13e, Abstracts

Geochemistry of dissolved noble and atmospheric gases in four diverse plutonic environments in the Precambrian Shield of Canada, J. D. Ross , Applied Geoscience Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Rcsearch Establ ishment , Pinawa, Manitoba ROE 1L0, Canada.

HVDROC;EOCnEMICAL and hydrogeological investigations in the Canadian Nuclear Fuel Waste Management Program have progressed, over a period of eight years, from equip- mcnt and methodology development to extensive, routine, ot~-sitc monitor ing and analysis of pumping tests from iso- lated borehole intervals to depths of over 1000 m.

During this period, over 70 samples for dissolved radiogenic and noble gases have been collected from four research areas across the Canadian Shield encompassing lhrce major plutonic rock types: granite-granodiori te , gabbro-anor thos i te , and gneiss-metagabbro .

Helium concentrat ions as high as 0.36 cm)/g H e n STP have been encountered, indicating the high subsurface- crustal production rate ofaHe, and the extensive rock-water interaction at depth. The radiogenic aHe and 3He/aHe ratio have clear signatures in each of the research arcas, but the deduced ratio of subsurface production of ~He and 4He is remarkably constant across the Canadian Shield and with depth. There appears to be no mantle derived component in the He, but there is evidence of some tritiagenic production of 3He in a few of the shallower waters.

Excess radiogenic ~'I'e:Ne is clearly indicated and corre- lates with the highest He conccntrations. Excess Ne cannot always be used as an indicator of air contaminat ion in noble gas samples.

In contrast with results from other investigators in Cana- dian Shield mine sampling, there is indisputable evidence of CH4 or higher hydrocarbons.

Uranium-series disequilibrium as a criterion for stability of radwaste sites. H. P. SChWARtZ, Depar tment of Geology, McMaster University, Hamil ton, Ontario L8S 4M1, Canada.

CRVSTXLt,INE rocks proposed as sites for radwaste disposal are commonly intersected by fractures along which fluids have moved and both altered the rock and precipitated filling minerals. If we wish to know the stability of these rocks with respect to fluid transport through them, it is important to know the timing of the more recent a l tera t ion- precipitation events. Over a time scale of 10 a to 1 Ma, we can use disequilibrium between daughters of U-isotopes to establish this timing. Neutral to acid groundwater will leach U from rock, and cause the 2>Th/234U activity ratio to deviate from unity: it recovers with a halfqife of 75 000 a. A similar effect is seen in the >~U/23sU ratio. Shorter-lived disturbances can be seen in deficiency/excess of 2-'~Ra (half- life = 1600 a). Other geochronometers for vein-filling or alteration minerals should also be considered, e.g. ESR analysis of quartz and calcite. Examples from test sites will be discussed.

U and its daughter products; (2) access of groundwater to these sites: (3) the volume of circulating water: and (4) the chemistry of the groundwater .

Studies of granitic samples from peralkaline complexes in the Arabian Shield have shown that most samples lost less than 20% of their U during recent exposure to the near- surface environment . Most of the U in these samples appears to be firmly bound in zircons. In contrast, most surface and shallow drill-core samples of the granite of Lankin Dome (Granite Mountains , Wyoming) have lost 70% of their U. Most of the U in these samples is weakly bound in biotite and epidote-family minerals. The granite recovered during the Illinois Deep Hole Project (Stephen- son County, Illinois) is mineralogically similar to the granite of Lankin Dome, but this granite lost radiogenic Pb rather than U, probably as a result of exposure to grotmdwater that had a markedly different chemistry from that in the Granite Mountains.

Studies of the Sherman Granite (Wyoming) and the Gotemar Granite (southeastern Sweden) have shown that lJ and/or Pb mobility is greatest in and near fractured rock. The greater mobility is interpreted to be the result of both a larger water-rock ratio in the fractured rock and exposure to water over an increased surface area (and consequently a greater number of uranium sites).

Several types of geochemical and mineralogical data can be used to identify rock-water interaction in granites: how- ever. if rock samples have favourable radiogenic to common Pb ratios, both the amount and approximate timing of U or Pb mobility can be obtained through the use of isotopic studies. Such information can be extremely important in the search for favourable hosts for containment of radioactive waste. Rocks such as the Gotemar Granite have undergone considerable rock-water interaction, most of which occurred approximately 400 Ma and little in recent times. Thus a search for zones that have experienced only a little interaction with water may provide a misleading prediction as to the ability of such zones to shield radioactive wastes from the modern biosphere.

From an isotopic point of view, an ideal candidate for evaluation as a host rock for radioactive wastes would have the following characteristics: (1) a high ratio (>2) of radiogenic to common Pb in order to optimize precision of the results: (2) a simple two-stage geologic history so that results could be interpreted without multiple working hypothesis: and (3) an originally high percentage (>50%) of labile U so that the results would be highly sensitive to even small amounts of rock-water interaction. These characteris- tics should produce rocks with marked radioactive dis- equilibrium in surface samples. The disequilibrium should grade to radioactive equilibrium with increasing depth until zones in which water has not circulated are found. Extensive regions of such zones must exist because U - T h - P b systema- tics of most analysed granitoids demonstra te closed-system behaviour for almost all of their history, except for their recent history in the near-surface environmcnt . [Abstract taken from Chem. Geol. 5~." 21.-~.5~ "~'~ (1986).]

Applications of U - T h - P b isotope systematics to the problems of radioactive waste disposal. J. S. STUCKLESS, U.S. Geo- logical Survey, Box 25046, Denver Federal Center , Mail Stop 963. Denver , CO 80225, U.S .A.

CONCENTRATIONS of U, Th and Pb, and the isotopic compos- ilion of Pb for whole-rock samples of granitoids show: (1) that open-system behaviour is nearly universal in the surface and near-surface envi ronments : and (2) that elemental mobility is possible to depths of several hundred metres. Several identified, or at least postulated, factors that control U and/or Pb mobility include: ( l ) the mineralogical sites for

Evidence for 14C uptake on fissure-filling calcites: a pilot study. EvA-LENA TULLBORG, Swedish Geological Company, Pusterviksgaten 2, S-403 01 Goteborg, Sweden.

THE UPTAKE of 14C on fissure-filling calcites from ground- water will seriously affect t4C datings, but can possibly be used as a "tracer" of water pathways. Accelerator mass- spectrometry has been used for HC analyses of four calcite samples. These were sampled from water-conducting fissures at depths ranging from about 90 to 39(11 m. The results show a decrease in ~4C content vs depth.