Retrograde alteration effects on rutile ore

Retrograde alteration (amphibolitisation) of eclogite may destroy a good rutile ore quality by altering rutile to ilmenite and in some cases also to titanite. Retrogression may be distinct particularly along the margin of the eclogite body and along shear zones (at all scales). Small-scale retrograde alteration is most commonly seen as dark veins; these are believed to have formed by fracturing of the eclogite during amphibolite facies metamorphic condition during uplift. In this process water-rich fluids had access along the fractures and reacted with the primary eclogite minerals to form alteration minerals, as illustrated below.

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Mosaic microphoto of thin section E211/106.9 (transmitted light, long side of photo is 31 mm) showing fractures with distinct retrograde alteration (amphibolitisation). At the beginning stage of the retrograde alteration omphacite is altered to very fine symplectitic intergrowths of diopside and albite; this is visible in the microphoto as dark cloudy areas particularly at the outer zone of the retrograde fractures (veins). As the alteration continues the symplectites transforms into a mixture of mainly amphibole (hornblende) and albite. The primary eclogite facies amphibole (barroisite or actinolite) and garnet is more resistant to alteration than omphacite, but finally also transforms into hornblende. In this process rutile alter to ilmenite.

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SEM back-scattered electron (BSE) images showing detailed mineral relationships. Further explanation is given on the photos.

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Section from the previous picture.

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Image showing silicate relationships.

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Variation in magnetic susceptibility along borehole 4 at Engebøfjellet. Retrograded parts of the cores, i.e. where there is a significant proportion of retrograded fractures/zones, tend to be distinctly magnetic due to secondary magnetite.

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