Structural geology of the Dugald River ZnPbAg deposit, Mount Isa Inlier, Australia

The sediment-hosted ZnPbAg deposit at Dugald River is situated 87 km northeast of Mount Isa, NW Queensland. It is a mid-scale base metal accumulation restricted to a black slate sequence of low metamorphic grade. The orebody is tabular and consists of fine- to medium-grained sulphides with a dominant mineralogy of sphalerite, pyrrhotite, pyrite, galena, quartz and muscovite. Three different ore types have been recognized based on mineralization textures; laminated, banded and brecciated. The present reserve stands at 38 million tons of ore averaging 13.0% Zn, 2.1% Pb and 42 g/t Ag. A structural investigation has revealed that six stages of deformation have affected the metasediments in the Dugald River area. The first four (D₁, D₂, D₃ and D₄) are characterized by the extensive development of folds and associated axial plane cleavage. They were all generated in a ductile regime and are of considerable significance for the structural evolution of this region as well as for the emplacement and localization of the sulphide mineralization. D₅ provides a transition towards brittle deformation developing strong kink folds with subhorizontal axial planes. D₆ was a brittle event, producing E-W-trending open folds and major NE and NW strike-slip faults crosscutting all the pre-existing structural elements plus segmenting the orebody. Correlation between the development of deformation and the formation of mineralization can be observed from macro- to microscales. Relationships of mineralization with folds and cleavage indicate a post-D₂ (dominant deformation event) and probably syn-D₄ deformation timing for the ZnPbAg mineralization at Dugald River, as suggested by the ubiquitous truncations of D₂ fabrics by ore mineral assemblages throughout the deposit.     

Microstructural evidence for an epigenetic origin of a Proterozoic zinc-lead-silver deposit, Dugald River, Mount Isa Inlier, Australia

The Zn-Pb-Ag deposit contained in the metasediments of middle Proterozoic age at Dugald River forms one of a number of significant zones of sulphide mineralization within the Eastern Fold Belt of the Mount Isa Inlier. It is characterized by its high Zn + Pb grade, high Zn/(Zn+Pb) ratio and strong structural controls with the present resource standing at 38 million tons averaging 13% Zn, 2.1% Pb and 42 g/t Ag. Microstructural timing relationships and a variety of microscopic ore textures indicate that the deposit resulted from replacement and partial infill of carbonaceous and pyritic host rocks by hydrothermal ore-forming fluids during the D₄ deformation event. This genesis is contrary to earlier syngenetic-based interpretations, but accords with the discordant nature and structural controls on emplacement of the mineralization. Key timing criteria include (1) truncation of S₂ and/or S₃ by the late ore minerals, (2) replacement textures in undeformed mineral paragenesis, (3) slightly preferred dimensional orientation and undulating extinction of quartz and muscovite that are intergrown but which crystallized earlier than sulphide minerals in veins. The presence of these microstructural relationships throughout the deposit and the complete absence of any syngenetic stratiform precursor to zinc-lead-silver mineralization indicates that the zinc, lead and silver at Dugald River were epigenetically introduced rather than just having undergone syngenetic deposition during sedimentation and remobilization during deformation. The regional distribution of the mineralization in a specific stratigraphic zone is most likely due to the partitioning of deformation between different rock types. This caused the weaker lithology to accommodate significantly higher strain than adjacent more competent units, resulting in fracturing and localization of the syntectonic mineralization. Received: 8 September 1995 / Accepted: 12 April 1996     

Dehydration-related deformation during regional metamorphism, NW Sardinia, Italy

The relationship between deformation and dehydration has been investigated in Hercynian regionally metamorphosed rocks exposed on NW Sardinia. Two episodes of prograde mineral growth (M₁ & M₂) involving dehydration are recognized: growth of chlorite/phengite porphyroblasts at anchizone metamorphic conditions, contemporaneous with the first phase of deformation, D₁, and growth of biotite from chlorite and phengite coincident with the second phase of deformation, D₂. Deformation during both episodes of dehydration is characterized by penetrative axial planar foliations defined by well-developed phyllosilicate preferred orientations quantified by XRD textural goniometry, tight to isoclinal similar folds (interlimb angles <40°), and mineral-filled veins (hydrofractures) orientated parallel to axial planar foliations, that formed contemporaneously with the development of the penetrative foliations. No prograde mineral growth occurred during D₂ at chlorite-zone conditions. D₂ deformation in the absence of dehydration is characterized by non-penetrative crenulation cleavages, poorly developed phyllosilicate preferred orientations, relatively open (interlimb angles >40°), low-strain similar folds and minor brittle deformation. Systematic variations in macrofold interlimb angles, with respect to the timing of mineral growth, indicate that enhanced shortening (c. 80%) occurred during dehydration. Microfabrics show that the onset of dehydration is associated with the transition from a crenulation cleavage to a penetrative foliation. The presence of axial planar hydrofractures that formed coevally with dehydration and fabric development requires that supralithostatic fluid pressures and low differential stresses (<c. 20 MPa) accompanied dehydration. These features demonstrate a connection between the timing of dehydration and the style of deformation.     

Structure of the Late Palaeozoic Coffs Harbour Beds,northeastern New South Wales

F₁ macroscopic folds in the Late Palaeozoic Coffs Harbour Beds in the SE portion of the New England Fold Belt are commonly transected by cleavage. These macroscopic folds are tight to isoclinal structures, with a consistent vergence to the NE. Axial surfaces are either steeply dipping to the SW or vertical, and are typically faulted. Anomalous bedding‐cleavage relations occur where the steeply dipping cleavage intersects overturned limbs of F₁ macroscopic and some F₁ mesoscopic folds. Elsewhere F₁ mesoscopic folds have a well developed, axial‐surface cleavage and are rarely downward facing. Cleavage is commonly strike‐divergent from axial surfaces of F₁ macroscopic folds, except adjacent to the Demon Fault System, where they are parallel. These anomalous cleavage‐folds relations possibly developed during the one deformation. D₁ structures are refolded by kink‐like folds that are steeply plunging. The structural style of the D₁ deformation indicates that it possibly resulted from accretionary processes at a consuming plate margin.     

胶东玲珑金矿田171号脉深部金矿床特征及构造控矿作用

玲珑金矿田171号脉赋存于招平断裂北段破头青断裂中,在其深部探明金资源储量150余吨,确定其为超大型破碎带蚀变岩型金矿床。为了揭示构造与成矿、深部矿与浅部矿、石英脉型矿与蚀变岩型矿等的关系,为深部找矿提供典型实例和理论依据,文章通过大量矿区地质勘探、野外地质调查和区域地质综合研究,剖析了矿床特征,揭示了构造控矿规律。171号金矿脉深部共有7个金矿体,均为盲矿体。其中,171_1号主矿体埋深120~1700m,呈似层状、大脉状分布,沿走向及倾向显舒缓波状。矿体总体走向60°,倾向SE,倾角36.5~43.5°。矿体长2500m,斜深510~3100 m,平均厚度4.23 m,平均金品位2.71×10~(-6),矿石主要为黄铁绢英岩型(蚀变岩型)。矿脉和矿体受断裂构造控制,矿化、构造、蚀变分带具有一致性。由控矿断裂的主裂面至远离主裂面,构造变形强度由强变弱,蚀变强度和矿化强度也由强变弱。招平断裂是一条切割早期韧性剪切带的脆性断裂,总体呈舒缓波状展布,大部分地段发育于早前寒武纪变质岩系与侏罗纪玲珑型花岗岩之间,断裂上盘脆性变形较弱,下盘脆性变形带宽大,是沿不同时代地质体之间发育的左行铲式正断层,类似于拆离断层,是胶东白垩纪伸展构造的组成部分。玲珑金矿田是典型的石英脉型金矿产地,但大型金矿床多为赋存于断裂构造中的蚀变岩型金矿床,石英脉型金矿床与蚀变岩型金矿床是同一构造系统中不同构造位置的产物;一般在主断裂中赋存蚀变岩型金矿床,在主断裂下盘的次级张裂隙中产出石英脉型金矿床。胶东以正断层为主的伸展构造系统为大规模成矿提供了有利条件,构造、蚀变分带和不同类型金矿床的关系是金矿找矿的重要标志。     

新疆康古尔金矿床时空四维结构模型

矿床时空四维结构模型是对矿床（体）与诸主要控矿因素时空配置关系的描述,它基于从时间演化的角度来考察矿体（体）的三维空间分布规律及其与诸控矿因素之间的内在联系的结构关系,它是矿床模型研究的重要前沿问题之一。本文通过对康古尔金矿床中矿体的形态空间展布及控矿构造型式、成矿阶段、成矿物质来源、成矿时代等方面的综合研究,提出火山岩、韧性挤压变形带、同构造花岗岩体、晚期韧－脆性正断剪切带是矿床金成矿的四个重要     

The structural evolution of the Northern Hastings Block and southern Nambucca Block,southern New England Orogen,eastern Australia

The Hastings Block is a weakly cleaved and complexly folded and faulted terrain made up of Devonian, Carboniferous and Permian sedimentary and volcanic rocks. The map pattern of bedding suggests a major boundary exists that divides the Hastings Block into northern and southern parts. Bedding north of this boundary defines an upright box-like Parrabel Anticline that plunges gently northwest. Four cleavage/fold populations are recognised namely: E–W-striking, steeply dipping cleavage S₁ that is axial surface to gently to moderately E- or W-plunging; F₁ folds that were re-oriented during the formation of the Parrabel Anticline with less common N–S-trending, steeply dipping cleavage S₂, axial surface to gently to moderately N-plunging F₂ folds; poorly developed NW–SE-striking, steeply dipping cleavage S₃ axial surface to mesoscopic, mainly NW-plunging F₃ folds; and finally_, a weakly developed NE–SW-striking, steeply dipping S₄ cleavage formed axial surface to mainly NE-plunging F₄. The Parrabel Anticline is considered to have formed during the D₃ deformation. The more intense development of S₂ and S₃ on the western margin of the Northern Hastings Block reflects increasing strain related to major shortening of the sequences adjacent to the Tablelands Complex during the Hunter–Bowen Orogeny. The pattern of multiple deformation we have recorded is inconsistent with previous suggestions that the Hastings Block is part of an S-shaped orocline folded about near vertically plunging axes.     

辽宁省本溪北台铁矿的构造控制规律——运用构造解析解决找矿问题的一个实例

本溪北台铁矿为产于太古宙鞍山群中的BIF.矿体形态、产状均甚复杂.构造解析表明,这种复杂性是由于经受了至少三期构造变形所引起的.本文在详细剖析各期构造变形及其对铁矿体的控制规律的基础上,提出了深部找矿方向,已得到实践验证,经济效果显著.     

Thrust-controlled, gold quartz-vein mineralisation at the Tom's Gully mine, Northern Territory, Australia

Metasedimentary and minor metavolcanic rocks of the Early Proterozoic Pine Creek Inlier rest unconformably on Late Archaean granitic basement. Three basin-wide, regional deformation events at ca.1885–1870 Ma are recognised: I) W- to NW-verging thrusts and recumbent folds (D₂), II) upright, open to tight, doubly-plunging, NNE- to NNW-trending folds (D₃), and III) open, upright, E-trending folds (D₄). In the centre of the Pine Creek Inlier, post-tectonic granites (1835–1820 Ma) are spatially, temporally and probably genetically associated with mesothermal gold-quartz vein deposits. The Tom's Gully deposit consists of a shallowly S-dipping quartz reef in graphitic shale and siltstone within the thermal aureole of the post-tectonic (1831 ± 6 Ma) Mt Bundey pluton. Gold mineralisation comprises two(?) SSW-plunging sulphidic ore-shoots which are intimately associated with brecciation and recrystallisation of early barren quartz. Where early quartz is absent from the thrust, gold mineralisation is not developed, indicating that this secondary brittle fracturing was essential to sulphide and gold deposition. The ore-shoots plunge parallel to the trend of D₃ fold axes. The reef is hosted by a D₂ thrust fault with transport to the NW. D₃ folds in the hangingwall and footwall decrease in amplitude toward the reef indicating that, during continued E-W compression, the thrust acted as a décollement zone. Field relationships and microstructural studies suggest that quartz and sulphide were deposited in a reactivated thrust during wrench shear along several NNE-trending faults associated with emplacement of the Mt Bundey pluton.     

洪江市大坪金矿床地质特征及其找矿前景

大坪金矿床产于黄茅园与中华山花岗岩体之间外接触带，赋矿地层为上元古界及震旦系浅变质绿片岩相碎屑岩系，金源物质丰富，区域构造变形强烈，褶皱断裂发育，矿床总体受NE向大型韧脆性剪切带（F8)控制，矿化明显分为NW向和NE向两组，以NW向矿脉蚀变强，矿物组合复杂，金含量较高，富矿体产于两者交叉处，主要矿体均分布于变形强带内部及附近，矿化与硅化，绢云母化，黄铁矿，毒砂关系密切，目前发现含放脉37条，矿体近30个，找矿前景十分广阔。     

Zircon U–Pb Age and Deformation Characteristics of the Jiama Porphyry Copper Deposit,Tibet: Implications for Relationships between Mineralization,Structure and Alteration

The Jiama copper deposit is one of the largest deposits recently found in Tibet and is composed of three types of mineralization including skarn, hornfels and porphyry. To investigate the relationship between mineralization, structure and alteration, we report new zircon U–Pb age and present field observations on the deformation characteritics associated with the copper mineralization in Jiama. Two main periods of deformation were identified, represented by D₁ and D₂ in Jiama. The first deformation (D₁) occurred around 50 Ma, whereas the second deformation (D₂) that was closely related to mineralization occurred later. Previous zircon U–Pb and molybnite Re–Os dating results indicate that the mineralizatoin occurred at ～15 Ma and thus the D₁ regional deformation significantly occurred before the mineralization time, although the D₁ deformation probably provided important space for the development of significant copper deposition. Our new mapping and observations on the D₂ deformation demonstrate that the mineralization was closely coeval with or slightly later than the time of D₂ deformation. The new U–Pb zircon age further indicates that the aplite formed in ～17.0 Ma and thus the D₂ deformation happened later than this time because the D₂ deformation cut across the aplite, which is proposed to be the key control for copper mineralization. Altered laminated hornfels including three types of alteration (A‐, K‐ and S‐type) were found spatially associated with the D₂ deformation. The type‐A is mainly silicification, with fine sericite or chlorite, as well as abundant disseminated sulphides on fracture surfaces; the type‐S is mainly fine‐grained silicification with patches of chlorite, epidote and common sulphides; the type‐K (potassic alteration) appears to be fine‐grained biotite. Such types of alteration indicate the presence of skarns at depth where ore shoots are located. Taken together, the multiple structural‐magmatic‐mineralization events contributed to the formation of the supergiant Jiama porphyry copper deposit in Tibet. The results have general implication for regional exploration.     

Structural features and metallogenesis of the Carlin-type Jinfeng (Lannigou) gold deposit, Guizhou Province, China

Jinfeng, previously known as Lannigou, is the largest Carlin-type gold deposit in the Yunnan–Guizhou–Guangxi region in southwestern China. Gold mineralization in the Jinfeng deposit is almost entirely fault-hosted and structurally controlled, with very little disseminated ore occurring in the adjacent host rocks. The structural elements in the Jinfeng deposit can be subdivided into 3 groups comprising NS-, NW-, and NE-striking faults and folds, with NW-striking structures controlling the overall framework of the deposit. Four tectonic stages have been recorded in the Jinfeng area, i.e., rifting, orogenic compression, lateral transpression, and lithospheric extension. A series of contemporaneous normal faults, such as the N-striking and east-dipping F1 and F7 faults developed along the edges of a carbonate platform during basin rifting (D₂–T₂). These structures provided an initial framework for subsequent basin evolution, and also represent the principal hydrothermal conduits. A gradual change of the compression direction during the orogenic stage (T₃) from E→W to NE→SW, gave rise to the NW-striking structures, including large, tight to overturned folds such as the Huangchanggou synclinorium and associated thrusts such as the F3 fault. The development of these orogenic, predominantly NE-dipping structures, as well as accompanying NE-striking dextral shear and transform faults (such as the F2 fault) along the margin of the Laizishan Dome established the structural pattern of the deposit area. The NW-striking folds were refolded by NE-striking superimposed folds during post-collisional lateral transpression (J₁) and NW–SE directed compression. Oblique stress distribution gave rise to NS-trending compression and EW-trending extension, with dilational zones developing at the intersection of the F2 and the F3 faults east of the Laizishan dome. It is these dextral- and sinistral-normal dilational zones in which gold was precipitated during the main ore-forming event at Jinfeng. Following the main ore stage lithospheric extension occurred during the Yanshan stage (J₂–K) resulting in minor reverse faults that in places cut pre-existing structures. The above four main structural deformation stages mirror the evolution of the Youjiang Basin from inception to basin inversion and post-orogenic collapse and renewed extension. Significant gold metallogenesis at Jinfeng occurred during the transition from collisional compression to extensional tectonics in the early Jurassic, and is focussed into intersections of F2 and F3 and fault splays adjacent to F3. This structurally controlled gold metallogenic model is likely to be applicable to analogous settings elsewhere in the Yunnan–Guizhou–Guangxi triangle area, and has implications for the targeting of Carlin-type gold mineralization in this region.     

Deformation history of the Paleoproterozoic Liaohe assemblage in the eastern block of the North China Craton

The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D₁ to D₃) and four episodes of metamorphism (M₁ to M₄). The earliest greenschist facies event (M₁) is recognized in undeformed or weakly deformed domains wrapped by the S₁ schistosity, suggesting that M₁ occurred before D₁. The D₁ deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F₁), an associated penetrative axial planar schistosity (S₁), a mineral stretching lineation (L₁) and regional-scale ductile shear zones. Concurrent with D₁ was M₂ metamorphism, which occurred before D₂ and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F₁ folds and other sense-of-shear indicators, together with the radial distribution of the L₁ lineation surrounding the Liaoji granitoids, imply that D₁ represents an extensional event. The D₂ deformation produced open to tight F₂ folds of varying scales, S₂ axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D₂ was M₃ metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D₃) formed NW-WNW-trending folds (F₃), axial planar kink bands, spaced cleavages (S₃), and strike–slip and thrust faults, which deflect the earlier D₁ and D₂ structures. D₃ occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M₄).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D₁) and the M₁ and M₂ metamorphism, and the final closing of which was associated with the D₂ and D₃ phases of deformation and M₃ and M₄ metamorphism.     

Structural assemblies and age of deformation in the western sector of the Anyui-Chukotka Fold System,Northeastern Asia

Indications of intense deformation in the Anyui-Chukotka Fold System and the South Anyui Suture Zone have been noted for a long time [3, 5, 19, 36]. The character and age of the deformation, however, remain a matter of debate. Using structural paragenetic and deformational kinematic analyses, we establish three deformation stages in the Anyui-Chukotka Fold System. The structural assembly comprising open folds and NW-trending axial-plane cleavage was formed during the stage of regional compression (D₁) related to the collision of the Chukotka-Arctic Alaska microcontinent with Eurasia. The assembly of the second stage in the Alyarmaut Rise is distinguished by isoclinal folds F₂, gently dipping metamorphic schistosity, and pervasive cleavage in combination with folded quartz veins and lenses. Planar structural elements of the second stage are disturbed by low-amplitude normal and reverse faults and kink folds of stage D₃. The U-Pb (SHRIMP-RG) and ^40/39Ar methods were used for determination of the isotopic age of the deformations. The Aptian-Albian zircon age (117–108 Ma) has been established for six postcollision granitic plutons of the Anyui-Chukotka Fold System and the South Anyui Suture. Syncollision deformation completed 125–117 Ma ago. The extensional tectonic stage D₂ accompanied by emplacement of the Lyupveem pluton occurred 120–105 Ma ago. The ^40/39Ar age of the biotite from the metamorphic rocks marks the age of syndeformation metamorphism (109–103 Ma). The lower limit of brittle failure and deformation D₃ is estimated at 105 Ma.     

新疆于田塔木其铜锌矿构造控矿规律探讨

新疆于田塔木其铜锌矿是西昆仑造山带内众多与火山作用有关的热水喷流沉积块状硫化物矿床之一,但该矿的空间位态与典型层控矿床不一致。研究区经历的主要构造运动期次可分为3期,S2期构造运动对现今岩石及矿体的空间分布状态起决定性作用。野外及镜下详细观察表明,S2期构造变形为强压应力兼具一定剪切性质的脆韧性变形,对研究区内原始层理及矿体进行了构造置换。S2期构造透镜体长轴及劈理产状统计表明,构造透镜体空间展布状态小受S2期劈理控制,透镜体最大压扁面平行于S2期劈理面分布,研究区在S2期遭受了强烈的南北挤压应力。通过对小型褶皱、透镜体的观测及劈理的统计,结合构造剖面的测制,推测矿体上一级构造样式为大型无根褶皱,矿体本身为加厚的Z型次级褶皱形成的透镜体,矿体所处部位可能为无根褶皱的背形南翼或向形北翼。S3期节理产状要素统计表明,在S3期研究区发生了南北挤压应力作用下的脆性破裂,这些破裂对矿体的分布有一定影响。     

Crustal thickening and attenuation as revealed by regional fold interference patterns: Ciudad Rodrigo basement area (Salamanca,Spain)

The structure of the Ciudad Rodrigo area (Iberian Massif, Central Iberian Zone) has been revisited in order to integrate new geological data with recent models of the evolution of the Iberian Massif. Detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the Variscan belt. The structural evolution shows, in the first place, the development of a regional train of overturned folds with associated axial planar foliation (D₁). Towards the lower structural levels, the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone (D₂), which extends at least to the deepest parts of the basement exposed in the study area. The amplification and rotation of D₁ folds about a horizontal axis also occurred within this shear zone. The flat-lying character of the D₂ structures accounts for the attenuation of the previously thickened crust, which developed following gravity gradients during thermal re-equilibration. Subsequent deformation led to the formation of two orthogonal sets of upright folds (D₃), representing a new shift between crustal thinning and crustal thickening in the region.     

Structure of the Ordovician succession around Aberdaron,southwest Llŷn,North Wales

Three deformation phases are recognizable within the Lower Ordovician metasedimentary sequence of the Aberdaron area and they are similar to those described for Lower Palaeozoic sequences in other parts of North Wales. There is no certain evidence however for a major Aberdaron Syncline as described by some previous workers. The first deformation phase produced southcast verging mesoscopic folds with steep to moderate dipping axial surfaces and a sporadic axial plane cleavage. The second deformation was relatively weak and produced only a low-dipping crenulation cleavage at a few favoured localities. The third phase gave rise to numerous small buckle folds, kinks in some pelitic units where the first cleavage was well developed, an axial plane cleavage, and a suite of quartz veins. The orientation of the third phase minor structures is not uniform and the fold trend and strike of axial plane cleavage varies from east-northeast to south-southeast, although retains a constant angular relationship to the local strike of bedding. The distribution of the third cleavage is bimodal and the third deformation phase may have been brought about by conjugate shears during a late brittle fracture stage of NW–SE compression. The structural sequence affecting the Ordovician cannot be correlated with that in the Mona Complex and it seems likely that the Mona Complex was deformed before the Arenig.     

Tectonic Imprints of the Hazara Kashmir Syntaxis on the Mesozoic Rocks Exposed in Munda,Mohmand Agency,Northwest Pakistan

Two well-developed mesoscopic folds, D_2 and D_3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D_2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D_3 folds, which developed during SE–NW shortening. Micro- to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D_3. The newly established NW–SE trending micro- to mesoscopic structures in Munda termed D_2, which postdated F_1/F_2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D_2 and D_3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D_2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D_2 and D_3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D_2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.     

Structural and kinematic analysis of the Early Paleozoic Ondor Sum-Hongqi mélange belt,eastern part of the Altaids (CAOB) in Inner Mongolia,China

We present a structural and kinematic study of an Early Paleozoic subduction mélange and a magmatic arc that form the main elements of the Southern Orogen Belt of Inner Mongolia, which lies in the eastearn part of the Altaids or Central Asia Orogenic Belt. The structural analysis of the mélange conducted in the Hongqi and Ondor Sum areas (western Inner Mongolia) shows two phrases of ductile deformation. The D₁ event is responsible for the pervasive S₁ foliation, NW–SE striking L₁ stretching lineation and F₁ intrafolial folds. These microstructures are coeval with a greenschist facies metamorphism. During D₂, NW-verging F₂ folds associated with a S₂ axial planar cleavage deformed S₁ and L₁. The D₁ kinematic criteria indicate a top-to-the-NW sense of shear. D₁ and D₂ developed before the unconformable deposition of the Early Devonian shallow water sandstone. A lithosphere scale geodynamic model involving an Early Paleozoic southeast-directed subduction beneath the North China Craton and late Silurian collision of the North China Craton with an hypothetical microcontinent is proposed to account for the microstructural evolution.     

Structural controls on carbonate-hosted Pb–Zn mineralization in the Dongmozhazhua deposit,central Tibet

Fault zones control the locations of many ore deposits, but the ore-forming processes in such fault zones are poorly understood. We have studied the deformation and ore textures associated with fault zones that controlled the lead–zinc mineralization of the Dongmozhazhua deposit, central Tibet, ∼100 km southwest of Yushu City. Geological mapping shows that the structural framework of the Dongmozhazhua area is defined by NW–SE-trending reverse faults and superposed folds that indicate at least two stages of deformation. The first stage is characterized by tight nearly E–W-striking folds that formed during the closure of the Jinshajiang Paleo-Tethyan Ocean in the Triassic. The second stage of deformation produced NW–SE-trending reverse faults and related structures of the Fenghuoshan–Nangqian fold-and-thrust belt associated with India–Asia collision in the late Eocene to Oligocene. Scanline surveys along the ore-controlling fault zones show an internal structure that comprises a damage zone, a breccia zone with clasts that have become rounded, and a breccia zone with lenticular clasts, and this complex architecture was formed during at least two compressional substages of deformation. The Pb–Zn mineralization in the Dongmozhazhua area occurs exclusively close to NW–SE-trending reverse fault zones. Microtextural observations reveal that mineralization occurred as veinlets and disseminated blebs in limestone clasts, and as continuous bands and cements in fractured rocks. Cataclastic sulfide grains also can be seen in the matrix of some fault zones. The types of mineralization differ with structural position. The fillings of the ore-bearing veinlets typify the products of hydraulic fracture and both types of mineralization took place concurrently with regional contraction. We consider, therefore, that the ore-bearing fluids in the Dongmozhazhua deposit were concentrated in fault zones during regional compression and that the ore minerals were precipitated during hydraulic fracturing of host rocks. Subsequent fault activity pulverized some pre-existing sulfide material into cataclastic grains in the matrix of a tectonic breccia that developed in the same faults.