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1.
塔河油田中下奥陶统碳酸盐岩储层在构造活动的作用下发育多期方解石脉体,为研究储层流体演化和油气成藏过程提供了重要素材.通过岩心观察、薄片鉴定、阴极发光和微区元素分析在塔河油田中下奥陶统碳酸盐岩储层中识别出3期裂缝方解石脉体,3期脉体在阴极发光颜色、稀土元素、碳氧锶同位素以及形成时间方面都存在差异.采用激光原位方解石U-Pb定年技术确定3期方解石脉形成的绝对年龄分别为466±11 Ma~458.6±8.8 Ma、329.7±1.6 Ma~308.1±4.1 Ma和249.3±2.6 Ma~220.5±7.3 Ma.第2期和第3期方解石成脉流体主要来源于深部地层成岩流体,并伴有成烃流体的影响.第3期方解石脉的成脉流体可能还有壳源锶的输入,导致其87Sr/86Sr比值高于第2期方解石脉.方解石脉体的绝对年龄可以用于约束多旋回叠合盆地断裂活动和油气成藏时间.在塔河油田中下奥陶统碳酸盐岩储层中识别的3期方解石脉体形成时间分别对应于加里东中期Ⅰ幕、海西中期Ⅰ~Ⅱ幕和海西晚—印支期构造运动造成的断裂活动时间.方解石脉体与烃类流体包裹体的共生关系指示中下奥陶统储层在海西中期Ⅱ幕发生过一期原油充注,在印支期之后又存在两期原油充注. 相似文献
2.
《地质科技情报》2021,40(5)
四川盆地东南部志留系油气成藏过程研究缺少成藏和破坏阶段的直接证据,丁山地区志留系石牛栏组储层内沥青和方解石脉体记录了储层内流体演化的信息,对研究油气藏油气成藏演化过程有重要的指示作用。运用显微岩相学、阴极发光、微区原位元素和流体包裹体分析方法,确定了古流体来源和活动时间,并恢复了甲烷包裹体古压力,结合烃源岩生烃演化史和地质特征揭示了石牛栏组油气成藏演化过程和主控因素。研究结果表明丁山地区石牛栏组储层内发育两期方解石脉体,原油充注发生在两期方解石脉形成之前。第一期方解石脉形成于距今约127 Ma,阴极发光颜色为褐红色,方解石脉形成于偏还原环境,成脉流体来源于同层位成岩流体。第二期方解石脉形成于距今约83 Ma,阴极发光颜色为褐黄色,方解石脉形成于偏氧化环境。龙马溪组烃源岩大量生油阶段在距今190～150 Ma之间,生成的石油充注到石牛栏组储层中并在距今约135 Ma裂解成气藏。甲烷包裹体捕获压力为51.77～57.46 MPa,储层压力系数为1.26～1.40。石牛栏组古气藏在燕山期以来,由于构造运动导致地层抬升剥蚀和断裂发育,促使天然气泄漏。 相似文献
3.
岩心观察鄂西黄陵背斜南翼下寒武统牛蹄塘组二段黑色页岩中发育构造挤压成因的高角度裂缝和顺层裂缝.选取不同产状的典型裂缝脉体样品,薄片镜下观察发现高角度裂缝中充填方解石脉,顺层裂缝中充填方解石-白云石复合脉.通过阴极发光、流体包裹体分析以及微区原位元素测定,开展了裂缝脉体成岩环境演化研究,分析了不同产状裂缝对页岩气层局部封闭性的影响.阴极发光结果表明,高角度方解石脉和顺层方解石-白云石复合脉均为2期流体活动形成.根据流体包裹体分析,高角度方解石脉早期成脉流体主要为变质较深的地层卤水,晚期成脉流体混入了低矿化度流体,而顺层方解石-白云石复合脉2期成脉流体均未受低矿化度流体的改造.利用不同阴极发光的裂缝脉体碳酸盐矿物的微区原位微量元素测定结果,对铁锰含量、稀土元素（REE）和氧化还原判别参数分析显示,高角度方解石脉成岩环境经历了由还原环境向氧化环境转化,而顺层方解石-白云石复合脉的成岩环境长期保持了较为还原的环境.因此认为,高角度裂缝对页岩气层的局部封闭条件可能产生破坏作用,从而造成页岩气一定程度的散失,而顺层裂缝对局部封闭条件的破坏作用相对有限,裂缝发育特征及其经历的成岩演化过程研究可为页岩气层自封闭和页岩气微观保存条件的评价提供依据. 相似文献
4.
碳酸盐矿物中的同期烃类包裹体共生盐水包裹体均一温度变化范围较大,导致采用流体包裹体均一温度结合储层埋藏史和热演化史确定的油气成藏时间具有多解性.以塔里木盆地塔河油田奥陶系碳酸盐岩油气藏为例,基于方解石脉体中发育的流体包裹体岩相学、荧光分析和显微测温,结合激光原位方解石U-Pb定年结果,提出利用同期烃类包裹体共生盐水包裹体最小均一温度确定油气成藏时间,并确定塔河油田奥陶系碳酸盐岩储层油气充注期次和时间.塔河油田奥陶系储层共存在4期油充注,第1期至第3期油充注时间分别与3期方解石脉体形成时间一致,第4期油充注发生于3期方解石脉形成之后.对发育原生烃类包裹体的方解石脉进行激光原位U-Pb同位素绝对定年,结果指示采用同期油包裹体共生盐水包裹体最小均一温度确定的油气充注时间与方解石脉形成时间一致,说明采用同期盐水包裹体最小均一温度确定的油气充注时间更可靠.运用同期油包裹体共生盐水包裹体最小均一温度得到,塔河地区奥陶系碳酸盐岩油气藏4期油气充注时间分别对应加里东、海西、印支和燕山构造运动时期. 相似文献
5.
霸县凹陷牛驼镇凸起潜山内幕储层发育且勘探程度低,通过牛驼镇凸起潜山内幕方解石脉体及围岩的岩石学、同位素、包裹体等特征的研究,分析了古流体来源、期次和演化特征,再结合现今地层水特征,总结了流体活动对油气成藏的影响。研究结果表明：研究区发育3种类型方解石脉体(Cal1、Cal2和Cal3),其中加里东运动早期形成的Cal1型脉体与围岩同位素特征相似,认为流体来源于海相围岩。加里东运动晚期形成的Cal2型脉体可分为2期,其中裂缝边部的早期脉体Cal2 1与围岩同位素特征也相似,流体也来源于围岩;位于中心的晚期脉体Cal2 2相对围岩具有δ13C相似、δ18O亏损和87Sr/86Sr富集的特征,流体包裹体具有较低均一温度(<50 ℃)和盐度(13wt%～35wt%NaCl),认为流体来源于大气水和围岩的混合。喜马拉雅运动Es3-Es4期形成的Cal3型脉体相对围岩具有δ13C和δ18O亏损、87Sr/86Sr富集的特征,流体包裹体具有高均一温度(142～210 ℃),认为流体来源于岩浆活动。这些特征表明研究区存在3次流体活动,流体分别来源于海相围岩、大气水和岩浆活动,没有烃类流体活动的证据。研究区现今地层水具有低矿化度(2～3 g/L)、高钠氯系数(1～12)的特征,与邻近霸县生油洼槽的古近系地层水有很大差异,说明油气的保存条件及其与霸县洼槽水动力联系均较差。综合古流体和现今流体特征,认为研究区潜山内幕的流体活动特征不利于油气的成藏。 相似文献
6.
为了查明川东南盆缘常压页岩气区五峰组页岩裂缝脉体记录的古温度及古压力特征,揭示常压页岩气区储层压力降低的主要影响因素和页岩气散失机理.以松坎地区五峰组黑色碳质页岩裂缝脉体内的流体包裹体为主要研究对象,通过显微镜光学观察、阴极发光测试、包裹体测温和激光拉曼分析,结合埋藏-生烃-热史模拟,对裂缝脉体中的气-液两相盐水包裹体和高密度甲烷包裹体进行了系统研究.研究表明:松坎地区五峰组裂缝中主要存在着两期脉体充填,M1裂缝脉体形成时间距今约103~86 Ma,脉体较宽、阴极发光颜色为橘红色,其内甲烷包裹体捕获压力为82.6~91.5 MPa,反映燕山中期盆缘地区页岩气藏处在超压状态;M2裂缝脉体生长于M1裂缝脉体两侧,形成时间距今约90~72 Ma,脉体较窄、阴极发光颜色为暗红色,其内甲烷包裹体捕获压力为43.6~47.3 MPa.综合分析认为,川东南盆缘松坎地区五峰组在燕山期经历了由超压至常压的转变.燕山期以来的抬升剥蚀与褶皱变形导致地层内大量页岩气沿着滑脱层发生侧向运移和散失,地层压力快速下降.燕山晚期时,盆内稳定构造区页岩气藏仍处于超压状态,而盆缘地区已降至常压. 相似文献
7.
塔里木盆地热液流体活动及其对油气运移的影响 总被引：3，自引：0，他引：3
塔里木盆地奥陶系灰岩孔洞和裂缝中都可以见到大量的方解石充填,可以分为三种类型,即孔中方解石胶结物、CⅠ方解石脉和CⅡ方解石脉.CⅡ方解石脉具有较高的87Sr/86Sr比值,位于0.709103 ～0.710593之间,平均值为0.709538;较轻的碳氧同位素组成,其δ13CPDB值位于-5.67‰～-1.70‰之间,平均值为-2.95‰,δ18OPDB值位于-14.28‰～-7.88‰之间,平均值为-10.39‰.CⅡ方解石脉中的流体包裹体具有较高的均一温度,各样品平均值位于120.O～ 180.0℃之间.孔中方解石胶结物和CⅠ方解石脉在同位素组成和流体包裹体均一温度上较为一致,但都与CⅡ方解石脉有着显著的差别.综合比较各项分析测试结果,认为CⅡ方解石脉的形成与深部热液流体作用有关,而孔中方解石和CⅠ方解石脉则是从地层水中沉淀形成的.从热液流体中沉淀形成的CⅡ方解石脉中可见一定数量的油气包裹体,并且包裹体气相成分中除含有CO2外,还含有一定量的CH4和C2H6等有机组分.这些特征表明了热液流体在从深部向浅部活动过程中携带并促使了油气向浅部地层的运移.CⅡ方解石脉所具有的较轻的碳同位素组成是有机成因的CO2/CO32-在热液流体溶解携带油气时混入进了热液流体中的结果.热液流体主要通过降低原油粘度、减小油水界面张力来减小油气运移阻力,并能携带部分油气,从而促使油气沿断裂裂缝体系向浅部地层运移. 相似文献
8.
构造驱动大巴山前陆烃类流体排泄:含烃包裹体纤维状方解石脉证据 总被引：3，自引：0，他引：3
含烃包裹体纤维状方解石脉被认为是超高压下油气流体形成和排泄的标志。大巴山前陆构造带一些断裂和下古生界黑色泥岩和泥灰岩烃源岩微裂隙中分布有含烃包裹体纤维状方解石脉,成分分析表明其为低镁方解石。纤维状方解石脉δ13CVPDB和δ18OVPDB比围岩碳酸盐岩的明显变轻,前者δ13CVPDB和δ18OVPDB变化范围分别为-1.9%～-4.8‰和-8.4%～-12.8‰,后者分别为-1.7%～+3.1‰和-8.7%～-4.5‰,且δ13C与δ18O具有明显的线性关系,反映出纤维状方解石脉具有成岩有机流体与浅部流体混合的流体特征。纤维状方解石脉含有共生的固体沥青包裹体、含甲烷液相包裹体和气液二相盐水包裹体等多相态包裹体,其中沥青包裹体为油气运移的残余沥青。气液二相盐水包裹体均一温度主要位于140℃和196℃之间（峰值为179℃）,盐度较高（平均为9.7wt% NaCl）。分别应用盐水包裹体和甲烷包裹体等溶线P-T相图确定出含烃包裹体纤维状方解石脉形成的流体压力为150~200 MPa,属于异常超高压流体。地质和地球化学特征分析认为,大巴山含烃包裹体纤维状方解石脉不具有泥岩因压实成岩作用而形成的超高压流体特征。结合沉积和构造演化历史分析认为,印支碰撞造山运动和燕山前陆构造作用导致大巴山褶皱隆起并伴随天然气藏破坏和改造,挤压环境下的超高压构造应力驱动天然气流体排泄,大巴山前陆构造带含烃包裹体纤维状方解石脉就是超高压构造应力驱动天然气排泄的产物。 相似文献
9.
因古隆起周缘页岩生烃时间晚、热演化程度低,目前已成为中扬子地区油气勘探的重要领域.雪峰古隆起形成于加里东末期,经历了后期复杂的沉积-构造演化,白垩系覆盖区勘探方向尚未明确.以雪峰古隆起西侧沅麻盆地北部为主要研究对象,通过对不整合于寒武系页岩之上的白垩系底部方解石脉与寒武系页岩方解石脉地球化学和包裹体对比分析,研究了古流体形成环境、来源及其对寒武系生烃演化过程的指示.研究结果表明:（1）白垩系脉体碳氧同位素与围岩具有相似性,REE无明显的Eu异常,与寒武系脉体Eu强正异常对比鲜明,表明白垩系脉体主要来自白垩系,受寒武系页岩的影响弱;（2）白垩系方解石脉采样位置位于构造高位,但是包裹体类型显示以纯水溶液包裹体为主,形成于弱还原-弱氧化的环境,未见沥青和明显的纯甲烷包裹体,表明寒武系大规模生排烃在白垩系沉积之前;（3）以包裹体均一温度和Ro为制约,结合区域地质特征恢复了沅麻盆地寒武系两种类型埋藏史,即沅麻盆地北部草堂凹陷早期深埋藏,白垩纪以来不具二次生烃的条件;辰溪凹陷南部早期浅埋藏,白垩系以来Ro可能由1.2%升至2.5%,二次生烃潜力较大,是下一步勘探的有利区. 相似文献
10.
深部热流体活动与碳酸盐岩储层改造及油气成藏具有密切的关系.通过对塔里木盆地古城墟隆起奥陶系的11块样品的成岩观测和流体包裹体系统分析, 识别出3期古流体活动.结合埋藏史, 确定这3期流体活动的发生时间分别为: 第1期以第1世代高角度裂缝及网状裂缝充填方解石为代表, 推测与加里东晚期构造运动有关; 第2期为构造-热液白云岩化流体, 可能与塔里木盆地经历二叠纪末大规模的火山活动有关; 第3期以充填于孔、缝中央的晚期方解石为代表, 记录了晚期天然气充注事件, 发生于喜山期.第1期流体活动伴随的构造运动导致了早期充注的油气沥青化; 第2期流体活动对该区域的储层有建设性改造作用, 所形成的鹰山组内幕储层成为了区内重要的勘探目的层; 第3期流体活动记录了晚期天然气充注事件.因此, 工区应以寻找喜山晚期天然气藏为主要勘探目标. 相似文献
11.
Centimetre‐ to decimetre‐wide quartz+calcite veins in schistes lustrés from Alpine Corsica were formed during exhumation at 30–40 Ma following blueschist facies metamorphism. The δ18O and δ13C values of the veins overlap those of the host schistes lustrés, and the δ18O values of the veins are much higher than those of other rocks on Corsica. These data suggest that the vein‐forming fluids were derived from the schistes lustrés. Fluids were probably generated by reactions that broke down carpholite, lawsonite, chlorite and white mica at 300–350 °C during decompression between c. 1400 and 800 MPa. However, the δ18O values of the veins are locally several per mil higher than expected given those of their host rocks. The magnitude of oxygen isotope disequilibrium between the veins and the host rock is inversely proportional to the δ18O value of the host rock. Additionally, calcite in some schists is in isotopic equilibrium with calcite in adjacent veins, but not with the silicate fraction of the schists. Locally, the schists are calcite bearing only within 1–20 cm of the veins. The vein‐forming fluids may have been preferentially derived from calcite‐bearing, high‐δ18O rocks that are common within the schistes lustrés and that locally contain abundant (>15%) veins. If the fluids were unable to completely isotopically equilibrate with the rocks, due to relatively rapid flow at moderate temperatures or being confined to fractures, they could form veins with higher δ18O values than those of the surrounding rocks. Alteration of the host rocks was probably inhibited by isolation of the fluid in ‘quartz‐armoured’ veins. Overall, the veins represent a metre‐ to hectometre‐scale fluid‐flow system confined to within the schistes lustrés unit, with little input from external sources. This fluid‐flow system is one of several that operated in the western Alps during exhumation following high‐pressure metamorphism. 相似文献
12.
Metasomatic and Sr-isotopic changes, associated with formation of zoned alteration halos along hydrothermal veins, are documented for a gneiss from the Artenberg quarry near Steinach (Kinzigtal, Schwarzwald, SW Germany). Veins are postorogenic, SW-NE-oriented, and cut straight through metaquartzdioritic Variscan gneiss, where flow of low-temperature fluids (～100–200°C) caused adularia-quartz-sericite-type alteration. Fluid-rock interaction occurred nearly 50 Ma after Variscan metamorphism, as constrained by a Rb–Sr multimineral isochron for unaltered gneiss of 327.1?±?3.1 Ma, and by two independent ages of 279.2?±?3.1 Ma and 274?±?13 Ma, based on Rb–Sr systematics of late-stage quartz from the veins. In a profile from unaltered gneiss towards a vein, alteration-induced mineralogical changes correlate with metasomatic net addition of K, Rb, and Cl to the alteration zone, combined with net loss of Na, Ca, and Sr. Strontium isotopes give a more detailed insight into the fluid-rock interaction process. 87Sr/86Sr ratios in a profile across the alteration zone are incompatible with simple Sr leaching but reflect partial replacement of the rocks’ Sr by fluid-derived Sr, the isotopic composition of which varied with time. Early fluids, with high 87Sr/86Sr ratios compared to unaltered gneiss, evolved into fluids with somewhat lower ratios, and finally reached a second maximum in 87Sr/86Sr ratios. This Sr-isotopic fluid evolution is equally revealed by the mineral sequence of the vein mineralization. It appears that the compositional evolution of the fluids correlates with the sequence of mineral breakdown reactions in the gneissic host rock, and that the Sr-isotopic evolution of the fluids can be fully explained as the result of internal, progressive reaction of fluid with the local rocks. Results also show that the spatial distributions of Sr isotopes in metasomatic alteration zones may reflect the complex evolution of fluid-rock interaction systems, and ultimately constrain the factors controlling both fluid compositions and alteration patterns. 相似文献
13.
R. Lira M.F. Poklepovic M.J. Dorais H.A. Millone G.M. Gomez 《Journal of South American Earth Sciences》2009,28(3):263-276
A deep epithermal vein system hosted in Late Proterozoic to Cambrian granodiorite has been identified in the Sierra Norte de Córdoba, the easternmost range of the Sierras Pampeanas Orientales of Argentina. The vein swarm extends over an area of 3 km2 parallel to a mylonitic belt and formed in fractured granodiorite. Thicknesses of veins are less than 0.5 m and their visible strike length is less than 100 m. Veins are either barren or weakly mineralized in base-metal sulfides. Most veins have mineral associations dominated by calcite and quartz with lesser amounts of chlorite, sericite, pyrite, and minor illite. In other less exposed albite-rich, adularia-bearing veins, chalcopyrite, bornite, galena, sphalerite, chalcocite and covellite may occur. The widespread occurrence of bladed calcite without any petrographic or microthermometric evidence of boiling implies that this particular habit of calcite may also develop under sub-near boiling fluid conditions. Thermometric calculations based on fluid inclusion data, chlorite composition and oxygen isotopes in the quartz–calcite pair, constrain the formation of the system between 300 and 350 °C, at pressures between 42 and 64 MPa (1.5–2.3 km). Stable isotope data suggest that W/R interaction might have been the most probable mechanism of alteration, involving the participation of meteoric fluids; nevertheless, the metallic signature of some weakly mineralized veins as well as intermediate fluid inclusion salinities favor a magmatic input and a mixed origin for the fluids. Textures and mineral associations, as well as the absence of evidence of boiling in fluid inclusions, all suggest that the silica–carbonate vein system formed deeper than typically shallow Au and Ag-bearing boiling solutions. A 485 (±25) Ma lamprophyre dike crosscuts some of these veins locally producing metasomatic reactions and skarn formation, which constrains the age of the hydrothermal system to the Cambrian-Early Ordovician time span. 相似文献
14.
《Chemie der Erde / Geochemistry》2016,76(3):391-404
The quartz veins and pegmatites of the Sierra de Comechingones (Sierras de Córdoba, NE Argentina) belong to the Comechingones Pegmatite field (CPF). For the quartz veins and the zoned pegmatites related parental granites are missing. The country rock of the quartz veins are mylonitic augengneisses in granulite to upper amphibolite facies. Field relations, microscopy, cathodoluminescence, radiometric age data, fluid inclusion, chemical and isotopic composition and literature define the quartz veins as synorogenic formed during the high-temperature phase of the Famatinian (480–460 Ma) event. During the Famatinian up to the Achalian (382–366 Ma) event the synorogenic quartz veins were subjected to high temperature ductile deformation documented by folding, boudinage and finally brittle shearing. K-Ar ages of illite from the shear zones of about 166 Ma document the final cooling of the Sierras Pampeanas below 100 °C. The long lasting thermal and deformational history of the study area is reflected by very different populations of fluid inclusions in vein quartz with remarkably high contents of thermogenic hydrocarbons in the early-formed fluid inclusions. LA–ICP–MS analysis reveals very low lattice-bound trace element contents, i.e. high purity quartz. 相似文献
15.
裂缝脉体中蕴藏着天然裂缝演化与古流体充注活动等重要信息,对于深层页岩气保存条件也有指示意义.针对泸州区块龙马溪组深层页岩裂缝脉体,综合运用光学薄片、阴极发光、流体包裹体及微区原位等分析手段,分析了裂缝脉体的发育特征,研究了成脉流体活动及其成岩环境演化,探讨了页岩气保存条件.研究发现,深层页岩裂缝脉体主要为石英与白云石或方解石组成的复合脉体,不同矿物之间表现出较复杂的切割关系,盐水包裹体与高密度甲烷包裹体大量发育,且裂缝脉体主要形成于还原性环境.整体看来,构造抬升背景下,深层页岩裂缝开始形成且处于不断开启或闭合过程,3期不同性质的古流体多次充注胶结,较封闭的成岩体系对深层页岩气的保存有利. 相似文献
16.
Johannes Glodny Alexander Kühn Håkon Austrheim 《Contributions to Mineralogy and Petrology》2008,156(1):27-48
Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general. 相似文献
17.
Origin and granite alteration effects of hydrothermal fluid: isotopic evidence from fluorite veins, Co. Galway, Ireland 总被引：1，自引：0，他引：1
The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway
Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene 40Ar-39Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal
fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite,
disturbing both its Rb-Sr and Sm-Nd isotopic systems. The 87Sr/86Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios
are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source
for more than 2% of the hydrothermal Sr. The initial 143Nd/144Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically
distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in
cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot,
dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous
limestone which then overlay the granite.
Received: 3 August 1995 / Accepted: 11 April 1996 相似文献