Timing and thermochemical constraints on multi-element mineralisation at the Nori/RA Cu–Mo–U prospect, Great Bear magmatic zone, Northwest Territories, Canada

Abstract

The timing of Cu–Mo–U mineralisation at the Nori/RA prospect in the Paleoproterozoic Great Bear magmatic zone has been investigated using Re–Os molybdenite and ⁴⁰Ar–³⁹Ar biotite geochronology. The Re–Os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the Great Bear magmatic zone. Cu–Mo–U mineralisation is hosted in early to syn-deformational hydrothermal veins consisting of quartz and K-feldspar or more commonly tourmaline-biotite-quartz-K-feldspar, with associated wall-rock alteration assemblages being predominantly biotite. Sulphide and oxide minerals consist of chalcopyrite, molybdenite and uraninite with lesser pyrite and magnetite. Elevated light rare earth elements and tungsten concentrations associated with the Cu–Mo–U mineralisation have also been reported at the prospect by previous workers. Molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins, attesting to their syngenetic nature (with respect to hydrothermal veining). Two molybdenite separates yield Re–Os model ages of 1,874.4 ± 8.7 (2σ) and 1,872.4 ± 8.8 Ma (2σ) with a weighted average model age of 1,873.4 ± 6.1 Ma (2σ). Laser step heating of biotite from the marginal alteration of the wall-rock adjacent to the veins yields a ⁴⁰Ar–³⁹Ar maximum cooling age of 1,875 ± 8 Ma (MSWD = 3.8; 2σ), indistinguishable from the Re–Os molybdenite model age and a previously dated ‘syn-tectonic’ aplitic dyke in the region. Dravitic tourmaline hosts abundant primary liquid–vapour–solid-bearing fluid inclusions. Analytical results indicate liquid–vapour homogenisation at >260°C constraining the minimum temperature of mineralisation. The solids, which are possibly trapped, did not homogenise with the liquid–vapour by 400°C. Salinities in the inclusions are variable. Raman spectra identify that at least some of the solids are calcite and anhydrite. Raman spectra also confirm the vapour phases contain some CO₂; whereas clathrates or CH₄ was not observed or detected. Quartz grains only host secondary fluid inclusions, which fluoresce under ultraviolet light, indicating trapped hydrocarbons. We speculate that these resulted from Phanerozoic fluid circulation through the Proterozoic basement. The collective interpretation of the age, hydrothermal character and associated metals, high temperature and variable salinity suggests that the Nori/RA Cu–Mo–U mineralisation can be linked with the earliest stages of plutonism in the Great Bear magmatic zone. From a regional perspective, the mineralisation may pre-date the extensive multi-element mineralisation now recognised as part of the iron oxide copper–gold (IOCG) spectrum of deposits. As IOCG provinces generally contain a variety of mineralisation styles, we interpret this as the earliest phase of the extensive mineralising system.

Appendix: Materials and methods

Mineralogy and petrography

Three fist-sized samples were collected (06ns2145′A′, 06ns2145′B′ and 06ns2145′C′) from Trench 101 in the Main Zone of the Nori/RA prospect (Figs. 2 and 4). Polished thin sections were made from samples 06ns2145′A′ and 06ns2145′B′ and investigated using a transmitted and reflected-light polarising microscope. The chemical composition of tourmaline was determined using a JEOL 8900 electron microprobe at the University of Alberta, operated at 20 kV accelerating voltage and 20 nA probe current and focussed beam. In addition, a 10-µm spaced transect was completed across an individual grain to help constrain compositional zoning (Table 1). Peak and background count times were 20 and 10 s, respectively. For calibration, a set of microbeam standards (natural minerals) from the Smithsonian Institution were utilised (Jarosewich 2002). Data reduction was performed using the Φ(ρΖ) oxide correction of Armstrong (1995). The instrument calibration was deemed successful when the composition of internal standards was reproduced within the error margins defined by the counting statistics.

Fluid inclusions

Abundant fluid inclusions are hosted by both quartz and tourmaline. Microthermometric data were collected from sample 06ns2145′B′ at the University of Alberta. This was completed using a Linkam THMSG 600 heating and freezing stage calibrated at −56.6°C, 0.0°C, 374.1°C and 573.4°C using synthetic fluid inclusion standards. Reported temperature measurements have an accuracy of ±0.1°C on cooling runs, and for heating runs within ±1°C (Table 2).

Raman spectroscopy was carried out using a Renishaw inVia Reflex Raman spectrometer fitted with a CCD detector. The exciting source was an ionised Ar laser operating at 514 nm with an output power of ∼50 mW. Spectra were collected with a ×50 objective lens, providing ∼  mW at the sampling point. Calibration was performed daily using the 520 cm⁻¹ line for silicon.

Geochronology

Samples 06ns2145′A′ and 06ns2146′B′ have fine-grained (<0.5 mm), disseminated molybdenite throughout the groundmass (Figs. 2c and 4) and were selected for Re–Os molybdenite dating (Stein et al. 1998, 2001). Samples were crushed in a ceramic shatterbox and a molybdenite separate was prepared by gravity separation methods. Approximately 20–30 mg of molybdenite was produced from each sample, and the ¹⁸⁷Re and ¹⁸⁷Os concentrations in aliquots of molybdenite from these separates were determined by isotope dilution mass spectrometry at the University of Alberta Radiogenic Isotope Facility. Methods followed the analytical protocol of Selby and Creaser (2001a, b, 2004). The decay constant used for ¹⁸⁷Re was 1.666 × 10⁻¹¹ year⁻¹ (Smoliar et al. 1996). All uncertainties are reported at 2σ levels, and model ages include an uncertainty of 0.31% in the value of λ ¹⁸⁷Re.

Sample 06n2145′C′ is from the biotite alteration halo enveloping the molybdenite + uraninite-bearing tourmaline-biotite vein (Figs. 2c and 4b). The sample is composed of >80% biotite and was selected for laser ⁴⁰Ar–³⁹Ar dating. A portion of the sample was crushed, and biotite was hand-picked using a binocular microscope and irradiated at the McMaster Nuclear Reactor in Hamilton, ON. The irradiated samples and standards were placed in holes in an aluminium disc for loading and analysis in the mass spectrometer at the Argon Geochronology Laboratory, at the Department of Physics, University of Toronto. Samples and Hb-3gr standards (Turner et al. 1971; assigned age = 1,071 Ma) were analysed by step-heating with a 532 nm, frequency-doubled, Nd-YAG laser. After laser extraction, purified gas fractions were inlet to a VG 1200 noble-gas mass spectrometer equipped with an ion multiplier, and the isotopic ratios were measured. Data reduction included corrections for atmospheric contamination and Ca and Cl interferences from the irradiation (Smith et al. 1996). The J value for the irradiation was 1.0998 ± 0.0022 × 10⁻². Errors in ages of the individual step-heated gas fractions (which all have the same J value) do not include the error in J, while the errors of plateau ages do include the error in J. Individual step heating data errors are reported at ±1σ (Table 4), and plateau age errors are reported at ±2σ.