REE fractionation,mineral speciation,and supergene enrichment of the Bear Lodge carbonatites,Wyoming, USA

The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ¹⁸O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO₄³⁻/CO₃²⁻ and F⁻/CO₃²⁻ ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.     

Mineralogical and isotopic evidence for phenocryst-matrix disequilibrium in the Garner Mountain andesite

Petrographic, trace element and isotopic evidence demonstrates that magma mixing preceded the eruption of the Garner Mountain andesite. The flow contains reversely zoned plagioclase phenocrysts and amphibole pseudomorphs composed of plagioclase, clinopyroxene, orthopyroxene and opaque oxides. Partially resorbed quartz grains are also present. In contrast to the isotopically uniform matrix, plagioclase phenocrysts have ⁸⁷Sr/⁸⁶Sr ratios that correlate negatively with matrix Sr and positively with matrix Rb abundances. These observations demonstrate increasing isotopic disequilibrium between the plagioclase and matrix in the more evolved varieties of the flow.Plagioclase phenocrysts and matrix are assumed to record fractionation-assimilation events in different parts of the magma chamber. Early formed plagioclase phenocrysts crystallized under AFC conditions close to the roof of the chamber and were subsequently entrained in a liquid mixture composed of evolved interstitial liquid held in the partly crystallized roof zone and newly injected parental magma.     

Galena mineralization in the Orcadian Basin,Scotland: Geological and isotopic evidence for sources of lead

Galena and other sulphides are widespread in the Old Red Sandstone (Devonian) Orcadian Basin. Syndiagenetic occurrences are associated with organic-rich sedimentary rocks, especially algal limestones. Epigenetic galenas are often spatially related to these syndiagenetic occurrences, particularly in the vicinity of Permo-Carboniferous igneous intrusions. Mineralization is concentrated about the granite-gneiss basement inlier in southwest Orkney.Orcadian Basin galenas have lead isotope compositions which show a linear trend related to Lewisian granulite facies metamorphism at about 2,700 Ma. The most radiogenic leads must include a component of average orogene or upper crustal lead. The isotopic compositions are notably similar to those of Caledonian granite feldspar leads measured by Blaxland et al. (1979). Granite was probably a source for lead in galena deposits about the basement in south-west Orkney, but away from the basement an average orogene component would have become increasingly important. There was no major input of magmatic lead to the basin during Permo-Carboniferous times, only a remobilization of existing lead.     

Origin and evolution of 2 Ga old carbonatite complex (Ihouhaouene,Ahaggar, Algeria): Nd and Sr isotopic evidence

Associated syenitic rocks and carbonatites from Ihouhaouene, Algeria, have been investigated for their Sr and Nd isotope and trace element geochemistry. A zircon U-Pb emplacement age (1994 +22 -17 Ma) has been obtained from the carbonatites. The REE characteristics, among which the significant and approximately constant negative Eu anomaly and the evolution of (La/Ce)N and (Yb/Lu)N ratios which increase sympathetically with total REE abundances, are considered to be of purely magmatic origin. They are used to constrain the genetic links between syenites and carbonatites. Sr and Nd isotopes suggest a similar source for carbonatites and syenites, which is enriched compared to a chondritic reference: Nd(T)=-6.4 to -8.6 and ISr(T)=0.7097. These features are interpreted as evidence of contamination of a mantle-derived magmatic precursor by continental crust, occurring in a magmatic chamber.     

Mineralogical and geochemical evidence for coupled bacterial uranium mineralization and hydrocarbon oxidation in the Shashagetai deposit,NW China

Unusual mineral structures have recently been found in a sandstone-hosted roll-type uranium deposit in the Middle Jurassic Zhiluo Formation in the Shashagetai deposit, the northern Ordos basin, NW China. The structures possess a chemical composition and crystal structure characteristic of mineral coffinite [(USiO₄)_1−x(OH)_4x], which occurs as nanoparticles with size ranging from 5 to 25 nm. These structures are interpreted to be fossilized microorganisms, based on mineralogical biosignatures including morphology, size, occurrence of biogenic coffinite as nano-crystals, and biological elements such as P. The intimate intergrowth of coffinite with secondary pyrite of bacterial origin, as defined by low δ³⁴S values, and calcite cements with petroleum-derived carbon supports its biogenic origin. Oil inclusions in the host sandstone are characterized by abundant n-alkanes, slightly increased Pr/nC₁₇ and Ph/nC₁₈ ratios, significant amounts of demethylated hopanes and tricyclic terpanes, and the existence of unresolved complex mixtures. These characteristics are interpreted to be a result of mixing of an earlier, heavily degraded oil with a later charged fresh oil; subsequently the oils were slightly degraded. These lines of evidence lead to the proposal that the reduction of sulfate and oxidization of petroleum are likely synchronous with reduction of hexavalent [U(VI)] to tetravalent [U(IV)] uranium by sulfate-reducing bacteria (SRB). The discovery of a natural association of microorganism-like structures, a uranium mineral, and biodegraded petroleum has implications for uranium biomineralization and fossil fuel exploration.     

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt-hosted gold mineralization at the Tau deposit of the Mupane gold mine,Botswana

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite-rich zones associated with biotite-chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold-thiocomplexes in the ore-forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two-stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ³⁴S of gold-bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi-processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.     

西藏班公湖-怒江成矿带主碰撞期成矿作用:荣嘎钼矿岩石地球化学及同位素年龄的证据

荣嘎钼矿是新近在班公湖-怒江成矿带发现并证实的首例斑岩型钼矿床。为了查明班公湖-怒江成矿带主碰撞期的成岩成矿作用,探讨荣嘎钼矿的富Mo机制,对该矿床进行岩石地球化学以及同位素年代学的研究。本次研究获得荣嘎含矿二长花岗斑岩的锆石SHRIMP206Pb/238U加权平均年龄为99. 8±1. 9Ma (MSWD=2. 0),属晚白垩世早期。7件辉钼矿样品的模式年龄较为一致,介于98. 0±1. 6Ma～101. 8±1. 7Ma之间,加权平均年龄为99. 7±1. 1Ma。荣嘎含矿二长花岗斑岩样品主量元素表现为富硅(SiO_2=72. 83%～75. 49%)、富碱(Na_2O+K_2O=8. 12%～8. 55%)、过铝质(A/CNK=1. 08～1. 13)的高钾钙碱性系列岩石。样品明显富集Rb、K、Th、U等元素,中等亏损Nb、Ta,强烈亏损Ba、Sr、P、Ti等元素,具中等程度的负Eu异常(δEu=0. 53～0. 63),稀土元素配分曲线呈轻稀土相对较陡,重稀土较为平坦的右倾型。荣嘎含矿二长花岗斑岩样品的(~(87)Sr/~(86)Sr)i值为0. 7042～0. 7055,ε_(Nd)(t)值介于0. 03～0. 68之间,锆石ε_(Hf)(t)值介于+12. 4～+17. 9之间,两阶段模式年龄t_(DM2)较为年轻(31～314Ma)。基于以上同位素以及岩石地球化学特征研究,荣嘎含矿二长花岗斑岩很可能是幔源岩浆诱发了富Mo洋底沉积物的熔融并与壳源熔体混合形成母岩浆,再经历结晶分异作用而形成。结合本文以及区域上的研究资料,荣嘎钼矿可能形成于拉萨地块与南羌塘地块碰撞的大地构造背景,富Mo大洋沉积物熔体的加入及随后的结晶分异是荣嘎含矿岩浆中金属Mo富集的主要原因。班公湖-怒江成矿带南缘侏罗-白垩纪海相地层分布的地区,可能为勘查钼矿床的有利靶区,应重点评价晚白垩世早期高分异中酸性岩体的含矿性。     

The chemical and isotopic record of rock-water interaction in the Sherman Granite,Wyoming and Colorado

Chemical, isotopic, radiographic, and rock-leaching data are combined to describe the effects of rock-water interactions in core samples of petrographically fresh, 1.43 b.y.-old Sherman Granite. The data serve to identify sensitive indicators of incipient alteration and to estimate the degree, pathways, and timing of element mobilization. Unfractured core samples of Sherman Granite are remarkably fresh by most chemical or isotopic criteria, but incipient alteration is indicated by the abundance and distribution of uranium and the degree of radioactive equilibration of uranium with its decay products. Uranium abundances which are out of equilibrium with lead decay products indicate remobilization of a portion (3 to 60 percent) of original uranium in late Phanerozoic time. Association of uranium with minor but pervasive secondary alteration products also indicates some remobilization. The amount of apparent uranium mobility in unfractured Sherman Granite (3 to 60 percent) is small compared to the results of similar studies of Archean granites from nearby localities. Chemical and isotopic data evaluated as a function of core-sample depth suggest a uranium migrational pathway involving near-surface leaching and reconcentration at depth. Movement of solutions through the upper 200 ft (60 m) of Sherman Granite is fracture controlled, and brecciated granite shows more obvious petrographic, chemical, and isotopic evidence of alteration and multi-element redistribution. Laboratory experiments using freshly crushed Sherman Granite confirm that uranium is leached in preference to elements such as Si, Mg, Ca, and K, and that leachable uranium is situated close to the solid-liquid interface; perhaps as uranium along grain boundaries, in crystal defects, or on cleavage traces of minerals that exclude uranium from their structure.     

安哥拉Bonga铌矿床成矿碳酸岩岩石地球化学和C-O同位素研究

安哥拉Bonga碳酸岩型铌矿床位于Parana'-Angola-Etendeka碱性岩-碳酸岩火成岩省东部,是一个孤立产出的中心式岩栓,侵入于元古宙花岗岩基底中。岩石地球化学研究表明,Bonga岩体由钙碳酸岩和少量的镁碳酸岩组成,岩体成分从钙碳酸岩向镁碳酸岩演化。矿物组合上,钙碳酸岩以方解石为主,副矿物有磷灰石、磁铁矿、烧绿石和少量稀土矿物;镁碳酸岩以白云石为主,烧绿石含量降低,稀土矿物含量增高。富钙碳酸岩(摩尔比值Ca O/Ca O+Mg O+Fe O+Mn O0.83)中Nb含量较高,变化于148.1×10~(-6)~8394×10~(-6),平均为2127×10~(-6),∑REE变化于1441×10~(-6)~9452×10~(-6),平均为2791×10~(-6),LREE/HREE变化于16.7~58.3,平均为25.0;富镁碳酸岩(摩尔比值Ca O/Ca O+Mg O+Fe O+Mn O0.83)Nb含量降低,变化于300.9×10~(-6)~3910×10~(-6),平均为1502×10~(-6),∑REE升高,变化于1659×10~(-6)~18849×10~(-6),平均为7111×10~(-6),轻稀土更加富集,LREE/HREE增大,变化于19.1~114,平均为57.6。铌在碳酸岩浆演化的早期富集,铌矿化主要与富钙碳酸岩有关;稀土元素的富集相对较晚,主要与富镁碳酸岩有关。对碳酸岩碳氧同位素的瑞利分馏模拟计算(RIFMS模型)结果表明,Bonga碳酸岩的铌矿化(烧绿石沉淀)主要受岩浆作用控制,其温度不低于600℃。     

Jurassic magmatism related Pb-Zn-W-Mo polymetallic mineralization in the central Nanling Range,South China: Geochronologic,geochemical, and isotopic evidence from the Huangshaping deposit

The Nanling Range in South China is characterized by extensive Mesozoic magmatism and coeval nonferrous and rare metal mineralization. Huangshaping is a world-class Pb-Zn-W-Mo polymetallic skarn deposit in the central Nanling Range. Magmatic rocks occurring in this ore district include quartz porphyry, granite porphyry, granophyre, dacite porphyry, and aplite, with only the first three granitoids genetically associated with polymetallic mineralization. Most of the orebodies are constrained within the contact zones as skarn and veins between these granitic stocks and the carbonate wall rocks.Since the age of the quartz porphyry is still controversial, and studies of the dacite porphyry and aplite are absent, we focus on these magmatic rocks first. LA-ICP-MS zircon U-Pb dating suggests that the crystallization ages of the quartz porphyry, dacite porphyry, and aplite are 154.3 ± 1.9 Ma, 158.1 ± 0.8 Ma, and 148.4 ± 3.4 Ma, respectively. Combined with previously published age data, we infer the evolutionary sequence of magmatic rocks should be dacite porphyry → quartz porphyry → granite porphyry (granophyre) → aplite. The quartz porphyry, dacite porphyry, and aplite yield high contents of high field strength elements (Zr + Nb + Ce + Y = 255–440 ppm), high ratios of 10,000 × Ga/Al (2.6–3.2), and prominent depletions in Ba, Sr, Eu, P, and Ti, indicating their crustal affinities to A-type granites. They have negative ε_Nd(t) values (−9.4 to −7.0) and high initial Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb_i = 18.307–18.644, ²⁰⁷Pb/²⁰⁴Pb_i = 15.689–15.742, ²⁰⁸Pb/²⁰⁴Pb_i = 38.589–38.986), suggesting that they were probably derived by partial melting of ancient granulitic crustal materials.The sulfide minerals exhibit a wide range of δ³⁴S_V-CDT values from −22.6 to 24.2‰, with ²⁰⁶Pb/²⁰⁴Pb of 17.669–19.708, ²⁰⁷Pb/²⁰⁴Pb of 15.492–15.714, and ²⁰⁸Pb/²⁰⁴Pb of 37.880–39.789, indicating that sulfur, lead, and other associated metals were derived from a mixture of magmatic components and the Carboniferous wall rocks. Fluid inclusions in pyrrhotite, sphalerite, and marmatite samples have ³He/⁴He ratios of 0.12 to 1.53 Ra, with calculated mantle helium proportions of 1.3 to 18.9%, indicating a predominantly crustal origin for the ore fluids, with minor inputs from the mantle. The Huangshaping deposit is a typical example of the genetic relationship both spatially and temporally between Jurassic magmatism and polymetallic metallogeny in the Nanling Range.     

Evolution of zoned micas and associated silicates in the Oka carbonatite

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

Evolution of the sublayer of the Sudbury Igneous Complex: geochemical, Sm–Nd isotopic and petrologic evidence

The mineralized sublayer at the base of the Sudbury Igneous Complex (SIC) consists of two variants, the noritic contact sublayer and radial and concentric quartz dioritic offset dykes. Both are characterized by the presence of significant quantities of Ni–Cu–PGE sulphides and by a prominent population of recrystallized diabasic-textured and melanocratic to ultramafic fragments. The two variants of the sublayer contain compositionally distinct inclusion populations and inclusion-bearing matrices. Contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments. The compositional variation in SIC rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt(s).

The basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase, and is geochemically and isotopically identified with Huronian basalts which comprise the south range footwall, with (Ce/Yb)_N ratios of around 2.5 and _Nd¹⁸⁵⁰ between −2 to −5. The melanocratic inclusions in the sublayer are typically coarse-grained and undeformed, with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet, which has (Ce/Yb)_N ratios of around 10 and _Nd¹⁸⁵⁰ around −9. Calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet. It is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact. This basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling. Offset dykes were emplaced and the magmas effectively removed from the system. Subsequently, continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer. No extracrustal (e.g., mantle) component is envisioned in this model to explain the silicate compositional distributions in the SIC, and mafic crustal rocks in the target zone are implicated as the metal source for the SIC deposits.     

Abundance and distribution of the rare-earth elements and yttrium in the rocks and minerals of the Oka carbonatite complex,Quebec

are-earth (REE) and yttrium abundances were determined, by an ion-exchange-X-ray fluorescence procedure, for whole-rock (14) and mineral (87) samples from the Oka carbonatite complex. Whole-rock and mineral data indicate a trend of total REE + Y enrichment, and relative enrichment in light REE, in the order: ultrafenites < ijolites < okaites. The sövites may show wide variations in total REE + Y concentrations, but relative REE abundance patterns will be similar. The greatest REE and Y concentrations occur in apatite, niocalite, perovskite and pyrochlore. Many of the minerals show europium anomalies (both positive and negative), and these are believed to be the result of closed system competition between the various minerals for divalent Eu. The partition coefficients for mineral pairs are quite variable, indicating that the Oka rocks were emplaced through a wide-range of physicochemical and/or nonequilibrium conditions. A reasonable model for the origin of the complex involves a limited partial melting of mantle material, emplacement of the melt in a magma chamber, crystallization of mafic minerals resulting in a residual liquid which produced ijolite and subsequently okaite, and crystallization of the carbonatites from a volatile-rich, possibly immiscible, phase.     

Mineralogical and isotopic properties of inorganic nanocrystalline magnetites

Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF₅ lines. At low total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite. At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10 nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope fractionation factor between magnetite crystals and water was of 0-1‰, similar to the fractionation factor associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH.     

Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe²⁺)(CO₃)₂ cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe₃O₄. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (⁸⁷Sr/⁸⁶Sr=0.7033; )Nd=0.2-1.0; ²⁰⁶Pb/²⁰⁴Pb=16.37; ²⁰⁷Pb/²⁰⁴Pb=15.42; ²⁰⁸Pb/²⁰⁴Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '¹⁸O=8.0 and '¹³C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)_n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.     

Jurassic extensional tectono-magmatism and associated mineralization of the El Faldeo polymetallic district, Chilean Patagonia: geochemical and isotopic evidence of crustal contribution

The El Faldeo is the southernmost Andean mineral district and consists of Paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the Ibáñez formation. In addition, two epizonal intrusive units and related subvolcanic porphyries, sills and dikes which intrude both the metamorphic basement and the Ibáñez formation, are recognized: the Quebrada Colorada Granodiorite (QCG) and the Cerro Esmeralda Tonalite (CET). The polymetallic mineralization was formed in four stages and includes pyrite, arsenopyrite, gold, silver, sphalerite, galena and chalcopyrite hosted in felsic tuffs of the Ibáñez formation, hydrothermal breccias and epizonal intrusions. Field observations and geochronological data (U-Pb, Ar-Ar and fission track) on CET rocks suggest: (1) the development of subsiding pull-apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid-Jurassic time; (2) a tectonic exhumation of the rocks of the district, probably triggered by the Chile ridge–trench interactions that started in Late Miocene time. Sr-Nd isotopic data from CET samples, and the Pb isotope characteristics of galenas from the second and third mineralization stages, are indicative of crustal contribution to the origin of the magmas and galena mineralization. Rocks of the Paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas.     

The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O¹⁸ by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC¹³ values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C¹³ content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.     

Late Hercynian U-vein mineralization in the Alps: fluid inclusion and C,O, H isotopic evidence for mixing between two externally derived fluids

Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with D –34 to –49. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with D –50 to –65. All fluids contain trace N₂, CO₂ and probably CH₄. The carbonates have ¹³C –8 to –14. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich ( –35) low fO₂, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.     

The Malines Cambrian carbonate-shale-hosted Pb-Zn deposit,France: Thermometric and isotopic (H,O) evidence for pulsating hydrothermal mineralization

Microthermometric, stable isotope (D/H, ¹⁸O/¹⁶O), and Raman spectroscopic data are given for four different mineralization events located beneath a Triassic unconformity. These events include the Pb-Zn-bearing K_II mineralization which fills geodes and fractures in the main karstic mineralization K_I and the F mineralization which fills fractures in the Cambrian carbonates and shales. Post-K_I, pre-K_II, and F mineralization fluids were hot (150°C), moderately saline (10 equiv. wt% NaCl), and precipitated red dolomite. Subsequently, equally hot but more saline (20 equiv. wt% NaCl) fluids deposited white dolomite and then sphalerite of the F mineralization; they were followed by another generation of dolomite at about 70°C from fluids with about 15 equiv. wt% NaCl and, even later, barites (165°C, 10 equiv. wt% NaCl). Variation in the homogenization temperature with salinity for the different dolomite generations suggests that the hot saline fluids were repeatedly diluted by a cooler, less-saline fluid. The fluids are interpreted to be of formation-water origin having possibly developed in the deep levels of the Rhone basins to the southeast, which was at least 3 km thick at the time of mineralization (post-Hetangian). The inferred metal-bearing hot brines were probably episodically expulsed during the dewatering of the basin, depositing their mineralization in the more permeable fault and karstic zones associated with the uplift of the St. Bresson horst. Cold surface waters probably invaded the mineralized zones between the pulses of hydrothermal solutions. This hydrothermal model with the introduction of sulfides can account for the precipitation and dissolution textures associated with the hydrothermal mineralizations.