Late Ductile Shear Research Articles

Evolution of gold-bearing splays of crustal-scale fault zones of the south-central Archean Abitibi subprovince: Constraints from the Kirana deformation zone, Kirkland Lake, Ontario, Canada

We explored the effects of brittle-ductile deformation processes on the localization of strain and hydrothermal fluid flow throughout the formation and evolution of a fault zone. This was accomplished through detailed structural analysis of the Kirana deformation zone (Kdz), a splay of the Archean crustal-scale, orogenic gold-hosting Larder Lake-Cadillac deformation zone (LLCdz) in Ontario, Canada. Investigations included mapping, quantification of fault-related breccia by particle size distribution analysis, microstructural and petrographic characterization, and kinematic analysis of brittle-ductile structures. The Kdz records three principal evolutionary stages. Stage 1 involved seismogenic processes that resulted in the formation of a >50 m wide fault core and damage zone. Stage 2 is characterized by transpressional ductile shear localized along texturally favorable, mechanically weakened zones that formed during stage 1 and/or along zones where coeval fluid flow facilitated reaction weakening. Native gold is observed within the mylonitized fault core that is interpreted to have formed syn-to late-ductile shear. All structures are crosscut by small-scale faults and shear veins with disseminated alteration halos, defining stage 3. The Kdz demonstrates how early fault processes may control the spatial pattern and temporal progression of strain and hydrothermal fluid localization in orogenic gold-bearing fault splays.

钦杭成矿带中新元古代海底火山喷流成矿作用—以江西万载兴源冲铜矿床为例<br>Meso-Proterozoic Marine Exhalative Mineralization in Qinhang Belt—An Example from Xingyuanchong Copper Deposit in Wanzai, Jiangxi

钦杭成矿带位于华南板内，属扬子与华夏两个古陆块在晚元古代碰撞拼接带。该带及旁侧矿床按成矿系列可归纳为新元古代海底喷流沉积型铜锌矿床成矿系列和燕山期与花岗岩有关的钨锡铜铅锌多金属矿床成矿系列。该带北东端的兴源冲铜矿床为中元古代海底喷流沉积变质叠改型铜矿床，其矿化与中元古界蓟县系宜丰岩组变细碧岩、变角斑岩有密切关系，受后期韧性剪切带控制，经变质热液作用，属多次叠加成矿。 The Qinhang belt is located along the Qinhang Neoproterozoic suture in South China, which separates the Yangtze craton and Cathaysian Blocks. Deposits along the belt can be divided into two episodes, i.e. Neoproterozoic marine exhalative Cu-Zn deposits, and Yanshanian granitoids related W-Sn-Cu-Pb-Zn deposits. Xingyuanchong copper deposit in its northeast belongs to the former series, is a Mesoproterozoic marine exhalative meta-sedimentary copper deposit, closely related with metamorphic spilite and quartz porphyry in Yifengyan Formation in Jixianian, controlled by late ductile shear zones, and superimposed mineralized after metamorphosed hydrothermal fluids processes.

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17–1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09–1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite , pyrite , pyrrhotite , and chalcopyrite mineralization . Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17–1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province. ► A ground gravity survey provides new data that aids mineral exploration. ► Most gravity anomalies are associated with mafic to intermediate intrusive rocks. ► A Gravity anomaly over mineralized outcrops may be generated by a deep gabbro. ► Mafic intrusion may be the driving source for mineralization or remobilisation.

Ultrapotassic Rocks along Late Ductile Shear Zones from the Eastern Ghats Belt, India

Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, Khyphen;feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midhyphen;crust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K 2O/Na 2O ratio of ∼12 (molar) and K 2O/Al 2O 3 ratio of ∼1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity. However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to Khyphen;feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt posthyphen;dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Panhyphen;African age. The extensive Khyphen;feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.

Ductile and brittle deformations in the northern snake range, nevada

The northern Snake Range, east-central Nevada, is one of the metamorphic core complexes of the Sevier hinterland. Within the range a major décollement separates an ‘upper plate’ composed of brittlely deformed Paleozoic sedimentary rocks (mostly carbonates), from a ‘lower plate’ composed of metamorphic Upper Precambrian-Lower Cambrian rocks, intruded by gneissic granites. A study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that: the northern Snake Range décollement has been a zone of intense non-coaxial E-vergent shear and transport in a ∼- N115°E direction. Outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains (γ) of at least 20. The interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition, but might result from two distinct phases of deformation. Ductile deformation in and below the décollement could have occurred before brittle deformation in the upper plate. Brittle faulting in the upper plate related to Basin and Range extension reactivated the upper surface of the ductile shear zone. High topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent Oligo-Miocene basins.

Parallel evolution of Archaean low- and high-grade terrane: A view based on relationships between the Abitibi, Wawa and Kapuskasing belts

Abstract The Kapuskasing structural zone (KSZ) is a region of high-grade Archaean gneiss transverse to the regional east—west structural trends of the adjacent lower-grade Abitibi, Wawa and Quetico belts. The structural zone is made up of granulite and amphibolite facies paragneiss, tonalitic gneiss and anorthosite-suite gneiss, whereas adjacent regions are metavolcanic—metasedimentary belts, with gneissic and plutonic components. Foliations in the KSZ strike northeast and dip moderately northwest. To account for the development of contrasting lithological, structural and metamorphic characteristics in adjacent areas, a unique Archaean history is postulated for the KSZ. Psammitic sedimentary rocks, possibly deposited in a fault-bounded basin, were intruded by anorthosite which may have had its erruptive equivalents in the tholeiitic volcanic rocks in the adjacent Abitibi belt. During the Kenoran orogeny the KSZ was metamorphosed and intruded at infrastructural levels while low-grade regions outside underwent sub-greenschist to amphibolite metamorphism at higher levels. Late ductile shear in the KSZ reoriented gneissic structural elements into northeast directions. Proterozoic fault movements juxtaposed low and high-grade regions.