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Document Type:Latin Dissertation
Language of Document:English
Record Number:53125
Doc. No:TL23079
Call number:‭NR45571‬
Main Entry:Ryan Michael Morelli
Title & Author:Rhenium-osmium geochronology of low-level sulfide minerals from hydrothermal ore deposits---Applications, limitations, and implicationsRyan Michael Morelli
College:University of Alberta (Canada)
Date:2008
Degree:Ph.D.
student score:2008
Page No:225
Abstract:Rhenium-osmium geochronology of common sulfide minerals is a technique that offers a theoretical means of directly dating many hydrothermal ore deposits in Earth's crust. In practice, however, the successful application of this technique has historically been impeded by major analytical challenges. This PhD thesis utilizes recent analytical developments that, for the first time, permit precise and accurate isotopic measurements of the low concentrations of rhenium and osmium typical of most crustal sulfide minerals. The objective of this study is to further develop this technique for mainstream application, and to assess the strengths and limitations of the technique with different sulfide minerals. To accomplish this, measurement of rhenium-osmium isotopic compositions have been performed for common sulfide minerals, including arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite, and gersdorffite, from multiple gold and base metal deposits in worldclass ore districts globally, and through geologic time. By comparison of determined rhenium-osmium results to existing geochronologic constraints for the deposits, and considering the thermal histories of the host terranes, a more comprehensive understanding of the utility of this technique has been achieved. The results of this study indicate that, using proper sampling, analytical, and data reduction protocols, both arsenopyrite and pyrite are very reliable chronometers of ore-forming hydrothermal events. Demonstrably accurate and relatively precise (better than 1.5%) rhenium-osmium mineralization ages have been determined for important orogenic gold deposits using arsenopyrite (Homestake, South Dakota; Muruntau, Uzbekistan) and for orogenic gold (Con, Northwest Territories) and volcanogenic massive sulfide (Trout Lake and Sherritt Gordon, Manitoba) deposits using pyrite. For these minerals, apparent minimum closure temperatures of 400°C for arsenopyrite and possibly as high as 675°C for pyrite can be inferred. Both the rhenium-osmium mineralization ages and initial Os isotopic compositions of these sulfides can be used to provide crucial petrogenetic information regarding ore genesis. In contrast, spurious rhenium-osmium results from coexisting pyrrhotite and sphalerite demonstrate that these minerals are readily disturbed during low temperature, post-crystallization thermal events, and are poor choices for rhenium-osmium geochronology. Results for chalcopyrite from the Konuto Lake volcanogenic massive sulfide deposit, Saskatchewan, are less conclusive, but suggest that chalcopyrite rhenium-osmium isotopic compositions may be reset at metamorphic temperatures of less than μ∼500°C.
Subject:Earth sciences; Rhenium-osmium geochronology; Sulfides; Hydrothermal ores; Ore deposits; Geology; Geochemistry; 0996:Geochemistry; 0372:Geology
Added Entry:University of Alberta (Canada)