Fe Ti Oxide Ores Research Articles

Petrology and geochemistry at the Lower zone-Middle zone transition of the Panzhihua intrusion, SW China: Implications for differentiation and oxide ore genesis

A sequence of gabbros showing isotropic, layered and fine-grained textures is exposed in the Nalaqing mine at the southern tip of the ∼260 Ma Panzhihua intrusion, SW China. The field relations, structure, texture and mineralogy of the rocks indicate that the sequence represents the transition between the Lower zone and Middle zone of the intrusion. Isotropic gabbros characteristic of the Lower zone pass upward to layered gabbros of the Middle zone through a ∼5 m-thick microgabbro sheet, within and close to which small-scaled, concordant Fe-Ti oxide ore horizons are identified. Strong fractionation between HFSE and REE in a subset of samples is ascribed to cumulus titanomagnetite into which HFSE are preferentially incorporated over REE, as reflected in the parallel relations between Nb/La, Hf/Sm and Ti/Ti*. Both the isotropic and layered gabbros display cumulate textures and have similar mineral compositions (Mg# of clinopyroxene = ∼76–79 and An59–61), isotopic compositions [(87Sr/86Sr)i = 0.7044–0.7045 and εNd(t) = +2.4 to +3.9] and trapped liquid contents inferred from Zr abundance (∼17–34 ppm). However, there are substantial variations in elemental abundances (V, Cr and PGE) and ratios (Ti/V, La/Yb, Ba/Y and Cu/Pd) between the two types of gabbros, features that cannot be explained by cumulate formation from a common magma in a closed system. The microgabbros generally resemble high-Ti Emeishan basalts in major element compositions, but their low trace element abundances indicate some lost of residual liquid is inevitable despite rapid nucleation and cooling. Combined with available data and observations, we propose a model involving in-situ crystallization, followed by magma recharge and closed-system fractionation to explain the formation of texturally distinctive gabbros at Nalaqing and the evolution of the lower part of the Panzhihua intrusion.

Petrogenesis and metallogenesis of the Taihe gabbroic intrusion associated with Fe–Ti-oxide ores in the Panxi district, Emeishan Large Igneous Province, southwest China

The Taihe layered gabbro intrusion in the northernmost part of the Panxi district in southwest China is part of the 260 million year old Emeishan Large Igneous Province. This intrusion hosts a giant Fe–Ti oxide deposit with 810 million tonnes of ore reserves, which makes it one of the largest deposits in the Panxi district. The intrusion covers an areal extent of ~13km2 and has a vertical stratigraphic thickness of ~1400m. It can be divided into a lower zone (LZ) of coarse-grained gabbro, apatite-bearing gabbro, troctolite and intercalated gabbro and clinopyroxene-bearing troctolite, followed upward by a middle zone (MZ) of gabbro and intercalated clinopyroxenite, plagioclase-bearing clinopyroxenite with major oxide layers, and an upper zone (UZ) of olivine gabbro and layered gabbro including unmineralized leucogabbro and melanogabbro, with some small oxide ore bodies in the lower part. Each of these ‘zones’ contains oxide minerals and relatively similar lithologies. Ore textures and associated mineral assemblages indicate that the ore bodies formed by crystallization of Fe–Ti–V-rich melt under high oxygen fugacity and a volatile-rich environment during the late-stage of magmatic differentiation. A general systematic variation of major oxides is seen through the intrusion as reflected by a slight overall decrease in MgO and Fe2O3 (as total iron) and an increase in SiO2, Na2O, Al2O3, and CaO upward in the layered sequences. Based on lithology and bulk-rock geochemical features, such as positive Eu anomalies, the Taihe intrusion is inferred to have been derived from a ferropicritic melt and became more evolved in chemistry upward following a tholeiitic differentiation trend with enrichment in Fe, Ti, and V. The Taihe gabbros define a small range of age-corrected εNd(t) (t=260Ma) from −0.6 to 0.7 and (87Sr/86Sr)t ratios ranging from 0.7040 to 0.7050. The relatively lower εNd(t) values and higher (87Sr/86Sr)t ratios compared to those from Lijiang picrite which represents the initial product of Emeishan plume head, combined with the enrichment in light rare earth element (LREE) relative to heavy rare earth element (HREE) as well as negative high field strength element (HFSE; e.g., Nb, Ta, Zr, and Hf) anomalies, suggest that subduction-related material was involved in the source region. We propose that the parental picritic magma was generated from the interaction of the ~260 million year Emeishan mantle plume with the lithospheric mantle, and the picritic magma interacted with an eclogitic component in the lithospheric mantle. In our view, the eclogitic component was derived from the earlier Neoproterozoic subduction. The junction of these subduction-modified lithospheric mantle sources and the Emeishan plume was a possible crucial factor leading to the production of large Fe–Ti oxide deposits in the Panxi area.

Volatile and C-H-O isotopic compositions of giant Fe-Ti-V oxide deposits in the Panxi region and their implications for the sources of volatiles and the origin of Fe-Ti oxide ores

The Panzhihua, Hongge, and Baima Fe-Ti-V oxide deposits in the Panzhihua-Xichang (Panxi) region are hosted in large layered mafic-ultramafic intrusions. The layered intrusions intrude either the Neoproterozoic Dengying Formation, composed mainly of limestone, or the Paleoproterozoic Hekou Formation, composed of meta-sedimentary-volcanic rocks. It remains unclear if the wall rocks have been involved during the fractionation of magmas and have affected the sequence of crystallization of Fe-Ti oxide. Volatiles and their C-H-O isotopic compositions of magnetite, apatite, clinopyroxene, and plagioclase of different types of ores from the three intrusions are analyzed using a technique of stepwise heating mass spectrometer to evaluate the role of wall rocks in the formation of Fe-Ti oxide ores. Volatiles released from magnetite are composed mainly of H2O and CO2, whereas the other minerals are composed mainly of H2O, CO2 and H2. At 800–1200°C temperature interval, the average δ13C values of CO2 of all the minerals from the three intrusions range from −7.7‰ to −13.5‰ and the average \(\delta ^{18} O_{CO_2 } \) values from 19.1‰ to 19.5‰, which are scattered in a mixed field with basalt and the two types of wall rocks as end-members, indicating that CO2 from the wall rocks may have been involved in the magmas from which the three intrusions formed. At 400–800°C temperature interval, both δ13C values (−13.7‰ to −17.9‰ on the average) and δ18O values (16.2‰ to 19.2‰ on the average) of CO2 of all the minerals are lower than those for 800–1200°C temperature interval, and much closer to the values of the wall rocks. Abundant H2O released at the 400–800°C temperature interval has relatively low δD values ranging from −90‰ to −115‰, also indicating the involvement of fluids from the wall rocks. The average bulk contents of volatiles released from magnetite of the Hongge, Baima, and Panzhihua intrusions are 4891, 2996, and 1568 mm3 STP/g, respectively, much higher than those released from other minerals in total, which are 382, 600, and 379 mm3 STP/g, respectively, indicating that magnetite crystallized from magmas with much more volatiles than other minerals. This can be interpreted as that crystallization of clinopyroxene and plagioclase in the early fractionation of magmas resulted in volatiles such as H2O that were eventually enriched in the residual magmas and, at the same time, fluids from the wall rocks may have been involved in the magmas and were trapped in magnetite, which crystallized later than clinopyroxene and plagioclase.

Petrogenetic implications of mineral chemical data for the Permian Baima igneous complex, SW China

The Late Permian Fe-Ti oxide ore-bearing Baima igneous complex (BIC) is one of three gabbro-granitoid complexes with the Emeishan large igneous province. Mineral compositions are determined for the BIC layered gabbro in order to constrain subsolidus and magma chamber processes. The averaged compositions of cumulus olivine, clinopyroxene and plagioclase within individual samples range from Fo65-76, Mg# = 75 to 82 and An49-64 but they are not correlative. The observed mineral compositions are consistent with those modeled using the pHMELTS program. Highly variable magnetite compositions are consistent with extensive subsolidus re-equilibration and exsolution. The occurrence of reversely-zoned granular olivine in Fe-Ti oxide ores is a manifestation of Mg transfer between Fe-Ti oxides and olivine at relatively high (<1150 °C) subsolidus temperatures. The primary oxide is inferred to be an aluminous titanomagnetite. Similar to other layered intrusions in the region, the gabbroic unit of the BIC displays Zr depletion which is consistent with loss of a residual liquid during magma differentiation. If the most Zr-rich syenites of the complex are taken into account, the Zr budgets between the combined gabbro-syenite and the basalts are similar. This indicates that the BIC most likely represents a closed system in terms of magma extraction.

Flood basalt-related Fe–Ti oxide deposits in the Emeishan large igneous province, SW China

In the Panzhihua–Xichang region (Sichuan Province, SW China), there are a number of world-class magmatic Fe–Ti oxide deposits. They are hosted as conformable masses in lower parts of layered mafic–ultramafic intrusions that are part of the end-Guadalupian (∼ 260 Ma) Emeishan large igneous province. The ore-bearing Panzhihua, Hongge, Baima, Taihe and Xinjie intrusions are spatially and temporally associated with flood basalts and granitoids in the province. New mineralogical data for Fe–Ti oxide ores in the Hongge and Baima intrusions, combined with an overview of previously published data, provide a better understanding on the formation of these enigmatic deposits. Geochemical and Sr–Nd isotopic data confirm a genetic relation between the intrusions and high-Ti Emeishan flood basalts. Parental magma compositions, estimated using the most Mg-rich cumulus olivine in the intrusions, are characterized by slight to moderate degrees of fractionation (MgO = 7.2–11 wt.%). Occurrences of rare Cr-bearing titanomagnetite (Cr 2O 3 = 1.2–10.7 wt.%) in the Panzhihua, Hongge and Xinjie intrusions are consistent with early crystallization of Fe–Ti oxides. Oxide-silicate equilibria and low V concentration (< 4800 ppm) in magnetite are consistent with relatively high oxygen fugacity. Flood basalt-related origin, early Fe–Ti oxide crystallization and relatively high oxygen fugacity, are characteristic of Fe–Ti oxide deposits in the Panxi region but are fundamentally different from those hosted in highly-differentiated, upper parts of large layered intrusions (e.g. the Bushveld Complex). Similar Fe–Ti oxide deposits have not been documented in other well-studied large igneous provinces to date, but a number of Fe–Ti oxide ore occurrences worldwide are potentially similar to this type of flood basalt-related oxide deposits.

U–Pb and 40Ar/ 39Ar geochronology of the Saint-Urbain and Lac Allard (Havre-Saint-Pierre) anorthosites and their associated Fe–Ti oxide ores, Québec: Evidence for emplacement and slow cooling during the collisional Ottawan orogeny in the Grenville Province

Intrusive suites containing anorthosite–mangerite–charnockite–granite (AMCG) are a characteristic feature of Mesoproterozoic magmatic activity in the Grenville Province of eastern North America. In this study, the U–Pb and 40Ar/ 39Ar geochronology of two AMCG suites (Saint-Urbain and Havre-Saint-Pierre), and associated rutile-bearing hemo-ilmenite deposits, within the allochthonous polycyclic belt of the Grenville Province in Québec are used to constrain their crystallization ages and cooling histories, and to assess their tectonic setting. Both AMCG suites, separated by 700 km along the St. Lawrence River, have similar crystallization ages (ca. 1050–1060 Ma) and durations of magmatism (10–15 million years). The 450 km 2 Saint-Urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite (1046.2 ± 3.1 Ma) compared with anorthosite to the north and south (1055.0 ± 2.4 Ma and 1053.6 ± 2.6 Ma, respectively). The age of a deformed oxide–apatite gabbronorite (1057.4 ± 1.5 Ma) along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites, whereas the oldest intrusion in the Saint-Urbain area is the regionally extensive Saint-Anne du Nord orthopyroxene granodiorite (1060.8 ± 2.8 Ma). The large 11,000 km 2 Havre-Saint-Pierre anorthosite suite comprises a number of intrusive phases, including the 2700 km 2 Lac Allard anorthosite, which contains the giant Lac Tio ilmenite deposit and Big Island ilmenitite dyke. U–Pb zircon ages of three anorthosites from the Lac Allard intrusion, including the host to the Lac Tio deposit, overlap within error (1061.6 ± 3.0 Ma, 1060.5 ± 1.9 Ma, 1057.4 ± 8.4 Ma) and the crosscutting Big Island dyke yields an age of 1052.9 ± 6.5 Ma. These results indicate that the Havre-Saint-Pierre anorthosite should be divided into an older geon 11 part (ca. 1129 Ma) and a younger geon 10 part (ca. 1060 Ma). Average cooling rates of 3–4 °C/million years were estimated for the two AMCG suites by combining crystallization ages (U–Pb zircon) and cooling ages (U–Pb rutile, 40Ar/ 39Ar biotite/plagioclase) and reflect emplacement of the intrusions into a hot, long-duration orogen. Emplacement of these two AMCG suites at ca. 1050–1060 Ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional Ottawan orogenic phase of the Grenville orogeny. The results of this study provide strong support for proposals that link AMCG magmatism associated with the Grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle.

Cumulate Origin and Polybaric Crystallization of Fe-Ti Oxide Ores in the Suwalki Anorthosite, Northeastern Poland

Fe-Ti oxide ores in the Proterozoic Suwalki massif-type anorthosite, northeastern Poland, have been recognized through geophysical exploration and extensive drilling down to 2,600 m depth. The Fe-Ti oxide-rich rocks from Suwalki consist of vanadium-poor Ti-magnetite in lenses varying from the centimeter- to the kilometer-scale. Fe-Ti‐rich rocks are commonly layered and have gradational contacts with the host anorthosite; they do not represent well-defined intrusions such as the major Tellnes and Lac Tio deposits. Based on petrography, modal proportions, whole-rock analyses, and liquidus phase compositions of Fe-Ti oxide ores, the sequence of crystallization is as follows: plagioclase + Ti magnetite + ilmenite, followed by orthopyroxene, then clinopyroxene, and finally apatite. The comparatively low V content (0.20‐0.67 wt % V) in Ti magnetite results from relatively oxidized crystallization conditions. The diapiric emplacement of the anorthositic pluton influenced the crystallization of the Fe-Ti ores and is responsible for the crystal sorting controlled by the density contrast of liquidus phases. Polybaric crystallization of Fe-Ti oxide ores is evidenced by variable Al2O3 (ca. 1‐5 wt %) content of the associated orthopyroxene. Polybarism is responsible for the appearance of Fe-Ti oxides before orthopyroxene, due to the decreasing stability field of orthopyroxene with pressure.

ORIGIN OF ZIRCON RIMS AROUND ILMENITE IN MAFIC PLUTONIC ROCKS OF PROTEROZOIC ANORTHOSITE SUITES

Zircon rims around ilmenite occur in a wide variety of mafic plutonic rocks (Fe–Ti oxide ores, anorthosite, and gabbro) from unmetamorphosed Proterozoic intrusions in the Grenville Province of eastern Canada (Havre-Saint-Pierre, Mattawa and Saint- Urbain anorthosites, Quebec; Twin Lakes intrusive complex, Ontario) and the western U.S. (Laramie anorthosite, Wyoming). The rims range from a few μm to 100 μm across and occur along grain boundaries between Ti-based oxide minerals (mainly ilmenite and exsolved ferrian ilmenite, but also rutile) or between silicate minerals (plagioclase and biotite) and Ti-based oxide minerals. The zircon rims contain very low concentrations of P (<447 ppm), U (0.9–1.1 ppm) and Th (15–19 ppm), which, coupled with the restriction of the rims to ilmenite grain boundaries, rules out formation from high-temperature, zircon-saturated interstitial liquid and from low-temperature hydrothermal fluids. There is no textural evidence that zircon replaced pre-existing baddeleyite that may have formed by an oxidation–exsolution mechanism from ilmenite. The textural and compositional characteristics of the zircon rims are consistent with an origin by diffusion of Zr from ilmenite to grain boundaries during slow cooling and subsequent reaction with available Si from adjacent silicate minerals (or from ilmenite itself) to form zircon. A positive correlation between whole-rock TiO2 and Zr contents in samples of Fe–Ti oxide ores from Saint-Urbain and Big Island (Havre-Saint-Pierre) (up to 500 ppm in massive ilmenite ore) confirms that Zr is compatible in ilmenite crystallized from ferrobasaltic magmas. Mass-balance constraints demonstrate that the observed quantities of zircon can be derived from Zr-bearing ilmenite crystallized from basaltic magmas with typical Zr concentrations (161–252 ppm) after fractionation of Zr-free minerals (150–247 ppm). Zircon rims are not expected to occur in ilmenite-bearing basaltic lavas and high-level intrusions owing to rapid cooling, but zircon rims may be a common feature of slowly cooled plutonic rocks that crystallize from ferrobasaltic magmas. The U–Pb dating of these rims may provide additional constraints on the thermal evolution of a given plutonic rock.

Magma chamber processes in the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the formation of Fe–Ti ores in massif-type anorthosites

The origin of igneous Fe–Ti oxide ores associated with massif-type anorthosites is investigated through a detailed study of the world-class Tellnes ilmenite deposit, part of the late-Proterozoic (930–920 Ma) AMC series of the Rogaland Anorthosite Province (SW Norway). More than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body. Four zones are defined, based on variations in modal proportions and cumulus mineral assemblages: the Lower and Upper Central Zones and the Lower and Upper Marginal Zones. Plagioclase and whole-rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase–ilmenite or plagioclase–ilmenite–orthopyroxene–olivine cumulates with a melt of ferrodioritic (jotunitic) composition with a content decreasing from 80 to 20% from the margins to the central part of the ore body. Phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb. The orthopyroxene–olivine liquidus boundary is a peritectic in the Bjerkreim-Sokndal layered intrusion and a cotectic in Tellnes and this explains the differences in the sequence of crystallization of the two intrusions. The high concentration of ilmenite, well above cotectic proportions, resulted from gravity-sorting in the Tellnes ore body, which represents the lower part of a larger magma chamber. Uniform Sr isotope ratios do not support magma mixing. The cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill-like magma chamber. The present trough-shape and mineral orientations result from deformation during gravity-induced subsidence and by up-doming of the anorthosite. Fractional crystallization of a TiO 2-rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of Fe–Ti deposits in Proterozoic massif-type anorthosites.

River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

The River Valley pluton is a ca. 100 km 2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An 60–70) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo 70–80. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560±155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ± 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ± 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ± 130 Ma and 2100 ± 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb-Pb systems from a younger event such as the Grenvillian orogeny of ca. 1.0 Ga. None of the isotopic systems we investigated, however, gives an age near 1.0 Ga, suggesting that neither the River Valley pluton, nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the Grenvillian orogeny. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of μ = 8.06, ϵ Nd = 0 to −3, and I Sr = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.