Ti-poor Magnetite Research Articles

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Rock Magnetism and Magnetic Fabric Study of the Icelandite and Rhyodacite Long Volcanic Sequence at Mauna Kuwale, Wai’anae Volcano, Oahu, Hawaii, USA

In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always <60 mT, which suggest the predominance of magnetic components of low coercivity, like magnetite. Usually, two coercivity components are identified in the specimens. In addition we also conducted magnetic granulometry analyses on 27 specimens to determine the domain state of the flows. The ratio of hysteresis parameters (Mr/Mrs versus Hcr/Hc) show that overall samples fall in the Pseudo-Single Domain (PSD) region with high values of Mr/Mrs and very low values of Hcr/Hc. Only two samples from cooling units 17 and specially 22 show a Single Domain (SD) magnetic behavior and a sample from one unit approaches the SD-MD mixture region. We measured the magnetic susceptibility of all cooling units and we found out that in all analyzed units the magnetic susceptibility is low 13.7 ± 8.8 (10−3 SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k3 axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K1 axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k3 axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation.

The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper–gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper–gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U–Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426–0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Tectonics, cooling rates and temperatures during emplacement of the Rajmahal traps, India

Paleomagnetism is used to decipher the emplacement setting of the Rajmahal traps. Sampling was restricted to flows with known age. Three magnetic minerals are identified, titanomaghemite, Ti-poor magnetite, and possibly Mg–Al-rich titanomagnetite. Detailed analysis of thermomagnetic curves revealed that the emplacement temperature for Rajmahal traps was lower than 600 °C. Two magnetization components are distinguished. The low coercivity component, probably chemical remanent magnetization, is erased in fields up to 3.1 mT. In contrast, the high coercivity thermal remanent magnetization is more stable, sometimes throughout the demagnetization. The paleomagnetic results show that the sites have suffered up to 11° of clockwise and 17° of counter-clockwise rotation. As there is no field evidence of faulting and pervasive deformation, we propose that this rotation is probably due to penecontemporaneous joints or faults. These structures could have acted as conduits for fluids that increased the cooling rate.

Geochemical constraints on the origin and evolution of the volcanic Rooiberg Group, Bushveld Large Igneous Province, South Africa

This study combines a new bulk-rock dataset of major and trace element compositions with an existing database for the Rooiberg Group, the latter representing one of the largest felsic volcanic provinces in the world and recording the earliest magmatic activity associated with the Bushveld Large Igneous Province (LIP), South Africa. The Rooiberg Group comprises four formations including, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formations. The Dullstroom Formation consists of a differentiated suite ranging from basalt to rhyolite, whereas the three upper formations are composed principally of rhyolite. Intrusion of the mafic–ultramafic rocks of the Rustenburg Layered Suite (RLS) physically divided the Dullstroom Formation into a lower portion beneath the RLS consisting mainly of basalts with minor felsic volcanics, and an upper section above the RLS dominated by dacite and rhyolite. Bulk-rock geochemical data combined with quantitative trace element modelling indicate that the low-Ti and high-Ti basalts and dacite-rhyolites of the Dullstroom Formation collectively represent an advanced degree of partial melting of a mafic, lower crustal source genetically linked to a Mg-rich magma referred to as the B1 parental magma of the Lower Zone and lower Critical Zone of the RLS and comprising mainly pyroxenite, norite and peridotite. In contrast, the silica-rich Damwal, Kwaggasnek and Schrikkloof Formations of the Rooiberg Group are the products of a fractionally crystallised melt derived from a different source rock to that for the Dullstroom suite, likely of intermediate composition. This study also suggests that the low-Ti and high-Ti basalts of the Dullstroom Formation were derived from a common source, and that the variation in the Ti content is due to the crystallisation of phenocrystic, Ti-rich magnetite present only in the high-Ti basalts, whereas the Ti-poor magnetite of the low-Ti basalts precipitated from a slurry dominated by interstitial liquid.

Nanomineralogy of hydrothermal magnetite from Acropolis, South Australia: Genetic implications for iron-oxide copper gold mineralization

Abstract Magnetite is the dominant Fe-oxide at the Acropolis IOCG prospect, Olympic Dam district, South Australia. Complementary microbeam techniques, including scanning transmission electron microscopy (STEM), are used to characterize titanomagnetite from veins in volcanic rocks and Ti-poor magnetite from a granite body with uplifted position in the volcanic sequence. A temperature of 670 ± 50 °C is estimated for Ti-poor magnetite using XMg-in-magnetite thermometry. Titanomagnetite, typified by Ti-rich trellis lamellae of ilmenite in magnetite, also displays sub-micrometer inclusions forming densely mottled and orbicular subtypes of titanomagnetite with increasing degree of overprinting. STEM analysis shows nanoparticles (NPs) of spinels and TiO2 polymorphs, anatase, and rutile. These vary as dense, finest-scale, monophase-NPs of spinel sensu stricto in Ti-poor magnetite; two-phase, ulvöspinelhercynite NPs in primary titanomagnetite; and coarser clusters of NPs (hercynite±gahnite+TiO2-polymorphs), in mottled and orbicular subtypes. Nano-thermobarometry using ilmenite-magnetite pairs gives temperatures in the range ~510–570 (±50) °C, with mineral-pair re-equilibration from primary to orbicular titanomagnetite constrained by changes in fO2 from ilmenite-stable to magnetite+hematite-stable conditions. Epitaxial relationships between spinel and Fe-Ti-oxides along trellis lamellae and among phases forming the NPs support exsolution from magnetitess, followed by replacement via mineral-buffered reactions. Lattice-scale intergrowths between ulvöspinel and ilmenite within NPs are interpreted as exsolution recording cooling under O2-conserving conditions, whereas the presence of both TiO2-polymorphs displaying variable order-disorder phenomena is evidence for subtly fO2-buffered reactions from anatase (reducing) to rutile (more oxidizing) stabilities. Transient formation of O-deficient phases is retained during replacement of ilmenite by anatase displaying crystallographic-shear planes. Development of dense inclusion mottling and orbicular textures are associated with NP coarsening and clustering during vein re-opening. Fluid-assisted replacement locally recycles trace elements, forming gahnite NPs or discrete Sc-Ti-phases. Hydrothermal titanomagnetite from Acropolis is comparable with magmatic magnetite in granites across the district and typifies early, alkali-calcic alteration. Open-fracture circulation, inhibiting additional supply of Si, Ca, K, and so on during magnetite precipitation, prohibits formation of silician magnetite hosting calc-silicate NPs, as known from IOCG systems characterized by rock-buffered alteration of host lithologies. Obliteration of trellis textures during subsequent overprinting could explain the scarcity of this type of hydrothermal magnetite in other IOCG systems.

Magnetic fabric from Quaternary volcanic edifices in the extensional Bransfield Basin: internal structure of Penguin and Bridgeman islands (South Shetlands archipelago, Antarctica)

SUMMARYStudying the magnetic fabric in volcanic edifices, particularly lava flows from recent eruptions, allows us to understand the orientation distribution of the minerals related to the flow direction and properly characterize older and/or eroded flows. In this work, the magnetic fabric from recent (Quaternary) lava flows (slightly inclined in seven sites and plateau lavas in two sites), pyroclastic deposits (two sites from a scoria cone) and volcanic cones, domes and plugs (three sites) from Penguin and Bridgeman islands, located in the Bransfield backarc basin, are presented. The volcanism in the two islands is related to rifting occurring due to the opening of the Bransfield Strait, between the South Shetlands archipelago and the Antarctic Peninsula. The direction of flow of magmatic material is unknown. Rock magnetic analyses, low temperature measurements and electron microscope observations (back-scattered electron imaging and Energy Dispersive X-ray analyses) reveal a Ti-poor magnetite (and maghemite) as the main carrier of the magnetic fabric. Hematite may be present in some samples. Samples from the centre of the lavas reveal a magnetic lineation either parallel or imbricated with respect to the flow plane, whereas in the plateau lavas the magnetic lineation is contained within the subhorizontal plane except in vesicle-rich samples, where imbrication occurs. The magnetic lineation indicates a varied flow direction in Bridgeman Island with respect to the spreading Bransfield Basin axis. The flow direction in the plateau lavas on Penguin Island is deduced from the imbrication of the magnetic fabric in the more vesicular parts, suggesting a SE–NW flow. The volcanic domes are also imbricated with respect to an upward flow, and the bombs show scattered distribution.

Magnetic anisotropy of an ancient volcanic system: Flow dynamics of post-collisional Ediacaran volcanism in southernmost Brazil

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.

Detailed palaeomagnetic record at Rose Cottage Cave, South Africa: Implications for the Holocene geomagnetic field behaviour and chronostratigraphy

Palaeomagnetic data from a sedimentary section spanning the Holocene and terminal Pleistocene (~13 kya) from Rose Cottage Cave, eastern Free State (South Africa), are reported. The palaeomagnetic analysis took into account rock magnetism and directional analysis. The former reveals that most samples show stable single domain and superparamagnetic particles of Ti-poor magnetite and haematite. Natural remanent magnetisation directions were determined by progressive alternating field demagnetisation methodology. Directional analysis shows normal directions between samples 18 to 39 and 85 to 92; however, during the Early and Late Holocene in samples principally from RC40 to 84 ‘anomalous’ directions occurred. There is a significant westward shift in declination of ~80°, and a conspicuous fluctuating inclination in the lower part of the section during the Early Holocene at ≥9.5 kya and before ~12.0/13.0 kya. This palaeomagnetic record might become a chronostratigraphical marker for latest Pleistocene/Holocene sedimentary deposits in South Africa. Our two new accelerator mass spectrometry radiocarbon dates for the sampled deposit are 9500±50 BP and 1115±30 BP.&#x0D; Significance&#x0D; &#x0D; The study provides new accelerator mass spectrometry dates on the chronological sequence existing in Rose Cottage Cave.&#x0D; The findings contribute to the knowledge of the geomagnetic field behaviour since the terminal Pleistocene to the Late Holocene in a period spanning the last 13 000/12 000 years.&#x0D; This palaeomagnetic record might become a chronostratigraphical marker for the latest Pleistocene/ Holocene sedimentary deposits in South Africa.&#x0D;

Formation of the Vergenoeg F–Fe–REE Deposit (South Africa) by Accumulation from a Ferroan Silicic Magma

AbstractSituated in the centre of the Paleoproterozoic Bushveld Large Igneous Province (LIP) of South Africa the Vergenoeg F–Fe–REE deposit is one of the largest, but at the same time most unusual, fluorite deposits on Earth. In situ major and trace element analyses of fayalite, magnetite, ilmenite, fluorapatite, fluorite and allanite from fayalite-rich rocks are combined with oxygen isotope data for fayalite, magnetite and ilmenite to unravel the complex evolution of the deposit. Textural and compositional characterization of the fayalite-rich rocks supports a magmatic formation as cumulates and an intense late hydrothermal overprint. Fayalite accumulated together with minor Ti-rich magnetite, ilmenite, fluorapatite and allanite from a highly evolved, H2O-poor felsic melt at low oxygen fugacity. Chondrite-normalized rare earth element (REE) patterns of fayalite and the recalculated parental melts, using fayalite–rhyolite partition coefficients, exhibit positive trends with strong enrichment of the heavy REE (HREE) relative to the light REE (LREE). Apart from the LREE depletion the patterns are similar to those of highly fractionated high-silica REE rhyolites that often occur in siliceous LIPs. We attribute the LREE depletion to crystallization of accessory allanite, the main host of the LREE in the cumulates. Chondrite-normalized REE patterns of the parental melt prior to fayalite accumulation, recalculated using allanite–rhyolite partition coefficients, resemble the composition of the rhyolites of the Rooiberg Group and therefore document a petrogenetic link to the Bushveld LIP. High δ18O values of fayalite (up to ≈7·4 ‰) are consistent with its crystallization in a rhyolitic melt that has formed by extensive fractionation from basic melts of the Rustenburg Layer Suite, the mafic member of the Bushveld LIP. Primary fluorite crystallized together with rare quartz, and a second generation of fayalite, magnetite and ilmenite from rare intercumulus melt in interstices between cumulate fayalite. Textural and mineral compositional data, as well as the generally negative δ18O values of magnetite (–2·9 to 0 ‰), are in agreement with the main magnetite–fluorite ore formation in Vergenoeg being related to a hydrothermal overprint, which was responsible for further F and Fe enrichments of the rocks. Fluorine-rich fluids, released from the crystallizing granites of the felsic member of the Bushveld LIP (Lebowa Granite Suite), caused the extensive alteration of fayalite to bowlingite and its replacement by Ti-poor magnetite and quartz. The hydrothermal overprint was associated with the widespread formation of secondary fluorite and minor fluorapatite. Our new petrogenetic model for the Vergenoeg deposit, as constrained from the primary fayalite cumulates, implies that the formation of the Vergenoeg deposit was directly linked to the evolution of the Bushveld LIP.

The long life of SAMBA connection in Columbia: A paleomagnetic study of the 1535 Ma Mucajaí Complex, northern Amazonian Craton, Brazil

In recent years, there has been a significant increase in the paleomagnetic data of the Amazonian Craton, with important geodynamic and paleogeographic implications for the Paleo-Mesoproterozoic Columbia supercontinent (a.k.a., Nuna, Hudsoland). Despite recent increase of paleomagnetic data for several other cratons in Columbia, its longevity and the geodynamic processes that resulted in its formation are not well known. A paleomagnetic study was performed on rocks from the ∼1535 Ma AMG (Anorthosite-Mangerite-Rapakivi Granite) Mucajaí Complex located in the Roraima State (Brazil), in the northern portion of the Amazonian Craton, the Guiana Shield. Thermal and AF treatments revealed northwestern/southeastern directions with upward/downward inclinations for samples from twelve sites. This characteristic remanent magnetization is mainly carried by Ti-poor magnetite and in a lesser amount by hematite. Site mean directions were combined with previous results obtained for three other sites from the Mucajaí Complex, producing the dual polarity mean direction: Dm = 132.2°; Im = 35.4° (N = 15; α95 = 12.7°; k = 10.0) and a paleomagnetic pole located at 0.1°E, 38.2°S (A95 = 12.6°; K = 10.2). The Mucajaí pole favours the SAMBA (South AMerica-BAltica) link in a configuration formed by Amazonia and Baltica in Columbia. Also, there is geological and paleomagnetic evidence that the juxtaposition of Baltica and Laurentia at 1.76–1.26 Ga forms the core of Columbia. The present paleomagnetic data predict a long life 1.78–1.43 Ga SAMBA connection forming part of the core of the supercontinent.

Magnetostratigraphy of a Middle Jurassic delta system (Lajas Formation), Portada Covunco section, southern Neuquén Basin, Argentina

A systematic paleomagnetic study was carried out in the Lajas Formation (Middle Jurassic), cropping out at the Covunco creek in the central part of the Neuquén basin. The Lajas Formation at the Portada Covunco section consists of a succession of conglomerates, sandstones and mudstones that reaches c. 290 m-thick. On the basis of lithological and textural features, sedimentary structures, strata geometries, and fossil content, 14 facies were defined that were grouped in two facies associations within a fluvial-dominated delta system. The age of the Lajas Formation is rather controversial, and has been assigned from sequence stratigraphy and palynological data. Thus from south to north, interpreted ages span the Upper Aalenian–Middle Bathonian through the Callovian. Therefore, magnetostratigraphy was used to better precise the age of the Formation at Portada Covunco. Paleomagnetic data obtained from 29 sampling sites present reliable behaviors. Rock magnetic studies (thermomagnetic curves, K vs H curves, IRM acquisition curves) suggest that the remanence is carried by Ti-poor magnetite. Anisotropy of magnetic susceptibility data show clustering of the Kmax axes produced by the action of paleocurrents. The characteristic remanent magnetization passed a tilt test and a reversal test that prove its primary origin. The resultant magnetostratigraphy is characterized by a reverse-dominant with minor normal polarity intervals. When compared with the Bajocian-Bathonian reference geomagnetic polarity time scale (GTS2016), the best fit is achieved in the Lower through Middle Bathonian (Chrons M41 to M39). A paleomagnetic pole was calculated for Portada Covunco located at 167.1°E, 59.4°S, N = 28, A95 = 3.6°, K = 57.2. When compared with the other available paleopole of the same age −170 Ma-derived from dykes exposed at Mamil Choique, Patagonia, Portada Covunco falls in the same paleomeridian although at a lower latitude. Thus, it is interpreted that the studied section was not subjected to rotations around vertical axis (R = 4° ± 10°), but was compacted, which produced inclination shallowing of the magnetic vector (F = 18° ± 11°). The calculated inclination shallowing and amount of compaction in the section are well within the expected values in medium to fine-grained sandstones. Using the paleomagnetic pole from Mamil Choique as reference, the locality of Portada Covunco would have been located at c. 33°S during the Bathonian, in agreement with paleoclimatic interpretations derived from palynological data.

Application of in situ titanite U–Pb geochronology to volcanic-hosted magnetite deposit: New constraints on the timing and genesis of the Zhibo deposit, Western Tianshan, NW China

The Awulale metallogenic belt within the Western Tianshan orogenic belt of northwestern China includes four large iron oxide deposits with a total resource of ∼1000 million metric tons (Mt) Fe. Among these, the Zhibo deposit is a large (337 Mt at 26–68 wt% Fe) volcanic-hosted magnetite deposit, where massive Ti-poor magnetite ores are hosted in the Carboniferous volcanic and volcaniclastic sequences. Here we use in situ U–Pb analyses of titanite and zircon by laser ablation ICP-MS to place tight constraints on the timing and genesis of iron mineralization at Zhibo. Titanite in the magnetite ore are closely associated with magnetite and Ca alteration assemblages consisting of actinolite, epidote, and calcite. The dated titanite exhibit strongly fractionated REE patterns with heavy REE enrichment, neutral to negative Eu anomalies, and have low Th and U concentrations, and low Th/U ratios. The textural and geochemical characteristics indicate that the titanite are hydrothermal in origin and coeval with magnetite in the paragenetic sequence. Titanite from three magnetite ores yield weighted mean 207Pb-corrected 206Pb/238U ages of 310.3 ± 1.8 Ma (MSWD = 0.17), 310.1 ± 1.8 Ma (MSWD = 0.30), and 315.3 ± 2.5 Ma (MSWD = 0.26), constraining the iron mineralization at Zhibo to a time interval between 315 Ma and 310 Ma. Magmatic zircon from a host andesite sample yield U–Pb age of 316.3 ± 3.4 Ma (MSWD = 0.079). The overlapping ages for magnetite ores and the host volcanic rocks confirm a genetic relationship between them, and are consistent with a magmatic contribution to the mineralization system, as also indicated by Fe and O isotope data of magnetite in previous work. These new U–Pb results are also consistent with age estimates for mineralization and igneous activity in other major magnetite deposits in the Awulale iron metallogenic belt, indicating a significant iron mineralization event related to the ca. 315–300 Ma volcanism. Combined with previous geological and geochemical evidence, we conclude that the Zhibo magnetite deposit was formed mainly by iron-rich fluids derived from a mafic to intermediate magma in a volcano-plutonic structure.

Magnetic fabric and flow direction in the Ediacaran Imider dyke swarms (Eastern Anti-Atlas, Morocco), inferred from the Anisotropy of Magnetic Susceptibility (AMS)

Located in the Imiter Inlier (Eastern Saghro, Anti-Atlas, Morocco), Ediacaran volcanic dykes have been studied for their petrofabric using Anisotropy of Magnetic Susceptibility (AMS) technique. Four dykes, namely TF, TD, FF and FE show andesitic compositions and are considered to belong to the same dyke swarm. They are oriented respectively N25E, N40E, N50E, and N10E and have been emplaced during a first tectonic event. The dyke FW, oriented N90E displays a composition of alkali basalt and its emplacement is attributed to a subsequent tectonic event.These rocks are propylitized under greenschist facies conditions forming a secondary paragenesis constituted by calcite, chlorite, epidote and sericite. The dykes TF, TD, FF and FE are sub-volcanic calc-alkaline, typical of post-collisional basalts/andesites, belonging to plate margin andesites. The FW dyke shows a within-plate basalt signature; alkaline affinity reflecting a different petrogenetic process.The thermomagnetic analyses show a dominantly ferromagnetic behaviour in the TF dyke core carried by single domain Ti-poor magnetite, maghemite and pyrrhotite. The dominantly paramagnetic susceptibilities in TF dyke rims and TD, FE, FF and FW dykes are controlled by ilmenite, amphibole, pyroxene and chlorite.The magnetic fabrics of the Imider dykes, determined by our AMS study, allows us to reconstitute the tectonic conditions which prevailed during the emplacement of these two generations of volcanic dykes. The first tectonic event was characterized by a roughly NE-SW compression and the second tectonic event is characterized by an E-W shortening followed by a relaxation recording the end of the Pan-African orogeny in the eastern Anti-Atlas.

New Constraints on the Timing of Host-Rock Emplacement, Hydrothermal Alteration, and Iron Oxide-Apatite Mineralization in the Kiruna District, Norrbotten, Sweden

High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30–70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05–5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores.

Immiscible iron- and silica-rich melts and magnetite geochemistry at the El Laco volcano (northern Chile): Evidence for a magmatic origin for the magnetite deposits

The Pliocene El Laco volcano (northern Chile) hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones (dykes) hosted by hydrothermally altered andesite lava flows. Phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility. Most of the pyroxene and the plagioclase phenocrysts contain abundant Fe-rich melt inclusions with low-Ti magnetite, pyroxene, apatite, and minor anhydrite. The glassy groundmass of these melt inclusions is chemically similar to sub-alkaline rhyolitic rocks (e.g., >70% SiO2; Si-Al-K-Na-rich). Large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica-rich groundmass but hosting evenly distributed immiscible and complex Fe-rich spheroidal globules that are enriched in Ti, P, Mg, Ca, and S. These immiscible globules and the Ti-poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide-melt liquid, after which their coalescence leads to the formation of magnetite melts, from which the magnetite that formed the El Laco iron oxide deposits crystallized and were emplaced.The mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the Fe-Ox melts and supports a magmatic origin for the El Laco magnetite. This is mainly recognized by the continuum of compositions between volcanic, separate immiscible Fe-oxide melts, and magnetite ores. The trace element composition of the Fe-rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite. The Ti content of the magnetite also records the separation and crystallization of the iron-rich melt. Typically, the magnetite in the orebodies and the melt inclusions is Ti-poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally-altered andesite; that is interpreted as being attributed to its crystallization under oxidizing and sulfur-rich conditions. The cause of this unusual chemical behaviour is probably due to the mechanism(s) governing the crystallization of the iron-rich melt, including the temperature, changes in chemistry prior to eruption, and an increase in the redox conditions of the silicate melt. Magma oxidation facilitates the prevalence of ferric iron and formation of Ti-poor magnetite, preventing the incorporation of Ti in the exsolving iron-rich melt. The likely reason for the formation of these large Ti-poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions, something that had a significant impact on the elemental distribution.

Magnetic fabric (AMS, AAR) of the Santa Marta batholith (northern Colombia) and the shear deformation along the Caribbean Plate margin

Anisotropy of low-field magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were measured in the Santa Marta Batholith formed by subduction of the Caribbean Plate beneath the northern South America. The batholith, elongated in the N-S direction, records multiple pulses of quartzdiorite to tonalite and granodiorite magmas between 58 and 49 Ma. The high mean magnetic susceptibility (4 × 10−3 SI) combined with thermomagnetic and partial magnetic remanence measurements indicate that the magnetic susceptibility depends on Ti-poor magnetite. AMS is defined by ellipsoids that are dominantly oblate. The foliation was used to distinguish a narrow band of E-trending magnetic structures that separate the batholith in two lobes. The southern lobe is characterized by foliations that are broadly parallel to the contact with the wall rocks, while the northern lobe by foliations oblique to the batholith elongation. Late tonalitic magmas dated at c. 50 Ma record, in turn, a fabric apparently controlled by E-trending tectonic events. Partial AAR indicates that the subfabrics of magnetite with different grain sizes are nearly parallel to AMS, therefore discarding the possibility of superposed fabrics with different orientations. The magnetic fabric pattern is consistent with a magma emplaced in an arc setting deformed by a dextral shear. Synthetic extensional shear bands localize the magmatic deformation along East-trending corridors that probably were exploited to emplace the late magmatic pulses. Accretion of the Eocene batholith and the Late Cretaceous metasedimentary host-rocks to the South American continent defines a major strike-slip shear suture that resulted from the oblique convergence of the Caribbean Plate.

Vertical AMS variation within basalt flow profiles from the Xitle volcano (Mexico) as indicator of heterogeneous strain in lava flows

The within-flow vertical variation of anisotropy of the magnetic susceptibility (AMS) of three basaltic flow profiles from the Xitle volcano were investigated in relation to the lava flow-induced shear strain. Rock magnetic properties and opaque microscopy studies have shown that the magnetic mineralogy is dominated by Ti-poor magnetite with subtle vertical variations in grain size distribution: PSD grains dominate in a thin bottommost zone, and from base to top from PSD-MD to PSD-SD grains are found. The vertical variation of AMS principal direction patterns permitted identification of two to three main lava zones, some subdivided into subzones. The lower zone is very similar in all profiles with the magnetic foliation dipping toward the flow source, whereas the upper zone has magnetic foliation dipping toward the flow direction or alternates between dipping against and toward the flow direction. The K1 (maximum AMS axis) directions tend to be mostly parallel to the flow direction in both zones. The middle zone shows AMS axes diverging among profiles. We present heterogeneous strain ellipse distribution models for different flow velocities assuming similar viscosity to explain the AMS directions and related parameters of each zone. Irregular vertical foliations and transverse to flow lineation of a few samples at the bottommost and topmost part of profiles suggest SD inverse fabric, levels of intense friction, or degassing effects in AMS orientations.

Spot Reading of the Absolute Paleointensity of the Geomagnetic Field Obtained from Potsherds (Age Ca. 500-430 AD) in Teotihuacan, Mexico

Archaeointensity data have been obtained successfully using the Thellier-Coe protocol from twelve potsherds recovered from the vicinity of the “Piramide del Sol”, Teotihuacan, Mexico. In order to understand the magnetic behavior of the samples, we have conducted low-field versus temperature (k-T) experiments to determine the magnetic carriers of the pre-Columbian artefacts, as well as Saturation Isothermal Remanent Magnetization (SIRM), hysteresis loops and back-fields TESTS. The Curie temperatures indicate the presence of at least three magnetic mineral phases (238°C - 276°C, 569°C - 592°C, and 609°C - 624°C). The predominant Curie temperatures for these samples are typical of Ti-poor magnetite. The results of the magnetic grain size analyses indicate that if the magnetic mineral in a sample is only magnetite, the distribution on the modified Day et al. (1977) diagram yields specimens in the Single (SD), Pseudo (PSD) and Superparamagnetic (SP) domain ranges. The successfully absolute paleointensity determinations in this study using the Thellier-Coe protocol have yielded an average paleointensity of 38.871 +/- 1.833 m-Teslas (N = 12), and a virtual geomagnetic dipole moment of 8.682 +/- 0.402 × 1022 A/m2 which is slightly lower than the present field strength and which corresponds to an age interval between 500 and 430 AD. Thus, our results correlate well with the recently published CALS3K.4 curve and the incipient archaeointensity reference curve for the Mesoamerican paleo-field results. Therefore, the age of the artefacts would correlate well with absolute the early classic Teotihuacan cultural period.

Age and magnetic fabric of the Três Córregos granite batholith: evidence for Ediacaran transtension in the Ribeira Belt (SE Brazil)

Anisotropy of magnetic susceptibility (AMS) and U–Pb (SHRIMP) zircon ages in the eastern part of the Tres Corregos batholith (Ribeira belt, SE Brazil) indicate a well-defined fabric pattern acquired between 600 and 595 Ma. The batholith consists mostly of porphyritic granites distributed in the Ribeirao Branco, Barra do Chapeu and Itaoca plutons. Late fluorite-bearing alkaline granites, some containing Sn-polymetallic greisen-type deposits, intruded the Ribeirao Branco pluton and the low-grade metasedimentary host rocks. The magnetic fabric of the Ribeirao Branco granite is dominantly oblate and oblique to the pluton elongation while that in Barra do Chapeu is mostly concentric. On both plutons, AMS records the preferred orientation of coarse, homogeneous Ti-poor magnetite grains. Such fabric patterns indicate a partitioned strain field dominated by strike-slip left-lateral shear deformation in the Ribeirao Branco and extension in the Barra do Chapeu pluton. The zircon ages of these plutons are 600 ± 6 Ma and 595 ± 4 Ma. Likewise, the ages of alkaline plutons were in the range of 597 and 595 Ma, registering a fast transition between the typical syntectonic batholithic magmatism to the late, highly evolved and specialized magmas. These results indicate that the Ribeira belt was deformed by transtension in the Middle Ediacaran. The geological setting is consistent with a continental arc with the Tres Corregos batholith emplacing at the middle-upper crust.