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1.
A particular variety of volcanogenic country rock breccia is described; a contact breccia that has been identified at Venetia, River Ranch and Wimbledon kimberlite pipes. The contact breccia is clast supported with no juvenile kimberlite component, has tightly packed, angular fragments (with occasional rounding of smaller particles), and has a shear-fabric dipping towards the center of each kimberlite pipe or volcanic event. Clasts have preferred orientations parallel to the fabric. Photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis. The fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism. Clast size distribution fractal dimensions in the coarser particles (greater than circa 3 cm) range from greater than 3 for nonsheared breccia, down to circa 2.3 for the sheared breccia. Breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high-energy explosions, followed by collapse and abrasion by subsequently gravity-induced rockmass subsidence. All studied contact breccias produced a distinctive fractal signature in the finer particles (<3 cm) of circa 1.6 that can be explained by a comminution fragmentation process in that particular particle size range. It is suggested that these subsidence breccias require a substantial volume deficit at depth within the volcanic pipe in order to explain their origin and size. The methodology used in this study could be used to characterize any other volcanic breccia and further model their origins. 相似文献
2.
江西九瑞矿集区硅质断裂磨砾岩带的厘定及其成岩成矿意义 总被引：4，自引：4，他引：0
在九瑞矿集区研究叠合断裂和叠加成矿作用的基础上,我们进一步详细研究了出露在洋鸡山-丁家山-望夫山一线的硅质角砾岩,指出它们不是原先认为的石炭系沉积硅质岩,而应属于一种断裂磨砾岩,并深入探究其形成过程及与成矿之关系.断裂磨砾岩是断裂分带结构成熟的标志之一,多在剪切作用和热液作用下,断裂岩石经硅化-破裂-碎裂-粉碎-研磨,形成具有一定圆度和球度,大小差异较大的磨砾或磨粒,且又会反复的集结-破碎,不断拓宽断裂构造形成磨砾-角砾岩带.本文研究的断裂磨砾岩,呈北东向展布,延长达十几千米.成分上以硅化角砾岩为主,SiO2含量一般大于90％,石英颗粒由隐晶到显晶.一些角砾岩中含Fe2O3较高,有可能是原先的硫化物经氧化形成的褐铁矿.本区洋鸡山-丁家山-望夫山一线产出的断裂磨砾-角砾岩带,很可能是燕山期构造-岩浆-成矿事件的产物.在城门山和武山铜矿,我们之前的工作己发现存在产于泥盆系五通组和石炭系黄龙组层滑构造体系中的黄铁矿角砾岩,则有可能属于海西期同生断裂活动的产物.因此,这些不同的角砾岩具有多阶段活动和叠加成矿的特征.本文还进一步指出,九瑞地区其他层位(如泥盆系与志留系之间、志留系与奥陶系之间)发育的层滑构造体系和断裂角砾岩及热液蚀变岩,也很可能是成矿有利部位,值得今后找矿工作的关注. 相似文献
3.
The unique combination of its large size (250-300 km diameter), deep levels of erosion (>7 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impact-generated melt rock occur in the Vredefort Structure: the Vredefort Granophyre - impact melt rock - and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome - the central uplift of the impact structure - contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the non-genetic term “pseudotachylitic breccia” has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift.In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U-Pb single zircon and Ar-Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites all formed at 2020±5 Ma - the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits. 相似文献
4.
Abundant and regionally unique dolostone lithoclast breccias occur throughout the shallow-marine, Lower to Middle Ordovician Pogonip Group in the Nopah Range and adjacent ranges in eastern California and southern Nevada. Breccia bodies display sharply cross-cutting relationships with host dolostone bedrock stratigraphy. They also show stratigraphic variability in size, shape and dolostone clast composition, but similarity in breccia matrix composition and framework texture and fabric. These characteristics are consistent with a palaeokarst origin. Upsection changes in breccia clast lithology as well as multiple occurrences of associated quartz sand-filled grikes (solution-widened fissures) indicate multiple episodes of carbonate platform exposure and karstification. Repeated karstification is also indicated by stratiform bodies of quartz sand and thin terra rossa palaeosols that locally truncate breccias and grike systems, thus bracketing karstified exposure surfaces. Facies successions and stacking patterns between recognized exposure surfaces are developed as transgressive–regressive cycles and thus show depositional sequence architecture. Hence, these breccias and other associated palaeokarst features are related to a succession of disconformities that provide a sequence-stratigraphic framework for assessing Ordovician relative sea-level history of the south-western Cordilleran margin of Laurentia. 相似文献
5.
广西桂林庙头上泥盆统融县组台地边缘相具鸟眼或溶孔砂屑生屑藻粘结灰岩中发育了大量灰岩脉，灰岩脉呈岩墙状近直立地切过围岩，露头上可见连续垂直延伸长度达十余米，脉内仍是上泥盆统融县组灰岩组分，仅比围岩稍晚或同时代的：角砾状退白云石化藻粘结灰岩、角砾状藻粘结灰岩（快速堆积，角砾多数来自围岩）、细鲕粒颗粒灰岩、含钙球砂屑粒泥灰岩、藻砂屑腹足泥粒灰岩（正常沉积或液化变形软沉积流，均高于围岩层位）及大量栉壳状方解石脉（张性环境）等，灰岩脉内可见软沉积物变形痕迹及液化脉（地震液化），晚期灰岩脉可穿插早期灰岩脉；在其西侧同层位共生有一套楔状（地裂缝）、不规则状角砾灰岩体，角砾棱角状，大小不等，成分是灰色中厚层角砾状藻粘结灰岩、含钙球砂屑粒泥灰岩等。笔者等注意到灰岩脉均出露在北东向正断层的下盘（东侧），而角砾灰岩体则均出露在该断层的上盘（西侧），事实上，灰岩脉及角砾灰岩体分布走向与北东向断层走向一致，角砾灰岩体中的角砾成分显示其来自较灰岩脉围岩稍高层位，与灰岩脉内充填岩性相近，因此，是该断层控制了当时的沉积，此断层是晚泥盆世台地边缘同沉积正断层；灰岩脉是与断层伴生的张性裂隙被围岩角砾或稍晚时段的沉积物充填；角砾灰岩体是同沉积正断层形成的断层崖崩落角砾岩，指示一个消失殆尽的晚泥盆世碳酸盐岩同沉积正断层陡崖，因而，桂林台地，至少西段台地边缘，是与右江各孤立台地边缘一样——在地质图上应标示为同沉积正断层边界。其构造意义是：庙头地区的灰岩脉、角砾灰岩体及同沉积断层是桂林台地对晚泥盆世构造伸展作用的响应，从而说明广西晚古生代的板块拉张，不是从二叠纪才开始的，而是至少从晚泥盆世就开始了；晚古生代至早三叠世持续的孤立台地与深水盆地相间的古地理格局，是在晚泥盆世就奠定了基础。 相似文献
6.
The Stonepark Prospect is located in County Limerick, south-central Ireland. Multiple zones of Zn–Pb mineralisation have been identified at Stonepark and these are approximately 5 km west of the Pallas Green Prospect. At Stonepark, the sulphide bodies are hosted within the Waulsortian Limestone and closely resemble other Irish-type deposits. The mineralisation is composed of pyrite-marcasite, sphalerite and galena with gangue Fe-dolomite and calcite cements. A key difference at Stonepark is the presence of Chadian-aged volcanic rocks (Knockroe Volcanics) that intrude into and overlie the Waulsortian Limestone. Subsequent hydrothermal brecciation of the Waulsortian Limestone and Knockroe intrusions resulted in the formation of tabular polymict breccia bodies containing mixed carbonate and clasts of intrusive rocks. These have then been overprinted by massive sulphide mineralisation. Further syn-mineralisation brecciation has overprinted the earlier breccias. Drilling has demonstrated a spatial relationship between the volume of intrusive rocks (dykes and polymict breccias) and Zn–Pb mineralisation. This association suggests that the intrusive rocks provided a mechanism for the introduction of the mineralising fluids into the breccia bodies. This is significant as to date no large controlling fault has been identified, as is seen at other Irish-type deposits. Further work is required to understand the alteration process of the intrusive rocks and how this may relate to the mineralising process. 相似文献
7.
The Polaris deposit, located on Little Cornwallis Island in the Canadian Arctic, was a Mississippi Valley-type Zn–Pb deposit hosted by brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Mapping indicates that strike-slip faults on the east side of the Polaris deposit were active during the last stage of the Late Devonian Ellesmerian Orogeny. Polaris is on a jog in the north-oriented, Early Devonian Boothia fault system and was the site of localized extension during south-directed Late Devonian Ellesmerian compression. This structural setting elsewhere in the district may be prospective for Zn–Pb mineralisation.Ore fluids rising in the Late Devonian interacted with the host rock causing dissolution, brecciation and collapse. Carbonate beds are thinned, indicating widespread removal of carbonate material. Five breccia types (crackle, pseudo, cobble, mega and collapse) are present in the vicinity of the deposit. Crackle breccia is preserved around the periphery of the deposit and is indicative of structural dilation or the early stages of mineralisation. Dolomite and pseudobreccias are also preserved around, and extend beyond, the periphery of the deposit. These are considered an early stage of alteration directly related to the mineralising fluids and could act as a vector to the centre of the mineralising system where collapse, mega and cobble breccias occur intimately with massive mineralisation. 相似文献
8.
《矿物学报》2013,(Z1)
The northern extension of the Semailophiolite in the UAE is dominated by tectonic mantle peridotites and gabbros. The peridotites are mostly harzburgites with little lherzolites, and contain various intermittentveins, dikes and pods. These veins, dikes and pods can be divided into three rock categories: the ultramafic group that includes dunites, chromitites and pyroxenites, the granitic dikes, and carbonate veins. Occurrences of these bodies are explicit and ubiquitous, but their distribution throughout the sequence is not consistent. Pyroxenites, chromitites, and granites occur in the upper half of the sequence, but are more pronounced and dominant close to the transition zone. Dunites pods and carbonate veins can be found at different stratigraphic levels. 相似文献
9.
金顶铅锌矿床是一个以沉积岩(灰岩与砂岩)为寄主岩石的超大型铅锌矿床。本文在对矿区构造和矿床基本特点研究的基础上,重点阐述了两种不同类型矿体(砂岩型和灰岩型)的构造类型、矿石显微构造及矿石组构的特点。本文研究表明,极其发育的断裂构造系统是兰坪盆地地质构造发育的基本特点,在盆地边缘、盆地内部普遍发育了具有薄皮式逆冲断层和推覆构造组合特点的成矿前断裂,它们造成矿区内大规模地层倒转与堆叠现象;盆地中发育了由构造角砾岩与透镜体带构成、宽度达近3km的NE-NNE向成矿期中轴断裂,它控制着矿体或矿床的发育与分布;近SN方向的成矿后地堑构造对于矿床具有破坏作用,并使之剥露于地表。金顶铅锌矿床主要有两种重要的矿床类型,即板状或层状砂岩型矿床和角砾状灰岩型矿床,它们分别位于白垩系钙质长石石英砂岩与三叠系沥青质灰岩构成的两个逆冲席体内。以凤凰山矿区为典型,金顶铅锌矿床具有3层结构:上部的三叠系白云质灰岩岩盖,中部的砂岩型矿体和下部的灰岩型矿体,三者之间被两条作为圈闭存在的重要逆冲断层带间隔。砂岩型矿体与灰岩型矿体之间由几个角砾岩筒连接,作为成矿时期的矿液运移通道存在。破裂与微破裂构造、块状构造、对生脉状构造及假晶构造等是最常见的显微构造型式,它们的存在揭示出破裂与碎裂、充填和交代作用过程在铅锌矿化过程中均起着重要的作用。金顶铅锌矿床的形成与就位,直接受构造作用控制。成矿前的逆冲推覆作用为高压成矿流体的形成以及成矿圈闭的发育创造了条件,成矿期中轴断裂的活动性促使成矿流体向上运移,而灰岩与砂岩的物理性质(包括孔隙度/渗透率以及力学属性)则制约了两种不同类型矿体的出现。致密的灰岩渗透率较低,含矿流体加热了灰岩并使之软化,同时高流体压力使得围岩易碎,形成由早期到晚期的细角砾岩、粗角砾岩和破裂,成矿物质同时沉淀结晶,胶结围岩角砾形成角砾型灰岩矿体。高压流体通过流体通道进入低孔隙度的砂岩后,便迅速弥散于其中,沿着颗粒边界或裂隙充填,并交代岩石中的钾长石碎屑及方解石胶结物,形成块状的砂岩型矿体。 相似文献
10.
Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites,French Pyrenees 总被引：1，自引：0，他引：1
In the Pyrenees, the lherzolites nowhere occur as continuous units. Rather, they always outcrop as restricted bodies, never more than 3 km wide, scattered across Mesozoic sedimentary units along the North Pyrenean Fault. We report the results of a detailed analysis of the geological setting of the Lherz massif (central Pyrenees), the type‐locality of lherzolites and one of the most studied occurrences of mantle rocks worldwide. The Lherz body is only 1.5 km long and belongs to a series of ultramafic bodies of restricted size (a few metres to some hundreds of metres), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The clastic formations also include numerous layers of polymictic breccias reworking lherzolitic clasts. These layers are found far from any lherzolitic body, implying that lherzolitic clasts cannot derive from the in situ fragmentation of an ultramafic body alone, but might also have been transported far away from their sources by sedimentary processes. A detailed analysis of the contacts between the Lherz ultramafic body and the surrounding limestones confirms that there is no fault contact and that sediments composed of ultramafic material have been emplaced into fissures within the brecciated carapace of the peridotites. These observations bear important constraints for the mode of emplacement of the lherzolite bodies. We infer that mantle exhumation may have occurred during Albian strike‐slip deformation linked to the rotation of Iberia along the proto‐North Pyrenean Fault. 相似文献
11.
黑龙江争光金矿床隐爆角砾岩特征及其地质意义 总被引：2，自引：0，他引：2
黑龙江争光金矿床位于大兴安岭东北缘的多宝山矿集区,矿体主要呈脉状产于燕山期闪长岩体与中奥陶统多宝山组的内外接触带上,受断裂构造控制明显。在野外地质调查过程中,发现了3个隐爆角砾岩体（J-1、J-2、J-3）,角砾岩体从中心向外侧有较显著的岩石类型分带,中心为热液隐爆角砾岩,向外角砾增大,逐渐过渡为震碎角砾岩。根据胶结物的不同和穿插关系可将其分为四期,最早期为硅质胶结,中期为绿泥石胶结,其次为矿质胶结,最后为灰黑色安山质胶结,其中第二期和第三期隐爆作用与矿化关系密切。通过对隐爆角砾岩地质特征的研究,结果表明在Ⅱ号矿带绿泥石胶结隐爆角砾岩体深部具有寻找与矿质胶结隐爆角砾岩有关金铜矿化体的潜力,这对争光金矿床深部及外围找矿具有一定的指导意义。 相似文献
12.
以野外观察描述为手段,系统研究了碳酸盐岩断裂变形机制的影响因素及断裂带结构演化过程,剖析了碳酸盐岩地层中断裂带结构与流体运移的关系。研究表明,影响碳酸盐岩内断裂变形机制的因素包括岩性、孔隙度、变形深度、温度、胶结作用、先存裂缝等,控制断裂带结构形成的因素包括滑动位移和破裂模式等。低孔隙度碳酸盐岩以裂缝发育为主,高孔隙度碳酸盐岩变形早期产生变形带,带内裂缝联接逐渐发育成断层带。随着埋藏深度的增加,断裂带结构不同：埋藏深度小于3 km,断层核主要发育无内聚力的断层角砾岩和断层泥;埋藏深度大于3 km,断层核普遍发育有内聚力的断层角砾岩和碎裂岩,破碎带发育多种成因的裂缝。随着位移的增加,破裂模式从早期的破裂作用变为后期的碎裂作用,最终形成碎裂流。断裂带演化是一个四维过程,断层核和破碎带发育情况直接影响断层对油气的运移和封闭的作用。断裂变形机制、断裂带内部结构以及与流体运移关系的研究,都可为封闭性提供重要的理论依据。 相似文献
13.
Pyroclastic rocks: another manifestation of ultramafic volcanism on Gorgona Island,Colombia 总被引：5，自引：0，他引：5
Tertiary ultramafic volcanism on Gorgona Island, Colombia, is manifested not only by komatiite flows, but also by a more voluminous sequence of tuff breccias, which is cut by comagmatic picrite dikes. The ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules. The entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst-laden picritic magma. MgO contents of tuff breccias and picrite dikes range from 21 to 27 wt%. Relative to nearby komatiite flows, these rocks are MgO-rich, and FeO-, TiO2- and Ni-poor. HREE concentrations are very low (N<0.2), indicating derivation from a more highly depleted mantle source region. Nd isotopic data suggest a cogenetic relationship between the picritic magma that formed the tuff breccia and associated dikes and that which produced the komatiite flows. Nevertheless Pb isotopic data as well as whole rock geochemistry preclude such a connection, either due to olivine fractionation/accumulation or to different degrees of partial melting. These ultramafic rock types crystallized from magmas which most likely were extracted from distinct mantle source regions. 相似文献
14.
The intramontane basins of the Betic Cordilleras (SE Spain) formed subsequent to the main phase of orogenic deformation during the middle Miocene in a close genetic relation to the Trans-Alboran Shear zone. Left lateral movements along a local branch (Carboneras fault zone, CF; strike NE–SW) of this zone played a major role in controlling the formation and dynamics of the Nijar-Carboneras Basin. To the south of the fault, a major phase of strike-slip faulting is recorded during the late Tortonian. The expression of this event is the Brèche Rouge de Carboneras (BRC), which seals a deep denudational surface on top of dislocated fault blocks formed by volcanics of the Cabo de Gata complex and early Tortonian shallow marine calcarenite. The sedimentary facies of this widely distributed unit in the Carboneras-Subbasin mirror the submarine topography and the distribution of the fault zones. Along strike-slip fault zones, autoclastic breccias and neptunian dikes preferentially oriented NW–SE and NE–SW occur, which are interpreted to represent the near-surface expression of the faults. Red limestone forms the groundmass of the autoclastic breccia and infills of neptunian dikes, which exhibit multiple phases of opening of fissures, gravitational sedimentary infill, lithification, and renewed creation of cracks. Steep relief, probably along fault scarps, was mantled by epiclastic volcanic conglomerate with a red carbonate matrix. Well-lithified coarse skeletal limestone rich in planktonic foraminifera formed pavements along sediment starved rocky surfaces in deep water. Laterally, within topographic depressions, the pavement limestone grades into thick accumulations of skeletal rudstone composed of fragmented azooxanthellate corals and stylasterid hydrozoans, which were concentrated by powerful bottom currents and gravitiy flows. Within the shallow water zone of dip slope ramps, cross-bedded calcarenite and calcirudite formed. Based on textures, fabrics and biota, rocks of the BRC were grouped into nine genetic lithofacies which document cryptic, deep-aphotic and shallow-photic environments typical of a sediment starved extensional basin. 相似文献
15.
湖南石门界牌峪地区雄黄矿床中伴有各类矿化角砾岩。从其岩石学、岩石化学、地球化学的特征来看，它是一种由热泉气体及部分物质在封闭还原条件下产生急剧爆发形成的特殊的角砾岩群。主要成因类型有“侵入状”角砾岩、爆破角砾岩、岩溶角砾岩和崩塌角砾岩。常见多种角砾岩叠加，它们是一种极好的控矿容矿载体，具有重要的找矿意义。 相似文献
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《International Geology Review》2012,54(10):893-909
Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 106 metric tons of Cu) are prominent, if not dominant, features in the three giant Andean Cu deposits of Los Pelambres, Los Bronces-Rio Blanco, and El Teniente of central Chile. At Los Bronces-Rio Blanco, over 90% of the >50x 106 metric tons of hypogene Cu occurs within the matrix of breccias and/or clasts and wall rock altered in association with the formation of these breccias, while at the other two deposits a lesser but still significant amount of Cu ore also is directly related to breccias. At both Los Pelambres and Los Bronces-Rio Blanco, high-grade (>0.5%) Cu occurs in zones of potassic alteration characterized by stockwork biotite veining and intense biotitization associated spatially, temporally, and genetically with biotite breccias. At Los Bronces-Rio Blanco, high-grade ore also occurs in younger tourmaline breccia pipes, emplaced both within and around the older central biotite breccia complex and potassic alteration zone after a period of uplift and erosion. Potassic alteration, sericitization, silicification, and mineralization of clasts in these tourmaline breccias occurred during their formation. At El Teniente, a significant amount of high-grade Cu ore also occurs in different tourmaline-rich breccias, including the marginal portion of the Braden breccia pipe and a related zone of quartz-sericite alteration that surrounds this pipe. Small, shallow, weakly mineralized or barren silicic porphyry intrusions occur in each of these three deposits, but their main role has been to redistribute rather than emplace mineralization. The mineralized breccia pipes in each deposit were emplaced into early and middle Miocene volcanic and plutonic rocks during the late Miocene and Pliocene by the expansion of boiling aqueous fluids. Fluid-inclusion and stable-isotope data indicate that the high-temperature, saline, metalrich fluids that produced the brecciation, precipitated the Cu ore in the matrix of the breccias, and generated the associated alteration and mineralization in clasts and wall rock were magmatic in origin. These magmatic fluids were not derived from the early and middle Miocene host plutons, which already were solidified at the time of breccia emplacement. Sr- and Nd-isotopic compositions of breccia-matrix minerals indicate that breccia-forming fluids were exsolved from magmas that were isotopically transitional between older volcanic and plutonic host rocks and younger silicic porphyry stocks, dikes, and extrusives. The fact that the roots of the breccias have not yet been encountered implies that these magmas cooled at depths >3 km to form plutons not yet exposed at the surface. The generation of the multiple mineralized breccias at each deposit occurred over a relatively short (but still significant) time period of 1 to 3 million years, during the final stages of existence of the long-lived (7gt;15 m.y.) Miocene magmatic belt in central Chile. The decline of magmatic activity in this belt was tectonically triggered, as subduction angle decreased in association with the subduction of the Juan Fernandez Ridge. This caused a decrease in the sub-arc magma supply and subsequently eastward migration of the magmatic arc, as well as crustal thickening, uplift, and erosion, which led to the superposition of younger and shallower alteration and mineralization events on older and deeper events in each deposit. The giant Cu deposits of central Chile cannot be explained by a static model in which their size is a function of the mass of a single pluton or the longevity of a single hydrothermal convection system. These deposits are giant because they were produced by multistage processes involving the formation, over a period of 1 to 3 million years, of multiple superimposed mineralized breccias and associated alteration zones resulting from the exsolution of metalrich magmatic fluids from independent magma batches cooling at depths >3 km. Neither an unusually large magma supply nor Andean magmas of unusually high Cu content is required to produce the sequence of multiple mineralization 相似文献
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Solution-collapse breccias of the Minkinfjellet and Wordiekammen Formations, Central Spitsbergen, Svalbard: a large gypsum palaeokarst system 总被引：3，自引：0，他引：3
Large volumes of carbonate breccia occur in the late syn-rift and early post-rift deposits of the Billefjorden Trough, Central Spitsbergen. Breccias are developed throughout the Moscovian Minkinfjellet Formation and in basal parts of the Kazimovian Wordiekammen Formation. Breccias can be divided into two categories: (i) thick, cross-cutting breccia-bodies up to 200 m thick that are associated with breccia pipes and large V-structures, and (ii) horizontal stratabound breccia beds interbedded with undeformed carbonate and siliciclastic rocks. The thick breccias occur in the central part of the basin, whereas the stratabound breccia beds have a much wider areal extent towards the basin margins. The breccias were formed by gravitational collapse into cavities formed by dissolution of gypsum and anhydrite beds in the Minkinfjellet Formation. Several dissolution fronts have been discovered, demonstrating the genetic relationship between dissolution of gypsum and brecciation. Textures and structures typical of collapse breccias such as inverse grading, a sharp flat base, breccia pipes (collapse dolines) and V-structures (cave roof collapse) are also observed. The breccias are cemented by calcite cements of pre-compaction, shallow burial origin. Primary fluid inclusions in the calcite are dominantly single phase containing fresh water (final melting points are ca 0 °C), suggesting that breccia diagenesis occurred in meteoric waters. Cathodoluminescence (CL) zoning of the cements shows a consistent pattern of three cement stages, but the abundance of each stage varies stratigraphically and laterally. δ18O values of breccia cements are more negative relative to marine limestones and meteoric cements developed in unbrecciated Minkinfjellet limestones. There is a clear relationship between δ18O values and the abundance of the different cement generations detected by CL. Paragenetically, later cements have lower δ18O values recording increased temperatures during their precipitation. Carbon isotope values of the cements are primarily rock-buffered although a weak trend towards more negative values with increasing burial depth is observed. The timing of gypsum dissolution and brecciation was most likely related to major intervals of exposure of the carbonate platform during Gzhelian and/or Asselian/Sakmarian times. These intervals of exposure occurred shortly after deposition of the brecciated units and before deep burial of the sediments. 相似文献
19.
Relatively few studies have examined fault rock microstructures in carbonates. Understanding fault core production helps predict the hydraulic behaviour of faults and the potential for reservoir compartmentalisation. Normal faults on Malta, ranging from <1 m to 90 m displacement, cut two carbonate lithofacies, micrite-dominated and grain-dominated carbonates, allowing the investigation of fault rock evolution with increasing displacement in differing lithofacies. Lithological heterogeneity leads to a variety of deformation mechanisms. Nine different fault rock types have been identified, with a range of deformation microstructures along an individual slip surface. The deformation style, and hence type of fault rock produced, is a function of host rock texture, specifically grain size and sorting, porosity and uniaxial compressive strength. Homogeneously fine-grained micrtie-dominated carbonates are characterised by dispersed deformation with large fracture networks that develop into breccias. Alternatively, this lithofacies is commonly recrystallised. In contrast, in the coarse-grained, heterogeneous grain-dominated carbonates the development of faulting is characterised by localised deformation, creating protocataclasite and cataclasite fault rocks. Cementation also occurs within some grain-dominated carbonates close to and on slip surfaces. Fault rock variation is a function of displacement as well as juxtaposed lithofacies. An increase in fault rock variability is observed at higher displacements, potentially creating a more transmissible fault, which opposes what may be expected in siliciclastic and crystalline faults. Significant heterogeneity in the fault rock types formed is likely to create variable permeability along fault-strike, potentially allowing across-fault fluid flow. However, areas with homogeneous fault rocks may generate barriers to fluid flow. 相似文献