40^Ar／39^Ar Ages of Auriferous Quartz Veins from the Fengyang and Zhangbaling Regions and Their Geological Significance

Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu oragene, respectively. Neo-Archean gneiss and amphibolite and metamor-phosed sea-facies sodic volcanic rocks axe the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozo-ic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in thetwo regions. Gold-beating quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhanghaling region.Generally, the formation of the auriferous quartz veins involved three stages. At the first stage,gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins wereformed; and at the third stage gold-poor barite and/or carbonate veins were formed. The 40^Ar/29^Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangeheng gold deposits range between 116.1 0.6 Ma and 118.3 0.5 Ma and are pretty close to their least apparent ages and isoehronal ages, respectively. All plat-eau, least apparent and isoehronal ages range between 113.4 0.4 Ma and 118.3 0.5 Ma,which are considered as the formation age range of the quartz. It is reasonable and reliable to takethe 40^Ar/39^Ar age range of the quartz as the formation age range of gold-bearing quartz veins onthe basis of spatial relationship between gold-bearing quartz veins and their country rocks. Thegold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiangfault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and expe-rienced very strong magnmtic process. It is shown that the magnmtic hydrothermal fluid is a veryimportant part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhanghaling re-gions. The formation of the gold ore deposits in the Fengyang and Zhanghaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the exten-sional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.     

张八岭隆起早白垩世变质核杂岩的厘定——对郯庐断裂带的制约及其与成矿作用关系

张八岭隆起广泛分布的平缓韧性剪切带与郯庐断裂带平移作用形成的陡立韧性剪切带明显不同。通过对平缓韧性剪切带的几何学、运动学分析,结合早白垩世盆地特征、中国东部变质核杂岩伸展拆离断层和同构造岩浆岩同位素定年结果,厘定出张八岭隆起早白垩世变质核杂岩。该变质核杂岩上盘由南华纪-奥陶纪沉积地层和早白垩世盆地组成,下盘为新元古代浅变质碎屑沉积岩、变海相火山岩（基底）以及早白垩世侵入岩,上下盘之间被一条主伸展拆离断层所分隔。变质核杂岩长轴为NE-SW向,指示构造反映上盘向SE剪切滑动,与中国东部变质核杂岩的伸展方向完全一致。通过本次变质核杂岩的厘定,结合野外地质事实,笔者认为管店-马厂断裂是郯庐断裂带的次级断裂,是对郯庐断裂带早白垩世末第三次左行平移的响应。在综合研究的基础上,建立了区域构造-岩浆-成矿关系模型,揭示了张八岭隆起早白垩世经历了早期伸展（变质核杂岩阶段）-挤压走滑（管店-马厂断裂形成阶段）-晚期伸展（闪长质脉岩侵位阶段）3个构造阶段,多期构造、岩浆的叠加作用下,形成了本区的金多金属矿产。     

~(40)/Ar~(39)Ar Ages of Auriferous Quartz Veins from the Fengyang and Zhangbaling Regions and Their Geological Significance

1IntroductionAuriferousquartzveinsarecommonintheFengyangandZhangbalingregionsofAnhuiProvinceandtheyconstitutetheRongdu ,Zhuding ,Dagongshan ,MaoshanandShangchenggolddepositsandalotofgoldperspectives .TheRb SrisochronagesofalterationmineralsfromtheRongduandDagongshangolddepositsare (10 9.0± 4 .4 )Maand (15 3.76± 11.2 )Ma ,respectively (Dongetal .,1995 ) .Thereisalargedifferenceinagebetweenthetwogolddeposits ,butitcanbeseenthatthegolddepositswereformedinLateJurassictoEarlyCretaceous .A…     

浙江诸暨璜山金矿构造特征及深部找矿预测

诸暨璜山金矿是浙江省著名的金矿之一,位于江山-绍兴断裂带北东段。本文通过对璜山金矿和庙下畈金矿的构造特征进行对比研究,认为这两个金矿形成的构造背景和成矿时代基本相同,但矿体走向和剪切动向截然不同,璜山金矿赋存于NE走向的剪切带内,庙下畈金矿位于NW走向的剪切带内,二者构成共轭剪切带,叠加在江山-绍兴主干韧性剪切带之上。成矿后该区先后受到NW-SE和NNE-SSW方向的挤压,形成NW走向和近SN方向的脆性断裂,对矿体起到破坏作用。依据共轭剪切带的空间分布,推测江绍断裂南东侧可能存在与璜山金矿对应的赋矿剪切带,该带位于新元古代陈蔡群变质岩之下,埋藏较深,是璜山地区深部找矿的有利勘探区。     

Structural setting,style and timing of vein-hosted gold mineralization at the Pogo deposit,east central Alaska

Gold-bearing veins within the Liese zone of the Pogo deposit display a two-stage evolutionary history that records temporal variation in kinematics, fluid chemistry and temperature. Several stacked shallow northwest-dipping shear veins are developed at Pogo, and collectively comprise the Liese Zone. Veins consist of: (1) early, narrow biotite-bearing shear veins; (2) white quartz veins with pyrite-arsenopyrite bands, referred to as main stage quartz veins, that have sericite-Fe-Mg carbonate alteration envelopes and which exploit the early shear veins; and (3) extension veins that form as steeper offshoots from the main stage veins. The presence and orientation of oblique fabrics developed in the older biotite-bearing shear veins are indicative of top-to-the-south displacement under ductile to semi-brittle conditions at higher temperatures. In contrast, the orientation of the extension veins and local sigmoidal shapes indicate a component of top-to-the-northwest normal displacement on the main stage veins in their present orientation, and brittle to semi-brittle conditions of formation. Dolomite-sericite alteration surrounding main stage veins may represent late to post-mineral hydrothermal fluid exploitation of vein margins during ongoing normal displacement along vein systems. All types of veining overprint 107–106 Ma, post-metamorphic granitic dykes. Molybdenite in main stage quartz assemblages has returned Re-Os ages of 104.2±1.1 Ma, significantly older than 96 to 91 Ma ⁴⁰Ar/³⁹Ar ages obtained from vein alteration assemblages that may reflect thermal resetting during post-mineral fault related hydrothermal activity, magmatism and/or retrograde cooling of the lithologic sequence. Unlike typical mesothermal shear vein hosted gold systems, Pogo is temporally and tectonically separated from metamorphic deformation events, and has a comparable kinematic and geometric architecture to Cretaceous plutonic gold deposits in the region. We interpret the deposit to have formed during a regional Cretaceous extensional event during multi-stage exploitation of extensional fault surfaces by hydrothermal fluid from a cooling magmatic source.Editorial handling: S.G. Hagemann     

辽南变质核杂岩主拆离断层的波瓦状构造(corrugation)及其成因

基于对辽南变质核杂岩主拆离断层及下伏韧性剪切带的几何学分析以及拆离断层带构造岩的显微构造研究,结合变质核杂岩构造—岩浆活动及其形成过程和力学机制,重点讨论了主拆离断层的波瓦状构造的特点及其形成条件。结果表明:波瓦状构造的发育与下盘晚期阶段花岗质岩体的侵位具有同时性。伸展构造发育初期就位的深成岩体加热和软化了上部地壳,此时岩石力学属性控制的钝角锯齿状断裂构成了波瓦状构造的雏形,晚期阶段花岗质深成侵入岩的不均匀侵位制约了波瓦状构造的发育。     

大别造山带东段扬子陆块和华北陆块间缝合带的位置

大别山为扬子陆块和华北陆块之间的碰撞造山带.构造-岩石单元的岩石组成、同位素年代学资料和构造关系表明, 大别山东段主要由扬子陆块北缘不同变质程度的变质基底和少量浅变质盖层组成, 没有代表蛇绿混杂岩和华北陆块南缘古生代活动大陆边缘的火山-侵入岩建造.各主要构造-岩石单元间的界线为超高压变质岩折返过程中形成的伸展型剪切带, 大别山北部的伸展-逆冲推覆构造也是超高压变质岩折返过程中伸展构造的一部分, 其中不存在具有缝合带意义的重要构造界线.因此, 在大别山东段, 华北陆块和扬子陆块间的缝合带既不是水吼-五河剪切带, 也不是磨子潭-晓天断裂.根据地球物理资料推测, 南北陆块间的缝合带应分布在信阳-舒城断裂的前缘, 但现在覆于合肥盆地中新生代沉积之下.      

云蒙山变质核杂岩抬升过程中伸展拆离和岩浆底辟联合作用的证据

北京地区云蒙山变质核杂岩在白垩纪阶段抬升的早期,伴随着沿四合堂剪切带由北向南的拆离滑脱和大型花岗闪长岩的垂向侵位,晚期变形发生在花岗闪长岩岩基周边及其邻近围岩中,形成云蒙山剪切带,并伴随大量同构造的花岗岩和伟晶岩岩脉灌入。剪切带中所有的岩脉都随时间发生了不同程度的变形,较老的岩脉形成紧闭的圆柱状褶皱,枢纽与剪切带的线理和面理接近平行。岩脉与剪切带中L-S组构的平行化作用主要是由于这种转动的结果。岩脉的成分和长英指数随它们的变形程度发生变化,说明持续的岩浆分异作用与韧性剪切变形是同时发生的。云蒙山剪切带由岩浆底辟引起的上盘岩石重力所驱动,并不断得到同构造侵位岩脉的补充,起到存储和不断改造侵位岩脉的作用。早期伸展体制下形成的四合堂剪切带局部遭受云蒙山剪切带的改造或复合。该地区的岩石、构造和同构造岩脉的变形几何学和运动学证据表明,太古宙结晶基底的抬升是下部岩浆底辟与上部地壳伸展拆离共同作用的结果。     

Geological features and origin of gold deposits occurring in the Baotou–Bayan Obo district, south-central Inner Mongolia, People's Republic of China

Located at western portion of northern margin of North China craton, the Baotou–Bayan Obo district is one of the most important Fe–REE–Nb and Au metallogenic provinces in China. Presently, about 52 gold deposits and prospects have been discovered, explored and mined, among which Shibaqinhao, Laoyanghao, Houshihua, Saiyinwusu, Wulashan and Donghuofang are the most important ones. All these gold occurrences can be subdivided into three groups (or types) according to its host rocks: (1) hosted by Archean high-grade metamorphic rocks; (2) hosted by Proterozoic sedimentary rocks; (3) hosted by or related to Hercynian alkaline intrusive rocks. The first group contains the Shibaqinhao, Laoyanghao and Houshihua gold deposits. Gold mineralization at these three deposits occurs within Archean amphibolite, gneiss and granulite as gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite and chalcopyrite. The Saiyinwusu deposit belongs to the second group, and occurs within Proterozoic sandstone, quartzite and carbonaceous slate as quartz veins and replacement bodies along the fracture zones. Pyrite, marcasite, arsenopyrite, native gold and electrum are identified. The third group includes the Wulashan, Donghuofang and Luchang deposits. Gold mineralization at these three deposits occurs predominantly within the Hercynian alkaline syenite or melagabbro stocks and dyke swarms or along their contacts with Archean metamorphic wall rocks as K-feldspar–quartz veins, dissemination and veinlets. Pyrite, galena, chalcopyrite, native gold and calaverite are major metallic minerals.δ³⁴S value of sulfides (pyrite, galena and pyrrhotite) separates from groups 1 and 2 varies from −4.01‰ to −0.10‰ and −3.01‰ to 2.32‰, respectively. δ³⁴S values of Archean and Proterozoic metamorphic wall rocks for groups 1 and 2 deposits range from −20.2‰ to −17.0‰ and −15.8‰ to −16.2‰, respectively. The values are much lower than their hosted gold deposits. All these pyrite separates from Hercynian alkaline intrusions associated with the gold deposits show positive δ³⁴S values of 1.3‰ to 4.8‰, which is higher than those Precambrian metamorphic wall rocks and their hosted gold deposits. δ³⁴S values of the sulfides (pyrite and galena) from the Donghuofang and Wulashan deposits (group 3) increase systematically from veins (−14.8‰ to −2.4‰) to the Hercynian alkaline igneous wall rocks (2.8‰ to 4.8 ‰). All of these deposits in groups 1, 2 and 3 show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the gold ores plot between the Hercynian alkaline intrusions and Precambrian metamorphic wall rocks. Data are interpreted as indicative of a mixing of lead from mantle-derived alkaline magma with lead from Precambrian metamorphic wall rocks.Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the groups 1 and 2 deposits were generated during the emplacement of the Hercynian alkaline syenite and mafic intrusions. The Hercynian alkaline magma may provide heat, volatiles and metals for these groups 1 and 2 deposits. Evolved metamorphic fluids produced by the devolatilization, which circulated the wall rocks, were also progressively involved in the alkaline magmatic hydrothermal system, and may have dominate the ore fluids during late stage of ore-forming processes. Most of these gold deposits hosted by Archean high-grade metamorphic rocks occur at or near the intersections of the NE- and E–W-trending fracture systems. The ore fluid of the group 3 deposits may have resulted from the mixing of Hercynian alkaline magmatic fluids and evolved meteoric waters. The deposits are believed to be products of Hercynian alkaline igneous processes along deep-seated fault zones within Archean terrain.     

内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。     

Extensional ductile tectonics of the Sioule metamorphic series (Variscan French Massif Central)

In the Northern part of the Variscan French Massif Central, the Sioule series, from top to bottom, consists of a pre-Viséan granite, migmatite, gneiss and mica schist. Two ductile deformations have been recognized. The earlier phase is characterized by a north-east-south-west trending stretching lineation; the second phase, characterized by a north-west-south-east trending mineral, stretching and crenulation lineation, is better marked in the lower mica schist part than in the upper granito-gneissic part. This second phase occurred during retrogression of the metamorphic rocks; related shear criteria indicate a top to the south-west shear. The Namurian-Westphalian magmatic bodies such as the Echassières leucogranite, Pouzol-Servant microgranite and numerous north-east -south-west trending microgranite dykes are emplaced in extensional fractures related to the same north-west-south-east maximum stretching direction. The asymmetrical shapes of the two granitic massifs indicate that they intruded towards the south-east. The synkinematic retrogression of the metamorphic rocks, the shape of the magmatic bodies and a re-examination of the numerous available data support the interpretation that the deformation is due to the extensional tectonic regime related to the Variscan crustal re-quilibration. This interpretation is in agreement with the correlation of the Sioule series with the Chavanon series. The two series belong to a unique tectono-metamorphic unit left-laterally offset by the Stephanian motion of the Sillon Houiller fault. This study also shows that the Sillon Houiller did not play a significant part during the Namurian-Westphalian extensional tectonics of the Massif Central. Correspondence to: M. Faure     

滇西墨江金厂金矿床时空四维结构模型

对墨江金厂金矿关键性地质问题研究结果表明,矿区金厂韧性剪切带（ＳＺｊｃ）、ＮＷＷ向褶皱构造和金矿超镁铁岩体侵位构造是不同成矿期的控矿构造,金矿床有三种成因类型,分别对应三期三种不同类型的成矿作用。     

Stick-slip model for kink band formation in shear zones and faults

Differential shear stresses acting along or adjacent to a non-planar fault surface or shear zone may cause uneven acceleration during slip. Alternatively, at the initiating and closing stages of motion of parts of a stick-slip fault, localised shear stresses may be variable. Stress variation of this nature causes zones of relative compression and tension, especially close to the “stick” zones on the fault. In fissile rocks adjacent to the fault, kink bands form in zones of local relative compression, while stratal extension features such as veins, fractures and extensional crenulations might be expected in the corresponding zones of relative tension. Repetitive motion on the fault should therefore cause the development of a suite of kink bands superimposed on each other and on any complementary extensional structures. Field evidence indicates that the extensional structures are not developed to the same extent as the kinks, perhaps due to ductile flow during layer-parallel extension of phyllosilicate rocks.

The advantages of this model are that it does not require bulk shortening of the shear zone relative to the enclosing less strained rocks, nor does it depend on complex stress orientation changes.     

Late Precambrian series and structures in the Navalpino Variscan Anticline (Central Iberian Peninsula)

The Navalpino Anticline is a major Variscan structure in the Central Iberian Zone of Spain. Three lithological groups are defined in the pre-Ordovician rocks of this anticline. The Rifean or Lower Vendian Extremeño Dome Group is unconformably overlain by the Upper Vendian Ibor-Navalpino Group. This latter group presents two different facies separated by a NW-SE trending synsedimentary fault. The Lower Cambrian Valdelacasa Group unconformably overlies both the Extremeno Dome and the Ibor-Navalpino Groups.Three pre-Variscan episodes of deformation have been defined in the area of the Navalpino Anticline. A major asymmetrical fold with a subvertical east-west-striking limb is the result of the first deformation event of pre-Late Vendian age. The second deformation event is of Cadomian (Late Precambrian) age and is composed of two stages; (i) an early extensional stage including NW - SE trending extensional fault and basin development in the north-eastern block; and (ii) a second compressive stage giving rise to north-south trending upright folds. This second compressive stage possibly inverted the basin. A final pre-Variscan deformation event took place between the Early Cambrian and the Early Ordovician resulting in a 5–10° tilting to the north-east.There are two main phases of Variscan deformation in the area. The first deformation event (D_v1) gave rise to a upright WNW - ESE trending folds on all scales, whereas the second (D_v2) gave rise to a brittle—ductile sinistral strike-slip shear zone tending subparallel to the axial trace of the D_v1 folds.     

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO₂-rich, H₂O-rich and mixed H₂O–CO₂ fluid inclusions in the gold-bearing quartz veins. The H₂O–CO₂ inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ³⁴S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO₂.     

Location of the Suture Zone between the South and North China Blocks in Eastern Dabie Orogen

INTRODUCTION TheDabieMountainsareacollisionalorogenbe tweentheSouthandNorthChinablocks.Theageof theultrahighpressuremetamorphism(Hackeretal., 1998;Rowleyetal.,1997;Amesetal.,1993;Liet al.,1993)andthelateststratainvolvedintheforeland fold thrustbelt(Xueta…     

Extensional Late Paleozoic deformation on the western margin of Pangea, Patlanoaya area, Acatlán Complex, southern Mexico

New mapping in the northern part of the Paleozoic Acatlán Complex (Patlanoaya area) records several ductile shear zones and brittle faults with normal kinematics (previously thought to be thrusts). These movement zones separate a variety of units that pass structurally upwards from: (i) blueschist-eclogitic metamorphic rocks (Piaxtla Suite) and mylonitic megacrystic granites (Columpio del Diablo granite ≡ Ordovician granites elsewhere in the complex); (ii) a gently E-dipping, listric, normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions; (iii) the Middle–Late Ordovician Las Minas quartzite (upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes and a leucogranite dike from the Columpio del Diablo granite) unconformably overlain by the Otate meta-arenite (lower greenschist facies psammites and pelites): roughly temporal equivalents are the Middle–Late Ordovician Mal Paso and Ojo de Agua units (interbedded metasandstone and slate, and metapelite and mafic minor intrusions, respectively) — some of these units are intruded by the massive, 461 ± 2 Ma, Palo Liso megacrystic granite: decussate, contact metamorphic muscovite yielded a ⁴⁰Ar/³⁹Ar plateau age of 440 ± 4 Ma; (iv) a steeply-moderately, E-dipping normal fault; (v) latest Devonian–Middle Permian sedimentary rocks (Patlanoaya Group: here elevated from formation status). The upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics, which varies from (i) three in the Piaxtla Suite, through (ii) two in the Las Minas unit (E-trending sheath folds deformed by NE-trending, subhorizontal folds with top to the southeast asymmetry, both associated with a solution cleavage), (iii) one in the Otate, Mal Paso, and Ojo de Agua units (steeply SE-dipping, NE–SW plunging, open-close folds), to (iv) none in the Patlanoaya Group. ⁴⁰Ar/³⁹Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma and show low temperature ages of  260 Ma. Post-dating all of these structures and the Patlanoaya Group are NE-plunging, subvertical folds and kink bands. An E–W, vertical normal fault juxtaposes the low-grade rocks against the Anacahuite amphibolite that is cut by megacrystic granite sheets, both of which were deformed by two penetrative fabrics. Amphibole from this unit has yielded a ⁴⁰Ar/³⁹Ar plateau age of 299 ± 6 Ma, which records cooling through  490 °C and is probably related to a Permo-Carboniferous reheating event during exhumation. The extensional deformation is inferred to have started in the latest Devonian ( 360 Ma) during deposition of the basal Patlanoaya Group, lasting through the rapid exhumation of the Piaxtla Suite at  350–340 Ma synchronous with cleavage development in the Las Minas unit, deposition of the Patlanoaya Group with active fault-related exhumation suggested by Mississippian and Early Permian conglomerates ( 340 and 300 Ma, respectively), and continuing at least into the Middle Permian (≡ 260 Ma muscovite ages). The continuity of Mid-Continent Mississippian fauna from the USA to southern Mexico suggests that this extensional deformation occurred on the western margin of Pangea after closure of the Rheic Ocean.     

A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Penedono), together with a deposit linked to sub-vertical structures in silicified metasediments at Três-Minas, and several intra-metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P–T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasediments). They are:Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315–310 Ma) at P–T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphism (P=300–350 MPa and T=500–550°C). No clear evidence was found for gold deposition during this stage.Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P–T conditions between 100 and 300 MPa and 300 and 450°C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous-carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found.Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P–T conditions were <100 MPa and <300°C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb–Bi–Ag dominated). The fractures frequently contain chlorite (± sericite) especially where they crosscut earlier sulphides (arsenopyrite).These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation.     

黑龙江乌拉嘎大型金矿床流体包裹体特征及矿床成因研究

乌拉嘎金矿床是受断裂构造和中酸性侵入体联合控制的浅成低温热液型矿床,区内黑龙江群变质岩为成矿提供物源．燕山期花岗斑岩是主要的容矿围岩．深部岩浆源为成矿提供了热动力。矿体受构造引张部位和构造交汇部位控制．在空间上侧伏斜列。矿石矿物主要有自然金、黄铁矿、白铁矿、辉锑矿、自然银等,脉石矿物有玉髓状石英、胶状蛋白石、碳酸盐、冰长石、绢云母、高岭土等;矿石结构主要为胶状结构、细粒状结构和碎裂结构,矿石构造为脉状、网脉状和角砾状构造．矿石的矿物组合和组构均显示典型的低温矿物组合和组构特点。流体包裹体研究表明,石英中主要发育气液两相及少量单液相包裹体．成矿流体属H2O—NaCl体系类型。成矿流体具有低盐度（1．22％-5．4％NaCl）、低密度（0．88--0．96g／cm^3）的特征,成矿温度为140～220℃,成矿平均压力为23．24MPa,形成深度为2．32km。通过氢、氧同位素分析认为,成矿流体δDSMOW,值为-78．25‰-132．64‰,6moHp值为-3．7‰～6．23‰,表明成矿流体主要来自大气降水。综合研究表明．乌拉嘎金矿属于由中性、近还原和低温流体形成的低硫化型浅成低温热液金矿床。