OTHER NORTHERN STUDIES

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Precious Metals in Vegetation from Northern Saskatchewan, 1987

by Colin E. Dunn1

Dunn, C.E. (1987): Precious metals in vegetation from northern Saskatchewan, 1981; in Sunmary of Investigations 1987, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report "117-4.

Studies have continued this year on the distribution of precious metals in and among common plant species of the northern forests.

The field component of the work comprised three days in the Rottenstone area. Other information reported here is derived from material collected by the author in 1986, and a few samples of interest collected by Antelope Oil and Gas this year.

Platinum

Rottenstone Lake

The Rottenstone area ("Hall" showing, Ni, Cu, and Pt mineralization in an ultramafic matrix) was first selected for the study of platinum-group metal (PGM) uptake by vegetation in 1983 (Dunn, 1983, l 986a), and has proved to be a valuable field laboratory for examining patterns of precious metal distribution in many of the common plants that comprise the boreal forest. Twigs of black spruce were found to accumulate PGMs to higher concentrations than other plant parts (up to l, 350 ppb Pd and 880 ppb Pt in ash). This year further research on this material and additional bulk samples collected in 1984 and 1986 have revealed certain analytical problems, as well as provided the

lExploration Geochemistry Subdivision, Mineral Resources Division, Geological Survey of Canada.

first data on the full suite of PGMs in vegetation (Dunn et al., in press). Table l summarizes these data, which were obtained by neutron activation analysis (NAA) following a nickel sulphide fire-assay of ash samples weighing from 5 to lD g.

Iridium is the only PGM which can be readily detected by the rapid instrumental neutron activation method (INAA) commonly employed for multi-element analysis. Iridium, although of very low cosmic abundance, can be detected by routine INAA at levels as low as 0.2 ppb Ir in 8 g pellets of dry vegetation, and as low as 2 ppb in l g of plant ash. Lower levels of detection are possible by performing a longer counting procedure. Background levels of Ir in vegetation are only a few parts per trillion but, in the vicinity of PGM mineralization, Ir levels may be elevated to above the INAA detection limits. As a result of these factors, and in view of its geochemical associations, iridium may be considered a pathfinder element since any detectable Ir in vegetation would suggest the presence of elevated concentrations of platinum in bedrock.

Preliminary data from wet chemical analysis of ashed samples from Rottenstone show that the order of Pt abundance is (from highest to lowest): black spruce twigs > black spruce bark > !abrader tea twigs > alder leaves > black spruce trunkwood = jack pine trunkwood > jack pine twigs > alder twigs > horsetail > jack pine needles > black spruce

Table 1 - Platinum Group Metals and Gold in Single Samples of Ashed Vegetation from the Data obtained by NAA Vicinity of the Rottenstone (Hall) Ni-Cu-PGM Deposit, Saskatchewan.

following N1S fire assay. All precious metal values 1n ppb.

Conrnon Name Botanical Name % Ash Pt Pd Os Ir Ru Rh Au

Spruce Twigs A Picea Mariana 2.84 830 l 100 28 24 30 34 l 01

Spruce Twigs 8 P1cea Mariana 2. 76 910 410 13 15 21 49 100

Spruce Twigs c Picea Mariana ,. 77 l l O <1001 7 4.4 l 5 9 20

Spruce Trunkwood Picea Mariana 0.70 300 270 10 5.6 12 9 28

Alder Twigs Al nus Cr1s11a l.46 100 65 3 ,. 7 8 6 28

Alder Leaves Al nus Cr1s11a 3.83 420 490 15 9.8 37 26 110

lhlgh detection limit due to small s~le weight.

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needles > birch twigs > willow twigs > labrador tea leaves > grass.

At the end of June 1987, a small-scale biogeochemical survey was conducted "down-ice" from the Hall deposit, in conjunction with a bulk till geochemistry sampling program by J. Campbell (Saskatchewan Research Council) and W. Coker (Geological Survey of Canada). The purpose of this study is to assess the relative value of the two procedures in defining down-ice dispersion trains of precious metals from the Hall deposit. Results are pending.

Tremblay Lake

Fifteen kilometres northeast of Rottenstone Lake there is a diabase dyke with some enrichment of PGM (> 100 ppb, L. Hulbert, pers. comm.). Part of this dyke forms a densely wooded island in Tremblay Lake. A brief biogeochemical survey of the west part of this island was conducted in June 1986. One sample of ashed black spruce bark yielded 31 ppb Pt and 10 ppb Rh, with 30 ppb Pt in twigs of the same tree. Labrador tea twigs a few hundred metres distant yielded 77 ppb Pt (Dunn et al., in press). The inference is that, in areas of thin overburden, the vegetation may be used as a preliminary screening tool for detecting bedrock enrichment of PGM in the order of a few hundred ppb.

Koliniak Lake

Phenomenal concentrations of gold have been found in ashed vegetation from near Koliniak Lake, 12 km east-northeast of Lower Waddy Lake. This area of high-grade felsic to intermediate gneisses with intercalated basic volcanics was mapped in detail by Johnston ( l 969), who reported gossans and some enrichment of base metals in soils and bedrock. A few samples of alder and pine twigs were collected by A. Frew (for Antelope Oil and Gas) in June l 987 from topographic hollows near disseminated pyrite and pyrrhotite occurrences, as part of a broad geochemical reconnaissance of areas of gold potential.

Combined samples of alder twigs and leaves were ashed at 600°C I and analyzed for gold by conventional lead fire assay cupellation, followed by solution in aqua regia. This work was conducted by Loring Laboratories, Calgary, exercising care to avoid contamination. Concentrations (over l 30,000 ppb Au) were so extreme that they were worthy of more detailed investigation, as well as comparison with the large database of similar material currently being compiled at the Geological Survey of Canada (GSC).

lAt this t~erature most of the Cs and Rb will vol at i 1 i ze.

Ashed samples weighing about 0.5 g were loaded into vials at the GSC and analyzed by INAA at Activation Laboratories Ltd., Brantford, Ontario.

Gold concentrations from these samples yielded from 130 to 231,000 ppb Au (equivalent to almost 5,000 ppb Au in dry material); this is higher than any previously recorded gold concentration in vegetation, except for a report by Babicka (l 943) which was later shown to be false (c.f., Brooks, 1982). The only concentration recorded of similar magnitude was 180,000 ppb Au in ashed pine twigs from Tasmania (Baker, 1986). Of particular interest is the coincident enrichment in the vegetation of anomalous concentrations of several elements commonly associated with gold mineralization (Table 2).

Table 2 - Elements in Ashed Alder and Leaf Samples (Combined) from near Koliniak Lake (n=l2; ash yield approximately 2 percent).

Au Ag As Br Co Zn

Koliniak Lake

130-213,000 31-380 <2-370 l l -600 8-62 3,000-8,200

Au in ppb, all other elements in ppm

Normal Back­ground (approx.)

10-20 <5 l 40 10 l ,000

It was noted earlier that bromine is enriched in alder ash near some zones of gold mineralization (Dunn, 1985). Six of the samples analyzed contained over 300 ppm Br. Cobalt was slightly enriched in several samples; antimony and molybdenum were significantly enriched in one sample. In addition to the trace element data, major element analyses indicate significant sodium enrichment, moderate iron enrichment and possibly slight potassium depletion.

This assemblage of elements is remarkably consistent with those commonly associated with certain styles of gold mineralization. Whereas gold mineralization t,as not as yet been uncovered in this area, the biogeochemical data indicate that the area warrants closer investigation in order to establish the reasons for such extreme enrichments in the vegetation.

Integrated Studv of the Star Lake Area - An Update

Biogeochemistry

In 1986 a study was launched in an attempt to integrate biogeochemical data with remotely sensed

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data, known mineralization and bedrock geology (Dunn et al., 1986). To date, multi-element data have been compiled for ashed spruce bark and alder twigs obtained from about 600 sites within a 150 km2 area encompassing the Star Lake gold mine, the Rod Zone and the newly discovered Transom Lake/Jasper mineralization (Fig. l). Spruce bark information has been summarized earlier (Dunn, l 986b), since which time the alder samples have been analyzed. The significant new information to emerge from the alder study is that many samples collected near the eastern edge of the Island Lake Pluton and into the pluton (near the old 'Decade Mine' trail, I to 2 km west of Pesimokan Lake) have yielded anomalous concentrations of gold (IO times background) and molybdenum (100 times background). This zone of anomalies is "!!Q-ice" from the Jasper showing, suggesting additional zones of mineralization to the northeast.

Throughout the study area, there is a broad coincidence of cesium enrichment associated with elevated concentrations of gold in the vegetation. Cesium is a rare element which forms few minerals, one of which is galkhaite. This extremely rare mineral has been found in association with the Hernia gold deposits (Harris, 1986). Although the cesium enrichment may not in it:;elf be highly significant, the association with gold is further encouragement that mineralization may be present.

Data Integration

Spectral reflectance data from four flights over part of the area are currently being processed. Details of the multi-detector electro-optical imaging scanner system (MEIS) used are given in Dunn et al. ( 1986).

Landsat (TM) data tapes have been received and registered, and data analysis will commence in the near future.

Within the past month a new spatial data analysis (TYDAC) system has been obtained at the GSC, and a post-doctoral fellow will establish procedures required to integrate the data sets, including the newly acquired stable base geology maps from the field programs of Thomas (1984, 1985) and Harper (1986), high resolution gamma-ray spectrometry surveys (Geological Survey of Canada, 1987) and aeromagnetic data.

It is hoped that this exercise will help to refine exploration targets, and assist in the discovery of more zones of mineralization.

Acknowledgements

I thank D. Surjik (Antelope Oil and Gas) and A. Frew for supplying the samples from the Koliniak Lake area and permitting release of the data.

Figure 1 - Location of the Star Lake study area.

References

Babicka, J. (1943): Gold in Lebewesen; Microchim. Acta, v31 (in German).

Baker, W.E. (1986): Gold in vegetation as a prospecting method in Tasmania; in Carlisle, D., Berry, W.L., Kaplan, I.R. and Watterson, J .R. (eds.), Mineral Exploration: Biological Systems and Organic Matter; Rubey Volume V, Prentice Hall, New Jersey, p!Sl-158.

Brooks, R.R. (1982): Biological methods of prospecting for gold; J. Geochem. Explor. v280, pl09-122.

Dunn, C.E. (l 983): Biogeochemical investigations in northern Saskatchewan: preliminary data on tungsten, gold, platinum, rare-earths and uranium; in Summary of Investigations 1983, Sask. Geol. Surv., Misc. Rep. 83-4, pl06-122.

____ (1985): Biogeochemical exploration for gold in the La Range belt, 1985; in Summary of Investigations 1985, Sask. Geo!. Surv ., Misc. Rep. 85-4, p37-49.

Dunn, C.E. (l 986a}: Biogeochemistry as an aid to exploration for gold, platinum and palladium in the northern forests of Saskatchewan, Canada; J. Geochem. Explor. v25, p2 l-40.

Dunn, C.E. (l 986b): Biogeochemical studies in the Saskatchewan Gold Belt, 1986; in Summary of Investigations 1986, Sask. Geo!. Surv ., Misc. Rep. 86-4, pl36-138.

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Dunn, C.E., Rencz, A., Bonham-Carter, G. and Belanger, J .R. (l 986): Integrated biogeochem­ical and remote sensing studies, Star Lake area, Saskatchewan (NTS 73P-l6 and 74A-l); in Summary of Investigations 1986, Sask. Geol. Surv., Misc. Rep. 86-4, pl36- l38.

Dunn, C.E., Hall, G.E.M. and Hoffman, E. (in press): Platinum group metals in common plants of northern forests: developments in analytical methods, and the application of biogeochemistry to exploration strategies; J. Geochem. Explor.

Geological Survey of Canada { l 987): Airborne geophysical survey - gamma-ray spectrometry -1986: Star Lake - Waddy Lake area, Saskatchewan; Geel. Surv. Can., Open File 1515.

Harper, C. T. (1985): Bedrock geological mapping, Windrum Lake area; in Summary of Investigations 1986, Sask. Geol. Surv ., Misc. Rep. 86-4, p8- l 9.

--~~ (1986): Bedrock geological mapping, Windrum Lake area (part of NTS 640-4, 73P- l 6 and 74A-l); in Summary of Investigations 1986, Sask. Geol. Surv ., Misc. Rep. 86-4, pS-17.

Harris, D.C. (1986): The minerals in the main Hemlo gold deposit, Ontario; in Current Research, Part A; Geol. Surv. Can., Pap. 86-lA, p49-54.

Johnston, W .G.Q. (1969): The geology of the eastern portion of the Waddy Lake area, Saskatchewan; Sask. Dep. Miner. Resour., Rep. 127, 43p.

Thomas, D.J. (1984): Geological mapping, Star Lake area {part of NTS 74P- l6 and 74A- l ); in Summary of Investigations l 984, Sask. Geol. Surv., Misc. Rep. 84-4, p21-31.

------,-,- (1985): Bedrock geological mapping, Roundish-Servin Lakes area (part of NTS 73P- l 5 and -16); in Summary of Investigations 1985, Sask. Geol. Surv., Misc. Rep. 85-4, pl8-27.