A Natural Analogue for Storage of Radwaste in Crystalline Rocks
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A NATURAL ANALOGUE FOR STORAGE OF RADWASTE IN CRYSTALLINE ROCKS
DOUGLAS G. BROOKINS, MARK S. ABASHIAN Dept. of Geology, Univ. N.M., Albuquerque, NM 87131 LEWIS H. COHEN Univ. Calif., Riverside, CA 92502 HAROLD A. WOLLENBERG, JR. Lawrence Berkeley Laboratories, Berkeley, CA 94720
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 O-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 ang permeability; this helped prevent elemental migration during the 10 - 10 years of stock crystallization. The petrographic and geochemical studies show that the Idaho Springs (or equivalent) metamorhpic rocks are well suited for radwaste storage. INTRODUCTION The Bryan-Eldora Stock, a composite body of quartz monzonite, syenodiorite, and granodiorite, intrudes crystalline metavolcanic and metasedimentary rocks of the Idaho Springs Formation. The stock has been modeled by Hart and co-workers (1) as a 26,000 ft. by 10,000 ft. 'brick' intruded as magma at a temperature of approximately 780 C at 55 MYBP (Millions of Years Before Present). Previous mineralogic and chemical studies indicate that the present surface exposures are only 3,000 ft. below the original depth of emplacement. The intruded rocks of the Idaho Springs Formation have been dated at 1,600 to 1,400 MYBP with secretion pegmatites formed between 1,200 and 1,400 MYBP. A strong regional metamorphic event affected the Idaho Springs Formation at about 1,350 to 1,400 MYBP but the rocks have been unaffected until the intrusion of the Bryan-Eldora Stock. The classic work of Hart (2) has shown that variations in mineralogy, chemistry and isotopic systematics of the Idaho Springs Formation can be explained by fairly simple heat flow models. The current interest in the contact metamorphosed zone of the Idaho Springs Formation lies in the use of the Bryan-Eldora Stock as a high temperature magma rich in volatiles which could possibly have pronounced effects in producing hydrothermal fluids which, in t
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