Colin E. Dunn

Multi-element and rare earth element composition of lichens, mosses, and vascular plants from the Central Barrenlands, Nunavut, Canada

Abstract Lichen (n=12) and moss (n=6) species from a remote region of northern Canada have remarkably similar multi-element patterns suggesting they are non-specific accumulators of metals under existing conditions. Within individual species the concentration of many metals analyzed range over an order of magnitude. Many elements have a positive correlation with multi-element (n=48) and REE (rare earth element) totals. Others, such as Cd, K, and Zn have relatively consistent concentrations across all lichen and moss species, and across all sampling sites, indicating different accumulation and/or retention processes. Lichens and mosses have REE concentrations 1–3 orders of magnitude less than those of the average upper continental crust (UCC) but yield identical patterns. The correlation of other poorly soluble elements and key elemental ratios in lichen and moss are also similar to UCC and modern river sediment values. Metals including Sc, V, Cr, Fe, Co, Ga, Y, Hf, W, Pb, Th, and U show strong positive correlations with REE in lichen and moss. Rare earth elements may be useful as reference elements in environmental studies because of transport in the particulate phase, lack of significant anthropogenic sources, coherent group geochemistry, generally robust concentrations, and upper crustal signatures. Further, the REE may be helpful in identifying particulate deposition related to anthropogenic activities and enrichment of other elements by biogenic processes. The multi-element compositions of vascular plants (leaves and twigs) are fundamentally different from those of lichen and moss, lack correlation with REE, and are extremely enriched for many elements (100–1000× average upper continental crust) relative to the REE; perhaps because of limited REE solubility and transport via root systems. Enrichment factors for most metals of environmental concern are low; Pb is elevated but may be an artifact of low concentrations in local bedrock. Trace metal concentrations in lichen and moss at Otter Lake are similar to those measured across the Northwest Territories over 25 a ago.

Biogeochemistry as an aid to exploration for gold, platinum and palladium in the northern forests of Saskatchewan, Canada

Abstract Twigs of shrub alders (Alnus crispa and Alnus rugosa) tend to contain more gold than other common species in the northern forests of Saskatchewan. Alders are not cyanogenic, hence samples can be ashed to preconcentrate gold without loss of the metal as the volatile gold cyanide. Samples from mineralized zones commonly have over 50 ppb Au in the ash of the outermost 50 cm of alder twig. Background values are about 10 ppb Au. In the absence of alder from a given locality, other species may provide useful information on gold in the substrate. Examples are given of surveys using balsam fir (Abies balsamea), white spruce (Picea glauca) and black spruce (Picea mariana). A reconnaissance scale survey in which alder twigs were collected at 2-km intervals has outlined an area, coincident with a major lithostructural domain, within which ashed twigs contained from 20 to 130 ppb Au ( x = 45 ppb Au ). In sharp contrast, alders in a neighbouring area contained less than 10 ppb Au. It appears that this regional approach to biogeochemical sampling in glaciated terrains may provide a quick appraisal of the gold potential of underlying bedrock. Platinum and palladium show a tendency to concentrate in twigs and trunk of black spruce (Picea mariana) and jack pine (Pinus banksiana), and in stems of labrador tea (Ledum groenlandicum). Spruce was sampled close to a worked out nickel-copper deposit that contained 3000 ppb Pt and 6000 ppb Pd. The ashed twigs yielded up to 880 ppb Pt and 1350 ppb Pd, compared to background levels of below 10 ppb Pt and 2 ppb Pd.

The Biogeochemical Expression of Deeply Buried Uranium Mineralization in Saskatchewan, Canada

Dunn, C.E., 1981. The biogeochemical expression of deeply buried uranium mineralization in Saskatchewan, Canada. In: A.W. Rose and H. Gundlach (Editors), Geochemical Exploration 1980. J. Geochem. Explor., 15: 437–452. Ten kilometres from the eastern edge of the Athabasca Sandstone, near McClean Lake, uranium mineralization (locally up to 27% U3O8) lies 150 m beneath the surface at the unconformity between the Athabasca and crystalline basement. A biogeochemical survey of the area sampled AH and BF soil horizons, peat moss, and plant organs from the dominant species, viz. black spruce (Picea mariana), jack pine (Pinus banksiana), labrador tea (Ledum groenlandicum), and leather leaf (Chamaedaphne calyculata). Uranium concentrations in the ash of various media are surprisingly high: spruce twigs up to 154 ppm U; labrador tea and leather leaf stems around 100 ppm U. Conversely, labrador tea roots yield < 5 ppm U and spruce trunk wood usually < 1 ppm U. Soils give values of 1—3 ppm U. Contoured U values reveal that highest concentrations occur in plants growing above, but laterally displaced from the mineralization. Track-Etch data show a similar pattern. Upward migration of ions along steeply inclined fractures is invoked to explain the phenomenon. Other elements are presents in varying concentrations, depending upon the plant species and the plant organ. High concentrations of several elements are recorded, most notably Cd and Ag in the conifers.

The determination of Au, Pd and Pt in ashed vegetation by ICP-mass spectrometry and graphite furnace atomic absorption spectrometry

Abstract Current methods to determine Au, Pd and Pt as a “package” in samples of ashed vegetation require a minimum of 10 g to achieve detection limits below 10 ppb. This precludes practical application of biogeochemistry as a prospecting method for these elements. This study is concerned with the development of a method whereby Au, Pd and Pt can be determined in 1–2-g samples of ash to detection levels of 0.5–2 ppb for these elements. The proposed procedure involves ashing at 870°C, decomposition in HF-aqua regia, separation of the analytes by coprecipitation on Te, and analysis by graphite furnace atomic absorption spectrometry (GFAAS) or inductively coupled plasma mass spectrometry (ICP-MS). The high-temperature ashing is employed to obtain full recovery for Pd. Alternatively, additional digestion and evaporation with HClO4 can be used but with the attendant risk of loss of Au through volatilization of the chloride. Sample introduction in ICP-MS by electrothermal vaporization allows for detailed studies down to 0.2–0.3 ppb for these elements but at the sacrifice of the high productivity enjoyed in analysis by ICP-MS compared to GFAAs. If samples contain high amounts of Cu (e.g., 1000 ppm), an element which is carried through the separation procedure to varying degrees, certain precautions are necessary. Copper causes an isobaric interference (possibly CuAr+) on 105Pd in analysis by ICP-MS and therefore measurement should be made at 106Pd or 108Pd. Copper also suppresses the absorbance for Au in analysis by GFAAS; matrix-matching or the method of standard additions overcomes this interference. Good accuracy is indicated by comparison of results, at higher concentrations of these elements, with independent methods such as NiS fire-assay/neutron activation analysis and Pb fire-assay/ICP-MS. The homogeneity of the sample in Au, Pd and Pt distribution is the dominant factor in the precision obtained. Control samples of ashed spruce twigs and needles averaged relative standard deviations (n = 7) of 15% for Au (at 40 and 230 ppb), 6% for Pd (at 230 and 1430 ppb) and 18% for Pt (at 90 and 800 ppb).

Biogeochemical exploration for gold in tropical rain forest regions of Papua New Guinea

Abstract Biogeochemical methods have been widely used for mineral exploration, particularly in boreal forests and semi-arid regions, but there have been fewer applications in tropical areas. This paper describes a biogeochemical method of exploration for Au in equatorial regions. After investigation of several plant species, Astronidium palauense, a small- to moderate-size tree, was found to have many suitable attributes. (1) It is widely distributed in the southwest Pacific, although its occurrence may be limited at elevations greater than 1000 m. (2) The tree is easy to identify and is sufficiently common (e.g., one tree per 100 m2 on Simberi and Lihir Islands, Papua New Guinea) for detailed sampling. (3) The outer bark is easy to obtain and the ashed bark reliably indicates Au concentrations in the substrate. (4) The root system reaches at least 4 m depth, allowing greater penetration than surface soil samples, which is important in volcanic terrains where geochemical targets may be buried by post-mineralization volcanic eruption or debris flows. (5) The areal distribution of the root system samples a large volume of soil (ca. 100 m3), which reduces the nugget effect for Au. (6) The ease of sampling and low weight of bark reduces the time and cost over soil surveys, for example 6 minutes per site compared with 15 minutes per 1 m deep soil. Bark can be ashed in the field, 200–500 samples in 2 to 4 days, then shipped for multi-element (Au, As + 32 elements) instrumental neutron activation analysis (INAA). Field tests on Simberi and Lihir Islands, PNG, show that biogeochemical surveys have a high level of reliability for identification of prospects.

The behaviour of platinum group elements in the surficial environment at Ferguson Lake, N.W.T., Rottenstone Lake, Sask. and Sudbury, Ont., Canada

Abstract A multidisciplinary geochemical study of the distribution, dispersion, and glacial dispersal, of the pge and associated elements has been undertaken within soil, till, humus, vegetation and water at Ferguson Lake, Northwest Territories, Rottenstone Lake, Saskatchewan and Sudbury, Ontario. As the pge generally are present at low levels in surficial materials, development work on analytical techniques was an essential part of this study. At Ferguson Lake, the spatial distribution patterns of Au, Pt and Pd in till clearly indicate the exposed gossan zones, as expected, but also indicate a possible extension of the zone beneath a peat bog- and till-covered area. Down-ice dispersal of Au, Pt and Pd is limited to one to two hundred metres, in the μ m component of the till samples collected from frost boils. In vegetation the pge enrichment extends for several hundred metres down-ice and is best defined by Pd in birch twigs. Detectable, although extremely low, levels of Pt (2.8 ppt) and Pd (2.0 ppt) are present in waters in the vicinity of the gossanous zones at Ferguson Lake. At Rottenstone Lake, moderate to high concentrations of pge , Au, and base metals were found in ashed twigs of black spruce and the hmc of the tills for a distance of less than two hundred metres down-ice of the mineralization. Low Pd and Au concentrations were present in ashed spruce twigs about one kilometre down-ice of the mineralization, where only the hmc of the tills yielded anomalous concentrations of Pt and Au. There appears to be only limited dispersal of the pge and Au. These data indicate that only the hmc and the spruce twigs are of value in detecting Pt and Au in this area. Palladium presents a different picture, being detectable in only some of the soils, absent in the tills and hmc , yet appreciably enriched in the twig ash. The inference is that Pd is moving in solution and is being somewhat adsorbed in the soil but is much more significantly being taken up by the plant roots. At the Sudbury areas the pge , hosted in the Ni-Cu mineralization, are best reflected by elevated levels in the ashed humus of almost all elements examined. There is only minimal response in pge and Au to the mineralization from any of the fractions of the soil; whereas the μ m fraction of the B-horizon soil reflects the mineralization by elevated levels of As, Sb, Se, Cr, Co, Ni, Cu, Pb and Zn. Only hmc from the tills show elevated pge , Au and variable enhancement in As, Sb, Se and the base metals. The μ m portion of the tills tends to be highest in As, Se, Cr and the base metals. This ongoing study shows that surficial materials and vegetation are effective in identifying areas of concealed pge mineralization. Various pathfinder elements, primarily Cu and Ni, but perhaps also As, Se, Sb and the other base metals, in the μ m B-horizon soils and tills, may be informative in a preliminary evaluation of the pge potential of an area, prior to undertaking the more expensive precious-metal analyses. Humus and vegetation both appear extremely effective, and most cost efficient, and heavy-mineral concentrates ( hmc ) appear effective, for identifying areas with pge potential, whereas hmc from tills appear most effective for zeroing in on the site of the pge mineralization.

Platinum group metals in common plants of northern forests: developments in analytical methods, and the application of biogeochemistry to exploration strategies

Abstract All platinum-group metals (PGM) in ashed plant tissues, from an area of platinum mineralization in Saskatchewan, have been measured by neutron activation analysis of a NiS fire assay bead. Concentrations of up to 1350 ppb (ng/g) Pd, 880 ppb Pt, 49 ppb Rh, 37 ppb Ru, 24 ppb Ir, and 15 ppb Os occur in ashed twigs of black spruce ( Picea mariana ), indicating that the ratios of PGM uptake are about the same as those occurring in the bedrock. Plants growing on drift-covered diabase, known to have about 100 ppb PGM, contain up to 77 ppb Pt in ash, demonstrating the potential value of biogeochemical methods in helping to delineate platiniferous zones. Twigs of black spruce, jack pine, and labrador tea appear to be the optimum sample media in this environment. A rapid multi-element neutron activation analysis by direct irradiation of tissue permits measurement of 0.05 ppb Ir in dry material, or 2 ppb Ir in ashed samples. In view of the normally very low concentrations of Ir in plants, any detectable Ir probably indicates enrichment of PGM in the substrate. Comparison of several wet-chemical analytical methods indicates that the optimum (cost-effective) technique for detecting 1–2 ppb Pt, Pd, or Rh in 2-g samples of ash is by Te co-precipitation (following fusion and dissolution), and analysis by ICP-MS with sample introduction by electrothermal vaporization. However, care must be taken to ensure complete dissolution of all PGMs in the ash.

Enrichment of platinum and associated elements in organic seepage soils of the Tulameen ultramafic complex, southern British Columbia

Abstract Concentrations of Pt have been determined in seepage waters, organic soil horizons and trees in and upslope from a seepage site on the dunite core of the platiniferous Tulameen ultramafic complex. The concentrations of Pt (together with Sb, Cr, Ni, Br, Co, As and Cu) are enhanced twenty-fold in the organic seepage soils compared to Pt concentration in both the LFH horizons of the upslope soils and Pt concentrations found in the ash of twigs and bark of trees growing on the seep. The seepage waters contain 0.4 to 0.8 ng/L Pt. The study provides evidence that weathering under temperate conditions can release Pt from platinum group minerals and that the released Pt can then be transported, either in solution or a colloidal form, to seepage sites where it is accumulated by organic matter. Organic-rich soils from seepage sites might therefore provide suitable sampling media for geochemical surveys for Pt.

Trace element chemistry of vegetation applied to mineral exploration in eastern Nova Scotia, Canada

Trace element analysis of conifer tissues demonstrates the application of biogeochemistry to mineral exploration for gold and other metals hosted by lower Paleozoic metasediments in eastern Nova Scotia. At the regional scale, enrichment of up to 170 ppb Au and 72 ppm As in the ash of twigs from balsam fir (Abies balsamea) coincides with known gold districts and major shear zones. These concentrations are approximately 30 times background level. The twig trace element chemistry indicates previously unsuspected Au potential in Carboniferous Horton Group sedimentary rocks, and around the contact aureoles of Devonian granitoids where the twigs are also enriched in Cs (×30), Sb (×3), Rb (×3) and W (×10). Along the Minas Fault, separating the Avalon and Meguma terranes, anomalous groupings of Ba and Cr in twigs suggest exploration potential for base metal (Cu-Pb-Zn) and mafic or ultramafic (Co-Ni-Pt group element) associations. Detailed biogeochemical studies at five till- and/or bog-covered localities near the Beaver Dam gold deposit outline mineralization buried beneath up to 25 m of surficial sediment. An association of Au, As and Cs in the vegetation provides consistent patterns equal to or better than the more erratic geochemical response in soil and till surveys over the same areas. Concentrations of up to 170 ppb Au in the ash of twigs indicate locations suitable for further exploration. The low shrubs Labrador tea (Ledum groenlandicum), leather leaf (Chamaedaphne calyculata) and bog laurel (Kalmia polifolia) were collected from two small boggy areas suspected to overlie gold-bearing bedrock. Typically, these species contain <10 ppb Au in ash. At one site they contained 22 to 36 ppb Au, and at the other they yielded 55 to 85 ppb Au, in association with Cs and weak Co enrichment. In the glaciated terrain of eastern Nova Scotia, biogeochemical exploration is an effective method for delineating metalliferous zones. Multi-site anomalous concentrations of Au and related pathfinder elements occur where subcropping mineralization is covered by till. Regional and prospect-level biogeochemistry can assist exploration by providing valuable data to aid in the discovery of buried or blind mineralization.

Multi-element study of vegetation from a zone of rare-earth rich allanite and apatite in northern Saskatchewan, Canada

Abstract Neutron activation analysis of 180 ashed components of common plants growing near Ba, Th and rare-earth-rich minerals in northern Saskatchewan has outlined the distribution patterns of 26 elements. Birch, jack-pine, black spruce, labrador tea, alder, willow and moss are all capable of absorbing moderate to high quantities of La, Ce, Sm, Ba, and Sr, but very little Th. Europium, Yb and Lu levels are low, but this is probably a function of the relatively low levels of these elements in the bedrock. In general, twigs and trunkwood concentrate the rare-earths more than leaves, needles and cones. Traverses were conducted across pods of mineralization to collect black spruce twigs from 55 sites. Data indicate a response to mineralization in the rare-earth element (REE) uptake by the twigs. Factor analysis of the biogeochemical data confirms that data variability occurs mainly in La, Ce, Sm, Eu and Ba, and secondly in elements associated with Fe. It is concluded that low cost biogeochemical methods can assist in locating pods of near-surface allanite-rich mineralization, but as the haloes are localized due to low REE mobility, vegetation sampling must be closely spaced.