Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

Abstract

a b s t r a c t a r t i c l e i n f oMagmatic rocks containing economic concentrations of iron, titanium, vanadium and phosphorous are commonly associated with massif-type anorthosites and related rocks. This rock association is part of the anorthositemangerite-charnockite-(rapakivi-)granite suites that are restricted to the Proterozoic. Understanding the geochemistry and emplacement mechanisms of ilmenite, magnetite and apatite ore deposits is crucial for exploration, efficient mining operations and ore processing. This review discusses the controlling factors on the grade of an ore, its mineralogy, and its major and trace element distribution. We present petrogenetic models of currently mined deposits (Lac Tio, Tellnes, Damiao) and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide (Grenville, North China Craton, East European Craton, Rogaland, Laramie). Models of formation of anorthosite and related rocks are presented, as well as the nature of the possible parental magmas of the suite. A mineralogical classification of Fe-Ti ores is proposed: (1) Gabbro-noritic ilmenite ore ± apatite ± magnetite; (2) Ti-magnetite-dominated ore; (3) Nelsonite (Fe-Ti oxides + apatite); and (4) Rutile-ilmenite ore. The stability of ilmenite and magnetite is then critically reviewed and the influence of various factors, particularly oxygen fugacity and crystallization pressure, is examined. We discuss liquidus compositions of Fe-Ti oxides and the behavior of important trace elements such as Cr and V, both of which are sensitive to fO 2 variations. Postcumulus evolution of both oxides can occur due to re-equilibration with trapped liquid, re-equilibration with ferromagnesian silicates, exsolution, oxidation, reaction between ilmenite and magnetite, and metamorphic overprinting. These various processes are described and their effects on the oxide geochemistry are emphasized. Several potential ore-forming processes have been invoked and can explain the formation of huge concentration of ilmenite, ±magnetite, ±apatite. Fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals. Silicate liquid immiscibility can segregate conjugate Si-rich and Fe-rich melts, the latter being enriched in Fe-Ti-P. Magma mixing can produce hybrid magmas located in a single-phase field of the phase diagram and precipitate a pure ilmenite cumulate. Alternative processes are also described, such as ejection of Fe-Ti-enriched residual melts by filter-pressing and compaction, solidstate remobilization of ilmenite in veins, and hydrothermal transport of Fe and Ti from the host anorthosite followed by concentration in veins and lenticular ore bodies. The magnetic properties of Fe-Ti ore deposits present contrasting signatures, depending on whether the natural remanent magnetization is dominated by hemo-ilmenite or multi-domain magnetite. Micro-and macro-scale deformation features of ore rocks are intimately correlated with magma emplacement, and...

Fe–Ti(–V) Oxide Deposits of the Kunene Anorthosite Complex (SW Angola): Mineralogy and Thermo-Oxybarometry

“…The mineralogy of the studied Fe-Ti(-V) oxide bodies is dominated by titaniferous magnetite, which is consistent with the KIC being a labradorite-type anorthosite [6]. As observed in Figure 3, the KIC contain less TiO 2 than Lac Tio and Tellnes, (which are ilmenite-dominated), and lower Fe/Ti than Lac Tio, because the KIC ore bodies (like Damiao and Tellnes) contain less hematite.…”

“…In this case (massive deposits formed by fractional crystallization and separation by density), the Fe-Ti(-V) oxide ore bodies would be concordant with the intrusive anorthosite. However, in some massif-type anorthosite Fe-Ti(-V) oxide deposits worldwide (e.g., Damiao in China, Tellnes in Norway and Lac Tio in Canada), the elongated bodies are discordant (or with intrusive relationships) with the anorthosite hosts [6]. Although the contact between the oxide bodies of the KIC and their hosts could not be determined in the field, layering in the anorthosite is not parallel to the elongation of the massive oxide bodies, so they can be considered discordant (Figure 2a).…”

“…In addition, the foliation observed in many anorthosite complexes, as in the KIC, are described as syn-emplacement features ( [6] and references therein). Magmas parental to anorthosites and their residual liquids are TiO 2 -enriched (>4 wt % TiO 2 ), which would promote early saturation of ilmenite, even at high f O 2 [97].…”

“…[3,52]; Kydlandsvatn and Hestnes: [54]; Panzhihua: [53]; Grader intrusion: [56]; Tellnes: [11]. (a) is based on [6]; and (b) is based on [3]. TiO2, SiO2, Fe2O3, CaO, P2O5 are in wt %, V and ∑REE (La-Lu and Y) are in ppm.…”

“…In addition, vanadium is a common byproduct (usually below 0.2% V) [4]. These deposits occur as massive oxide bands embracing some hundreds of million tons of ore with a mineralogy dominated by ilmenite and titaniferous magnetite [4][5][6]. Andesine (An ) anorthosites are described to contain the most important magmatic Fe-Ti oxide ores, even if significant resources are described also for labradorite metamorphism on the KIC [20].…”