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In mineral deposits the application of the Re-Os system has evolved on two fronts; as a geochronometer in molybdenite, and as a tracer of the source of metals by direct determination of the source of Os contained in the ore minerals. Results obtained from a wide variety and types of mineral deposits indicate that ore minerals in most deposits contain a high initial osmium composition, compared to the mantle value at the time of ore formation. The Re-Os data presented here for the Platreef, South Africa, adds to the growing notion that the crust plays a fundamental role in the formation of mineral deposits and as a source of ore minerals. Additional data from the Zambian Copperbelt illustrate the utility of the Re-Os system as a geochronometer of sulfide mineralization. Two isochron ages of ca. 825 Ma and 575 Ma are consistent with a long-lived period of multistage mineralization linked to basin evolution and support a model where brines play a fundamental role in the formation of sediment-hosted stratiform deposits.Numerous new Re-Os molybdenite ages have recently been reported; however, the behavior of Re and Os in molybdenites is still poorly understood and controversy remains regarding the possible disturbance of the Re-Os isotopic system. Previous studies indicate that the Re-Os system in molybdenites, and in other sulfides, can experience disturbance by Re and Os loss or Re gain (both examples of open system behavior), and that the analysis of these altered samples yields equivocal ages. Through replicate analyses of samples and/or comparison with other robust dating techniques, such as the U-Pb geochronometer, it is possible to differentiate between Re-Os molybdenite ages reflecting a mineralization age or a post depositional event. Once the reliability of the Re-Os molybdenite analyses is proven, it is possible to constrain the timing of mineralization and the identification of multiple molybdenite mineralization events, information that is relevant in assessing the longevity of porphyry systems.The examples presented in this work support the use of the Re-Os isotopic system as an important geochemical tool in the understanding of mineral deposits.

The general goal of this thesis was to examine and quantify the role of rooted submerged aquatic macrophytes in the cycling of metals in the littoral zone. An empirical study using data from an extensive literature survey showed that aquatic plants do not differ markedly in element composition from the sediments in which they grow, and that during uptake the plant's roots do not discriminate between elements essential and not essential for growth. The organic content, pH and redox potential of the sediments were shown to affect the bioavailability of metals to the plants. Biomass density was used to categorize species to assess broad patterns in plant-sediment metal relationships. It was found that the presence/absence of an understory contributed variation to the plant-sediment metal relationships. The role of rooted macrophytes as vectors for various elements between the sediments and attached epiphyte was quantified by growing Myriophyllum spicatum in radio-labelled sediments. This species was found to be a link between the sediments and attached epiphytes for the 4 elements studied (Co, Cs, Eu and Mn). Finally, the export of dissolved metals from weedbeds during plant senescence was quantified using a mass balance approach. It was found that approximately 15% to 30% (depending on the metal) of the metal inventory of the plants at maximum seasonal biomass was lost as dissolved compounds during senescence. During the growing season the macrophyte beds were net sources of metals to the lake, but were net sinks annually.

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