The Enåsen gold deposit,central Sweden

Stable isotope analyses of quartz, sulphides, and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the Palaeoproterozoic Enåsen gold deposit. Reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks. However, oxygen isotope geothermometry on a coexisting quartz-magnetite pair gave a minimum temperature for peak metamorphism of around 650 °C which agrees with Fe-Mg geothermometry. This suggests that grain-scale equilibrium is achieved. The variation in oxygen isotope ratios (¹⁸O = 7.3 – 10.5) on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher ¹⁸O-values in more superficial parts. Temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit. It is suggested that alteration, silicification, and mineralization at the Enåsen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200–250 °C and that the hydrothermal fluids consisted of meteoric and magmatic water. A tentative reconstruction of the fossil hydrothermal system is presented. Sulphur isotope ratios of sulphides from the fold-bearing quartz-sillimanite gneiss gave ³⁴S-values close to zero indicating a magmatic source of the sulphur.     

The dilemma of the Jiaodong gold deposits: Are they unique?

The ca. 126e120 Ma Au deposits of the Jiaodong Peninsula, eastern China, define the country＇s largest gold province with an overall endowment estimated as＆gt;3000 t Au. The vein and disseminated ores are hosted by NE-to NNE-trending brittle normal faults that parallel the margins of ca. 165e150 Ma, deeply emplaced, lower crustal melt granites. The deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilatational jogs. Country rocks to the granites are Pre-cambrian high-grade metamorphic rocks located on both sides of a Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous ＂Yan-shanian＂intracontinental extensional deformation and associated gold formation. 〈br〉 Classification of the gold deposits remains problematic. Many features resemble those typical of orogenic Au including the linear structural distribution of the deposits, mineralization style, ore and alteration assemblages, and ore fluid chemistry. However, Phanerozoic orogenic Au deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong de-posits, in contrast, formed within two Precambrian blocks approximately 2 billion years after devolati-lization of the country rocks, and thus require a model that involves alternative fluid and metal sources for the ores. A widespread suite of ca. 130e123 Ma granodiorites overlaps temporally with the ores, but shows a poor spatial association with the deposits. Furthermore, the deposit distribution and mineral-ization style is atypical of ores formed from nearby magmas. The ore concentration requires fluid focusing during some type of sub-crustal thermal event, which could be broadly related to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific plate subduction, and seismicity along the continental-scale Tan-Lu fault. Possible ore genesis scenarios include those where ore fluids were produced directly by the metamorphism of oceanic lithosphere and overlying sediment on the subducting paleo-Pacific slab, or by devolatilization of an enriched mantle wedge above the slab. Both the sulfur and gold could be sourced from either the oceanic sediments or the serpentinized mantle. A better understanding of the architecture of the paleo-Pacific slab during Early Cretaceous below the eastern margin of China is essential to determination of the validity of possible models.     

The Variscan Tamlalt–Menhouhou gold deposit,Eastern High-Atlas,Morocco

The Tamlalt–Menhouhou gold deposit belongs to the Neoproterozoic–Palaeozoic Tamlalt inlier located in the Eastern High-Atlas (Morocco). It occurs in altered Upper Neoproterozoic bimodal volcanic and volcano-sedimentary units outcropping in the Tamlalt–Menhouhou area. Gold mineralization has been identified in quartz veins related to shear-zones associated with a strong quartz-phyllic-argillic alteration. Visible free gold is related to goethite–malachite–barite boxworks in quartz veins. The other alteration minerals accompanying gold mineralization are mainly carbonates, chlorite, hematite, albite and pyrite whose relative proportion defines three alteration types. ⁴⁰Ar/³⁹Ar geochronology performed on phengite grains from phyllic alteration and the auriferous quartz veins, yields plateau ages ranging from 300 ± 5 Ma to 284 ± 12 Ma with a weighted mean age of 293 ± 7 Ma. This identifies a Late Variscan age for the Tamlalt–Menhouhou “shear zones-related” gold deposit and emphasizes the consequences of the Variscan orogeny for gold mineralization in the High-Atlas and Anti-Atlas Neoproterozoic inliers.     

The application of biogeochemistry for gold exploration in the Masjed–Daghi, Julfa, NW Iran

The aims of present study are investigation of endemic plants at Masjed–Daghi area introducing hyperaccumulator and indicator plants for Au, Ag, As, Cu, Mo, Hg, Re, Sb, and Te mineralization and also describe the biogeochemical response pattern over a known Au–Cu mineralized site. The Masjed–Daghi prospecting area is covered by Eocene flysch, andesite, trachyandesite, dacite, rhyodacite, Oligocene agglomerate, and Quaternary deposits. Previous researches reported copper porphyry mineralization and related epithermal gold veins in this area. This study presents that plants with high metal intake enabled us to obtain invaluable information about natural concentrations of chemical elements in the substrate and to recognize new potential areas for mineral prospecting. Stachys inflata has biological absorption coefficient mean exceeding or near hyperaccumulating criterion >1 for most of the elements investigated then could be as a hyperaccumulator. The indicator values belong to S. inflata, Artemisia sp., Salvia sp., Astragalus sp., Peganum harmala, Moltkia coerulea, and Cousinia sp.     

The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia

Wetar Island is composed of Neogene volcanic rocks and minor oceanic sediments and forms part of the Inner Banda Arc. The island preserves precious metal-rich volcanogenic massive sulfide and barite deposits, which produced approximately 17 metric tonnes of gold. The polymetallic massive sulfides are dominantly pyrite (locally arsenian), with minor chalcopyrite which are cut by late fractures infilled with covellite, chalcocite, tennantite–tetrahedrite, enargite, bornite and Fe-poor sphalerite. Barite orebodies are developed on the flanks and locally overly the massive sulfides. These orebodies comprise friable barite and minor sulfides, cemented by a series of complex arsenates, oxides, hydroxides and sulfate, with gold present as <10 m free grains. Linear and pipe-like structures comprising barite and iron-oxides beneath the barite deposits are interpreted as feeder structures to the barite mineralization. Hydrothermal alteration around the orebodies is zoned and dominated by illite–kaolinite–smectite assemblages; however, local alunite and pyrophyllite are indicative of late acidic, oxidizing hydrothermal fluids proximal to mineralization. Altered footwall volcanic rocks give an illite K–Ar age of 4.7±0.16 Ma and a ⁴⁰Ar/³⁹Ar age of 4.93±0.21 Ma. Fluid inclusion data suggest that hydrothermal fluid temperatures were around 250–270°C, showed no evidence of boiling, with a mean salinity of 3.2 wt% equivalent NaCl. The ³⁴S composition of sulfides ranges between +3.3 and +11.7 and suggests a significant contribution of sulfur from the underlying volcanic edifice. The ³⁴S barite data vary between +22.4 and +31.0, close to Miocene seawater sulfate. Whole rock ⁸⁷Sr/⁸⁶Sr analyses of unaltered volcanic rocks (0.70748–0.71106) reflect contributions from subducted continental material in their source region. The ⁸⁷Sr/⁸⁶Sr barite data (0.7076–0.7088) indicate a dominant Miocene seawater component to the hydrothermal system. The mineral deposits formed on the flanks of a volcanic edifice at depths of ~2 km. Spectacular sulfide mounds showing talus textures are localized onto faults, which provided the main pathways for high-temperature hydrothermal fluids and the development of associated stockworks. The orebodies were covered and preserved by post-mineralization chert, gypsum, Globigerina-bearing limestone, lahars, subaqueous debris flows and pyroclastics rocks.     

The Bagassi gold deposits on the eastern margin of the Houndé greenstone belt,Burkina Faso

The Bagassi gold deposits are situated on the West African craton and hosted in Palaeoproterozoic rocks of the Houndé greenstone belt, southwest Burkina Faso. High-grade gold mineralisation is hosted in quartz–gold ± pyrite veins-lodes (V_1A), in dilational zones and narrow shears in the Bagassi granitoid, and forms the majority of the resource–reserve portfolio in the Bagassi exploration permits, with gold grades of 18–21 g/t. Shear hosted gold-pyrite mineralisation in quartz veins in dilational jogs (V_1B) occurs along narrow discontinuous shear zones that trend north-northwest in Birimian-aged metabasaltic units, and forms a secondary gold resource. Gold mineralisation is restricted to formation in the late Eburnean Orogeny and formed during a change from east-west to transcurrent compression and shearing. The Bagassi deposits demonstrate that granitoids emplaced prior to onset of the Eburnean Orogeny represent viable gold mineralisation in host rocks that are increasingly seen to be associated with significant gold resources.     

The Muruntau gold deposit(Uzbekistan)——A unique ancient hydrothermal system in the southern Tien Shan

The Muruntau gold deposit in the Central Kyzylkum,Uzbekistan is one of the largest single gold deposits worldwide.Data available from the literature are reviewed with the aim to(1) integrate the present knowledge on this unique deposit from Russian and English literature;(2) show the considerable progress made in the understanding of the genesis of the Muruntau deposit during the last decades;and(3) point to problems still open for future research.Deposit formation occurred through a multi-stage process involving sedimentation,regional metamorphism including thrusting,magmatism with formation of hornfels aureoles and several stages of hydrothermal activity.According to recent knowledge,synsedimentary or pure metamorphic formation of gold mineralization seems unlikely.The role of granite magmatism occurring roughly within the same time interval as the main hydrothermal gold precipitation remains uncertain.There are no signs of interaction of matter between the magma(s) and the hydrothermal system(s).On the other hand,there was an intense,high-temperature(above 400 ℃)fluid- wall rock interaction resulting in the formation of gold-bearing,cone-like stockworks with veins,veinlets and gold-bearing metasomatites.Several chemical and isotope indicators hint at an involvement of lower-crustal or mantle-related sources as well as of surface waters in ore formation.Deposit formation through brecciation involving explosion,hydrothermal or tectonic breccias might explain these data.Further investigations on breccia formation as well as on the exact timing of relevant sedimentary,metamorphic,magmatic and hydrothermal events are recommended.     

The genesis of the slab window-related Arzular low-sulfidation epithermal gold mineralization （eastern Pontides, NE Turkey）

The Arzular mineralization is one of the best examples of epithermal gold deposits in the eastern Pontides orogenic belt.The mineralization is hosted by the subduction-related basaltic andesites and is mainly controlled by E-W and NE-SW trending fracture zones.The main ore minerals are galena, sphalerite,pyrite.chalcopyrite.tetrahedrite and gold.Homogenization temperatures of fluid inclusions are between 130 and 295℃ for quartz and between 90 and 133℃ for sphalerite.Sulphur isotope values obtained from pyrite,galena and sphalerite vary between 1.2‰ and 3‰.indicating that sulphur belongs to magmatic origin and was derived from the Lutetian non-adakitic granitic intrusions in the region.Oxygen isotope values are between 15.0‰ and 16.7‰ and hydrogen isotope values are between -87‰ and -91‰ The sulphur isotope thermometer yielded temperatures in the range of 244-291℃ for the ore formation.Our results support the hypothesis that the Arzular mineralization is a low-sulfidation epithermal gold deposit associated with non-adakitic subduction- related granitic magmas that were generated by slab window-related processes in a south-dipping subduction zone during the Lutetian.     

胶东和小秦岭:两类不同构造环境中的造山型金矿省

胶东和小秦岭是我国排名前两位的金矿产地,根据对这两个地区的实地野外考察、室内研究及对已有大量研究成果的总结,我们认为胶东与小秦岭地区的金矿床均可归入造山型金矿的范畴,它们分别形成于增生型造山体制和碰撞型造山体制.胶东金矿床形成于早白垩世(130～120Ma左右)与洋壳俯冲(增生)造山相关的活动大陆边缘环境,矿床主要产于中生代花岗岩岩体中,严格受断裂带(NNE向或NE向为主)控制,成矿流体具有低盐度高CO_2含量的特征,He-Ar同位素研究显示成矿过程有幔源物质的加入.综合金矿床及中生代岩浆岩(特别是与成矿近同时的早白垩世郭家岭花岗岩及基性岩脉)的地质地球化学特征与成岩成矿动力学,我们提出在俯冲的太平洋板块后退的背景下,胶东地区增厚地壳中的榴辉岩相下地壳及下伏岩石圈地幔发生两阶段拆沉,强烈的壳幔相互作用最终导致了早白垩世普遍的岩浆活动及金的爆发成矿的模式.小秦岭地区金矿床主要以大型含金石英脉的形式产出于太华群变质基底的脆性-韧性剪切带(EW向为主)中,而与区域内燕山期大型花岗岩岩基没有直接联系,矿床地质特征(如低盐度高CO_2,以变质流体为主的成矿热液)与造山型金矿吻合,He-Ar同位素特征表明金矿床形成时有幔源物质的加入.小秦岭地区脉状Au-Mo矿床印支期成矿年龄(215～256Ma,辉钼矿Re-Os)表明印支期是小秦岭地区金成矿的主要时期,小秦岭金矿属于陆陆(华北与扬子)碰撞造山过程中形成的造山型金矿.     

A scale-integrated exploration model for orogenic gold deposits based on a mineral system approach

Concept-based orogenic gold exploration requires a scale-integrated approach using a robust mineral system model.Most genetic hypotheses for orogenic gold deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model.Plausible models involve metamorphic fluids,but the fluid source has been equivocal.Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean deposits,and numerous Chinese provinces,including Jiaodong,where the only feasible fluid source is sub-crustal.If all orogenic gold deposits define a coherent mineral system,there are only two realistic sources of fluid and gold,based on their syn-mineralization geodynamic settings.These are from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package,or release from mantle lithosphere that was metasomatized and fertilized during a subduction event,particularly adjacent to craton margins.In this model,CO_2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 ℃.This orogenic gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales.Within these settings,it is then possible to define the critical gold mineralization processes in the system:fertility,architecture,and preservation.The geological parameters that define these processes,and the geological,geophysical and geochemical proxies and responses for these critical parameters can then be identified.At the geodynamic to province scales,critical processes include a tectonic thermal engine and deep,effective,fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin.At the district to deposit scale,the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems,with gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps.Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.     

造山型金矿研究进展:兼论中国造山型金成矿作用

造山型金矿指与大洋板块俯冲和陆块拼贴有关、产在汇聚板块边界变质地体内部或者边缘受韧-脆性断裂构造控制的,成矿流体以低盐度H2O-CO2-CH4为主要特征的,成矿深度(2~20 km)和温度(200~650℃)及其相应的蚀变矿化组合有较大变化的系列金矿床.造山型金矿形成与超大陆聚合时限具有一致性.由于围岩类型和控矿构造多样性、地球化学特征具有多解性、金属源区和演化的不确定性以及成矿就位和物质起源的空间差距,造山型金矿成因模式有以下两个主要观点.第一种为大陆地壳变质流体成因模式,认为造山型金矿形成于造山作用同变质阶段,并随岩石圈演化矿床的物质来源发生变化;富金流体的释放由上地壳岩石绿片岩相到角闪岩相的进变质作用导致,该过程中的黄铁矿向磁黄铁矿转变释放了大量的金,这种模式被广泛运用于赋存在绿片岩相中的显生宙造山型金矿.然而越来越多的实例证实造山型金矿主要形成于峰期变质的退变质阶段或者与区域变质没有任何关系,变质流体成因模式受到了强烈质疑;与大陆地壳变质模式相对立的是幔源流体模式,其认为流体起源于俯冲洋壳脱水或富集地幔再活化,不同时代和地区的成矿流体具有一致性;尽管该模式不符合传统的平衡条件下的相变原理,但是基于幔源流体的存在及其浅部运移的大量观测,初步认为成矿流体是在超临界和非平衡条件下完成了金属的幔→壳迁移.中国造山型金矿分布于江南造山带志留纪、天山-阿尔泰二叠纪、华北克拉通北缘三叠-侏罗纪、特提斯造山带二叠-侏罗纪、华南板块晚三叠世-侏罗纪、华北克拉通东南缘白垩纪、青藏高原及周缘古近纪等七大成矿带,主要受到了显生宙不同时代造山作用的控制,成矿时代晚于变质峰期,重要成矿带大型矿集区(胶东、哀牢山、扬子西缘)的实例解剖均支持幔源流体成因模式.     

克拉通和古生代造山带中的韧性剪切带型金矿：金矿成矿条件与成矿环境分析

文章总结了产在克拉通和古生代造山带中、受韧性剪切带控制的金矿的重要研究成果。通过对比这两类金矿的地质特征及其成矿作用发生的大地构造背景，探讨形成韧性剪切带型金矿的成矿环境与成矿机制。大型韧性剪切带型金矿一般就位于剪切带的脆一韧性转换位置，成矿作用一般不受围岩性质和变质程度的控制。剪切带既是成矿流体的通道，又是金的沉淀场所。克拉通中韧性剪切带型金矿的成矿模式有两类：同构造成矿和构造期后成矿，前者认为变质流体沿韧性剪切带迁移，最终在剪切带中沉淀形成矿床；后者则强调发生在韧性剪切带形成之后的地质过程如岩浆活动等对成矿作用的贡献。     

俯冲隧道内不同深度的壳幔相互作用:地幔楔超镁铁质岩的镁同位素记录

在不同的俯冲深度,俯冲板片会释放出不同来源和组成的熔/流体进入俯冲隧道中,并进而影响上覆地幔楔及衍生岛弧岩浆的地球化学组成.然而,如何识别俯冲隧道中不同深度熔/流体组分的来源一直是俯冲带研究中的难点.对不同深度来源的地幔楔超基性岩进行了Mg同位素研究,发现了Mg同位素具有示踪俯冲板块熔/流体来源的能力.首先,研究了美国加州Franciscan杂岩中一套经历了多期次流体交代作用的浅部来源（< ~60 km）的变质超基性岩.这些部分蛇纹石化的地幔楔超基性岩在蛇纹石脱水形成滑石的过程中会释放轻Mg同位素进入流体,而重Mg同位素更多地残留在滑石相中;随后进一步受俯冲板块来源流体的交代形成具有高CaO和轻Mg同位素组成的透闪石化变橄榄岩,暗示流体中含有源自俯冲板片的、富集轻Mg同位素的碳酸盐,说明在弧前~60 km深度,部分含Mg碳酸盐（方解石）可以在俯冲隧道中发生溶解并迁移交代上覆地幔楔橄榄岩.对深部地幔楔来源（~160 km）的大别造山带毛屋地区超镁铁质岩体岩相学和元素地球化学研究结果证实了其交代成因.结合多相包裹体、元素地球化学以及前人估计的温-压条件,推测交代介质更接近超临界流体.锆石U-Pb年代学研究揭示,交代作用主要发生在古生代洋壳俯冲阶段（454±58 Ma）,超高压变质作用则发生在三叠纪陆壳俯冲阶段（232.8±7.9 Ma）.古生代锆石中大量的碳酸盐矿物包裹体和重O同位素特征说明古生代洋壳俯冲交代过程中有沉积碳酸盐组分加入.全岩和单矿物的Mg同位素组成均显著低于地幔值以及大别新元古代榴辉岩,说明交代的碳酸盐组分来源应为循环的沉积富Mg碳酸盐,暗示了在俯冲带深部富Mg沉积碳酸盐在超临界流体中会发生溶解迁移.由于沉积碳酸盐具有独特的、显著富集轻Mg同位素组成的特征,这种交代作用会造成地幔楔局部具有异常的Mg同位素组成,从而解释目前观察到的岛弧火山岩的Mg同位素特征.因此,Mg同位素是示踪俯冲碳酸盐与上覆地幔楔相互作用的有效工具.      

Gold mineralization in China: Metallogenic provinces,deposit types and tectonic framework

We present a review of major gold mineralization events in China and a summary of metallogenic provinces, deposit types, metallogenic epochs and tectonic settings. Over 200 investigated gold deposits are grouped into 16 Au-metallogenic provinces within five tectonic units such as the Central Asian orogenic belt comprising provinces of Northeast China and Tianshan-Altay; North China Craton comprising the northern margin, Jiaodong, and Xiaoqinling; the Qinling-Qilian-Kunlun orogenic belt consisting of the West Qingling, North Qilian, and East Kunlun; the Tibet and Sanjiang orogenic belts consisting of Lhasa, Garzê-Litang, Ailaoshan, and Daduhe-Jinpingshan; and the South China block comprising Youjiang basin, Jiangnan orogenic belt, Middle and Lower Yangtze River, and SE coast. The gold deposits are classified as orogenic, Jiaodong-, porphyry–skarn, Carlin-like, and epithermal-types, among which the first three types are dominant.The orogenic gold deposits formed in various tectonic settings related to oceanic subduction and subsequent crustal extension in the Qinling-Qilian-Kunlun, Tianshan-Altay, northern margin of North China Craton, and Xiaoqinling, and related to the Eocene–Miocene continental collision in the Tibet and Sanjiang orogenic belts. The tectonic periods such as from slab subduction to block amalgamation, from continental soft to hard collision, from intracontinental compression to shearing or extension, are important for the formation of the orogenic gold deposits. The orogenic gold deposits are the products of metamorphic fluids released during regional metamorphism associated with oceanic subduction or continental collision, or related to magma emplacement and associated hydrothermal activity during lithospheric extension after ocean closure. The Jiaodong-type, clustered around Jiaodong, Xiaoqinling, and the northern margin of the North China Craton, is characterized by the involvement of mantle-derived fluids and a temporal link to the remote subduction of the Pacific oceanic plate concomitant with the episodic destruction of North China Craton. The Carlin-like gold metallogenesis is related to the activity of connate fluid, metamorphic fluid, and meteoric water in different degrees in the Youjiang basin and West Qinling; the former Au province is temporally related to the remote subduction of the Tethyan oceanic plate and the later formed in a syn-collision setting. Porphyry–skarn Au deposits are distributed in the Tianshan-Altay, the Middle and Lower Yangtze River region, and Tibet and Sanjiang orogenic belts in both subduction and continental collision settings. The magma for the porphyry–skarn Au deposits commonly formed by melting of a thickened juvenile crust. The epithermal Au deposits, dominated by the low-sulfidation type, plus a few high-sulfidation ones, were produced during the Carboniferous oceaic plate subduction in Tianshan-Altay, during Early Cretaceous and Quaternary oceanic plate subduction in SEt coast of South China Block, and during the Pliocene continental collision in Tibet. The available data of different isotopic systems, especially fluid D–O isotopes and carbonate C–O systems, reveal that the isotopic compositions are largely overlapping for different genetic types and different for the same genetic type in different Au belts. The isotopic compositions are thus not good indicators of various genetic types of gold deposit, perhaps due to overprinting of post-ore alteration or the complex evolution of the fluids.Although gold metallogeny in China was initiated in Cambrian and lasted until Cenozoic, it is mainly concentrated in four main periods. The first is Carboniferous when the Central Asian orogenic belt formed by welding of micro-continental blocks and arcs in Tianshan-Altay, generating a series of porphyry–epithermal–orogenic deposits. The second period is from Triassic to Early Jurassic when the current tectonic mainframe of China started to take shape. In central and southern China, the North China Craton, South China Block and Simao block were amalgamated after the closure of Paleo-Tethys Ocean in Triassic, forming orogenic and Carlin-like gold deposits. The third period is Early Cretaceous when the subduction of the Pacific oceanic plate to the east and that of Neo-Tethyan oceanic plate to the west were taking place. The subduction in eastern China produced the Jiaodong-type deposits in the North China Craton, the skarn-type deposits in the northern margin (Middle to lower reaches of Yangtze River) and the epithermal-type deposits in the southeastern margin in the South China Block. The subduction in western China produced the Carlin-like gold deposits in the Youjiang basin and orogenic ones in the Garzê-Litang orogenic belt. The Cenozoic is the last major phase, during which southwestern China experienced continental collision, generating orogenic and porphyry–skarn gold deposits in the Tibetan and Sanjiang orogenic belts. Due to the spatial overlap of the second and third periods in a single gold province, the Xiaoqinling, West Qinling, and northern margin of the North China Craton have two or more episodes of gold metallogeny.     

西秦岭地区金矿类型及其成矿作用

西秦岭地区是中国最重要的金矿矿集区之一,除产出少数夕卡岩型金矿床外,几乎所有的其他金矿床都可归并为造山型、卡林型和类卡林型3种类型。研究表明,西秦岭地区中生代花岗岩主要形成于中晚三叠世,而金矿成矿主要集中在晚三叠世,它们都是华北板块与华南板块碰撞导致的秦岭造山作用的产物。西秦岭地区造山型金矿床主要赋存在泥盆系和石炭系一套复杂的构造变形和区域变质的绿片岩相岩中,主要受北西西向脆韧性剪切带控制,成矿元素组合主要为Au-Ag。矿石中含有大量显微自然金、银金矿,明金可见。成矿流体主要为变质流体。由造山作用引起的强烈构造运动为成矿流体提供了运移通道,为矿质沉淀提供了有利的场所。虽然一些造山型金矿床与中酸性岩体相邻,但矿化与岩浆活动不具直接的成因关系。西秦岭地区卡林型金矿床主要产于轻微变质的寒武系至三叠系沉积岩中,明显受地层、岩性和构造控制。金矿床中的金以超显微金和存在于含砷黄铁矿与毒砂晶格中的固溶体金为主。成矿元素组合为Au-As-Hg-Sb-Ba。成矿流体由早期形成的地层水被后期大气降水补给活化形成,也有部分岩浆水或变质水的加入。在伸展背景下大气降水通过循环演化形成了较浅层次的流体系统,导致Au等成矿元素发生沉淀而形成浸染状矿石。西秦岭地区类卡林型金矿床主要产于浅变质沉积岩建造中,受脆韧性剪切带的控制,并形成于花岗岩岩体附近。与造山型、卡林型金矿床最大的不同之处在于,类卡林型金矿床的形成与同时期的岩浆活动有密切的成因关系。矿石中存在显微自然金,载金矿物主要为黄铁矿、含砷黄铁矿和碲化物。成矿热液主要是岩浆水与变质水、建造水的混合流体。与造山型金矿床类似,流体不混溶导致类卡林型金矿床的形成。     

Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits

With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ～10°-25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ～70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition.Anticlinal or antiformal fold hinges, particularly those of 'locked-up' folds with ～30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits.In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granitegreenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits.     

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.