Derek H.C. Wilton

Isotopic and elemental analysis of uraninite concentrates using inductively coupled plasma-mass spectrometry (ICP-MS)

Inductively coupled plasma-mass spectrometry (ICP-MS) is capable of measuring isotopic and elemental abundances in geologic materials easily and rapidly. Although the precision of isotope ratio data obtained by ICP-MS is inferior to that by thermal ionization mass spectrometry, it is adequate for application to a number of geochemical exploration problems. National Institute of Standards and Technology (NIST) Pb metal standard reference material 981 (NBS981), was used as the isotopic standard to correct the measured isotope intensities for mass discrimination. The mean relative standard deviation (RSD) of the determinations of the abundances of 206Pb, 207Pb, and 208Pb in the two other NIST Pb isotope reference materials, NBS981 and NBS982, was better than 0.3%, whereas the RSD for the determination of the less abundant 204Pb was 0.4%. Accuracy was demonstrated by repeated analysis of NBS981, NBS982, and NBS983. The Student t-statistic ranged between −1.75 and 2.04 for the abundances of the 4 Pb isotopes in the three NIST materials. Data from a suite of 13 uraninite-rich samples from Labrador demonstrate the ability of ICP-MS to determine age and geochemical information sufficient for regional interpretations. The determined radiogenic 207Pb/206Pb ratios of 12 of the samples give ages between 1697 and 1805 Ma with average uncertainties (one standard deviation) of 4 Ma, whereas one of the samples has an age of 495 ± 4 Ma. The average age of the 12 samples was 1752 ± 27 Ma. Along with the Pb isotope intensities, 232Th and 238U were measured and the U-Pb age determined from a fit of the 206Pb/238U vs. 207Pb/235U for 9 of the samples. The concordia intercept age of 1740 Ma for the best-fit line is in good agreement with the mean 207Pb/206Pb age of 1752 Ma.

Palaeoproterozoic, ∼ 1.88–2.0 Ga, organic matter from the Mugford/Kaumajet Mountain Group, northern Labrador

Abstract The Palaeoproterozoic, ∼ 1.88–2.0 Ga, Mugford Group of northern Labrador is a relatively undeformed cover sequence developed on Archaean crust of the Nain craton. The Cod Island Formation occurs at the base of the group and consists of mixed shale, chert, dolostone and siltstone. Thin layers of highly matured organic material similar to anthracite have been found within a shale and chert sequence. Based on the nonmigrated and sedimentological context of the layers, the material may represent some of the earliest intact, in situ, organic deposits known. δ 13C isotope ratios of the organic matter in this deposit range from −31.79 to −32.22‰ and the H/C atomic ratios from 0.05 to 0.08. The material represents the first truly biological remains known from the Nain Province of Labrador and the earliest organic material in northeastern North America.

The Midas Pond Gold Prospect, Victoria Lake Group, Central Newfoundland: A Mesothermal Quartz Vein System with Epithermal Characteristics

The Midas Pond gold prospect, central Newfoundland, is hosted by sheared and altered felsic and mafic pyroclastic rocks of the Tulks Hill volcanics, Victoria Lake Group, and has been trenched and tested by 19 diamond-drill holes. Gold values are sporadic, with the best diamond-drill intersection assaying 7.3 g/t over 0.9 m, and the highest grade channel sample containing 14.7 g/t over 1.15 m. Alteration and mineralization are confined to a 200 m wide brittle-ductile shear zone. This shear zone formed in response to regional D1 deformation, and the shear zone fabric parallels the regional S1 foliation. D2 deformation resulted in broad Z-shaped flexuring of the shear zone. The deformation was probably related to the ca. 396 Ma to 420 Ma Salinic Orogeny. Advanced argillic alteration and an extensive halo of Fe-carbonate and pyrite surround the gold mineralization. Alteration minerals include pyrophyllite, paragonite, quartz, plagioclase, chlorite, fluorite, Fe-carbonate, and pyrite. As such, the alteration seems to reflect combined epithermal (argillic) and mesothermal (carbonate) mineralizing environments. The argillic alteration and fluorite represent a regional hydrothermal effect on the felsic volcanic rocks; whereas, the Fe-carbonate and pyrite represent a mesothermal alteration associated with auriferous quartz veins. The Au, which is spatially associated with pyrite, occurs in three structurally-controlled vein sets. These veins are confined to the contact between a highly deformed breccia (the banded mafic tuff) and structurally overlying, altered felsic tuffaceous rocks. The vein sets include: V1 boudinage veins that parallel the shear-zone fabric (C-shear veins), and V2 and V3 extensional fracture veins which are controlled by an S2 fracture cleavage. V1 veins are the earliest and contain the lowest concentrations of Au. V2 and V3 veins are concentrated within the hinges of the D2 flexures. Pb isotope data for galena separates indicate that the Pb in the vein systems was probably of local derivation. Sulfur, C, and O isotope data for mineral separates are similar to data for typical mesothermal lode gold deposits. Fluid inclusion data suggest the presence of at least three hydrothermal fluid types in the region, viz., (1) an early saline fluid which produced the argillic alteration, (2) a CO2-bearing fluid directly related to the auriferous mineralization, and (3) H2O-NaCl fluids which flowed through the Midas Pond shear zone at a lateral distance from the zone of mineralization. The mineralizing fluids are interpreted to have been CO3−2-rich types which originated through metamorphic dehydration and decarbonation of mixed island-arc and continental crustal rocks. Gold precipitation resulted from the reaction of these fluids with the Fe-rich breccia unit. Pre-existing and simultaneously crystallizing pyrite served as loci for Au precipitation.

Burial and Exhumation History of the Labrador-Newfoundland Margin and Implications for Hydrocarbon Exploration

The stratigraphic record along the continental margin of Labrador and Newfoundland provides ample evidence for vertical movements both prior to and after break-up that could have importance for hydrocarbon exploration in the region. Publically available vitrinite reflectance datasets from a well in the Jeanne d’Arc Basin indicate that the Cenozoic and deeper sections has been hotter in the past, presumably due to deeper burial prior to Late Cenozoic cooling. Furthermore, one of the datasets shows evidence of a major degree of deeper burial on the baseTertiary unconformity, but additional data are required to investigate the reality of the base-Tertiary episode of burial and exhumation. Over much of the Labrador shelf, Miocene deposits are absent, and we show evidence based on vitrinite reflectance and sonic data that indicate that Miocene deposits of significant thickness may have been present prior to uplift and exhumation. Onshore Labrador, the presence of a Cretaceous outlier on Precambrian basement adds to the evidence of one or more events of exhumation that has removed pre-Cretaceous sediments on a regional scale, similar to the offshore Labrador where Cretaceous rocks rest on Precambrian basement over most of the shelf. We also present results from a pilot study comprising apatite fission-track analysis (AFTA) data that reveals a Phanerozoic history involving a series of burial and exhumation episodes. The pilot study is a forerunner for a study of the onshore and offshore domain with three components: (1) A thermochronological study based on samples from outcrops and from onshore and offshore boreholes, (2) A stratigraphic landform analysis of the onshore study area based on mapping of erosion surfaces that will provide evidence of the vertical motion onshore and a relative denudation chronology, and (3) An integrated interpretation of the geological, geomorphological and thermochronological data to provide a coherent model of the timing and magnitude of the vertical movements along the margin both prior to and after break-up. Failure to account for the effects of uplift and erosion, such as greater depths of burial prior to exhumation, may lead to serious underestimation of the petroleum resource maturity and to erroneous estimates of the timing of hydrocarbon generation, and not least to changes in migration routes and in the source-to-sink system of sediment input into offshore basins.

Measurement of 238U/235U ratio in black spruce seedlings to fingerprint the source of uranium taken up by black spruce trees as a biogeochemical sampling medium

This study explored the measurement of 238U/235U ratios in black spruce seedlings as a tracer to investigate the origin of U taken up by this biogeochemical sampling media. Inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) and neutron activation (NAA) analyses of the 238U/235U ratios in the total digested (HF/HNO3-aqua regia) portions of substrates (soil and peat) treated with 200 ml aliquots of 100, 250, and 400 mg/l of aqueous solutions of depleted uranium (DU) exhibited a substantial deviation from the accepted natural value of 137.88. Similarly, the measured 238U/235U ratios in stems of seedlings grown in the DU-dosed substrates ranged from 174 to 210 and are linearly correlated with the dose of DU applied. The elevated 238U/235U ratio in the seedlings suggests that U taken up by the plants is derived predominantly from the labile DU components in water rather than the component adsorbed onto mineral phases in the substrate. Consequently, it was concluded that the U contents in vegetation used as a biogeochemical sampling medium are derived from the labile components in groundwater and/or surface water. The precision and accuracy of the measured ratios were compared with the 238U/235U ratios of in-house granite and uraninite reference samples.