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

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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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Moreover there was an interesting point about the metamorphism of the volcanic system: the intensity of metamorphism was steadily decreasing from the bottom to the top (very weakly metamorphic pillows) . I suggested an eventual direct relationsnip between the depth of the crustal segment and a kind of late-magmatic auto alteration (deuteric alteration). These are the reasons why I remain personally interested in the results of dredging operations, close to any mid-oceanic ridge. If there were some metamorphic rocks already there, they may well be attributed to some process of alteration of that kind. As soon as you are occupied by such questions, generally many other interesting problems arise. Studying recently, in collaboration with the Bedford Canadian Oceanic Institute, a set of dredged specimens, I momentarily shift toward another topic of interest: the origin of the dredged serpentinized ultrabic fragments. Among many interesting and mysterious facts, we found evidence of an intense pre-serpentinization cataclasis, probably due to a creeping process at the base of the crust, and presumably related to the oceanic floor spreading. Many metamorphic specimens from N. lat. on the Mid Atlantic Ridge are still waiting for an investigation; we may say already that they uniformly belong to the green schist facies only. It is fascinating to meet, near Thunder Bay, in the heart of the Canadian Shield, greenstone belts showing perfectly preserved relics of submarine basic lavas, exactly similar to the one formed very recently. It is extremely promising to compare their detailed petrological properties, with those recent equivalent rocks, because we may suppose some kind of difference between now and 3 billion years ago in the superficial environment and in the relations between continents, oceanic crust and mantle. In this comparison, as a petrologist, I foresee the possibility of detecting an evolution of the earth ' s crust, evolution about which we do not even have a serious hypothetical model yet. The geology school at Lakehead has therefore, a wonderful field of investigations for a long and sucessful future o BONGARD LESLIE GO. LTD. 204 ARTHUR ST. THUNDER BAY (P), ONT. STOCKBROKERS BOND DEALERS MUTUAL FUNDS D. F. BARON MGR. W. N. AUBRY S. MALINOSKI J. M. SCOTT TELEPHONE 344-6618

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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