As with stable isotopes, researchers continue to ``push the sample size envelope'' for radiogenic isotopes. Brannon et al. [1992] and Nakai et al. [1993] used Rb/Sr isotopic data on sphalerites to date the age of mineralization of several Mississippi Valley-type Pb-Zn ore deposits; the technique is complicated by differential brine-mineral partitioning of Rb and Sr and the consequent need to remove inclusion fluids prior to analysis. Nonetheless, the diverse age determinations from different deposits suggest that the timing of Paleozoic orogenic activity and resulting regional brine migrations in North America, thought to be responsible for forming these types of ore deposits, may be more complicated than was previously assumed. Chelsey et al. [1994] used Sm-Nd isotopic data on fluorites to date the age of mineralization in the Illinois-Kentucky fluorite district; they obtained a Permian age identical to that obtained by Brannon et al. [1992] for Upper Mississippi Valley sphalerites, supporting a model for large-scale fluid movement from the Illinois basin related to the Alleghenian-Ouachita orogenies in North America.
Re-Os isotope systematics are proving useful in ore deposit studies because of the Os isotopic contrast between mantle and crust, the occurrence of Re in molybdenite, and the occurrence of Os in PGE deposits. Marcantonio et al. [1994] used Re-Os, Nd-Sm, Rb-Sr and O isotopic systematics to demonstrate that primary magmatic PGE mineralization in the Wellgreen intrusion, Yukon Territory, had been overprinted by post-crystallization hydrothermal processes which remobilized radiogenic crustal Re and Os from sedimentary wall-rock sources. They questioned earlier studies which had concluded that radiogenic or variable Os isotopic compositions in magmatic PGE deposits must reflect mantle heterogeneities or crustal assimilation, rather than hydrothermal remobilization from radiogenic crustal sources after magma emplacement. McCandless and Ruiz [1993] applied Re-Os isotopic systematics to determine the ages of molybdenite-bearing porphyry base metal deposits associated with the Laramide orogeny in Arizona. They used a variety of analytical techniques to identify molybdenites which had not been affected by post-crystallization remobilization. They found that in each deposit ore deposition consistently occurred in the late stages of magmatic activity. Two narrow episodes of mineralization were delineated: 74-70 million years ago (largely within older Precambrian basement) and 60-55 million years ago (largely within younger Precambrian basement). The synchroneity of this widespread mineralization implied that some type of fundamental crust-mantle interaction resulted in regional genesis of the metal-enriched magmas responsible for the deposits. Walker et al. [1994] applied Re-Os, Nd-Sm and Pb isotopic systematics to magmatic Cu-Ni-PGE sulfide ores and associated igneous rocks from three Permian Noril'sk-type deposits in Siberia. They found that the isotopic data required a hot-spot type asthenospheric mantle source for the primary igneous melts and PGE, with little or no crustal contribution for these elements.