Metamorphic Shear Zones Research Articles

Hypozonal lode gold deposits: A genetic concept based on a review of the New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granites deposits

The mineral system of hypozonal lode gold deposits has been a matter of scientific discussion in recent years, mainly centring around two models: (1) syn- to post-peak metamorphic hydrothermal mineralization (hypozonal orogenic gold) and (2) pre-peak metamorphic mesothermal mineralization and subsequent metamorphic overprint.Here we review the evidence for and against the existence of hydrothermal gold mineralization in high-temperature metamorphic terranes. The New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granite deposits are considered to represent hypozonal orogenic gold deposits based on petrographic and field observations. Gold mineralization is hosted in syn- to post-peak metamorphic shear zones. Ore and hydrothermal alteration assemblages replace the metamorphic mineral assemblages of the host rocks and follow the same retrograde PT path. This indicates that the mineralization forms an integral part of the terrane evolution from syn- to post-peak metamorphism to final exhumation.Comparison with other hypozonal deposits indicates that the hypozonal orogenic gold system is active at ca. 500–700°C and 2–7kbar, and forms a Ca–Se–Cu–Ni–Co–S-dominated enrichment compared to the mesozonal counterparts. The hypozonal deposits appear to be restricted to Precambrian terranes in variable host rocks ranging from amphibolite and banded-iron formation to marble and granite. The fluid source for hypozonal gold is metamorphic, magmatic or a combination of both. A complex tectonometamorphic evolution of the high-grade metamorphic host terranes is regarded critical for forming the deposits by: (1) generating permeability in shear zones; (2) creating PT gradients that promote fluid migration; (3) inverting the metamorphic gradient by thrusting or extensional shearing; and (4) initiating crustal anatexis and granite emplacement as source of heat and ore fluid. Hypozonal orogenic gold deposits form in syn- to post-peak metamorphic shear zones of high-grade terranes in the centre or foreland of Precambrian accretionary and collisional orogens during the evolved collision stage, when active subduction ceased or moved outboard and the terranes are uplifted.

Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U–Pb and 40Ar–39Ar geochronology

New U–Pb and 40Ar–39Ar geochronology and structural data from high- to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U–Pb age of 465±14Ma (Middle Ordovician) measured on migmatite from the Coimbra–Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age.This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra–Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5km to 16.6km over a period of ca. 10Ma in the Viséan (ca. 340–330Ma), indicating oblique slip exhumation rates of 8.5 to 10.6mm/yr (Campo Maior migmatites) and 3.2mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4km depth in the period ca. 344–334Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356±12Ma, 321±13Ma and 322±29Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35Ma.

Scale invariant sheath folds in salt, sediments and shear zones

Sheath folds are developed in a broad spectrum of geological environments in which material flow occurs, including gravity-driven surficial deformation in ignimbrites, unconsolidated sediments and salt, together with deeper level ductile shear zones in metamorphic rocks. This study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales. Elliptical closures defining eye-folds represent ( y– z) cross sections through highly-curvilinear sheath folds. Our analysis of the published literature, coupled with field observations, reveals remarkably similar ellipticities ( R yz ) for sheath folds in metamorphic shear zones ( R yz 4.23), salt ( R yz 4.29), sediment slumps ( R yz 4.34), glaciotectonites ( R yz 4.48), and ignimbrites ( R yz 4.34). Nested eye-folds across this range of materials ( N = 1800) reveal distinct and consistent differences in ellipticity from the outer- ( R yz ) to the inner-most ( R y ′ z′ ) elliptical “rings” of individual sheath folds. The variation in ratios from outer to inner rings ( R′ = R yz / R y ′ z′ ) in gravity-driven surficial flows typically displays a relative increase in ellipticity to define cats-eye-folds ( R′ < 1) similar to those observed during simple and general shear in metamorphic rocks. We show that sheath folds develop across a range of scales within these different environments, and display elliptical ratios ( R yz ) that are remarkably constant ( R 2 > 0.99) across 9 orders of magnitude (sheath y axes range from ∼0.1 mm to >75 km). Our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain, with R′ also reflecting the rheological significance of layering associated with original buckle fold mechanisms. The scaling pattern of sheath folds reflects the length scales of the precursor buckle folds (and width of deformation zones) across a broad range of materials and environments. With continued deformation, the layering marking the original folds may become increasingly passive to define sheath folds. These empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials, strain rates and scales encompassing a variety of deformation settings.

Discovery of Llandovery (Silurian) graptolites and probable Devonian corals in the Southalpine Metamorphic Basement of the Eastern Alps (Agordo, NE Italy)

We report the discovery of Aeronian (Middle Llandovery) graptolites, and corals of probable Devonian age, in boudins hosted by greenschists, within the Southalpine Metamorphic Basement. These discoveries provide key constraints to the depositional age range of the protoliths. This remarkable occurrence of almost undeformed graptolites and compound corals in boudins within a metamorphic shear zone indicates very marked local strain partitioning.

Origin of deformation bands in argillaceous sediments at the toe of the Nankai accretionary prism, southwest Japan

Deformation bands in argillaceous sediments at the toe of the Nankai accretionary prism are roughly planar, ∼1–10 mm thick, bedding-oblique zones. The bands are typically developed as two oppositely dipping sets. A band of one set can be displaced by the slickenlined surface of the oppositely dipping band, showing a reverse slip with respect to a horizontal plane. Deformation bands commonly consist of thinner subbands that are oblique or parallel to the boundaries of the deformation bands. The subbands also display a reverse slip with respect to a horizontal plane, and particles within subbands are oriented perpendicular to the maximum principal stress ( σ 1), regardless of the dips of the subbands. Physical property analysis using an X-ray computed tomography scanner demonstrates that porosity reduction in the deformation bands relative to the surrounding undeformed material ranges from 1 to 6%. The deformation bands are compactive shear bands formed under subhorizontal σ 1 associated with northwest-directed plate convergence, and subbands arise from a combination of reverse sense of shear and flattening perpendicular to σ 1, with their geometries producing the S–C aspect observed in sheared clayey sediments or ductile metamorphic shear zones.

High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland

The redox state of sillimanite zone (650–700°C, 5–6 kbar) metasediments of the Barrovian type area, Scotland, was investigated using estimates of metamorphic oxygen fugacity (fO2), sulfur fugacity (fS2), and fluid chemistry based on new determinations of mineral and rock compositions from 33 samples. A total of 94% of the samples lack graphite, contain both ilmenite–hematite solid solutions (RHOMOX) and magnetite, and had metamorphic fO2 about 2 log10 units above the quartz–fayalite–magnetite (QFM) buffer. The regional variation in metamorphic fO2 for these rocks was minimal, about ±0·3 log10 units, reflecting either a protolith that was homogeneous with respect to redox state, or an initially variable protolith whose redox state was homogenized by metamorphic fluid–rock interaction. RHOMOX inclusions in garnet porphyroblasts that become richer in ilmenite from the interior to the edge of the host porphyroblast suggest that at least some syn-metamorphic reduction of rock occurred. Significant variations in bulk-rock oxidation ratio (OR) that are probably inherited from sedimentary protoliths are found from one layer to the next; OR ranges mostly between ∼20 and ∼50 [OR = molecular 2Fe2O3 × 100/(2Fe2O3 + FeO)]. These OR variations are uncorrelated with fO2 and do not indicate that large, order-of-magnitude gradients in fO2 and redox state existed or were preserved between layers during metamorphism. The other 6% of the samples contain ilmenite, lack magnetite, and had low fO2 0–1 order of magnitude below QFM in the stability field of graphite. They are characterized by combinations of the following: large fluid HF/H2O; metasomatic, tourmaline-bearing veins; absence or rarity of primary organic matter; and crosscutting late metamorphic shear zones rich in carbonaceous material. Such observations suggest that locally low fO2 conditions may have been related to the influx of reducing fluids from elsewhere in the area.

Gold mineralization in high-grade metamorphic shear zones of the Renco Mine, southern Zimbabwe

The Renco gold mine in southern Zimbabwe is the only known major gold deposit in the granulite facies terrane of the northern marginal zone of the Late Archean to mid-Proterozoic Limpopo belt. Gold mineralization is confined to a system of high-temperature mylonite zones characterized by two distinct geometries. These include a series of southeasterly dipping, anastomosing, north-northeast- to east-northeast- trending tabular lodes, termed shallow reefs, and subvertically inclined, easterly plunging pipelike lodes, termed steep reefs. The kinematics and orientation of the mineralized shear zones are consistent with a lateral and frontal thrust zone geometry that formed during the Late Archean thrusting of the northern marginal zone onto the Zimbabwe craton. Gold is spatially and temporally closely associated with sulfide mineralization, including pyrrhotite as the dominant sulfide with minor amounts of chalcopyrite and pyrite. Associated wall-rock alteration comprises a garnet-biotite-quartz + or - siderite mineral assemblage. Mineral textures within the host mylonites, as well as garnet-biotite thermometry, indicate gold deposition at temperatures of about 600 degrees C under mid-amphibolite conditions, slightly postdating regional peak metamorphic conditions. Fluid flow in the high-temperature shear zones was largely controlled by fracture permeabilities. Transient episodes of brittle fracturing during conditions of close to lithostatic fluid pressures were promoted by a pronounced strain partitioning within the narrow shear zones into ductile mylonite bands and brittle-ductile lithons that contain the bulk of the gold sulfide mineralization. The high-grade metamorphic ore and alteration mineral assemblages are overprinted by lower greenschist facies parageneses along brittle faults and cataclasites that are related to the mid-Proterozoic tectonism of the northern marginal zone. It is concluded that the mineralization at Renco illustrates the rare case of a midcrustal high-grade metamorphic gold mineralization in southern Africa where the vast majority of Late Archean lode gold deposits are related to low-grade metamorphic granite-greenstone terranes.

Metamorphism and deformation along the emplacement thrust of the Samail ophiolite, Oman

The base of the Samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement. The classic exposure of this metamorphic sole at Wadi Tayin consists of a few meters of garnet-clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite, overlying 150 m of lower amphibolite facies mafic and sedimentary schist. The sole is bounded by faults but is internally coherent. Peak metamorphic temperatures decrease exponentially, from ∼ 825°C at the top of the sole to ∼ 500°C at its base, as documented by changes in grain size, hornblende composition, the disappearance of chlorite and epidote, garnet-clinopyroxene element partitioning, and garnet-hornblende partitioning. The inverted peak temperature gradient decreases from > 4000°C/km in the upper 50 m to ∼ 100°C/km in the lower 150 m. Contrary to earlier reports, greenschist-facies tectonites are not present at Wadi Tayin, although the entire sole underwent greenschist-facies retrogression, driven by the influx of fluids from the overlying peridotite or underlying carbonate. Calculations based on an amphibolite constitutive relation reveal that, although the section represents a lithologic continuum, the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column.

Oscillatory zoning in metamorphic minerals: an indicator of infiltration metasomatism

AbstractExamples of oscillatory zoning in metamorphic minerals, imaged using a Back-Scattered Electron Detector on the SEM, are described from a series of contrasting environments. These are a prehnite vein sampled by drilling in the Mirvalles geothermal field, Costa Rica, a pyroxene vein developed in a regional metamorphic shear zone in the Yilgarn block, Western Australia, and a bedded metasomatic diopside rock from regionally metamorphosed metasediments in Connemara, Ireland. In each case the formation of oscillatory zoning can be ascribed to mineral growth under supersaturated conditions due to fluid infiltration. Oscillations can be related in the first example to periodic episodes of pressure release and boiling in the geothermal field, but in the regional metamorphic examples actualistic models are harder to define. The development of oscillatory zoning is likely to be a characteristic feature of infiltration metasomatism and can be used as a criterion in the recognition of metasomatic mineral growth in metamorphic rocks outside the vein environment.