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Article

Pb-Isotopic Study of Galena by LA-Q-ICP-MS: Testing a New Methodology with Applications to Base-Metal Sulphide Deposits

by
Christopher R. M. McFarlane
*,
Azam Soltani Dehnavi
and
David R. Lentz
Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
*
Author to whom correspondence should be addressed.
Minerals 2016, 6(3), 96; https://doi.org/10.3390/min6030096
Submission received: 31 May 2016 / Revised: 26 August 2016 / Accepted: 2 September 2016 / Published: 15 September 2016
(This article belongs to the Special Issue Advances in Mineral Analytical Techniques)
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Abstract

:
In situ laser ablation quadrupole inductively coupled plasma mass spectrometry was used to measure Pb isotopes in galena. Data acquisition was optimized by adjusting spot size, energy density, and ablation time to obtain near steady-state low relative standard deviation (%RSD) signals. Standard-sample bracketing using in-house Broken Hill galena as external reference standard was used and offline data reduction was carried out using VizualAge for Iolite3. Using this methodology, galena grain in polished thin sections from selected massive sulphide deposits of the Bathurst Mining Camp, Canada, were tested and compared to previously published data. Absolute values and errors on the weighted mean of ~20 individual analyses from each sample compared favourably with whole-rock Pb-Pb isotope data. This approach provides a mean to obtain rapid, accurate, and moderately (0.1% 2σ) precise Pb isotope measurements in galena and is particularly well suited for exploratory or reconnaissance studies. Further refinement of this approach may be useful in exploration for volcanogenic massive sulphides deposits and might be a useful vectoring tool when complemented with other conventional exploration techniques.

1. Introduction

Application of in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in the realm of compositional and isotopic investigations of sulphide minerals has expanded in the last decade [1,2,3,4,5]. The initial Pb isotope composition of an ore fluid is best determined from Pb-rich minerals genetically associated with the mineralizing event, e.g., galena (initial U/Pb = 0) [6,7,8,9,10,11] as well as pyrite [4,5,12,13,14,15,16,17], sphalerite [18], chalcopyrite, and pyrrhotite [19]. In contrast to conventional whole-rock Pb isotope determination of sulphides, which suffers from laborious sample preparation procedures as well as mixing of sulphide phases of potentially disparate petrogenesis, micro-analytical techniques offer the advantage of better textural and intragranular control on the selection of target material. LA-ICP-MS developments continue to be made in terms of Pb-isotope determination [4,5,20]. For example, a comparison study by [20] demonstrated the feasibility of analysing Pb ores (galena) by LA sector-field (SF) ICP-MS. They measured Pb isotope ratios on NBS981 wire at the start, middle, and end of a laser ablation session and obtained errors on the mean of 0.4 on 206Pb/204Pb to 1.2% on 207Pb/204Pb. They also encountered significant bias between Pb isotope ratios measured by TIMS (thermal ionization mass spectrometry) and LA-SF-ICP-MS with both datasets corrected using NBS981 for mass bias correction, but no documentation of the effect of variable fluence on ablation behaviour was provided by [20]. To date, no investigation of Pb isotope analyses on galena using an optimized matrix-matched LA-quadrupole (Q) ICP-MS methodology has been conducted.
Isotope ratios measured by Q-ICP-MS have lower precision compared to multiple-collection (MC)-ICP-MS which can achieve precision <0.01%. However, if instrumental noise can be minimized and mass bias accurately corrected, single-collector SF-ICP-MS and Q-ICP-MS have been shown to produce reliable and precise Pb-isotope data [9,20]. Pooled weighted means for >20 individual measurements of moderate (~0.2 to 0.4% 2σ) precision can be used to lower errors even further. Thus, it is theoretically possible to obtain high-precision (≤0.1% 2σ) Pb-isotope ratios with LA Q-ICP-MS by targeting Pb-rich minerals and optimizing ablation and collection conditions to generate near-steady-state low %RSD signals. In massive sulphide deposits containing galena, a single polished thin section offers hundreds of potential targets, which, because of the expected homogeneity of Pb-isotopes at this scale, should yield identical isotope ratios. Therefore, the ultimate precision of pooled data is not limited by sample supply, but rather by instrumental conditions that minimize internal errors. In LA-Q-ICP-MS, the stability of the measured Pb ion beams is a complex function of processes occurring in system components starting at the site of ablation all the way to the ICP-MS detector: pulse-to-pulse laser energy stability, laminar versus turbulent flow in transfer tubing, efficiency of smoothing devices, torch flicker, constant interface region vacuum, ion optics tuning, and quadrupole mass filter settings. This contribution reports on our attempts to optimize these instrumental conditions with the ultimate objective of exploiting this technique for reconnaissance-level Pb-isotope measurements in galena-bearing ore deposits.
Some of the additional challenges involved in Pb-isotope measurement of galena (and other sulphides) in massive sulphide deposits are: (1) textural complexity and occurrence of Pb- and/or U-bearing inclusions and domains; (2) achieving controlled ablation condition with minimal laser-induced melting; and (3) standardization and choice of data reduction strategy (see [5]). Nevertheless, LA-ICP-MS analysis offers a number of advantages in Pb-isotopic studies of sulphides [5]. First, quadrupole ICP systems are capable of analysing minerals with a wide variety of Pb content due to the large dynamic range (typically nine orders of magnitude) of their detection system. Second, fast sequential measurement of analytes (e.g., <10 ms per mass) ensures that time-dependent variations in the ion beam can be captured on a <0.5 s timescale. The fast scanning quadrupole filter also allows a broader suite of analytes to be measured beyond just the Pb masses of interest facilitating identification of Pb-bearing micro-inclusions, such as zircon, monazite, and uraninite. Furthermore, the elemental imaging capability of this technique is a great advantage as it provides quick semi-quantitative data to be used in revealing textural complexities and best selection of single spot targets. Therefore, LA-Q-ICP-MS offers a promising technique to be invested in terms of developments in sulphide Pb-isotopes methodology that could be ultimately incorporated into high-precision LA MC-ICP-MS measurements.

2. Summary of the Bathurst Mining Camp Geology and Sampling

Middle Ordovician bimodal volcanic and sedimentary sequences are host to the volcanogenic massive sulphide deposits of the BMC, New Brunswick, Canada. The BMC contains 45 deposits and 95 occurrences, in which 25 deposits are more than 1 Mt, including the supergiant Brunswick No. 12, containing a pre-mining massive sulphide resource in excess of 290 Mt ([21] and references therein). The Bathurst Supergroup including Tetagouche, California Lake, and Sheephouse Brook groups, hosted the massive sulphides of the BMC [21]. The Bathurst Supergroup originally formed distinct rift-related basins within the greater Tetagouche-Exploits back-arc basin [21,22,23,24,25,26,27] and were subsequently tectonically juxtaposed in a west-dipping subduction complex. The BMC deposits have been affected by the Salinic Orogeny [22] and subsequent Acadian Orogeny [23,24,25,26,27]. Metamorphism varies from lower- to upper greenschist facies with peak metamorphic conditions from 325 to 400 °C and 6 to 7 kbars [28]. The simplified geological map of the Bathurst Mining Camp, northeastern New Brunswick, showing the location of massive sulphide deposits examined during this study (black circles), is presented in Figure 1 (modified from [26,29]). Samples for this study were selected from representative massive sulphide deposits of the BMC, specifically: Brunswick No. 6, Halfmile Lake Deep zone, Key Anacon East zone, Caribou, and Restigouche deposits.
In general, the massive sulphide samples of the BMC are fine-grained and texturally complex. Thus, targeting individual minerals for LA-Q-ICP-MS trace-element or isotope measurements necessitates crater diameters in the 20 (or less) to 70 µm range. The selected samples contain predominantly five minerals that comprise 95% of the rock: pyrite, sphalerite, galena, chalcopyrite, and pyrrhotite. Minor amounts of arsenopyrite, marcasite, and tetrahedrite-tennantite, are also present and sulphosalts occur at trace levels. In the VMS deposits of the BMC, galena occurs as blebs, veins, disseminations, and inclusions associated with other phases. Regional metamorphic effects and the relative softness and ductility of galena explain why it occurs interstitial to other phases in BMC deposits. The proportion of galena is higher in the bedded Zn-Pb sulphide facies than in the bedded pyrite and vent complex facies (see [21] and references therein) (Figure 2).

3. Methodology

3.1. LA-Q-ICP-MS Instrumentation

The LA Q-ICP-MS instrumentation at the Department of Earth Sciences, University of New Brunswick, Fredericton, Canada includes an ASI (formerly Resonetics) Resolution™ M-50 193 nm ArF (excimer) laser system (Canberra, Australia) synchronized to an Agilent 7700x quadrupole inductively coupled plasma mass spectrometer equipped with dual external rotary pumps. The system has been described in detail in a number of previous publications (e.g., [30,31]). Ablation is conducted into a two-volume low-volume Laurin Technic Pty S-155 sample cell capable of holding six 45 × 25 mm polished thin sections and five 25 mm round standard pucks. The cell is repeatedly evacuated and backfilled with He to remove traces of air from the cell after each sample exchange. Ablated aerosol is transported out of the cell using 300 mL/min He and 930 mL/min Ar that is carried to the ICP-MS in 4 mm OD Nylon™ tubing. A Laurin Technic Pty “Squid” smoothing device (Canberra, Australia) was placed mid-way between the ablation cell and the ICP-MS torch. No carry-over of Pb was observed from one ablation to the next or on a day-to-day timescale despite total Pb ion beams of > 1 × 108 cps.
The ICP-MS was operated at 1550 W and a torch depth of 4.0 mm. The ICP-MS was tuned, while rastering across NIST610 glass, to achieve ThO+/Th+ < 0.15% (monitor of oxide production), 238U+/232Th+~1.0 (monitor of plasma robustness), and 44Ca++/44Ca+ < 0.3% (monitor of double-charged production). This tuning step was also used to establish lens settings and carrier gas flows that yielded a raw 207Pb/206Pb for NIST610 that overlapped the accepted value (0.9095 ± 0.0005). The ICP-MS is equipped with a dual-mode discrete dynode detector with a pulse counting mode for low ion beam intensities (<2 × 106 cps) and an analogue mode for high-intensity signals. Thus the pulse-analogue factors (P/A) are set for each analyte in the acquisition at the start of every new sequence (or when detector voltages are adjusted) by rastering across NIST610 and varying the spot size and repetition rate to obtain ~2 × 106 cps for each analyte so that the cross-over between detector modes can be calibrated. In the case of galena analysed here, all Pb masses were recorded in analogue mode, whereas Ag, Hg, Tl, Th, and U were collected in pulse mode.

3.2. Ablation Condition

Ablation conditions were additionally optimized by trial-and-error for the acquisition of Pb isotopes of galena. Galena has a low melting point, so that a low and stable energy density setting is critical in order to avoid the effects of incomplete vaporization and ionization of large particles in the plasma [5,32,33]. In addition, the size of ablation crater is important with respect to improving the accuracy and precision, and for control downhole fractionation: craters with high diameter/depth ratio (e.g., >10) typically yield minimal fractionation and more steady-state signals. This near-steady-state signal ensures that the sequential counting of the relevant isotopes is measured on a time-invariant (or nearly so) ion beam.
The first round of Pb isotope data were obtained under the conditions of 23 µm diameter crater size, 25 s ablation with a laser pulse rate of 3 Hz, and a fluence of ~1 J/cm2 as measured using a hand-held energy meter above the ablation cell (Table 1). At least 20 ablations on galena per sample were obtained, although some that hit inclusions were discarded. This short ablation time on galena was based on the observation that after ~20 s the signal became very noisy, presumably because of zone heating, melting, and spalling of agglomerates. A background collection time of 60 s was used to allow for washout of the high-intensity Pb signal, so that accurate Hg and Pb background levels could be determined prior to the next ablation. Errors resulting from surface contamination due to polishing of sulphides are inevitable, but can be typically eliminated by pre-ablating spots with a single laser pulse that removes ~100 nm of material and allowing 10 s for washout. The isotopes measured in the first round included 200Hg, 201Hg, 202Hg, 203Tl, 204(Pb + Hg), 205Tl, 206Pb, 207Pb, 208Pb, 232Th, and 238U. Individual dwell times were set at 100 ms except for 208Pb (50 ms) and 232Th and 238U (20 ms). The total quadrupole sweep time per measurement (including detector dead-time and settling time) was 0.89 s. This extended list of analytes was chosen so that a full assessment of instrumental mass-bias was possible and an accurate 204Hg correction could be obtained. For example, the isotopes of Hg can be used to apply (if necessary) a mass-bias correction to the 202Hg/204Hg based on the measured/reference 200Hg/201Hg or 203Tl/205Tl ratio. Correcting for mass bias prior to 204Hg peak-stripping in this way should provide the most accurate net 204Pb ion beam measurement.
In the second round of analysis of the same samples, the ablation conditions were changed to 44 µm diameter crater size, 40 s ablation time, ~0.35 J/cm2 fluence and a laser pulse rate of 2 Hz (Table 1). The lower laser fluence conditions showed very controlled ablation and low %RSD signals for the Pb/Pb ratios. In this run, 203Tl, 238U and 232Th were not included because the results of the first analytical session showed that U and Th are sufficiently low in galena that they have no measurable effect on the Pb isotope ratios. Although Tl is a potentially important substituent in galena, the measured 203Tl/205Tl ratio was too imprecise for assessment of mass bias. The dwell times of the analytes were reduced to give a total quadrupole sweep time of 0.33 s. This is necessary to avoid the isotope ratios “pulsing” when 2 Hz laser frequency is used and increased the total number of measurements per ablation up to 120.
Finally, all of the samples were analysed using a 60 µm diameter crater size and 60 s ablation time, but with all other parameters kept constant as the second round (Table 1). Silver (107Ag) was additionally analyzed and added to the analyte list. Increasing the diameter of the ablation crater improved the errors even more as a result of larger ion beams measured with minimal loss of intensity over the duration of the ablation. In addition, ablation of galena with varying crater sizes (23, 44 and 60 µm), provides a good test of the reproducibility of the method and a plot of the error on weighted mean of Pb isotopic ratios versus crater size suggests that additional increases in the crater size and ablation time could help to incrementally decrease weighted mean errors to <0.1% 2σ (Figure 3). In addition, a plot comparing differences in ablation results for 23 vs. 60 µm diameter craters and 1 vs. 0.35 J/cm2 shows increases in sensitivity, more controlled ablation, lower %RSD signals, and consequently higher precision (Figure 4).

4. Standardization

The use of NIST glasses for external calibration of sulphide compositional data has been examined previously (e.g., [37,38]); however, a reference material with sulphide-based composition is preferred (e.g., [33,39,40]). Unfortunately, there has been only limited investigation of matrix matching issues in the context of Pb isotope analyses of sulphides (see [41]). Natural in-house pyrite standards were preferred by [5] rather than NIST glasses to provide a matrix-matched standardization approach. Recently, Pb isotopic ratios for the standard MASS-1 have been reported (GeoReM database); however, using MASS-1 is not a common approach in data reduction of Pb-isotope of sulphides. Examining MASS-1 during this study shows that the high Hg (compared to other targets) causes long washout for Hg (~5 min) that raises backgrounds and obscures low-level Hg signals during ablation of unknowns. Following [5], we standardized the galena Pb-isotope measurements relative to an in-house Broken Hill galena [42] to ensure matrix-matching. The reference values for Broken Hill galena were taken from [42]. Since the ablation was performed at low energy density, effects of matrix-mismatching are expected to be significant [5]. Thus, although MASS-1 and other glass standards (610 and GSE1-G) were analysed in the first session for comparison, the results displayed significant laser-induced biases that could not be corrected. NIST610 does not ablate under the laser fluence of 0.35 J·cm−2 used for the 44 and 60 µm laser crater diameter sessions.
Data were collected in 2 to 4 h long runs with standards evenly spaced after every ten unknowns to allow an accurate instrument drift correction. The measured raw Pb-Pb ratios on Broken Hill galena show no significant drift, although sensitivity did decrease over the course of the runs (based on the total ion beam measured on BH galena). The integrations for the standard (Broken Hill) and unknowns were inspected offline and the integration selected based on the lowest internal error (2 S.E.) on the 207Pb/204Pb ratio, which typically comprised all but a 2 s rise time at the start of the ablation. One of the challenges of Pb isotope measurement is inherent difficulties related to resolving the interference effect of 204Hg on the 204Pb signal at low concentration. In addition, Hg impurities in the carrier gases have been an issue for many laser-based ICP-MS laboratories [43]. At the University of New Brunswick, all carrier gas lines are fitted with dedicated high-capacity (up to 20 L/min) in-line Hg traps and maintenance and cleaning of the instrument is conducted on the routine basis. The 202Hg gas backgrounds remain <400 cps under the highest sensitivity conditions. In the analysed galena, the Hg occurs in low concentrations (up to 3.2 ppm, unpublished data from the LA-ICP-MS analysis on galena, BMC), so induced-Hg interference is not a significant issue.
Offline data reduction was conducted using Iolite3™ (http://www.iolite.org.au; [44]) running as a plugin in Wavemetrics IgorPro 6.32™ software (http://www.wavemetrics.com). Iolite facilitates visualization of ion beam intensities versus time and allows integration of specific portions of the ablated signal. Consequently, inclusions and surface related impurities can be avoided. Of the several data reduction schemes developed for Iolite, calculation of Pb isotopes was tested using two approaches: the VizualAge DRS [45] and Laser_Pb_Faraday. VizualAge applies a drift correction, but no downhole fractionation correction to the Pb-isotope ratios and uses an empirical correction, based on the measured/true values of the standard to correct the isotope ratios for instrumental mass bias. The Laser_Pb_Faraday approach is designed for multiple-collector ICP-MS instruments and uses a fractionation correction, based on the measured/true value on the standard and an assumed exponential fractionation that is a function of the mass difference between isotopes: k = (1/Δm)ln(Rm/Rt) where Δm is the mass difference and Rm and Rt are measured and true ratios, respectively. Using the background to calculate both 200Hg/202Hg and 201Hg/202Hg and assuming canonical values of 0.7736 and 0.4414, the mean exponential factor (k) for the first analytical session is 0.0009 ± 0.0030. This implies negligible instrumental mass bias over this mass range. A similar result was obtained for 203Tl/205Tl (canonical value = 0.4189) measured on MASS-1. The measured/true 203Tl/205Tl yielded a weighted mean of 1.000 ± 0.051, again implying negligible mass bias over this mass range. Finally, raw Pb-Pb values on Broken Hill galena are very close to the assumed true values. The measured 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 207Pb/206Pb values for Broken Hill were within ~2% of the true values, attesting to a small, but measureable mass bias for the Pb mass range under the analytical conditions employed. This mass bias varies from day-to-day and future work will involve an additional tuning step on Broken Hill galena to obtain ion lens settings that minimize mass bias. Thus, because of the small mass bias and matrix-matched nature of the external calibration, a simple empirical correction similar to that employed by VizualAge is preferred compared to an exponential-type mass bias correction.
Typically, the precision of quadrupole ICP systems in determination of Pb isotopes is low compared to multi-collector systems. This can be compensated, to some extent, by increasing the number of spot analysis and pooling the data, ensuring that the mean square weighted deviations (MSWD) on the weighted average (using the Isoplot software; [46]) remains below 1.3 [47]. To ensure fully propagated errors, at least 15 standards need to be run. Figure 5 shows probability density plots of the Pb isotopes of galena from the Halfmile Lake Deep zone. The measured Pb isotope ratios are demonstrably normally distributed justifying the use of weighted means for the populations. All errors in Table 1 are reported at the 95% confidence interval.

5. Results

The Pb isotope composition of galena (n = 308) from representative massive sulphide deposits of the BMC is summarized in Table 1 and the whole dataset in Supplementary Materials 1, along with reference values for the BMC VMS deposits based on the unpublished conventional whole-rock Pb-Pb data collected by the Geological Survey of Canada [34,35,36].
The data are reported herein as drift and externally corrected values relative to Broken Hill galena standard with fully propagated 2σ (95% CI) errors on the weighted means. Weighted means were calculated by Isoplot for the 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 207Pb/206Pb and 206Pb/208Pb ratios. The mean 202Hg ion beam in galena was <10 cps, whereas 207Pb was ~3.0 × 106 icps (integrated count per second). The data for the BMC samples show a range of Pb isotope ratios that are unique to each deposit, consistent with the previously documented array of Pb-isotope values recorded for the BMC. This array of values is distinguishable at both 44 µm and 60 µm crater diameters, with the latter having better precision. The results are plotted on conventional 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams, showing the variation of Pb isotopic ratios based on the [48] (Figure 6) and relative to the terrestrial Pb-isotope growth curve [49] and other terrestrial reservoirs from [48]. Error correlations between 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb are currently not reported by VizualAge, but estimated correlation coefficients of 0.0003 and 0.001, respectively, were calculated from each final corrected dataset. An error correlation of 0.1 was assumed for the conventional data for the BMC. The results of galena ablation with 44 µm crater sizes shows that galena from Restigouche yields the most radiogenic 207Pb/204Pb ratio (15.697), whereas galena from Key Anacon East zone yields the lowest 207Pb/204Pb ratio (15.595). The 208Pb/204Pb ratio is highest in galena from the Restigouche deposit (38.140) and the lowest in galena from Halfmile Lake Deep zone and Brunswick No. 6 (38.000). The 206Pb/204Pb results are highest in galena from Restigouche (18.320) and the lowest in galena from Brunswick No. 6 (18.123).

6. Discussion

6.1. Limits of Precision and Accuracy

In contrast to previous studies that attempted to analyse Pb isotopes in galena by LA-ICP-MS, we have shown that fluence >1 J·cm−2, repetitions rates >3 Hz, and <30 µm crater diameters are sub-optimal for controlled ablation of galena. Here, optimization at 44 and 60 µm diameter craters produced more accurate and precise data, with Pb-Pb data for all five samples falling within or just outside the main BMC Pb-Pb array, especially in 206Pb/204Pb versus 208Pb/204Pb space. Isotopic differences among deposits are resolvable and all data measured by LA-Q-ICP-MS clearly points to an upper crustal source of Pb. Ablation of galena samples at 60 µm diameter resulted in an error on the pooled data that is on par with conventional Pb-Pb data: 2σ errors of ~0.10% on 206Pb/204Pb and 207Pb/206Pb, ~0.13% on 207Pb/204Pb and 206Pb/208Pb, and ~0.18% on 208Pb/204Pb. The errors on 207Pb/204Pb and 208Pb/204Pb could be further improved by using better optimized dwell times for 206Pb, 207Pb, and 208Pb.
Whereas the measurements clearly fall within error of the regional BMC array, there remain inconsistencies between the measured and reference values on a deposit scale. Whereas data for the Halfmile Lake and Caribou deposits overlap within error the conventional values, we measured anomalously low 206Pb/204Pb for Brunswick No. 6 and Key Anacon East zone and anomalously high 206Pb/204Pb for Restigouche. The Brunswick No. 6 deposit also gave a lower than expected 208Pb/204Pb value. There was, however, a significant improvement of both the precision and accuracy of isotope ratios measured for the Caribou deposit using a 60 µm diameter crater. Thus, further testing at larger crater diameter (if possible given the grain size distribution) may help resolve some of these inconsistencies. It is nonetheless possible that there is a measureable difference between the conventional whole-rock data and the in situ data given the possibility of mixing between different Pb-isotope sources (e.g., host-rock volcaniclastic detritus vs. hydrothermal mineralization) using a bulk sampling approach. If these differences are still manifest once the LA-Q-ICP-MS measurements are further optimized, in situ testing using high precision LA-MC-ICP-MS may be warranted to resolve the different source components in these deposits. As discussed below, this approach could provide new insight into the VMS deposits exploration.

6.2. Exploration Implications

Pb isotopic studies have been applied as a powerful tool in exploration of metallogenic terrains ([6] and references therein). The Pb isotopic investigation of mineral systems is based on comparing the isotopic compositions of existing deposits within a district to prospective showings. In these situations, the Pb-isotope signature can assist in differentiating among mineralization types, as well as the scale of deposition [6,51,52]. In ancient mineralization occurrences, the difference between mean Pb isotopic compositions of ore and host rocks increased with time. Thus, in metamorphic terrains, Pb isotope signatures can potentially help in tracing footprints of major mineralization ([51,52] and references therein).
The microanalytical method described here provides rapid screening of galena, with levels of accuracy and precision to establish the Pb-isotopic composition of galena with respect to camp-scale Pb-Pb arrays. Although galena was the focus of this study, other Pb-bearing sulphides (pyrite, sphalerite, chalcopyrite) could also be targeted if grain sizes are sufficiently large to permit the use of large ablation craters to obtain >106 cps of 207Pb. This methodology can be applied in determination of Pb isotopic compositions of a wide range of sulphide phases occurring in the ore horizons, the associated host rocks, iron formation, and surficial materials. By assessing the background level of Pb isotopes of the main sulphide horizons, any systematic variation in Pb isotopes within the host rock or sediments can be potentially used as a vectoring tool in VMS exploration [53]. Variations in Ag/Tl or other trace metals in galena (e.g., Cd, In, and Te) analysed simultaneously with Pb isotopes could also reveal petrogenetically distinct pulses of galena mineralization. Hence, this approach provides a complementary technique to conventional Pb isotope methods and to other geochemical exploration tools applied to VMS deposits.
Foremost, this study has established important instrumental settings necessary to achieve the highest possible precision and accuracy on Pb-isotopes analysed in situ by laser ablation, independent of the ICP-MS platform. Critically, the use of low laser fluence, low laser pulse rate, and an expanded analyte list that includes isotopes of Ag, Hg, Tl, Pb, Th, and U are important features of the development of this method. Internal precision could be further improved by using larger laser craters (e.g., >60 µm) when galena grain size is amenable. Additional optimization could include measurement of key elemental discriminants, such as concentrations of Ag, Cd, Te, Tl, and Bi. Extension of this approach to in situ analysis using higher-sensitivity sector-field ICP-MS or to multiple-collector (MC)-ICP-MS equipped with an array of ion counters and Faraday detectors would be expected to reduce internal precision significantly, potentially to variations at the <0.1% level on individual spots and <0.05% on weighted means for pooled results. This level of precision is ideally required for more detailed Pb-isotope investigations, especially in samples where several textural generations of galena are present. Independent of the plasma-source platform; however, optimization of ablation conditions on galena is critically important in ensuring precise and accurate data.

7. Conclusions

The methodology outlined herein offers a robust technique for the evaluation of the Pb isotopes in galena. This approach allows rapid data acquisition and correction, with sufficient precision and accuracy to resolve Pb-isotope differences among a suite of massive sulphide deposits. In VMS deposits of the BMC, Pb isotopes are typified by radiogenic signatures related to derivation from an upper crust (including metasedimentary components) lead reservoir with minor mixing with a mantle or orogene source. This approach can be potentially applied as a reconnaissance tool for VMS exploration to identify sulphide mineralization that lies along regional Pb-Pb arrays associated with major deposits. Further refinement of the technique is required; however, results thus far suggest that accurate and relative precise Pb-isotope analysis by LA-Q-ICP-MS is now within reach.

Supplementary Materials

The following are available online at www.mdpi.com/2075-163X/6/3/96/s1.

Acknowledgments

We gratefully acknowledge the Geological Survey of Canada for funding this project through NRCan TGI-4 (Targeted Geoscience Initiative IV) program grant to David R. Lentz and Christopher R.M. McFarlane. We sincerely appreciate colleagues at the Geological Survey of Canada; particularly Jan Peter (VMS Ore System Science Leader) for his cooperation and advice. We also appreciate colleagues at New Brunswick Department of Energy and Resource Development in Bathurst, especially Jim Walker and Sean McClenaghan for assistance in locating previous dataset and samples. We are deeply thankful to Walker for English proofreading and suggestions on the first draft. We are grateful to Brandon Boucher at the final stage of the project, assisting in LA-ICP-MS laboratory at the University of New Brunswick. We acknowledge anonymous reviewers for their meticulous and constructive reviews of the draft manuscript.

Author Contributions

Christopher R.M. McFarlane designed and carried out the LA-ICP-MS analysis, data correction and developments of methodology, and contributed in interpretations and writing. Azam Soltani Dehnavi conducted the sample selection, petrography, and prepared the first manuscript. David R. Lentz provided samples and contributed in interpretations and writing.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Simplified geological map of the Bathurst Mining Camp, northeastern New Brunswick, showing the location of massive sulphide deposits examined during this study (black circles) (modified from [26,29]).
Figure 1. Simplified geological map of the Bathurst Mining Camp, northeastern New Brunswick, showing the location of massive sulphide deposits examined during this study (black circles) (modified from [26,29]).
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Figure 2. Photomicrographs (plane-polarized reflected light) of representative samples from massive sulphide deposits of the BMC. (a) A massive sulphide sample containing intergrowth of galena (Gn) with sphalerite (Sp). Sample is from drill hole 62-55-115 @ 35.2 m (Caribou deposit). (b) A bedded sulphide sample containing masses of sphalerite, showing intergrowth with pyrite (Py) and galena. Sample is from drill hole 94-DL-32 @ 402 m (Key Anacon East zone deposit).
Figure 2. Photomicrographs (plane-polarized reflected light) of representative samples from massive sulphide deposits of the BMC. (a) A massive sulphide sample containing intergrowth of galena (Gn) with sphalerite (Sp). Sample is from drill hole 62-55-115 @ 35.2 m (Caribou deposit). (b) A bedded sulphide sample containing masses of sphalerite, showing intergrowth with pyrite (Py) and galena. Sample is from drill hole 94-DL-32 @ 402 m (Key Anacon East zone deposit).
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Figure 3. Plot of the error on weighted mean of Pb isotopic ratios vs. crater size (sample 62-55-115, Caribou deposit).
Figure 3. Plot of the error on weighted mean of Pb isotopic ratios vs. crater size (sample 62-55-115, Caribou deposit).
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Figure 4. Comparison of results of different ablation condition of galena: (a) 23 µm, 2.5 Hz, and 1 J·cm−2; and (b) 60 µm, 2 Hz, and 0.35 J·cm−2.
Figure 4. Comparison of results of different ablation condition of galena: (a) 23 µm, 2.5 Hz, and 1 J·cm−2; and (b) 60 µm, 2 Hz, and 0.35 J·cm−2.
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Figure 5. Probability density plots of Pb isotope ratios of galena from the Caribou deposit. MSWD stands for Mean Square Weighted Deviations.
Figure 5. Probability density plots of Pb isotope ratios of galena from the Caribou deposit. MSWD stands for Mean Square Weighted Deviations.
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Figure 6. Plot of 206Pb/204Pb versus (a) 207Pb/204Pb and (b) 208Pb/204Pb of galena by LA-Q-ICP-MS from representative massive sulphide deposits of the BMC. The Pb isotopic ratios presented for BMC VMS (grey area) ([34,35,36] (R.I. Thorpe, unpublished data), [48]) and New Brunswick, Canada (NB pluton) [50]. The average Pb evolution curves for mantle (M), orogene (O), and upper crust (UC) are from [48]. Thick dashed lines present the ablation of galena under the 44 µm, 2 Hz, and 1 J·cm−2 condition. Thick solid lines present the ablation of galena under the 60 µm, 2 Hz, and 0.35 J·cm−2 condition. Dotted line presents the reference values of Pb isotopes of examined deposits for comparison ([34]).
Figure 6. Plot of 206Pb/204Pb versus (a) 207Pb/204Pb and (b) 208Pb/204Pb of galena by LA-Q-ICP-MS from representative massive sulphide deposits of the BMC. The Pb isotopic ratios presented for BMC VMS (grey area) ([34,35,36] (R.I. Thorpe, unpublished data), [48]) and New Brunswick, Canada (NB pluton) [50]. The average Pb evolution curves for mantle (M), orogene (O), and upper crust (UC) are from [48]. Thick dashed lines present the ablation of galena under the 44 µm, 2 Hz, and 1 J·cm−2 condition. Thick solid lines present the ablation of galena under the 60 µm, 2 Hz, and 0.35 J·cm−2 condition. Dotted line presents the reference values of Pb isotopes of examined deposits for comparison ([34]).
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Table
Table 1. LAICP-MS Pb isotope ratios (all data) for galena from representative massive sulphide deposits of the BMC.
Table 1. LAICP-MS Pb isotope ratios (all data) for galena from representative massive sulphide deposits of the BMC.
No.DepositDrill Hole206Pb/204Pb2σ207Pb/204Pb2σ208P/204Pb2σ207Pb/206Pb2σ206Pb/208Pb2σ
Ablation condition: 23 µm, 3 Hz, 1 J·cm−2, 25 s ablation duration
n = 20Caribou62-55-11518.2940.04415.7380.04438.2700.1000.85930.00220.47830.001
n = 19Key Anacon East zone94-DL-3218.2430.04015.7250.04338.3200.1200.86230.00200.47690.001
n = 20Brunswick No. 6B-25918.1550.03815.6280.04338.0600.1100.86270.00200.47620.001
n = 20RestigoucheCP39-12318.1810.03715.6800.03738.0900.1000.86210.00210.47710.001
n = 20Halfmile Lake Deep zoneHN-11918.1570.04415.5680.04538.0700.1100.86140.00230.47650.001
Ablation condition: 44 µm, 2 Hz, 0.35 J·cm−2, 40 s ablation duration
n = 18Caribou62-55-115 18.1930.03815.6630.03338.1200.1000.85720.00150.47870.001
n = 20Key Anacon East zone94-DL-3218.1560.02615.5950.03038.0820.0970.85990.00150.47730.001
n = 20Brunswick No. 6B-25918.1230.03115.6680.03338.0000.1000.86050.00160.47930.001
n = 19RestigoucheCP39-12318.3200.03015.6970.03338.1400.1000.85860.00150.47840.001
n = 9Halfmile Lake Deep zoneHN-99-11918.1700.04215.6110.04838.0000.1400.85940.00230.47940.002
Ablation condition: 60 µm, 2 Hz, 0.35 J·cm−2, 60 s ablation duration
n = 23Caribou62-55-11518.2640.01815.6370.02138.1700.0680.85630.00100.47820.001
n = 20Caribou62-55-11518.2670.01915.6450.01638.1570.0460.85660.000630.47830.001
n = 20Key Anacon East zone94-DL-3218.1900.01615.6340.01538.1300.0590.85890.000730.47650.001
n = 20Brunswick No. 6B-25918.2030.01715.6530.01638.0700.0470.86030.000620.47810.001
n = 20RestigoucheCP39-123 18.2470.01715.6730.01638.1850.0470.85860.000650.47860.001
n = 20Halfmile Lake Deep zoneHN-11918.2030.01715.6610.01738.1350.0480.86060.000630.47630.001
Reference values [34,35]
Brunswick No. 12 18.1870.01815.6490.01638.100.04
Austin Brook 18.2470.01815.6580.01638.140.04
Heath Steele A1 18.1990.01815.6520.01638.120.04
Stratmat 18.2070.01815.6410.01638.120.04
Wedge 18.2750.01815.6620.01638.200.04
Armstrong A 18.2870.01815.6550.01638.170.04
Rocky Turn 18.3190.01815.6680.01638.270.04
Orvan Brook 18.3050.01815.6530.01638.200.04
Murry Brook 18.3090.01815.6690.01638.220.04
Chester 18.3020.01815.6590.01638.220.04
Strachans Lake BK. 18.2230.01815.6490.01638.190.04
Canoe Landing Lake 18.2430.01815.6530.01638.190.04
Willett Showing 18.2950.01815.6510.01638.190.04
Armstrong B 18.2980.01815.6600.01638.230.04
Portage Lakes 18.2300.01815.6470.01638.140.04
Captain 18.2790.01815.6570.01638.160.04
Brunswick No. 6 18.2770.01815.6630.01638.220.04
Key Anacon 18.2470.01815.6580.01638.140.04
Caribou 18.2760.01815.6540.01638.190.04
Halfmile Lake 18.2170.01815.6590.01638.150.04
Restigouche 18.2410.01815.6480.01638.160.04
Reference values [36]
Wedge 18.2810.00715.7250.0138.1590.01
Heath Steele 18.2290.00715.6580.0138.130.01
Brunswick 12 18.2370.00715.6710.0138.2030.01
Restigouche 18.2570.00715.70.0138.1970.01

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McFarlane, C.R.M.; Soltani Dehnavi, A.; Lentz, D.R. Pb-Isotopic Study of Galena by LA-Q-ICP-MS: Testing a New Methodology with Applications to Base-Metal Sulphide Deposits. Minerals 2016, 6, 96. https://doi.org/10.3390/min6030096

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McFarlane CRM, Soltani Dehnavi A, Lentz DR. Pb-Isotopic Study of Galena by LA-Q-ICP-MS: Testing a New Methodology with Applications to Base-Metal Sulphide Deposits. Minerals. 2016; 6(3):96. https://doi.org/10.3390/min6030096

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McFarlane, Christopher R. M., Azam Soltani Dehnavi, and David R. Lentz. 2016. "Pb-Isotopic Study of Galena by LA-Q-ICP-MS: Testing a New Methodology with Applications to Base-Metal Sulphide Deposits" Minerals 6, no. 3: 96. https://doi.org/10.3390/min6030096

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