Lakehead University Geology - Journal Yearbook (Thunder Bay, Ontario Canada)

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Analysis of Material Balance in Segregated Bodies Existence of material balance is one of the most convincing evidences in favour of segregational origin of the differentiated features like veins or lenses of mineral concentrates bordered by characteristic aureoles . Analysis of material balance is therefore crucially important and it cannot be overemphasized that the methods employed for such analysis ought to be reliable as well as capable of revealing the true state of balance within the differentiated bodies. By use of spherical and triaxial ellipsoidal models of segregation the reliability of the methods commonly used for balance analysis are tested. It is demonstrated that none of them can yield correct results. The reason for this is that these methods do not take the true volume ratio of the concentrate and aureole into account. This volume ratio is an indispensable factor for correct balance analysis. Accordingly, modified procedures to evaluate the state of balance are suggested. Determination of the above mentioned volume ratio in natural specimens is extremely difficult and imposes severe restrictions on the scope of balance analysis. The state of balance across a hornblende vein surrounded by a feldspathic aureole has been deter- mined by the suggested method as well as by one of the existing methods for a comparative evaluation of their reliabilities. The results obtained by the suggested methods show that material balance exists, thus indicating the segregational origin of the vein. This agrees well with the conclusion derived from mineralogical and chemical evidences. But, as predicted from the model studies, the existing method shows a lack of material balance across the vein. GRADUATE TO FALCONBRIDGE Geologists, Geophysicists, Geochemists, Mineralogists Mining and Electronic Technologists FALCONBRIDGE NICKEL MINES LIMITED 23 TOMLINSON BLOCK SEVEN KING STREET EAST, sa n. Cumberland st. -thunder bay. ont. TORONTO 210, CANADA 22

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Dr. K. Chakraborty, M.Sc. (Jad»); Ph.D. (l»I«T. ) Assistant Professor, Lakehead University A Statistical Study of Crystal Contacts Across a Segregated Hornblende Vein in Amphibolite and its Implication The pattern of spatial distribution of crystals in rocks depends on the energies of crystal contacts and entropy of distribution. The stable equilibrium patterns possess minimum distri- butional free energy. For a linear unidimensional system consisting of equal numbers of A and B crystals of the same size, the distributional free energy can be expressed as F = i n P( u M + U BB - 2U ab ) + nU AB + nkT [p log (-gp) + (l - p) log(l - p)] where U = energy of A-B contact, etc., and p = probability of A having another A as neighbour. Thus, for given contact energies, the value of p corresponding to the minimum of F would determine the spatial distribution of crystals in the rocks. The energies associated with different types of crystal contacts in natural rocks are unknown. However, if p can be determined, it might be possible to decipher the relative energies of the crystal contacts. A possible way to determine p is to carefully evaluate the frequencies of different crystal contac ts in a given rock. Frequencies of contacts depend on the preferred crystal associations as well as on the modal percentage of the minerals and crystal sizes. By suitable statistical device (Markov Chain) the frequency of crystal contacts only due to preferred crystal association can be evaluated. Crystal association during crystallization of a rock is governed by other factors apart from contact energies. Hence evaluation of contact energies would be plausible where rearrangement of initial crystal association is apparent. An attempt has been made to evaluate relative energies of hornblende-hornblende, plagioclase- plagioclase and homblende-plagioclase contacts from a specimen of amphibolite (hornblende and plagioclase together make up more than 90 o by volume). The specimen contains a differentiated zone consisting of a hornblende vein bordered by a feldspathic aureole. It has been concluded elsewhere that the differentiation is later than the crystallization of the amphibolite. Frequencies of crystal contacts across the differentiated zone are analyzed statistically employing Markov Chain concept. It is observed that homblende-plagioclase contacts are minimum in the differentiated zone and gradually increase and assume maximum value away from it. The reverse is true for hornblende-hornblende and plagioclase-plagioclase contacts. Thus the distribution patterns of crystals in the amphibolite away from, adjacent to and within the differentiated zone are ordered, random and segregated respectively. This suggests that segregational pattern possesses minimum distributional free energy for this system which is possible if the mean energy of hornblende-hornblende and plagioclase-plagioclase contacts is less than the energy of homblende-plagioclase contact. 21

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Mr. R. Bennett. Honours Practical Geology, Heriot Watt College. Edinburgh, Scotland . Background : Chief Laboratory Technician, 1936-1967 The Grant Institute of Geology, University of Edinburgh. Researching many kinds of technical approaches to Ph. D. studies. Presently, Chief Laboratory Technician, Lakehead University. Thin Section Making with the Aid of Araldite Epoxy Resin. Araldite Epoxy (Resin 502, Hardener 956)has the ability to penetrate into the cracks and cleavage planes of minerals and it will bond efficiently the angular particles of semi-consoli- dated sedimentary rocks, as well as recent sediments and soil samples. The adhesive properties of the Resin are such that it can be used as a mounting media, where such thermo-plastics as Lakeside 70 cannot be used or are not advisable due to the re-arrangement or twisting of the surface to be mounted. IGNEOUS ROCKS The following are some of the ways in which Araldite Resin was used in making thin sections of serpentinites from the Atlantic Ridge of the Azores. These rocks were very fragile and had a coating of manganese over the whole or part of the sample. The Preparation for the thin section: Due to the fragmental nature of the sample, it was necessary to bond a part of it even before attempting to cut it. This is done by mixing Araldite Resin ( 10 parts of resin to 2 parts hardener, by volume) in a thin-walled pliable polythene jar. The SDecimens to be cut are placed in an oven set at 60 degree Centigrade until the surface is hot to the touch. Remove from the oven and immerse the portion of the rock to be sectioned in the resin. Return the polythene jar with the specimen to a vacuum oven and impregnate the sample. When the oven chamber starts to evacuate, the resin will start to froth. This frothing is controlled by periodical closing of the vacuum and slowly allowing the air to force the resin into the sample ' s surface. Repeat this procedure until it becomes apparent that there is sufficient penetratio n of the resin into the surface of the sample, so that when cold, it can be cut without breaking. The Cutting: Remove the sample from its container. A surrounding mass of excess resin is beneficial, as it will preserve the outside edges of the sample. Clamp the unimpregnated part in the cutting machine and cut off the required part. If the specimen is holding together, then cut a second slab for future work. Wash the cut-off part and examine it under the microscope. Ensure that the surface cracks have been impregnated and if not, dry the slab and return it to the oven to complete drying and warming of the surface for coating with a fresh mix of resin. If the slab is used, it may suffice to place it on the plate to warm for surface coating. Surface coat the slab with resin, impregnating it as before if it was on the hot plate or in the oven. Allow it to set, strip off any unwanted resin with a razor blade while the slab is still hot, to save unnecessary grinding. 23