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1.
Breccias from the central uplift at the Kentland, Indiana impact structure have outcrop and microscopic characteristics that give insight into events that may occur in a carbonate-dominated sedimentary sequence in the moments following hypervelocity impact. Three distinct types of brecciated rock bodies — fault breccias, breccia lenses, and breccia dikes — suggest multiple mechanisms of fragmentation. The fault breccias occur along steeply dipping faults that coincide with compositional discontinuities in the stratigraphic succession. The breccia lenses and dikes are less localized in occurrence and show no systematic spatial distribution or orientation. The fault breccias and breccia lenses show no consistent cross-cutting relationships, but both are transected by the breccia dikes. Textural analysis reveals significant differences in particle size distributions for the different breccias. The fault breccias are typically monomict, coarsest and least uniform in grain size, and yield the highest power-law exponent (fractal dimension) in plots of particle size vs. frequency. The polymict dike filling is finest and most uniform in grain size, has the lowest power-law exponent, and is locally laminated and size-sorted. SEM images of the dike-filling breccia show that fragmentation occurred to the scale of microns. Material within the breccia lenses has textural characteristics intermediate between the other two types, but the irregular morphology of these bodies suggests a mechanism of formation different from that of either of the other breccia categories. The breccia lenses and dikes both have sub-mm-scale spheroidal vugs that may have been formed by carbon dioxide bubbles released during sudden devolatilization of the carbonate country rock. Collectively, these observations shed light on the processes that occur during the excavation and modification phases of crater formation in carbonate strata — heterogeneous, polyphase, multiscale deformation accomplished over a time interval of seconds. 相似文献
2.
The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Québec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining. 相似文献
3.
研究了隐爆角砾岩与断层角砾岩、火山爆破角砾岩的区别,隐爆岩筒上、中、下部隐爆角砾岩的区别,含矿隐爆角砾岩筒特征,以及与隐爆、成矿作用有关的次火山(超浅成)岩特征。 相似文献
4.
Formation of the fragments of the wall-rock during dyking is one of the important manifestations of instantaneous magmatic events. This process is well documented at shallower depths of Earth’s crust but not at deeper levels. In this paper the in situ xenoliths of host rock nepheline syenite within a micro-shonkinite dyke emplaced at mid-crustal depths is described and the fractal theory applied to evaluate origin of the xenoliths. The nepheline syenite xenoliths are angular to oval shaped and sub-millimetre to ~50 cm long. The xenoliths are matrix supported with clasts and matrix being in equal proportions. Partly detached wall-rock fragments indicate incipient xenolith formation, which suggested that the model fragmentation processes is solely due to dyke emplacement. Fractal analytical techniques including clast size distribution, boundary roughness fractal dimension and clast circularity was carried out. The fractal data suggests that hydraulic (tensile) fracturing is the main process of host rock brecciation. However, the clast size and shape are further affected by postfragmentation processes including shear and thermal fracturing, and chemical erosion. The study demonstrates that dyking in an isotropic medium produces fractal size distributions of host rock xenoliths; however, post-fragmentation processes modify original fractal size distributions. 相似文献
5.
Taking No.50 kimberlite pipe of Wafangdian diamond deposit in Liaoning Province as an example, the authors systematically analyzed its geological characteristics. Based on the petrogeochemical analysis of porphyry phlogopite kimberlite, breccia porphyry kimberlite with surrounding rocks and kimberlite tuff breccia, it is found that there are less ultrabasic components in carbonated kimberlite tuff breccia and more ultrabasic components in kimberlite tuff breccia mixed with steatitization, serpentinization and carbonation. The content of Cr, Ni and Ti is relatively lower in kimberlite tuff breccia, slightly higher in breccia porphyrg phlogopite kimberlite with surrounding rocks and the highest in porphyry phlogopite kimberlite and porphyry kimberlite. This deposit is mainly composed of breccia porphyry kimberlite with surrounding rocks and porphyry phlogopite kimberlite, followed by kimberlite tuff breccia, breccia porphyry phlogopite kimberlite with surrounding rocks and kimberlite breccia. Chromite bearing pyrope, chromite and moissanite are associated minerals of the diamond deposit. The kimberlite ore-bearing grade is high in the western part and low in the eastern part in the horizontal direction, while the kimberlite ore-bearing grade changes little in the vertical direction. Through the three-dimensional modeling, it is inferred that instead of the root phase, No.50 kimberlite pipe is the fault dislocation caused by the EW nappe structural force with the No.50-1 kimberlite body at the depth of 600 m in the eastern pipe. 相似文献
6.
以辽宁瓦房店金刚石矿床50号岩管为例,系统分析了该矿床的地质特征。通过对斑状富金云母金伯利岩、含围岩角砾斑状金伯利岩和金伯利凝灰角砾岩进行岩石地球化学分析发现: 碳酸盐化金伯利凝灰角砾岩超基性成分较少,滑石化、蛇纹石化及碳酸盐化混合金伯利凝灰角砾岩超基性成分较多; 铬、镍、钛在金伯利凝灰角砾岩中的含量较低,在含围岩角砾斑状金云母金伯利岩中的含量略高,在斑状富金云母金伯利岩和斑状金伯利岩中的含量最高。该矿床主要为含围岩角砾斑状金伯利岩和斑状富金云母金伯利岩,其次为金伯利凝灰角砾岩、含围岩角砾斑状金云母金伯利岩和含金伯利物质角砾岩。含铬镁铝榴石、铬铁矿和碳硅石是金刚石的伴生矿物。水平方向上,金伯利岩含矿品位西部较富,东部较贫; 垂直方向上,金伯利岩含矿品位变化较小。通过三维建模,推测50号岩管不是根部相,而是受EW向推覆构造作用影响发生的断层错位,在其东侧600 m深处存在50-1号金伯利岩体。 相似文献
7.
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
8.
The Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross‐cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain‐size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the ～70 m thick stratiform crater‐fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater‐fill are interpreted as rock‐fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub‐unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub‐unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small‐scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra‐basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub‐aerial settings. 相似文献
9.
火山碎屑物的粒度、粒形和分布特征蕴含着其形成机制和喷发的环境信息。基于镜泊湖地区蛤蟆塘火山的一个空落堆积剖面的野外地质和岩相学,以粒度分析和分形理论定量研究了火山碎屑物的粒度分布、粒形几何及其分形特征。蛤蟆塘火山空落碎屑粒度分布均为单峰式,由岩浆爆炸形成的空落浮岩粒度峰值较小,而由射气岩浆喷发形成的含细花岗岩碎屑夹层的碎屑粒度峰值较大。空落浮岩颗粒的类球度、长宽比和凸度都小于含细花岗岩碎屑夹层的数值,表明空落浮岩颗粒相对不规则的特点。利用多段幂律方法拟合了蛤蟆塘火山空落碎屑颗粒分布规律,发现空落浮岩颗粒存在四个幂律分布段（即对应四个分形维数）,这是由于岩浆初始破碎、火山通道内的二次破碎以及风力筛选作用等造成的;含细花岗岩碎屑夹层的碎屑分布有两个幂律分布段（对应两个明显不同的分形维数）,即浮岩和花岗岩碎屑的形成是因不同破碎机制造成的。 相似文献
10.
The Catoca kimberlite pipe is among the world’s largest primary diamond deposits. The Catoca volcanic edifice is only slightly
eroded. Kimberlitic rocks of various facies compose a crater of about 1 km in diameter and a diatreme. The structure of the
pipe and mining conditions of the deposit are complicated by intense intrapipe tectonic processes related to large-amplitude
subsidence. Based on geological data, we propose a structural model of the deposit and a paleovolcanological model of the
Catoca pipe formed during a full cycle beginning with a stage of active volcanism and completed by stages of gradually waning
volcanic activity and sedimentation. It is suggested that the banded tuffisitic kimberlite of the crater zone was deposited
at the stage of active volcanic eruption from specific pyroclastic suspension as a low-viscosity mixture of crystals and aqueous
sol rich in serpentine. 相似文献
11.
马来西亚沙捞越邦达、什兰江控矿角砾岩筒构造均为典型的浅成—半浅成角砾岩筒构造，角砾岩及控矿类型具有明显的垂向分带特点。邦达控矿角砾岩筒构造发育角砾岩筒上段的塌陷角砾岩，角砾岩筒剥蚀深度较浅，控制典型的高硫化型浅成低温金铜矿床，且其深部具有较大的找矿潜力。什兰江控矿角砾岩筒构造发育角砾岩筒下段的爆发角砾岩和中段的流化角砾岩，分别控制斑岩型金矿化和低硫化型浅成低温热液型金矿化，角砾岩筒剥蚀深度较大，对矿床的保存和找矿极为不利。 相似文献
12.
晋东北地区出露一系列与燕山期富碱的酸性、中酸性浅成相-次火山相有成因联系的爆破角砾岩筒。耿庄金银矿区隐爆角砾岩筒是其中较典型且研究较详的实例之一。它位于两条断裂交叉处,平面上呈似椭圆状,纵剖面上呈不对称的漏斗状,岩筒中心直径约为150m,延深600m以上。该岩筒由四种成因类型包括七种不同成分的角砾岩组成,它们在水平上大致呈环状分布。角砾岩筒的隐蔽爆破作用至少发生过两次。主要的矿化作用产生在第二次爆破作用之后,形成了一个含Pb、Zn、Cu等的多金属金银矿床。 相似文献
13.
从区域尺度和矿床尺度两个方面论述了斑岩铜矿系统的特点.区域尺度上：1）斑岩铜矿多呈矿带或成矿域出现,带内众多斑岩铜矿呈簇或组合呈线状产出,这是构造作用控制下不连续岩株呈线状侵入就位的表现; 2）主要产于俯冲作用形成的岛弧和陆缘环境,构造应力属挤压但与中等拉张作用也有关,最近的研究证实大陆碰撞造山带也是斑岩型矿床产出的重要环境;3）其形成是通过具氧化性,S饱和,富含金属的岩浆熔体侵入所致,岩浆侵入作用为成矿提供了物质来源; 4）围岩的物理性质以及化学组成对矿床的规模、品位以及矿化类型具有极强的控制作用,碳酸盐岩围岩主要赋存近源Cu-Au夕卡岩矿床,少量远程Zn-Pb或Au夕卡岩矿床,在夕卡岩前缘还形成交代型Cu和Zn-Pb-Ag±Au矿床.矿床尺度上：1）含矿斑岩与斑岩型矿床时空相依,成因相联,是斑岩铜矿重要的含矿母岩和金属-S的可能载体;2）火山角砾岩筒在深部与矿化体平行或斜交,其与围岩的接触带,一般也是富硫金成矿带的一部分;3）与矿化有关的斑岩成矿系统内的角砾岩主要有爆发角砾岩、侵入角砾岩、爆发侵入角砾岩、热液角砾岩和热液卵石脉;4）斑岩铜矿系统中的热液蚀变自下而上可分为不含矿的早期钠质-钙质蚀变→含矿的钾化→绿泥石化-绢云母化→绢云母化→高级泥化,热液蚀变互相套合,矿化互相叠加;5）岩帽是斑岩型热液-成矿活动-蚀变体系的重要组成部分,是重要的找矿标志. 相似文献
14.
Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 10 6 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 >50 x 10 6 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
15.
前人研究提出金顶超大型铅锌矿床发育出了多种角砾岩,其成因与盐底辟作用有关。文章对这些角砾岩进行了分形分析,确定了不同角砾岩的角砾形态分维值 Dr和粒径大小分维值 Ds,来进一步探讨角砾岩的形成过程。其中, Dr表征形态复杂性,可指示角砾迁移距离长短或后化学作用改造情况, Ds表征角砾破碎情况,指示形成角砾的能量大小。研究表明,金顶矿床中一部分角砾岩为蒸发盐或膏砂近原位底辟、破碎灰岩围岩形成,它们具有较小的 Dr和 Ds值。与这些角砾岩空间过渡的层状砂质胶结灰岩砾角砾岩(或称层状含灰岩角砾砂岩),为蒸发盐而非水流体携带灰岩角砾流出地表,并进入含水沉积系统后形成,角砾未经历水流体搬运、磨圆,具有较小的 Dr和较大的 Ds值。另外一部分角砾岩为蒸发盐和砂泥质底辟破碎围岩,并共同迁移了一定距离而形成,但总体未发生长距离的迁移,随后进入地表沉积系统中。上述部分角砾岩还经历了形成后的流体交代溶蚀作用,导致 Dr值变大。本研究显示,在盐底辟有关铅锌矿床内,赋矿围岩包含多种由不同底辟过程形成的角砾岩。 相似文献
16.
The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Québec, Canada, forming the major component of the Mineral Resource of Stornoway Diamond Corporation’s Renard Mine. Renard 2 is infilled with two distinct kimberlite units that exhibit Kimberley-type pyroclastic kimberlite and related textures. Hypabyssal kimberlite also occurs as smaller cross-cutting sheets and irregular intrusions. The units are distinguished by their rock textures, groundmass mineral assemblages, olivine macrocryst size distributions and replacement products, mantle and country rock xenolith contents, whole rock geochemical signatures, bulk densities and diamond grades. These differences are interpreted to reflect different mantle ascent and near-surface emplacement processes and are here demonstrated to be vertically continuous from present surface to over 1000 m depth. The distinctive petrological features together with sharp, steep and cross-cutting internal contact relationships, show that each unit was formed from a separate batch of mantle-derived kimberlite magma, and was completely solidified before subsequent emplacement of the later unit. The mineralogy and textures of the ultra-fine-grained interclast matrix are consistent with those described at numerous Kimberley-type pyroclastic kimberlite localities around the world and are interpreted to reflect rapid primary crystallization during emplacement of separate kimberlite magmatic systems. The units of fractured and brecciated country rock surrounding the main kimberlite pipe contain kimberlite-derived material including carbonate providing evidence of subsurface brecciation. Together these data show that Renard 2 represents the deeper parts of a Kimberley-type pyroclastic kimberlite pipe system and demonstrates that their diagnostic features result from magmatic crystallisation during subsurface volcanic emplacement processes. 相似文献
17.
The 613±6 Ma Anuri kimberlite is a pipelike body comprising two lobes with a combined surface area of approximately 4–5 ha. The pipe is infilled with two contrasting rock types: volcaniclastic kimberlite (VK) and, less common, hypabyssal kimberlite (HK). The HK is an archetypal kimberlite composed of macrocrysts of olivine, spinel, mica, rare eclogitic garnet and clinopyroxene with microphenocrysts of olivine and groundmass spinel, phlogopite, apatite and perovskite in a serpentine–calcite–phlogopite matrix. The Ba enrichment of phlogopite, the compositional trends of both primary spinel and phlogopite, as well as the composition of the mantle-derived xenocrysts, are also characteristic of kimberlite. The present-day country rocks are granitoids; however, the incorporation of sedimentary xenoliths in the HK shows that the Archean granitoid basement terrain, at least locally, was capped by younger Proterozoic sediments at the time of emplacement. The sediments have since been removed by erosion. HK is confined to the deeper eastern parts of the Anuri pipe. It is suggested that the HK was emplaced prior to the dominant VK as a separate phase of kimberlite. The HK must have ascended to high stratigraphic levels to allow incorporation of Proterozoic sediments as xenoliths. Most of the Anuri kimberlite is infilled with VK which is composed of variable proportions of juvenile lapilli, discrete olivine macrocrysts, country rock xenoliths and mantle-derived xenocrysts. It is proposed that the explosive breakthrough of a second batch of kimberlite magma formed the western lobe resulting in the excavation of the main pipe. Much of the resulting fragmented country rock material was deposited in extra crater deposits. Pyroclastic eruption(s) of kimberlite must have occurred to form the common juvenile lapilli present in the VKs. The VK is variable in nature and can be subdivided into four types: volcaniclastic kimberlite breccia, magmaclast-rich volcaniclastic kimberlite breccia, finer grained volcaniclastic kimberlite breccia and lithic-rich volcaniclastic kimberlite breccia. The variations between these subtypes reflect different depositional processes. These processes are difficult to determine but could include primary pyroclastic deposition and/or resedimentation. There is some similarity between Anuri and the Lac de Gras kimberlites, with variable types of VK forming the dominant infill of small, steep-sided pipes excavated into crystalline Archean basement and sedimentary cover. 相似文献
18.
在外荷载等因素作用下,粗粒土易发生颗粒破碎。对泥岩和砂岩颗粒进行了一系列的单颗粒破碎试验,基于尺寸效应和颗粒破碎分形模型,研究了单颗粒破碎强度、破碎能量及Weibull模量等与分形维数间的关系。利用PFC3D对单颗粒破碎过程进行分析,并与试验数据对比,验证了数值程序的可靠性;随后扩展到大颗粒粒径,分析了其单破碎强度及破碎能量。结果表明：在同类试验条件下,不同材料所得的分形维数是不同的;不同粒径砂岩的破碎程度均大于泥岩;单颗粒破碎强度具有明显的尺寸效应;单颗粒破碎强度与破碎能量均可通过分形维数与颗粒粒径预测得出;修正后的Weibull模量也可通过分形维数得出;数值模拟结果与试验结果及预测结果基本一致;大颗粒粒径单颗粒破碎强度模拟结果与预测结果基本一致,破碎能量稍有差异,需进一步试验验证。研究成果可为获取大粒径粗粒土的单颗粒强度与变形特性提供参考。 相似文献
19.
中酸性岩浆侵入体隐蔽爆破作用形成角砾岩体(筒)的同时于其中形成隐爆角砾岩型金矿床.侵入岩体规模一般较小,岩性主要为闪长岩类、石英(二长)斑岩类或正长岩类的中酸性岩石.角砾岩体在平面上呈近圆形或椭圆形,面积相对较小,在剖面上呈垂直地表的筒状,深部一般与中酸性侵入岩体相连.角砾的成分在浅部多为围岩角砾,往深部逐渐变为以花岗质角砾为主,胶结物主要是细岩屑和蚀变矿物.中酸性侵入体是矿床的成矿母岩,隐爆作用的发生及形成的隐爆角砾岩体(筒)是容矿构造,角砾岩体中的次级断裂是控矿构造,各因素对于此类型金矿床的形成起着重要的作用. 相似文献
20.
The majority of the Mio-Pleistocene monogenetic volcanoes in Western Hungary had, at least in their initial eruptive phase, phreatomagmatic eruptions that produced pyroclastic deposits rich in volcanic glass shards. Electron microprobe studies on fresh samples of volcanic glass from the pyroclastic deposits revealed a primarily tephritic composition. A shape analysis of the volcanic glass shards indicated that the fine-ash fractions of the phreatomagmatic material fragmented in a brittle fashion. In general, the glass shards are blocky in shape, low in vesicularity, and have a low-to-moderate microlite content. The glass-shape analysis was supplemented by fractal dimension calculations of the glassy pyroclasts. The fractal dimensions of the glass shards range from 1.06802 to 1.50088, with an average value of 1.237072876, based on fractal dimension tests of 157 individual glass shards. The average and mean fractal-dimension values are similar to the theoretical Koch-flake (snowflake) value of 1.262, suggesting that the majority of the glass shards are bulky with complex boundaries. Light-microscopy and backscattered-electron-microscopy images confirm that the glass shards are typically bulky with fractured and complex particle outlines and low vesicularity; features that are observed in glass shards generated in either a laboratory setting or naturally through the interaction of hot melt and external water. Textural features identified in fine- and coarse-ash particles suggest that they were formed by brittle fragmentation both at the hot melt-water interface (forming active particles) as well as in the vicinity of the interaction interface. Brittle fragmentation may have occurred when hot melt rapidly penetrated abundant water-rich zones causing the melt to cool rapidly and rupture explosively. 相似文献
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