Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA

Abstract The North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low-grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite-scolecite-smectite, (2) heulandite-stilbite-smectite, (3) laumontitechlorite-albite, (4) laumontite-chlorite-albite ± prehnite ± pumpellyite and (5) epidote-chlorite-albite ± actinolite zone. In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca-rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed-layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed-layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior. The above features suggest that the overall pattern is one of burial-type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern. Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low-grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions.     

The geochemistry,geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay,Ontario, Canada

The Midcontinent Rift (MCR) of North America comprises a series of basaltic sheets, flows and intrusive rocks emplaced in the Lake Superior region during the Mesoproterozoic. The mafic rocks preserved on the northern flank of Lake Superior represent the older portions of the rift sequence and offer insights into the early development of the rift. New geochronological, geochemical and paleomagnetic data are presented for the dikes and sills located in and south of Thunder Bay, Ontario. Three sill suites are recognized within the study area; an earlier, spatially restricted ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the north of the study area. In addition three dike sets are recognized, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt. Mollie dike. The geochemical data demonstrate that the majority of sills south of Thunder Bay are of Logan affinity and distinct from those of broadly similar age in the Nipigon Embayment to the north. The Pigeon River dikes that intrude the sills are geochemically coherent but distinct from the Logan sills and could not be feeders to the sills. The new age of 1109.2 ± 4.2 Ma for the Cloud River dike and its R polarity are consistent with published magnetostratigraphy. The Mt. Mollie dike age (1109.3 ± 6.3 Ma) indicates that it is not coeval with the spatially associated Crystal Lake gabbro as previously thought. The complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south. The geochemistry and geochronology of the intrusions suggest a long-lived and complex magmatic history for the Midcontinent Rift.     

Highly evolved hypabyssal kimberlite sills from Wemindji,Quebec, Canada: insights into the process of flow differentiation in kimberlite magmas

Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo₉₀. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).     

Sulfide mineralogy and sulfur isotope geochemistry of layered sills in the Deer Lake Complex,Minnesota

The Deer Lake Complex, located in north-central Minnesota, consists of a series of layered peridotite-pyroxenite-gabbro sills. Sulfide minerals occur as fine disseminations throughout pyroxenite and gabbro units, and occur more sporadically in peridotite and basal chilled margin units. Sulfide volume percentage rarely exceeds 0.5. A distinct zonation in sulfide mineralogy and sulfur isotopic composition characterizes the sills. Cobaltian pentlandite is the dominant sulfide mineral in peridotite (pd) units, with Ni-enrichment most likely linked to the serpentinization process. δ³⁴Spd values are variable, ranging from ?3.5 to +2.8‰. Sulfide assemblages in pyroxenite (px) and lower gabbro units consist of chalcopyrite, pyrrhotite, and minor pentlandite. δ³⁴S_px values range from ?1 to +1 ‰. Pyrite is the principal sulfide mineral in upper gabbro (μg) units. Its origin may be related to increased f0₂ conditions of the remaining melt and to reaction between a S-bearing volatile phase and mafic silicates. δ³⁴S_ug values range from 1 to 3.5 ‰. Sulfur isotopic values of chilled margin (2–9 ‰) and peridotite units, together with the erratic spatial distribution of sulfide minerals in these zones, suggests that the parent magma was not initially saturated with sulfur, and that local sulfide concentrations are the result of incorporation of sulfur derived from metasedimentary country rocks. Sulfide saturation was more uniformly reached during pyroxenite formation, with contained sulfur being of magmatic origin. Enrichment in ³⁴S of pyrite from upper gabbro may be explained by buildup of isotopically heavy sulfur following a Rayleigh process, coupled with possible involvement of a SO₂-rich fluid phase during hydrothermal alteration.     

Geochemical and isotopic evidence for the age,orogenic setting and petrogenesis of the Nychum Volcanic association,North Queensland

Rb‐Sr and K‐Ar age determinations agree with palaeobotanical evidence in assigning an age of 270 Ma (Lower Permian) to the Nychum Volcanics, North Queensland. Geological and geochemical evidence (moderate ‐REE, La/Yb, Th, Zr/Y, Hf/Yb, Sc/Ni) indicate that the volcanics were erupted on a thin, active continental margin. Five magma groups are present: high‐alumina basalts, andesite‐dacites, acid rocks, tholeiitic andesites, and pitchstone with a high La/Yb ratio. The high‐alumina basalts and calc‐alkaline andesites have a parent‐daughter (source rock‐partial melt) relationship. The acid rocks share high HREE, Zr, Hf, Zn, Sc, Fe/Mg and low Al with, and may be fractionated from, the tholeiitic subduction‐zone andesites. Low‐pressure fractional crystallisation is evident in all five magma groups.     

西藏措勤地区则弄群火山岩源区——地球化学及Sr-Nd同位素制约

西藏中冈底斯带出露大面积早白垩世则弄群火山岩。措勤地区则弄群火山岩具有较高的SiO2含量(61.91%~74.28%),轻稀土元素相对富集,富集大离子亲石元素Rb、Ba、Th、U,亏损高场强元素Nb、Ta等,并具Sr负异常,同时具有高的初始Sr同位素(0.707 89~0.710 17)和低的Nd的同位素(-3.4~-9.3)特征,与壳源岩浆的地球化学特征类似。在εNd(t)-(87Sr/86Sr)i图解上,措勤地区则弄群火山岩所有样品均投于第四象限,而且都分布在上地壳与雅鲁藏布江MORB型亏损地幔混合线附近。微量元素构造环境判别图解显示,则弄群火山岩形成于岛弧环境。综合已有研究成果以及最新1∶25万区域地质调查资料,认为措勤地区则弄群长英质火山岩很可能是幔源基性岩浆的热量促使地壳物质脱水发生重熔的产物,其形成的大地构造背景很可能与班公湖-怒江特提斯洋南向的俯冲作用有关。     

Hydrothermal hydrocarbon gases: 2, Application in the East African Rift System

Hydrocarbon gases are ubiquitous in the hydrothermal systems of the East African Rift System (EARS), though often found at very low concentrations in the ‘volcanic’ eastern branch as compared to the ‘sedimentary’ western branch. Study of the chemical and isotopic compositions of these hydrocarbons from sites in Kenya, Ethiopia, Djibouti and Uganda reveals considerable homogeneity over hundreds of km of the various rift units. Consideration of C and He isotopic evidence points to a predominantly crustal thermogenic origin for the hydrocarbons, there being no evidence of mantle inputs in either the MORB or ‘hotspot’ sectors of the EARS. Temperature information from geothermal wells has been utilised to investigate the relationship between reservoir temperatures and ratios of CH₄ to C₂H₆. The general C₁/C₂ geothermometric relationship proposed in Part 1 of this study holds reasonably well, and is shown to give results equal to or better than the ‘inorganic’ gas geothermometers presently in use, both in the wellfields and undeveloped high-enthalpy geothermal areas. Results from low-enthalpy hot spring systems are less well correlated with apparent deep temperatures, but consistent with data from similar systems elsewhere in the world.     

氢氧同位素在地下水流系统的重分布：从高程效应到深度效应

大气降水的氢氧同位素含量具有高程效应,降水入渗后参与地下水循环,其高程效应如何受地下水流系统的影响转化为地下水氢氧同位素的深度效应？现有研究对于这个问题缺少定量认识。文章构建单向倾斜盆地和双峰波状盆地的稳态地下水循环理论模型,采用MODFLOW模拟剖面二维地下水流场、采用MT3DMS模拟重同位素分子的对流-弥散过程,得到地下水D和¹⁸O含量的空间分布,探讨了氢氧同位素高程效应在地下水流系统转化为深度效应的机理。结果表明：在单斜盆地,补给区大气降水D和¹⁸O含量的高程效应转化为排泄区地下水δD和δ¹⁸O值随埋深增大而指数型衰减的深度效应;在双峰波状盆地,当含水层渗透性相对入渗强度较大时(K₀/w=1 000),仅发育一个区域地下水流系统,在区域地下水的排泄区δD和δ¹⁸O随埋深增大呈现S形曲线分布;当含水层渗透性相对入渗强度较小时(K0/w=250),双峰波状盆地发育多个局部地下水流系统,区域地下水的排泄区δD和δ¹⁸O随埋深增大呈现S形曲线,而局部地下水排...     

Hydrothermal sedimentation in the caldera of Santorini, Hellenic Volcanic Arc

Sediments in the caldera of Santorini are receiving a hydrothermal input of iron and manganese from presently active hydrothermal vents off the Kameni Islands, and are enriched in these elements in their surface layer. However, greater Fe-Mn enrichments occur in discrete layers at depth in the cores separated from the surface by Fe-Mn poor sediments, suggesting that a past hydrothermal event may have been more intense than the present one. The buried Fe-Mn enriched layers occur above a turbidite thought to have resulted from sediment slumping due to a major volcanic eruption and earthquake in 1650, and are thought to have formed consequent on the activation of faults related to the magma chamber by the eruption facilitating seawater-rock interaction processes and the formation of metal-rich hydrothermal solutions.     

贵州龙动物群产出层位碳氧同位素研究

贵州龙动物群是二叠-三叠纪大绝灭后海洋生态系统辐射发展阶段最晚期的典型代表,为了解其古环境信息,对贵州省兴义地区顶效及周边产地开展了精细剖面测量,采集碳酸盐岩进行了全岩碳氧同位素测试。测试结果显示：泥麦谷剖面δ¹³C值分布范围-6.3‰~2.6‰,δ¹⁸O分布范围在-5.7‰~-1.2‰;谢米剖面δ¹³C值分布范围在-4.5‰~2.5‰,δ¹⁸O在-5.7‰~-1.2‰;坝尾剖面δ¹³C值分布范围在0.5‰~2‰,δ¹⁸O在-6.4‰~0.1‰;顶效剖面δ¹³C值分布范围在-4.3‰~2.1‰,δ¹⁸O在-7.3‰~-2.5‰。根据碳氧同位素数据相关性分析显示泥麦谷、谢米、坝尾剖面的同位素值受后期成岩作用影响较小。在泥麦谷、谢米及顶效贵州龙动物群产出层位下段均出现明显的δ¹³C负异常,与低水位时期海洋生物快速死亡密切相关,也与火山作用存在一定的联系。由于顶效剖面受后期成岩作用影响,笔者利用泥麦谷、谢米及坝尾剖面δ¹⁸O数据对兴义地区古海水温度进行了恢复,结果显示温度分布在19~27℃,平均为23℃左右,说明当时属于温暖气候环境。     

Sulfide petrology of basal chilled margins in layered sills of the Archean Deer Lake Complex,Minnesota

Sulfide minerals in amounts up to 3 vol% are found in basal, chilled marginal zones of two layered peridotite-pyroxenite-gabbro sills in the Early Precambrian Deer Lake Complex, northcentral Minnesota. The sulfides occur interstitially to silicate minerals, and consist of pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite, gersdorffite, and marcasite±pyrite as an alteration product of pyrrhotite.The basal chilled units (3–6 m) of the sills are divisable into three zones based primarily on textures. The lowermost zone is an equigranular basalt, whereas the overlying zone is characterized by skeletal, spinifex-like actinolite after clinopyroxene. The upper zone of the basal margins contains elongate and swallow tail plagioclase, and is barren of sulfide minerals.Electron microprobe analyses of sulfide minerals and modal data suggest that sulfide bulk compositions at 1,100–1,000 ° C represent a pyrrhotite solid solution and a coexisting Cu-rich sulfide liquid. Cooling of the Cu-rich liquid and low temperature transformations are thought to have produced chalcopyrite or chalcopyrite plus pyrrhotite. The sulfide minerals have reequlibrated to temperatures near 300 ° C or less.Analyses of sulfur content and ³⁴S values suggest that assimilation of sulfur from adjacent country rocks was the principal mechanism responsible for anomalous concentrations of sulfides in the basal chilled margins. The distribution of sulfides in the peridotite-pyroxenite-gabbro portions of the sills, and calculations of settling rate preclude an origin involving gravitational settling of immiscible droplets through the magma body.     

华北克拉通北缘裂谷渣尔泰群LA-ICP-MS碎屑锆石U-Pb测年及地质意义

华北克拉通自1.85Ga形成之后,经历了广泛的拉伸,形成了一系列中-新元古代裂谷。其中北缘裂谷由渣尔泰群、白云鄂博群及化德群组成,发育了一系列大型-超大型多金属矿床。该裂谷系形成演化的研究对认识华北克拉通中-新元古代演化及区域找矿具有重要意义。但是对于该裂谷系中渣尔泰群的时代,一直存在争议。早期认为渣尔泰群属于中元古代,主要分布于狼山地区和渣尔泰山地区。但最新的研究已将狼山地区的渣尔泰群限定为新元古代,更名为狼山群,并据此确定华北克拉通北缘存在新元古代裂谷。因此,必须对渣尔泰山地区渣尔泰群的时代进行重新限定。在此基础上可综合分析华北克拉通北缘裂谷的形成与演化。系统的LA-ICP-MS锆石U-Pb测年结果表明,渣尔泰山地区渣尔泰群碎屑锆石年龄峰值主要为1.8~1.9Ga和2.5Ga。与北缘裂谷长城系,包括白云鄂博群下部及化德群下部的碎屑锆石年龄组成特征一致。而蓟县系、待建系及青白口系存在1.1~1.35Ga、1.5~1.6Ga等较年轻的碎屑锆石年龄峰值,如白云鄂博群上部、新元古代狼山群及化德群上部。因此,渣尔泰山地区渣尔泰群整体可与长城系对比。碎屑锆石年龄组成特征表明,渣尔泰群物质主要来自华北克拉通内部的太古宙-古元古代结晶基底。综合前人对白云鄂博群、化德群及狼山群地层年龄及岩浆岩的研究成果,可确定北缘裂谷是中-新元古代多期裂解事件形成的复杂裂谷。     

Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine,Peru

The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ¹⁸O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ¹⁸O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ¹⁸O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ¹⁸O_H2O and δD_H2O values range from 8 to 12%. and ?3 to +42%., respectively.δ¹⁸O_H2O values calculated from δ¹⁸O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ¹⁸O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are ¹⁸O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ¹⁸O_H2O and δD_H2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. ¹⁸O-enrichment in andesites may be produced by isotopic exchange with high ¹⁸O waters at elevated temperatures and sufficiently high water/rock ratios.     

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

⁸⁷Sr/⁸⁶Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ¹⁸O−δ²H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids.Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same ⁸⁷Sr/⁸⁶Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.     

新疆萨热克砂砾岩型铜矿床成矿流体碳、氢-氧同位素组成及其意义

新疆萨热克铜矿为砂砾岩型铜矿床,赋矿地层为上侏罗统库孜贡苏组砂砾岩.矿体以层状为主,并有裂隙、碎裂构造岩相伴生,常见辉铜矿等金属硫化物与石英、方解石和白云石等胶结物沿砾石间隙或岩石裂隙充填分布.为了揭示成矿流体的性质、来源,文章对矿体中呈脉状产出的石英和碳酸盐矿物(方解石和白云石)中的流体包裹体和碳、氢-氧同位素进行了测定.研究表明:石英和方解石中流体包裹体的气相分数通常为5％～15％,均一温度变化范围88～249℃,平均为172℃;盐度变化范围为6.30％～12.51％,平均为9.46％,为中低温、中低盐度成矿流体.脉状方解石中的δD变化范围为-72‰～-62‰,平均-67.3‰;δ18O变化范围为3.9‰～8.0‰,平均5.3‰,脉状石英中 δD变化范围为-101.7‰～-87.1‰,平均-93.3‰;δ18O变化范围5.1‰～7.5‰,平均6.6‰,氢-氧同位素组成主要落在岩浆水和变质水重叠区及其附近.方解石(白云石)中的δ18O变化范围为-15.1‰～-9.5‰,平均为-12.9‰;δ13C变化范围为-2.5‰～-2.0‰,平均为-0.8‰.同位素组成显示其位于海相碳酸盐岩区左侧附近,与研究区石炭系等碳酸盐岩的碳、氧同位素相似,表明铜矿石中的脉状方解石脉(无机碳)主要与海相碳酸盐岩的溶解作用有关.综合考虑上述结果,文章推测在萨热克铜矿的北矿段深部可能存在隐伏岩体,成矿流体为变质流体与岩浆热液叠加形成的具中、低温,中、低盐度特征的混合流体,该研究区深部存在具有进一步找矿的潜力.     

新疆萨热克砂砾岩型铜矿床成矿流体碳、氢-氧同位素组成及其意义

新疆萨热克铜矿为砂砾岩型铜矿床,赋矿地层为上侏罗统库孜贡苏组砂砾岩.矿体以层状为主,并有裂隙、碎裂构造岩相伴生,常见辉铜矿等金属硫化物与石英、方解石和白云石等胶结物沿砾石间隙或岩石裂隙充填分布.为了揭示成矿流体的性质、来源,文章对矿体中呈脉状产出的石英和碳酸盐矿物(方解石和白云石)中的流体包裹体和碳、氢-氧同位素进行了测定.研究表明:石英和方解石中流体包裹体的气相分数通常为5％～15％,均一温度变化范围88～249℃,平均为172℃;盐度变化范围为6.30％～12.51％,平均为9.46％,为中低温、中低盐度成矿流体.脉状方解石中的δD变化范围为-72‰～-62‰,平均-67.3‰;δ18O变化范围为3.9‰～8.0‰,平均5.3‰,脉状石英中 δD变化范围为-101.7‰～-87.1‰,平均-93.3‰;δ18O变化范围5.1‰～7.5‰,平均6.6‰,氢-氧同位素组成主要落在岩浆水和变质水重叠区及其附近.方解石(白云石)中的δ18O变化范围为-15.1‰～-9.5‰,平均为-12.9‰;δ13C变化范围为-2.5‰～-2.0‰,平均为-0.8‰.同位素组成显示其位于海相碳酸盐岩区左侧附近,与研究区石炭系等碳酸盐岩的碳、氧同位素相似,表明铜矿石中的脉状方解石脉(无机碳)主要与海相碳酸盐岩的溶解作用有关.综合考虑上述结果,文章推测在萨热克铜矿的北矿段深部可能存在隐伏岩体,成矿流体为变质流体与岩浆热液叠加形成的具中、低温,中、低盐度特征的混合流体,该研究区深部存在具有进一步找矿的潜力.     

新疆萨热克砂砾岩型铜矿床成矿流体碳、氢-氧同位素组成及其意义

新疆萨热克铜矿为砂砾岩型铜矿床,赋矿地层为上侏罗统库孜贡苏组砂砾岩.矿体以层状为主,并有裂隙、碎裂构造岩相伴生,常见辉铜矿等金属硫化物与石英、方解石和白云石等胶结物沿砾石间隙或岩石裂隙充填分布.为了揭示成矿流体的性质、来源,文章对矿体中呈脉状产出的石英和碳酸盐矿物(方解石和白云石)中的流体包裹体和碳、氢-氧同位素进行了测定.研究表明:石英和方解石中流体包裹体的气相分数通常为5％～15％,均一温度变化范围88～249℃,平均为172℃;盐度变化范围为6.30％～12.51％,平均为9.46％,为中低温、中低盐度成矿流体.脉状方解石中的δD变化范围为-72‰～-62‰,平均-67.3‰;δ18O变化范围为3.9‰～8.0‰,平均5.3‰,脉状石英中 δD变化范围为-101.7‰～-87.1‰,平均-93.3‰;δ18O变化范围5.1‰～7.5‰,平均6.6‰,氢-氧同位素组成主要落在岩浆水和变质水重叠区及其附近.方解石(白云石)中的δ18O变化范围为-15.1‰～-9.5‰,平均为-12.9‰;δ13C变化范围为-2.5‰～-2.0‰,平均为-0.8‰.同位素组成显示其位于海相碳酸盐岩区左侧附近,与研究区石炭系等碳酸盐岩的碳、氧同位素相似,表明铜矿石中的脉状方解石脉(无机碳)主要与海相碳酸盐岩的溶解作用有关.综合考虑上述结果,文章推测在萨热克铜矿的北矿段深部可能存在隐伏岩体,成矿流体为变质流体与岩浆热液叠加形成的具中、低温,中、低盐度特征的混合流体,该研究区深部存在具有进一步找矿的潜力.     

新疆萨热克砂砾岩型铜矿床成矿流体碳、氢-氧同位素组成及其意义

新疆萨热克铜矿为砂砾岩型铜矿床,赋矿地层为上侏罗统库孜贡苏组砂砾岩.矿体以层状为主,并有裂隙、碎裂构造岩相伴生,常见辉铜矿等金属硫化物与石英、方解石和白云石等胶结物沿砾石间隙或岩石裂隙充填分布.为了揭示成矿流体的性质、来源,文章对矿体中呈脉状产出的石英和碳酸盐矿物(方解石和白云石)中的流体包裹体和碳、氢-氧同位素进行了测定.研究表明:石英和方解石中流体包裹体的气相分数通常为5％～15％,均一温度变化范围88～249℃,平均为172℃;盐度变化范围为6.30％～12.51％,平均为9.46％,为中低温、中低盐度成矿流体.脉状方解石中的δD变化范围为-72‰～-62‰,平均-67.3‰;δ18O变化范围为3.9‰～8.0‰,平均5.3‰,脉状石英中 δD变化范围为-101.7‰～-87.1‰,平均-93.3‰;δ18O变化范围5.1‰～7.5‰,平均6.6‰,氢-氧同位素组成主要落在岩浆水和变质水重叠区及其附近.方解石(白云石)中的δ18O变化范围为-15.1‰～-9.5‰,平均为-12.9‰;δ13C变化范围为-2.5‰～-2.0‰,平均为-0.8‰.同位素组成显示其位于海相碳酸盐岩区左侧附近,与研究区石炭系等碳酸盐岩的碳、氧同位素相似,表明铜矿石中的脉状方解石脉(无机碳)主要与海相碳酸盐岩的溶解作用有关.综合考虑上述结果,文章推测在萨热克铜矿的北矿段深部可能存在隐伏岩体,成矿流体为变质流体与岩浆热液叠加形成的具中、低温,中、低盐度特征的混合流体,该研究区深部存在具有进一步找矿的潜力.     

Hydrothermal alteration and zeolitization of the Fohberg phonolite,Kaiserstuhl Volcanic Complex,Germany

The subvolcanic Fohberg phonolite (Kaiserstuhl Volcanic Complex, Germany) is an economic zeolite deposit, formed by hydrothermal alteration of primary magmatic minerals. It is mined due to the high (>40 wt%) zeolite content, which accounts for the remarkable zeolitic physicochemical properties of the ground rock. New mineralogical and geochemical studies are carried out (a) to evaluate the manifestation of hydrothermal alteration, and (b) to constrain the physical and chemical properties of the fluids, which promoted hydrothermal replacement. The alkaline intrusion is characterized by the primary mineralogy: feldspathoid minerals, K-feldspar, aegirine–augite, wollastonite, and andradite. The rare-earth elements-phase götzenite is formed during the late-stage magmatic crystallization. Fluid-induced re-equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of secondary phases. Feldspathoid minerals are totally replaced by various zeolite species, calcite, and barite. Wollastonite breakdown results in the formation of various zeolites, calcite, pectolite, sepiolite, and quartz. Zeolites are formed during subsolidus hydrothermal alteration (<150 °C) under alkaline conditions. A sequence of Ca–Na-dominated zeolite species (gonnardite, thomsonite, mesolite) is followed by natrolite. The sequence reflects an increase in \(\log [(a_{{{\text{Na}}^{ + } }} )/(a_{{{\text{H}}^{ + } }} )]\) and decrease in \(\log [(a_{{{\text{Ca}}^{2 + } }} )/(a_{{{\text{H}}^{ + } }}^{2} )]\) of the precipitating fluid. Low radiogenic ⁸⁷Sr/⁸⁶Sr values indicate a local origin of the elements necessary for secondary mineral formation from primary igneous phases. In addition, fractures cut the intrusive body, which contain zeolites, followed by calcite and a variety of other silicates, carbonates, and sulfates as younger generations. Stable isotope analysis of late-fracture calcite indicates very late circulation of meteoric fluids and mobilization of organic matter from surrounding sedimentary units.     

Strontium and oxygen isotopic systems in waters of mud volcanoes of the Taman Peninsula

Ten of eleven analyzed water samples from mud volcanoes of the Taman Peninsula are characterized by ⁸⁷Sr/⁸⁶Sr ratio within 0.70734–0.70957, which overlaps the values typical of the Mesozoic and Cenozoic sedimentary carbonates, but sharply differs from the value in the clayey sediments of the Maikop Group (0.7157 ± 0.0022). These data indicate that the strontium isotopic composition is mainly defined by carbonate reservoirs, with relatively little effect of elision solutions, input of which is noticeable only in the water of Gladkovsky Volcano (⁸⁷Sr/⁸⁶Sr = 0.71076). The high δ¹⁸O in mud volcanic waters (up to 14.2‰) can also be attributed to ionic exchange with sedimentary carbonates at temperatures around 150°C.