Stable isotopes and noble gases in the Xishimen gold deposit,central North China Craton: metallogeny associated with lithospheric thinning and crust–mantle interaction

Quartz-vein type gold mineralization at Xishimen is a recently discovered gold deposit in the central North China Craton. More than 50 auriferous quartz veins occur in this region within a NNW–SSE-trending fault zone 4600 m in length and 3–10 m wide. Wall rocks are mainly Precambrian tonalite–trondhjemite–granodiorite (TTG) gneisses and associated supracrustals, modified by K-feldspathization and pyrite-phyllic hydrothermal alteration. Based on detailed field and petrographic studies, we identify five episodes of mineralization: pyrite-phyllic stage (I), coarse-grained pyrite-milky white quartz stage (II), fine-grained smoky grey quartz-pyrite stage (III), fine-grained smoky grey quartz-polymetallic sulphide stage (IV), and quartz-carbonate stage (V). We present results of δ³⁴S analysis of sulphide minerals from the different stages which show tightly clustered values in the range of –1.0‰ to 2.1‰, close to those of mantle and meteorite sulphur. Lead isotopic ratios of pyrite from the early to main stages also show restricted ranges with ²⁰⁶Pb/²⁰⁴Pb of 16.289–17.286, ²⁰⁷Pb/²⁰⁴Pb of 15.217–15.453, ²⁰⁸Pb/²⁰⁴Pb of 37.012–38.232, implying lower crustal input. ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios of fluid trapped in pyrite are 0.68 Ra to 1.20 Ra (where Ra is the ³He/⁴He ratio of air = 1.4 × 10^?6) and 540.9–1065, respectively. ³He and ⁴Ar concentrations vary from 10.05 to 18.5 (10^?7 cm³STP/g) and 6.15 to 17.4 (10^?7cm³STP/g), respectively, with calculated mantle helium ranging from 8.47% to 14.96% (average 11.01%). δ¹⁸O_Q and δ¹⁸D_Q values of quartz range from 8.0‰ to 13.2‰ and –101.9‰ to –70.5‰, respectively, with calculated δ¹⁸O_W values of the mineralizing fluid ranging from 1.11‰ to 5.72‰, suggesting the mixing of magmatic aqueous fluid with meteoric water during gold precipitation. We correlate the mixed crust–mantle signature of the ore-forming sources to magmatism and metallogeny associated with Mesozoic inhomogeneous lithosphere thinning in the central North China Craton.     

Geochronology and geochemistry of the Cambrian (~518 Ma) Chagantaolegai ophiolite in northern West Junggar (NW China): constraints on spatiotemporal characteristics of the Chingiz–Tarbagatai megazone

The Chingiz–Tarbagatai megazone is a well-known early Palaeozoic unit in eastern Kazakhstan that extends into northern West Junggar along the Tarbagatai–Xiemisitai–Sharbuti Mountains. Here, we report new geochronological and geochemical data for the Chagantaolegai ophiolite, discovered in the south Xiemisitai Mountains, north of the Heshituoluogai Valley in West Junggar. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages of a gabbro in the Chagantaolegai ophiolite are 517 ± 3 and 519 ± 3 Ma, corresponding to the early to middle Cambrian. Serpentinized peridotites in the ophiolite show low rare earth element (REE) abundances (0.61–0.94 μg/g) and slight loss of middle REEs. Chagantaolegai metagabbro samples have flat to very slightly depleted patterns in light REEs, with a narrow range of REE abundances. The Chagantaolegai spilites and metagabbros display similar flat patterns, which show a geochemical signature similar to mid-ocean ridge basalt (MORB) between Yb and Zr, and diverge from MORB towards the most incompatible elements in MORB-normalized multi-element diagrams. The Th/Yb vs. Nb/Yb diagram for the spilite samples also suggests a mid-ocean ridge (MOR) setting. To summarize, all the geochemical characteristics of the Chagantaolegai ophiolite point to MOR ophiolite. The positive ε_Nd(t) values (0.78 to +9.85) suggest that the gabbros and spilites were derived from a depleted mantle source with chemical inhomogeneity. The Chagantaolegai ophiolite spatiotemporal characteristics correlate well with the Kujibai and Hongguleleng ophiolites in the northern part of West Junggar, and the Zhaur and Balkybek ophiolites in eastern Kazakhstan. These data show that early Palaeozoic oceanic crust started to develop in the early to middle Cambrian in northern West Junggar. Stratigraphic correlations also suggest that the South Saur Fault and an inferred fault buried in the Heshituoluogai depression are the northern and southern boundary of the Chingiz–Tarbagatai megazone, respectively.     

Geomorphologic assessment of relative tectonic activity in the Maharlou Lake Basin,Zagros Mountains of Iran

Spatial differences of Quaternary deformation and intensity of tectonic activity are assessed through a detailed quantitative geomorphic study of the fault‐generated mountain fronts and alluvial/fluvial systems around the Maharlou Lake Basin in the Zagros Fold–Thrust Belt of Iran. The Maharlou Lake Basin is defined as an approximately northwest–southeast trending, linear, topographic depression located in the central Zagros Mountains of Iran. The lake is located in a tectonically active area delineated by the Ghareh and Maharlou faults. Combined geomorphic and morphometric data reveal differences between the Ghareh and Maharlou mountain front faults indicating different levels of tectonic activity along each mountain front. Geomorphic indices show a relatively high degree of tectonic activity along the Ghareh Mountain Front in the southwest, in contrast with less tectonic activity along the Ahmadi Mountain Front northeast of the lake which is consistent with field evidence and seismotectonic data for the study area. A ramp valley tectonic setting is proposed to explain the tectonosedimentary evolution of the lake. Copyright © 2011 John Wiley & Sons, Ltd.     

中条山涑水杂岩的同位素年代学研究及其地质意义

中条山是中国重要的铜多金属成矿带,拥有著名的晚太古代“变斑岩型”铜矿峪大型铜矿床和元古代“层控型”胡家峪-篦子沟铜矿床,近年来又在中条山西南段的涑水杂岩中发现以变基性火山岩（斜长角闪岩）为容矿岩石的桃花洞铜矿。为了弄清楚该铜矿的形成时代及其成矿地质背景,利用电感耦合等离子质谱（LA-MC-ICP—MS）方法对桃花洞铜矿容矿围岩涑水杂岩中的寨子-西姚灰色片麻岩、斜长角闪岩及二长花岗岩进行了同位素年代学研究。结果表明：寨子-西姚灰色片麻岩、斜长角闪岩脉和二长花岗岩中锆石的岩浆结晶年龄分别为（2625±11）、（2592±16）、（2548±15）Ma;岩石中古老继承性锆石的年龄分别为（2790±15）、（2773±24）、（2782±34）Ma;三种不同成生顺序的岩石中均有大于2700Ma的锆石存在,且年龄相当接近;中条山地区涑水杂岩可能最初形成于太古代。涑水杂岩作为与成矿关系密切的容矿围岩,其成岩时代可以间接指示成矿时代,由此得出赋存在变基性火山岩层中的古火山含矿原岩建造型铜矿有可能是中国最古老的铜矿之一。     

新疆西天山巴音沟与蛇绿岩伴生的洋岛玄武岩——主微量元素证据

对巴音沟蛇绿混杂岩内发育的两套玄武岩进行了主、微量元素研究,结果表明,其中一套TiO2的含量较低,在0.81%~1.00%之间,REE含量低,LREE相对于HREE具轻微亏损,稀土元素配分图和微量元素的原始地幔标准化图上呈平坦分布型式,Nb、Ta略亏损,为大洋中脊玄武岩。结合其他微量元素特征,初步认为该玄武岩源于大洋形成的初期,源区是还没有经历大规模岩浆提取阶段的洋中脊玄武岩。另一套玄武岩则具有典型的洋岛玄武岩特征,具有较高的TiO2含量(1.89%~3.14%),富碱质,同时富集LREE和HFSE元素,高Nb、Ta含量,在微量元素原始地幔标准化图上显示明显的Nb、Ta正异常,微量地球化学元素具有EMⅡ型OIB特征。     

阿尔泰山南缘白垩纪以来的剥露历史和古地形恢复

应用裂变径迹技术研究阿尔泰山南缘中新生代以来的隆升剥露历史,并对古地形进行恢复。对所获得的11个磷灰石样品的裂变径迹年龄分析表明,裂变径迹年龄变化分布于99.1~43.7 Ma之间。该地区晚白垩世以来平均视剥露速率为0.050 mm/a。热史模拟结果表明,阿尔泰山南缘自白垩纪以来经历了多期冷却剥露历史:早白垩世至晚白垩世晚期(约120~75 Ma),剥露速率为0.044 mm/a;晚白垩世晚期至始新世(约75-70~50 Ma),剥露速率为0.070 mm/a;中新世以来(约20-15 Ma~现今),剥露速率为0.081 mm/a。对研究区进行古地形恢复显示,自白垩纪(120 Ma)至今,平均剥露幅度达约5 km,古地形海拔降低约0.8 km。阿尔泰山南缘白垩纪以来古地形海拔仅在17.5~50 Ma期间保持基本稳定,其他阶段均有降低的趋势。早白垩世的构造抬升与蒙古-鄂霍茨克海最后阶段的闭合以及西伯利亚板块和中朝-蒙古板块的最终收敛和碰撞有关;晚白垩世晚期至始新世的构造活动则是受拉萨地块、Kohistan-Dras岛弧增生的远距离影响;中新世以来快速隆升可能与印度板块与欧亚板块碰撞的远程效应有关。     

论岷山地区冰缘岩溶

岷山地区冰缘岩溶是处于岷山雪线下部冰缘环境的碳酸盐岩之溶蚀与冻融相互渗合、叠加的岩溶地质类型,具有碳酸盐岩溶场和冰缘岩屑与冰期、间冰期堆积物岩溶场的二元结构。岷山地区大多数碳酸盐岩处于较低的海拔区,在第四纪气温振荡回暖的影响下冰缘岩溶已经衰退,仅残留着早期冰缘岩溶形成的地貌和钙华体,现在的黄龙沟冰缘岩溶因其碳酸盐岩海拔高,仍处在第四纪末期冰川退缩后的冰缘环境,冰缘岩溶地貌和钙华体还在发育,不过现今全球性气温升高和人类的活动对冰缘岩溶都将产生负面影响,以钙华体为核心景观的黄龙、九寨沟风景区的管理部门应予加强监测和保护。     

新疆古伦沟地区古仍格萨拉东岩体的地球化学及锆石U-Pb年龄

古伦沟地区古仍格萨拉东花岗闪长斑岩体位于中天山构造带北缘。地球化学和锆石U-P年龄测定结果显示: 岩石高碱富Na、贫Fe和Mg、弱过铝质(ACNK=0.98~1.11), 稀土总量较低(∑REE=61.28×10-6~99.50×10-6)且分异明显(LaN/YbN=7.82~22.80)、铕弱负异常(δEu=0.72~0.97), 相对富集Rb、Ba、Th、U、K等, 亏损Nb、Ta、P、Ti等, 具火山弧花岗岩的特征。岩体具有较均一的Sr、Nd同位素组成: (87Sr/86Sr)i=0.70677~0.70685, εSr(t)=40.10~41.21, (143Nd/144Nd)i=0.51190~0.51191, εNd(t)= –2.62 ~ –2.30, tDM＝1.31~1.38 Ga。其锆石U-Pb年龄为(488.9±1.7)~(470.5±3.1) Ma。表明古仍格萨拉东岩体形成于早奥陶世陆缘弧环境, 可能是先存的中元古代幔源基性下地壳部分熔融的产物, 其形成与古准噶尔洋向伊犁—中天山板块下的俯冲作用有关, 标志着中天山北缘于早奥陶世时期已进入与俯冲消减有关的活动陆缘演化阶段。此次岩浆活动导致区内发生斑岩型铜矿化。     

西南天山东德沟地幔楔环境异剥钙榴岩的形成过程

中国西南天山昭苏县东德沟地幔楔环境的斜长角闪岩与蛇纹岩接触带中发现了异剥钙榴岩.按异剥钙榴岩化程度,可将研究的岩石分为三种类型:斜长角闪岩、异剥钙榴岩化斜长角闪岩和异剥钙榴岩,它们的矿物组合分别为:角闪石+中长石+石英+榍石+磷灰石±次透辉石;角闪石+钙长石+透辉石+斜黝帘石+绿泥石±方解石;斜黝帘石+钙铝榴石+透辉石+绿泥石+方解石.异剥钙榴岩化岩石的矿物化学及主量元素特征反映出交代流体中富Ca、Al.与洋底变质环境和俯冲带中发生的异剥钙榴岩化作用相比,此处的异剥钙榴岩化岩石中的流体有限.质量平衡计算也表明斜长角闪岩在富Ca、Al流体的交代下形成异剥钙榴岩,这一过程中,早期有Mg元素加入,而晚期异剥钙榴岩形成时,Mg又大量流失.     

东天山红石金矿床石英Rb-Sr同位素定年

红石金矿床产于东天山秋格明塔什—黄山韧性剪切带中,其成矿作用主要受韧性剪切带控制.前人通过金矿石绢云母Ar-Ar法将该矿床主成矿时代限定在254 ～ 259Ma之间,本文从讨论Rb-Sr法测年在韧性剪切带型金矿成矿时代研究中的可行性角度出发,对红石金矿主成矿期形成的金矿石进行石英流体包裹体Rb-Sr法测年.Rb-Sr法测年得到等时线年龄为257±4Ma,n(87Sr)/n(86Sr)=0.70997±0.00016.该年龄位于红石金矿床254～259Ma的主成矿时代范围内,说明石英流体包裹体Rb-Sr测年对于解决韧性剪切带型金矿床成矿时代具有很好的可行性.另外,红石金矿床与同一剪切带内的康古尔金矿在成矿时代、空间位置关系以及成矿物质来源等方面均具有一致性,认为红石金矿与康古尔金矿属相同成矿事件的产物.通过分析红石金矿床成矿时代、韧性剪切带作用时代、右行走滑以及后期的冷却降温作用时代的关系,推测红石金矿床及其剪切带内所在的康古尔等金矿床成矿作用主因是秋格明塔什—黄山韧性剪切带内晚期的右行走滑剪切变形及走滑作用后的快速抬升和缓慢冷却作用.从韧性剪切带型金矿成矿类型来看,属于早期韧性剪切变形,后期脆—韧性变形、成矿作用叠加这一类型.