Origin and diagenetic evolution of Ca–Mg–Fe carbonates in some coalfields of Japan

Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca²⁺ and Mg²⁺ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.     

Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.     

Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan

Magnesite, siderite and dolomite are characteristic alteration minerals occurring in Miocene hanging wall rocks of dacitic composition which host the Kuroko orebodies. These carbonates generally occur in a more stratigraphically upper horizon than chlorite alteration zone surrounding the orebodies. The Mg/(Mg+Fe) ratios of the carbonates decrease from the central alteration zone to marginal zone. The Mg/(Mg+Fe) ratios of carbonates and chlorite positively correlate. The δ¹⁸O and δ¹³C values of magnesite, siderite and dolomite positively correlate with each other and lie between the igneous and marine carbonate values. The petrographic, isotopic and fluid inclusion characteristics and thermochemical modelling calculations indicate that magnesite and dolomite formed in the central zone close to the orebodies due to the interaction of hydrothermal solutions with the biogenic marine carbonates. Calcite formed further from the orebodies from hydrothermal fluids which did not contain a biogenic marine carbon component. The compositional and textural relationships indicate that superimposed alterations (chlorite alteration and carbonate alteration) occurred in hanging wall rocks. The mode of occurrences and the Mg/(Mg+Fe) ratios of magnesite and dolomite occurring in hanging wallrocks are useful in the exploration for concealed volcanogenic massive sulfide-sulfate deposits. Received: 9 September 1997 / Accepted: 23 September 1997     

内蒙古二连盆地白音查干凹陷下白垩统腾格尔组热水沉积过程中矿物沉淀顺序及其控制因素*

基于岩心、岩石薄片及元素分析等资料,通过偏光显微镜、定量矿物扫描(QemScan)、电子探针、能谱和流体包裹体分析等技术手段,探讨了二连盆地白音查干凹陷下白垩统腾格尔组热水沉积岩中热水沉积矿物的沉淀顺序。研究区热水沉积矿物主要为铝硅酸盐和碳酸盐,含少量黄铁矿、重晶石、萤石等矿物。这些矿物在空间上具有特定的先后关系或位置关系: (1)在纹层状岩层中,由下到上从铝硅酸盐过渡为碳酸盐。在铝硅酸盐矿物组合中,首先出现的是方沸石,其次是钠沸石,再次为钠长石;在碳酸盐矿物中,首先出现的是菱镁矿,其次是菱铁矿,再次为铁白云石,有时还可见到方解石,但其含量很低。(2)在凝絮状团块中,团块中心为黄铁矿,从中心向外依次过渡为方沸石、钠沸石、菱镁矿、菱铁矿,最外层为铁白云石。在纹层状岩层和凝絮状团块中,有时还能见到上述顺序的逆顺序。(3)在后期充填的裂缝中,边部为沸石或绿泥石等铝硅酸盐矿物,向中心逐渐过渡为铁白云石碳酸盐。综合研究区所有的矿物序列来看,最完整的序列应为黄铁矿—钾长石—方沸石—钠沸石—钠长石—菱镁矿—菱铁矿—白云石—方解石,但是这种完整的序列是不常见的,通常仅为其中某几种矿物的组合,然而不管是哪些矿物进行组合,其出现的顺序都不变。推测该矿物序列的类型受热液喷流和溢流阶段、时间、温度和热水化学性质控制,早期高温、高盐度条件下出现的铝硅酸盐矿物序列较多,而晚期主要形成碳酸盐矿物序列。     

Silicate and carbonate mineral weathering in soil profiles developed on Pleistocene glacial drift (Michigan, USA): Mass balances based on soil water geochemistry

Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO₂. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg²⁺/Ca²⁺ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca²⁺ and Mg²⁺ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg²⁺ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.     

X射线粉晶衍射仪在大理岩鉴定与分类中的应用

大理岩主要有方解石大理岩、白云石大理岩和菱镁矿大理岩三种。以往大理岩是依据偏光显微镜下观察岩石结构构造及矿物成分进行分类定名,由于方解石、白云石、菱镁矿都属于三方晶系,具有闪突起、高级白干涉色、一轴晶负光性和菱形解理等相同晶体光学特征,偏光显微镜下区分十分困难。为了准确鉴定大理岩中碳酸盐矿物种类及其相对含量,本文利用岩石薄片偏光显微镜和X射线粉晶衍射技术对32件大理岩岩石样品进行分析测试。岩石薄片鉴定结果表明:大理岩造岩矿物主要有方解石、白云石、菱镁矿、石英、斜长石、白云母、黑云母、绿泥石、黏土和金属矿物。根据岩石结构构造及矿物组分特征,可把32件大理岩样品划分为方解石大理岩、长英质方解石大理岩、石英绿泥白云石大理岩、白云石大理岩、云英质白云石大理岩和菱镁矿大理岩等15个类型。X射线粉晶衍射分析表明:大理岩造岩矿物主要有方解石、白云石、菱镁矿、石英、斜长石、钾长石、云母、绿泥石、滑石和蒙脱石。综合分析认为:岩石薄片偏光显微镜鉴定技术很难区分方解石、白云石和菱镁矿等碳酸盐矿物,以及细小的石英、钾长石和斜长石、滑石和白云母等鳞片状硅酸盐矿物;X射线粉晶衍射分析技术不仅能准确检测出大理岩中方解石、白云石和菱镁矿等碳酸盐矿物种类及相对含量(方解石、白云石和菱镁矿的X射线衍射主峰有明显差异,d值分别为0.303 nm、0.288 nm和0.274 nm),而且能够有效鉴别岩石中粉砂级斜长石、钾长石与石英(三种矿物的X射线衍射主峰d值分别为0.319 nm、0.324 nm、0.334 nm);且能区分蒙脱石、绿泥石、云母和滑石等层状硅酸盐矿物(四种硅酸盐矿物的X射线衍射主峰d值分别为1.400 nm、0.705 nm、0.989 nm、0.938 nm)。综合岩石薄片偏光显微镜鉴定和X射线粉晶衍射分析结果,最终确定32件大理岩样品划分为22个岩石类型。研究认为:仅根据岩石薄片偏光显微镜鉴定或X射线粉晶衍射技术其中一种方法不能准确鉴定大理岩岩石,应将大理岩岩石野外观察、岩石薄片鉴定和X射线粉晶衍射技术结合起来,才能准确确定大理岩岩石类型。     

Carbonate minerals in Bulgarian coals with different degrees of coalification

Coals of various rank (ranging from lignite to anthracite), and originating from seven coal basins in Bulgaria, were investigated for the occurrence and types of carbonate minerals. The following carbonates are present in their characteristic forms: Calcite (clastic, organogenic, cryptograined, xenomorphic, massive and infiltrational); siderite (cryptograined, xenomorphic both infilling and replacing, massive, euhedral, framboidal and infiltrational both fracture-filling and cleat-filling); dolomite (cryptograined, xenomorphic, massive and infiltrational) and magnesite.Carbonate mineralization depends on the penetration of ready mineral forms and mineralized solutions during peat genesis and the infiltration of mineralized solutions during coalification. The mineral composition of the carbonates in the coals studied and the occurrences of the various carbonate minerals depend on the combination of a series of factors such as: (1) the presence of carbonate rocks in the province feeding the peat bogs; (2) the way and capacity of feeding; (3) acidity (pH) and reducing conditions (Eh) of the environment, (4) tectonic situation during the peat genesis; and (5) availability of infiltrational mineral solutions during coalifications.No infiltrational carbonate mineralization was found in the lignites studied. Due to the combination of the above five factors, bituminous coals from the Balkan basin and the anthracite from Svoge are characterized by the greatest variety of carbonate minerals and their forms.     

Aragonite and magnesite in eclogites from the Jæren nappe,SW Norway: disequilibrium in the system CaCO3–MgCO3 and petrological implications

Eclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz‐rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clinozoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO‐, MgO‐ and CO₂‐bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg‐calcite. The well‐preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite‐magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions.     

Diagenetic crystallization and oxidation of siderite in red bed (Buntsandstein) sediments from the Central Iberian Chain,Spain

Compactional deformation facilitated replacement of dolomite and calcite by siderite and its subsequent oxidation in carbonate cemented red beds of the Triassic Buntsandstein in the Iberian Chain. Locally, the sedimentary clasts were cemented by carbonate that was derived from dissolution of locally exposed dolomite in the basement. Microstructures indicate that during sedimentation of the rocks, oxidizing conditions prevailed in the sediments and the basement was reddened by impregnation of hematite. Reducing conditions prevailed during deformation of the sediments. Ferric iron was reduced to Fe²⁺, that reacted with deformed dolomite and calcite cement to produce fine grained siderite. At a later stage, siderite crystallites were (partly) oxidized to form a secondary phase of brown ferric oxide (goethite). Locally, goethite transformed to fine grained hematite that caused secondary reddening of the sediments. The reactions are associated with a combined volume loss of the solid phases of c. 50% per reaction mol; this was accommodated by the formation of pores. Oxidation of siderite was associated with release of CO₂; localized dissolution took place of feldspar and concurrently growth of kaolinite occurred by acidifying condition during release of CO₂. The relation of redox reactions and deformation is comparable to those in red bed conglomerates in the region. Reductive dissolution occurred at sites of stress concentration, particularly at contact points of pebbles. Late stage precipitation of ferric oxides and pyrolusite took place at oxidizing conditions in association with uplift.     

祁连山木里三露天天然气水合物层段碳酸盐矿物学地球化学特征

祁连山木里三露天天然气水合物与碳酸盐相伴产出是一种常见现象。为探讨这些碳酸盐的形成环境,对碳酸盐进行了矿物X射线衍射分析、主微量元素 测试以及碳氧同位素测试。X射线衍射结果表明,样品中碳酸盐矿物主要由方解石、白云石以及少量菱铁矿组成。微量元素和稀土元素特征显示碳酸盐形成于干旱氧化环境。δ¹³C_VPDB为-2.3 ‰~3.77‰,平均为2.43‰;δ18OVPDB为-17.90‰~-10.69‰,变化范围不大,碳氧同位素显示碳酸盐矿物中的碳可能为天然气水合物分解、白云石沉淀和湖水混染等联合作用所致,氧的 来源可能受大气降水影响。     

Exsolution of dolomite and application of calcite–dolomite solvus geothermometry in high‐grade marbles: an example from Skallevikshalsen,East Antarctica

Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ～460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.     

东营凹陷中央隆起带沙河街组碳酸盐胶结物发育特征及其形成机制

碳酸盐胶结物是东营凹陷中央隆起带砂岩储层中重要的自生矿物,但其形成机制目前尚无深入研究.首先在岩相观察下针对东营凹陷中央隆起带沙河街组的碳酸盐胶结物进行期次划分并归纳其发育特征,再依据各期碳酸盐胶结物的共生矿物、碳氧同位素组成、流体包裹体温度、元素化学成分等信息对其形成机制进行研究.研究结果表明:研究区碳酸盐胶结物的碳不仅受到有机质成熟过程中排放的有机酸的影响,还受到沙四段沉积的湖相碳酸盐岩溶解的影响.研究区沙河街组主要发育四期碳酸盐胶结物:第一期碳酸盐胶结物以白云石为主,其形成过程与甲烷细菌对有机质的分解作用有关;第二期碳酸盐胶结物以方解石为主,与第一期碳酸盐胶结物之间夹有一层绿泥石薄膜,胶结物的形成与孔隙流体的浓缩导致的Ca、HCO₃-过饱和现象相关;第三期碳酸盐胶结物主要为方解石、白云石和铁白云石,以充填长石溶孔和原生孔隙为特征,其物质来源于长石的溶蚀及泥岩的脱水作用;第四期碳酸盐胶结物多为铁方解石和铁白云石,以充填早期碳酸盐溶蚀后形成的次生孔隙为特征,物质来源于粘土矿物的转化,多与黄铁矿颗粒共生,其形成过程受到烃类流体活动的影响.      

Origin of giant stratabound deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by low-grade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics.     

Hydrothermal mixing,carbonate dissolution and sulfide precipitation in Mississippi Valley-type deposits

A large number of Mississippi Valley-Type (MVT) deposits are located within dissolution zones in carbonate host rocks. Some genetic models propose the existence of cavities generated by an earlier event such as a shallow karstification, that were subsequently filled with hydrothermal minerals. Alternative models propose carbonate dissolution caused by the simultaneous precipitation of sulfides. These models fail to explain either the deep geological setting of the cavities, or the observational features which suggest that the dissolution of carbonates and the precipitation of minerals filling the cavities are not strictly coeval. We present a genetic model inspired by the textural characteristics of MVT deposits that accounts for both the dissolution of carbonate and precipitation of sulfides and later carbonates in variable volumes. The model is based on the mixing of two hydrothermal fluids with a different chemistry. Depending on the proportion of the end members, the mixture dissolves and precipitates carbonates even though the two mixing solutions are both independently saturated in carbonates. We perform reactive transport simulations of mixing of a regional groundwater and brine ascending through a fracture, both saturated in calcite, but with different overall chemistries (Ca and carbonate concentrations, pH, etc). As a result of the intrinsic effects of chemical mixing, a carbonate dissolution zone, which is enhanced by acid brines, appears above the fracture, and another zone of calcite precipitation builds up between the cavity and the surrounding rock. Sulfide forms near the fracture and occupies a volume smaller than the cavity. A decline of the fluid flux in the fracture would cause the precipitation of calcite within the previously formed cavities. Therefore, dissolution of carbonate host rock, sulfide precipitation within the forming cavity, and later filling by carbonates may be part of the same overall process of mixing of fluids in the carbonate host rock.Editorial handling: C. Everett     

Mineralogy of primary carbonate-bearing hypabyssal kimberlite, Lac de Gras, Slave Province, Northwest Territories, Canada

A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis. The samples have excellent preservation of textures and primary mineralogy and are archetypal or Group 1 kimberlite. Five kimberlite localities are identified as calcite-bearing based on the presence of high Sr–Ba calcite as phenocrysts, microphenocrysts and in segregations. Three kimberlite localities are identified as dolomite-bearing based on the presence of mixed calcite–dolomite segregations containing oscillatory and banded textures of calcite–dolomite solid solution and dolomite (±magnesite). Sr–Ba calcite are characterized by high X_Ca (>0.95) and are enriched in Sr (4900–11,100 ppm) and Ba (3200–14,200 ppm). The calcite–dolomite and dolomite–magnesite solid solution compositions span the X_Ca range from 0.42 to 0.95, and typically have Sr and Ba contents in the range of 1000–4000 ppm. The carbonate, silicate and oxide mineral compositions suggest that the origin of the calcite-bearing versus dolomite-bearing kimberlites studied is related to subtle differences in parent magma composition, in particular, the CO₂/H₂O ratio. Formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence, in which Sr–Ba calcite precipitates from an evolved kimberlite melt. Subsequently, calcite–dolomite solid solution and dolomite is precipitated from localized, Mg-rich carbonate fluids at relatively high temperatures (higher than serpentine stability).     

Metal release from dolomites at high partial-pressures of CO2

The potential for metal release associated with CO₂ leakage from underground storage formations into shallow aquifers is an important consideration in assessment of risk associated with CO₂ sequestration. Metal release can be driven by acidification of groundwaters caused by dissolution of CO₂ and subsequent dissociation of carbonic acid. Thus, acidity is considered one of the main drivers for water quality degradation when evaluating potential impacts of CO₂ leakage. Dissolution of carbonate minerals buffers the increased acidity. Thus, it is generally thought that carbonate aquifers will be less impacted by CO₂ leakage than non-carbonate aquifers due to their high buffering potential. However, dissolution of carbonate minerals can also release trace metals, often present as impurities in the carbonate crystal structure, into solution. The impact of the release of trace metals through this mechanism on water quality remains relatively unknown. In a previous study we demonstrated that calcite dissolution contributed more metal release into solution than sulfide dissolution or desorption when limestone samples were dissolved in elevated CO₂ conditions. The study presented in this paper expanded our work to dolomite formations and details a thorough investigation on the role of mineral composition and mechanisms on trace element release in the presence of CO₂. Detailed characterization of samples from dolomite formations demonstrated stronger associations of metal releases with dissolution of carbonate mineral phases relative to sulfide minerals or surface sorption sites. Aqueous concentrations of Sr²⁺, CO²⁺, Mn²⁺, Ni²⁺, Tl⁺, and Zn²⁺ increased when these dolomite rocks were exposed to elevated concentrations of CO₂. The aqueous concentrations of these metals correlate to aqueous concentrations of Ca²⁺ throughout the experiments. All of the experimental evidence points to carbonate minerals as the dominant source of metals from these dolomite rocks to solution under experimental CO₂ leakage conditions. Aqueous concentrations of Ca²⁺ and Mg²⁺ predicted from numerical simulation of kinetic dolomite dissolution match those observed in the experiments when the surface area is three to five orders of magnitude lower than the surface area of the samples measured by gas adsorption.     

Magnesite diagenesis in redbeds: a case study from the Permian of the Northern Calcareous Alps (Tyrol, Austria)

The Upper Permian Gröden Formation of the Northern Calcareous Alps (Austria) is composed of alluvial fan and playa lake sediments that were deposited in intramontane basins. A conspicuous feature of these redbeds is the abundance of magnesite in the form of nodules and discrete layers in mudstones as well as intergranular cement in sandstones. Sedimentological observations indicate that the bulk of these carbonates formed during early diagenesis and were probably syndepositional. Petrographically, most magnesites consist of micrite or, less commonly, microspar. An early non-ferroan magnesite is post-dated by later stage ferroan magnesites. Nodules consisting of recrystallized, sparry magnesite were observed only at one location. The general absence of relics of a non-magnesite precursor mineral and the occurrence of shrinkage features suggest that the fine grained magnesites formed by transformation of a hydrated magnesium carbonate mineral, e.g. hydromagnesite. Carbon, oxygen, sulphur and strontium isotope ratios in conjunction with sedimentological criteria support a model of (hydro)magnesite precipitation in an inland playa lake system, which was fed by run-off from the surrounding hinterland. The scarcity of evaporites and the dominance of magnesite over calcite and dolomite suggest that the playa lake brines were low in sulphate and had high Mg/Ca ratios. The source for the high magnesium concentrations is thought to be the weathering of Devonian dolostones and associated massive magnesite deposits in the catchment area.     

87Sr/86Sr in kimberlitic carbonates by ion microprobe: Hydrothermal alteration,crustal contamination and relation to carbonatite

Carbonates in a 30 cm wide zoned kimberlite dyke from the De Beers Mine, Kimberley, S. Africa were studied by cathodoluminescence and electron microprobe techniques and their ⁸⁷Sr/⁸⁶Sr ratios were measured using an AEI-IM20 ion microprobe. Primary carbonates (including calcite dendrites, rhombohedral calcites in segregation vesicles and mosaic dolomite) have high Sr (0.69–1.35 wt.% SrO) and Ba (0.24–0.44% BaO) and ⁸⁷Sr/⁸⁶Sr ratios in the range 0.7046 to 0.7056. Secondary sparry calcite in amygdales and veins is characterised by low Ba (<0.05% BaO) and ⁸⁷Sr/⁸⁶Sr near 0.72. Rhombohedral calcite 0.5 cm from a contact with 2,900 my. old biotite-gneiss has minor element chemistry like that of primary carbonate, but an elevated ⁸⁷Sr/⁸⁶Sr ratio of 0.7103, possibly indicating crustal contamination in a boundary layer of the kimberlite magma. Amygdale-like segregations of carbonate and/or serpentine originated as gas-cavities and were not formed by liquid immiscibility. They are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid. However, dendritic calcite and primary dolomite and calcite with high Sr, Ba and low ⁸⁷Sr/⁸⁶Sr demonstrate shared chemical characteristics between these carbonates and carbonatite. The primary kimberlite magma had initial ⁸⁷Sr/⁸⁶Sr close to 0.7046.     

Manganese removal from mine waters – investigating the occurrence and importance of manganese carbonates

Manganese is a common contaminant of mine water and other waste waters. Due to its high solubility over a wide pH range, it is notoriously difficult to remove from contaminated waters. Previous systems that effectively remove Mn from mine waters have involved oxidising the soluble Mn(II) species at an elevated pH using substrates such as limestone and dolomites. However it is currently unclear what effect the substrate type has upon abiotic Mn removal compared to biotic removal by in situ micro-organisms (biofilms). In order to investigate the relationship between substrate type, Mn precipitation and the biofilm community, net-alkaline Mn-contaminated mine water was treated in reactors containing one of the pure materials: dolomite, limestone, magnesite and quartzite. Mine water chemistry and Mn removal rates were monitored over a 3-month period in continuous-flow reactors. For all substrates except quartzite, Mn was removed from the mine water during this period, and Mn minerals precipitated in all cases. In addition, the plastic from which the reactor was made played a role in Mn removal. Manganese oxyhydroxides were formed in all the reactors; however, Mn carbonates (specifically kutnahorite) were only identified in the reactors containing quartzite and on the reactor plastic. Magnesium-rich calcites were identified in the dolomite and magnesite reactors, suggesting that the Mg from the substrate minerals may have inhibited Mn carbonate formation. Biofilm community development and composition on all the substrates was also monitored over the 3-month period using denaturing gradient gel electrophoresis (DGGE). The DGGE profiles in all reactors showed no change with time and no difference between substrate types, suggesting that any microbiological effects are independent of mineral substrate. The identification of Mn carbonates in these systems has important implications for the design of Mn treatment systems in that the provision of a carbonate-rich substrate may not be necessary for successful Mn precipitation.