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1.
Deep-sea volcaniclastic sedimentary systems: an example from La Fournaise volcano, Réunion Island, Indian Ocean 总被引：1，自引：1，他引：0
A volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores, camera surveys, high-resolution sonar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Réunion Island, a basaltic shield volcano in the SW Indian Ocean. Three main areas are identified from the study: (1) the proximal fan extending from 500 m water depth down to 2000 m water depth; (2) the outer fan extending from 2000 m water depth down to 3600 m water depth; (3) the basin extending beyond 3600 m water depth. Within these three main areas, seven distinct submarine environments are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep-water deltas, debris-avalanche deposits, and eroded floor in the valley outlets; the outer fan is characterized by a discontinuous fine-grained sedimentary cover overlying coarse-grained turbidites or undifferentiated volcanic basement; the basin is characterized by hemipelagic muds and fine-grained turbidites interbedded with sandy and gravelly turbidite lobes. The evolution of the deep-sea volcaniclastic fan is strongly influenced by sector collapses, such as the one which occurred 0·0042 Ma ago. This collapse produced a minimum of 6 km3 of debris-avalanche deposit in the proximal area. The feeding regime of the deep-sea fan is ‘alluvial dominated’ before the occurrence of any sector collapse and ‘lava-dominated’ after the occurrence of a sector collapse. The main deep-water lava-fed delta is prograding among the blocks of the debris-avalanche deposits as a result of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave-action, earthquakes and accumulation of new volcanic debris on top of the deltas. Both turbidity currents triggered on the deep-water delta slope, and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line. 相似文献
2.
G. Camus Dr. M. Diament Dr. M. Gloaguen Dr. A. Provost Dr. P. Vincent Dr. 《GeoJournal》1992,28(2):123-128
Conclusion The data collected during the Mentawai cruise help to clarify understanding of the 1883 eruption of Krakatau. We have previously discussed the weaknesses of the interpretation of Williams (1941) and others (Self and Rampino 1981) and emphasized that only a Mount St. Helens-type collapse during the course of the eruption could account for all the characteristics of the eruption and of the related deposits.The discovery on land of deposits attributable to a debris-avalanche, in the stratigraphic position where they were expected, is a strong argument for the validity of our scenario.Marine surveys confirm that the sea bottom around Krakatau is covered by a thick ignimbritic deposit. But the presence of this deposit does not invalidate the presence of a debris-avalanche deposit under the ignimbrites. The hummocky morphology favours this hypothesis.Flank-failure of volcanoes is generally considered as a very efficient mechanism for triggering tsunamis (Kienle et al. 1987; Siebert et al. 1987). However, the majority of the volcanoes where flank-failure has been described are tall and bulky and the collapse of a broad edifice like Krakatau may be surprising. However the geological evidence shows that such a mechanism can act at various scales; for example the flank collapse of Mayu Yama volcano (height 700 m, volume 0,3 km3), a parasitic cone of Unzen volcano (Japan), triggered a debris-avalanche into the sea that was 1 km long, with a characteristic hummocky surface; the resulting tsunami killed 9528 people (Katayama 1974). In the same way, a partial collapse of Iliwerung volcano, Indonesia (50 × 106 m3) in July 1979, triggered a tsunami which killed several hundred people (McClelland et al. 1989). At Krakatau, the main summit was 822 m asl; the collapse took place along the edge of the prehistoric caldera and this structural unconformity probably facilitated the triggering of the process. 相似文献
3.
Landscape and sedimentary response to catastrophic debris avalanches, western Taranaki, New Zealand 总被引：1，自引：1，他引：0
Catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre-collapse cone volumes being removed. Deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks. On the weakly-incised Mt. Taranaki ring-plain, volcanic debris avalanche deposits typically form a large, wedge shape (in plan view), over all flat-lying fans. Following volcanic debris avalanches a period of intense re-sedimentation commonly begins on ring-plain areas, particularly in wet or temperate climates. This is exacerbated by large areas of denuded landscape, ongoing instability in the scarp/source region, damming of river/stream systems, and in some cases inherent instability of the volcanic debris avalanche deposits. In addition, on Mt. Taranaki, the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism, generating large volumes of juvenile tephra onto unstable and unvegetated slopes, or construction of new domes with associated rock falls and block-and-ash flows. The distal ring-plain impact from these post-debris avalanche conditions and processes is primarily accumulation of long run-out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character. Common to these post-debris avalanche units is evidence for high-water-content flows that are typically non-cohesive. Hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography. The post-collapse deposits flank large-scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain. These deposits on Mt. Taranaki provide a record of landscape response and ring-plain evolution in three stages that divide the currently identified Warea Formation: 1) the deposition of broad fans of material adjacent to the debris avalanche unit; 2) channel formation and erosion of Stage 1 deposits, primarily at the contact between debris avalanche deposits and the Stage 1 deposits and the refilling of these channels; and 3) the development of broad tabular sheet flows on top of the debris avalanche, leaving sediments between debris avalanche mounds. After a volcanic debris avalanche, these processes represent an ever changing and evolving hazard-scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in. 相似文献
4.
Secondary precipitates (iron oxide, calcite, etc.) are currently observed in cold-climate Pleistocene deposits. Some have micro- and ultrastructures quite different from precipitates of vadose, phreatic and biogenic origin, and seem to have originated by freezing processes. The microstructures of calcite coatings from a Pleistocene cryopediment in the Mendoza Pre-Cordillera, from a Lower Pleistocene cryogenic slope deposit in Western Transbaikalia and from the present active layer in Antarctica are described. They show similar patterns: fibrous crystals often consisting of piles of platelets, some with internal holes, assembled in millimetre-scale fringes on the lower face of clasts. Observational (mainly fabric) evidence confirms that such peculiar crystals are formed during freezing. The features are unknown in other climates and, when found in past sediments, can be diagnostic of cryogenic palaeoenvironments. 相似文献
5.
Rock avalanches are a form of hazardous long-runout landslide and leave fragmented deposits of complex sedimentology that, if studied in detail, can provide insight into their emplacement processes. Complexity arises due to the myriad overlapping factors known to contribute to the final deposit fabric, such as source structures, lithology (i.e. material properties), topographic feedback, substrate interaction and emplacement processes (i.e. internal factors), as well as our reliance on (un)suitable exposures. Herein, we present sedimentological data from two carbonate rock avalanche deposits (Tschirgant in Austria and Flims in Switzerland), where changes in lithology can be eliminated from the causal equation due to their largely mono-mineralic composition. We further eliminated the effects of external influences such as topography or substrate interactions by detailed facies mapping of the deposit interior. Since sedimentary properties locally vary within less than 1-m2 outcrop area, emplacement processes are the only causes that remain to explain the different fabrics. Characteristic (fractal) grain size distributions of three distinctive sub-facies in the interior of these, and other, rock avalanche deposits—jigsaw-fractured, fragmented, and shear zone facies—can be linked to specific processes acting during emplacement. We suggest that a heterogeneously distributed and progressively increasing particle breakage in the moving granular mass best explains the ranges of fractal dimensions and associated features for the respective sub-facies, from simple breakage along pre-existing planes, through dynamic fragmentation which locally minimises coordination number, to zones of shear concentration. No exotic emplacement mechanisms (such as air-layer lubrication or fluidised substrates) are required to produce these features; continued, heterogeneous degrees of fragmentation of an initially intact source rock best explains the sedimentary record of rock avalanches. 相似文献
6.
David M. Pyle Tjeerd H. van Andel Panayiotis Paschos Paul van den Bogaard 《Quaternary Research》1998,49(3):280-286
A newly recognized 2-m-thick trachytic volcanic ash deposit from northwestern Greece is dated at 374,000 ± 7000 yr and correlated with the Middle Pleistocene volcanic activity of central Italy. The deposit represents ash fallout from one of the largest volcanic eruptions in Europe of the past 400,000 yr and should provide an important stratigraphic marker within the poorly dated Middle Pleistocene deposits of Italy and Greece. 相似文献
7.
《Earth》2002,57(1-2):1-35
Landslides have been a key process in the evolution of the western Canary Islands. The younger and more volcanically active Canary Islands, El Hierro, La Palma and Tenerife, show the clearest evidence of recent landslide activity. The evidence includes landslide scars on the island flanks, debris deposits on the lower island slopes, and volcaniclastic turbidites on the floor of the adjacent ocean basins. At least 14 large landslides have occurred on the flanks of the El Hierro, La Palma and Tenerife, the majority of these in the last 1 million years, with the youngest, on the northwest flank of El Hierro, as recent as 15 thousand years in age. Older landslides undoubtedly occurred, but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments. Landslides on the Canary Island flanks can be categorised as debris avalanches, slumps or debris flows. Debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence, up to a maximum thickness of 1 to 2 km. They are the commonest type of landslide mapped. In contrast, slumps move short distances and are deep-rooted landslides which may affect the entire thickness of the volcanic edifice. Debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks. Some landslides are complex events involving more than one of the above end-member processes.Individual debris avalanches have volumes in the range of 50–500 km3, cover several thousand km2 of seafloor, and have runout distances of up to 130 km from source. Overall, debris avalanche deposits account for about 10% of the total volcanic edifices of the small, relatively young islands of El Hierro and La Palma. Some parameters, such as deposit volumes and landslide ages, are difficult to quantify. The key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks, broadening into a depositional lobe below 3000 m. Debris avalanche deposits have a typically blocky morphology, with individual blocks up to a kilometre or more in diameter. However, considerable variation exists between different avalanche deposits. At one extreme, the El Golfo debris avalanche on El Hierro has few large blocks scattered randomly across the avalanche surface. At the other, Icod on the north flank of Tenerife has much more numerous but smaller blocks over most of its surface, with a few very large blocks confined to the margins of the deposit. Icod also exhibits flow structures (longitudinal shears and pressure ridges) that are absent in El Golfo. The primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes. Observations in experimental debris flows show that the differences between the El Golfo and Icod landslide deposits are probably controlled by the greater proportion of fine grained material in the Icod landslide. This, in turn, relates to the nature of the failed volcanic rocks, which are almost entirely basalt on El Hierro but include a much greater proportion of pyroclastic deposits on Tenerife.Landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands, with landslides propagating perpendicular to the rift orientation. However, this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a ‘weakness’ that can be carried forward during island development. This may occur because certain island flanks are steeper, extend to greater water depths or are less buttressed by the surrounding topography, and because volcanic production following a landslide my be concentrated in the landslide scar, thus focussing subsequent landslide potential in this area. Landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight. Although the actual triggering factors are poorly understood, they may include or be influenced by dyke intrusion, pore pressure changes related to intrusion, seismicity or sealevel/climate changes. A possible relationship between caldera collapse and landsliding on Tenerife is not, in our interpretation, supported by the available evidence. 相似文献
8.
9.
西藏自治区谢通门县斯弄多铅锌矿区地球物理特征及找矿方向 总被引：1，自引：0，他引：1
浅成低温热液型矿床是一类主要产于陆相火山岩或与陆相火山岩有关的低温热液矿床,矿床埋藏深度浅是其最大的特点,这也使地球物理勘探方法在这类矿床的勘查中具有先天优势。本文通过对西藏斯弄多Pb-Zn-Ag多金属浅成低温热液型矿床的地球物理参数特征研究,采用激发极化法等综合物探技术在矿体外围探获具有巨大前景的Pb-Zn-Au-Ag浅成低温热液型矿体。目前单工程控制有3段矿体:顶部Au-Ag矿体,中部Au矿体,底部Pb-Zn-Ag矿体。矿体最高Au品位可达5.3g/t,Ag最高品位410g/t,Pb+Zn综合平均品位5.9%。研究结果表明,斯弄多地区林子宗群火山岩物性参数稳定,对矿体异常干扰较小,产于该套火山岩中极强硅化体中的浅成低温热液型Au矿多具有中高极化率和中低阻特征。 相似文献
10.
笔者在该文中着重表达了３方面研究成果：在陈述清水沟－白柳沟地区成矿地质背景基础上，描述了已知工业矿床的基本特征，并将其类比为两类矿床，其一为Ｚｎ－Ｐｂ－Ｃｕ型小铁山式矿床，以展布于郭密寺北火山穹隆北东和南西两侧的下柳沟、弯阳河、下沟及郭密寺矿床为代表；其二为Ｃｕ（－Ｚｎ）型白银厂式矿床，以出露在黑河古火山穹隆北翼近核部的香子沟矿床为实例。成矿前期、成矿时期，以及成矿后期地质条件的分析与概括表明，该地区与白银厂地区在海相火山岩的成岩环境─—裂谷岛弧、火山岩组合──双峰式、古火山机构类型──古火山穹隆，以及成矿机制──海底热液对流循环成矿等诸多方面都具相似性，是寻找铜多金属硫化物矿床的良好地段。提出了该区此类矿床的综合成矿模式，相信它在本地区扩大寻找小铁山式矿床和突破性寻找白银厂式矿床中具有重要指导意义和实际操作意义． 相似文献
11.
A. V. Zernack S. J. Cronin V. E. Neall J. N. Procter 《International Journal of Earth Sciences》2011,100(8):1937-1966
The >25 ka volcaniclastic ring-plain succession in south-west Taranaki has been remapped to establish a much more detailed
understanding of the older stratigraphic record of Mt. Taranaki. Coastal cliff exposures show a range of volcaniclastic lithofacies,
including debris-avalanche and lahar deposits, and allow a detailed chronological reconstruction of past volcanic and sedimentary
events. Five new debris-avalanche deposits were identified, and their distribution in coastal cross-sections mapped. In addition,
four previously described units were renamed and their stratigraphic position and lateral extent redefined. Chronostratigraphic
control of the younger (<50 ka) sequence was obtained by radiocarbon dating of wood found within, or peat interbedded with,
the deposits. Emplacement ages of the older units were estimated from their stratigraphic position and underlying marine wave-cut
surfaces. Overall, at least 14 widespread debris-avalanche deposits occur within the <200 ka ring-plain record of Mt. Taranaki,
suggesting one major edifice failure on average every 14,000 years, with an increase in frequency since 40 ka. The stratigraphic
reconstruction of the ring-plain succession showed that the same pattern of deposition was repeatedly produced throughout
the existence of Mt. Taranaki. Depending on their sedimentological characteristics, the different volcanic and sedimentary
lithofacies can be related to phases of edifice-construction or collapse events. Based on the identified cyclic sedimentation
pattern, we present a new episodic stratigraphy that integrates existing and new lithostratigraphic units into a coherent
chronostratigraphic framework that can be applied to the entire volcanic and volcaniclastic succession at Mt. Taranaki. This
model takes into account the complex geological processes that have taken place on the volcano and provides a more uniform
stratigraphic terminology that could be applied to repeatedly collapsing stratovolcanoes elsewhere. 相似文献
12.
祁连优地槽区块状硫化物矿床形成地质环境的初步对比研究和探讨 总被引：3，自引：0，他引：3
在成矿规律学的研究中,许多研究者常把优地槽的发展演化与矿床形成阶段相结合并力图给予模式化,认为块状硫化物矿床是优地槽发展初期的坳陷阶段,地槽基底下坳带来巨厚沉积和强烈火山作用时期,与火山和沉积作用有关的形成物(毕利宾,1950;斯米尔诺夫,1973)。 相似文献
13.
PENG Runmin ZHAI YushengKey Laboratory of Lithospheric Tectonics Lithoprobing Technology China University of Geosciences Ministry of Education Beijing 《《地质学报》英文版》2004,78(2)
Most ore-forming characteristics of the Langshan-Zha'ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha'ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical 相似文献
14.
PENG Runmin ZHAI YushengKey Laboratory of Lithospheric Tectonics Lithoprobing Technology China University of Geosciences Ministry of Education Beijing 《《地质学报》英文版》2004,78(2)
Most ore-forming characteristics of the Langshan-Zha'ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha'ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical 相似文献
15.
新疆北部陆相火山岩型金矿床的主要特征 总被引：1，自引：1，他引：1
陆相火山岩型金矿目前是新疆最为重要也最富有特色和发展前景的金矿新类型。与金矿有关的火山岩，主要是一套以玄武岩－安山岩－流纹岩及其相应火山碎屑岩组合为特征的陆相火山岩。新疆北部存在两期（华力西中期和中晚期－晚期）陆相火山岩，相应发育两期金矿化。矿床地质和地球化学的一系列特征表明，该类金矿是与陆相火山岩密切相关，形成于岛弧或活动大陆边缘型挤压造山环境及近地表条件下的浅成低温热液金矿床。成岩成矿物质具源自上地幔或下部地壳的深源特征。 相似文献
16.
阿尔泰南缘麦兹泥盆纪火山_沉积盆地成矿特点及其铅锌、铁、金找矿潜力分析 总被引：7，自引：1，他引：7
麦兹盆地位于西伯利亚板块南缘阿尔泰陆缘活动带中,是一个重要的铅、锌、铁矿化集中区,其中著名的有蒙库大型铁矿床、可可塔勒大型铅锌矿床,是阿尔泰南缘多金属成矿带中最重要的成矿盆地之一。盆地内矿床、矿点多为火山喷流热水沉积成因,层控特征明显,具有规模大、形态规则、厚度及有用组分稳定等特点。铁矿成矿与早泥盆世早期细碧角斑质火山作用相伴,而铅锌成矿则与早泥盆世晚期长英质火山喷流沉积作用相伴。通过对麦兹盆地成矿地质特征和典型矿床中矿体分布变化规律的研究,认为北西向同生断裂、火山沉积洼地、火山喷发中心(火山机构)共同控制铅锌矿的产出,矿体还受到后期褶皱、变质作用的改造。根据有利层位、火山岩相、喷流沉积岩、热液蚀变、矿化组合、铁帽、控矿构造和地球化学异常等综合找矿评价标志,分析了可可塔勒矿区深部、什根特、铁热克萨依、H-48号异常的铅锌找矿潜力,并探讨了盆地内铁、金的找矿潜力,指出了进一步找矿的方向与有利地段。 相似文献
17.
构造-流体耦合关系对水上亚系统金矿成矿环境的影响 总被引：2，自引：0，他引：2
流体温度-密度-盐度关系化学模式显示,水下亚系统是一种火山作用形成的富含NaCI向海底释放的流体系统,该系统形成块状硫化物矿床;水上亚系统是陆相火山岩地区的地下低盐度低温流体系统,形成了浅层低温热液型和热泉型矿床.由流体的质量方程、能量方程和运动方程的特点可以看出,在成矿作用过程中的流体运动,受到构造-流体-成矿系统的关系中多种因素的影响,如介质的孔隙度、流体密度、渗透率、流体的黏度和流体内压力等.以世界一些陆相火山地区的金矿为例进行讨论,充分证明了陆相火山地区的金矿流体的运动方式具有两种类型:一类是成矿流体沿岩石的孔隙流动,如伊尔曼得金矿等;另一类是成矿流体沿裂隙流动,如阿希金矿等.虽然这些金矿形成于同一火山岩地区,但是,流体在围岩的孔隙和断裂中以不同的方式运动,流体的内压力、流体的密度将发生不同的变化,同时引起流体的氧逸度、pH值、盐度产生不同的改变.由此表明流体运动的方式和扩散影响了成矿环境. 相似文献
18.
内蒙古狼山—渣尔泰山中元古代被动陆缘热水喷流成矿特征 总被引：22，自引：1，他引：21
狼山—渣尔泰山中元古代被动陆缘产有东升庙、炭窑口、霍各乞和甲生盘等热水喷流沉积矿床 ,与世界中元古代的SEDEX型矿集区有许多相似之处 :①矿床的产出受华北古陆北缘裂陷槽内三级断陷盆地控制 ;②各大矿床都具有鲜明的层控特征 ,所有矿体总体呈层产在中元古界的白云石大理岩、碳质千枚岩 (或片岩 )中 ;③矿石具有细纹层状、条带状构造 ,喷流沉积成矿特征十分明显 ;④成矿过程中伴有明显的同生断裂活动 ,它在一定程度上控制了矿体的空间分布及其组合 ;但不同矿床同生断裂活动的强度、时限、规模都不同 ,从而导致不同矿床在相同含矿岩组中矿体产出的先后顺序不同和大量层间砾岩与同生角砾状矿石的形成 ;⑤厚大Zn ,Pb ,Cu复合矿体具有明显的分带性 ,自下至上 ,Cu/ (Zn +Pb +Cu)比值由高→低 ;⑥重晶石层发育 ,多与黄铁矿层互层状产出 ,也有与闪锌矿层互层 ,但与世界典型SEDEX型矿床又有重要差别。成矿期间火山活动明显 ,在霍各乞、东升庙、炭窑口矿床惟一容矿的狼山群第二岩组中先后发现了具有变余斑状或聚斑状结构、变余杏仁构造的基性火山岩、钠质“双峰式”火山岩和钾质“双峰式”海相火山岩及凝灰岩夹层。结合①各种硫化物的铅同位素主要分布在地幔和下地壳铅演化曲线附近 ;②部分黄铁矿的Co/Ni值远 相似文献
19.
PENGRunmin ZHAIYusheng 《《地质学报》英文版》2004,78(2):534-547
Most ore-forming characteristics of the Langshan-Zha‘ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha‘ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical zonation of sulfides; (5) The Cu/(Pb Zn Cu) ratio of the large and thick Pb Zn Cu orebodies gradually decreases from bottom to top; and (6) barite is interbedded with pyrites and sometimes with sphalerite. However, some characteristics such as the Co/Ni radio of the pyrites, the volcanism, for example, of the Langshan-Zha‘ertalshan metallogenic belt, are different from those of the typical SEDEX deposits of the world. The meta-basic volcanic rock in Huogeqi, the sodic bimodal volcanic rocks in the Dongshengmiao and potassic bimodal-volcanic rocks with blastoporphyfitic and blasto-glomeroporphyritic texture as well as blasto-amygdaloidal structure in the Tanyaokou deposits have been discovered in the only ore-bearing second formation of the Langshan Group in the past 10 years. The metallogeny of some deposits hosted in the Langshan Group is closely related to syn-sedimentary volcanism based on the following facts: most of the lead isotopes in sphalerite, galena, pyrite, pyrrhotite and chalcopyrite plot on both sides of the line for the mantle or between the lines for the mantle and lower crust in the lead isotope composition diagram; cobalt content of some pyrites samples is much higher than the nickel content (Co/Ni= 11.91-12.19). Some volcanic blocks and debris have been picked out from some pyritic and pyrrhotitic ores. All Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying metamorphic volcanic rocks in the only ore-bearing second formation. In the Jiashengpan deposit that lacks syn-sedimentary volcanic rocks in the host succession only Pb and Zn ores occur without Cu ore, but in the Dongshengmiao, Tanyaokou and Huogeqi deposits with syn-sedimentary volcanic rocks in the host succession Cu ores occur. This indicates a relatively higher ore-forming temperature. The process of synsedimentary volcanic eruption directly supplied some ore-forming elements, and resulted in secular geothermal anomaly favorable for the circulation of a submarine convective hydrothermal system, which accounts for the precipitation of deep mineralizing fluids exhaling into anoxidic basins along the syn-sedimentary fault system in the Langshan-Zha‘ertai rift. The Dongshengmiao, Tanyaokou, and Huogeqi deposits hosted in the Langshan Group appear to be a transitional type of mineral deposit between SEDEX and VMS-types but with a bias towards SEDEX, while the Jiashengpan deposit hosted in the Zha‘ertai Group is of a characteristic SEDEX type. This evidence, together with other new discoveries of Mesoproterozoic volcanic rocks and the features of lithogeny and metallogeny of the Bayun Obo deposit in the neighborhood emphasize the diversity, complexity and uniqueness of the Mesoproterozoic Langshan-Zha‘ertal-Bayun Obo ore belt. 相似文献
20.
《International Geology Review》2012,54(4):287-306
In the Sierras Pampeanas of San Luis, Argentina, Late Tertiary volcanic rocks extend along a 80-km NW-SE-trending belt, between La Carolina and Sierra del Morro. Several gold deposits, among which those in the western end of the belt are better known, are genetically related to the volcanic rocks, formed during a volcanic episode that occurred between 9.5 Ma and 1.9 Ma. Located 600 km from the Peru-Chile trench, the volcanic belt represents the easternmost and youngest mineralized magmatic manifestation associated with the shallowing of the Nazca plate in the flat-slab Andean segment extending from 28° to 33° S Lat. The volcanic complex includes lavas and volcaniclastic rocks. Small-volume lavas were emplaced as domes, flows, and dikes. Pyroclastic deposits are associated with them in certain areas, such as at La Carolina, Cerro Tiporco, and Sierra del Morro. At La Carolina, phreatomagmatic breccias and base-surge deposits define a maar-diatreme volcanic setting. At Cerro Tiporco and Sierra del Morro, the volcaniclastic units are related to the formation of calderas. Mesosilicic magmas (SiO2 = 59% to 68%) belong to normal to high-K calc-alkaline and shoshonitic magma types. At both local and regional scales, K enrichment accompanies progressively decreasing age. Although the volcanic rocks differ from the typical Andean series, some geochemical features, such as Ta and Ti depletion, high large-ion-lithophile-element (LILE) contents, and arc-like Ba/La and La/Ta ratios, indicate an arc signature. In the La Carolina zone, the most important mineralization is the La Carolina volcanic-hosted, low-sulfidation, epithermal gold deposit. Here, several gold and base-metal-bearing epithermal veins cut basement rocks. In the Canada Honda district, the most important mineral deposits are the Diente Verde gold-rich porphyry copper deposit and low-sulfidation epithermal gold and base-metal veins hosted by both basement and coeval volcanics. There is no strong evidence of gold-bearing mineral deposits on the eastern side of the volcanic belt. However, there are hydrothermal alteration zones at Cerros del Rosario and El Morro as well as traces of gold at the Santa Isabel calcareous onyx deposit and inside the Sierra del Morro caldera. In addition, favorable volcanic structures, such as the calderas at Tiporco, Cerro Lomita, and El Morro, make the eastern side of the belt an interesting target for mineral exploration. 相似文献