Industrial mineral quality of limestone; the effect of contact metemorphism on textural properties, brightness and geochemistry

Abstract

When carbonate are altered by contact metamorphism the industrial mineral quality of the rock can be affected. Based on two contrasting limnestone units from Oslo Rift, Norway, a study has been made of the impact that contact metamorphism has on the quality of limestone with respect to its textural properties, brightness and geochemistry. Samples with variable metamorphic imprints have been examinated and compared to their non-metamorphosed equivalents. The result indicate that, in the context of limestone utilisation for industrial mineral applications, contact metamorphism can have a favourable effect on some properties. It can reduce the content of organic matter and give the rock a lighter colour and increased brightness values, and the calcite can attain more even grain-shape and -boundaries and increased grain size. However, there are no indications from the present study that contact metamorphism improves the quality with respect to the occurrence and abundance of contaminants. These limestones show that the impurities can remain very fine-grained and partly intergrown with the calcite, implying that contact-metamorphosed limestone may be no simpler to purify than non-metamorphosed limestone. If the marble has also been metasomatically altered, which is not uncommon in contact-metamorphic environments, the rock is actually likely to be more contaminated, and of correspondingly poorer quality, than it was originally. Overall, the implications are that contact metamorphism need not improve the limestone quality sufficiently to produce an economic industrial mineral deposit. It may be required that the limestone was of very high purity prior to metamorphism and also has been unaffected by any accompanying metasomatism.