APPLICATIONS OF THE RE-OS ISOTOPIC SYSTEM IN THE STUDY OF MINERAL DEPOSITS: GEOCHRONOLOGY AND SOURCE OF METALS

摘要

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.