Structural geochemistry of gold mineralization in the Linglong-Jiaojia district, Shandong Province, China

The Linglong-Jiaojia district is one of the most important regions containing gold deposits in China. These gold deposits can be divided into: a) the pyrite-gold-quartz vein type (Linglong type), which is controlled by brittle-ductile to ductile deformation structures, and b) the alteration-zone type (Jiaojia type), characterized by small veinlets, or the disseminated type recognized in brittle shear zones. Lode gold deposits in the Jiaojia area occur in NE brittle fracture zones, formed in a dominantly simple shear deformation regime, mainly in thrust attitude with a minor sinistral strike slip component. In the Linglong area, the lode gold deposits are located at the intersection of three types of structures: NNE and NE brittle-ductile fault zones and the ENE ductile reverse shear zone in the south of the area. The structural characteristics of these brittle shear zones are consistent with a tectonic NNW-SSE principal stress field orientation. Similar stresses explain the ENE Qixia fold axes, the Potouqing and several other ENE reverse ductile shear zones elsewhere in the region, the Tancheng-Lujiang fault zone and its subsidiaries in the vicinity of the Linglong-Jiaojia district, as well as the southern ENE suture zone north of Qingdao. Therefore these structural systems occurred as part of different major tectonic events under NNW-SSE compression principal stress fields in the area. Gold deposits are hosted in smaller-scale structures within the brittle fault zones and brittle-ductile shear zones. Although ore bodies and, on a smaller scale, quartz ore veins often seem to be randomly oriented, it is possible to explain their distribution and orientation in terms of the simple shear deformation process under which they were developed. The progressive simple shear failure is characterized by various fracture modes (tension and shear) that intervene in sequence. The tension and shear fractures are influenced by the stress level (depth of burial beneath the paleosurface) in their structural behavior, show variable dilatancy (void openings) and extend on all scales. By making use of these characteristics, a progressive failure analysis can be applied to predicting the shape and extent of ore bodies as well as the styles of mineralization at any given location.     

Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy

The controversial relationship between the orogenic segments of the Western Alps and the Northern Apennines is here explored integrating recently published 3D tomographic models of subduction with new and re-interpreted geological observations from the eclogitic domain of the Voltri Massif (Ligurian Alps, Italy), where the two belts joint each other. The Voltri Massif is here described as an extensional domain accommodating the opposing outward migration of the Alpine and Apennine thrust fronts, since about 30–35 Ma. Using tomographic images of the upper mantle and paleotectonic reconstructions, we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time. Our tectonic model suggests that the westward shift of the Alpine thrust front from the Oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the Apenninic slab.     

Late Alpine evolution of the central Menderes Massif, western Turkey

The central Menderes Massif (western Turkey) is characterized by an overall dome-shaped Alpine foliation pattern and a N-NNE-trending stretching lineation. A section through the southern flank of the central submassif along the northern margin of Büyük Menderes graben has been studied. There, asymmetric non-coaxial fabrics indicate that the submassif has experienced two distinct phases of Alpine deformation: a top-to-the N-NNE contractional phase and a top-to-the S-SSW extensional event. The former fabrics are coeval with a regional prograde Barrovian-type metamorphism at greenschist to upper-amphibolite facies conditions. This event, known as the main Menderes metamorphism, is thought to be the result of internal imbrication of the Menderes Massif rocks along south-verging thrust sheets during the collision of the Sakarya continent in the north and the Anatolide-Tauride platform in the south across the Gzmir-Ankara suture during the (?)Palaeocene-Eocene. Top-to-the S-SSW fabrics, represented by a well-developed ductile shear band foliation associated with inclined and/or curved foliation, asymmetric boudins, and cataclasites, were clearly superimposed on earlier contractional fabrics. These fabrics are interpreted to be related to a low-grade (greenschist?) retrogressive metamorphism and a continuum of deformation from ductile to brittle in the footwall rocks of a south-dipping, presently low-angle normal fault that accompanied Early Miocene orogenic collapse and continental extension in western Turkey. A similar tectono-metamorphic history has been documented for the northern flank of the dome along the southern margin of the Gediz graben with top-to-the N-NNE extensional fabrics. The exhumation of the central Menderes Massif can therefore be attributed to a model of symmetric gravity collapse of the previously thickened crust in the submassif area. The central submassif is thus interpreted as a piece of ductile lower-middle crust that was exhumed along two normal-sense shear zones with opposing vergence and may be regarded as a typical symmetrical metamorphic core complex. These relationships are consistent with previous models that the Miocene exhumation of the Menderes Massif and Cycladic Massif in the Aegean Sea was a result of bivergent extension.     

江南造山带西南段摩天岭穹隆脆韧性剪切与铀成矿作用

在造山带构造体制转换环境下,铀元素的活化、迁移和沉淀富集过程多与脆韧性剪切带的递进发育过程紧密相关。鉴于此,详细的区域和矿田构造解析是正确理解铀矿床成因的关键。华南江南造山带及其周缘分布着大面积低温铀矿床,既有晚古生代铀矿床,又有后期叠加的新生代铀矿床,均受脆韧性剪切转换带控制。华南铀矿床成矿作用具有多期叠加成矿特点,以往对单个铀矿床成矿地质和赋矿特征的研究较多,但对铀成矿作用与区域地质、构造体制及其剪切转化关系的系统研究则较为薄弱,造成对矿床成因认识很不统一,制约了对这一区铀成矿作用的深入理解。为深入了解这类铀矿床的形成机制,本文选取了摩天岭穹隆376矿床和374矿床两种不同地质特征的代表性铀矿床开展研究。通过详细地构造解析,我们认为摩天岭穹隆至少发育了五期构造变形,分别为新元古代(～820Ma)(D1期)近东西向褶皱与同构造岩浆侵位、加里东期顶部向NW的逆冲(453～426Ma)(D2期)、后加里东期NE走向的正向韧性剪切(426～295Ma)(D3期)、燕山晚期-喜马拉雅期的脆-韧性伸展(87～47Ma)(D4期)及喜马拉雅期以来的构造隆升与剥蚀(47Ma～至今)(D5期);结合显微构造与电子探针分析,认为D3期和D4期为关键铀成矿期。通过系统野外调查,以构造地质解析为主线,对新元古代三防花岗岩体及其周缘主要含矿构造与典型铀矿床的关系进行了详细解析,提出了摩天岭两种类型铀矿床由脆韧性递进变形控制的统一铀成矿模式,以期对华南铀矿勘查工作提供借鉴。     

Granitoids of the Magba shear zone,West Cameroon,Central Africa: Evidences for emplacement under transpressive tectonic regime

The Magba Shear Zone is made up of granites, migmatites, orthogneiss, metagabbro, mafic dyke and mylonites with coarse grained texture, porphyroblastic, granoblastic, cataclastic and mylonitic texture respectively. Structural features and kinematic indicators testify the syntectonic emplacement of Magba granitoids and also provide detailed information on the relative timing of deformation as follows: (1) D1 of tangential movement immediately followed by (2) the D2 phase which is heterogeneous simple shear in dextral transpressive context with a NW-SE direction (3) D3 tectonic phase is marked by sinistral transpressive tectonic and superposed folding with a NE-SW kinematic direction. Combined ductile NE-SW shear movements and NWSE compressional movements defined a transpressional tectonic regime during the D3 deformation (4) A brittle stage D4 is controlled by transcurrent tectonics and responsible for the emplacement of faults, and joints. The Magba granites would have intruded along sub-vertical mid-crustal feeder channels and were emplaced as a sheet or sheets along the shear zone during the early stage of the C3 shearing, followed by gabbro and mafic dyke at the late stage. Strike-slip dilatancy pumping under transpressive tectonic is suggested as a possible mechanism for the emplacement of the Magba granites.     

Syn‐extension leucogranite deformation during convergence in the Eastern Central Alps: example of the Novate intrusion

The Novate intrusion is a Late Alpine leucogranite that intruded the structures related to dextral back‐thrusting along the Periadriatic Fault System in the Eastern Central Alps. The Novate granite was heterogeneously deformed from amphibolite to greenschist facies conditions during cooling of the intrusion. The deformation inside the granite is characterized by strongly localized and anastomosed ductile shear zones surrounding lenses of weakly deformed granite and by late faults formed at the brittle–ductile transition. The fault kinematic analysis of conjugated shear zones suggests that the Novate leucogranite was emplaced at 25 Ma in an extensional regime along the southern tip of the Forcola Fault. A model of extensional jog opening by vertical shearing along the Forcola Fault provided the space for magma accommodation. The Novate granite is the first evidence for orogen‐parallel syn‐extensional leucogranite emplacement during the Oligocene collision in the Alps.     

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.     

郯庐断裂带张八岭隆起北段的左旋走滑挤压变形及其~(40)Ar/~(39)Ar定年

郯庐断裂带张八岭隆起北段,自西向东分别出露北北东向的韧性滑脱变形带、脆-韧性过渡带和脆性的前陆褶断带。韧性滑脱变形带内的张八岭群片岩,广泛发生了低绿片相背景下的糜棱岩化。其中呈现为平缓的糜棱面理和近南北向的矿物拉伸线理。显微构造及石英C轴组构分析显示,该韧性滑脱带一致为上盘向南的运动。该带以东依次变为上盘向南南东→南东的逆冲活动,总体上为左旋走滑挤压变形带。张八岭群所在的韧性变形带为深部陡立走滑构造与浅部脆性构造之间的滑脱变形带,其间的差异走滑变形,造成了该滑脱层在总体向北运动中出现上盘向南的剪切变形。对6处张八岭群片岩中15个不同粒级白云母的40Ar/39Ar定年指示,变形发生在(236.2±0.5)~(238.0±0.4)Ma的中三叠世晚期。这表明郯庐断裂带的左行平移发生在华北与华南板块碰撞的深俯冲阶段,起源于陆内转换断层。     

Phanerozoic polyorogenic deformation in southern Jiuling Massif,northern South China block: Constraints from structural analysis and geochronology

The structure of the Jiuling Massif has been investigated in order to delineate the polyorogenic deformation and discuss its geodynamic evolution and orogenic mechanisms. Detailed structural analysis indicates that the D₁ event is characterized by top-to-the NNW ductile shearing with pervasive foliation, and mineral and stretching lineation developed in the entire region. Compared with the D₁ deformation, D₂ structures are localized in ductile shear zones with subvertical foliation and subhorizontal E–W trending lineation, indicating a dextral ductile shearing. The D₃ event, marked by folds and thrusts mainly in a brittle domain, modified the D₁ structures by asymmetrical folds. The dominant D₄ structures are gravitational folds and normal faults, corresponding to a later extension. Our new geochronological data suggest that the D₁ event occurred between 465 and 380 Ma with D₂ dextral shearing at the end of this Early Paleozoic orogen, and the D₃ event has been constrained at 245–215 Ma. The final uplift of the Jiuling Massif by the D₄ event can be correlated with the Late Mesozoic extension across the eastern South China block. Along with previous studies in the South China block, the structural pattern of the Jiuling Massif elucidates the influence of the Early Paleozoic and Early Mesozoic intracontinental belts triggered by repeated reactivation of the Jiangshan–Shaoxing Fault. Combined with deformation to the south, the Early Paleozoic belt shows a positive flower pattern, with opposing kinematics, rooted in the Jiangshan–Shaoxing Fault. During the Early Mesozoic, a general intracontinental belt was developed with uniform kinematics in both the Jiuling Massif and the Xuefengshan Belt, possibly resulted from the far-field effect of the Triassic NW-directed Paleo-Pacific subduction.     

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

The inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.     

西藏罗布莎豆荚状铬铁矿成矿演化的构造过程

摘要：通过对罗布莎铬铁矿区的构造解析,揭示出了豆荚状铬铁矿的成矿演化规律。由地幔韧性剪切带和脆－韧性剪切带组成的含铬剪切带是成矿期构造,被造山期发生的变形分解作用和脆性断裂作用改造,成矿演化经历了从上地幔到上地壳所发生的５个构造变形相的转换,即熔融流变变形相→地幔韧性剪切变形相→壳幔脆→韧性剪切变形相→塑性挤压变形相→脆性断裂变形相,可划分为中生代改造成矿和新生代矿床改造两个阶段,并概括为包含６个变形世代的构造成矿序列。     

Structural analyses of the crystalline rocks between Dirang and Tawang,West Kameng district,Arunachal Himalaya

In Kameng Valley of Arunachal Pradesh, the crystalline rocks of Se La Group of Higher Himalaya are thrust over the Lesser Himalayan rocks of Dirang Formation, Bomdila Group along the Main Central Thrust and exhibit well preserved structures on macro- to microscopic scales. Detailed analysis of structures reveals that the rocks of the area have suffered four phases of deformation D₁, D₂, D₃ and D₄. These structures have been grouped into (i) early structures (ii) structures related to progressive ductile thrusting and (iii) late structures. The early structures which developed before thrusting formed during D₁ and D₂ phases of deformation, synchronous to F₁ and F₂ phases of folding respectively. The structures related to progressive ductile shearing developed during D₃ phase of deformation, when the emplacement of the crystalline rocks took place over the rocks of Dirang Formation along the Main Central Thrust. Different asymmetric structures/kinematic indicators developed during this ductile/brittle-ductile regime suggest top-to-SSW sense of movement of the crystalline rocks of the area. D₄ is attributed to brittle deformation. Based on satellite data two new thrusts, i.e. Tawang and Se La thrusts have been identified parallel to Main Central Thrust, which are suggestive of imbricate thrusting. Strain analysis from the quartz grains of the gneissic rocks reveals constriction type of strain ellipsoid where k value is higher near the MCT, gradually decreases towards the north. Further, the dynamic analysis carried out on the mesoscopic ductile and brittle-ductile shear zones suggest a NNE-SSW horizontal compression corresponding to the direction of northward movement of Indian Plate.     

安徽江淮地区北部韧性剪切带与金矿关系

区内发育有东西向、南北向两类韧性剪切带,前者为韧性平移剪切带,后者为韧性逆冲剪切带,它们分别形成于早、中元古代末。南北向韧性剪切带是区内金矿的主要控矿构造,控制了区域的矿化分带及矿田、矿床、矿体的分布。金矿主要形成在地洼期,与叠加在早期韧性剪切带上的脆-韧性变形作用有关。     

Tectonic transport directions of the Lycian nappes in southwest Turkey constrained by kinematic indicators

The orientation, asymmetry and cross-cutting relationships of the structures along the contact zone between the Lycian nappes and the Menderes Massif suggest the presence of three deformation phases in the Milas region of southwest Turkey. The first deformation phase (D1) is characterized by a ductile deformation with top-to-the-NE sense of shear. Structural data of the first deformation measured along the uppermost part of the Menderes Massif and the base of the Lycian nappes suggest that the lowermost unit of the Lycian nappes was emplaced initially from southwest to northeast onto the Menderes Massif during the Early Eocene. The second deformation phase (D2) is also ductile in nature and is characterized by an E–W-trending stretching lineation with a bivergent sense of shear, which is probably related to the load of the overlying nappes. A third deformation phase (D3) is characterized by south-dipping normal faults with top-to-the-S sense of movement. This third deformation phase can be related to southward movement of the Lycian nappes along a low-angle décollement zone. The tectonic contact between the Menderes Massif and the Lycian nappes and their strongly-deformed rocks are unconformably covered by approximately flat-lying, coal-bearing Early–Middle Miocene sedimentary rocks, which constrains the upper time limit for all three deformation phases.     

夹皮沟剪切带化学成分演化研究

野外地质填图及室内工作表明,夹皮沟剪切带可以划分为早期韧性剪切带(>2500Ma)和晚期脆—韧性剪切带(2000～1600Ma).韧性剪切作用是在角闪岩相条件下形成,以位错、扩散蠕变变形为主,体系内岩石成分与体积基本保持不变,为封闭体系,与矿化关系不密切.脆—韧性剪切作用形成一系列片糜岩,岩石以微破裂及溶解—沉淀变形为主,体系内岩石成分与体积都发生变化,为开放体系,与矿化关系密切.韧性及脆—韧性剪切带的基础岩石是长英质片麻岩,但脆—韧性剪切带被韧性剪切带所限定,并在韧性剪切带基础上形成.     

Geometric consequences of ductile fabric development from brittle shear faults in mafic melt sheets: Evidence from the Sudbury Igneous Complex,Canada

Compared to felsic igneous rocks the genetic relationship between brittle and ductile fabric development and its influence on the geometry of deformed mafic melt sheets has received little attention in structural analyses. We explore these relationships using the Sudbury Igneous Complex (SIC) as an example. The SIC is the relic of a layered impact melt sheet that was transformed into a fold basin, the Sudbury Basin, during Paleoproterozoic deformation at the southern margin of the Archean Superior Province. We studied brittle and ductile strain fabrics on the outcrop and map scales in the southern Sudbury Basin, notably in the Norite and Quartz Gabbro layers of the SIC. Here, deformation is heterogeneous and occurred under variable rheological conditions, evident by the development of brittle shear fractures, brittle-ductile shear zones and pervasive ductile strain. The mineral fabrics formed under low- to middle greenschist-facies metamorphism, whereby brittle deformation caused hydrolytic weakening and ductile fabric development. Principal strain axes inferred from all structural elements are collinear and point to a single deformation regime that led to thinning of SIC layers during progressive deformation. Ductile fabric development profoundly influenced the orientation of SIC material planes, such as lithological contacts and magmatic mineral fabrics. More specifically, these planar structural elements are steep where the SIC underwent large magnitudes of thinning, i.e., in the south limb of the Sudbury Basin. Here, the actual tilt component of material planes is likely smaller than its maximum total rotation (60°) inferred from inclined igneous layering in the Norite. Our field-based study shows that ductile fabric development from brittle faults can have a profound influence on the rotational components of primary material planes in deformed igneous melt sheets.     

中国南天山萨恨托亥-大山口成矿带韧性剪切带与金成矿作用

以南天山中段萨恨托亥-大山口成矿带内控矿韧性剪切带为例,对韧性剪切带的金成矿作用进行了初步探讨.通过对地质体的构造变形特点、变形演化过程的分析表明,韧性剪切带的构造属性控制了金矿的产状及规模,金矿化阶段与韧性剪切带的变形演化过程密切相关.矿化类型、矿化强度及矿化方式受韧性剪切带发展阶段制约,剪切带内物质组分迁移变化揭示出韧性剪切带与金在剪切带内的迁移富集、沉淀成矿的内在联系.韧性剪切带成矿作用是南天山成矿带中段重要的金矿成矿作用.     

Structural analysis of tin-bearing mineralization in Bou El Jaj district (Hercynian Central Massif,Morocco)

The aim of this paper is to characterize the geological setting of tin-bearing mineralization at the Bou El Jaj (BLJ) sector, located in the NE termination of the Moroccan Central Massif, South of Meknes city, along the NE-SW-striking shear zone. The main tin mineralization corresponds to a NNE-SSW altered corridor of tourmaline, about 10 to 12 km long, from BLJ to Achmmach mount. The geological structures are affected by three ductile deformation phases D1, (E-W shortening), D2 (NW-SE shortening), and D3 (N-S shortening), overprinting folds, and one brittle deformation event D4 (NW-SE shortening), which was synchronous with alteration and mineralization. The tourmaline-altered sediments occur in two parallel veins, about 2.5 km long and 200 m wide for each one, and are controlled by structures such as bedding, main cleavage, thrusts, and joints. Tin mineralization as cassiterite is always associated with tourmaline alteration and is controlled by the different structures.     

Alpine tectonic evolution and thermal water circulations of the Argentera Massif (South-Western Alps)

Three groups of thermal springs with temperatures close to 70 °C discharge both in the core (at Bagni di Vinadio and Terme di Valdieri) and on the external margin (at Berthemont-Les-Bains) of the Argentera Massif. Detailed structural field analysis carried out on the hydrothermal sites allows us to delineate both a model of Alpine tectonic evolution of the Argentera Massif and the patterns of hydrothermal circulation that were active during its final exhumation. The observed fault rock assemblages provide information relative to deformation that occurred in viscous, frictional-to-viscous and frictional crustal regimes. During the Early Miocene, the Bersezio Fault Zone and the Fremamorta Shear Zone, two main mylonitic shear zones, mainly accommodated regional transpression and provided pathways for fluid flow promoting mineral reactions in greenschist facies. During the Late Miocene–Early Pliocene, frictional-to-viscous deformation affected the massif, which underwent predominant transpression in the internal sectors and extension on the external margin. During the Plio-Pleistocene, deformation in frictional condition accompanied the final exhumation of the massif in a transpressive regime and resulted in the development of the NW–SE striking cataclastic zones. The hydraulic properties of these structures mainly influence the patterns of the active thermal circulations and the localization of the recharge and discharge zones. At Berthemont these faults represent conduits, whereas at Vinadio and Valdieri they form complex systems of conduits and barriers. In these two latter sites, the cataclastic faults compose flower structures that constrain laterally the thermal fluid flows while intensely fractured granites sited at depth constitute a highly-transmissive geothermal reservoir. Less permeable migmatitic gneisses overlaying the granites prevent a massive infiltration of the cold fluids at depth. This context favours within the high-permeability fractures granites the development of buoyancy-driven flows which combined with topographically-driven flows, provided the conditions for the upflow of the high-temperature waters.