Metamorphic Magnetite Research Articles

Magnetic Properties and Magnetic Signatures of BIFS of the Hamersley Basin and Yilgarn Block, Western Australia

Magnetite-rich banded iron-formations (BIFs) exhibit characteristic magnetic properties, including strong anisotropy. Interpretation of the geological structure of BIF units from the associated magnetic anomalies is complicated by anisotropy of susceptibility and, frequently, by remanent magnetisation. The timing of remanence acquisition relative to folding exerts a crucial influence on the form of the anomalies. Neglect of susceptibility anisotropy and remanence can lead to large errors in interpreted dips and thicknesses of BIF units. The effective susceptibility of BIFs parallel to bedding exceeds the susceptibility normal to bedding, typically by a factor of 2 to 4. Bedding-parallel susceptibilities of magnetite-rich BIFs are typically 0.5 to 2.0 SI (0.05–0.16 G/Oe). Remanence directions in BIFs usually lie close to the bedding plane. Koenigsberger ratios (Q) of BIFs vary widely, but characteristic values can often be determined for individual units. Q values in the range 1 to 2 are common. The magnetisations of the haematite-rich supergene-enrichment iron ores are much lower than those of their BIF precursors.Magnetic properties of outcropping BIFs are usually greatly modified by weathering, which substantially decreases the bulk susceptibility, the degree of anisotropy and the remanence intensity. Deeper and more intense weathering of BIFs is encouraged by faulting and can be associated with reduced magnetic response over intensely faulted zones.The remanence of BIFs from the Hamersley Basin is carried by late diagenetic to low-grade metamorphic magnetite after primary haematite. At Wittenoom and Paraburdoo, the remanent magnetisation of Brockman Formation BIFs is pre-folding, whereas in an area of higher metamorphic grade, the Turner Syncline, the remanence of the BIFs is probably post-folding. Aeromagnetic signatures over the Turner Syncline clearly reflect anisotropy and remanence.Anisotropy and remanence effects are also evident in observed magnetic signatures over Archaean BIFs in the Yilgarn Block. Magnetic property measurements on samples from the Mount Magnet area and elsewhere confirm the high anisotropy and strong remanent magnetisations of these rocks.

U-Pb Dating of Magnetite from the Kinsman Quartz Monzonite, New Hampshire, USA: Implications for Dating Natural Remnant Magnetization

We have studied the U P b systematics of magnetites separated from the Acadian Kinsman Quartz Monzonite (KQM) in the northern Appalachians, USA. The main objective of the present study was to investigate the U P b geochronology of magnetite and to evaluate its potential for constraining the timing of magnetic acquisition in rocks. This may prove to be helpful in the reconstruction of apparent polar wander paths (APWP) for use in plate tectonic reconstruction (e.g., van der Voo, 1993) and may also shed light on the nature of remagnetization events. A few attempts have been made recently to date metamorphic and ultramafic magnetite directly using the 4~ method (e.g., (>zdemir and York, 1990; 1992) and Pb isotopes (Garitpy et al., 1990). Previous studies have identified two major episodes of plutonism during the Acadian Orogeny in New Hampshire. The KQM belongs to the earliest phase of the plutonism and has been dated at ~,410 Ma (Lyons and Livingston, 1977; Barreiro and Aleinkoff, 1985), whereas the second phase of plutonism (the Concord-type) took place at ~ 355 Ma (Harrison et al., 1987). It has been suggested that the KQM is the product of syntectonic anatexis of metasediments at midcrustal levels (Lathrop et al., 1994).

The distribution, petrology, and genesis of nickel ores at the Juan Complex, Kambalda, Western Australia

Massive and disseminated Fe-Ni-Cu ores in the Juan complex occur in two belts of contrasting Ni tenor that are interpreted as original sea-floor depressions in basalt flows into which separate sulfidic komatiitic magmas were extruded. Similarities in composition and coincidental areal distribution of massive and disseminated ores indicate that their emplacement was nearly simultaneous. These features, the distribution of metasomatic reaction zones at basalt-ultramafic contacts, and the presence of interpillow massive Fe-Ni sulfides in the underlying basalt, all point to the existence of abundant primary massive sulfides on top of the basalt.A magmatic genetic model is favored for the Juan ores in which massive sulfides were extruded and emplaced slightly in advance of viscous komatiitic magma rich in olivine phenocrysts, as dictated by contrasts in flow rates. The thick komatiitic flow came to rest upon massive sulfides that were partly crystalline, due to rapid marginal chilling. Residual sulfide droplets settled during flow at the base of the flow, thus forming disseminated ore.During ensuing prograde regional metamorphism, massive ore was thickened in some folds and mobilized into some faults and previously barren contacts, but overall redistribution was very limited. Metamorphism did not create new massive ore.During declining metamorphic temperatures, oxidation reactions and attendant sulfur and copper diffusion generated metamorphic magnetite along sulfide-silicate contacts, pyritic layers and lenses in massive ore, and Cu-rich sulfide stringers in wall rocks. On final cooling, sulfide ores were annealed and many tectonic surfaces were overprinted by static recrystallization of silicates and sulfides.