Robert H. McNutt

Water-rock interaction and chemistry of groundwaters from the Canadian Shield

The chemical and isotopic compositions of groundwaters in the crystalline rocks of the Canadian Shield reflect different degrees of rock-water interactions. The chemistry of the shallow, geochemically immature groundwaters and especially of the major cations is controlled by local rock compositions, whereby dissolution reactions dominate. Conservative constituents, such as chloride and bromide, however, are not entirely a result of such reactions but appear to be readily added from leachable salts during the initial stages of the geochemical evolution of these waters. Their concentration changes little as major cations increase, until concentrations of Total Dissolved Solids (TDS) reach 3000 to 5000 mg 1−1. The isotopic composition of these shallow waters reflects local, present day precipitations. In contrast to the shallow groundwaters, the isotopic and chemical compositions of the deep, saline waters and brines are determined by extensive, low-temperature rock-water interactions. This is documented in major ion chemistries, 18O contents and strontium isotopic compositions. These data indicate that the deep brines have been contained in hydrologically isolated “pockets”. The almost total loss of primary compositions make discussions on the origin of these brines very speculative. However, all brines from across the Canadian Shield have a very similar chemical composition, which probably reflects a common geochemical history. The concentrations of some major and most minor elements in these fluids appear to be governed by reactions with secondary mineral assemblages.

The 87Sr86Sr values of Canadian Shield brines and fracture minerals with applications to groundwater mixing, fracture history, and geochronology

Analyses of saline waters, fracture minerals, and host rocks from seven localities on the Canadian Shield demonstrate the utility of the 87Sr86Sr ratio in the study of groundwater systems in crystalline rocks. The ratios range from 0.704 to 0.753 and have obtained their signatures by mineral/rock interactions, primarily involving the feldspars. We have identified brines from isolated pockets in the same mines where extensive flow regimes exist. There is mixing of different brines as well as mixing with meteoric waters. The isotopic results on calcites from fractures and shear zones show more than one generation of mineral growth in a given fracture. The 87Sr86Sr ratios of the calcites vary from values identical to the present-day brine in the fracture zone to ratios with Archean signatures. This implies that activity may occur in fault zones over a very long time. The brines are very rich in Sr (up to 2400 mg/1), very low in Rb, and have relatively radiogenic 87Sr86Sr ratios. They are ubiquitous in Shield rocks and, if they were present throughout geological time, they may be one reason why RbSr ages of felsic plutons are commonly younger than associated UPb ages.

Partition of circulating lead between serum and red cells is different for internal and external sources of lead

Serum lead, whole blood lead, and lead in both tibia and calcaneus were measured in each of 49 active lead workers. Serum lead correlated more strongly with both in vivo bone lead measurements than did whole blood lead. The ratio of serum lead to whole blood lead varied from 0.8% to 2.5% and showed a positive correlation with tibia, and an even stronger correlation with calcaneus lead. This implies that lead released from bone (endogenous exposure) results in a higher proportion of whole blood lead being in serum than is the case for exogenous exposure. This observation needs to be confirmed, and the relationships amongst the parameters must be studied further, particularly in former or retired lead workers. If confirmed, since at least a portion of lead in serum is readily diffusible and thus toxicologically more immediately significant than lead bound to red cells, the health implications of endogenous exposure may have to be reassessed.

Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26° and 29° south

Initial87Sr/86Sr ratios have been determined for 34 plutonic and volcanic rocks covering the entire age span of magmatic events associated with the Andean orogeny between latitudes 26° and 29° south. The igneous rocks, the majority dated by K/Ar mineral techniques, range in age from Lower Jurassic (190 m.y.) to Quaternary (0.89 m.y.). In addition, initial ratios were determined for three granitoid plutons and one metasediment from the pre-Mesozoic basement which underlies the entire Andean orogen in this transect at shallow depth. The compositions vary from basalt to rhyolite, and from quartz diorite to granodiorite or trondjemite, for the extrusives and intrusives, respectively. Mid-Cretaceous to Quaternary rocks exhibit a systematic west to east increase in mean strontium isotope ratio from 0.7022 to 0.7077, whereas the initial ratios of Jurassic plutons vary from 0.7043 to 0.7059, and do not correlate with age. The existence of unusually low initial ratios (e.g. 0.7022, 0.7023) for several Mesozoic plutonic rocks strongly implies a sub-crustal source for at least some of the Andean magmas. The time-dependent post-Jurassic increase in initial ratio is considered to reflect a systematic change in the composition of partial melts generated in response to the progressive subduction of a lithospheric slab. It is suggested that a systematic change in the locus of melting takes place from along or close to the upper surface of the subduction slab into hanging-wall mantle peridotite as subduction continues.

Isotopic evidence for the crustal evolution of the Frontenac Arch in the Grenville Province of Ontario, Canada

Abstract Nine late-tectonic granitoid bodies from the Central Metasedimentary Belt (CMB) of the Grenville Province were studied by the Sr, Nd and O isotopic systems. They occur on either side of the Rideau Lake Fault (RLF). Four bodies south of the fault were dated by the U-Pb zircon method. Three of the ages are tightly constrained at 1166 ± 3 Ma. This is in contrast to ages reported for late-tectonic plutons north of the fault between 1090 and 1076 Ma. t Nd DM model ages range from 1211 to 1480 Ma, indicating that the plutons were derived from a mixed source involving older crust. Exposed pelitic and psammitic metasediments in the area have t DM ages averaging 1790 Ma. The observed Sr-Nd-O relationships indicate that marble was involved in the evolution of the granitoids. The granites are chemically similar to the Mid-Proterozoic anorogenic plutons that occur elsewhere in North America. Their crystallization ages are not found elsewhere in the CMB in Ontario, but they are similar to ages found to the east in the Adirondacks, New York. This suggests that the Frontenac-Adirondack area represents a separate Penokean-age block assembled with the rest of the CMB ∼ 1070 Ma ago.

Groundwater 87Sr/86Sr values in the Eye-Dashwa Lakes pluton, Canada: Evidence for plagioclase-water reaction

The Eye-Dashwa Lakes pluton is a Sr-rich felsic intrusive of the sanukitoid suite located in NW Ontario in the Superior Province of the Canadian Shield. The 87Sr/86Sr values of the CaNaCl-type saline, deeper groundwaters fall in the narrow range of 0.7057–0.7070, similar to the 87Sr/86Sr value in plagioclase but not to that in K-feldspar, biotite, or the whole rock. This suggests that the water isotopic composition is the result of plagioclase-water interaction. The fracture minerals gypsum and calcite have similar 87Sr/86Sr ratios, indicating that they have crystallized from waters similar to the present deep groundwaters. The shallow, CaHCO3-type, dilute groundwaters show a wider spread in the Sr ratio of 0.7065–0.7278, reflecting their derivation from minerals in the overlying soil. The results for the saline groundwaters are consistent with thermodynamic and kinetic models that demonstrate the relatively rapid dissolution of plagioclase with respect to other rock-forming silicates, and should have application to most brines located in silicate host rocks.

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

Abstract 87 Sr/ 86 Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ 18 O−δ 2 H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids. Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same 87 Sr/ 86 Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.

Electrothermal isotope dilution inductively coupled plasma mass spectrometry method for the determination of sub-ng ml–1 levels of lead in human plasma

An isotope dilution technique coupled with electrothermal volatilization and inductively coupled plasma mass spectrometric detection is reported for the quantitative determination of Pb in human blood plasma with a calculated limit of detection of 16 × 10–15 g. Sample preparation is simple, requiring only addition of concentrated nitric acid and a measured amount of 204Pb spike in solution. Analysis time is typically 2.5 min, the relative standard deviation is <2%(n= 5), calculations require only simple numeric manipulations and the isotopic ratio of the Pb isotopes can be monitored. The Pb concentration of a commercially available dehydrated human plasma was found to be 1.27 ± 0.02 ng ml–1.

Strontium isotopic composition of some brines from the Precambrian Shield of Canada

Abstract Twenty-four groundwater samples from seven operating mines at Sudbury, Yellow-knife and Thompson (Ontario, North West Territories and Manitoba, resp.), all from depths greater than 1 km and ranging in total dissolved solids (TDS) from 1900 to 250,000 mg l−1, were measured for their 87 Sr 86 Ar values. Each geographic location gives a limited range in values and each location is distinct from the others. This is interpreted as the result of extensive water-rock interaction on a local scale. For most of the time, these brines were isolated and only recently have been exposed to surface water as a result of the mining operations. The extent of the isolation is shown by the contrasting isotopic values of two “pockets” of water (0.711 vs. 0.716) located on opposite sides of the same fault system on the North Range at Sudbury. The exchange at all sites probably has continued until the present, as indicated by the close agreement between water and present-day 87 Sr 86 Sr whole-rock values. If so, it suggests that there is no single age for such brines, but it may be possible to date stages in the waters evolution by determining the age of secondary minerals that equilibrated with the water.

Osmium isotope evidence for a crustal origin of platinum group elements in the Sudbury nickel ore, Ontario, Canada

Abstract Sulphide ores from the International Nickel Companys (INCO) Creighton Mine, Sudbury, were analysed for osmium isotope ratios by ICP-MS. Rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix. Calculated initial 187 Os 188 Os ratios at 1.85 Ga (the emplacement age of the Sudbury complex) cluster around 0.60. The occurrence of less radiogenic compositions is attributed to post-emplacement, open-system behaviour of the Re Os system. The Creighton results strongly overlap the initial ratios of published osmium data from two other Sudbury mines, suggesting a narrow range of isotope ratio in the original ore, within the range of estimated osmium isotope compositions in the country rock at 1.85 Ga, using published and new data. Therefore, the sulphide ores of the Sudbury complex can have an entirely crustal source without a mantle-derived contribution. This conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis, since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex. The extraction of platinum group elements (PGE) from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure.