness. These micro-layers represent a mixture of adjacent wall rock material, variably dissolved and reacted wall rock (e.g., fluid inclusions crowded with solids) and new mineral grow. Thus, these lenses are micro-domains where highly variable amounts of fluid:rock interaction has occurred. Where fibre textures have been observed, oblique extension is inferred for vein opening. Fluid inclusions in clear quartz between laminae correspond to aqueous-carbonic types and are similar to fluid inclusions in non-laminated quartz. Thermometric data for these inclusions were used to construct isochores and infer fluid pressures at the time of fluid entrapment and, hence, vein formation. The calculated pressures are often 0.5 to 2 kbars above that inferred for vein emplacement during regional deformation and metamorphism of the Meguma Terrane and imply excessive fluid overpressures (i.e., supra-lithostatic). Such overpressures, although rarely reported in the literature, are common in these laminated quartz veins throughout the Meguma Group. It is suggested that such conditions of overpressure may reflect a combination of the layered nature of the host rocks (e.g., impermeable of shale beds) and the inability of the host rocks to dissipate stress, especially if sudden increases in applied stress occurred during rupture.
Shear-Zone-Hosted Gold Deposits Revisited
The spatial association between regional-scale shear zones and large economic gold deposits has long been recognized and examined. From a structural perspective, the cause-and-effect relationship between ductile shear zone deformation and mineralization remains an area of inquiry. Experience from structural studies on ductile shear zones sheds light on the critical conditions and mechanisms for mineralization. Quartz-vein-hosted gold deposits in brittle shear zones form the basis for understanding hydrothermal models for mineralization in a network of open fractures. The paradox with ductile shear zones is their formation at mid to lower crustal conditions, in the realm of regional metamorphism, where porosity and permeability are generally lacking. The theoretical study of ductile shear zones is based on concepts of continuum mechanics and strain compatibility. At depths below the brittle-ductile transition, ductile strength is so low that strain occurs in response to very small differential stress. In ideal conditions, ductile shear zones would expel fluids. In fact, gold mineralization in ductile shear zones is associated with strain incompatibilities on various scales. For some deposits, strain incompatibility is caused by competency contrasts along lithologic contacts. In other examples, strain incompatibilities occur on a microscopic (grain) scale between adjacent minerals with different deformation mechanisms operating under the same conditions. In either case, strain incompatibilities create local loss of cohesion and sparse, temporary porosity in spite of high pressure conditions. Although ductile deformation will eliminate this porosity, the difference in time scales for fluid flow and crystal-plastic flow creates the possibility for mineralization. Geochemical evidence points to large volumes of fluid interaction to produce economic gold deposits. Since fluid volumes are so extremely low in ductile structures, these structures must be long-lived as loci of active deformation in order to concentrate gold. Therefore, crustal-scale structures in the vicinity of plate or terrane boundaries, with large-scale displacements accumulating over millions of years, are most likely to be prospective. Empirically, a survey of giant gold deposits suggests that steeply dipping structures associated with orogen-parallel displacements are especially likely to be mineralized.
Structural Controls on Syenite-hosted Gold Mineralization of the Matachewan Area, a Westernmost Exposure of the Cadillac-Larder Lake Deformation Zone, South Abitibi Greenstone Belt, Canada
The structural controls of gold deposits in the Cadillac-Larder Lake Deformation Zone (CLLDZ) in general are established. However, the syenite-hosted gold deposits in this region remain poorly constrained. The Matachewan area is located on the western extension of the CLLDZ, characterized by polyphase deformation, pervasive alteration and syenite-hosted gold mineralization. Detailed structural studies were undertaken to establish the relationships between the deformation and mineralization and to further define the tectonic evolution of the CLLDZ. Three stages of deformation (D1 to D3) were identified based on field observations. D1 predated the Timiskaming assemblage and was due to nearly NE-SW-orientated compression. F1 fold and L1 lineation are rarely preserved because of the strong overprinting by the subsequent D2 deformation. S1 foliation strikes NW- SE, overprinting S0 bedding at a small angle. D2 occurred under a dextral transpressional environment, characterized by top-to-the-NW oblique thrusting and E-W-orientated strike-slip dextral shearing. F2 folds plunge moderately to steeply to the SW or S. S2 foliation dominantly strikes NE-SW to nearly E-W and dips steeply to the south, which is consistent with the major structures of the CLLDZ. L2 mineral stretching lineations have two preferred orientations. The NW-SE-orientated lineation is kinematically consistent with the D2 reverse faults, defined by chlorite, mica aggregates or striations, moderately to steeply plunging to SE. The E-W-orientated lineation is kinematically related to the EW dextral shearing, defined by elongated clasts or mineral aggregates. D3 was a sinistral ductile to brittle deformation and generated NW-SE-trending subvertical kink bands, indicating a lower temperature condition than that of the D1 and D2. The Matachewan syenite-hosted gold deposit is associated with the CLLDZ that developed during D2. Three main stages of veining occur in the syenite. The earliest stage of veining (V1) is characterized by boudinaged quartz-albitite-iron carbonate veins, dipping to the SW at moderate to high angles. The second stage (V2) is represented by folded quartz-pyrite veinlets and disseminated sulfide, dipping to the NNE at shallow to moderate angles. The third stage (V3) is of similar feature with V2 and is comprised of en echelon or planar quartz-carbonate veins with sulfide minerals, dipping to the NE at moderate angles. Petrological studies reveal that the main gold mineralization is associated with V2 veins, and partially with V3 veins. Structural overprinting relationships indicate that V2 and V3 veins developed during D2 and late D2, respectively, involving subhorizontal NW-SE compressional stress under high fluid pressure. The attitude of these auriferous veins essentially controls that of the mineralization. In general, the structural features of the Matachewan area can be well correlated with those of other areas of the CLLDZ. The intimate relationship between the deformation and the geometry of auriferous veins can help to examine other syenite-hosted gold deposits of the CLLDZ and guide gold exploration.
Structural Evolution of The Garson Ni-Cu-PGE Deposit, Sudbury, Ontario
The Garson deposit is one of several deformed Ni-Cu-PGE deposits in the South Range of the Sudbury Structure. The deposit is located along the contact between the Sudbury Igneous Complex (SIC), and underlying metabasalt and metasedimentary rocks of the Paleoproterozoic Huronian Supergroup. The ore bodies are hosted by steeply south-dipping shear zones that record three deformation events (D1 - D3) following their formation as magmatic deposits at the base of the SIC. D1 is characterized by a S1 foliation defined by amphibole. The shear zones originally formed as shallowly north-dipping D1 thrusts, possibly in response to flexural slip during NW-directed compression and folding of the SIC during the Penokean Orogeny. D1 thrusts imbricated the ore zones and emplaced slivers of ore, metabasalt, and metasedimentary rocks into the overlying SIC norite. S1 is crenulated and locally preserved in microlithons bounded by a S2 cleavage defined by chlorite and altered amphibole. S2 contains a strong down-dip L2 lineation defined by elongate chlorite aggregates. S2 and L2 formed as the D1 shear zones were reactivated at greenschist grade during north-directed D2 thrusting, which overturned the SIC/Huronian Supergroup contact in the vicinity of the Garson Mine and locally thickened massive ores in F2 fold hinges and in other dilatant structures. S2 foliation locally terminate against massive Po-Pn-Ccp sulfide zones, and together with the local thickening of the ore in F2 fold hinges, indicate that the massive ores behaved ductilely and may have recrystallized during D2. D2 was probably synchronous with reactivation of contiguous shear zones at the Thayer Lindsley Mine, which accommodated a reverse, south-over-north movement during the Mazatzal-Labradorian Orogeny. Late, south- side-down, normal D3 shear zones formed during gravitational collapse under greenschist facies metamorphic conditions following cessation of D2. Because D3 structures occur only on a local scale, D3 had no significant impact on the overall geometry of the Garson ore bodies other than local remobilization of sulfides in the hinges of F3 folds. The geometry of the ore bodies is, to a large extent, controlled by the first order D1 shear zones, but significant remobilization of the ore occurred during D2 through ductile flow of the sulfides.
Three dimensional strain analysis using deformed veins
We present a new method for representing flow and finite deformation with stereographic projections and apply it to geological problems. Surfaces of no stretching (SNSa) are calculated from the velocity gradient tensor of any given flow, and are plotted in lower-hemisphere equal-angle projections. The diagrams show clearly the instantaneous shortening (s) and extension (e) fields and the shapes of the SNSa of the flow. The shape of the SNSa depends on the flow symmetry and the kinematic vorticity number (Wk). For an orthorhombic (pure shear) flow, the SNSa consists of one or two planes or a circular or elliptical conical surface symmetrical about the principal planes of the strain rates. For a monoclinic flow, the SNSa consists of two planes or a circular or elliptical conical surface symmetrical about the vorticity- normal section. For volume-constant plane strain flows, the SNSa are mutually perpendicular planes that intersect along the principal strain rate axis with zero stretching. For triclinic strain, the SNSa has no symmetry. For a finite deformation, the material surface defined by material lines that were originally parallel to the surface of no stretching can be calculated (SNSb). For a volume-constant flow with a Wk < 1, SNSb represents the boundary between the shortening followed by extension field (se) and (e), and SNSa the boundary between (s) and (se). SNSa and SNSb only touch at the centre of the strain ellipsoid. For Wk = 1, the two surfaces touch along a line. For Wk > 1, the two surfaces intersect and a field of extension followed by shortening (es) exists. Volume change may also introduce an (es) field. If it is assumed that the finite deformation resulted from a steady-state deformation path, then, from the asymmetry of SNSb with respect to SNSa, Wk can be determined, using the Mohr circle for position gradient tensors F or H. While, for finite deformation, methods for two-dimensional strain analyses are well developed, three- dimensional analyses are less commonly performed. The methods we developed will not only aid in three- dimensional strain analyses of veins, but they also aid in the three-dimensional visualization of flow and strain geometries.
Recognizing synsedimentary deformation features in tectonically deformed sedimentary rocks of orogens
Studies of modern continental margins, both active and passive, show abundant evidence for deformation of unconsolidated and weakly lithified sediments at a variety of scales. At the largest scales these structures include tectonically displaced sheets with sizes and displacements comparable to those in convergent orogens, accompanied by faults and folds comparable to those of thrust belts. At outcrop scale they include synsedimentary folds, faults, debris flows, and intraformational breccias. At microscopic scale the effects of synsedimentary deformation are more subtle, but include changes in grain packing and the destruction of depositional fabrics and structures. Ancient sedimentary rocks deposited on continental margins that were subsequently incorporated in orogens should include widespread records of such deformation. However, distinguishing synsedimentary structures from effects of post-lithification deformation can be extraordinarily difficult. Controversies surrounding the origin of individual structures have sometimes led to radical differences in interpretation of both sedimentary and tectonic history, at orogen scale. Debate over the origin of outcrop-scale structures has been confused by assumptions and terminological ambiguities. We suggest that two distinct questions need to be answered in interpreting potential synsedimentary deformation structures: (i) What was the lithification state of the rocks involved? (ii) Was the deformation confined to a surficial layer on a slope, or rooted in a tectonic structure? The answers to these questions have often been confused; outcrop evidence for pre-lithification deformation has been incorrectly used to infer superficial, down-slope movement, whereas studies of modern trench environments have shown that unlithified sediment can easily be incorporated in 'rooted' tectonic deformation zones. In ancient orogens, several potential criteria may assist in answering the first question. Deformed slope sediments in the Neoproterozoic Windermere Supergroup, and the Jurassic Hazelton and Bowser Lake Groups, both in the Canadian Cordillera of British Columbia, contain structures that can be categorized broadly as pre-lithification, post-lithification, and hybrid. We suggest that pre-lithification structures can be recognized where sandstone geometries indicate more ductile behaviour than interbedded mudstones. These features include: class 3 folds in sandstone interlayered with parallel or near-parallel (class 1) folds in mudstones; sandstone sills; sandstone dikes; and breccias of angular mud clasts in sandstone. Post-lithification structures include regional folds with axial planar cleavage, in which mudrocks were more ductile than interbedded coarser clastics, and which have resulted in distortions of individual sand grains. Hybrid structures arise where synsedimentary deformation has rotated layered rocks into orientations within the shortening field of subsequent tectonic strain. The resulting 'tectonic' folds are nonetheless distributed in movement zones whose geometry is related to earlier, synsedimentary deformation. Regional mapping of such zones is the only way to answer to the second question: whether the structures originated from down- slope or tectonically rooted deformation.
Metamorphic evolution of the Archean Bird River Greenstone Belt, Southeastern Manitoba.
The Archean Bird River greenstone belt (BRGB) is located on the southwestern edge of the Northern Superior province between the 3.2 Ga old Winnipeg River microblock to the south and the metasedimentary belt of the English River subprovince to the north. The BRGB underwent 3 main deformation phases. The D1 event took place ca 2698 Ma and displays a north-side-up shearing. The D2 event occurring at ca 2684 Ma in a transpressive context presents a complex structural pattern mixing vertical tectonics in the BRGB and strike- slip tectonics along the boundaries of the greenstone belt with other subprovinces. At ca 2640 Ma, the D3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare-elements pegmatitic plutons in a still hot (400°C-500°C) country rock. Metamorphism affected the BRGB even before the D1 event as the crystallisation-fabric relationships show that garnet porphyroblast grew in a rock devoided of any fabric. Pseudosections and garnet isopleths modelling with the THERMOCALC software coupled with average PT mode showed that the entire BRGB underwent an increase of P and T during D1. The maximum P-T conditions (up to 5 kbar and 550°C) were reached in the northeastern part of the belt, in the Peterson Creek Unit. The other units experienced P-T conditions around 2-3 kbar and 450-500°C. During the D2 event, units metamorphosed at greater depth are exhumed and superimposed onto units metamorphosed at lower pressure. This exhumation was coeval with a transpressive deformation and occurred during a strong increase in temperature. The thermobarometric calculations show a temperature increase parallel to the trend of the belt from west to east. The average temperature on the west part is 450°C and reaches 650°C on the eastern edge of the belt. This temperature gradient is related to the widespread plutonism in the Marijane plutonic complex in the east and in the nearby English River Subprovince. To the southeastern part of the belt, the Eaglenest shear zone provides the best evidence of a polyphase metamorphism. Textural analyses of garnet porphyroblasts show a crystallisation-dissolution cycle with the crystallisation of two generations of garnet associated to D1 and D2 events. Metamorphism lasted until and after the D3 event since porphyroblasts of hornblende, biotite and cordierite overprinted the D1 to D3 regional composite fabric.
Lithology, stratigraphy and structure of the Aillik domain: Implications on the tectonic evolution of the Makkovik Province, Labrador
The Makkovik Province of Labrador is considered part of a Paleoproterozoic accretionary belt that developed on the southern margin of the North Atlantic craton during the ca. 1.9-1.7 Ga Makkovikian-Ketilidian orogeny. The Aillik domain represents one of three domains that characterize province. Recent regional bedrock mapping has further defined the lithological units that occur within the Aillik domain. The Aillik domain largely comprises: a) the Aillik Group (previously termed the Upper Aillik Group), a supracrustal assemblage consisting of metasedimentary and metavolcanic rocks; and, b) abundant, syn- and post-deformation Paleoproterozoic intrusive suites which intrude the Aillik Group. The ca. 1883йд1856 Ma Aillik Group comprises polydeformed, upper-greenschist to lower-amphibolite facies, bi-modal volcanic rocks and sedimentary lithologies and hosts abundant base-metal and uraniferous showings. Paleoproterozoic intrusive suites in the Aillik domain include syn-volcanic quartz-feldspar- porphyritic granites (ca. 1858 Ma), ca. 1805-1795 Ma foliated to massive intrusions, and non-foliated ca. 1720 Ma and Labradorian (ca. 1650-1640 Ma) intrusions. Deformation in the Aillik Group is typically characterized by regional-scale, open to isoclinal, moderately plunging, upright to overturned folds (F1 in Aillik domain) that were subsequently refolded (F2). The Big Island shear zone (BISZ) is thought to represent one of a series of ductile shear zones that excised part of the Aillik Group, with another parallel shear zone possibly occurring at Ford's Bight and Pomiadluk Point. The direction of fold vergence of the regional folds changes about the BISZ. West of the shear zone, folds verge to the northwest; whereas, to the east folds verge to the northeast. Folding and development of regional-scale shear zones was contemporaneous with the development of an axial-planar fabric in the Aillik Group and regional amphibolite-facies metamorphism, all of which are attributed to compressional stages during the Makkovikian orogeny. With the exception of the porphyritic, syn-volcanic granites, all of the plutonic rocks appear to postdate regional-scale folding in the area. Although some of the ca. 1800 Ma plutons have acquired the regional penetrative fabric, indicating they intruded synchronous to deformation, they do not appear to be folded. Most of the post-Makkovikian, northeast-trending faults appear to cut all of the plutonic units except the ca. 1650-1640 Ma Labradorian suites that appear to have escaped this early faulting/shearing event(s). The southern exposures of the Aillik domain preserve evidence for brittle deformation associated with Labradorian orogeny. Lithological characteristics of the Aillik group support formation in a transitional environment, from a shallow marine to a marginal marine or subaqueous environment. Bedrock mapping, geochronology, and geochemistry suggest that the Aillik Group formed in a back-arc (rifted-arc?) setting built on a juvenile terrane (possibly an older accreted arc). Although depositional basement has not been identified, it cannot be coincident with present-day basement, as folding and shearing during Makkovikian orogenesis transported these Aillik Group rocks northwestward. Further work on constraining the timing of formation, deformation and metamorphism of the Aillik Group is ongoing and will be instrumental in unraveling the tectonic evolution of the Makkovik Province.