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Paleohydrological inferences from fracture calcite analyses: an example from the Stripa Project, Sweden
Applied Geochemistry, Vol. 2~ pp. 33-36, 1987.
0883-2927/87$3.lX/t .00 Pergamon Journals Ltd.
Printed in Great Britain
Paleohydrological inferences from fracture calcite analyses: an example from the Stripa Project, Sweden G. M.
MILTON
EnvironmentalResearch Branch, Atomic Energy of Canada Limited, Research Co., Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ 1JO, Canada Abstract--Three samples of calcite fracture coatings (all less than 100 mg), recovered from the N1 borehole drill-core at the Stripa Project, Sweden, have been analyzed for their uranium, thorium and radium contents and isotopic ratios. The data indicate significant differences in uranium concentrations a n d 234U/238U activity ratios between the precipitating solutions and the groundwaters recently pumped from nearby fractures in this borehole. Higher concentrations and low activity ratios would be expected in these groundwaters at the onset of weathering in the Stripa granite; subsequent large increases in activity ratios are probably the result of near surface microfracturing caused by post glacial uplift and more recent extensive mining operations. Mean times of calcite deposition as short as 95 500 a, estimated from 23°Th/234Uratios, support the contention that these changes are of fairly recent origin.
INTRODUCTION
Stripa Project borehole N1 is a subhorizontal borehole 300 m in length, drilled at the main SGU site located at the 360 m mine level, near the leptitegranite contact. The average of two 234U/238U activity ratio measurements for water pumped over the entire length of this borehole is 4.4. NORDSTROMet al. (1985) have suggested that present day N1 waters are a mixture of deeper, more saline waters and shallower groundwaters, and the range of values reported for these two possible end members (Table 1) does not totally discredit this hypothesis. However, waters from more discrete intervals beyond 150 m, which may make only minor contributions to the total borehole throughput, have 2 3 4 U / 2 3 8 U activity ratios close to 10, considerably higher than present values for those of either hypothesized source. It seemed likely, therefore, that a study of any such coprecipitated U in the region of highest present day activity ratios could aid considerably in our understanding of the evolution of these waters. To check this hypothesis, and also to estimate their mean times of deposition, three calcite samples
ANY MINERALformed by precipitation from groundwater or surface water containingmeasurable U can potentially be dated by U-series methods. This topic has been well reviewed by GASCOYNEand SCHWARCZ (1982). Most fracture calcites contain small but measurable quantities of U. In addition to their usefulness in the estimation of mean ages of recent depositions, a recent study (MILTONand BROWN, 1987a) has shown that the U and Th isotopic ratios observed in these calcites can also provide information on the variability of water chemistry in the fracture since the time of deposition. If substantial disagreement exists between deposition times estimated from the difference between 2 3 4 u / Z 3 8 U activity ratios in present-day groundwaters and in co-existing calcites, and those calculated from 23°Th/Z34U activity ratios in the calcite, some evidence is gained for major changes in uranium activity ratios in these flow paths over that time span.
Table 1. Dissolved uranium in Stripa groundwaters Water sampled Shallowgroundwaters Intermediate groundwaters
Borehole N1
Interval (m)
Depth below ground (m)
0-100m
U concentration (p~g.1-i)
234U/238U activity ratio
0.81-90.4
3.3 av. (range 2.18-3.23)
2-300 3-300 152-251 252-300
357-401 357-401 379-394 397-401
8.46 1.83 1.24 1.27
3.22 + 0.10 5.64 _+0.26 9.65 -+0.20 9.0 +_0.18
409-506 100-505
766-863 457-862
<0.5 av. < 0.5 av.
3.5 av. (range 3.06-3.96) 4.2 av. (range 3.2O-5.41)
Deep saline waters
Borehole V1
All data taken from NORDSTROMet al. (1985). 33
G. M. Milton
34
Table 2. Major cation and radioelement content of borehole N1 calcites Sample location (A) Calcites
N1-259.35 N 1-275.09 N1-286.0
(B) Groundwater*
N1, interval 252-300
Weight of sample dissolved (mg)
Ca/Mg
Fe (%)
U (p~g. I-1)
Th (tzg. 1-L)
50.7 28.9 81.5
392 277 362
0.3 0.23 0.10
104.8 30.9 (i) 218.5 (ii) 217.4 1.27
33.9 <4 <6 <3
115
* Data taken from NORDSTROMet al. (1985).
scraped from fracture surfaces originally existing at positions between 259 and 286 m in borehole N1 have been subjected to U-series analysis. These samples were supplied by S. K. Frape and P. Fritz of the University of Waterloo.
METHOD
Each sample was weighed before dissolution in dilute HCI (final volume ~ 15 ml, pH ~ 4). Corrections were made for the weight of small insoluble residues, presumed to be rock flour or weathered material dislodged from the fracture surface by the action of the dental pick. The solutions were weighed prior to the addition of 5 ml of a toluene-based scintillator (PPO-POPOP), shaken, and counted in an inverted position in a low background L.S. counter for a period of 3-4 weeks to follow the ingrowth of Rn and its daughters. Total Ra present in each sample was estimated from measured equilibrium activities. Subsequently, most of the aqueous phase was recovered from the scintillation vial, weighed, and divided into portions equivalent to ~25 mg of sample, for U, Th and Ra purification procedures and measurements by a spectrometry and/or liquid scintillation counting. Details of the radiochemical procedures are described by MILTON(1985). Measured portions were also set aside for Ca, Mg and Fe determinations by atomic absorption spectroscopy.
RESULTS AND DISCUSSION
Mineralogical and elemental studies of these fracture coatings have shown them to be very pure calcites (S. K. FRAPE, private communication). Ratios of Ca/Mg for calcites deposited in granitic fractures
generally range from 25 to 150; the unusually high ratios reported here for borehole N1 calcites (Table 2) may in fact be as much as an order of magnitude lower than the true values as a result of partial leaching of Mg from accompanying clay minerals. Present day groundwaters sampled from this borehole also exhibit unusually high Ca/Mg ratios suggesting little change in major element composition of these waters since the time of calcite deposition. In contrast, the U-series analytical results and measured isotopic activity ratios, reported in Table 3, indicate that a substantial change has occurred in uranium water chemistry, although the extent and nature of the change is open to interpretation. The chief discussion points arising from these data are as follows. (1) The U concentrations of two of these calcites are considerably greater than those reported elsewhere for similar fracture coatings (e.g. SZABO and KYSER, 1985; NORDSTROM et a l . , 1985; MILTON and BROWN, 1987a); this is unexpected in the light of the average present day concentrations in groundwaters from nearby intervals in this borehole (1.27/zg. l-t). Although it is recognized that some surface incorporation of U will accompany calcite dissolutionreprecipitation reactions, it seems unlikely that a U concentration of 220 p~g. 1 i calcite could result from this mechanism alone. O t h e r more probable causes are: (a) U reduction and subsequent UO2 deposition on the calcite surface; (b) surface adsorption of U by thin coatings of iron oxyhydroxide or ferrihydrite laid down on top of the calcite; and (c) higher U concen-
Table 3. Uranium, thorium and radium activity ratios in borehole N1 calcites* Sample location (A) Calcites
N1-259.35 N1-275.09 N1-286.0
(B) Groundwater+
Yield tracer?
234u/Z38u
Yes No Yes Yes Yes No
1.31_+0.05 1.28_+0.07 1.24 _+0.07 0.91 _+0.05 0.87 _+0.05 0.98 _+0.10
N1, interval 252-300
* a errors from counting statistics only. ? Corrections applied for detrita123°Th and 228Th. $ Data taken from NORDSTROMet al. (1985).
9.0 _+0.18
23°Th/232Th 228Th/232Th 23°Th/234U 226Ra/Z3°Th 7.3_+0.46 8.2_+0.13 >25 >90 >200 No 232Th observed
-1.8_+0.36 ---No 228Th observed
0.60_+01t -0.89 _+0.07 0.96 _+0.07 0.90 _+0.04
1.13_+0.02 -0.93 _+0.16 0.54 _+0.04 0.63 _+0.03
--
--
Paleohydrological inferences from fracture calcite, Stripa Project, Sweden trations in the groundwaters at the time of calcite deposition. (a) The oxidation potentials of the waters sampled from this region of the borehole ( - 150 mV < Eh < 40 mV), are in the reducing range for U (LANGMUIR, 1978); however, the measured U concentrations of 1-10 /xg. 1-1 are indicative of relatively oxidizing conditions. This apparent contradiction may be the result of slow reduction rates for U known to exist above - 1 5 0 mV (MILTON, 1985). In addition, the major differences in redox potentials observed between neighbouring packed-off intervals may indicate that reducing conditions are transitory. Unless the redox potentials of these waters have been markedly different from present day values at some time since calcite deposition, it seems unlikely that U reduction is the cause of high U concentrations in fracture calcites. (b) At an Fe concentration of 0.1% it would be necessary to assume a distribution coefficient of >1.7 x l0 s m l . g -1 in the case of sample Ni-286 (Ko = mass of solute on solid phase per gram of solid phase/mass of solute per ml of solution (FREEZE and CHERRY (1979))). This value is several orders of magnitude greater than that measured in laboratory studies (MELTONand BROWN, 1987b); in addition, the lack of correlation observed between Fe and U concentrations in the samples appears to rule out this latter mechanism of enrichment. (c) High concentrations of U in paleo groundwaters appear to be the most plausible explanation for U-enriched calcites. Uranium concentrations in the Stripa granite are about 15 times the global average (~40 /xg.g 1 at depth, frequently concentrated as uraninite in micro-fractures (NELSON et al., (1979)). However, the grey granite outcrop shows strong evidence of U loss and 234U depletion by leaching over a period in excess of 150 000 a. Uranium concentrations measured in present day shallow groundwaters at Stripa have varied from 1 to 100 /xg. 1-~ . It is conceivable that at the time of deposition of these calcites the recharging waters generally contained U in concentrations nearing 100 b~g'l-a as a result of more aggressive leaching of newly exposed surfaces. 2. The 234U/238U activity ratios measured in these calcite samples are very low relative to present day groundwaters. If no change in activity ratio of the circulating water has occurred since the time of
35
deposition, these precipitates must have an age in excess of 106 a, in order to allow time for decay of 234U excess from 8-9 to <0.3. Alternatively, flow conditions and groundwater activity ratios may have changed substantially since deposition took place. It has already been pointed out that continued recoil of 234Th atoms into groundwater, due to large concentrations of U in microfractures, is the most likely cause of high activity ratios in Stripa waters (NORDSTROM et al., 1985). Enhanced 234u/Z38u activity ratios have been attributed elsewhere to greatly increased porosities in recharge areas as the result of post-glacial uplift with accompanying faulting and fracturing (MILTON, 1985); such an explanation is well suited to the enhanced activity ratios of the shallow groundwaters at Stripa. Extensive blasting and drilling operations during 400 a of mining operations may have made a more significant contribution to very high activity ratios observed at the 360 m level, especially because not all the boreholes exhibit similar effects. In both cases the decreased concentrations of U presently entering the groundwater flow allow the effects of recoil to dominate over preferential etching--this would not have been possible in waters containing 100/zg. 1-j o f U . 3. Samples 259 and 275 have 234U/23sU ratios >1 and 23°Th/Z34u ratios less than unity. The depositional ages calculated from these ratios (Table 4) are internally consistent with the difference between their depositional "ages" calculated solely from the decline in 234U excess. However, both sets of ages may be in error for a number of reasons, most notably if the U is adsorbed on the surface rather than incorporated at the time of deposition, or if extensive losses of 234U and 23°Th have occurred as a result of recoil from the calcite surface, not necessarily to the same extent. Although the depletion of 234U in sample 286 is not statistically significant, it appears to be matched by a similar depletion of 23°Th relative to 234U. Similar losses in the other 2 samples will have caused underestimations of residence times for the calcites. An even larger depletionin 226Ra relative to 23°Th may be indicative of recent selective leaching of Ra from sample 286. If the N1 waters are indeed a mixture of deep and shallow recharge waters it seems unlikely that the shallow contribution has decreased in recent times. Consequently, it can be hypothesized that the activity ratio of the depositing waters was between 3 and 4 if
Table 4. Depositional "ages" of borehole N1 calcites Activity ratios Sample location N1259.35 N1 275.09 N1 286.0
234U/238U
230Th/238U
1.31 _+0.05 1.24 _+0.07 0.90 _+0.03*
0.79 _+0.05 1.11 _+0.09 0.83 _+0.04*
*Weighted mean values.
Mean time of deposition
"Age" difference between locations (a)
(aB.P.)
(a) 23°Th/234Udata
(b) 234U/238Udata
95 520 _+9659 ] 209 210 _+59 700J > 350 000
113 690 + 60 500
92 300 +_120 200
36
G.M. Milton
no change has occurred to either contributor, or between 1.5 and 4 if the activity ratio of the shallow c o m p o n e n t has increased substantially, as suggested in Section 2, above. The approximate time required for these activity ratios to decline to those measured in the calcites is between 800000 and 180000 a. Because the m a j o r ion chemistries of present day groundwater in the 150-250 m and 270-276 m intervals of this borehole do bear a strong resemblance to that of the shallow waters analyzed, the 180000 a value, based almost entirely on a shallow source, is reasonable and is not out of line with the mean times estimated by the 23°Th/234Umethod. Stable isotope measurements on these calcites have confirmed that the 259 m sample is of comparatively recent origin, with dL~O of -11%o, while sample 286 can be subdivided into several generations, one recent, and one of possible hydrothermal origin with d ~ O = - 14.5%o (P. FRITZ, private communication).
CONCLUSIONS
Uranium-series measurements in fracture calcites can provide significant insights into the long-term stability of current hydrological regimes, as well as providing a check on stable isotope indicators of calcite depositional "ages". Analyses of several fracture coatings from the 360 m level at the Stripa Project mine suggest that aggressive leaching of U from the near-surface rock by recharge waters has decreased very considerably over the past 150000 a, while at the same time increased microporosity has allowed more opportunity for recoil addition of ~-34Uat these depths. During the longer residence times of these waters at intermediate depths ( ~ 3 5 0 m level), recent microfracturing resulting from mining operations has been responsible for even larger recoil additions of 234U.
The changing nature of the U isotopic signature in these waters sharply limits its usefulness as a tool in estimating their mean subsurface residence times.
REFERENCES
GASCOYNEM. and SCHWARCZH. P. (1982) Carbonate and sulphate precipitates. In Uranium Series Disequilibrium: Applications to Environmental Problems (eds M. IvaNovlcn and R. S. HARMON), Chap. 11, pp. 268-301. Clarendon Press. FREEZE R. A. and CHERRY J. A. (1979) Groundwater. Prentice-Hall. LANGMUIRD. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42,547-569. MILTONG. M. (1985) Uranium series disequilibrium in rock water systems of the Canadian Shield. PhD Thesis, University of Waterloo, Waterloo, Ontario. Mn,mN G. M. and BROWNR. M. (1987a) U series dating of calcite coatings in groundwater flow systems of the Canadian Shield. lsot. Geosci. (accepted for publication). MILTON G. M. and BROWNR. M. (1987b) Adsorption of uranium from groundwater by common fracture secondary minerals. Can. J. Earth Sci. (accepted for publication). NELSON P., PAULSSONB., RACHIELER., ANDERSSONL., SCHRAUFT., HUSTRULIDW., DURAND. and MAGNUSSON K. A. (1979) Preliminary report on the geophysical and mechanical borehole measurements at Stripa. LBL Report 8250. Lawrence Berkeley Lab., University of California. NORDSTROMD. K.. ANDREWSJ. N., CARLSSONL., FONTES J-C., FRITZ P., HOSERH. and Ot,SSONT. (1985) Hydrogeological and hydrogeochemical investigations in boreholes-final report of the phase 1 geochemical investigations of the Stripa groundwaters. SKB Tcch. Report, Stripa Project 85-06. SZABO B. J. and KYSER T. L. (1985) Uranium, thorium isotopic analyses and uranium series ages of calcites and opal and stable isotopic compositions of calcite from drillcores UE 25a #1, USW G-2 and USW G-3/GU-3, Yucca Mountain, Nevada. U.S. Geol. Surv. Open File Rep. 85-224.
0883-2927/87$3.lX/t .00 Pergamon Journals Ltd.
Printed in Great Britain
Paleohydrological inferences from fracture calcite analyses: an example from the Stripa Project, Sweden G. M.
MILTON
EnvironmentalResearch Branch, Atomic Energy of Canada Limited, Research Co., Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ 1JO, Canada Abstract--Three samples of calcite fracture coatings (all less than 100 mg), recovered from the N1 borehole drill-core at the Stripa Project, Sweden, have been analyzed for their uranium, thorium and radium contents and isotopic ratios. The data indicate significant differences in uranium concentrations a n d 234U/238U activity ratios between the precipitating solutions and the groundwaters recently pumped from nearby fractures in this borehole. Higher concentrations and low activity ratios would be expected in these groundwaters at the onset of weathering in the Stripa granite; subsequent large increases in activity ratios are probably the result of near surface microfracturing caused by post glacial uplift and more recent extensive mining operations. Mean times of calcite deposition as short as 95 500 a, estimated from 23°Th/234Uratios, support the contention that these changes are of fairly recent origin.
INTRODUCTION
Stripa Project borehole N1 is a subhorizontal borehole 300 m in length, drilled at the main SGU site located at the 360 m mine level, near the leptitegranite contact. The average of two 234U/238U activity ratio measurements for water pumped over the entire length of this borehole is 4.4. NORDSTROMet al. (1985) have suggested that present day N1 waters are a mixture of deeper, more saline waters and shallower groundwaters, and the range of values reported for these two possible end members (Table 1) does not totally discredit this hypothesis. However, waters from more discrete intervals beyond 150 m, which may make only minor contributions to the total borehole throughput, have 2 3 4 U / 2 3 8 U activity ratios close to 10, considerably higher than present values for those of either hypothesized source. It seemed likely, therefore, that a study of any such coprecipitated U in the region of highest present day activity ratios could aid considerably in our understanding of the evolution of these waters. To check this hypothesis, and also to estimate their mean times of deposition, three calcite samples
ANY MINERALformed by precipitation from groundwater or surface water containingmeasurable U can potentially be dated by U-series methods. This topic has been well reviewed by GASCOYNEand SCHWARCZ (1982). Most fracture calcites contain small but measurable quantities of U. In addition to their usefulness in the estimation of mean ages of recent depositions, a recent study (MILTONand BROWN, 1987a) has shown that the U and Th isotopic ratios observed in these calcites can also provide information on the variability of water chemistry in the fracture since the time of deposition. If substantial disagreement exists between deposition times estimated from the difference between 2 3 4 u / Z 3 8 U activity ratios in present-day groundwaters and in co-existing calcites, and those calculated from 23°Th/Z34U activity ratios in the calcite, some evidence is gained for major changes in uranium activity ratios in these flow paths over that time span.
Table 1. Dissolved uranium in Stripa groundwaters Water sampled Shallowgroundwaters Intermediate groundwaters
Borehole N1
Interval (m)
Depth below ground (m)
0-100m
U concentration (p~g.1-i)
234U/238U activity ratio
0.81-90.4
3.3 av. (range 2.18-3.23)
2-300 3-300 152-251 252-300
357-401 357-401 379-394 397-401
8.46 1.83 1.24 1.27
3.22 + 0.10 5.64 _+0.26 9.65 -+0.20 9.0 +_0.18
409-506 100-505
766-863 457-862
<0.5 av. < 0.5 av.
3.5 av. (range 3.06-3.96) 4.2 av. (range 3.2O-5.41)
Deep saline waters
Borehole V1
All data taken from NORDSTROMet al. (1985). 33
G. M. Milton
34
Table 2. Major cation and radioelement content of borehole N1 calcites Sample location (A) Calcites
N1-259.35 N 1-275.09 N1-286.0
(B) Groundwater*
N1, interval 252-300
Weight of sample dissolved (mg)
Ca/Mg
Fe (%)
U (p~g. I-1)
Th (tzg. 1-L)
50.7 28.9 81.5
392 277 362
0.3 0.23 0.10
104.8 30.9 (i) 218.5 (ii) 217.4 1.27
33.9 <4 <6 <3
115
* Data taken from NORDSTROMet al. (1985).
scraped from fracture surfaces originally existing at positions between 259 and 286 m in borehole N1 have been subjected to U-series analysis. These samples were supplied by S. K. Frape and P. Fritz of the University of Waterloo.
METHOD
Each sample was weighed before dissolution in dilute HCI (final volume ~ 15 ml, pH ~ 4). Corrections were made for the weight of small insoluble residues, presumed to be rock flour or weathered material dislodged from the fracture surface by the action of the dental pick. The solutions were weighed prior to the addition of 5 ml of a toluene-based scintillator (PPO-POPOP), shaken, and counted in an inverted position in a low background L.S. counter for a period of 3-4 weeks to follow the ingrowth of Rn and its daughters. Total Ra present in each sample was estimated from measured equilibrium activities. Subsequently, most of the aqueous phase was recovered from the scintillation vial, weighed, and divided into portions equivalent to ~25 mg of sample, for U, Th and Ra purification procedures and measurements by a spectrometry and/or liquid scintillation counting. Details of the radiochemical procedures are described by MILTON(1985). Measured portions were also set aside for Ca, Mg and Fe determinations by atomic absorption spectroscopy.
RESULTS AND DISCUSSION
Mineralogical and elemental studies of these fracture coatings have shown them to be very pure calcites (S. K. FRAPE, private communication). Ratios of Ca/Mg for calcites deposited in granitic fractures
generally range from 25 to 150; the unusually high ratios reported here for borehole N1 calcites (Table 2) may in fact be as much as an order of magnitude lower than the true values as a result of partial leaching of Mg from accompanying clay minerals. Present day groundwaters sampled from this borehole also exhibit unusually high Ca/Mg ratios suggesting little change in major element composition of these waters since the time of calcite deposition. In contrast, the U-series analytical results and measured isotopic activity ratios, reported in Table 3, indicate that a substantial change has occurred in uranium water chemistry, although the extent and nature of the change is open to interpretation. The chief discussion points arising from these data are as follows. (1) The U concentrations of two of these calcites are considerably greater than those reported elsewhere for similar fracture coatings (e.g. SZABO and KYSER, 1985; NORDSTROM et a l . , 1985; MILTON and BROWN, 1987a); this is unexpected in the light of the average present day concentrations in groundwaters from nearby intervals in this borehole (1.27/zg. l-t). Although it is recognized that some surface incorporation of U will accompany calcite dissolutionreprecipitation reactions, it seems unlikely that a U concentration of 220 p~g. 1 i calcite could result from this mechanism alone. O t h e r more probable causes are: (a) U reduction and subsequent UO2 deposition on the calcite surface; (b) surface adsorption of U by thin coatings of iron oxyhydroxide or ferrihydrite laid down on top of the calcite; and (c) higher U concen-
Table 3. Uranium, thorium and radium activity ratios in borehole N1 calcites* Sample location (A) Calcites
N1-259.35 N1-275.09 N1-286.0
(B) Groundwater+
Yield tracer?
234u/Z38u
Yes No Yes Yes Yes No
1.31_+0.05 1.28_+0.07 1.24 _+0.07 0.91 _+0.05 0.87 _+0.05 0.98 _+0.10
N1, interval 252-300
* a errors from counting statistics only. ? Corrections applied for detrita123°Th and 228Th. $ Data taken from NORDSTROMet al. (1985).
9.0 _+0.18
23°Th/232Th 228Th/232Th 23°Th/234U 226Ra/Z3°Th 7.3_+0.46 8.2_+0.13 >25 >90 >200 No 232Th observed
-1.8_+0.36 ---No 228Th observed
0.60_+01t -0.89 _+0.07 0.96 _+0.07 0.90 _+0.04
1.13_+0.02 -0.93 _+0.16 0.54 _+0.04 0.63 _+0.03
--
--
Paleohydrological inferences from fracture calcite, Stripa Project, Sweden trations in the groundwaters at the time of calcite deposition. (a) The oxidation potentials of the waters sampled from this region of the borehole ( - 150 mV < Eh < 40 mV), are in the reducing range for U (LANGMUIR, 1978); however, the measured U concentrations of 1-10 /xg. 1-1 are indicative of relatively oxidizing conditions. This apparent contradiction may be the result of slow reduction rates for U known to exist above - 1 5 0 mV (MILTON, 1985). In addition, the major differences in redox potentials observed between neighbouring packed-off intervals may indicate that reducing conditions are transitory. Unless the redox potentials of these waters have been markedly different from present day values at some time since calcite deposition, it seems unlikely that U reduction is the cause of high U concentrations in fracture calcites. (b) At an Fe concentration of 0.1% it would be necessary to assume a distribution coefficient of >1.7 x l0 s m l . g -1 in the case of sample Ni-286 (Ko = mass of solute on solid phase per gram of solid phase/mass of solute per ml of solution (FREEZE and CHERRY (1979))). This value is several orders of magnitude greater than that measured in laboratory studies (MELTONand BROWN, 1987b); in addition, the lack of correlation observed between Fe and U concentrations in the samples appears to rule out this latter mechanism of enrichment. (c) High concentrations of U in paleo groundwaters appear to be the most plausible explanation for U-enriched calcites. Uranium concentrations in the Stripa granite are about 15 times the global average (~40 /xg.g 1 at depth, frequently concentrated as uraninite in micro-fractures (NELSON et al., (1979)). However, the grey granite outcrop shows strong evidence of U loss and 234U depletion by leaching over a period in excess of 150 000 a. Uranium concentrations measured in present day shallow groundwaters at Stripa have varied from 1 to 100 /xg. 1-~ . It is conceivable that at the time of deposition of these calcites the recharging waters generally contained U in concentrations nearing 100 b~g'l-a as a result of more aggressive leaching of newly exposed surfaces. 2. The 234U/238U activity ratios measured in these calcite samples are very low relative to present day groundwaters. If no change in activity ratio of the circulating water has occurred since the time of
35
deposition, these precipitates must have an age in excess of 106 a, in order to allow time for decay of 234U excess from 8-9 to <0.3. Alternatively, flow conditions and groundwater activity ratios may have changed substantially since deposition took place. It has already been pointed out that continued recoil of 234Th atoms into groundwater, due to large concentrations of U in microfractures, is the most likely cause of high activity ratios in Stripa waters (NORDSTROM et al., 1985). Enhanced 234u/Z38u activity ratios have been attributed elsewhere to greatly increased porosities in recharge areas as the result of post-glacial uplift with accompanying faulting and fracturing (MILTON, 1985); such an explanation is well suited to the enhanced activity ratios of the shallow groundwaters at Stripa. Extensive blasting and drilling operations during 400 a of mining operations may have made a more significant contribution to very high activity ratios observed at the 360 m level, especially because not all the boreholes exhibit similar effects. In both cases the decreased concentrations of U presently entering the groundwater flow allow the effects of recoil to dominate over preferential etching--this would not have been possible in waters containing 100/zg. 1-j o f U . 3. Samples 259 and 275 have 234U/23sU ratios >1 and 23°Th/Z34u ratios less than unity. The depositional ages calculated from these ratios (Table 4) are internally consistent with the difference between their depositional "ages" calculated solely from the decline in 234U excess. However, both sets of ages may be in error for a number of reasons, most notably if the U is adsorbed on the surface rather than incorporated at the time of deposition, or if extensive losses of 234U and 23°Th have occurred as a result of recoil from the calcite surface, not necessarily to the same extent. Although the depletion of 234U in sample 286 is not statistically significant, it appears to be matched by a similar depletion of 23°Th relative to 234U. Similar losses in the other 2 samples will have caused underestimations of residence times for the calcites. An even larger depletionin 226Ra relative to 23°Th may be indicative of recent selective leaching of Ra from sample 286. If the N1 waters are indeed a mixture of deep and shallow recharge waters it seems unlikely that the shallow contribution has decreased in recent times. Consequently, it can be hypothesized that the activity ratio of the depositing waters was between 3 and 4 if
Table 4. Depositional "ages" of borehole N1 calcites Activity ratios Sample location N1259.35 N1 275.09 N1 286.0
234U/238U
230Th/238U
1.31 _+0.05 1.24 _+0.07 0.90 _+0.03*
0.79 _+0.05 1.11 _+0.09 0.83 _+0.04*
*Weighted mean values.
Mean time of deposition
"Age" difference between locations (a)
(aB.P.)
(a) 23°Th/234Udata
(b) 234U/238Udata
95 520 _+9659 ] 209 210 _+59 700J > 350 000
113 690 + 60 500
92 300 +_120 200
36
G.M. Milton
no change has occurred to either contributor, or between 1.5 and 4 if the activity ratio of the shallow c o m p o n e n t has increased substantially, as suggested in Section 2, above. The approximate time required for these activity ratios to decline to those measured in the calcites is between 800000 and 180000 a. Because the m a j o r ion chemistries of present day groundwater in the 150-250 m and 270-276 m intervals of this borehole do bear a strong resemblance to that of the shallow waters analyzed, the 180000 a value, based almost entirely on a shallow source, is reasonable and is not out of line with the mean times estimated by the 23°Th/234Umethod. Stable isotope measurements on these calcites have confirmed that the 259 m sample is of comparatively recent origin, with dL~O of -11%o, while sample 286 can be subdivided into several generations, one recent, and one of possible hydrothermal origin with d ~ O = - 14.5%o (P. FRITZ, private communication).
CONCLUSIONS
Uranium-series measurements in fracture calcites can provide significant insights into the long-term stability of current hydrological regimes, as well as providing a check on stable isotope indicators of calcite depositional "ages". Analyses of several fracture coatings from the 360 m level at the Stripa Project mine suggest that aggressive leaching of U from the near-surface rock by recharge waters has decreased very considerably over the past 150000 a, while at the same time increased microporosity has allowed more opportunity for recoil addition of ~-34Uat these depths. During the longer residence times of these waters at intermediate depths ( ~ 3 5 0 m level), recent microfracturing resulting from mining operations has been responsible for even larger recoil additions of 234U.
The changing nature of the U isotopic signature in these waters sharply limits its usefulness as a tool in estimating their mean subsurface residence times.
REFERENCES
GASCOYNEM. and SCHWARCZH. P. (1982) Carbonate and sulphate precipitates. In Uranium Series Disequilibrium: Applications to Environmental Problems (eds M. IvaNovlcn and R. S. HARMON), Chap. 11, pp. 268-301. Clarendon Press. FREEZE R. A. and CHERRY J. A. (1979) Groundwater. Prentice-Hall. LANGMUIRD. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42,547-569. MILTONG. M. (1985) Uranium series disequilibrium in rock water systems of the Canadian Shield. PhD Thesis, University of Waterloo, Waterloo, Ontario. Mn,mN G. M. and BROWNR. M. (1987a) U series dating of calcite coatings in groundwater flow systems of the Canadian Shield. lsot. Geosci. (accepted for publication). MILTON G. M. and BROWNR. M. (1987b) Adsorption of uranium from groundwater by common fracture secondary minerals. Can. J. Earth Sci. (accepted for publication). NELSON P., PAULSSONB., RACHIELER., ANDERSSONL., SCHRAUFT., HUSTRULIDW., DURAND. and MAGNUSSON K. A. (1979) Preliminary report on the geophysical and mechanical borehole measurements at Stripa. LBL Report 8250. Lawrence Berkeley Lab., University of California. NORDSTROMD. K.. ANDREWSJ. N., CARLSSONL., FONTES J-C., FRITZ P., HOSERH. and Ot,SSONT. (1985) Hydrogeological and hydrogeochemical investigations in boreholes-final report of the phase 1 geochemical investigations of the Stripa groundwaters. SKB Tcch. Report, Stripa Project 85-06. SZABO B. J. and KYSER T. L. (1985) Uranium, thorium isotopic analyses and uranium series ages of calcites and opal and stable isotopic compositions of calcite from drillcores UE 25a #1, USW G-2 and USW G-3/GU-3, Yucca Mountain, Nevada. U.S. Geol. Surv. Open File Rep. 85-224.