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Chemical and Petrographic Characterization of Ilmenite and Magnetite in Oxide-Rich Cumulates of the Sokndal Region, Rogaland, Norway

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Chemical and Petrographic Characterization of Ilmenite and Magnetite in Oxide-Rich Cumulates of the Sokndal Region, Rogaland, Norway NGU-BULL 436, 2000 - PAGE 49 Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndal Region, Rogaland, Norway SUZANNE A. McENROE, PETER ROBINSON & PETER T. PANISH McEnroe, S. A., Robinson, P., & Panish, P. T. 2000: Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndal Region, Rogaland, Norway. Norges geologiske undersøkelse Bulletin 436, 49-56 Igneous ilmenite and magnetite in some oxide-rich cumulates of the Sokndal region show chemical features and microtextures that reflect the progress of magmatic evolution and subsolidus re-equilibration. These features also relate to the patterns of aeromagnetic anomalies in the region. The most primitive oxides studied occur in the Tellnes ilmenite norite. These are followed by oxides in the Bjerkreim-Sokndal layered intrusion, first in ilmenite norites and finally in magnetite-rich norites. The trend of magmatic oxide crystallization in Bjerkreim-Sokndal, as previously determined by Wilson et al. (1996), was from hematite-rich ilmenite with minor end-member magnetite to hematite- poor ilmenite with titanomagnetite, a trend indicating progressive diminution of Fe2O3. This trend was accompanied by decreasing MgO and Cr2O3, and by gradually increasing V2O3 to the point where magnetite became a primary magmatic precipitate: beyond this V2O3 also decreased. In primitive samples the ilmenite has abundant hematite exsolution lamellae, whereas the magnetite has end-member composition and is free or nearly free of lamellae. Highly evolved samples contain near end-member ilmenite without exsolution, and titanomagnetite with fine exsolution of ulvöspinel subsequently oxidized to ilmenite and/or oxidation-exsolution lamellae of ilmenite. MgO is most strongly concentrated in ilmenite; Cr2O3 and V2O3 are strongly concentrated in hematite exsolution lamellae, but even more strongly in coexisting magnetite. The most chemically pure ilmenite, with the exception of MgO, occurs in small abundance in the most evolved titanomagnetite-rich cumulates. Suzanne A. McEnroe & Peter Robinson, Geological Survey of Norway, N-7491 Trondheim, Norway. Peter T. Panish, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA. ilmenite-rich rocks. The stable remanence and high Q values Introduction found in hematite-ilmenite rocks could have consequences In 1995, NGU completed a detailed aeromagnetic survey of for the interpretation of long- and short-wavelength mag- the Sokndal region as part of a broad investigation into netic anomalies. ilmenite resources (Rønning 1995). The resulting aeromag- netic map showed a dramatic pattern of aeromagnetic highs and lows. McEnroe et al. (1996) demonstrated that the mag- Groups of samples netic lows are produced by rocks with a high ratio of natural Mineral analyses are presented from five parts of the region remanent magnetization (NRM) to susceptibility (X), com- (Fig. 1). The first group of samples is from the Tellnes ilmenite monly referred to as the Koenigsberger ratio (Q value) where norite orebody in a magnetic low dominated by the reversed Q = NRM/ X • F (F = present local magnetic field strength). remanent magnetization of the Åna-Sira anorthosite. The Rocks with high Q values include ilmenite-rich norites and remaining four groups are from the Bjerkreim-Sokndal lay- anorthosites with very fine-grained oxides, and the anoma- ered intrusion. Of these four, two groups are from more lies are dominated by strong remanent negative inclination. primitive ilmenite-bearing norites that form magnetic lows in Conversely, the extreme magnetic highs are produced by the lower part of the intrusion as exposed at Heskestad and rocks with a very low ratio of natural remanent magnetiza- Mydland, though well above the lowest levels exposed in the tion to susceptibility. Rocks with these low Q values are mag- Bjerkreim lobe. The remaining two groups are from the more netite-rich, including norites, mangerites and quartz manger- evolved magnetite-rich norites that form the magnetic highs ites, and give rise to induced magnetic anomalies. During the in the upper part of the layered intrusion at Mydland and at study of the rock-magnetic properties and paleomagnetism Bakka. of these rocks, a series of detailed electron-probe analyses was performed on the oxide minerals, with special attention General oxide compositions to analytical precision for major elements and the economi- cally significant trace elements Cr and V. These are summa- and magma evolution rized here and will be explored in greater detail elsewhere. A The derivation and evolution of the magma from which the detailed study is also in progress to understand the Tellnes ilmenite norite was formed has been much debated extremely strong and stable remanent magnetization in the and is still unsettled (Krause et al. 1985, Wilmart et al. 1989, NGU-BULL 436, 2000 - PAGE 50 SUZANNE A. McENROE, PETER ROBINSON & PETER T. PANISH Fig. 1. Simplified geological map from NGU report 96.059 (modified from Duchesne & Michot 1987), showing the sam- pling localities at Tellnes Mine, Heskestad, Mydland and Bak- ka. Duchesne 1999). On the other hand, the magmatic deriva- result is an appearance of progressive reduction in the tion and evolution of the Bjerkreim-Sokndal layered intrusion oxides. This was probably not produced by a change of exter- has been thoroughly documented (Wilson et al. 1996). Nev- nal environment, but by extraction of cumulates, driving the ertheless, the stages of relative magmatic evolution of all the magma toward more reducing compositions, even in this samples can be roughly worked out using such ratios as example where early ilmenite dominated over magnetite. MgO/ (MgO + FeO) in the silicates, and the relative contents This trend is not to be confused with apparent oxidation or of MgO, V2O3, and Cr2O3 in the oxides. These show that the reduction effects within individual oxide grains, that were oxide minerals of the Tellnes ilmenite norite are more primi- mainly the product of localized subsolidus exchange reac- tive than those sampled by us in the layered intrusion. Thus, tions. the Tellnes deposit cannot be correlated with late stages in The oxide compositions and petrographic observations evolution of the Bjerkreim-Sokndal intrusion, nor with the of this study illustrate very well the conclusions of Wilson et internal mangerite dike system in the Åna-Sira Anorthosite, al. (1996). The overall chemical results are illustrated by four as suggested by Krause et al. (1985). Detailed geochemistry tie lines in Fig. 2, meant to show our best estimate of the equi- on the Tellnes ilmenite norite and the associated mon- librium tie lines of the oxides in the triangle FeO (+MgO) - 1 zonorite to quartz mangerite dike (Wilmart et al. 1989), indi- Fe2O3- TiO2 under high-temperature conditions. These val- cates they cannot be co-magmatic and suggests the ilmenite ues were obtained qualitatively from typical analyses of norite could be derived from crystallization of the enclosing oxide grains, as much as possible including minor exsolution anorthosite or possibly the most primitive parts of the Bjer- lamellae, and will be followed up by more precise estimates kreim-Sokndal intrusion not covered in our sampling. More based on modal analysis of oxide grains. Duchesne (1999), by recent precise U-Pb geochronology (Schärer et al. 1996) indi- contrast, made bulk XRF analyses of high-purity oxide min- cates that the Tellnes ilmenite norite is about 10 m. y. eral separates which should give a good measure of the bulk younger than the other intrusions in the region, so that there composition of host plus lamellae. His ilmenite and magnet- are no exposed rocks available for study representing its ite compositions (his Table 4) from the Tellnes deposit com- source magma. Further extensive consideration of the origin pare favorably with ours, although the ilmenite actually plots of the oxide-rich deposits was given by Duchesne (1999). closer to our TE-41 from Heskestad than to our TE-3, the most Based on the broader study of the Bjerkreim-Sokndal lay- Cr-rich of our three samples from the Tellnes deposit. ered intrusion by Wilson et al. (1996), the Heskestad, Mydland The most oxygen-rich tie line (TE-3) in Fig. 2 is for the and Bakka locations of this study fall either in or slightly below their MCU-IV cyclic unit or in the upper transition from 1. A maddening and easily overlooked numerical feature of Fig. 2 is norite to mangerite and quartz mangerite. The magmatic that the distance of an analysis point from the ilmenite toward the evolution trend of the oxides of the layered intrusion (Wilson hematite end-member is an indirect and non-linear function of et al. 1996) involves early precipitation of hematite-rich hematite content. For example, a grain of composition hematite 50, Fe2+ Ti4+ Fe3+ O , plots at 0.33 mol. % Fe O . The ilmenite TE-3, ilmenite together with minor Ti-poor magnetite. These are 0.5 0.5 1 3 2 3 plotting at .077 mol. % Fe2O3, actually contains 14.5% of the hema- followed by a gradual decrease in hematite content of ilmen- tite end-member. This problem is eliminated by plotting on a less ite coupled with increasing Ti content of magnetite. The end traditional cation basis. SUZANNE A. McENROE, PETER ROBINSON & PETER T. PANISH NGU-BULL 436, 2000 - PAGE 51 quently oxidized to ilmenite. The explanations based on Fig. 2 are supported in detail, both by petrographic observations and by electron-probe analyses. Detailed oxide petrography Reflected- and transmitted-light microscopy were performed to understand better the magnetic phases and correlative magnetic properties, to establish if the magnetic carriers were in equilibrium with the mineral assemblage, and to determine the degree of alteration. In all samples the oxides are well equilibrated showing coarse anhedral grains. Dis- crete ilmenite grains are usually >1mm. Some oxides are rimmed by pyroxene, and many are in clusters with orthopy- roxene and/or clinopyroxene.
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