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

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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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Two uses of such a scheme might be found in mineral deposit exploration and research into early- crustal conditions. For example, the exploration geologist may not have been aware of the possibility of breccia-pipe porpbyrycopper deposits associated with Neohelikian rocks of the Lake Superior area. Examination of available maps indicates the presence of many crypto-volcanic features; more deposits of the Tribag type might be found at appropriate structural loci. Metallogeny might also be useful as an indicator of a spe cific tectonic stage. For example, if anomalous concentrations of molybdenum are found only in post-orogenic, high level salic intrusive rocks, then the presence of this metal in certain Archean granite may suggest that these granites formed much later than the predominant volcanic rocks, in a post-island arc, continental setting. We may thus investigate the possibility of two igneous events in the Archean which may have occurred at widely separated times. Metallogeny of Proterozoic Rocks in the Lake Superior Area Tectonic Time m.y. Stage 1000 1350 1650 Cratonic Sedimentary and Intrusive Multiple Stage Deposits Effusive Rock Rock Structures Syngenetic Deposits Source Bed External forces, for applied, formation of deposits coarse continental minor, alkalic Cratonic fault- a) Cu in alkali Cu a) tilting allows sediments, fine. complexes car- ing and tilting bodies . updlp migration, lamellar interflow bonatite. Major due to deep b) Cu-Ni in layered precipitation of sediments (after layered gabbroic fracturing. mafic bodies. Pb-Zn at ' in situ ' weather- bodies, diabase c) Cu in basalt. structural trap. ing of volcanics). sheets . d) Cu-Mo in breccia pipes (Tribag) 1 3 b) Mafic intrusive Flood basalt, minor sills and dykes rhyolite . possibly as gas- cause remobili- eous effusions. zation of Ag to structural loci formed due to contemporaneous cratonic fault- ing. c) Cu to interflow sediments . Continental red bed sedimentation none uplift, conse- Pb-Zn-Ba in red quent weather- ing beds granite deformation. granodiorite simple folding, pegmatites faulting. Meta- morphism in deepest parts of basin, minor anatexis. Protogeosyn- blackshale, iron clinal formation, minor basalt, limestone, greyvacke. deeper basin Iron sediment added (minor Cu in at same rate as gabbro) basinal subsi- dence. Ag in shale 2100 Protobasin orthoquartzite, cgl, greyvacke minor gabbro intermontain (Uranium, Elliot basins, shallow Lake) - rapid weathering transport in streams 2500 NOTE: Brackets indicate deposits not in region of this study. 1 - Armbrust, 1969 - James et al. , 1968 3 - Roscoe, 1969 20

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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