Halfmile Lake Research Articles

Chlorite-White Mica Pairs’ Composition as a Micro-Chemical Guide to Fingerprint Massive Sulfide Deposits of the Bathurst Mining Camp, Canada

The compositions of phyllosilicates, with a focus on fluid-mobile elements, were evaluated as a means to fingerprint the Middle Ordovician metamorphosed (greenschist facies) volcanogenic massive sulfide deposits of the Bathurst Mining Camp (BMC), Canada. Ninety-five drill-core samples from six of the major deposits of the Bathurst Mining Camp (Brunswick No. 12, Heath Steele B zone, Halfmile Lake Deep zone, Key Anacon East zone, Louvicourt, and Restigouche) were analyzed using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. Typically, phyllosilicates (chlorite, white mica, and to a lesser extent biotite) are ubiquitous phases in the host rocks of the massive sulfide deposits of the BMC. Electron microprobe analysis results show a wide compositional variation in chlorite and white mica. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis was performed to measure fluid-mobile elements, showing that Tl is distinctly enriched in all white mica (up to 719 ppm) relative to chlorite (up to 50.1 ppm). Chlorite hosts Sn (up to 4600 ppm), Hg (up to 7.3 ppm), Sb (up to 35.4 ppm), As (up to 1320 ppm), In (up to 307 ppm), Cd (up to 83.2 ppm), and Se (up to 606 ppm). White mica hosts Sn (up to 1316 ppm), Hg (up to 93 ppm), Sb (up to 1630 ppm), As (up to 14,800 ppm), In (up to 1186 ppm), Cd (up to 98 ppm), and Se (up to 38.8 ppm). Limited LA-ICP-MS analysis on biotite indicates a higher overall concentration of Tl (mean = 14.6 ppm) relative to co-existing white mica (mean = 2.18 ppm). On average, biotite is also more enriched in Hg, Sn, and Ba relative to chlorite and white mica. Laser Ablation ICP-MS profiles of chlorite, white mica, and biotite demonstrate smooth time-dependent variations diagnostic of structural substitution of these elements. Compositional variation of chlorite-white mica pairs presented in the current study shows systematic variations as a function of distance from the mineralized horizons. This highlights the potential to use trace-element signatures in these phyllosilicate pairs to identify proximal (chlorite) and distal (white mica) footprints for volcanogenic massive sulfides exploration.

Reprocessing legacy three-dimensional seismic data from the Halfmile Lake and Brunswick No. 6 volcanogenic massive sulphide deposits, New Brunswick, Canada

We reprocessed legacy three-dimensional (3D) seismic data from the Halfmile Lake and Brunswick areas, both of which were acquired for mineral exploration in the Bathurst Mining Camp, New Brunswick. Each 3D seismic survey was acquired over known volcanogenic massive sulphide deposits and covered areas with strong mineral potential. Most improvements resulted from a reduction of coherent and random noise on prestack gathers and from an improved velocity model, combined with re-imaging with dip moveout corrections and poststack migration or prestack time migration. At Halfmile Lake, the new imaging results show the Deep zone and a possible extension of the sulphide mineralization at greater depth. True amplitude processing has shown that this anomaly has strong amplitudes and is offset from the Deep zone by a shallowly dipping fault (<15°). With the clearer geological context provided by our results, this anomaly, which appears as a stand-alone anomaly on an original image obtained by Noranda Exploration Ltd., becomes a defendable exploration target. Nonorthogonal acquisition geometry and receiver patches of the Brunswick No. 6 3D seismic survey generated artefacts after dip moveout processing that reduced the overall quality of the seismic volumes. By using a filtering approach based on the application of a weighted Laplacian-Gaussian filter in the Kx–Ky domain, we reduced the noise and improved the continuity of reflections. We also imaged the short and flat reflections observed previously only in the shallow part of prestack time migrated data. These short reflections appear as diffractions on the filtered stacked section with dip moveout corrections, indicating that they originate from small geological bodies or discontinuities in the subsurface.

Application of portable XRF to the direct analysis of till samples from various deposit types in Canada

In this study, results by direct portable XRF (‘pXRF’) on unsieved till samples were compared with those by established laboratory methods (aqua regia or fusion ICP-MS and ICP-ES) on the <0.063-mm fraction to determine if the application of direct pXRF in the field would serve as an acceptable guide for immediate follow-up work. Four test sites in Canada were chosen: the Halfmile Lake Cu-Pb-Zn VMS deposit; the intrusion-hosted W-Mo Sisson deposit; a Pb-Zn Mississippi Valley–type (MVT) deposit in the Pine Point district; and the Triple B kimberlite. Unsieved till samples from the GSC archive collection were used for this study and included samples from background areas, immediately overlying, and at various distances down-ice of each deposit. Ziploc® and Whirl-Pak® bags that were used to contain the samples in the field were tested for their properties of X-ray attenuation and contamination. In general, the performance of pXRF in the four test areas was very good where concentrations of elements of interest (indicator or pathfinder elements) were substantially above detection limits by this technique (in the low ppm range for many elements). The following elements, shown to be useful indicator elements (important constituents of the ore/commodity) or pathfinder elements (those associated with the commodity elements) by the established methodology, showed similar patterns by pXRF on the unsieved material: Zn, Cu, Pb, and As at Halfmile Lake; W, Mo, Cu, Zn, Pb, and As at the Sisson deposit; Zn, Pb, and Fe at Pine Point; and Ca, Sr, Cr, and Ni at Triple B. Pathfinder elements whose concentrations were too low for determination by pXRF include: Ag and Sb at Halfmile Lake; Ag and Cd at Sisson; Cd, S, and Se at Pine Point; and Co, Mg, P, U, and Th at Triple B. The high background for Bi by pXRF, equivalent to c . 50 ppm, and its noisy signal precluded its use at Halfmile Lake and Sisson. Elements which tended to show poor precision (three analyses each sample) by pXRF in some samples due to sample heterogeneity include Sn, V, and W. Mercury was erroneously reported for the majority of samples in the low ppm range by pXRF whereas its concentration in fact was in the low ppb range. Several Pb-, Zn- ( c . 1% Pb, Zn) and Fe-rich (up to 16% Fe) samples demonstrated spectral interferences by: Pb on As, Th and Se; Zn on Cu; and Fe on Co. Results for six till samples analysed in Ziploc® and Whirl-Pak® bags showed that Ziploc® absorbs fewer low-energy photons and hence is preferable for determining light elements such as Si, K and Ca. Supplementary material: Four data-sets are available at http://www.geolsoc.org.uk/SUP18897

Principal component analysis of magnetite composition from volcanogenic massive sulfide deposits: Case studies from the Izok Lake (Nunavut, Canada) and Halfmile Lake (New Brunswick, Canada) deposits

Magnetite grains from the Izok Lake (Nunavut, Canada) and the Halfmile Lake (New Brunswick, Canada) volcanogenic massive sulfide deposits, and from till covering the nearby areas were investigated using the scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and optical microscopy. The method of robust estimation for compositional data (rob-composition) was applied to censored geochemical data, and the results were analyzed by principal component analysis (PCA). Textural relationships and mineral association of magnetite reveal the history of formation, and contribute to the explanation of characteristic compositional differences of magnetite from different geological settings. The integration of petrography and mineral chemistry allows discriminating magmatic, metamorphic and hydrothermal magnetite grains in the VMS deposits bedrock samples. Magmatic magnetite is found in Izok Lake gabbro, and Halfmile Lake syenite, felsic ash tuff and gossan samples, whereas magnetite in Izok Lake massive sulfides, gahnite-rich dacite and iron formations formed during the amphibolite facies metamorphism. In Halfmile Lake andesite, magnetite recrystallized during greenschist facies metamorphism. In the magnetite alteration zone associated to the Halfmile Lake deposit, hydrothermal magnetite has been overprinted by metamorphic magnetite. Halfmile Lake massive sulfides in chloritic argillite contain hydrothermal magnetite.PCA identifies discriminator elements and their contributions to magnetite composition from different Izok Lake Lake and Halfmile Lake bedrock samples. The results suggest that Si, Ca, Zr, Al, Ga, Mn, Mg, Ti, Zn, Co, Ni and Cr are discriminator elements for VMS deposits and their host bedrocks. The distinct chemical signatures for magnetite from various bedrock lithologies demonstrate that magnetite grains of the same origin share more similarities in chemistry, as high Ti indicates magmatic sources for magnetite, whereas high Si, Ca and Mg are indicative of hydrothermal settings. Variable compositions of metamorphic magnetite suggest that the chemistry of this type of magnetite is controlled by the composition of host rocks, the grade of metamorphism and oxygen fugacity. PCA of EPMA and LA-ICP-MS data of magnetite from the Izok Lake and Halfmile Lake bedrock samples yield discrimination models for classification of magnetite grains from till. Decreases in the proportion of magnetite grains with the chemical signature of the Izok Lake massive sulfides and gahnite-rich dacite down-ice from the Izok Lake deposit show the use of magnetite chemistry in geochemical exploration. In the Halfmile Lake area, till magnetite grains with the signature of VMS mineralization make a glacial dispersal train more than 2km down-ice from the deposit.

3D seismic reflection imaging of volcanic-hosted massive sulfide deposits: Insights from reprocessing Halfmile Lake data, New Brunswick, Canada

Three-dimensional seismic reflection data from the Halfmile Lake area, New Brunswick, Canada, was reprocessed over an 18- [Formula: see text] grid to improve the seismic signatures of a 5-million-ton volcanic-hosted massive sulfide (VHMS) deposit located at 1200-m depth, known as the deep zone, as well as key host-rock structures. We chose a prestack dip moveout (DMO) and poststack migration processing sequence to preserve the possible diffraction signature of the deep VHMS zone. Despite the high level of source-generated noise and large statics caused by near-surface conditions, our processing results revealed improved 3D seismic images for shallow and deep structures. Many of the imaged structures were easily correlated with known lithological con-tacts constrained by boreholes and petrophysical measurements. A short, flat-lying segment of high-amplitude reflection at about 800-m depth in the unmigrated cube was interpreted to originate from a small portion of the lower VHMS zone. The DMO stack was characterized by a large, high-amplitude asymmetric diffraction signature originating from the deep VHMS zone. The asymmetry of the diffraction hyperbola relative to the location of the deep zone was interpreted as resulting from a shape effect from the zone, with the strongest amplitudes along the diffraction hyperbola found north-northwest of the apex. This indicated that the deep VHMS zone dips in a similar direction. This diagnostic diffraction signature was not preserved with the prestack migration approach previously implemented for processing Halfmile Lake data.

Lithogeochemistry and Hydrothermal Alteration at the Halfmile Lake South Deep Zone, a Volcanic-Hosted Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick

The Halfmile Lake South Deep zone, Bathurst Mining Camp, New Brunswick, was discovered by Noranda Inc. (Exploration) as a result of a 3-D seismic survey in 1998 and the subsequent drilling of ten diamond-drill cores. The deposit consists of massive, breccia, and stockwork Pb-Zn-Cu sulfide minerals, and is hosted by a volcano-sedimentary sequence belonging to the overturned Ordovician Tetagouche Group. Epiclastic rocks and interbedded fine-grained felsic pyroclastic rocks dominate the stratigraphic footwall. Locally, crystal-rich felsic tuffs and subordinate epiclastic rocks comprise the immediate stratigraphic hanging wall. The entire stratigraphic sequence was intruded by quartz-feldspar porphyritic intrusions, and cut by intermediate and basic dikes. The volcanic and sedimentary rocks can be discriminated geochemically using trace element ratios such as Zr/TiO2 and Nb/TiO2, despite intensive alteration and cleavage development. These ratios indicate that four protolith volcanic compositions exist: rhyolite, dacite, andesite, and basalt. The aphyric and feldspar- and quartzphyric volcanic rocks are dacitic and rhyolitic in composition; epiclastic rocks have trace element ratios consistent with dacitic compositions. Pearce element ratio diagrams, general element ratio diagrams, and petrographic observations demonstrate that the rhyolitic volcanic rocks exhibit evidence of albite, potassium feldspar, and quartz fractionation, dacitic volcanic rocks exhibit little evidence of any fractionation, and epiclastic sedimentary rocks exhibit evidence of quartz sorting only. Hydrothermal alteration is best represented by the presence of phengitic muscovite and daphnitic chlorite. Minor calcite occurs in the stratigraphic hanging wall, deep in the stratigraphic footwall, and in post-mineralization dikes, and thus is not interpreted to be part of the causative hydrothermal event. Element additions and losses during alteration have been used to determine net alteration reactions, and these have been used to identify alteration parameters that are independent of other forms of alteration and fractionation/sorting, and which can be used to guide exploration. These parameters include: a bulk hydrolysis measure, (2Ca+Na+K–2CO2)/Al; a muscovite alteration measure, K/Al; an albite destruction measure, Na/Al; a chlorite alteration measure, (Fe+Mg–S/2)/Al; a chlorite composition measure, (Fe–S/2)/Mg; a sulfidization measure, S/Ti; and a carbonatization measure, CO2/Ti. With the exception of the carbonatization measure, these alteration parameters define a distinct lateral and vertical zone of intense hydrothermal alteration in the stratigraphic footwall of the deposit, and demonstrate that the epiclastic rocks are predominantly chlorite altered, and the volcanic rocks are predominantly muscovite altered. Within the alteration halo, element additions of Fe and H, and element losses of Na are characteristic. Potassium was added to muscovite-altered rocks, but subsequently removed from chlorite-altered rocks. The results from this study demonstrate that metamorphism and deformation have not significantly obscured hydrothermal alteration signatures.

Stable isotope geochemistry of ground and surface waters associated with undisturbed massive sulfide deposits; constraints on origin of waters and water–rock reactions

Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ 18O VSMOW and δ 2H VSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ 18O VSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ 18O VSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ 13C VPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO 2 and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ 13C VPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ 13C VPDB), indicating carbon isotopic fractionation from methanogenesis via CO 2 reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH 4-degassing during sampling.

Mobility and fractionation of rare earth elements during supergene weathering and gossan formation and chemical modification of massive sulfide gossan

Primary massive sulfide gossans (MSG) in the Bathurst Mining Camp (BMC), New Brunswick, Canada, are characterized by relative enrichment of Au, Sb, and As, formation of jarosite group minerals (jarosite, plumbojarosite, and argentojarosite) and little or no fractionation in the rare earth elements (REE), including preservation of large positive Eu anomalies (average [Eu/Eu*] NASC = 4.14 in MSG; 6.61 in massive sulfide mineralization; 0.60 in host rocks). The chemical and mineralogical characteristics of MSG (e.g., Halfmile Lake deposit) imply low pH (<3) and relatively oxidizing conditions during gossan formation; oxidation of a volcanogenic massive sulfide body (comprising pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite) with a falling water table. The lack of light REE or heavy REE fractionation and preservation of positive Eu anomalies characteristic of the original (465 Ma) hydrothermal fluid is consistent with relatively large water–rock ratios during massive sulfide mineralization oxidation, and removal of the REE predominantly as sulfate complexes (LnSO 4 +, Ln(SO 4) 2 −). Low pH groundwaters recovered from past producing mines in the BMC display REE patterns reflecting those inferred to have occurred during gossan formation. Gossan at the Restigouche deposit, in contrast to the Halfmile Lake deposit, displays mineralogical and chemical evidence for having been chemically reworked since primary gossan formation. Evidence for chemical reworking includes loss of primary massive sulfide mineralization textures, replacement of plumbojarosite with anglesite, almost complete removal of jarosite minerals, loss of Au, Sb, and As and apparent preferential removal of Eu, resulting in loss of positive Eu anomalies for most samples (average [Eu/Eu*] NASC = 1.21 in the gossan, with many displaying strong negative anomalies; 3.65 in massive sulfide mineralization; 0.54 in host rocks). Based on geochemical modeling, conditions inferred for the chemical reworking of the Restigouche deposit include near neutral conditions and either relatively oxidizing conditions with Eu 2+ hosted in a preferentially weathered mineral host (possibly through substitution for Pb in plumbojarosite and beudantite) or cycling between reduced and oxidized conditions during gossan reworking.

Downhole seismic imaging of a massive sulfide orebody with mode‐converted waves, Halfmile lake, New Brunswick, Canada

Multioffset, multiazimuth downhole seismic data were acquired at Halfmile lake, New Brunswick, to image known massive sulfide lenses and to investigate the potential existence of a steeply dipping mineralized zone connecting them. The massive sulfide lenses, which have significantly higher elastic impedances than host rocks, produce strong scattering. The downhole seismic data show prominent scattered (P‐P and S‐S) and mode‐converted (P‐S and S‐P) waves originating from the deposit. Such complex scattering from massive sulfide ore was not observed previously in vertical seismic profiling data. Migration of the scattered and mode‐converted waves from several shot points imaged different parts of the deepest lens. The scattered S‐waves and mode‐converted waves provide additional imaging capabilities that should be considered when selecting downhole seismic methods for mining exploration.

Sulphide oxidation and groundwater transport of base metals at the Halfmile Lake and Restigouche Zn–Pb massive sulphide deposits, Bathurst Mining Camp, New Brunswick

A detailed hydrogeochemical study at the undisturbed Halfmile Lake and Restigouche Zn–Pb deposits, Bathurst Mining Camp, New Brunswick, was initiated to elucidate processes controlling the oxidation of sulphide minerals, factors influencing base-metal dispersion in ground and surface waters, and to improve hydrogeochemical exploration methods. Groundwaters were collected using flow-through bailer and straddle-packer technologies; the latter of which proved to be the most effective under the fracture-flow dominated conditions. Groundwaters from the two deposits are compositionally distinct. At the Halfmile Lake deposit, groundwaters are dominated by low total dissolved solids (TDS &lt; 250 mg l −1 ) and Ca–HCO 3 -type compositions. Groundwaters at the Restigouche deposit vary in composition from shallow Ca–HCO 3 - and Ca–SO 4 -type waters to high TDS Na–Cl-type waters. Elevated sulphate and base metal contents (up to 4200 μg l −1 Zn and 1400 μg l −1 Pb) of shallow Ca–SO 4 -type groundwaters proximal to massive sulphides indicate that the sulphides are undergoing natural oxidation. The degree to which the massive sulphides are oxidized depends on the depth of penetration of dissolved oxygen and flux rates, which are controlled by the permeability of the rocks and local hydrological gradients. Transport of metals away from massive sulphides depends on physical factors that include hydrology, groundwater flow, and the degree of connectivity between groundwater and surface water environments, as well as groundwater pH, metal adsorption and colloid mobility.

MINERALOGY AND GEOCHEMISTRY OF SUSPENDED SEDIMENTS FROM GROUNDWATERS ASSOCIATED WITH UNDISTURBED Zn Pb MASSIVE SULFIDE DEPOSITS, BATHURST MINING CAMP, NEW BRUNSWICK, CANADA

Suspended sediments from groundwaters were recovered from two undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp, New Brunswick, Canada. Suspended sediments in most groundwaters are dominated by quartz and aluminum silicate minerals. Fe oxyhydroxides are probably present as surface coatings in addition to amorphous phases. Suspended sediments from groundwaters recovered from boreholes that intersect the Restigouche orebody near its surface expression consist primarily of Fe, Pb, and Zn sulfate and Fe sulfide minerals. In general, suspended sediments are depleted in the mobile elements K, Rb and Cs compared to host rock-types from both deposits. Suspended sediments show generally flat element-enrichment patterns normalized to respective felsic host-rocks for most other elements, but exhibit strong enrichments in Pb, Fe, Co, Zn, Ag, and Eu. Iron-oxide-rich suspended sediments from a borehole that penetrates massive sulfide gossan at the Halfmile Lake deposit are more strongly depleted in most elements relative to other sediments. At the Restigouche deposit, the degree of enrichment in Pb, Zn, Cu, and Ag in the suspended sediment is greater than for the deeper Halfmile Lake deposit. Compared to coexisting groundwaters, suspended sediments are relatively enriched in Al, Fe, Pb, and relatively depleted in Ca, Sr, and Mg. Potassium and Rb are generally more strongly fractionated into the suspended sediment phase than other mobile elements, suggesting that their abundance in the groundwaters is controlled by secondary phases (illite – white mica), consistent with SEM and XRD data. Comparisons of groundwaters, suspended material and host rocks yield important information regarding relative fractionation of major and trace elements among these reservoirs. Suspended sediments proximal to massive sulfide ore have elevated Fe/Ti and base-metal contents; with increasing distance away from mineralization, Al/(Al + Fe + Mn) values increase, base-metal contents decrease, and Zn/Pb values increase. The elevated base-metal content of suspended sediments associated with massive sulfides is important, as mining activities in the Bathurst mining camp can result in changes in groundwater Eh–pH conditions, potentially resulting in greater dispersion of metals that are presently strongly sorbed onto sediments.

Physical properties and seismic imaging of massive sulfides

Laboratory studies show that the acoustic impedances of massive sulfides can be predicted from the physical properties ([Formula: see text], density) and modal abundances of common sulfide minerals using simple mixing relations. Most sulfides have significantly higher impedances than silicate rocks, implying that seismic reflection techniques can be used directly for base metals exploration, provided the deposits meet the geometric constraints required for detection. To test this concept, a series of 1-, 2-, and 3-D seismic experiments were conducted to image known ore bodies in central and eastern Canada. In one recent test, conducted at the Halfmile Lake copper‐nickel deposit in the Bathurst camp, laboratory measurements on representative samples of ore and country rock demonstrated that the ores should make strong reflectors at the site, while velocity and density logging confirmed that these reflectors should persist at formation scales. These predictions have been confirmed by the detection of strong reflections from the deposit using vertical seismic profiling and 2-D multichannel seismic imaging techniques.

Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn–Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*] NASC as low as 0.08), produced by oxidation of Ce 3+ to Ce 4+ and preferential removal of Ce 4+ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.

Hydrogeochemical, isotopic, and rare earth element evidence for contrasting water–rock interactions at two undisturbed Zn–Pb massive sulphide deposits, Bathurst Mining Camp, N.B., Canada

A detailed hydrochemical study at two undisturbed Ordovician massive sulphide (Zn–Pb) deposits (Restigouche and Halfmile Lake deposits, Bathurst Mining Camp, New Brunswick) was initiated to elucidate the processes controlling the oxidation and dissolution of sulphide minerals, the subsequent dispersion of metals in ground and surface waters, and the precipitation of secondary minerals. Groundwater hydrogeochemical signatures are different for the two deposits. Elements that form sulphide minerals in the deposits correlate positively in surface waters and increase with proximity to the Restigouche deposit, whereas metal contents for Halfmile Lake surface waters are lower, though still typically higher than background levels. Variations in the composition of ground and surface waters draining the two deposits reflect in part differences in the depth and geometry of the massive sulphides and host lithologies. The Halfmile Lake deposit is deeper, more steeply dipping, and overturned compared to the shallower Restigouche deposit. Groundwaters at the Halfmile Lake deposit up to 760 m below surface are low-TDS (<250 mg l −1), Ca–HCO 3 to Na–HCO 3 waters with oxygen and hydrogen isotopic compositions identical to surface waters. Groundwater REE patterns are flat to LREE-enriched and are similar to host lithologies, unlike the surface waters which are LREE-depleted compared to shale. The Restigouche groundwaters range up to 21,000 mg l −1 TDS (Na–Cl waters) and display heavier oxygen and hydrogen isotopic compositions than local surface waters. Variations in groundwater composition require that the two deposits have different hydrologies which influence the natural oxidation of the massive sulphides and thus the environmental hydrochemical signature.