A feature that makes the Re-Os isotope system uniquely suitable for addressing ore deposit research is that Re and Os are strongly chalcophile, and are therefore concentrated in sulfides as compared to silicates (e.g. Morgan and Lovering, 1967; Allègre and Luck, 1980). The fact that Re and Os are concentrated in sulfides allows for two important possibilities for research: (1) innovative geochronological work on sulfides aimed at better understanding the duration of ore-forming events, and
(2) tracing the source of ore-forming components by comparing the initial 187Os/188Os ratios of sulfides with those of the probable sources. At present, the most important mineral in Re-Os geochronology is molybdenite, which was recognized early to have potential because of its enrichment in Re and near absence of initial Os. Some of the early molybdenite Re-Os ages reported give fairly reasonable ages using today's better known decay constant for 187Re. However, these studies also showed that there were some problems with this mineral for dating (e.g. Luck and Allègre, 1982). McCandless et al. (1993) were
the first to recognise that problems with closure of molybdenite were due to microinclusions in the mineral that could not be recognised by conventional microscopic techniques. The open system behaviour of molybdenite was also demonstrated by the fact that the mineral could lose Re even in low temperature (e.g. supergene) environments. As for the closure temperature of molybdenite for Re-Os, this is difficult to evaluate because it is a function of the diffusional behaviour of the elements in the presence of a significant fluid phase. In addition, the Re-Os system may be disturbed by alteration phenomena. Mobilization
of Re and Os is demonstrated in ores in mafic-ultramafic igneous complexes, where magmatic processes dominate but
remobilization of Os by hydrothermal processes is recognised. Although there are these complications, the strength of Re-Os is evident in the study of base metal porphyry deposits. These type of deposit is directly related to subduction processes and the Re-Os isotope composition of base metal sulphides document that a significant proportion of the Os in these systems is from the crust even though it has been thought that the mantle is the most important source for the ore-forming metals.
Allègre, C.J. & Luck, J-M., Earth Planet. Sci. Lett. 48, 148-154 (1980).
Luck, J-M. & Allègre, C.J., Earth Planet. Sci. Lett. 61, 291-296 (1982).
McCandless, T.E., Ruiz, J. & Campbell, A.R., Geochimica et Cosmochimica Acta 57, 889-905 (1993).