Alain Plouffe

Comparison of Cu–Hg–Ni–Pb concentrations in soils adjacent to anthropogenic point sources: examples from four Canadian sites

The extent and variation of Cu, Hg, Ni, and Pb loading in soil profiles (humus, B-horizon, C-horizon) were examined in the vicinity of four Canadian industrial sources of airborne metal particulates located in different ecozones and geological terrains: the Cu smelter at Rouyn-Noranda, Québec; the Cu-Zn smelter at Flin Flon, Manitoba; the Pb–Zn smelter at Trail, British Columbia; and the inactive Pinchi Lake Hg Mine, British Columbia. Three major controlling factors on the metal concentrations in soils have been assessed: (1) distance from the anthropogenic point source, (2) organic matter content, and (3) geology of the substrate. Distance from source largely controls smelter-related metal concentrations in humus, with concentrations decreasing with increasing distance according to specific parameters for each element and at each location. In the B-horizon, variations in organic matter content and substrate geology are important controls on metal concentrations, except in areas close to the source where sub-surface contamination was recognized for certain metals. The metal content of the C-horizon is predominantly determined by the nature and composition of the substrate. Variations in humus/C-horizon and B-horizon/C-horizon metal ratios are useful to help distinguish anthropogenic from natural sources of metal enrichments in soils.

Pb and S isotopic composition of indicator minerals in glacial sediments from NW Alberta, Canada: implications for Zn-Pb base metal exploration

ABSTRACT Results from an indicator mineral survey conducted with till samples in NW Alberta were used to identify a glacial dispersal train of sand-sized sphalerite and minor galena. High concentrations of dark grey to black, angular, brittle grains of sphalerite were found (100 to >1000 grains) in nine of ninety 30 kg till samples. The presence of high sphalerite grain counts in nine samples situated within a geographically restricted area argues against long-distance glacial transport, comminution, and deposition of erratic material from the carbonate-hosted Pine Point Zn-Pb deposits, located 330 km to the NE. The Pb isotopic composition of the galena grains recovered from till is similar to values obtained from Mississippi Valley-type deposits in the northern Canadian Cordillera and Pine Point and indicates that the galena is derived from similar basement sources situated along the Great Slave Lake Shear Zone. Sphalerite grains from the till have a sulphur isotopic composition significantly different from Mississippi Valley-type deposits in the northern and southern Cordillera, and are significantly different than the Pine Point deposits. These results highlight the potential to discover base metal mineralization hosted within the Cretaceous shale bedrock of northern Alberta.

Till sampling and geochemical analytical protocols used by the Geological Survey of Canada

The Geological Survey of Canada (GSC) has developed field and lab methods protocols to guide till sample collection, processing, geochemical analysis of the till matrix, monitoring of quality assurance/quality control, and archiving procedures for reconnaissance- to local-scale geochemical surveys. The most significant concepts and procedures are described in this paper. Continued and long-term use of these protocols will ultimately allow GSC researchers to integrate and contrast multiple datasets and ensure minimum levels of quality assurance and control for all till geochemical data. This set of protocols is the first established for Canadian till sampling and analysis and represents a contribution to the GSC’s Geo-mapping for Energy and Minerals (GEM) Program. Sharing the GSC’s knowledge on till sampling and analysis with the international community will allow other researchers and explorationists to adopt similar procedures. This sharing of knowledge will ultimately allow comparison of till geochemical datasets from various parts of Canada and internationally as well as ensuring a minimum level of quality assurance and control for all till geochemical data.

Processing of glacial sediments for the recovery of indicator minerals: protocols used at the Geological Survey of Canada

A successful method of mineral exploration in glaciated terrain is the use of indicator minerals recovered from carefully selected glacial sediments, and subsequently traced back to their bedrock source. The successful application of indicator mineral methods relies on efficient and effective recovery as well as the correct identification of a wide variety of indicator minerals. The Geological Survey of Canada (GSC) has developed protocols for ongoing and future research projects to achieve the highest quality for reporting indicator mineral data. Such protocols include the use of field duplicate samples, blank samples, and base material spiked with known numbers, morphologies, species, and sizes of indicator minerals. Field duplicate samples serve to estimate sediment heterogeneity. Spiked samples are used to monitor the accuracy of the sample processing and mineral identification methods for recovering specific minerals. Blank samples serve to detect potential carry-over contamination. In certain instances, a specific sample processing order is essential and should be communicated to the commercial processing laboratory. Ore-rich samples collected near known mineralization are to be processed last, to reduce chances of carry-over contamination. Repeated indicator mineral counts should be carried out on at least 10% of the heavy mineral concentrates to measure reproducibility (precision) of the mineral counts. All indicator mineral data, original laboratory reports, heavy mineral concentrates, unmounted picked grains, and grain mounts are now archived at the GSC, using specific guidelines.

Geochemical and mineralogical dispersal in till from the Mount Polley Cu-Au porphyry deposit, central British Columbia, Canada

The Quesnel terrane in the Interior Plateau of British Columbia, Canada, is highly prospective for locating new porphyry deposits; however, the bedrock in this region is obscured by a nearly continuous blanket of till, making mineral exploration challenging. Located within the Quesnel terrane is the Mount Polley deposit. It is an alkaline, silica-undersaturated, Cu-Au porphyry deposit mined by Imperial Metals Corporation. Eighty-six basal till samples were collected for geochemical and mineralogical analyses in the region of this deposit. Ore elements (Ag, Au and Cu), as well as pathfinder element (Hg and Zn) contents in till reflect detrital glacial dispersal from the Mount Polley deposit. The distribution of anomalous mineral counts of andradite garnet, apatite, chalcopyrite, epidote, jarosite and native gold also reflect glacial dispersal from the deposit. Outcrop-scale ice-flow indicators indicate a dominant ice-flow event to the NW that was preceded by a southwestward glacial advance. The element and mineral signatures of the Mount Polley Cu-Au porphyry deposit are dispersed in sub-glacial surface tills up to 12 km in the down-ice (NW) direction. We demonstrate that till geochemistry and mineralogy can serve for mineral exploration of Cu-Au porphyry mineralization in drift covered areas. Supplementary material: Complete data set for element and mineral results and ice-flow measurement is available at www.geolsoc.org.uk/SUP18828

Physical partitioning of mercury in till: an example from central British Columbia, Canada

Abstract The physical partitioning of Hg into different grain size fractions of till is predominantly controlled by the primary bedrock mineralogy, the distance of glacial transport, and the relative stability of cinnabar (HgS) in the soil weathering environment. At sites located short distances down-ice from bedrock cinnabar mineralization, the highest Hg concentrations in unoxidized till were measured in the sand- and granule-sized fractions reflecting the abundance of cinnabar in those size ranges. Similar partitioning was measured in oxidized till as cinnabar was found to be relatively resistant to postglacial weathering. Discrete clay-sized cinnabar grains obtained from the unoxidized till were viewed under the scanning electron microscope and suggest that the terminal grade of cinnabar is in the clay-sized range. In till collected from areas barren of cinnabar mineralization, the highest Hg levels were found in the clay-sized fraction which is attributed to the high adsorption of Hg by clay minerals.

Mercury and antimony in soils and non-vascular plants near two past-producing mercury mines, British Columbia, Canada

Mercury and Sb in soils (humus, B horizon, C horizon) and environmental biomonitors (moss and epiphytic lichen) were examined near two past-producing Hg mines located in British Columbia, Canada: Pinchi Lake and Bralorne Takla mines. Sequential extraction analyses and scanning electron microscopy were used to determine the chemical associations of Hg and Sb and the mineralogical forms of Hg in soils. Mercury and Sb in the C horizon are derived mostly from natural sources because their concentrations are controlled by the bedrock geology, glacial transport, and sediment types. Distribution of Sb in the B horizon is similar to that of the C horizon suggesting a common geogenic source. Mercury in the B horizon at Pinchi Lake mine could be derived in part from anthropogenic sources, because (1) only at that site, and not at any other sites of cinnabar occurrences in bedrock, is there a labile Hg enrichment in the B compared to the C horizon, and (2) there is a weak correlation between non-labile Hg concentrations in the B horizon and distance from the mine which occurs independently of the natural concentrations of the C horizon. Mercury and Sb concentrations in humus near Pinchi Lake mine (<10 km) appear to be derived in part from anthropogenic sources because there are strong correlations between labile and non-labile Hg and Sb concentrations with distance from the mine which occurs independently of natural concentrations in the C horizon. Near Pinchi Lake mine, biogeochemical cycling and gaseous Hg derived from the substrate are thought to be minimal pathways and sources of Hg to the humus compared to anthropogenic sources, otherwise the strong Hg enrichment in the humus compared to the C horizon only observed near Pinchi Lake mine would also be present at several sites of cinnabar occurrences in bedrock along Pinchi Fault. Anthropogenic Hg and Sb enrichment in the soils near Bralorne Takla mine is indistinguishable from natural enrichment. Mercury and Sb levels in the sampled moss and lichen species reflect locally derived wind-borne soil and rock dust.

Leaching of loosely bound elements during wet grain size separation with sodium hexametaphosphate: implications for selective extraction analysis

Sodium hexametaphosphate, Na6(PO3)6, is often used as a dispersant in wet grain size separation procedures to obtain the clay-sized (<0.002 mm) fraction of till and soil samples. However, the complexing properties of this reagent can lead to a loss of elements through chemical leaching during the separation process. This work examines potential losses of Cr, Zn, Fe, Mn, Ni, Sb, and Hg from glacial sediment (till) to dispersant solution. Compared to dissolution by conventional aqua regia digestion, these losses are insignificant at <0.3%. However, if the elements in a loosely bound form are under study using selective or sequential leaches, then the losses can be viewed as substantial. Sodium hexametaphosphate solution, used as dispersant at a strength of 0.5%, was found to dissolve as much as 68% of the Hg in a bulk till sample (<4 mm) that would be brought into solution by a 1 M sodium acetate extractant at pH 5, designed to dissolve the adsorbed/exchangeable/carbonate phase of a geological sample. Up to 10% of the Hg and 7% of the Cr leached from the clay-sized fraction by 1 M NaOAc could have been lost to the Na6(PO3)6 dispersant during the clay-sized separation. Consequently, the concentrations of labile or weakly bound forms of elements reported in a sequential extraction scheme could be reduced because of a loss to the Na6(PO3)6 dispersant.

Till geochemistry and mineralogy: vectoring towards Cu porphyry deposits in British Columbia, Canada

Regional till sampling was completed near four Cu porphyry mineralized zones in south-central British Columbia, Canada: Highland Valley Copper (Cu-Mo), Gibraltar (Cu-Mo), and Mount Polley (Cu-Au-Ag) deposits, and the Woodjam (Cu-Au-Mo) prospect. At all sites, Cu concentrations in the clay-sized fraction and chalcopyrite grains (0.25 – 0.5 mm; >3.2 specific gravity) are found in greater abundance in till near and down-ice from mineralized zones compared to surrounding background regions. At Mount Polley, the abundance of gold grains in till defines a dispersal train extending at least 3 km down-ice (SW and NW) from mineralization. At three sites out of four, epidote in till heavy mineral concentrates occurs in greater percentage near and down-ice from mineralized zones compared to background regions suggesting that this mineral could be an indicator of propylitic alteration associated with porphyry mineralization. The distribution pattern of Cu concentrations and chalcopyrite grains in till is controlled by the distribution of Cu-porphyry mineral occurrences in bedrock and the direction of ice-flow movements which prevailed during the last glaciation. By comparing study sites, there is a positive relationship between the areal extent of bedrock mineralization that was exposed to glacial erosion and the absolute values of Cu concentrations and chalcopyrite grain counts in till. In the Woodjam region where the till is thick (>10 m), eight samples with background Cu concentrations in the clay-sized fraction of till contain >4 grains of chalcopyrite/10 kg which is indicative of mineralization. This study demonstrates that a combination of till geochemistry and mineralogy is an efficient method for mineral exploration for Cu porphyry deposits covered by variable amounts of glacial sediments. Supplementary material: The full data sets on till geochemistry and mineralogy are available at: https://doi.org/10.6084/m9.figshare.c.3291503