西秦岭大水金矿床方解石Sm-Nd等时线年龄及其地质意义

大水金矿床位于西秦岭造山带南缘,矿床中金矿化与硅化、去碳酸盐化和方解石化密切相关。硅化主要表现为石英或玉髓交代(白云质)灰岩,在石英中常见灰岩或白云质灰岩的交代残余碎屑,在石英颗粒内或裂隙中见有自然金颗粒,说明硅化、去碳酸盐化和金矿化是同步的;成矿期的脉状-网脉状载金方解石主要分布于硅化-去碳酸盐化蚀变带的边缘或附近裂隙中,说明方解石的结晶与金矿化同步或稍晚。对该矿床成矿期的8件载金方解石样品进行了Sm-Nd同位素定年体系研究,方解石的147Sm/144Nd值变化于0.069 489~2.243 873,143Nd/144Nd值变化于0.511 673~0.514 369,获得Sm-Nd等时线年龄为(189.4±1.4)Ma,代表了大水金矿床的成矿年龄。大水金矿床的成矿时限与秦岭造山运动相关的碰撞型花岗闪长岩岩浆活动时限一致;此外,前人的稀土元素地球化学、黄铁矿Co/Ni比值及H-O同位素研究亦表明成矿物质来源与花岗闪长岩岩浆活动关系密切,因而本次获得的大水金矿床成矿年龄,在一定程度上为"大水金矿床的金成矿与花岗闪长岩岩浆活动存在成因联系"提供了年代学证据。     

康滇铜成矿带大红山IOCG矿床石英脉中黄铜矿Re-Os定年与意义

扬子西南缘康滇铜成矿带是我国重要的铁铜金矿产资源产区,分布许多小-大型铁氧化物-铜-金(IOCG)矿床,而本次研究的大红山矿床属康滇铜成矿带内最大的IOCG矿床之一。由石英脉中黄铜矿测年结果可以看出,Re-Os等时线年龄为1115±28 Ma,加权平均年龄为1113±14 Ma。因此,石英脉中黄铜矿矿化年龄为1115 Ma,结合扬子西南缘成岩成矿关系、背景研究,表明该黄铜矿为Grenvillian造山运动期碰撞后板内伸展环境产物。Re-Os同位素体系中高187Re/187Os比值、低含量的普通Os和高射性成因187Os的组成,指示成矿物质可能主要为壳源。     

贵州交犁-拉峨汞矿床方解石Sm-Nd同位素年代学

三丹汞矿带位于贵州省南部,是继湘黔汞矿带之外西南大面积低温成矿域内发现的又一重要汞矿带。因此,阐明带内汞矿的成矿物质来源、成矿时代及成矿动力学背景等问题,对深入探讨西南大面积低温成矿域的形成机理有着重要的作用。热液方解石是带内汞矿床的主要脉石矿物之一,作者尝试对三丹汞矿带中段交犁-拉峨汞矿床中的热液方解石进行Sm-Nd同位素测年,获得等时线年龄为(129±20)Ma,MSWD=0.21,εNd=-12.9,说明成矿作用主要发生在燕山晚期。该年龄的报道为深入探讨汞矿床以及西南大面积低温成矿域的形成和演化提供了重要的信息和依据。结合前人研究认为,西南低温成矿作用主要存在两期,早期为145~155 Ma的Sb成矿作用,晚期为120~135 Ma的Sb、Au、Hg、As成矿作用。围岩下奥陶统锅塘组灰岩样品的εNd值为-12.6,与方解石εNd值极为接近,指示该矿床成矿物质可能主要来自奥陶系赋矿海相碳酸盐岩。     

川西尔呷地吉Pb-Zn矿床方解石Sm-Nd等时线年龄及其地质意义

尔呷地吉中型铅锌矿床位于扬子地块西南缘川滇黔交界地带.矿体主要呈似层状、透镜状赋存于震旦系灯影组白云岩中,顶板为下寒武统筇竹寺组含炭质砂页岩.主要金属矿物为闪锌矿和方铅矿,Zn平均品位5.30%,Pb平均品位3.25%;主要非金属矿物为方解石和石英.矿石结构主要有自形晶结构、交代-侵蚀结构和固溶体分离结构,矿石构造主要有细脉-浸染状、块状和角砾状.矿床中铅锌矿化与硅化、沥青化和方解石化密切相关.对尔呷地吉Pb-Zn矿床成矿期8件与铅锌矿共生的方解石样品进行了Sm-Nd同位素体系研究,结果表明方解石的¹⁴⁷Nd/¹⁴⁴Nd值变化于0.058489~0.663246,¹⁴³Nd/¹⁴⁴Nd值变化于0.511852~0.512626,获得Sm-Nd等时线年龄为203±6 Ma （MSWD=1.2）,该年龄代表了尔呷地吉MVT Pb-Zn矿床的成矿年龄.尔呷地吉Pb-Zn矿床是典型的MVT Pb-Zn矿床,其矿床成矿作用与峨眉山玄武岩岩浆活动无关,与古特提斯洋闭合背景下的造山运动密切相关.     

四川拉拉铁氧化物-铜-金-铀矿床的铀成矿时代与成矿环境

位于扬子地块西南缘康滇地轴的拉拉矿床是我国典型的铁氧化物-铜-金-铀(IOCG)矿床。该矿床主要产出铁铜矿石,并共(伴)生有一定规模的铀矿化。本文在野外地质调查的基础上,综合利用微区X射线荧光光谱法(μ-XRF)、扫描电镜(SEM)、电子探针(EPMA)和激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)等测试技术,针对拉拉矿床中的铀矿化开展了详细的矿相学、地球化学和同位素年代学研究,并进一步探讨了铀成矿构造背景和成矿环境。研究表明,晶质铀矿、钛铀矿和铀石是拉拉矿床中主要的铀矿物,晶质铀矿同时产于铁铜矿石和铀矿化样品中,但钛铀矿和铀石仅产于铀矿化样品中。晶质铀矿主要呈立方体晶形产出,具有高U、Pb和Y,低Th、Ca、Si、Ti和P的地球化学组成,属于低钍晶质铀矿。U-Pb同位素定年表明拉拉矿床发育两期铀矿化,早期铀矿化产于铁铜矿石中,形成年龄为990±4 Ma,与早期铁铜矿化(～1.05 Ga)同步,晚期为后期叠加的热液铀矿化,以晶质铀矿-方解石-萤石脉的形式穿插于铁铜矿石中,形成年龄为～850 Ma,与晚期铁铜矿化(～850 Ma)同步,早晚两期矿化分别形成于大陆裂谷环环境和活动大...     

西秦岭甘南早子沟金锑矿床白云石Sm-Nd同位素地球化学及其意义

西秦岭造山带广泛发育脉状金锑矿化,但其成矿时代和成矿物质来源仍存在争议,影响了对该区矿床成因的深入认识。甘南早子沟超大型金锑矿床位于西秦岭造山带,矿体赋存于三叠系古浪堤组板岩和中酸性侵入岩内,包括蚀变岩型矿化和脉状矿化两种样式,是研究脉状金锑矿化成矿年代学和成矿物质来源的理想选区。本研究选择早子沟金锑矿床脉状矿化中与辉锑矿、金密切共生的白云石为研究对象,系统分析了其Sm、Nd同位素组成。结果表明,早子沟金锑矿床脉状矿化白云石Sm含量为0. 3221×10~(-6)～4. 918×10~(-6),Nd含量为0. 5667×10~(-6)～22. 76×10~(-6)。147Sm/144Nd和143Nd/144Nd分别为0. 1306～0. 2891和0. 511847～0. 512141,Sm-Nd假等时线年龄为282. 5±5. 1Ma。白云石εNd(0)为-15. 42～-9. 70,εNd(t)为-13. 66～-12. 19,tDM2为2. 03～2. 10Ga,变化范围相对较窄,表明样品源区相同。εNd(t)值与秦岭及其周边地区对比显示,早子沟金锑矿床成矿流体与英安斑岩围岩具有相同的Nd源区,可能来源于扬子板块北缘或南秦岭的古元古代结晶基底。白云石Sm/Nd分馏很弱且成矿流体混染了古元古代结晶基底物质可能是Sm-Nd假等时线产生的原因。这表明白云石Sm-Nd同位素体系可能无法为西秦岭广泛发育的脉状(金)锑矿床提供正确年代学信息。     

瑞典北部比约克达尔金矿床地质特征及成因

比约克达尔(Bjokdal)金矿位于瑞典北部古元古代斯科勒费特(Skellefte)地块的东部。成矿母岩为花岗岩类侵入体。该金矿的石英、白钨矿、方解石中的包体有3类。石英的δ18O值为8.3‰~14.3‰,2个全岩样品的δ18O值分别为9‰和10.4‰。Sm-Nd同位素数据得出矿石中白钨矿等时线年龄为1915±32Ma(MSWD=0.25),白钨矿的ε(Nd)(t=1.88Ga)的值为+1.1~+1.7。通过对矿化石英脉中流体包裹体的研究,将矿化分为3个主要阶段。第一阶段发育的石英脉伴随少量的白钨矿和硫化物,矿化作用的主要阶段发生了岩浆流体和地表水的混合,混合的同时伴随压力下降和p H值增加,第三阶段发生在130~220℃,这时产生了金-方解石-碲化物矿化,并对原生矿石进行了部分改造。     

贵州苗龙金锑矿床方解石微量元素、Sr-Nd同位素地球化学特征及其意义

苗龙金锑矿床位于贵州三都-丹寨金锑汞成矿带,矿体赋存于上寒武统三都组海相碳酸盐岩中。本文对该矿床的成矿期方解石微量元素含量、Sr-Nd同位素组成进行了研究。结果表明,该矿床成矿期方解石稀土元素总量变化范围大(5.98~139μg/g,平均值为43.7μg/g),在球粒陨石标准化稀土元素配分模式图中大多数方解石样品具有轻稀土富集特征((La/Yb)_N=1.84~9.18,(Gd/Yb)_N=2.55~6.14),少量样品表现为中稀土富集特征((La/Yb)_N=0.33~1.39,(Gd/Yb)_N=1.29~2.24)。该矿床成矿期方解石样品不具有Ce异常(δCe=0.97~1.19),但具有Eu异常(δEu=0.61~1.72),指示其形成于相对还原条件下。方解石样品的~(87)Sr/~(86)Sr值为0.7108~0.7144,平均值为0.7119,高于赋矿围岩的~(87)Sr/~(86)Sr值(0.7090),表明成矿流体相对富放射成因锶,其不可能主要由赋矿的三都组灰岩提供,而可能来自富放射成因锶的前寒武纪基底。Sm-Nd同位素研究表明,该矿床成矿期方解石样品可构筑两条Sm-Nd等时线,其对应的等时线年龄分别为273±14 Ma和272±43 Ma,但与地质背景不符,因此该次方解石Sm-Nd同位素定年等时线无地质意义。这种假等时线现象可能是由成矿流体初始~(143)Nd/~(144)Nd不均一造成的。方解石ε_(Nd)(0)(-14.02~-9.48)远小于0,指示成矿流体中的Sm和Nd来源于陆壳。     

华北太古代上地幔钕同位素组成、演化及对该区岩石圈地幔不均一性的制约

近年所获得的华北各地区太古代变质火山岩系9个较准确的Sm-Nd同位素年龄及初始Nd同位素比值表明,华北太古代(3500—2500Ma)上地幔都是亏损的,但在1000Ma范围內其ε_(Nd)值一直恒定在+3左右,与全球太古代亏损地幔Nd同位素演化规律一致。这表明华北太古代的构造体制有可能使大量地壳物质重新进入地幔从而保持ε_(Nd)值的恒定。如果华北大陆岩石圈地幔的主体是在太古代时从对流上地幔中分离出来的,利用该区太古代上地幔Nd同位素组成及其可能的sm/Nd值进行计算,可获得该区岩石圈非交代地幔应是与N型大洋中脊玄武岩(以下简称MORB)源类似或更亏损的高亏损地幔,其现代ε-(Nd)值最大变化范围在+7—+23之间。至今在华北我们尚未发现由这种高亏损非交代大陆岩石圈地幔产生的大陆玄武岩。如果某些大陆玄武岩可能产生于大陆岩石圈地幔,则其源区必定是经过地幔交代作用再富集了的地幔。     

扬子陆块庙湾蛇绿岩中橄榄岩的同位素年代学及其构造意义

扬子陆核崆岭高级变质地体内出露一套强变形的基性-超基性岩岩石组合, 主要呈似层状、透镜状分布于崆岭群中, 该套变基性-超基性岩组合对扬子陆块早期构造演化过程具有重要意义.通过同位素稀释法(isotope dilution thermal ionization mass spectrometry, 简称ID-TIMS)获得该套岩石组合中蛇纹石化方辉橄榄岩全岩Sm-Nd等时线年龄为1063±12Ma, 说明Sm-Nd同位素体系可用于对超低含量、发生强蚀变作用的超基性岩样品进行定年.样品Nd同位素组成相对均一(ε_Nd(t)值为6.90~7.32), 表明形成于封闭体系中, 其对应Nd同位素两阶段模式年龄为1.13~1.09Ga, 与形成年龄接近, 说明来自亏损软流圈地幔部分熔融.结合区域上已有的中元古代末期到新元古代早期构造岩浆事件研究, 认为在该时期扬子陆块可能由多个微陆块组成, 就扬子陆核而言, 其与扬子陆块西侧之间很可能存在分隔的大洋.      

Ferroan calcite replacement indicates former magnesian calcite skeletons

The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high-magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low-magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods. Using this criterion, an original low-magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high-magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high-magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty-seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes. Ferroan calcites contain up to 6 mol % FeCO₃ and up to 5 mol % MgCO₃. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0-03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite. Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate-reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ¹⁸C and δ¹⁸O values, and low Mg concentrations point more to a meteoric-phreatic origin of many ferroan calcite occurrences.     

Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO₃ (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg_0.03Ca_0.97CO₃ results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO₃ (K'_sp = 10?6.30).The surface layer precipitated on a synthetic Mg_0.08Ca_0.92CO₃ equilibrates finally with a carbonate more soluble than calcite (K'_sp = 10?5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Superstructures in ankerite and calcite

Two rhombohedral carbonates, ferroan dolomite (ankerite) and magnesian calcite from lower Jurassic ammonites, have been studied by transmission electron microscopy. The samples show small domains with the morphology of platelets parallel to (11 \(\bar 2\) 0) which have exceptionally well ordered superstructures and characteristic c-reflections. Diffraction patterns and contrast analysis of atomic resolution images indicate that the domains observed in ankerite are rich in Ca [Ca_0.75(Mg, Fe)_0.25CO₃] and those in calcite rich in Mg, and there is strong evidence that cation ordering is the cause for the observed superstructures which are proposed mainly on diffraction evidence. Rhombohedral carbonates have a close similarity to NaCl, with CO₃-groups substituting for Cl. Cation ordering patterns can therefore be discussed in analogy to f.c.c. alloys. We make use of ordering waves to describe superstructures and to qualitatively interpret kinetic conditions for their formation.     

Hyperfiltration-induced precipitation of calcite

Hyperfiltration is sometimes cited as a mechanism to explain high degrees of calcite cementation at shale/sandstone contacts. To test this cementation mechanism, a series of experiments were performed in which solutions undersaturated with respect to calcite were hydraulically forced through a Ca-bentonite at different flow rates. Calcite precipitate was observed on the bentonite membrane from hyperfiltrated stock solutions having initial calcite saturation indices of 0.91 and 0.59. Supersaturation conditions at the clay's high-pressure interface are likely provided by establishment of a concentration polarization layer.In the subsurface, the driving force for hyperfiltration is a differential hydraulic pressure gradient acting across a shale membrane. This hydraulically-induced flux of solution causes a build-up of solute at the shale's high-pressure interface to levels that may exceed saturation indices of common cementing minerals like calcite. Although the source of the hydraulic pressure is likely due to compaction within the sedimentary pile, directional flow constraints suggest that hyperfiltration-induced precipitation of calcite occurs at sand/shale boundaries away from areas of active compaction.     

Raman identification of calcite grains in the Chelyabinsk meteorite

Samples of the Chelyabinsk meteorite fallen in February 15, 2013, near Chelyabinsk, Russia, are analyzed by Raman spectroscopy. Olivines (forsterite), orthopyoxenes (enstatite), plagioclases (albite), iron oxides and sulfides, as well as calcite and, possibly, parisite were identified among mineral phases. Results were verified using scanning electron microscope with an energy-dispersive system (SEM-EDX). Data were compared with available materials on calcites from ordinary meteorites.     

CO2 emission accompanying the fracture of calcite

Time resolved mass spectroscopy of the emissions accompanying the fracture of calcite (rhombohedral CaCO₃) show that the principle volatile product, CO₂, is released in bursts milliseconds after the fracture event. Similar measurements during the abrasion of calcite and during low temperature thermal decomposition of pulverized calcite show similar CO₂ bursts. We argue that the observed bursts reflect localized decomposition of the calcite during the relaxation of reversible plastic deformation created by fracture and abrasion. This implies that mechanical, non-thermal processes play an important role in producing the observed decomposition products.     

Mechanical twinning in calcite considered with the concept of ferroelasticity

The mechanical twinning of calcite is compared with ferroelastic behavior. For calcite a paraelastic prototype phase does not exist, therefore a virtual prototype is defined with cubic m3m symmetry. Using this condition the strain tensors of all domain states of mechanical twinning are calculated. With the use of the strain tensors, application of Sapriel's strain compatibility law gives the crystallographic orientations of all possible twin planes between different domain states. The findings indicate that the twin walls are {1 0 0} and {1 1 0} planes with respect to the morphological rhombohedral lattice. These twin plane orientations from the equated strain tensors are in accordance with the r-and e-twin systems commonly observed in calcite. Received: 17 December 1998 / Revised, accepted: 24 April 1999     

The energetics of nanophase calcite

Calcium carbonate (CaCO₃) is an important component of the near-surface environment. Understanding the nature of its precipitation is important for a variety of environmental processes, as well as for the geologic sequestration of anthropogenic carbon dioxide. Calcite is the most thermodynamically stable bulk polymorph, but energy crossovers may exist that could favor the precipitation of vaterite or aragonite with decreasing particle size. The purpose of this study is to determine the surface energy of calcite, which is the first step towards understanding the effect of particle size on thermodynamic stability in the calcium carbonate system. The enthalpies of five well-characterized calcite samples (four nanophase and one bulk) were measured by acid solution isothermal and water adsorption calorimetric techniques. From the calorimetric data, the surface energies of calcite were determined to be 1.48 ± 0.21 and 1.87 ± 0.16 J/m² for hydrous and anhydrous surfaces. These values are similar to those measured for many oxides but larger than predicted from computational models for idealized calcite surfaces. The surfaces of synthetic CaCO₃ particles contain a range of planes and defect structures, which may give rise to the difference between the experimental and modeled values.