A Natural Analogue for Storage of Radwaste in Crystalline Rocks

Abstract

The Bryan-Eldora stock (Colorado) intruded the Precambrian Idaho Springs Formation metamorphic rocks 58 million years ago. Geochronologic-geochemical work by Hart et al. (1) has demonstrated that the heat from the cooling intrusive rocks was sufficient to affect mineral isotopic systematics up to 2,000 m. from the contact, and the nature of these isotopic perturbations can be explained by a simple diffusion model in turn based on various heat flow models. Our new studies are focused on elemental exchange between stock and intruded rock as a function of distance from the contact; the assumption is made that the stock is a very large, high heat source analogous to a waste form emplaced in the metamorphic rocks without benefit of canister or engineered backfill. Data for U, Th and the REE indicate actinide and lanthanide immobility except perhaps in the 0–2m. contact zone where some infiltration of the country rocks by stock-derived fluids occurred. Beyond 4 m. no stock-derived U, Th, REE or *Pb are noted. Further, whole rock Rb-Sr and stable 0 isotopic data indicate conductive cooling as opposed to convective, water-induced cooling. The intruded rocks possess low porosity and permeability; this helped prevent elemental migration during the 10⁵ − 10⁶ years of stock crystallization. The petrographic and geochemical studies show that the Idaho Springs (or equivalent) metamorhpic rocks are well suited for radwaste storage.