Granulite and amphibolite facies gneisses and migmatites of the Levack Gneiss Complex occupy a zone up to 8 km wide around the northern part of the Sudbury Igneous Complex (SIC). Orthopyroxene- and garnet-bearing tonalitic and semipelitic assemblages of granulite facies grade occur within 3 km of the SIC together with lenses of mafic and pyroxenitic rock compositions normally represented by an amphibole +/- cpx-rich assemblage; amphibolite facies assemblages dominate elsewhere in this terrain. These 2.711-Ga gneisses were introduced by (1) the Cartier Granite Batholith during late Archaean to early Proterozoic time and (2) the SIC, at 1.85 Ga, which produced a contact aureole 1-1.5 km wide in which pyroxene hornfelses are common within 200-300 m of the contact. A suite of 12 samples including both the opx-gt and amphibole-rich rock compositions have been studied. Garnets in the semipelitic gneisses are variably replaced by a plg-bio assemblage. Thermobarometric calculations using a variety of barometers and thermometers reported in the literature suggest that the granulite facies assemblages formed at depths in the 21-28 km range (6-8 kbar). Textures and mineral chemistry in the garnet-bearing semipelitic rocks indicate that this terrain underwent a second metamorphic event during uplift to depth in the 5-11 km range (2-3 kbar) and at temperatures as low as 500-550 C. This latter event is distinct from thermal recrystallization caused by the emplacement of the SIC; it probably represents metamorphism attributable to intrusion of the Cartier Granite Batholith. These data allow two interpretations for the crustal uplift of the Levack Gneisses: (1) The gneisses were tectonically uplifted prior to the Sudbury Event (due to intrusion of the Cartier Batholith); or (2) the gneisses were raised to epizonal levels as a result of meteorite impact at 1.85 Ga.
The Ni-Cu-platinum-group element ores in the sublayer and footwall breccia of the Sudbury structure have distinctive geochemical halos. In the Fraser mine, F/Cl ratios of the sublayer rocks appear to decrease, Na2O/K2O ratios of the footwall breccia samples decrease, and Cl in both units increases with proximity to mineralization. These halos are interpreted to result from interaction of residual magmatic fluids with host rocks, which altered the host-rock mineralogy and precipitated Cl-bearing minerals. The results of this study, together with the results of previous studies, suggest that most ores at Sudbury, including sublayer- and footwall breccia-hosted contact ores and Sudbury breccia-hosted footwall ores, are surrounded by Cl and possibly alkali halos and that these geochemical signatures may be used to identify rocks that are proximal to mineralization.
Research Article| November 01, 2002 The East Bull Lake Intrusive Suite: Remnants of a ~2.48 Ga Large Igneous and Metallogenic Province in the Sudbury Area of the Canadian Shield R. S. James; R. S. James Department of Earth Sciences and Mineral Exploration Research Center, Laurentian University, Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada †Corresponding author: e-mail, rjames@nickel.laurentian.ca Search for other works by this author on: GSW Google Scholar R. M. Easton; R. M. Easton Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada Search for other works by this author on: GSW Google Scholar D. C. Peck; D. C. Peck Anglo American Exploration (Canada) Ltd., Suite 800-700 West Pender Street, Vancouver, British Columbia V6C 1G8, Canada Search for other works by this author on: GSW Google Scholar J. L. Hrominchuk J. L. Hrominchuk Department of Earth Sciences and Mineral Exploration Research Center, Laurentian University, Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information R. S. James Department of Earth Sciences and Mineral Exploration Research Center, Laurentian University, Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada R. M. Easton Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada D. C. Peck Anglo American Exploration (Canada) Ltd., Suite 800-700 West Pender Street, Vancouver, British Columbia V6C 1G8, Canada J. L. Hrominchuk Department of Earth Sciences and Mineral Exploration Research Center, Laurentian University, Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada †Corresponding author: e-mail, rjames@nickel.laurentian.ca Publisher: Society of Economic Geologists First Online: 02 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 Economic Geology Economic Geology (2002) 97 (7): 1577–1606. https://doi.org/10.2113/gsecongeo.97.7.1577 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation R. S. James, R. M. Easton, D. C. Peck, J. L. Hrominchuk; The East Bull Lake Intrusive Suite: Remnants of a ~2.48 Ga Large Igneous and Metallogenic Province in the Sudbury Area of the Canadian Shield. Economic Geology 2002;; 97 (7): 1577–1606. doi: https://doi.org/10.2113/gsecongeo.97.7.1577 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The 2.49 to 2.475 Ga intrusions of the East Bull Lake intrusive suite occur in an east-northeast-trending discontinuous belt along the presently exposed boundary between the Archean Superior and the Proterozoic Southern provinces of the Canadian Shield near Sudbury, Ontario. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic magmatic event, extending from 2.49 to 2.44 Ga, which also includes bimodal volcanic rocks, felsic plutons, and the regionally extensive Hearst and Matachewan dike swarms. This regional magmatic event is thought to be the result of a mantle plume-driven, intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the Huronian Supergroup. The East Bull Lake intrusive suite appears to have been emplaced along a major axial-rift fault related to this rifting event.Compelling field and geochemical evidence summarized here indicates that the three largest East Bull Lake suite intrusions, the East Bull Lake, Agnew Lake, and River Valley intrusions, crystallized from similar, low Ti, high Al tholeiitic parent magmas that originated in deeper, more basic chambers. It is proposed that the primary magmas for the East Bull Lake suite intrusions were second-stage melts derived from a sublithospheric depleted mantle source that had been modified by Neoarchean subduction and accretion events.Most of the petrological characteristics of the East Bull Lake suite intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe enrichment trend in the residual magmas. Orthopyroxene is the high-temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite. Local olivine-rich cumulates, present in all of the major East Bull Lake suite plutons, in some instances appear to be comagmatic with associated plagioclase-rich cumulates. A distinctive marginal facies, comprising brecciated and locally thermally recrystallized and/or partially melted footwall rocks, is a common feature of the East Bull Lake suite plutons and indicates that a high-energy flow regime existed during the initial stages of emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite that contains erratically distributed mafic autoliths and footwall-derived xenoliths. Chemical contamination of the mineralized matrix of these inclusion- bearing rocks can be extreme at a local scale. The inclusion-rich zone is overlain by a thick interval of undifferentiated, plagioclase-rich cumulates that locally display spectacular glomerocrystic textures. Rhythmically and irregularly, modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy. Massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the Agnew Lake intrusion. Varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy.The East Bull Lake suite is currently being explored for contact-type platinum-group element (PGE)-Cu-Ni mineralization that has a clear spatial association with orthopyroxene-rich cumulates which are otherwise poorly represented in these intrusions. Disseminated PGE- and Cu-rich sulfide mineralization is concentrated within the inclusion-bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense, mafic residual liquid. The resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma. Feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background PGE contents compared to those for other known magmatic PGE deposits; however, the economic potential of the contact-type mineralization hinges on the efficacy of the physical concentration process, which upgraded the metal content and size of the original sulfide particles inside the magma chambers. Based on the proposed basal accumulation model, the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Zones of mylonites are a prominent feature of rocks that occur at the Grenville Front in central Ontario. They represent periods of northwest-directed thrusting during the Mesoproterozoic, the latest period due to the Grenville Orogeny at ∼1.0 Ga. We describe the strain and folding of a 300 m wide zone of mylonites southeast of Coniston, Ontario that occur in the foreland to the Grenville Province. They are greenschist facies quartz mylonites and quartz feldspar mylonites formed in the upper crust from sandstones and granites in the Southern Province. They are bounded to the north and south by the Murray Fault and Grenville Front Boundary Fault, respectively, steeply dipping reverse faults, which are post-mylonitization in age. The latter fault marks the southern boundary of the foreland mylonites with zones of mylonite in high grade, lower crustal schists, and gneisses of the Grenville Province. The Murray Fault marks their northwestern boundary against a middle Proterozoic metamorphic terrane. Shape fabric measurements of quartz indicate that strain intensity is random across the mylonite zone. Open angle quartz c-axis fabrics, the dominance of subgrain rotation recrystallization of quartz, and the metamorphic paragenesis quartz–muscovite–chlorite indicate that the mylonites formed at low greenschist facies temperature (350–430 °C). The measured variation in strain is attributed to variations in strain rate and water weakening. The mylonites are folded; hinge lines and axial planar stretching lineations plunge southeast perpendicular to the Grenville Front. The mylonites are similar to those that occur in the foreland of the Moine Thrust (northwest Scotland) and the Munsiari Thrust (northwest India).
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