Jan Olov Nyström

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.

Multiple sources for the Coastal Batholith of central Chile (31–34°S): geochemical and Sr–Nd isotopic evidence and tectonic implications

The Coastal Batholith of central Chile between 31 and 34°S represents a locus of long-lived (c. 200 Ma) plutonic activity. It is composed of four plutonic complexes: Santo Domingo, Limari, Papudo–Quintero and Illapel. These complexes, which occur as north-trending discontinuous belts that decrease in age eastward, were emplaced during four major episodes of arc magmatism. The Santo Domingo Complex (Carboniferous) comprises hornblende–biotite bearing tonalites and granodiorites with abundant mafic enclaves, and enclave-free granites. The association of enclave-bearing granitoids and enclave-free granites resulted from different degrees of mixing of crustal- and mantle-derived magmas. The Limari Complex (Lower Jurassic) is a bimodal association of crustal leucogranites and mantle-derived gabbros. The lithology of the Papudo–Quintero Complex (Middle Jurassic) varies from hornblende–pyroxene diorite to biotite granite. The main rock types, namely tonalites and granodiorites, commonly contain mafic enclaves. The granitoids and the enclaves have remarkably similar REE patterns and are similar in Sr–Nd isotope composition, which suggest that they represent a cogenetic suite of mantle origin. The Illapel Complex (Cretaceous) consists of hornblende bearing tonalites, trondhjemites and granodiorites that resemble the rocks of TTG suites. The eNd values and initial ratios (Sri) for the mafic rocks of the Carboniferous Complex (eNd: −2.0 and −3.5; Sri: c. 0.7057) differ markedly from those of the Mesozoic mafic rocks (eNd: +1.9 to +5.4; Sri: 0.7033 to 0.7039), the Cretaceous rocks being the most depleted with regard to Sr–Nd. A change from Carboniferous lithosphere-dominated sources to Mesozoic asthenosphere-dominated sources of the magmas that started in the Early Jurassic is recognized. Slab components can be recognized in the source of the Cretaceous rocks. The crustal source of the Lower Jurassic leucogranites was more isotopically depleted and refractory than that of the Carboniferous granites. On the other hand, no crustal participation in the origin of the Middle Jurassic and Cretaceous felsic granitoids is recognized; they are differentiated from mantle-derived magmas. Progressive removal of old lithospheric mantle during the Mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source, as well as the melting and modification towards a more refractory nature of the continental crust.

Jurassic and Early Cretaceous island arc volcanism, extension, and subsidence in the Coast Range of central Chile

More than 2000 km 3 of acid and 9000 km 3 of basic volcanic rocks formed during the Jurassic and Early Cretaceous in the Coast Range of central Chile, between 32°30′S and 34°S. These rocks, which constitute the major part of an •15-km-thick pile of alternately marine and continental deposits, issued from volcanic arcs situated between a land area with Paleozoic basement in the west and a marginal sea in the east. Asthenospheric upwelling led to extension and bimodal volcanism; the volcanic products were deposited in intra-arc basins subsiding at high rates (100–300 m/m.y.). The source of the magmas became more depleted with time due to an increase in degree of partial melting, and their compositions were modified by subduction-related fluids and contamination with a progressively thinner and younger crust. The basic lavas are of high-K calc-alkaline to shoshonitic affinity, chemically resembling the lavas found in some mature island arcs in the western Pacific. The extension and subsidence resulted in a low-relief topography close to sea level, in contrast with the present-day convergent type of Andean volcanism at the same latitude where calc-alkaline intermediate lavas erupt from volcanoes at great height above a thick crust.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15′–33°45′S)

This lithologic and geochemical study treats two Tertiary volcanic formations in the Andean foothills of central Chile deposited during and after an inferred culmination of crustal attenuation. The Abanico and Farellones Formations, which are described in their type localities just east of Santiago, formed from volcanic arcs in continental basins during the Oligocene and Early Miocene, respectively. Aphyric basic lavas of tholeiitic affinity, acid pyroclastic rocks, and lacustrine deposits constitute the >3100-m-thick Abanico Formation. The overlying >2100-m-thick Farellones Formation consists of calc-alkaline lavas (basalts absent) with thick pyroclastic deposits at the base. Both formations have Nd-Sr isotope signatures within the mantle array; the Abanico rocks (eNd ≈ +5.7) plot closer to N-MORB (normal mid-oceanic-ridge basalt) than the Farellones rocks (eNd = +3.9 to +5.1). The REE (rare earth element) patterns indicate greater depth to the mantle source and a smaller degree of partial melting with time. The Abanico lavas segregated within the stability field of spinel, whereas the lavas of the upper Farellones member show residual garnet in their source. Geochemical changes with time are systematic: the greatest contrast is between the middle and upper Farellones members in 1–2 m.y., e.g., for basaltic andesites, La/Yb increases from 4.3 (Abanico) to 5.6, 6.0, and 11.6 (lower, middle, and upper Farellones members). The bimodal composition of the Abanico Formation and the lower Farellones member indicates that volcanism took place during episodes of extensional conditions. Extension with subsidence is independently shown by the burial metamorphic pattern. Voluminous pyroclastic flows, structural relationships, and other evidence suggest recurrent caldera collapse. The first extensional episode ended with contraction and folding of the Abanico rocks, and the second episode resulted in uplift of the lower and middle Farellones members, followed by a more passive tectonic regime. Sequences showing many similarities with the Abanico and Farellones Formations occur along the Andean foothills of Chile. They decrease in age from Late Cretaceous– Paleocene at 23°S to Early Miocene–late Miocene at 35°S and might be explained by oblique subduction of oceanic ridge.

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...

Oxygen isotope composition of magnetite in iron ores of the Kiruna type in Chile and Sweden

Abstract Magnetite-apatite iron ores of the Kiruna type, unaffected by deformation, have structures and textures similar to those of igneous rocks. The best examples are the El Laco deposits in northern Chile which resemble lava flows, pyroclastic deposits and dikes. El Laco magnetites have δ18O values between 2.3 and 4.2‰ (V-SMOW). Magnetite from ore with a magmatic texture has a mean of 3.7‰, and the mean for magnetite intergrown with pyroxene in veins is 2.4‰. Oxygen isotope data given here, fluid inclusion results and geological evidence indicate that ore formation took place in a cooling magmatic system. Major orebodies resembling lava flows and near-vent pyroclastic deposits crystallized from magma at ca. 1000°C. Fluids from cooling magma deposited magnetite and pyroxene (±apatite) at ca. 800°C in fissures and open spaces, now present as veins cutting major orebodies. There is little evidence for significant magnetite precipitation during hydrothermal conditions. A large province of magnetite-apatite iron ore in central Chile (the Cretaceous iron belt) and the Kiruna district in northern Sweden also contain primary ore of magmatic appearance. Major deposits in the Chilean iron belt and Kiruna contain magmatic-textured magnetites with the following δ18O means: Algarrobo = 2.2‰, Romeral = 1.2‰, Cerro Imán = 1.6‰, and Kiirunavaara = 1.5‰. We consider all oxygen isotope data for unoxidized, magmatic-textured magnetite as representative of the Fe-rich magmas. Magnetites affected by hydrothermal alteration, recrystallization and subaerial oxidation have modified isotope signatures.

Geochemical trends in Mesozoic-Tertiary volcanic rocks from the Andes in central Chile, and tectonic implications

Abstract A study of approximately 300 samples of basic to intermediate lavas of Mesozoic and Tertiary age from five profiles across the Andes of central Chile (25°30′–35°S) shows that the ascribed “normal” geochemical trends transverse to this active continental margin and, with time (up to the Miocene), are oversimplified or reversed. The Jurassic-Paleogene lavas show no clear trend towards more evolved compositions in the east (away from the paleotrench), nor with time. There are even indications that the crust was thicker towards the south during part of the Mesozoic, contrary to the present situation. The tectonic evolution during the Jurassic-Early Cretaceous is characterized by episodic changes in the subduction regime, expressed among other things by a recurring non-organic shoshonitic fissure volcanism. The Late Cretaceous history constitutes a transition to the Cenozoic development with calcalkaline volcanic rocks erupting from central volcanoes in a caldera-graben setting.

Subvolcanic contact metasomatism at El Laco Volcanic Complex, Central Andes

Studies of drill cores from the Pasos Blancos area at El Laco in the central Andes, northern Chile, give evidence of an intense and extensive subvolcanic contact-metasomatic process. This process resulted from shallow-level emplacement of very volatile-rich iron-oxide magma, with discharge of volatiles that resulted in extensive fracturing of overlying volcanic rocks. The brecciated rocks were altered (mainly extensive scapolitization and formation of pyroxene) by hot magmatic fluids emitted from the cooling intrusion, and accompanied by magnetite deposition. With time and decreasing temperature, the metasomatic fluids evolved to fluids of hydrothermal character, and a final recent geothermal event took place that deposited superficial gypsum over a large part of the El Laco Volcanic Complex.

The Ordovician chondrite from Brunflo, central Sweden: III. Geochemistry of terrestrial alteration

The fossil H chondrite Brunflo, found in a slab of Ordovician limestone from central Sweden, is pervasively altered to an assemblage dominated by calcite and barite. The meteorite is surrounded by a 15-20 cm wide zone of lighter colors than the unaffected limestone due to dissolution of hematite. Here we present detailed geochemical analyses of two meteorite samples, 14 limestone samples at distances from 0 to 29 cm along two profiles from the meteorite, and a reference sample of Brunflo limestone. Element concentrations in Brunflo and surrounding bleached limestone have been strongly disturbed . during two stages of alteration early oxygenated and deep burial . In the meteorite, the Ni rCo ratio has changed from an initial value of 20 to 0.8 and redox sensitive elements like V, As, Mo, Re and U are strongly enriched. The sulfur isotope 34 . composition of barite from Brunflo d S sq 35‰ indicates initial loss of meteoritic sulfide, followed by later accumulation of sea water sulfate as barite. During deep burial under more reducing conditions, reduction processes supported by an externally derived reductant possibly derived from alum shale underlying the limestone, were largely responsible for the observed redox phenomena. In spite of massive redistribution of many elements, concentrations of Pt, Ir and Au remain at chondritic levels. The geochemistry and mineralogy of alteration determined for Brunflo are similar to .

Magnetite bombs at El Laco volcano, Chile

Abstract The origin of apatite iron ores of the Kiruna type remains controversial, especially the formation of almost pure magnetite that constitutes large parts of major deposits. The best preserved ores of this type occur on the flanks of El Laco, a Plio-Pleistocene volcano in the Chilean Andes. Based on recent studies a group of workers argues that the El Laco deposits were formed by hydrothermal fluids. However, existence of ballistic volcanic bombs composed of radiating porous aggregates of magnetite crystals in some of the orebodies, demonstrates that apatite iron ores can form directly from a melt.