Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non-planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky-white pure saddle dolomites (b1) which are often covered by pore-lining zoned dolomite (b2). The b phases often possess a saddle-shaped morphology. In contrast to the replacement origin of the a dolomite, the zoned b2 dolomite rims are interpreted as a cement formed in open cavities. The b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non-carbonate components.



All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (TH = 130–200 °C), saline (20–23 wt% CaCl2 eq.) fluids. A diagenetic high temperature origin is also supported by depleted δ18O values (−20 to −14‰ VPDB). A contribution of 87Sr-enriched fluids is reflected in the 87Sr/86Sr values (0·7091–0·7123).



Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin-platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra-zebra fracturing at decimetre to micrometre scale.

Zebra dolomitization as a result of focused fluid flow in the Rocky Mountains Fold and Thrust Belt, Canada

Abstract Internal sectors of the Alpine-Balkan-Carpathian-Dinaride (ABCD) orogen comprise fundamentally different ore deposits along strike in three temporally and spatially distinct belts. These were formed by several short-lived, late-stage collisional processes (including slab break-off) during the Late Cretaceous and Oligocene to Neogene times. Reconstruction of Late Cretaceous (c. 92–65 Ma) collisional structures, magmatic features and mineralization reveals contrasting variations along strike, including the following. (1a) The Late Cretaceous ‘banatite’ magmatic belt, which extends from the Apuseni mountains to the Balkans, associated mainly with porphyry Cu-Au, massive sulphide and Fe-Cu skarn mineral deposits. In respect to their country rocks and geodynamic setting, the magmatism is interpreted to represent either post-collisional or Andean-type calc-alkaline due to continuous subduction or break-off of the subducted lithosphere. (1b) The Alpine-West Carpathian sector, which is characterized by strong Late Cretaceous metamorphic/deformational overprint, lack of magmatism and both syn- and late-orogenic formation of metasomatic and metamorphogenic talc, magnesite, siderite and vein- and shear zone-type Cu and As-Au due to the exhumation of metamorphic core complexes. (2a) The Oligocene-Miocene Serbomacedonian-Rhodope metallogenic zone extends across several structural units from the Bosnian Dinarides to the Rhodopes and to Thrace. It includes both a belt with volcanic-hosted and vein-type Pb-Zn deposits and a belt of porphyry Cu-Au-Mo and epithermal Au mineralization, which is more common in the south. Both belts appear to relate to microcontinent collision and associated subsequent magmatism, again possibly due to slab break-off. (2b) Different types of mineralization were also formed along the internal Inner Carpathian and Alpine sectors during Late Oligocene to Miocene collision. In the Alps, mineralization formed due to eastward extrusion of fault-bounded blocks into the Carpathian arc. Associated mineral deposits are always related to exhumation of metamorphic core complexes and include: sub-vertical mesothermal Au-quartz veins and replacement As-Ag-Cu ore bodies within the metamorphic core complex, fault-bounded mineralization (Pb-W-Au) along low-angle ductile normal faults along the upper margins of the metamorphic core complex, mineralized (Sb-Au) strike-slip faults and sub-vertical Au-Ag-Sb-bearing tension veins. (2c) In contrast, nearly all Miocene ore deposits within the Carpathians are related to volcanic activity contemporaneous with the invasion of fault-bounded blocks into the Carpathian arc. These have been related to slab break-off and cessation of subduction. Mineral deposits include structurally controlled Au-Sb-Cu-Pb-Zn ore bodies within shallow volcanic edifices, with a preference for steep tension veins parallel to the motion direction of laterally escaping crustal blocks.

Contrasting Late Cretaceous with Neogene ore provinces in the Alpine-Balkan-Carpathian-Dinaride collision belt

Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights

We report the template-free, low-temperature synthesis of a stable, amorphous, and anhydrous magnesium carbonate nanostructure with pore sizes below 6 nm and a specific surface area of ~ 800 m2 g−1, substantially surpassing the surface area of all previously described alkali earth metal carbonates. The moisture sorption of the novel nanostructure is featured by a unique set of properties including an adsorption capacity ~50% larger than that of the hygroscopic zeolite-Y at low relative humidities and with the ability to retain more than 75% of the adsorbed water when the humidity is decreased from 95% to 5% at room temperature. These properties can be regenerated by heat treatment at temperatures below 100°C.The structure is foreseen to become useful in applications such as humidity control, as industrial adsorbents and filters, in drug delivery and catalysis.

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A template-free, ultra-adsorbing, high surface area carbonate nanostructure.

Origin of ultramafic-hosted vein magnesite deposits

Magnesite is relatively frequent as a mineral, there are, however, only two types of economically important deposits:

Genesis of magnesite deposits — models and trends

The specularite deposit at Waldenstein is an epigenetic replacement/vein-type hematite mineralization with distinct alteration zones. It is situated in highly metamorphosed schists and, gneisses of the Koralm Crystalline Complex. Sericitization and chloritization are strictly bound to this alteration and occur within a few meters of the hematite veins. Radiometric dating of the sericitization and the implicity of the mineralization yield a Middle Eocene age. Petrographic evidence proves an early, relatively reduced paragenesis (magnetite, ilmenite) being partly replaced by hematite and chlorite during the main phase of ore formation. Fluid inclusion and stable isotope investigations indicate that formation temperatures were approximately 300°C during the main stage and decreased to 200°C in the latest stage of the hydrothermal event. The H-isotope compositions of chlorites and of fluid inclusions in quartz indicate the influence of seawater. This also explains the high salinities determined by fluid inclusion studies (20–30% NaCl equiv.). According to the genetic model presented in this paper the hydrothermal activity started when the deformation accompanying the uplift of the Koralm Crystalline Complex passed from a ductile to brittle regime (Middle Eocene). The brittle faults acted as channelways for rising, deep fluids, probably of metamorphic origin. Temperature differences between the different uplifting crustal segments supported fluid circulation. Precipitation of the ore occurred at shallow crustal levels where the hydraulic regime was dominated by seawater, and oxidation of the original reducing fluids took place.

W. Pohl

Papers

Genesis of magnesite deposits — models and trends

Tertiary metallogenesis in the Eastern Alps: the Waldenstein hematite deposit (Austria)

Tertiary metallogenesis in the Eastern Alps: the Waldenstein hematite deposit (Austria)