Mineral Deposits and Precambrian Geology

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Browsing Mineral Deposits and Precambrian Geology by Subject "anatexis,"

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    Assessing primary versus secondary features in two pegmatite swarms with implications for the nature of pegmatitic systems
    (2023-03-24) Pfister, Jérémie Damien
    Understanding the processes behind the formation and evolution of pegmatites, which host globally significant resources of strategic metals (e.g., Li, Ta, Nb, Sn, Cs, Rb, Y, REE) critical to the global renewable energy transition, is paramount for their exploration and exploitation. This study investigates the nature, origin and evolution of two distinct pegmatite swarms (i.e., the LCT-type Little Nahanni Pegmatite Group, NWT, Canada, and the NYF-type Highway 69 Pegmatite Group, Ontario, Canada) in order to address several unresolved aspects of pegmatite genesis, such as the origin of pegmatite-forming melts (i.e., anatexis versus fractional crystallization), nature of various pegmatitic textures (i.e., primary versus secondary), metasomatism and re-mobilization of rare-metal mineralization, and influence of the host rocks on pegmatites (i.e., open- versus closed systems). This study integrates various aspects of field observations that include detailed mapping, field and petrographic observations (transmitted light, BSE, SEM-CL), bulk whole-rock and trace-elements geochemistry (ICP-AES, ICP-MS), in situ mineral analysis (SEM-EDS), in situ trace-elements analysis (LA-ICP-MS), bulk and in situ stable isotopes (O, H) geochemistry (DI-IRMS, SIMS), and in situ U-Pb geochronology (LA-ICP-MS). The principal findings suggest that: (1) both anatexis and fractional crystallization of a parental melt are viable processes of pegmatite formation; (2) most macroscopic pegmatitic textures are primary and are generally preserved during sub-solidus evolution of pegmatites; (3) late-stage, fine-grained albitic and micaceous units associated with significant rare-metal mineralization are secondary; (4) secondary micro-features related to coupled dissolution-precipitation processes in pegmatites are profuse, despite being rarely reported, and are frequently associated with accessory rare-metal mineralization; and (5) pegmatites evolve as partially open systems during both magmatic and metasomatic stages with influence from the host rocks, and in some cases, external fluids.
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    Understanding the South Range Sudbury impact structure: a study of shear zones, impact breccias and PGE occurrences in the Vermilion and Crean Hill mines area, Sudbury, Canada
    (2022-12-15) Généreux, Carol-Anne
    The Sudbury impact structure formed when a bolide collided with the southern Superior craton margin ca. 1.85 Ga ago. During the impact, the target rocks were brecciated and melted, which resulted in the formation of a thick impact melt sheet, the Sudbury Igneous Complex (SIC), underlain by brecciated basement rocks. Nickel-copper-platinum group elements (Ni-Cu-PGE) deposits formed at the base of the SIC as contact deposits, in association with offset dikes that flowed from the melt sheet into the basement rocks (offset deposits), and as sulphide veins and pods of disseminated PGE mineralization that were emplaced into the brecciated basement rocks (footwall deposits). The deposits and their host rocks were subsequently modified by orogenic events that reworked the South Range of the impact structure. The thesis discusses the formation of syn-impact breccias below the SIC-basement contact, the structural and metamorphic modification of the impact structure during subsequent orogenic events, and the effect of syn- and post-impact processes on the formation and remobilization of (Ni-)Cu-PGE mineralization at the contact-type Crean Hill deposit and offset-type Vermilion deposit. The breccias formed by in situ shock melting of the footwall rocks during the impact, and were modified by contact metamorphism during cooling of the SIC and by syn-tectonic deformation and metamorphism after the impact. The latter resulted in the formation of regional folds and south-side-up-dextral shear zones during northwest-directed shortening of the impact structure. (Ni-)Cu-PGE mineralization at the Crean Hill and Vermilion deposits was emplaced as sulphide melts immediately after the impact and was remobilized by metamorphic semi-metal melt and hydrothermal fluids into the breccias and shear zones during and after regional deformation. These results illustrate how both syn- and post-impact processes contributed to the formation and modification of Ni-Cu-PGE deposits in Sudbury.