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

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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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Dr. James M. Franklin, B.Sc. (Carleton); M.Sc. (Carleton); Ph.D. (Western) . Background Geologist for G.S.C. Assistant Professor, Lakehead University- Research Metallogeny of the Lake Superior Crustal Traverse, Shebandowan to Pickle Lake Origin of low temperature silver deposits, Thunder Bay area Stratigraphy of the Sibley Group, Thunder Bay District Metallogeny, Its Concepts and Uses Two concepts of metallogenesis are (l) the genesis of a single metal in a variety of geological environments, and (2) the examination of all mineral deposits within a geologically or geographic- ally defined region. The single metal concept does not facilitate documentation of variations in mode of occurrence with time, and may preclude comparison or integration of genetic ideas related to one metal with with those related to another. This concept does, however, allow for complete examination of the chemistry of concentration of a metal in all geological processes. For example, Gross (1965) in his study of iron deposits, is able to document the processes operative in concentrating iron in igneous (iron-titanium deposits associated with Grenville anorthosites), metamorphic (contact meta somatic deposits of Vancouver Island) and sedimentary (Algoma, Superior and Minette type deposits) bodies. Such a study contributes much to fundamental geochemistry, but may, in certain circumstances, be of less significance in deposit exploration. For example, in searching for copper, the exact nature of the chemical control on deposition of the metal is less important than the stage of development of a eugeocyncline or facies of cratonic cover sediments affiliated with copper deposition. Recognition of the appropriate lithofacies associated with a deposit is a fundamental factor in delineating new areas of exploration. The second metallogenetic concept involves examination of variations in mineral deposit type within a time-stratigraphic, litho stratigraphic or petrogenetic province. Basically an accurate inter- pretation of source and time of deposition of mineral deposits is integrated with a regional tectonic history including geosynclinal and post-orogenic evolution. Clearly, a prime difficulty in such a study is. selecting useful co-incident geographic and geologic limits. All lithological and structural variations in any time— unit should be included within the geographic bounds of the study. The area must have adequately outlined mineral deposit genesis, paleogeographic and tectonic reconstruction. Convenient geologic limits might be set by systemic boundaries and orogenic events. For example, the Aphebian era is defined at its initiation by the Kenoran orogeny, and at its end by the Penokean and Hudsonian orogeny. The Helikian era is defined by the latter orogenies at its inception, but the termination of dominant continental volcanism and sedimentation at its end. Together these eras include many conventional tectonic elements. The problem is to select an area in which the complete geosynclinal, mountain building, and continental deposition events are preserved. The Lake Superior and Central Labrador areas meet these requirements. A metallogenic scheme for the former region is outlined in Table 1. 19

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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