D2 Shear Zones Research Articles

The Paleoproterozoic MacLellan deposit and related Au-Ag occurrences, Lynn Lake greenstone belt, Manitoba: An emerging, structurally-controlled gold camp

The Paleoproterozoic Era is important for Au-Ag mineralization, including deposits in the Trans-Hudson Orogen of central Canada. The association between Au-Ag deposits and regional structures that formed during this era has seen markedly less research than Archean counterparts. The MacLellan deposit and related Au-Ag occurrences occur within the northern part of the Paleoproterozoic Lynn Lake greenstone belt in northwest Manitoba. Field relationships, combined with a detailed characterization of the host rocks, alteration, and structure, along with mass balance calculations, indicate that the host rocks were largely derived from a common amphibole-plagioclase schist protolith (originally a porphyritic basalt) that was variably deformed and altered to initially biotite ± quartz during D2 deformation, and subsequently to Cr-bearing chlorite (± carbonate, ± amphibole, ± quartz) during the late stages of D2 and D3. Gold-silver mineralization is associated with these alteration events and consists predominantly of native Au, aurostibite (AuSb2), Au in arsenopyrite, and Ag in galena. These can be separated into two distinct Au mineralizing events: 1) early disseminated arsenopyrite aligned with D2 fabrics that contains structurally-bound Au; and 2) late fracture-filling native Au, aurostibite, and Au-rich rims and fractures on and within arsenopyrite. The highest concentrations of Au and Ag occur where E-W trending dextral D2 shear zones intersect NNE-SSW trending brittle faults (D4), suggesting late remobilization of precious metals by hydrothermal fluids into dilatant zones. The structural control on formation and distribution of Au-Ag deposits along the MacLellan Shear Zone is similar to that observed along the regionally significant Johnson Shear Zone, which occurs in the southern portion of the Lynn Lake greenstone belt, suggesting similar processes operated within both portions of the belt. Integrating detailed geologic and structural mapping, petrography, geochemistry, and alteration, demonstrates that these Au-Ag occurrences formed as a result of structurally-controlled mineralizing systems associated with major shear zones, rather than being primarily syngenetic in origin (i.e., seafloor exhalative), as previously suggested. These major Paleoproterozoic shear zones display similar characteristics and relationships to well-documented structural ‘breaks’ in Archean terranes (e.g., Porcupine-Destor Deformation Zone and Larder Lake-Cadillac Deformation Zone), suggesting that, at least in terms of the Lynn Lake region, structural and mineral deposit forming processes may have occurred on comparable spatial scales to those in Archean systems.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

The Golden Mile in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, has produced 385 million tonnes of ore at a head grade of 5.23 g/t gold (1893–2016). Gold-pyrite ore bodies (Fimiston Lodes) trace kilometre-scale shear zone systems centred on the D2 Golden Mile Fault, one of three northwest striking sinistral strike-slip faults segmenting upright D1 folds. The Fimiston shear zones formed as D2a Riedel systems in greenschist-facies (actinolite-albite) tholeiitic rocks, the 700-m-thick Golden Mile Dolerite (GMD) sill and the Paringa Basalt (PB), during left-lateral displacement of up to 12 km on the D2 master faults. Pre-mineralisation granodiorite dykes were emplaced into the D2 shear zones at 2674 ± 6 Ma, and syn-mineralisation diorite porphyries at 2663 ± 11 Ma. The widespread infiltration of hydrothermal fluid generated chlorite-calcite and muscovite-ankerite alteration in the Golden Mile, and paragonite-ankerite-chloritoid alteration southeast of the deposit. Fluid infiltration reactivated the D2 shear zones causing post-porphyry displacement of up to 30 m at principal Fimiston Lodes moving the southwest block down and southeast along lines pitching 20°SE. D3 reverse faulting at the southwest dipping GMD-PB contact of the D1 Kalgoorlie Anticline formed the 1.3-km-long Oroya Shoot during late gold-telluride mineralisation. Syn-mineralisation D3a reverse faulting alternated with periods of sinistral strike-slip (D2c) until ENE-WSW shortening prevailed and was accommodated by barren D3b thrusts. North-striking D4 strike-slip faults of up to 2 km dextral displacement crosscut the Fimiston Lodes and the barren thrusts, and control gold-pyrite quartz vein ore at Mt. Charlotte (2651 ± 9 Ma).

Geochronological constraints on the trans-Hudsonian tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston-Mudjatik Transition Zone, Saskatchewan

The Hudsonian Pressure-Temperature-Deformation-time-(P-T-D-t) evolution of the pre-Athabasca crystalline basement (>∼1700Ma) of the Wollaston-Mudjatik Transition Zone (WMTZ) highlights two major tectono-metamorphic events M1-D1 and M2-D2. The ages of these two event have been obtained by in-situ LA-ICPMS U-Th-Pb dating of monazite from Grt-Crd bearing pelitic gneiss and U-Pb analyses performed on zircons from pegmatites, using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The M1-D1 event, interpreted as the burial of the thinned Hearne margin via southward thrusting to pressures varying from 10 to 6kbar, occurred between ca.1840 and 1813Ma. The M2-D2 event, producing the northeast structural trend of the WMTZ, was formed in a sinistral transpressional tectonic regime during the late stage of the Hudsonian oblique collision between ca. 1813 and 1770Ma. Thermobarometric estimates on the M2–D2 assemblages show that the entire studied area was reequilibrated at about 5kbar and 750–825°C. trans-Hudsonian pegmatites are viewed as the main proto-ore of the uranium-rich Athabasca unconformity-type deposits. Formation, transfer and differentiation of these trans-Hudsonian pegmatites are replaced in this P-T-D-t evolution. During the 1840–1813MaM1-D1 partial melting event, the first batches of melt produced in the deep crust are the most likely enriched in uranium. Then, these melts were transferred upwards to the upper crustal levels owing to the development of crustal scale steeply-dipping D2 shear zones, and finally were differentiated to form uranium-enriched pegmatites between 1813 and 1770Ma. Some monazite and zircon grains within retrogressed migmatites recorded a later event at ca. 1720Ma, interpreted as the terminal cooling event down to 300–400°C responsible for partial retrogression of metamorphic assemblages. This age provides new insights into the timing of the onset of the Athabasca sedimentation that should therefore be at least 1710Ma old or younger.

Granite emplacement at the termination of a major Variscan transcurrent shear zone: The late collisional Viseu batholith

A major event of plutonic activity occurred all across the Central Iberian Zone of the Iberian Variscan Belt at the end of Late Paleozoic Variscan collisional tectonism. The present study focuses on the western sector of the Viseu late-post-tectonic batholith (central Portugal), a large composite intrusion comprising three main plutonic units: (a) small bodies of mafic to intermediate composition preferentially concentrated along the northern border, (b) a wide ring of coarse porphyritic biotite monzogranite (Cota-Viseu granite) and (c) a more evolved medium porphyritic, biotite-muscovite monzogranite occupying the central part of the intrusion (Alcafache granite).The compositional zonation pattern of the whole batholith and the complex mixing/mingling relationships between the voluminous Cota-Viseu porphyritic granite and the mafic/intermediate rocks suggest that these melts were withdrawn from a lower crustal source region undergoing partial melting, invasion by mantle-derived mafic magmas, mixing and fractional crystallization. New CA-ID-TIMS U-Pb zircon ages indicate that pluton assembly via multipulse injection of successive magma batches took place between 299.4 ± 0.4 Ma and 296.0 ± 0.6 Ma.A detailed anisotropy of magnetic susceptibility (AMS) survey suggests that pluton emplacement occurred at the extensional termination of a regional-scale, ENE-WSW trending, sinistral D3 shear zone - the Juzbado-Penalva Shear Zone (JPSZ). A dilational opening model involving the development of “en-échelon” tensional gashes at the extensional termination of the fault, followed by progressive opening and widening of north-south trending fractures, provided the space into which the successive magma batches arriving from below were emplaced. Vertical inflation was accommodated by depression of the pluton floor. The proposed model is consistent with the asymmetric wedge-shaped geometry of the intrusion (steep root zone on the northern side, discordant subvertical walls and a shallowing pluton floor towards the south).

Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso

The Kiaka gold deposit is a major resource in West Africa, with measured and indicated resources of 124 Mt at 1.09 g/t Au (3.9 Moz) and inferred resources of 27 Mt at 0.83 g/t Au (0.8 Moz). Located within the Manga-Fada N'Gourma greenstone and plutonic belt in south of the Burkina Faso, the deposit is hosted by a metamorphosed volcano-sedimentary sequence of lithic-, quartz-biotite metagreywackes, aluminosilicate-bearing metapelites and garnet-orthopyroxene-bearing schists and volcanic units.Structural observations indicate four local deformation events: DK1, DK2 and DK3 and DK4. Respectively, these events are linked to regional D1 E-W compression, D2 NW-SE compression and lastly, D3- and D4-related reactivations along D2 shear zones. The S2 foliation and D2 shear zones are developed during lower amphibolite facies metamorphism whereas retrogression occurs during D3-4 reactivations along these shear zones at upper greenschist facies conditions. The emplacement of a dioritic intrusion, dated at 2140 ± 7 Ma (Concordia U-Pb age on magmatic zircon), is interpreted to be contemporaneous with sinistral displacement along mineralized, NE-trending D2 shear zones. The intersection of these shears zones and the Markoye shear zone (dextral-reverse D1 and sinistral-reverse D2 reactivations) controlled the final geometry of the host rocks and the ore zones.Four subparallel elongated ore bodies are mainly hosted within D2-related shear zones and some are developed in an apparent axial plane of a F2 isoclinal fold. Detailed petrographic studies have identified two main types of hydrothermal alteration associated with two stages of gold mineralization. The stage (1) corresponds to replacement zones with biotite and clinozoisite during the D2 event associated with pyrrhotite ± pyrite, chalcopyrite (disseminated gold stage). The stage (2) occurs during reactivations of the D2-related auriferous shear zones (vein stage) and is characterized by diopside ± actinolite D3 veins and veinlets and D4 pervasive muscovite, ± chlorite, ± calcite in quartz-carbonate vein selvages and associated with pyrrhotite + arsenopyrite ± electrum, ± native gold and tellurobismuthite. The latter stage (2) could be divided into two sub-stages based on mineralogy and crosscutting relationship. A weighted average Re-Os pyrrhotite age at 2157 ± 24 Ma (Re-Os age based on 3 replicates) constraints the timing of the disseminated gold stage and represents the first absolute age for gold mineralization in the Manga Fada N'Gourma area. The timing of gold at Kiaka may be also coeval with one of the two lode gold event at ∼ ca. 2.16–2.15 Ga and occurred concomitant with tectono-thermal activity during Eo-Eburnean orogeny.The study of the Kiaka gold deposit emphasizes the importance of a multi-scale and multidisciplinary approach (field observations, petrography geothermobarometry and geochronology) to decipher the polyphase character of some Paleoproterozoic gold deposits.

Extensional orogenic collapse captured by strike-slip tectonics: Constraints from structural geology and U[sbnd]Pb geochronology of the Pinhel shear zone (Variscan orogen, Iberian Massif)

The late Paleozoic collision between Gondwana and Laurussia resulted in the polyphase deformation and magmatism that characterizes the Iberian Massif of the Variscan orogen. In the Central Iberian Zone, initial continental thickening (D1; folding and thrusting) was followed by extensional orogenic collapse (D2) responsible for the exhumation of high-grade rocks coeval to the emplacement of granitoids.This study presents a tectonometamorphic analysis of the Trancoso-Pinhel region (Central Iberian Zone) to explain the processes in place during the transition from an extension-dominated state (D2) to a compression-dominated one (D3). We reveal the existence of low-dipping D2 extensional structures later affected by several pulses of subhorizontal shortening, each of them typified by upright folds and strike-slip shearing (D3, D4 and D5, as identified by superimposition of structures). The D2 Pinhel extensional shear zone separates a low-grade domain from an underlying high-grade domain, and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust, crustal thinning, decompression melting, and magma intrusion. Progressive lessening of the gravitational disequilibrium carried out by this D2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the Pinhel shear zone by strike-slip tectonics during renewed crustal shortening. High-grade domains of the Pinhel shear zone were folded together with low-grade domains to define the current upright folded structure of the Trancoso-Pinhel region, the D3 Tamames-Marofa-Sátão synform. New dating of syn-orogenic granitoids (SHRIMP UPb zircon dating) intruding the Pinhel shear zone, together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca. 331–311Ma, with maximum tectonomagmatic activity at ca. 321–317Ma. The capture and apparent cessation of movement of the Pinhel shear zone occurred at ca. 317–311Ma.

Tectonic evolution of the middle crust in southern Tibet from structural and kinematic studies in the Lhagoi Kangri gneiss dome

Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ∼3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.

Tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston–Mudjatik Transition Zone, Saskatchewan

The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (∼1.7 Ga) within the Wollaston–Mudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1–D1 and M2–D2. M1–D1 is associated with the development of a gently dipping foliation striking N90°–N100° and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly–McClean exploration drilling project. The M2–D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2–D2 assemblages show that the studied area was reequilibrated at about 4–5 kbar and 750–825 °C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly–McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1–D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites.

Wall-Rock Structural Controls on the Genesis of the Voisey's Bay Intrusion and its Ni-Cu-Co Magmatic Sulfide Mineralization (Labrador, Canada)

The Voisey’s Bay intrusion of the Mesoproterozoic Nain Plutonic Suite hosts a world-class magmatic Ni-Cu-Co sulfide deposit. Here we document the brittle and ductile structure of gneisses surrounding the intrusion using field data, borehole intersections, and subsurface structural data obtained from acoustic and optical televiewer surveys. We elucidate the role that wall-rock structure played on controlling the geometry of the intrusion and the formation of structural traps that host mineralization. The Voisey’s Bay intrusion consists at first order of two km-scale magma chambers, interpreted to represent an upper and lower chamber of the intrusion, that are connected by a system of dikes. Three types of gneiss, associated with the Paleoproterozoic Torngat orogen, form the wall rocks of the intrusion: Archean orthogneiss, Paleoproterozoic Tasiuyak paragneiss, and Paleoproterozoic enderbitic orthogneiss. Three phases of ductile deformation (D1–D3), predating intrusion emplacement, are recorded in the wall-rock gneisses. D1 produced the dominant S1 fabric, subparallel to original bedding (S) in the Tasiuyak gneiss. D2 shortening caused shallowly plunging, upright to moderately inclined, F2 folds in the enderbitic gneiss, the prototype of which was likely emplaced toward the end of D1. D3 ductile deformation resulted in steeply dipping shear zones, and flat lying to shallowly dipping C3 shear planes. At least two later phases of brittle deformation (D4 and D5) are present. D4 is a preemplacement event characterized by fractures with orientations similar to D3 shear zones and shear planes. D5 faults and joints formed during syn- to postemplacement brittle deformation likely associated with E-W brittle sinistral transtension associated with the Mesoproterozoic Gardar-Voisey’s Bay fault zone. The geometry of the Voisey’s Bay intrusion and its mineralization was controlled by pre- to synemplacement brittle structures and preemplacement ductile structures, which acted as passive wall-rock anisotropies. Mineralization hosted within Tasiuyak gneiss is controlled by the intersection of a dike with S1/S, D3 shear zones and D4 fractures. Where the intrusion occurs within enderbitic gneiss, mineralization locally coincides with shallow fabrics associated with F2 fold closures. The margins of the Voisey’s Bay dikes are controlled by steeply and shallowly dipping D4 fractures. Changes in dike thickness, which control the locations of mineralization, can be explained by thermomechanical erosion of the walls of the dike, which occurs by thermal expansion and contraction of wall rock due to repeated magma pulses, and the mechanical spalling of this wall rock along anisotropies. The strong brittle structural control on emplacement, along with previous geothermobarometry, is consistent with a mid-crustal (9–11 km) depth of emplacement. Lithosphere-scale E-W–trending D5 faults may have acted as conduits for the crustal-scale transport of primitive Voisey’s Bay magmas from their source, rather than an older orogenic suture zone.

Deformation, metamorphism, and mobilization of Ni–Cu–PGE sulfide ores at Garson Mine, Sudbury

The Garson Ni–Cu–platinum group element deposit is a deformed, overturned, low Ni tenor contact-type deposit along the contact between the Sudbury Igneous Complex (SIC) and stratigraphically underlying rocks of the Huronian Supergroup in the South Range of the 1.85-Ga Sudbury structure. The ore bodies are coincident with steeply south-dipping, north-over-south D1 shear zones, which imbricated the SIC, its ore zones, and underlying Huronian rocks during mid-amphibolite facies metamorphism. The shear zones were reactivated as south-over-north, reverse shear zones during D2 at mid-greenschist facies metamorphism. Syn-D2 metamorphic titanite yields an age of 1,849 ± 6 Ma, suggesting that D1 and D2 occurred immediately after crystallization of the SIC during the Penokean Orogeny. The ore bodies plunge steeply to the south parallel to colinear L1 and L2 mineral lineations, indicating that the geometry of the ore bodies are strongly controlled by D1 and D2. Sulfide mineralization consists of breccia ores, with minor disseminated sulfides hosted in norite, and syn-D2 quartz–calcite–sulfide veins. Mobilization by ductile plastic flow was the dominant mechanism of sulfide/metal mobilization during D1 and D2, with additional minor hydrothermal mobilization of Cu, Fe, and Ni by hydrothermal fluids during D2. Metamorphic pentlandite overgrows a S1 ferrotschermakite foliation in D1 deformed ore zones. Pentlandite was exsolved from recrystallized polygonal pyrrhotite grains after cessation of D1, which resulted in randomly distributed large pentlandite grains and randomly oriented pentlandite loops along the grain boundaries of polygonal pyrrhotite within the breccia ore. It also overgrows a S2 chlorite foliation in D2 shear zones. Pyrrhotite recrystallized and was flattened during D2 deformation of breccia ore along narrow shear zones. Exsolution of pentlandite loops along the grain boundaries of these flattened grains produced a pyrrhotite–pentlandite layering that is not observed in D1 deformed ore zones. The overprinting of the two foliations by pentlandite and exsolution of pentlandite along the grain boundaries of flattened pyrrhotite grains suggest that the Garson ores reverted to a metamorphic monosulfide solid solution at temperatures ranging between 550 and 600 °C during D1 and continued to deform as a monosulfide solid solution during D2.

Structural and Stratigraphic Controls on Magmatic, Volcanogenic, and Shear Zone-Hosted Mineralization in the Chapais-Chibougamau Mining Camp, Northeastern Abitibi, Canada(,)

The structure and stratigraphy of the Chapais-Chibougamau mining camp in the Archean Abitibi greenstone belt of the Superior Craton, Quebec, Canada, is reevaluated from new aeromagnetic data, geochronology, and field studies to provide a framework for syngenetic and epigenetic mineralization. Mineralization in the Chapais-Chibougamau area is synvolcanic (volcanogenic massive sulfide (VMS)), synmagmatic (Fe-Ti-V, Ni-Cu, Au-Ag-Cu-Zn-Pb veins, Chibougamau-type Cu-Au veins), and shear zone hosted (including Opemiska Cu-Au veins subtype). VMS deposits occur within felsic volcanic rocks of tholeiitic affinity and mafic to felsic volcanic rocks of transitional to calc-alkaline affinity at the top of three volcanic cycles of the Roy Group. VMS deposits and early polymetallic veins (Au-Ag-Cu-Zn-Pb) are associated with NNW- to NNE-striking synvolcanic faults. Fe-Ti-V deposits occur within the Layered zone of the Lac Dore Complex, especially where it thickens in areas of interpreted synmagmatic faults. Subeconomic magmatic Ni-Cu deposits occur at the contacts of mafic-ultramafic or tonalitic intrusions. “Chibougamau-type” Cu-Au veins formed through magmatic-hydrothermal processes. They are cut by dikes that predate regional D2 deformation but are located within, and are deformed by, NW- and NE-trending D2 shear zones. Shear zone-hosted Au deposits are developed preferentially within regional E-W–trending deformation corridors and along NNE-striking sinistral shear zones while “Opemiska-type” Cu-Au veins occur within regional overturned anticlines in mafic sills of the Cummings Complex. U-Pb dating of detrital zircons provides maximum depositional ages for metasedimentary rocks of the Blondeau Formation of the Roy Group and the Stella Formation of the Opemisca Group (which unconformably overlies the Roy Group). The Blondeau Formation is <2721 ± 3 Ma, which is coeval to the age of the Windfall Member in the Urban-Barry Belt. Both show similar exploration potential as they host early polymetallic (Au-Ag-Cu-Zn-Pb) veins. The Stella Formation is <2704 ± 2 Ma and is unaffected by first generation folds, thus providing a time constraint for the earliest folding event (D1). This new age also indicates that sedimentation in the Chapais-Chibougamau area occurred earlier than for the Timiskaming-type assemblages in the southern Abitibi greenstone belt, but the structural setting and presence of alkaline intrusions suggests similar potential for syenite-associated Au deposits in the Stella Formation.

Silver-rich telluride mineralization at Mount Charlotte and Au–Ag zonation in the giant Golden Mile deposit, Kalgoorlie, Western Australia

The gold deposits at Kalgoorlie in the 2.7-Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear zone system of the Golden Mile (1,500 t Au) in the southeast from the younger (D4) quartz vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill and display inner sericite–ankerite alteration and early-stage gold–pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20 % of the total gold in the first, but for 30 g/t Au) is characterized by Au/Ag = 2.54 and As/Sb = 2.6–30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault, but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag = 0.12–0.35) due to abundant hessite. We describe another array of silver-rich D2 shear zones which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz–carbonate veins and are surrounded by early-stage pyrite + pyrrhotite disseminated in a sericite–ankerite zone more than 6 m wide. Gold grade (0.5–0.8 g/t) varies little across the zone, but Au/Ag (0.37–2.40) and As/Sb (1.54–13.9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has a much lower Au/Ag (0.13) and As/Sb (0.48) and comprises scheelite, pyrite, native gold (830–854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250–300 °C, gold–hessite compositions indicate a fluid log f Te2 of −11.5 to −10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold–quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one.

Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea

Research Article| November 01, 2011 Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea Nathan R. Daczko; Nathan R. Daczko † 1Geochemical Evolution and Metallogeny of Continents Australian Research Council (GEMOC ARC) National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Sydney, Australia †E-mail: nathan.daczko@mq.edu.au Search for other works by this author on: GSW Google Scholar Peter Caffi; Peter Caffi 1Geochemical Evolution and Metallogeny of Continents Australian Research Council (GEMOC ARC) National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Sydney, Australia Search for other works by this author on: GSW Google Scholar Paul Mann Paul Mann § 2Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758-4445, USA §Current address: Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007; pmann@uh.edu Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (11-12): 2335–2351. https://doi.org/10.1130/B30326.1 Article history received: 07 May 2010 rev-recd: 13 Oct 2010 accepted: 21 Oct 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Nathan R. Daczko, Peter Caffi, Paul Mann; Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea. GSA Bulletin 2011;; 123 (11-12): 2335–2351. doi: https://doi.org/10.1130/B30326.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A shallow-dipping ductile mylonitic shear zone and concordant brittle detachment fault (Mai'iu fault) together make up the dominant geological structure that controls the orientation of dip slopes on the flanks of Mount Dayman, eastern Papuan Peninsula, Papua New Guinea. The dip slopes dip in all directions from the peak of Mount Dayman and form a domed landform that is much less dissected by streams compared to the adjacent Mount Suckling domed landform. The orientation of megacorrugations on the domed surface of Mount Dayman (footwall) is consistent with NNE-directed transport of the hanging-wall block, which is composed of low-grade undifferentiated volcanic and sedimentary rocks and minor ultramafic rocks. Though previously documented as a thrust surface, the geometry and style of structures and map relations presented in this study indicate an extensional origin for the domed mylonitic foliation (S1) and mineral elongation lineation (L1). The field relationships are consistent with the domed landform comprising the core of a metamorphic core complex. Observations of dominantly NNE-trending regional lineaments in aerial photography and Shuttle Radar Topography Mission (SRTM) data correlate with detailed field analysis of mineral elongation lineations (L1) in the main metamorphic core complex–bounding shear zone. Field relationships show a crosscutting sequence of structures that includes: (1) ductile S2 folia with ESE-plunging blue sodic-calcic amphibole mineral elongation lineations; (2) narrow, steeply dipping ductile D2 shear zones; and (3) semibrittle to brittle fault zones. S-C′ fabrics, asymmetric strain shadows around porphyroclasts, and fault drag indicate a top-down-to-the-NNE sense of shear for most structures. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (Wk) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). The NNE-directed lineaments and L1 mineral elongation lineations are consistent with the Australia-Woodlark Eulerian pole for periods between the early Pliocene (3.6 Ma) and Pleistocene (0.52 Ma). This observation is consistent with ca. 3.3 Ma granite and monzonite intrusions that cut the mylonitic fabrics and limit the age of the mylonitic fabrics to older than 3.3 Ma on Mount Suckling. A SE-dipping sedimentary sequence (Gwoira Conglomerate) characterizes part of the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The unit is in fault contact with the metabasite footwall across prehnite-bearing D3 brittle fault zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea

Research Article| November 01, 2011 Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea Nathan R. Daczko; Nathan R. Daczko † 1Geochemical Evolution and Metallogeny of Continents Australian Research Council (GEMOC ARC) National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Sydney, Australia †E-mail: nathan.daczko@mq.edu.au Search for other works by this author on: GSW Google Scholar Peter Caffi; Peter Caffi 1Geochemical Evolution and Metallogeny of Continents Australian Research Council (GEMOC ARC) National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Sydney, Australia Search for other works by this author on: GSW Google Scholar Paul Mann Paul Mann § 2Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758-4445, USA §Current address: Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007; pmann@uh.edu Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (11-12): 2335–2351. https://doi.org/10.1130/B30326.1 Article history received: 07 May 2010 rev-recd: 13 Oct 2010 accepted: 21 Oct 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Nathan R. Daczko, Peter Caffi, Paul Mann; Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea. GSA Bulletin 2011;; 123 (11-12): 2335–2351. doi: https://doi.org/10.1130/B30326.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A shallow-dipping ductile mylonitic shear zone and concordant brittle detachment fault (Mai'iu fault) together make up the dominant geological structure that controls the orientation of dip slopes on the flanks of Mount Dayman, eastern Papuan Peninsula, Papua New Guinea. The dip slopes dip in all directions from the peak of Mount Dayman and form a domed landform that is much less dissected by streams compared to the adjacent Mount Suckling domed landform. The orientation of megacorrugations on the domed surface of Mount Dayman (footwall) is consistent with NNE-directed transport of the hanging-wall block, which is composed of low-grade undifferentiated volcanic and sedimentary rocks and minor ultramafic rocks. Though previously documented as a thrust surface, the geometry and style of structures and map relations presented in this study indicate an extensional origin for the domed mylonitic foliation (S1) and mineral elongation lineation (L1). The field relationships are consistent with the domed landform comprising the core of a metamorphic core complex. Observations of dominantly NNE-trending regional lineaments in aerial photography and Shuttle Radar Topography Mission (SRTM) data correlate with detailed field analysis of mineral elongation lineations (L1) in the main metamorphic core complex–bounding shear zone. Field relationships show a crosscutting sequence of structures that includes: (1) ductile S2 folia with ESE-plunging blue sodic-calcic amphibole mineral elongation lineations; (2) narrow, steeply dipping ductile D2 shear zones; and (3) semibrittle to brittle fault zones. S-C′ fabrics, asymmetric strain shadows around porphyroclasts, and fault drag indicate a top-down-to-the-NNE sense of shear for most structures. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (Wk) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). The NNE-directed lineaments and L1 mineral elongation lineations are consistent with the Australia-Woodlark Eulerian pole for periods between the early Pliocene (3.6 Ma) and Pleistocene (0.52 Ma). This observation is consistent with ca. 3.3 Ma granite and monzonite intrusions that cut the mylonitic fabrics and limit the age of the mylonitic fabrics to older than 3.3 Ma on Mount Suckling. A SE-dipping sedimentary sequence (Gwoira Conglomerate) characterizes part of the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The unit is in fault contact with the metabasite footwall across prehnite-bearing D3 brittle fault zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Metamorphism and deformation on western Mt Chapple, Arunta Region, central Australia: A record of multiple high-grade tectonic events

Extensive exposures of deep crustal rocks in the south-central Arunta Region record a complex tectonic history from Proterozoic terrane assembly to Paleozoic exhumation. Western Mt Chapple was previously inferred to include evidence of much of this history. Detailed mapping and structural and metamorphic analysis, however, demonstrate three structurally distinct, high-grade deformational events, but no indication of the youngest, lowermost amphibolite to greenschist facies, Alice Springs exhumational overprint. Evidence of the oldest deformation is confined to local exposures of NE-striking gneissosity; neither metamorphic grade nor kinematics are constrained. The second event produced a steeply dipping, WNW-striking, NNE-dipping foliation, steeply E-plunging mineral lineation, and kinematic indicators recording dextral transpression in present-day geographic coordinates. Mineral assemblages preserved in blocks between D3 shear zones record D2 metamorphic conditions of 691–746 ± 50°C and 550–900 MPa. Northwest-striking D3 shear zones are characterised by an exceptionally well developed, NW-striking, steeply NE-dipping foliation and steeply SE-plunging mineral lineation with kinematic indicators that demonstrate sinistral-reverse, triclinic shear. Shear zone mineral assemblages record metamorphism at 702 ± 50°C. These data demonstrate a history of high-temperature deformation, including significant strike-parallel movement, which predates the Alice Springs orogeny.

Constraining the timing of deformation in the southwestern Central Zone of the Damara Belt, Namibia

Abstract Structural investigations and U–Pb sensitive high-resolution ion microprobe (SHRIMP) dating of rocks from the southwestern Central Zone of the Damara Belt, Namibia, reveal that a major SE-verging deformation event (D2) occurred at between 520 and 508 Ma. During D2, SE-verging simple shear and NE–SW pure shear extension in a constrictional stress field produced recumbent, south- to SE-verging, kilometre-scale folds and ductile shear zones, a NE–SW extensional lineation and conjugate shear bands, and was coeval with granitoid emplacement and high-grade metamorphism. The timing of this event is constrained by anatectic leucosomes in D2 shear zones (511±18 Ma) and extensional shear bands (508.4±8.7 Ma) as well as by syntectonic grey granites (520.4±4.2 Ma), and is similar to ages for high-grade metamorphism in the Central Zone. An upright folding event (D3) occurred at c . 508 Ma, resulting in the formation of basement-cored fold interference domes. The timing of deformation and metamorphism at 520–508 Ma in the mid-crustal SW Central Zone contrasts with ages of 560–540 Ma for shallow crustal NW-verging folding and thrusting elsewhere in the Central Zone that was concomitant with voluminous magmatism. This magmatism led to metamorphism and anatexis of the basement and the emplacement of anatectic red granites at 539±17 to 535.6±7.2 Ma, which contain 1013±21 Ma inherited zircons. The Central Zone therefore contains a record of crustal thickening, heating of the mid-crust, exhumation and orogen-parallel extension over the life of an orogen.

A D3 SHEAR ZONE IN THE MARGATE TERRANE AND ITS IMPLICATIONS FOR REGIONAL DEFORMATION IN THE NATAL METAMORPHIC PROVINCE (SOUTH AFRICA)

The Vungu shear zone is one of only two D3 shear zones in the Margate Terrane, and is here named and examined in detail for the first time. Detailed mapping indicates that this ~10 km wide shear zone can be subdivided into a high-strain core (~2 to 4 km thick) and lower-strain southeastern and northwestern marginal (each ~3 to 4 km thick) subzones. Macro- and microstructures are similar to other D3 shear zones in the Natal Metamorphic Province. Part of the northwestern marginal subzone shows D3-parallel magmatic foliation developed in the Oribi Gorge granite, thereby providing a reliable upper age of ~1.07 to 1.08 Ga for D3 in the Margate Terrane. This upper age is ~20 to 47 Ma older than that determined for the D3 event in the Mzumbe Terrane. Integration with newly constrained ages of the D1 (~1.09 Ga) and D2 (1.09 to 1.06 Ga) events illustrate their temporal overlap and support models that propose the D1– D3 structures comprise different expressions of the same tectonic event.

Structure and evolution of the Lewisian Gairloch shear zone: variable movement directions in a strike-slip regime

Synopsis The Gairloch shear zone (GSZ) has long been regarded as a dextral strike-slip shear zone with a northwesterly plunging shear direction. However, the original strain analysis applies only to the northeastern third of the zone and the structure of the remaining two-thirds is considerably more complex. The 6.5 km wide GSZ was formed during late Laxfordian (D3) deformation at c . 1.7 Ga. It was a product of the interaction of D3 folding and dextral strike-slip shear under retrogressive greenschist-facies metamorphism on rocks previously intensely deformed by early Laxfordian (D2) deformation in amphibolite-facies. The GSZ is divided into four units across strike: the Northeastern shear zone with NW-plunging D3 linear fabric; the Flowerdale belt with SE-plunging D2 linear fabric and low D3 strain; the Ard belt with steeply plunging F3 folds; and the Southwestern shear zone, with SE-plunging D3 lineation. Thus the GSZ consists of two high-strain D3 shear zones on either side of a central low-strain belt. Although consistently dextral strike-slip throughout, the D3 shear regime varies from NE-up in the Northeastern shear zone through sub-horizontal strike-slip in the Ard belt to SW-up in the Southwestern shear zone. This variation may have been caused by local uplift or depression induced by variations in geometry of the walls of the strike-slip system. A revised very approximate minimum estimate of the horizontal component of dextral shear is c . 9.5 km with a net NE-up vertical component of c . 5.5 km. Similarities in structure between the GSZ and the South Harris belt strengthen the case for these two zones to have been part of the same structure.

Tourmaline 40Ar/39Ar chronology of tourmaline-rich rocks from Central Iberia dates the main Variscan deformation phases

During crustal thickening, metapelites taken to depth release boron-bearing hydrothermal fluids because of progressive heating and dehydration. These fluids swiftly percolate upwards, especially if the crust is being actively deformed, to form tourmaline where the PT conditions and the chemical composition of the host-rock are favorable. The age of the so-formed tourmaline would record the age of the upward admittance of B-bearing fluids and, presumably, the age of the deformation. This process has been documented in the Martinamor Antiform of Central Iberia, a region where tourmaline-bearing rocks are particularly abundant. Metasomatic tourmaline from the Late Cambrian San Pelayo orthogneisses (zircon U-Pb age of 496 ± 5 Ma) yielded 40Ar/39Ar plateau ages at 370 ± 5 Ma and 342 ± 5 Ma. The first value represents the crystallization age of the tourmaline and is so far the most precise estimation of the age of crustal thickening in Central Iberia (D1). The second value reflects a partial loss of Ar caused by the second deformation phase (D2). Tourmaline from mylonitized and folded tourmalinites developed above D2 shear zones yield perturbed spectra with mean “plateau” ages of 347 ± 9 Ma and 342 ± 9 Ma which may represent either the resetting of older tourmaline or the formation of new tourmaline by focused boron metasomatism. After the metamorphic peak and simultaneously with the emplacement of the main granitoids of the Avila Batholith (310-315 Ma), another episode of boron metasomatism precipitated a new generation of tourmaline, which appears either concentrated in fine-layered tourmalinites (318 ± 2 Ma) or disseminated within Ediacaran-Cambrian metasediments (316 ± 2 Ma). The source of boron was the breakdown of previously formed tourmaline during melting reactions. Lastly, tourmaline from a leucogranitic body yielded a saddle-shaped age spectrum with a minimum age of ca. 296 Ma, roughly coeval with the youngest leucogranites. Although further work is required, our results suggest that tourmaline can be a useful chronological marker for dating deformation and magmatism.

Analysis of deformation fabric in an Alkaline Complex (Koraput): Implications for time relationship between emplacement, fabric development and regional tectonics

The Koraput Alkaline Complex (KAC) lies on the NE-SW trending Sileru Shear Zone (SSZ) separating the Proterozoic Eastern Ghats Province from the Archaean Indian craton. The core of the KAC is made of hornblende gabbro, which is rimmed by a band of nepheline syenite in the east and syenodiorite in the west. The timing of magmatism with respect to the SSZ is disputed. The KAC was deformed during emplacement, and a magmatic foliation related to the syn-emplacement deformation, D1, is present in the gabbroic core. The dominant D2-related field fabric strikes NESW and is penetrative in parts of the gabbro and marginal lithologies. E-W trending D3 shear zones cut across the complex. Distinct textural domains resulted from strain partitioning during deformation. Parts of the complex with magmatic textures constitute Domain-1, while D2 and D3 fabric zones comprise Domains-2 and 3 respectively. Temperatures in the KAC initially decreased following D1, but increased through D2 and D3. Anisotropy of magnetic susceptibility (AMS) studies show that the magnetic fabric generally follows S1 in Domain-1. While the magnetic fabric in Domain-2 is dominantly parallel to S2, some of it parallels S1. The latter is a relict D1 fabric that is recognized from AMS analysis but is obliterated in the field, which confirms that the KAC pre-dates the SSZ. The response of magnetic fabrics to temperature and implications of the study for Indo-Antarctica amalgamation are discussed.