The petrogenesis of magmatic Fe-Ti oxide ores associated with massif-type andesine anorthosites is investigated through detailed studies of the world-class Tellnes ilmenite deposit (SW Norway), the Grader layered intrusion (Quebec, Canada) and Fe-Ti ores from the Suwalki anorthosite (NE Poland). Extensive sampling in the field and in drill-cores reveals significant petrographical and compositional variations within a single ore body and between deposits from different anorthosite complexes. The composition of phases from bulk XRF analyses on mineral separates and from in situ LA-ICP-MS, Sr isotopic composition of plagioclase, bulk rocks major and trace element contents and the spatial variation of these data are used to understand controlling factors on ore composition. Phase diagrams and experimental data on ferrobasalts as well as comparisons with the well-documented Bjerkreim-Sokndal layered intrusion (SW Norway) are systematically used to further refine our understanding on the genesis of Fe-Ti ores.
More than 100 samples from drill-cores in the Tellnes ilmenite deposit, part of the late-Proterozoic (930-920 Ma) Rogaland Anorthosite Province (SW Norway), 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. Its content decreases 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 kbar. 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.
Major and trace element XRF and in situ LA-ICP-MS analyses of ilmenite in the Tellnes ilmenite deposit further constrain the two-stage fractional crystallization model of a ferrodioritic Fe-Ti-P rich melt. Stage 1 is characterized by ilmenite-plagioclase cumulates, and stage 2 by ilmenite-plagioclase-orthopyroxene-olivine cumulates. The concentration of V and Cr in ilmenite, corrected for the trapped liquid effect, (1) defines the cotectic proportion of ilmenite to be 17.5 wt.% during stage 1, and (2) implies an increase of during stage 2, most likely related to a shift in fO2. The proportion of 17.5 wt.% is lower than the modal proportion of ilmenite (ca. 50 wt.%) in the ore body, implying accumulation of ilmenite and flotation of plagioclase. The fraction of residual liquid left after crystallization of Tellnes cumulates is estimated at 0.6 and the flotation of plagioclase at 26 wt.% of the initial melt mass. The MgO content of ilmenite (1.4-4.4 wt.%) is much lower than the expected cumulus composition. It shows extensive postcumulus re-equilibration with trapped liquid and ferromagnesian silicates, correlated with distance to the host anorthosite. The Zr content of ilmenite, provided by in situ analyses, is low and uncorrelated with stratigraphy or Cr content. The data demonstrate that zircon coronas observed around ilmenite formed by subsolidus exsolution of ZrO2 from ilmenite. The U-Pb zircon age of 920 ± 3 Ma probably records this exsolution process.
The Grader layered intrusion belongs to the Havre-Saint-Pierre anorthosite in the Grenville Province (Quebec, Canada). This intrusion has a basin-like morphology and contains significant resources of Fe-Ti-P in ilmenite and apatite. Outcropping lithologies are massive oxide alternating with anorthosite layers, banded ilmenite-apatite-plagioclase rocks and layered oxide apatite (gabbro-) norites. Several drill cores provide evidence for stratigraphic variations of mineral and bulk cumulate compositions controlled by fractional crystallization and importantly the successive appearance of liquidus phases: plagioclase and ilmenite followed by apatite, then orthopyroxene together with magnetite, and finally clinopyroxene. This atypical sequence of crystallization results in the formation of plagioclase-ilmenite-apatite cumulates or “nelsonites” in plagioclase-free layers. Fine-grained ferrodiorites which cross-cut the coarse cumulates are shown to be in equilibrium with the noritic rocks. The high TiO2 and P2O5 contents of these liquids explain the early saturation of ilmenite and apatite before Fe-Mg silicates, which implies that nelsonites actually represent cumulates rather than Fe-Ti-P-rich immiscible melts. The location of the most evolved mineral and bulk cumulates compositions at several tens of meters below the top of the intrusion, forming a sandwich horizon, suggests crystallization both from the base and top of the intrusion. The concentrations of V and Cr in ilmenite display a single fractionation path for the different cumulus assemblages and define the cotectic proportion of ilmenite to 21 wt.%. This corresponds to bulk cotectic cumulates with ca. 8 wt.% TiO2, which is significantly lower than what is commonly observed in the explored portion of the Grader intrusion. The proposed mechanism of ilmenite-enrichment is lateral removal of plagioclase due to its buoyancy in the dense ferrodiorite. This plagioclase has probably accumulated in other portions of the intrusion or has not been distinguished from the host anorthosite.
Fe-Ti deposits in the Proterozoic Suwalki massif-type anorthosite (NE Poland), recognized through geophysical exploration, have been sampled in deep cores reaching 2800 m depth. Bulk cumulate analyses and liquidus phases composition of 70 Fe-Ti ores support their cumulate origin. The sequence of crystallization is: plagioclase, orthopyroxene, Ti-magnetite and ilmenite (64:36 on average), apatite and clinopyroxene. Fe-Ti-rich cumulates are commonly layered and display continuous relation with the host anorthosite. They do not represent well-defined intrusions such as the major Fe-Ti Tellnes and Lac Tio deposits. Fe-Ti oxides microtextures show conspicuous subsolidus re-equilibration, particularly external granule exsolution of pleonaste from Ti-magnetite. The composition of associated fine-grained ferrodiorites reveals relatively low Ti content and similar Mg# compared to jotunitic rocks associated with hemo-ilmenite ores in the Rogaland Anorthosite Province. Geochemical characteristics of these plausible parental magmas can account for the high Ti-magnetite/ilmenite ratio in cumulates. The diapiric emplacement of anorthositic plutons clearly influences the crystallization of Fe-Ti ores and is responsible for crystal sorting controlled by the density contrast of liquidus phases. Polybaric crystallization is evidenced by the high and variable Al2O3 content of orthopyroxene and by the occurrence of olivine corona around orthopyroxene interpreted from phases diagram to result from adiabatic decompression. The comparatively low V content in Ti-magnetite results from highly oxidized crystallization conditions.
It thus emerges that principal controlling factors on the formation of Fe-Ti ore and on their characteristics are parental magma composition, sequence of crystallization, crystal sorting, crystallization of trapped liquid, oxygen fugacity and postcumulus re-equilibration. Indeed, fine-grained rocks of Fe-Ti-P-rich ferrodioritic (jotunitic) composition, interpreted as parental melt composition, are responsible for atypical sequence of crystallization with ilmenite as an early liquidus mineral and apatite saturation for high fraction of residual liquid. The trace element content of ilmenite, particularly V and Cr, has been used to calculate cotectic proportion of ilmenite during fractional crystallization of ferrodiorites. These proportions are usually lower than those observed in Fe-Ti ores, which implies ilmenite sorting. This occurs by plagioclase flotation due to its buoyancy in the dense ferrodiorite. This plagioclase may have accumulated in other portions of the intrusion or has not been distinguished from the host anorthosite. Extensive postcumulus re-equilibration with trapped liquid and ferromagnesian silicates strongly modifies the primary liquidus composition of Fe-Ti oxides. The data also demonstrate that zircon coronas commonly observed around ilmenite in Fe-Ti ores formed by subsolidus exsolution of ZrO2 from ilmenite. The basin-like morphology of most Fe-Ti ores hosting intrusions results from the deformation during gravity-induced subsidence and by up-doming of the anorthosite. As immiscibility of a Fe-Ti-P-rich melt and magma mixing have not been evidenced in the studied Fe-Ti ores, early ilmenite saturation accompanied by ilmenite sorting due to plagioclase buoyancy are thus the only mechanisms responsible for the formation of Fe-Ti deposits in Proterozoic massif-type anorthosites.