Genesis of the Maanqiao gold deposit in the western Qinling orogen,central China: Constraints from fluid inclusions and in-situ sulfur isotopes

The western Qinling orogen (WQO) is one of the most important prospective gold provinces in China. The Maanqiao gold deposit, located on the southern margin of the Shangdan suture, is a representative gold deposit in the WQO. The Maanqiao deposit is hosted by the metasedimentary rocks of the Upper Devonian Tongyusi Formation. The EW-trending brittle-ductile shear zone controls the orebodies; they occur as disseminated, and auriferous quartz–sulfide vein. The ore-related hydrothermal alteration comprises silicification, sulfidation, sericitization, chloritization, and carbonatization. Native gold is visible and mainly associated with pyrite and pyrrhotite. Mineralization can be classified into the following three stages: bedding-parallel barren quartz–pyrite–(pyrrhotite) (early-stage), auriferous quartz–polymetallic (middle-stage), and carbonate–(quartz)–sulfide (late-stage).Detailed fluid inclusion (FI) studies revealed three types of inclusions in quartz and calcite: aqueous (W-type), CO₂–H₂O (C-type), and pure carbonic (PC-type) FIs. The primary FIs in the early-stage quartz are C- and PC-type, in the middle-stage quartz are mainly W- and C-type, and in the late-stage calcite are only W-type. During gold mineralization, the total FI homogeneous temperatures evolved from 189–375 °C (mostly 260–300 °C) to 132–295 °C (mostly 180–240 °C) to 123–231 °C (mostly 130–150 °C), and the salinities varied among 2.2–9.1 wt.% NaCl equiv. (mostly 5–8 wt.%) to 0.2–9.0 wt.% NaCl equiv. (mostly 3–6 wt.%) to 0.3–3.6 wt.% NaCl equiv. (mostly 2–4 wt.%). The ore-forming fluid was characterized as an H₂O–NaCl−CO₂−CH₄–(N₂) system with medium-low temperature and low salinity. The fluid immiscibility and fluid-rock interaction may be responsible for the precipitation of the sulfides and gold at the Maanqiao gold deposit. Three types of pyrite corresponding to the three mineralization stages, as well as pyrrhotite and arsenopyrite in the middle stage, are micro-analyzed for in-situ sulfur isotopic composition by LA-ICP-MS. Py1 yield near-zero δ³⁴S values of −2.5‰ to 3.0‰, which are somewhat lower than that of the granite hosted pyrites (Py-g, 4.8‰ to 6.6‰). The result suggests a mixed sulfur source from magmatic-hydrothermal fluids and the metamorphism of diagenetic pyrite. Pyrite + pyrrhotite + arsenopyrite assemblages in the middle-stage have relatively higher δ³⁴S values (6.6‰ to 12.3‰) and are mainly developed due to the metamorphism of the ore-host and underlying Devonian sedimentary sequences. The low δ³⁴S values of the late-stage fracture-filled Py3 (−21.9‰ to −17.0‰) resulted from an increasing oxygen fugacity, which was caused by the inflow of oxidized meteoric waters.Based on our studies, the Maanqiao gold deposit is considered to be an orogenic type and closely related to the Indosinian Qinling orogeny.     

北山马鬃山地区野马泉和勒巴泉韧性剪切带构造变形特征与年代学约束

野马泉和勒巴泉韧性剪切带作为北山造山带微陆块碰撞的产物，在北山造山带早古生代碰撞—造山过程中具有重要的地质意义。前人对其研究主要集中于勒巴泉地区构造变质核杂岩以及其中产出的蛇绿岩带。为了更好确定其构造变形时代，文章通过对野马泉和勒巴泉韧性剪切带野外构造剖面和显微构造特征的研究表明：野马泉和勒巴泉韧性剪切带具有由北西西向南东东逆冲，兼具右行剪切运动特征。通过对韧性剪切带中糜棱岩带中未变形花岗岩脉的锆石U-Pb定年，确定了野马泉和勒巴泉地区韧性剪切变形年龄上限为416 Ma，结合前人所报道的年龄，将韧性剪切变形时代大致限定在423~416 Ma之间，可能与牛圈子洋壳往北北东方向朝野马泉弧俯冲有关。     

北山马鬃山地区野马泉和勒巴泉韧性剪切带构造变形特征与年代学约束

野马泉和勒巴泉韧性剪切带作为北山造山带微陆块碰撞的产物,在北山造山带早古生代碰撞—造山过程中具有重要的地质意义。前人对其研究主要集中于勒巴泉地区构造变质核杂岩以及其中产出的蛇绿岩带。为了更好确定其构造变形时代,文章通过对野马泉和勒巴泉韧性剪切带野外构造剖面和显微构造特征的研究表明：野马泉和勒巴泉韧性剪切带具有由北西西向南东东逆冲,兼具右行剪切运动特征。通过对韧性剪切带中糜棱岩带中未变形花岗岩脉的锆石U-Pb定年,确定了野马泉和勒巴泉地区韧性剪切变形年龄上限为416 Ma,结合前人所报道的年龄,将韧性剪切变形时代大致限定在423~416 Ma之间,可能与牛圈子洋壳往北北东方向朝野马泉弧俯冲有关。     

Occurrence of gold in hydrothermal pyrite,western Taupo Volcanic Zone,New Zealand

The Taupo Volcanic Zone (TVZ) on the North Island of New Zealand is located above the subducting Pacific slab and hosts hydrothermal systems related to subduction, arc magmatism and crustal extension. In these systems, gold is transported primarily as sulphide complexes, with gold being deposited in response to boiling and mixing of the deep geothermal fluids. Conglomerate cobbles and hydrothermal fumaroles from the upper Retaruke River in the western TVZ are mineralised deposits that have been eroded from the nearby Miocene alluvial sedimentary plateau. Abundant gold-bearing pyrite was precipitated in the conglomerates and fumaroles by late hydrothermal process, primarily occurring as veinlets, disseminations and fine-grained aggregates which consists discrete euhedral microcrystals. Scanning electron microscopy combined with X-ray energy disperses spectroscopy shows that pyrites are commendably affected by late hydrothermal fluids, possibly by the carbonate fluid. Electron probe microanalysis revealed that the pyrite contains 20–120 ppm Au (averaging 60 ppm). Wavelength dispersive spectral elemental mapping suggests that gold is distributed uniformly in pyrite, indicating structurally bound gold (solid solution) in pyrite. Gold mineralisation recognised in the conglomerates and fumaroles demonstrates that the upper Retaruke River is a promising target for future gold exploration.     

东秦岭鮸鱼咀韧性剪切带构造变形特征及其年代学约束

东秦岭鮸鱼咀韧性剪切带是秦岭造山带晚古生代构造带武关杂岩和刘岭群的分界线,其变形作用的研究对揭示秦岭造山带晚古生代的构造演化具有重要意义.本文对这条剪切带进行了详细的几何学、运动学、锆石U-Pb和白云母40Ar/39Ar年代学研究.几何学和运动学指示鮸鱼咀韧性剪切带经历了两期主要的构造变形:早期由北向南的逆冲推覆变形和晚期左行走滑剪切变形.锆石U-Pb年代学研究表明构成剪切带主体的长英质糜棱岩中碎屑锆石主要的峰值年龄为448 Ma,最年轻的峰值年龄为390 Ma,其碎屑物质来源为北秦岭构造带,因此其原岩应该为一套沉积岩.眼球状白云母40Ar/39Ar同位素测年获得的坪年龄为(263±2)Ma.结合前人的变质变形年代学数据,认为早期的逆冲推覆与石炭纪大洋俯冲有关,晚期左行走滑是秦岭造山带晚二叠世"软碰撞"的构造响应.     

Ductile Shearing in Attur Shear Zone and its Relation with Moyar Shear Zone, South India

In the eastern part of southern Peninsular India, the charnockitic hills of the Madras block are cut across by the E-W trending Attur shear zone (ASZ) which is characterised by a thick (1 to 1.5 km) phyllonite zone, showing intense mylonitisation due to ductile shearing. Steeply plunging (70°–80°) stretching lineation on steeply dipping mylonitic foliation within this zone indicates a relative vertical upliftment of the adjacent blocks. A dextral shearing event from west to east is envisaged from the kinematic analysis of shear sense indicators such as S-C fabric, asymmetric folds, asymmetric augens and asymmetric porphyroclasts. Simultaneous development of these features, related to vertical and horizontal movements may be explained by the mechanism of transpressional deformation. The Attur shear zone may be correlated with the Moyar shear zone based on distinct lithological and structural similarities.     

西准噶尔萨吾尔地区二叠纪火山活动规律

西准噶尔萨吾尔地区位于新疆阿勒泰的吉木乃县及塔城地区和丰县。区内泥盆纪—二叠纪均有火山活动,其中二叠纪火山作用尤为强烈。二叠纪火山岩地层包括哈尔加乌组和卡拉岗组,哈尔加乌组为一套陆相中基性-中性火山岩及火山碎屑岩,卡拉岗组为一套陆相中基性-中酸性火山岩及火山碎屑岩。哈尔加乌—卡拉岗旋回火山岩主要岩性包括橄榄玄武岩、玄武岩、粗玄岩、安山岩、粗安岩、流纹岩、火山碎屑岩等。根据火山岩地层综合剖面以及火山岩的岩石学、岩相学特征,萨吾尔地区二叠纪火山活动由早至晚可分为5个阶段:中性喷发阶段、间歇性基性喷发阶段、酸性爆发及喷溢阶段、小规模中性间歇性爆发及喷溢阶段、基性喷发阶段;火山岩为陆相火山岩,具有双峰式特征,形成于伸展的构造背景下。     

南帕米尔喀拉苏沟韧性剪切带构造变形特征及年代学分析

帕米尔西构造结经历了古生代—中生代多个地体的复杂拼合过程, 又遭受了新生代印度-亚洲碰撞的改造, 在整个青藏高原构造演化中扮演了重要的角色, 但该地区中生代构造演化的研究仍然相对较少。通过研究南帕米尔一条北西—南东走向宽约1.5 km的韧性剪切带中野外和显微变形特征, 判定剪切带为左行剪切, 剪切以走滑分量为主, 兼具部分逆冲分量, 韧性变形呈现中心强, 边缘弱的特点。根据剪切带中花岗岩岩株的形态、变形、岩石地球化学特征以及与区域其它岩体的对比, 判定其为同构造花岗岩。该同构造花岗岩中测得的两个锆石U-Pb同位素年龄为(233.2±1.8) Ma和(235.7±1.4) Ma, 代表韧性剪切带形成于晚三叠世初期, 指示南帕米尔地块内部在乳山洋闭合初期已经发生明显的构造变形。     

Geophysical anomalies associated with uranium mineralization from Beldih mine, South Purulia Shear Zone, India

Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW-SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ～100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ.     

滇东黔西晚二叠世聚煤区潮坪沉积特征

滇东黔西晚二叠世聚煤区为整个华南地区重要的成煤区域。根据地质资料及工作实践,论述和总结了该区潮坪沉积特征及成煤规律。结果显示,本区潮坪体系特有的沉积构造十分丰富,主要有潮汐作用形成的单粘土层、双粘土层和潮汐周期层序、涨落潮沉积的二元韵律结构、扁平状和浑圆状泥砾、双向泥砾、递变式的潮汐周期层序和泥冲槽等;潮坪体系的沉积层序和平面砂体特征明显,潮沟砂体一般垂直于海岸展布,且常朝陆地方向分岔尖灭;潮坪环境在相对稳定的构造条件下,多形成广泛稳定的优质煤层,但潮沟或泻湖海湾等长期被海水侵漫的区域,常使下伏煤层的含硫量变高。指出潮坪体系是该区富集优质煤层的重要成煤环境之一。     

Timing of gold mineralisation in the Ballarat goldfields,central Victoria: Constraints from 40Ar/39Ar results

⁴⁰Ar/³⁹Ar data for muscovite separates and hydrothermally altered whole‐rock samples from the Ballarat West and the Ballarat East goldfields indicate that mesothermal gold mineralisation at Ballarat occurred during several episodic pulses, ranging in age from the Late Ordovician to the Early Devonian. Initial formation of auriferous structures in the Ballarat goldfields coincided with folding and thrusting associated with the development of the western Lachlan Fold Belt between 460 and 440 Ma. Subsequent fault reactivation and magmatism resulted in remobilisation and additional mineralisation between 410 and 380 Ma, and around 370 Ma. The results presented herein are in agreement with findings for other major gold deposits in central Victoria and further constrain the history of deformation, metamorphism and mineralisation in the western subprovince of the Lachlan Fold Belt.     

藏南冈底斯岩浆带中段曲水韧性剪切带的变形特征及其年代学约束

藏南曲水地区鸡公-色甫韧性剪切带系统的运动学和动力学研究,不仅对造山带构造研究具有重要理论意义,而且对青藏高原中-南部区域地质研究具有推进作用.作为藏南冈底斯岩浆带曲水岩基中的重要断裂构造,曲水韧性剪切带经历了新生代以来的大规模构造变形,其中走滑剪切作用最为显著,整体表现为右行走滑为主.通过对曲水剪切带中的构造片岩、初糜棱岩、糜棱岩以及长英质脉体等野外观测并结合室内镜下薄片以及石英EBSD(electron backscatter diffraction)组构分析,认为曲水剪切带主要经历了一期韧性变形事件.根据石英-长石变形矿物对并结合石英EBSD组构分析得出,构造变形发生的温度大约为500~550 ℃,高绿片岩相到角闪岩相.剪切带内普遍发育有不对称的褶皱、构造透镜体、σ碎斑、S-C组构和石香肠等变形组构,显示出右行走滑的特征.对研究区34组糜棱面理和9组拉伸线理进行极射赤平投影,糜棱面理的优选方位约为355°∠70°,拉伸线理产状约为95°∠8°.此外,在糜棱岩中发育两类长英质脉体,根据野外分布特征、显微组构、锆石成因学以及岩石地球化学特征综合研究认为,两类长英质脉体为同构造剪切脉体,其年代学可以对剪切带的形成时限起到很好的制约.通过对两套长英质脉体分别进行LA-ICP-MS锆石U-Pb定年,得到了38.67±0.88 Ma和35.05±0.29 Ma两组加权平均年龄,表明了曲水剪切带韧性变形发生于35~38 Ma的始新世末期(普利亚本期).这一年龄值处于印度-亚洲大陆晚碰撞期,因此曲水韧性剪切带右行走滑事件可能是印度板块持续向北俯冲,构造应力在欧亚板块边缘释放引起的陆内构造响应.      

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

 The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism. Received: 11 May 1998 / Accepted: 19 January 1999     

西秦岭凤太矿集区丝毛岭金矿床地质地球化学特征

西秦岭凤太矿集区丝毛岭金矿床位于八卦庙造山型金矿床西侧5km左右,是一个新探明的剪切带型金矿。其成矿作用过程可分为早期石英-绢云母-硫化物阶段、中期多金属-硫化物阶段和晚期碳酸盐阶段。对早、中期的石英流体包裹体测试结果表明,丝毛岭金矿床成矿流体以富CO2、中温、低盐度为特征,总体上属于中温低盐度CO2-H2O体系,流体包裹体类型的多样性是流体不混溶性的产物。从早阶段到主成矿阶段成矿流体的温度、压力和盐度均有降低,硫逸度增高,有利于金的沉淀富集。H、O、S、C同位素研究结果,以及与八卦庙金矿床的对比分析表明,二者的成矿流体具有相似性和同源性,都是以深部来源为主的多源流体。由于丝毛岭金矿床产出的层位高于八卦庙金矿床,其成矿环境相对开放。     

白垩纪羌塘地体西南缘的陆-陆碰撞:来自班怒带西段的花岗岩约束

羌塘地体西南缘布木错—雄巴日—羌多—赛登地区分布着大量呈岩株或岩枝状的中酸性-基性侵入岩,其岩石地球化学特点是低SiO₂(67.99%~73.32%)、高Al₂O₃(12.82%~15.02%),属于过铝质花岗岩,里特曼指数(2.09~2.63)σ<3.3,属钙碱性花岗岩类;稀土元素显示为轻稀土较强富集和重稀土强亏损的特征,岩浆结晶分异程度较高。岩石地球化学特征和稀有元素特征图解指示花岗岩构造环境有一定的差异性,以I型为主,其形成及侵位应与古特提斯洋的闭合、新特提斯洋的扩张有关,属同碰撞型造山花岗岩。通过对该区多个花岗岩体锆石LA-ICPMS 测定,测得其形成年代为116~107 Ma,与早白垩世晚期羌塘地体南缘的陆-陆碰撞作用时间相吻合。     

Integrated geophysical studies to elicit the subsurface structures associated with Uranium mineralization around South Purulia Shear Zone,India: A review

South Purulia Shear Zone in India is an important zone that hosts uranium mineralization. Since detailed geophysical studies have not been carried out in this region, an integrated geophysical study using self-potential, resistivity, very low frequency electromagnetic and radiometric methods was performed to identify the subsurface structures that could host the hydrothermally altered uranium deposits in the area. The study reveals the wide and large magnitude of negative self-potential anomaly across the South Purulia Shear Zone. The peak negative self-potential anomalies are correlated with the low gravity and low resistivity anomalies measured along various profiles. The low self-potential, gravity and low resistivity anomaly zones are also correlated with conducting zones inferred from very low frequency electromagnetic measurements. Interpretation of self-potential data reveals multiple thick sheet-type vertical and/or inclined structures which might be associated with uranium mineralization. Schlumberger resistivity sounding data reveals an increasing trend of apparent resistivity with current electrode separations/depth. Apparent conductance measured simultaneously with resistivity measurement reveals an increase in current flow of current as depth increases. This exhibits the presence of thin conducting layers at these depths, which is not reflected in resistivity sounding data due to suppression problem. Also this conducting layer is consistent in various soundings and is connected from Raghunathpur to the South Purulia Shear Zone. Correlation of very low frequency and self-potential data shows that the structures are comparable and a radiometric profile also advocate that the conducting structure is associated with radioactive minerals. These structures are likely to be mineralized zones as hydro-uranium anomaly has also been reported from various locations in the area. Moreover, drilling results at a specific mine near the study area also confirms the presence of uranium mineralization. The hydrothermal activity associated with uranium mineralization seems to be still active in the area. Such combined geophysical studies are essential to understand this highly complex shear zone for the economic exploitation of its natural resources.     

Time-scales for magmatic differentiation at the Snaefellsjökull central volcano, western Iceland: Constraints from U-Th-Pa-Ra disequilibria in post-glacial lavas

We present major and trace element and Sr-Nd-Pb and U-Th-Pa-Ra isotope data for a small sample suite of primarily post-glacial, mildly alkalic volcanic rocks from the Snaefellsjökull central volcano situated off the main rift systems in western Iceland. The volcanic rocks are crystal-poor and range from olivine alkali basalt to trachyte and show tight correlations of major and trace elements that are explained by fractional crystallization involving removal of olivine, plagioclase, clinopyroxene, Fe-Ti oxide and apatite. Sr-Nd-Pb isotopes are practically invariant, consistent with derivation from the same source region. During fractionation from primitive basalt to evolved trachyte, (²³⁰Th/²³²Th), (²³⁰Th/²³⁸U) and (²³¹Pa/²³⁵U) decrease progressively at broadly constant (²³⁸U/²³²Th). A continuous closed-system fractionation model that assumes constant initial (²³⁰Th/²³²Th) in the basaltic precursor melt indicates that hawaiite was derived from olivine basalt by ∼50% fractional crystallization within and trachyte by ∼80% fractionation within . An overrepresentation of evolved basalts and hawaiites with young inferred magma ages in the dataset is consistent with the parental precursor to these magmas intruded into the sub-volcanic magma plumbing system as a consequence of lithospheric rebound caused by deglaciation. Lavas affected by apatite removal have higher (²³¹Pa/²³⁵U) than predicted for simple radioactive decay, suggesting apatite significantly fractionates U from Pa. The proposed fractionation model consistently explains our U-series data assuming and and . If applicable, these D values would indicate that the effect of apatite fractionation must be adequately considered when assessing differentiation time scales using (²³¹Pa/²³⁵U) disequilibria data.     

西秦岭地区脉岩成因与金成矿关系——来自李坝金矿年代学、地球化学及Nd-Hf-S同位素的约束

甘肃李坝金矿位于西秦岭北成矿亚带礼岷金矿带东部,矿区内脉岩主要包括斜闪煌斑岩、云斜煌斑岩、闪长(玢)岩、细晶闪长岩、石英闪长岩、花岗(斑)岩、花岗细晶岩等。根据各脉岩与金矿的产出关系可划分为成矿前脉岩、成矿期脉岩和成矿后脉岩3类。不同种类、不同期次脉岩的成因及其对金成矿作用的制约存在不同认识。LA-ICP-MS锆石U-Pb定年结果显示,李坝金矿区花岗斑岩脉的年龄为(210±1)Ma(MSWD=0.15),与中川岩体同属晚三叠世构造岩浆活动的产物。成矿期花岗(斑)岩脉富碱、高钾,属于过铝质、高钾钙碱性系列,与成矿密切相关的煌斑岩脉同属高钾钙碱性系列。Nd-Hf同位素结果显示,李坝金矿区花岗斑岩脉的~(143)Nd/~(144)Nd比值为0.512 084±0.000 007,~(147)Sm/~(143)Nd比值为0.0903,ε_(Nd)(t)为-7.98,Nd同位素亏损地幔模式年龄T_(DM2)为1.64 Ga;锆石ε_(Hf)(t)=-6.30～+1.47,T_(DMC)为1.65～1.15 Ga,显示花岗斑岩脉的源区以古老的壳源物质为主,并有年轻物质的加入。硫同位素和矿床地球化学结果显示,黄铁矿的δ~(34)S值变化于4.3‰～11.6‰,平均值为8.45‰,表明其成矿硫源具有深源特点,金矿成矿物质来源于与脉岩有关的深部岩浆。综上,笔者认为矿区内脉岩与金矿化具有深部同源性,成矿同期的煌斑岩可能为金矿化提供部分成矿物质或成矿流体。