Curt Broman

Apatite-monazite relations in the Kiirunavaara magnetite-apatite ore, northern Sweden

The magnetite–apatite ores in the Kiruna area, northern Sweden, are generally considered to be of magmatic origin formed in a subvolcanic–volcanic environment during the early Proterozoic. They are thought to have crystallised from volatile-rich iron oxide magmas derived by immiscibility in calc-alkaline to slightly alkaline parental magmas. Three major morphological types of the magnetite–apatite ore (primary, brecciated, and banded) have been investigated for textural relations and mineral chemistry using transmitted light, back-scattered electron imaging (BSE), electron microprobe analysis (EMPA), and laser ablation–inductively coupled plasma-mass spectrometry (LA–ICPMS). In all three types, Th- and U-poor monazite is present as small inclusions in the apatite. Larger (up to 150 μm) recrystallised monazite grains, both along apatite grain boundaries and intergrown with magnetite and silicate minerals, are present in the brecciated and banded samples. Primary apatite grains, without monazite inclusions, are generally enriched in light rare earth elements (LREEs) together with Na and Si. Petrological and mineralogical evidence suggest that the Kiruna magnetite–apatite ore experienced successive stages of fluid–rock interaction. The first stage occurred under high-temperature conditions (700–800 °C) shortly after emplacement and crystallisation of the ore magmas and involved concentrated, probably Cl-dominated brines expelled from the magma. This fluid is held to be responsible for the nucleation of the numerous small monazite inclusions within the apatite due to high-temperature leaching of Na and Si, while the LREEs were concentrated in the monazite. The large monazite grains in the brecciated and banded samples are proposed to be the product of recrystallisation from the much smaller monazite inclusions. During greenschist-facies metamorphism (T=300–400 °C), fluids from the surrounding country rock caused strong (LREE+Na+Si) depletion along apatite grain boundaries and cracks in the apatite. LREEs were either redeposited as monazite grains along apatite grain boundaries or were flushed out of the ore. This fluid interaction also introduced the silicate components responsible for the interstitial formation of allanite, talc, tremolite, chlorite, serpentine, muscovite, quartz, and carbonates along apatite grain boundaries.

Lead and bromine enrichment in eclogite-facies fluids: Extreme fractionation during lower-crustal hydration

Hydration reactions associated with eclogite-facies metamorphism of granulites in the Norwegian Caledonides led to the formation of saline brines with exceptional compositions. Primary omphacite- and garnet-hosted fluid inclusions from Norwegian eclogites formed by hydration of granulites contain a plethora of solid phases. A total of 18 different minerals have been identified in multiphase brines, and 12 of these occur at the Badsholmen locality in Sunnfjord. Fluid inclusions from this locality are characterized by a high Br/Cl mass ratio (0.03) and extreme Pb enrichment as demonstrated by the presence of Pb-bearing daughter minerals including galena, a Pb-Cl–bearing phase, and a Pb-Cl-Br–bearing phase. The fluid compositions may reflect enrichment of elements incompatible with the silicate structure during consumption of H 2 O by eclogite-forming hydration reactions. Pb is believed to have been scavenged from the parent granulite by the increasingly saline brine during K-feldspar breakdown.

Fluid inclusions in magnetite-apatite ore from a cooling magmatic system at El Laco, Chile

Abstract The Pliocene El Laco deposits of magnetite-apatite iron ore of the Kiruna type, situated on the flanks of a volcano in northern Chile, can morphologically and structurally be described as extrusive and intrusive magmatic orebodies with hydrothermal overprinting. Fluid inclusions in pyroxene and apatite record different stages in the transition from a late-magmatic remnant fluid, probably exsolved during crystallization of an ore magma, to hydrothermal fluids of successively lower temperature and salinity. Hydrous saline Na-K chloride melt inclusions with anhydrite daughter crystals in pyroxene intergrown with magnetite from ore lava at Laco Sur and ore breccia in a dike-vein system at Cristales Grandes were formed at >800°C. Pyroxene-magnetite veins in the Laco Sur ore indicate crystallization in fissures during degassing of a flow emplaced at still higher temperatures. Melt inclusions like those in pyroxene also occur in an early generation of apatite from San Vicente Bajo, a subvolcanic orebody...

Low P-T Caledonian resetting of U-rich Paleoproterozoic zircons, central Sweden

Abstract Uranium-rich zircons from a Paleoproterozoic, high-grade deformation zone in the Fennoscandian Shield, central Sweden, show an almost complete resetting of the U-Pb system in early Phanerozoic time. A mylonitic gneiss in the deformation zone contains two types of highly discordant (>70%), U-rich zircons: large, brown, cloudy prisms, and small milky-white irregularly shaped grains. The gneiss also contains mostly clear prismatic zircon of lower U content with mildly discordant to concordant U-Pb ages. Laser Raman spectroscopy reveals that the dark cathodoluminescent areas in brown zircons have a highly metamict crystal structure, whereas the structures of both the dark cathodoluminescent milky-white grains and the bright cathodoluminescent clear prisms have higher degrees of crystallinity. Age dates obtained by U-Pb SIMS analysis of 40 zircons of the three types described above range continuously from concordant at 1871 ± 11 Ma to 98% discordant at 384 ± 15 Ma. The strongly discordant zircons clearly have suffered severe disturbance at about the time of the Caledonian orogeny. However, Caledonian metamorphic temperatures and pressures in this region did not exceed 150-200 °C and 1-3 kbar, too low to strongly disturb the U-Pb systematics in nonmetamict zircon by thermal means alone. Independent evidence indicates that saline fluids were circulating in the Paleoproterozoic basement rocks at this time, possibly driven by hydrological gradients generated in front of the encroaching Caledonian orogenic wedge. These low-temperature saline fluids are inferred to be responsible for causing both strong Pb loss in the mostly metamict brown zircons via a diffusive process, and the formation of small milky-white zircon via a lowtemperature recrystallization or dissolution/re-precipitation process.

Fossilized iron bacteria reveal a pathway to the biological origin of banded iron formation

Debates on the formation of banded iron formations in ancient ferruginous oceans are dominated by a dichotomy between abiotic and biotic iron cycling. This is fuelled by difficulties in unravelling the exact processes involved in their formation. Here we provide fossil environmental evidence for anoxygenic photoferrotrophic deposition of analogue banded iron rocks in shallow marine waters associated with an Early Quaternary hydrothermal vent field on Milos Island, Greece. Trace metal, major and rare earth elemental compositions suggest that the deposited rocks closely resemble banded iron formations of Precambrian origin. Well-preserved microbial fossils in combination with chemical data imply that band formation was linked to periodic massive encrustation of anoxygenic phototrophic biofilms by iron oxyhydroxide alternating with abiotic silica precipitation. The data implicate cyclic anoxygenic photoferrotrophy and their fossilization mechanisms in the construction of microskeletal fabrics that result in the formation of characteristic banded iron formation bands of varying silica and iron oxide ratios.

Extreme 13 C depletion of carbonates formed during oxidation of biogenic methane in fractured granite

Precipitation of exceptionally 13C-depleted authigenic carbonate is a result of, and thus a tracer for, sulphate-dependent anaerobic methane oxidation, particularly in marine sediments. Although these carbonates typically are less depleted in 13C than in the source methane, because of incorporation of C also from other sources, they are far more depleted in 13C (δ13C as light as −69‰ V-PDB) than in carbonates formed where no methane is involved. Here we show that oxidation of biogenic methane in carbon-poor deep groundwater in fractured granitoid rocks has resulted in fracture-wall precipitation of the most extremely 13C-depleted carbonates ever reported, δ13C down to −125‰ V-PDB. A microbial consortium of sulphate reducers and methane oxidizers has been involved, as revealed by biomarker signatures in the carbonates and S-isotope compositions of co-genetic sulphide. Methane formed at shallow depths has been oxidized at several hundred metres depth at the transition to a deep-seated sulphate-rich saline water. This process is so far an unrecognized terrestrial sink of methane.

A hydrothermal system associated with the Siljan impact structure, Sweden--implications for the search for fossil life on Mars.

The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

Fossilized Microorganisms from the Emperor Seamounts: Implications for the Search for a Subsurface Fossil Record on Earth and Mars

We have observed filamentous carbon-rich structures in samples drilled at 3 different seamounts that belong to the Emperor Seamounts in the Pacific Ocean: Detroit (81 Ma), Nintoku (56 Ma), and Koko Seamounts (48 Ma). The samples consist of low-temperature altered basalts recovered from all 3 seamounts. The maximum depth from which the samples were retrieved was 954 meters below seafloor (mbsf). The filamentous structures occur in veins and fractures in the basalts, where they are attached to the vein walls and embedded in vein-filling minerals like calcite, aragonite, and gypsum. The filaments were studied with a combination of optical microscopy, environmental scanning electron microscopy (ESEM), Raman spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Minerals were identified by a combination of optical microscopy, X-ray diffraction, Raman spectrometry, and energy dispersive spectrometry on an environmental scanning electron microscope. Carbon content of the filaments ranges between approximately 10 wt % and approximately 50 wt % and is not associated with carbonates. These results indicate an organic origin of the carbon. The presence of C(2)H(4), phosphate, and lipid-like molecules in the filaments further supports a biogenic origin. We also found microchannels in volcanic glass enriched in carbon (approximately 10-40 wt %) compatible with putative microbial activity. Our findings suggest new niches for life in subseafloor environments and have implications for further exploration of the subseafloor biosphere on Earth and beyond.

Magmatic-hydrothermal fluids in the Pahtohavare Cu-Au deposit in greenstone at Kiruna, Sweden

The Proterozoic Pahtohavare Cu-Au deposit is located in the greenstone belt near Kiruna, northern Sweden. The greenstone consists of mafic volcanic rocks with pillow lavas, mafic sills and albitized rocks, including tuffites, black schists and mafic sills, together with carbonates and mineralized zones. Mineralization occurs as impregnations, epigenetic quartz-rich breccias and fracture fillings with pyrite, chalcopyrite, pyrrhotite and gold in a complex tectonic environment. Fluid inclusions indicate an early formation of quartz and pyrite at temperatures initially near 500°C and a pressure of 2–2.4 kbar from a supersaturated aqueous solution of magmatic origin. In addition to halite cubes, daughter minerals of sylvite, calcite, hematite, graphite and two unknown phases are found. The main stage of chalcopyrite and gold deposition is characterized by aqueous fluids of variable salinity (up to 30 eq. wt.% NaCl including CaCl2), at temperatures below 350°C and pressures between 1 and 2 kbar. A minor CO2 phase with some N, accompanies this stage. Gold was transported as a chloride complex which destabilized due to an increase in pH (as a consequence of the CO2 loss) as well as cooling and dilution of the solution. The ore deposition occurred as a result of mixing with a low salinity aqueous solution during tectonic fracturing with pressure fluctuations and CO2 unmixing. Late oxidation of ores was caused by low to moderately saline (3 to 13 eq. wt.% NaCl) low temperature aqueous solutions.

Deep-biosphere consortium of fungi and prokaryotes in Eocene subseafloor basalts

The deep biosphere of the subseafloor crust is believed to contain a significant part of Earths biomass, but because of the difficulties of directly observing the living organisms, its composition and ecology are poorly known. We report here a consortium of fossilized prokaryotic and eukaryotic micro-organisms, occupying cavities in deep-drilled vesicular basalt from the Emperor Seamounts, Pacific Ocean, 67.5 m below seafloor (mbsf). Fungal hyphae provide the framework on which prokaryote-like organisms are suspended like cobwebs and iron-oxidizing bacteria form microstromatolites (Frutexites). The spatial inter-relationships show that the organisms were living at the same time in an integrated fashion, suggesting symbiotic interdependence. The community is contemporaneous with secondary mineralizations of calcite partly filling the cavities. The fungal hyphae frequently extend into the calcite, indicating that they were able to bore into the substrate through mineral dissolution. A symbiotic relationship with chemoautotrophs, as inferred for the observed consortium, may be a pre-requisite for the eukaryotic colonization of crustal rocks. Fossils thus open a window to the extant as well as the ancient deep biosphere.