Sparry Magnesite Research Articles

Heritage Stone 8. Formation of Pinolitic Magnesite at Quartz Creek, British Columbia, Canada: Inferences from Preliminary Petrographic, Geochemical and Geochronological Studies

Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.

Mineral control on the geochemistry of the Rock Canyon Creek REE-F-Ba deposit, British Columbia, Canada

The Rock Canyon Creek carbonate-hosted REE-F-Ba deposit has tectonic, stratigraphic and structural similarities with Mississippi Valley-type and sparry magnesite deposits in the SE Rocky Mountains. The main REE-fluorite zone is a steeply dipping body, extending 1100 m along-strike, 50 m wide and 100 m deep. It spatially coincides with pre-existing crackle breccias in carbonate rocks, and consists of dolomite, fluorite, barite, pyrite, quartz, K-feldspar, calcite, porous apatite, REE-fluorocarbonates and REE-phosphates. The main fluorocarbonates are bastnaesite, parisite and synchysite. Monazite, crandallite group minerals and apatite are the main phosphates. Fluorite content varies from less than 1 to 13.5% (by weight) and ∑REE + Y concentrations vary from trace to 1.95% (by weight). The mineralized zone is heterogeneous on the deposit scale, as indicated by three-dimensional geochemical modelling combined with a geochemical assessment based on 89 mineralized samples and detailed downhole mineral and geochemical profiles of a key borehole. Chemical heterogeneity and key elemental co-variations are explained by strong mineralogical control and have implications for the design of exploration and development programmes for this type of deposit. The chondrite-normalized REE pattern of samples from the mineralized zone shows enrichment in LREE, similar to typical carbonatite-related mineralization; however, no carbonatite is exposed nearby.

Indicator Isotope–Geochemical Characteristics of Sulfides from the Satka Magnesite Ore Field (South Urals Province)

The stable enrichment of pyrite from magnesite ores in δ34S isotope (from 5.4 to 6.9‰) compared with pyrite from the host (sedimentary and igneous) rocks was established in the classical Satka sparry magnesite ore field. Concretionary segregations of fine-grained pyrite in dolomite are depleted in the heavy sulfur isotope (δ34S, from–9.1 to–5.8‰). Pyrite from dolerite is characterized by δ34S values (–1.1 and 1.7‰) close to the meteorite sulfur. The δ34S values in barite from the underlying dolomite horizon vary in the range of 32.3–41.4‰. The high degree of homogeneity of the sulfur isotope composition in pyrite from magnesite is a result of thermochemical sulfate reduction during the syngenetic crystallization of pyrite and magnesite from epigenetic brines, formed during dissolution of evaporite sulfate minerals at the stage of early catagenesis of the Riphean deposits.

Geology, composition, and physicochemical model of sparry magnesite deposits of the Southern Urals

The metasomatic nature of magnesite formation, sequence and timing of geological processes, and solution sources have been established by comprehensive geological and geochemical study of the typical Satka and Ismakaevo deposits of sparry magnesite in the South Ural province. The hydrothermal metasomatic formation of magnesite is related to injection of high-Mg evaporite brine into heated carbonate rocks within permeable rift zones. The numerical physicochemical simulation of solution–rock interaction allowed us to determine the necessary prerequisites for sparry magnesite formation: the occurrence of marine salt solutions with a high Mg/Ca ratio and heating of solutions before or during their interaction with host carbonate rocks. The contribution of compositionally various solution sources, the temperature variation regime, proportions of CO2 and H2S concentrations in solution created specific features of particular deposits.

Review of the Metallogenic Regularity of Magnesite Deposits in China

AbstractChina has abundant reserves of magnesite, making it the world's leading source of this strategic mineral. Sparry magnesite is the main type of magnesite deposit, and is easy to exploit. It occurs mainly as the sedimentary‐metamorphic type. Production is centred on eastern Liaoning Province, where a world‐class large to super large magnesite ore processing and production facility has been developed. Hydrothermal metasomatic deposits, associated with ultramafic complexes and eluvial deposits produced by weathering, are two other important types found in China. The Western section of the Bangonghu‐Nujiang metallogenic belt is an important target region for prospecting lake‐sedimentary magnesite deposits. Based on a systematic analysis of material from 62 magnesite production areas, this study investigated the metallogeny of magnesite and delineated 13 magnesite metallogenic belts. Maps were produced showing metallogenic regularities in magnesite deposits, the metallogenic system of the magnesite deposits, and the distribution of the metallogenic belts of Chinese magnesite deposits. It provides a theoretical basis for forecasting the location of potential magnesite resources in China. Finally, it explores some key scientific issues, including the formation processes of ultra magnesite ore‐concentrated areas, and their sources of magnesium.

Mineralogical, geochemical, fluid inclusion and isotope study of Hohentauern/Sunk sparry magnesite deposit (Eastern Alps/Austria): implications for a metasomatic genetic model

The magnesite deposit of Hohentauern/Sunk, hosted in Visean carbonate rocks, is one of the best-exposed examples of “Veitsch type” sparry magnesite deposits, which are located in the Veitsch nappe of the Austroalpine Greywacke zone (Eastern Alps/Austria). The sparry magnesite is stratiform and stratabound within the Visean Steilbachgraben Formation and displays distinct metasomatic features and textures. The sparry magnesite of the Hohentauern/Sunk deposit is characterized by pinolites, rosettes and banded (zebra) textures. Due to microinclusions of dolomite and redolomitzation, the CaO/MgO of magnesite is elevated. Concentrations of selected major and trace elements (Fe-Mn, Sr-Ba, Cr-Ni) in sparry magnesite indicate formation in a marine/evaporitic environment and by Mg2+-metasomatism. The REE patterns of magnesite are characterized by low LREE/HREE, depletion of LREE and a negative Ce anomaly. The δ18O (9.59 to 12.32 ‰ SMOW) and δ13C (−2.23 to −0.02 ‰ PDB) values of magnesite overlap with those published for magnesite formed by metasomatic replacement of dolomite. Fluid inclusions in the sparry magnesite indicate a high salinity (22.4 mass% NaCl equivalent) of the Mg-rich fluid; it is of marine/evaporitic origin. All geological, petrographical and geochemical features support a diagenetic dolomitization of the carbonate host rocks followed by magnesite formation via metasomatic replacement and redolomitization. Sm-Nd geochronology indicates a Late Carboniferous to Early Permian age for magnesite formation triggered by intraformational circulation of fluids derived from buried Carboniferous evaporites.

Sm–Nd dating of hydrothermal carbonate formation: An example from the Breitenau magnesite deposit (Styria, Austria)

The geochemical evolution and chronology of the Breitenau sparry magnesite deposit has been investigated by means of major and trace element abundances, stable C–O isotope, and Sm–Nd isotope analysis. Chemically separated magnesite fractions (leachates) show consistent light-REE depleted REY patterns (marine carbonate normalized) and nearly constant normalized values for the middle and heavy REE without significant Ce, Eu or Y anomalies. Non-carbonate residues (composed of dark matrix material) are high in Al2O3 and show variably light to middle REE enriched REY patterns similar to shales. Leachates of limestone from nearby country rocks have normalized REY patterns similar to marine carbonates. Only Ce and Y are slightly fractionated. Based on their trace element chemistry and normalized REY patterns, leachates of dolomite from the margin of the deposit closely resemble either limestone (Type A) or magnesite (Type B). With one exception, δ18OVPDB for both dolomite types is within the range of the respective magnesite values (δ18OVPDB c. −14.2 to −15.7‰). Leachates of dolomite from druses within the deposit (Type C) show low REY abundances with normalized patterns similar to those of magnesite. Sm–Nd isotope data for magnesite leachates yield a well-defined isochron age of 222.5±9.8Ma (MSWD 3.1). Isochron calculations for leachate(s) and residue(s) from individual magnesite samples (“internal isochrons”) yield ages between 236±16Ma and 193.5±8.6Ma with a low degree of scatter (MSWD<1.3). The most precise estimate for the time of Sm–Nd isotopic equilibration between carbonates and their host assemblages, both on hand specimen and deposit scale, is obtained from the mean of internal isochron calculations for eight samples (229.3±2.4Ma; MSWD 0.65) and is interpreted to date hydrothermal formation of the magnesite deposit. The scatter in initial Nd isotopic composition of the eight internal isochrons (0.511891±0.000014; MSWD 7.7) indicates geologically based heterogeneities in isotopic composition on the deposit scale. Sm–Nd isotope data for Type C dolomite scatter off the upper right of the magnesite isochron. In combination with the respective O isotopic composition, the Sm–Nd isotope data of Type C dolomite can be modeled in terms of three distinct age groups: 1) 130±20Ma (δ18OVPDB c. −15‰), 2) 37.9±5.4Ma (δ18OVPDB c. −12.5‰), and 3) −1.7±6.8Ma (δ18OVPDB c. −11‰). Type C dolomite data indicate that druses were formed by open-system fluid flow throughout the deposit.

Genetic and metamorphic conditions constrained by fluid inclusions from Paleoproterozoic (c. 1.8 Ga) magnesite ore deposits, NE Brazil

Precambrian magnesite occurrences hosted by metadolomites from the Orós belt, Ceará, Brazil, are part of a greenschist–amphibolite, metavolcano-sedimentary terrain, dated at 1.8 Ga, cut by Meso- to Neoproterozoic Brasiliano granites and Neoproterozoic basic sills. These rocks were affected by a shear zone between 580 and 500 Ma. The magnesite-bearing marbles can be grouped as medium-grained (1–9 mm) at the Riacho Fundo ore deposit or sparry magnesite (1–15 cm) at the Cabeça de Negro ore deposit. The sparry magnesite shows textural characteristics related to original sedimentary structures. Both types of magnesite-bearing marbles contain aqueous and aqueous-carbonic fluid inclusions that yield homogenization temperatures between 170 and 370 °C. Applying a pressure correction, these temperatures are compatible with the evolution from greenschist to amphibolite facies metamorphic conditions, as described in previous work on the Orós region. It also agrees with data in specialized literature on the metamorphism of carbonate rocks. Fluid inclusion distribution, composition, and physical-chemical characteristics suggest temperature increase, probably related to metamorphism on these rocks. The medium-grained magnesite records partial contamination of CO 2-rich inclusions by relict carbonaceous material (bitumen, hydrocarbons?) that favors, but does not confirm, a syngenetic sedimentary origin and could have caused the lowering of CO 2 melting point in these inclusions. Therefore, though textural evidence points to a sedimentary-diagenetic model, fluid inclusions record conditions of a metamorphic event.

Geology and geochemistry of paleoproterozoic magnesite deposits (∼1.8Ga), State of Ceará, Northeastern Brazil

Five important magnesite mines and small occurrences in the state of Ceará form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Orós region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ∼550Ma). Two types of magnesite marble may be distinguished: (1) medium-grained (1 to 9mm) and (2) sparry-magnesite marble (1 to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite of their deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant. The sparry-magnesite marble is less enriched in SiO2, Fe2O3, Al2O3 and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (lagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater. Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeça de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases. The calcitic marble, deposited on open-marine environment, displays positive13C (+0.5 to 2.1 ‰ PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to −3.2‰ PDB). This13C fluctuation is within the range for carbonates deposited around 1.8Ga. The depositional environment of these carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, through dolomitic marbles. This points to a process of chemical differentiation within a paralic system, from open sea to landwards, more calcic in the first to more magnesian in the confined environment, in which lagoon waters are progressively purified from Ca through preferential precipitation of calcic carbonates and secondly of gypsum/anhydrite, which increases the Mg/Ca ratio, and lead carbonate deposits towards the magnesite pole. These metacarbonates also reveal a fractionation of the light REE, starting from the calcitic, through the dolomitic to magnesitic marbles. In the calcitic marbles, REE are in association with the clay fraction, while in magnesitic marbles, they seem to be in association with organic and/or soluble complexes. The magnesitic marbles are of sedimentary origin and underwent important diagenetic evolution before being metamorphosed during the Brasiliano orogenic cycle.

Genesis of magnesite deposits in the view of isotope geochemistry

Isotopic data, i.e. δ 13 C and δ 18 O including 86 Sr/87 Sr, are the fundamental data to characterize geochemically magnesite mineralizations. Other geochemical data, e.g. chemical data, REE and fluid chemistry, are supplementary characteristics for genetic models. The geological setting and the origin of magnesium are relevant to classify magnesites genetically. The δ 13 C - δ 18 O - diagrams of magnesite and siderite are significantly different. Stratabound magnesite is dominated by the isotopic signature of carbonate sediments, while siderites from occurrences with economic importance preferably show light carbon isotope like the ultramafic-hosted vein type mineralization. The δ 18 O original values of sparry magnesites are strongly influenced by burial and grade of metasomatism. Thus, the origin of sparry magnesite hosted by metasediments is to be considered as a sedimentary enrichment influenced by diagenetic and metamorphic processes. An approach to the best genetic model of each magnesite deposit needs the synopsis of all geological facts and geochemical data.

Comparison of Lower and Middle Riphean sparry magnesite deposits of the Southern Urals province

Sparry magnesite deposits (SMD) of the Lower Riphean (LR) and Middle Riphean (MR) sequences in the western slope of the South Urals have some distinguished peculiarities, which allows to compare them and speculate about the mechanism of ore formation and Mg-source origin. LR magnesites are located in some stratigraphic levels and lie within widespread dolostone horizons. MR magnesites that occur in the lower carbonate member of Avzyan fm. are represented by dolomitized limestones. The shape of LR magnesite body is very often layer-like with sharp contacts. The shape of MR one is sometimes lens like, contacts of ore body are complecated with zones of impregnable magnesites near the hosting dolomites. The crystalline structures of LR magnesites are typically corse-grained with average size of crystalls 10 mm. The magnesite grains of the MR one have 1-3 mm of average size. The chemical composition of MR magnesite ore shows increase of SiO 2 , Al 2 O 3 , FeO and decrease of LREE fractionation in comparison with the LR magnesites. As we assume, metasomatic magnesites of the Lower Riphean time are connected with sedimentation/early diagenesis stage. The source of Mg 2+ came from a crust of weathering of basic and ultramafic rocks in humid climate. Magnesite occurrences in Middle Riphean time are connected with high Fe-contents metasomatic fluids, which were resulted of evaporite Mg-enriched solutions transformation during kathagenesis.

Petrographic, REE, fluid inclusion and stable isotope study of magnesite from the Upper Triassic Burano Evaporites (Secchia Valley, northern Apennines): contributions from sedimentary, hydrothermal and metasomatic sources

Sparry and microcrystalline magnesite are minor constituents of the Upper Triassic Burano Evaporite Formation of the northern Apennines in Italy. Petrography and geochemistry of magnesite suggest three modes of formation. (1) Evaporitic precipitation of stratified microcrystalline magnesite layers associated with sulfate and carbonate rocks. Most REE are below ICP-MS detection limits. δ18O is +20.2‰ (SMOW) and δ13C is –2.6‰ (PDB). (2) Hydrothermal infill of Fe-rich (9.78 wt% FeO) lenticular sparry magnesite. This type of magnesite is characterized by very low LREE concentrations, whereas HREEs are relatively high. The fluid inclusion composition is NaCl–MgCl2–H2O, salinity is ~30 wt% NaCl equiv., and total homogenization temperatures range from 204–309 °C; δ18O is +17.5‰ and δ13C is +1‰. (3) The partial or total replacement of dolostones by lenticular sparry magnesite. LREEs are lower in magnesite compared with the partly replaced dolostones. Magnesite yields δ18O and δ13C compositions of +17.3 to +23.6‰ and +0.5 to +1.4‰, respectively, whereas the partly replaced dolostones yield δ18O and δ13C values of +25.0 to +26.2 and +1.3 to +1.9, respectively. Complete replacement of dolostones produced massive lenticular sparry magnesite rock containing ooids and axe-head anhydrite relicts; LREEs are depleted compared to unaffected dolostones; δ18O and δ13C compositions range from +16.4 to +18.4‰ and +0.4 to +0.9‰, respectively. These data and the association between fracture-filling and replacive magnesite suggests a metasomatic system induced by hydrothermal circulation of hot and saline Mg-rich fluids. These processes probably occurred in the Oligocene–Miocene, when the Burano Formation acted as main detachment horizon for the Tuscan Nappe during the greenschist facies metamorphism of the Apuane complex. Thrusting over the Apuane zone produced large scale fluid flow focused at the Tuscan Nappe front. Sources of Mg-rich fluids were metamorphic reactions in the Apuane complex and dissolution of Mg-salts at the thrust front. Considering a maximum tectonic burial depth of 10 km, as inferred from the geometry of the chain, the pressure-corrected temperature of magnesite precipitation (380 to 400 °C) and the calculated fluid composition (δ18O=+13.3±1.2‰) are in the range of the published Apuane metamorphic temperatures (300–450 °C) and fluid compositions (δ18O=7–16‰). The results of this study support the hydrothermal-metasomatic model for the formation of sparry magnesite deposits at the expense of dolostone units involved in thrusting and low-grade metamorphism, as proposed for the Northern Graywacke Zone (Alps) and the Eugui deposit (western Pyrenees).

Timing of post-depositional events in the Burano Formation of the Secchia valley (Upper Triassic, Northern Apennines), clues from gypsum–anhydrite transitions and carbonate metasomatism

Abstract The Burano Evaporite Formation from the Secchia River Valley is an up to 2200 m-thick sequence composed of meter-to decameter-scale interbeds of gypsum–anhydrite and dolostones with minor halite. The deposit has been affected by a complex array of post-depositional modifications, thermal events and large-scale evaporite dissolution, preventing a satisfactory reconstruction of the environment of deposition. The modifications are intense because these rocks were the main decollement horizon during the formation of the Northern Apennines chain. The carbonate rocks are massive and-or laminated dolomitic mudstone, wackestone, oolitic packstones and oolitic, peloidal, bioclastic grainstones, which commonly appear as mega-boudins within a sulfate groundmass. The dolostones (δ 18 O =−5.7 to −3.7‰; δ 13 C =+1.3 to +3.0‰; PDB) have been affected by Mg-metasomatic replacement by magnesite (δ 18 O =−14.0 to −2.6‰; δ 13 C =−2.6 to +1.4‰; PDB) induced by hydrothermal circulation. Total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310°C. The anhydrite rocks are characterized by flow structures such as centimeter-scale pseudo-lamination composed of aligned prismatic crystals with transposed isoclinal folds outlined by dolostones fragments. Homogenization temperatures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305°C (Emilia) and from 230 to 315°C (Tuscany). The gypsum rocks are composed of xenotopic irregular cloudy crystals and, more rarely, by centimeter-scale idiotopic crystals showing the same structures as the anhydrite rocks. The origin of the gypsum rocks is due to late alteration of anhydrite by migration of sharp hydration fronts. The hydration is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro-relics and authigenic quartz crystals which include anhydrite. The role of the Burano Evaporites during the Apennines tectogenesis can be depicted as follows: (a) prevalent deposition of gypsum in the Upper Triassic; (b) gypsum dehydration at burial conditions to form anhydrite (Cretaceous?); (c) syn-tectonic flow of anhydrite rocks, brecciation of dolostones; syn-tectonic growth stage of quartz euhedra at deep burial conditions possibly related to the development of the Oligocene–Miocene greenschist facies Apuane metamorphic complex; (d) hydrothermal deposition of sparry magnesite and partial Mg-metasomatic replacement of dolostones by magnesite; (e) sub-surface dissolution of halite to form thick matrix-supported residual caprock-like anhydrite mega-breccias; (f) complete gypsification of anhydrite at sub-surface conditions; and (g) evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of most clasts (“Calcare cavernoso”).

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.

GEOLOGIA E GEQQUÍMICA DOS ELEMENTOS MAIORES DOS DEPÓSITOS DE MAGNESITA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS ( CEARÁ)

The five most important magnesite mines and some small occurrences of the Ceara State form an array of discontinuous lenses extending for about 140 km. The magnesite-rich rocks are hosted in metadolomites with lutecite, fibroradiated quartz pseudomorph after sulfate nodules, scapolite and dissolution breccias. The marbles are hosted by a metavolcanosedimentary sequence of the greenschist to amphibolite facies, and were intruded by basic sills and granites of size, form, composition and age that vary from the Meso to Neoproterozoic. Two types of magnesite marbles may be distinguished: (1) magnesite marble of medium grain-size (1 to 9mm) and (2) sparry magnesite marble (1 to 15cm). The sparry magnesite marble has porphyric, rosette, layered, and palisade textures, the latter having remains of the original sedimentary features. In spite of deformation, the sparry crystals are hipidiomorphic and pinolitic. The rocks vary from white to light gray or dark gray or even red. The dark banded terms have traces of microfossils and stromatolitic structures. The texture and color of the medium-grained magnesite marbles are more homogeneous and the crystals are more anhedral. The sparry magnesite marble is in general poorer in SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO and richer in MgO as compared to the medium-grained types. The paleogeographic interpretation suggests that these rocks formed near a paralic system (lagoon), with strong evaporitic conditions. The depressions were of variable depth and length and probably isolated by stromatolite barriers, dried and flooded by seawater with continental inflows. The largest and deepest deposits correspond to the sparry magnesite. Regionally, the magnesite deposits give place to dolomitic marbles, which grade into almost pure calcitic marbles, indicating a chemical differentiation within the paralic system, by which the lagoon waters turned progressively poorer in Ca by the precipitation of calcium carbonate, thus increasing the Mg/Ca ratio and leading to the precipitation of magnesite. The magnesite marble is than interpreted as of sedimentary origin, and underwent strong diagenesis before metamorphism and Neoproterozoic deformation.

A Note on Oxygen and Carbon Isotopes of Magnesite and Dolostone of Mesoproterozoic Gangolihat Dolomite (Formation), Pithoragarh District, Uttar Pradesh

Abstract Nine samples of sparry magnesite and five samples of dolostone of the Mesoproterozoic Gangolihat Dolomite (Formation) of the Chandak area, Pithoragarh are analysed for stable isotopes of carbon and oxygen. d18O for dolostone varies from -10.8‰ to -9.0‰ (SMOW) with mean as -10.1‰ (SMOW) and for magnesite it varies from -16.8‰ to -11.8‰ (SMOW) with mean as -14.0‰ (SMOW). d13C for dolostone varies from -1.6‰ to 0.6‰ (PDB) with mean as -0.0‰ (PDB) and for magnesite it varies from -2.2‰ (PDB) to 0.1‰ (PDB) with mean as -1.0‰ (PDB). Thus, magnesite and dolostone show distinctly different isotopic signatures and hence magnesite must have formed from isotopically different fluids than those which produced dolostones. Magnesite is not syndepositionally produced but must have formed at some later stage by hydrothermal solutions.

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

ABSTRACTThe 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.

Origin of the sparry magnesite deposits around Bauri, Almora district, Uttar Pradesh, India

Seven pockets of variable dimensions of strata-bound sparry magnesite within the Middle Proterozoic Gangolihat Formation around Bauri in the Almora district, Kumaun, Lesser Himalaya, have been investigated petrographically and geochemically. The lenses and pockets of megacrystalline, bladed, occasionally stellate, magnesite aggregates invariably enclosed by stromatolitic or massive dolostones, often exhibit a concordant relationship with the latter. Besides the sharp contrast in crystal-linity of magnesite and dolostones and the patches of the latter in the former, relict features such as layers of chert, cryptocrystalline silica veins and stromatolitic structures are discernible in the magnesite. There is a gradual increase in MgO and FeO with a corresponding decrease in CaO, and a striking depletion in Sr from dolostone to magnesite but no noteworthy variation in other major or minor elements nor in insoluble contents. Both the dolostones and magnesites are characterised by the same range of oxygen isotope ratios. However, a marked enrichment of lighter carbon isotopes in magnesites is noted. Based on these observations, it is inferred that the magnesite around Bauri is a product of diagenetic magnesitisation of penecontemporaneous dolomite in a restricted biohermal tidal flat environment.

Geochemistry of magnesites and dolomites at the Oberdorf/Laming (Austria) deposit and implications for their origin

The major and trace element geochemistry of magnesite and dolomite samples from alternating rock masses at the Oberdorf/Laming (Styria, Austria) mineralization has been studied. The rare earth elements (REE) are considered to be of importance for the discussion of the origin of sparry magnesite deposits. The Al and REE contents are positively correlated, indicating a clay component as the main source for REE in the magnesites and their precursor rocks. Our data support the view that the Oberdorf and probably other magnesite mineralizations in the Eastern Alps have been formed by metasomatism. Through this process calcite (which was deposited by sedimentation in a marine environment) underwent dolomitization. Subsequently, Mg-rich solutions circulating through the rock masses formed the magnesite bodies. During the formation of the dolomites and the conversion to magnesites, the REE patterns of the original carbonates remained unchanged. The magnesites exhibit patterns that are very similar to sedimentary carbonates (which contain a REE-bearing clay component). Magnesites of purely sedimentary origin, formed in a predominantly evaporitic environment, should have lower total REE contents and different patterns due to the short residence times of these elements in seawater. Analyses of talc pseduomorphs after magnesite indicate as well that the REE patterns are not significantly altered during the talcification. The Mg-rich solutions that led to the formation of the magnesites have probably been derived from serpentinites below the gneiss complexes of eastern Styria and the South Penninic units of the Tauern window.

Chemical and tectonic controls on the formation of sparry magnesite deposits — the deposits of the northern Greywacke Zone (Austria)

The sparry magnesite deposits of the northern Greywacke Zone are situated at the base of thrust sheets. Model calculations and natural examples suggest that an inverse temperature gradient can only be established at the base of a thrust sheet if thrusting is so rapid as to be geologically unrealistic. Independently of this thrusting induces a fluid flow from the lower units to the front of the moving thrust sheet. Stability data of magnesite and dolomite in contact with fluids with different Ca/(Ca+Mg) ratios suggest that this tectonically induced fluid flow produce magnesite from preexisting dolomite by metasomatism.