Biogeochemical Studies in the Saskatchewan Gold Belt, by Colin E. Dunn1

Transcription

1 Biogeochemical Studies in the Saskatchewan Gold Belt, 1986 by Colin E. Dunn1 Dunn, C.E. (1986): Biogeochemical studies in the Saskatchewan gold belt, 1986; in Sunmary of Investigations 1986, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report Biogeochemical studies conducted in recent years have shown that chemical analyses of vegetation comprising Canada's northern forests can assist in delineating zones of gold mineralization (Dunn, l 984, l 985, l 986; Cohen et al., in press). Detailed surveys (50 m spacing) and those at reconnaissance levels (up to 2 km spacing) have both provided positive indications of mineralization. For surveys covering tens of square kilometres, a 200 m sample interval along traverses normal to the bedrock strike is a practical compromise between the accumulation of vast quantities of material at the 50 m detailed scale, and sampling at 2 km reconnaissance scale which may miss many zones of interest. Hence, for this study, samples were collected at intervals of 200 m within a l 50 km2 area (Fig. l), referred to here as the "Star Lake" area, that includes the mineral occurrences between Neyrinck and J ojay Lakes. This survey constitutes part of an integrated biogeochemical/ remote sensing program, details of which are given in Dunn et al. (this volume) The limits to sample collection in the Star Lake area were determined by ease of access from roads and trails; the distribution of bogs, lakes, and burn areas; and "down -time" due to bad weather. Four weeks (from late May) was considered the maximum period permissible for sample collection, because of seasonal variations in plant chemistry (Dunn l 984, l 985). If the survey had been extended for a longer period, data from samples collected at the start and finish of the survey period could not have been readily compared, as gold concentrations in some plant organs decrease through the growing season. In theory, samples spaced at 200 m intervals for 10 km along 16 lines spaced 1000 m apart (normal to the bedrock strike) would provide 816 samples over the l 50 km2 area, giving a sample density of 5.4 per km2. However, this does not take into account lakes, bogs and burnt areas; moreover, such a systematic coverage requires extensive air support. In practice, two teams of two persons were able to collect samples of two media per site and record appropriate geological and environmental notes from about 600 locations within the allotted time, thereby providing a coverage of 4 per km2. In addition, brief studies were made near zones of gold mineralization at Laonil and Puswawao Lakes, outside of the main study area. 1Resource Geophysics and Geochemistry Division, Geological Survey of Canada. Studies in northern Saskatchewan have shown that, locally, twigs of alder (Alnus crispa) are enriched in gold near zones of gold mineralization (Dunn, 1984). Elsewhere, the outer bark scales of black spruce (Picea mariana) reflect more strongly the presence of mineralization (e.g., Waddy Lake; Dunn, l 985). Environment can play an important role in the distribution of elements among plant species, causing species other than alder and spruce to be more sensitive to gold under certain conditions. For example, prior to development of the Star Lake gold mine, labrador tea and leather leaf collected from the peat bog over the 2 l Zone contained more gold than other species (Dunn, 1985). The most common species in the Star Lake area are black spruce and alder. Hence, alder twigs and spruce bark scales were the principal media selected for this survey, plus spruce twigs from about 20 percent of the sample locations. The spruce bark is a particularly suitable medium in that it comprises dead material which is not subject to seasonal changes in chemistry. More useful information could probably be obtained from sampling additional media (e.g., labrador tea), but a reasonable limit had to be set on the scope of the study. Sample Collection, Preparation, and Analysis Procedures for collecting, drying, separating, ashing and analysis (neutron activation) used in previous years were adopted for this survey (Dunn, l 984, l 985). A multi-element neutron activation package of Nuclear Activation Services Ltd. provided data on Ag, As, Au, Ba, Br, Ca, Co, Cr, Cs, Fe, Hf, Mo, Na, Ni, Rb, Sb, Sc, Se, Sr, Ta, Th, U, W, Zn, and the rare-earth elements La, Ce, Nd, Sm, Eu, Yb, Lu. Detection limits and background levels of these elements in alder twigs and spruce bark are given in Dunn (l 985, Table 1 ). Data presented in this report are concentrations in ash. Reasons for ashing samples rather than analyzing dried material are discussed in an earlier report (Dunn, 1985). Preliminary Data from the Main Survey Area Analytical data for 32 elements in spruce bark have been received, and a preliminary evaluation made of the gold data. At many sites, gold concentrations are below the detection limit of 5 ppb and, in general, the geochemical data

2 ' Extent of area surveyed Areas with anomalous gold in Spruce bark (>3 x background) 15' ' t I Sk~ L.~ - 2 km 55 55' GSC figure l - Location map showing extent of area surveyed, and areas with gold in spruce bark greater than three times background.

3 exhibit low variability. Throughout the survey area the concentrations of gold in spruce bark ash are generally lower than those in the southern part of the La Range Domain (cf. Dunn, 1984, 1985). Whereas 10 to 15 ppb Au constitutes background concentrations to the south, similar levels obtained in the present survey are considered anomalous, because background is less than 5 ppb Au. This difference may be the result of a greater mobility of gold around the more southerly deposits, due to their chemical melting and the nature of the disseminated mineralization. Given this mobility, the plants are able to pick up and accumulate more gold. Gold deposits in the Star Lake region tend to be "cleaner" (i.e., native gold trapped in quartz veins, and with generally few associated trace metals). The resultant lower background levels need to be taken into account when evaluating biogeochemical data from the current survey area. Some areas, however, do show multi-sample enrichment of gold (Fig. l): l) West and south J ojay Lake, notably in the area around the Rod Zone, where there is up to 690 ppb Au (see next section entitled "Rod Zone - Mallard Lake Area"), and 500 m southwest of Rush Lake with 380 ppb Au. 2. Around Neyrinck Lake (to 23 ppb Au) and for 4 km to the southwest, where several samples have yielded around 20 ppb Au and isolated values of 81 and 160 ppb Au. 3. Between J aysmith and Scriver Lakes, numerous samples have fairly low (10 to 16 ppb Au) yet regionally anomalous (2 to 3 times background) gold concentrations. To the west of Scriver Lake, near the eastern edge of the Island Lake Pluton, gold levels are generally higher with up to 58 ppb Au. 4. Around Liggett Bay (central part of Mclennan Lake), gold levels are slightly anomalous (10 to 17 ppb Au). 5. East of Skalicky Lake (northwest corner of survey area), values are similar to those at Liggett Bay. Table l - Gold Content (ppb) of Ashed Bark Scales from Black Spruce Along the Rod Zone. Cut-line Di stance (m) Gold (ppb) Coordinate Northeastward in Spruce Along from Trench Bark Ash Baseline with Au and Mo E E 10 + OOE E OOE E OOE E t OOE E OOE No other element shows enrichment in the spruce bark. The alder twigs yielded one to three times background concentrations of gold along the traverse, and indicate local enrichment in Mo (61 ppm) and Co (28 ppm). Subsequent exploration in this area by Mahogany Minerals has led to the discovery of gold mineralization (0.1 oz./ton Au) close to the site that gave 600 ppb Au in the bark, and a probable economic deposit (0.5 oz./ton Au) has been found less than 50 m from the site with 450 ppb Au in spruce bark (G. Burrill, pers. comm.). Farther south, about 150 m from the main trench, drilling has intersected mineralization with locally over 1.0 oz./ton Au at a location which had previously yielded 200 ppb Au in bark. Clearly the amount of gold in the vegetation is not proportional to the grade of gold mineralization, but this is to be expected unless all the mineralization is at bedrock surface. The deeper the mineralized body occurs, the more dissipated the geochemical signature becomes. The important point is that anomalous biogeochemical values (whether high or low in magnitude) can be evidence of local mineralization. Rod Zone - Mallard Lake Area In September 1985, a traverse was made along the Rod Zone, northward from the main trenched occurrence that contains native gold and molybdenite in quartz veins. Since both outcrop and gold mineralization are sporadic along this shear zone, spruce bark and alder twigs were sampled in an attempt to identify auriferous sections. Table l shows the great variation in gold content of spruce bark along the transect. The strong response above the main trenched occurrence is repeated elsewhere, which suggests that mineralization is also located at sites lo+oon, 11 +DON, and l 3+00N. l Laonil Lake - Seabee Gold Property (NTS 63M/l 2) A brief visit was made to the Laonil Lake area to collect vegetation from the vicinity of the VB Shear Zone. The purpose was to establish the trace element uptake of several species in this geological lrhe sample containing 690 ppb Au has been returned frlmi neutron activation analysis and analyzed by a wet chemical method in laboratories of the Geological Survey of Canada, Ottawa. The analyst was not informed of the NAA value, yet returned a c1m1parable assay of 687 ppb Au (G. Hall and J. Vaire, pers. COITlll.).

4 environment, and subsequently assess the potential usefulness of the biogeochemical method in assisting exploration in this area. Mineralized zones occur in sheared gabbroic rocks cut by quartz veins and stockworks. Tourmaline occurs in the veins and wall rock, and the quartz contains gold, pyrrhotite, pyrite and rare chalcopyrite. Details of the geology are given in Lewry ( 1977) and Delaney (this volume). Thirty-eight samples were collected from 18 sites (Fig. 2) across and along the V2 Zone, which forms a well-drained northwesterly-facing ridge. The most common species is black spruce; hence spruce bark scales were obtained at all sample sites, and spruce twigs from the twelve sites where the branches were within arms reach. In addition, samples of alder twigs, birch twigs, and jack pine bark were taken from the few places where they were growing near the spruce sample locations. Figure 2 shows strongly anomalous gold concentrations (45 to 330 ppb Au) in spruce bark ash from six locations, with anomalies of lesser magnitude (22 to 26 ppb Au) from three more. It is noteworthy that the highest value is similar to values recorded from spruce bark at several sites near the Komis deposit at Waddy Lake (Dunn, 1985). It appears that background values for the Seabee area are less than l O ppb Au in the bark. Gold in spruce twig ash ranges from 2 to 750 ppb Shear zone 110 Gold (ppb) in Spruce bark 330 : m ' '.5 1 N I Laonil Lake 1 km 55 40' Figure 2 - Gold (ppb) in ashed spruce bark, Laonil Lake - Seabee Property. GSC

5 Au, with the maximum concentration from a tree adjacent to that which yielded 3 30 ppb Au in its bark (Table 2). Table 2 - Laonil Lake (V2 Zone) - Comparison of Gold Content (ppb) of Ashed Vegetation (see Fig. 2 for locations). Jack Site Black SQruce Alder Birch Pine No. Bark Twigs Twigs Twigs Twigs l < < Arsenic (up to 12 ppm) is commonly above the usual background levels (2 to 4 ppm) for spruce bark, but cannot be considered highly indicative of mineralization, since above-background concentrations are quite common throughout the Saskatchewan gold belt. Data from several thousand samples in Saskatchewan show that tungsten is rarely above the l ppm detection limit of the analytical method. Three spruce bark samples from near the V2 Zone yielded 6, 3 and 3 ppm W, suggesting some tungsten enrichment in the bedrock. A preliminary assessment of REE data from spruce bark suggests an unusual enrichment of neodymium, with respect to lanthanum and cerium concentrations. Whereas background levels are usually less than 5 ppm Nd, several samples with only 3 to 5 ppb Au have over 10 ppm Nd. Thus it appears that there is an imbalance of some REE concentrations, the significance of which is unknown. Other elements are locally concentrated in some species: l) Molybdenum up to 4 30 ppm (background usually l to 5 ppm) in alder twigs (cf. l lppm Mo at Star Lake); 34 ppm Mo around the Rod Zone; and up to 230 ppm Mo at Waddy Lake; 2) Cobalt up to 66 ppm (background usually less than 10 ppm) in birch twigs (cf. dwarf birch twigs from close to the Lupin gold mine (NWT), 62 ppm Co; from the Tundra gold mine (NWT), 33 ppm Co; and the Star Lake gold mine, 32 ppm Co). Alder twigs (similar background values to birch) contain up to 66 ppm Co above the decline at Mallard Lake, and 28 ppm Co along the Rod Zone. Molybdenite and cobalt-bearing minerals have been reported from several of these gold deposits. In summary, the vegetation indicates that, near mineralized veins in the Laonil Lake area, there are enrichments of Au, Co, Mo and W, with possibly some chemical differentiation of the rare-earth elements. Slight enrichments of As and Cr are noted. Multi-element biogeochemical surveys in this area may assist in outlining mineralized zones, and could help in differentiating between auriferous and barren shear zones. Puswawao Lake (NTS 74A/l) In light of information supplied by the Saskatchewan Mining Development Corporation (SMDC) from encouraging drillhole intersects of gold and base metal mineralization near Puswawao Lake, and earlier reports of gold traces in that area (Miller, 1949), a visit was made to this locality (4 km west-northwest of Windrum Lake, Fig. 3) to test the vegetation response to mineralization. The topography consists of low hills and ridges with dry woodland separating boggy depressions. Only one outcrop, identified as a felsic metavolcanic, was encountered on the 3 km traverse to collect vegetation. The area has been mapped as metaandesite and metadacite, cut by a shear zone containing white quartz-sericite schist that has yielded traces of gold and silver (Miller, 1949; Beck, l 959). The shear zone may be an extension of that which transects J ojay lake to the south (Miller, l 949). Pyrite is present, and a green chrome-mica, typical of that commonly found with gold mineralization, has been observed (SMDC, pers. comm.). Spruce bark samples were obtained from 21 locations, and alder twigs from seven of these. Data received from the bark samples show unusually strong enrichment of gold (1300 ppb) at one site, and above background concentrations at five others (Fig. 3). Moderate arsenic enrichment is present (up to 17 ppm As or about 5 times background), with locally anomalous levels of antimony (6 ppm Sb or 5 times background), cobalt (23 ppm Co or 3 times background), chromium (46 ppm Cr or 3 times

6 ' 1 km STUDY AREA ~s 56 0s ' <5 < e 29 Shear zone 1 20 Gold (ppb) in Spruce bark 500 m t I PUSWAWAO L. Figure 3 - Gold (ppb) in ashed spruce bark, Puswawao Lake. 13 GSC background), cesium (6 ppm Cs or 3 times background), rubidium (360 ppm Rb or 5 times background), thorium (6 ppm Th or 3 times background) and possibly selenium. Preliminary data indicate 18 ppm Se which, if verified, is at least ten times background. These biogeochemical data indicate the presence of mineralization in this area. Acknowledgments I thank the many property owners on whose land we have worked this summer, for the various pieces of information supplied and for their cooperation whenever requested. In particular I am grateful to the Saskatchewan Geological Survey for the loan of two assistants (D. Toop and D. Dyck) for part of the field season, and for the generous use of the laboratory facilities in Regina.

7 T. Katzer, N. Richmond and R. Banville are the assistants who really deserve all the credit for the successful completion of this field season, to them I extend my gratitude for all their troubles. References Beck, L.S. ( 1959): Mineral occurrences in the Precambrian of northern Saskatchewan (excluding radioactive minerals); Sask. Dep. Miner. Resour., Rep. 36, 46p. Cohen, D.R., Hoffman, E.L., and Nichol, T. (In press): Biogeochemistry: A geochemical method for gold exploration in the Canadian Shield; in Geochemical Exploration, 1985, Ed. R.G. Garrett, J. Geochem. Explor. Dunn, C.E. ( 1984): Biogeochemical methods and surveys, southern La Range Belt: in Summary of Investigations 1984, Sask. Geol. Surv., Misc. Rep. 84-4, p ( 1985 ): Biogeochemical exploration for gold in the La Range Belt, 1985; in Summary of Investigations 1985, Sask. Geol. Surv., Misc. Rep. 85-4, p ( 1986): Biogeochemistry as an aid to exploration for gold, platinum and palladium in the northern forests of Saskatchewan, Canada; J H. Geochem. Explor., v25, p2 l -40. Lewry, J.F. ( 1977): The geology of the Glennie Lake area; Sask. Dep. Miner. Resour., Rep. 143, 53p. Miller, M.L. ( 1949): The geology of the Windrum Lake area (Maribelli Lake, east half), Saskatchewan; Sask. Dept. Miner. Resour., Rep. 3, 24p.