Preliminary study on the occurrence of geothermal systems in the tectonic compressional regions: an example from the Derman geothermal field in the Biga Peninsula, Turkey

There are numerous hot springs with temperatures ranging from 30 to 100 °C in Biga peninsula and they occur throughout the peninsula. The result of this study shows that the region is under a tectonic compressional regime. The investigation of the faults and fractures in the region indicates that the region has been affected first by N–S and then E–W compression since the Middle Miocene. Opening fractures and antithetic and synthetic faults due to the compressional movements provide paths for the deep circulation of water. In addition, the tectonic movements, granitic intrusion and volcanic activity have also played important roles as heat sources for the geothermal systems.     

Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity

Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic low-velocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity, high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east–west contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc.     

Origin and evolution of Pliocene–Pleistocene granites from the Larderello geothermal field (Tuscan Magmatic Province, Italy)

Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct ε_Nd(t) and ⁸⁷Sr/⁸⁶Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low ε_Nd(t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher ε_Nd(t) value (−7.9). Finally, a muscovite-rich source with high ε_Nd(t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H₂O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems.     

Mineralogy and geochemistry of a Late Permian coal in the Dafang Coalfield, Guizhou, China: influence from siliceous and iron-rich calcic hydrothermal fluids

This paper describes the influence of siliceous and iron-rich calcic low-temperature hydrothermal fluids (LTHF) on the mineralogy and geochemistry of the Late Permian No. 11 Coal (anthracitic, R_r=2.85%) in the Dafang Coalfield in northwestern Guizhou Province, China. The No. 11 Coal has high contents of vein ankerite (10.2 vol.%) and vein quartz (11.4 vol.%), with formation temperatures of 85 and 180 °C, respectively, indicating that vein ankerite and vein quartz were derived from low-temperature calcic and siliceous hydrothermal fluids in two epigenetic episodes. The vein quartz appears to have formed earlier than vein ankerite did, and at least three distinct stages of ankerite formation with different Ca/Sr and Fe/Mn ratios were observed.The two types of mineral veins are sources of different suites of major and trace metals. Scanning electron microscope and sequential extraction studies show that, in addition to Fe, Mg, and Ca, vein ankerite is the dominant source of Mn, Cu, Ni, Pb, and Zn in the coal, and the contents of these five elements are as high as 0.09% and 74.0, 33.6, 185, and 289 μg/g, respectively. In contrast, vein quartz is the main carrier mineral for platinum-group elements (PGEs) Pd, Pt, and Ir in the coal, and the contents of Pd, Pt, and Ir are 1.57, 0.15, and 0.007 μg/g, respectively. Sequential extraction showed a high PGE content in the silicate fraction, up to 10.4 μg/g Pd, 1.23 μg/g Pt, and 0.05 μg/g Ir, respectively. It is concluded that the formation of ankerite and quartz and the anomalous enrichment of trace elements in the No. 11 Coal in the Dafang Coalfield, Guizhou, result from the influx of calcic and siliceous low-temperature hydrothermal fluids.     

福建行洛坑钨矿床白钨矿Sm-Nd等时线年龄及微量元素地球化学特征

行洛坑超大型钨矿床位于福建省宁化县东部,是武夷山成矿带内最大的钨矿床,具有储量大、品位低、黑钨矿与白钨矿资源量相近的特点。白钨矿作为含钨矿物,其形成年龄可以代表矿床的成矿时代,其地球化学特征反映了成矿作用环境和过程。文章在对行洛坑钨矿床钨矿脉特征、黑钨矿与白钨矿赋存状态开展调查的基础上,挑选白钨矿单矿物进行了Sm-Nd等时线测年和微量元素测试。研究结果显示,白钨矿结晶年龄为（142.6±2.8）Ma,略小于黑钨矿原位U-Pb年龄（150.5±8.1）Ma和石英流体包裹体Rb-Sr等时线年龄（147.5±2.9）Ma,与矿区隐伏岩体成岩年龄143.5~149.5 Ma在误差范围内完全吻合,也与矿物结晶世代关系吻合,都属于晚侏罗世的产物,说明钨成矿与隐伏岩体成岩近于同时或稍晚发生。白钨矿中微量及稀土元素特征显示第一世代成矿流体中Eu³⁺ < < Eu²⁺,指示成矿流体为还原性流体;第二世代成矿流体中Eu³⁺ > > Eu²⁺,指示成矿流体为氧化性流体。行洛坑钨矿成矿与华南中生代大规模成矿作用及其大地构造背景和动力学环境密切相关。     

华南花岗岩型铀矿床成矿机理研究进展

本文介绍了华南花岗岩型铀矿床地质特征,系统总结了前人对其成矿热液来源、物质来源,铀的迁移沉淀机制、碱交代及花岗岩与成矿的关系等方面的研究成果。指出铀源体中的晶质铀矿及富铀矿物在深部相对还原的环境中被氧化而进入成矿热液中起主要作用的可能是大量幔源F和放射性衰变诱发产生的氧及碱交代溶蚀作用,热液中的大部分U6 主要被S2-和Fe2 等还原剂在浅部相对氧化的环境中还原成矿;给出了一种较简易的物质来源定量方法和铀成矿模式。     

灰岩碳氧同位素特征与隐伏花岗岩的关系——以栗木水溪庙矿区为例

对栗木水溪庙矿区泥盆系上统融县组灰岩的碳氧同位素进行了研究,该地区灰岩的碳氧同位素组成可提供隐伏花岗岩隆起及其相关流体的重要信息。受隐伏花岗岩侵入驱动的流体与上覆融县组灰岩发生反应的温度在110℃左右,流体的初始同位素组成为δ18OSMOW=-3‰,δ13CPDB≤-7‰,反应的水岩比值（w/r）可能小于5。这种岩浆水与大气降水的混合流体与围岩之间的水岩反应使得地表灰岩的δ18O和δ13C值降低,产生负异常。研究表明,围岩的δ18O值降低受反应的水岩比值和温度控制;δ13C值降低主要与反应的水岩比值有关。反应的温度越高,w/r值越大,灰岩的碳氧同位素负异常越明显。因此,水溪庙矿区地表出露的碳酸盐地层中的碳氧同位素变化可在地球化学勘查中用于指示下伏花岗岩岩脊的隐伏位置。     

初论兰坪盆地构造流体与成矿作用的时空格架及可能的成矿模式

滇西兰坪盆地中新生代形成了丰富的金属与盐类矿床，是我国著名的热液矿床成矿带。成矿系统时─空结构的统一性是进行成矿规律研究和成矿预测的前提和指导思想，综合分析兰坪盆地的成矿时代、矿床的空间分布及区域地质背景，初步认为：晚白垩世至早第三纪，盆地西缘出现以铜为主的成矿作用；兰坪盆地主要成矿作用发生在早、晚第三纪期间，因盆地东、西部边界向两边地块的俯冲碰撞及中轴断裂带的强烈活动，导致盆地中央地带出现大规模成矿流体活动，成矿流体因物理化学性质的改变而发生分异、运移，及盆地受挤压隆升，变成山间盆地，出现不同类型、不同大小的流体圈闭，因而形成不同种类，不同规模的矿床；同时，含有挥发性组分汞、锑、砷的流体从深部沿中轴断裂带上升向两侧的次级构造裂隙带扩散，导致盆地东部复式背斜带中出现锑、汞、砷的成矿作用；其后，矿床受到改造和氧化作用。盆地内发育深大断裂；兰坪盆地的成矿作用是在构造流体演化的制约下完成的，同时成矿作用的发生改变了流体的性质和构造环境，促使流体的再次循环，出现成矿作用的多期性、分带性。     

埋藏有机酸性流体对四川盆地东北部飞仙关组储层的溶蚀改造作用：溶解动力学实验研究

通过白云岩化鲕粒灰岩与0.1 mol/L乙酸的旋转盘溶解动力学实验,研究了深埋藏中有机质热演化过程中伴生的有机酸对鲕粒灰岩储层的改造作用机理.结果显示,白云岩化鲕粒灰岩溶解反应的速率在3.26×10-7～6.66×10-7 mol/(cm2·s)之间,并且溶蚀速率随温度和转速的增加而增大.反应前后样品表面的扫描电子显微...     

西藏羊八井热田地热流体成因及演化的惰性气体制约

地热流体中惰性气体的相对丰度和同位素组成，不仅可以揭露热田的热源性质，而且还能够揭示深，浅层地热流体的内在联系和演化过程等。在西藏羊八井热田的地热气体中，已检测出大量的^4He组分，3He/^4He值是大气的0．087－0．259倍，表明深部地壳物质的局部熔融为热田提供能量，浅层地热流体的3He/4He 值自西北向东南呈降低趋势，与热储温度的变化相一致，反映出侧向运移时补充了更多的壳源氦，热田北区深层地热流体具有稍高的3He/4He值，是浅层地热流体的母源，气体中氪和氙的相对丰度具有大气降水成因的特征，结合现有的实际资料，建立了热田地热流体的概念模型。