Biogeochemical exploration using Douglas-fir tree tops in the Mabel Lake area, southern British Columbia (NTS 82L09 and 10)
Within the northeast quadrant of Vernon map sheet NTS 82L and southern part of 82M (Seymour Arm) are five known groups of Zn/Pb deposits within a cover sequence that mantles Paleoproterozoic core gneiss. Of these five, Kingfisher is the only group below the tree-line and it is the only group within the present survey area. The widespread regional distribution of thin prospective homotaxial calcareous stratigraphic units that host these deposits lends credence to the concept that similar mineralization may lie beneath the extensive area that is covered by a veneer of glacial deposits and dense forest. Exploration in this environment, illustrated on the frontispiece, is challenging because of the rugged terrain, poor access, forests and till cover. Geophysical signatures provide some guidance, but the ore-bearing lithologies do not generate a distinctive geophysical signature. An additional exploration tool is needed. The chemical analysis of tree tissues can provide insight to the composition of rocks concealed by overburden. The roots of trees can be perceived as natural drills that penetrate the substrate and extract metals from overburden, groundwater and locally bedrock. These metals are translocated through the roots into the aerial tissues where they are sequestered in differing proportions according to the metabolic requirements of an individual species of tree - in this case Douglas-fir. Chemical signatures are usually subtle and data should be viewed with this in mind, especially when considering that mineralized targets are long, but only a few metres wide. Therefore, the chances of locating a mineralized body from the sampling grid for this airborne survey of one site per 1 km2 are exceedingly small. The objective is, therefore, to identify groups of 'pathfinder' elements, characteristic of Sedex Zn/Pb mineralization that provide focus for more detailed exploration efforts. During a four day period in late March, 2006, a helicopter-supported survey was conducted within 700 km2 of an area centred upon Mabel Lake B.C., to collect 562 samples of foliage and stems from the tops of Douglas-fir trees. Sampling was at 1 km on an offset grid, with closer spacing over the Kingfisher deposits and an area of interest with similar stratigraphic units that borders the Tsuius River. Total helicopter time, including daily mobilization and refuelling, was 35 hours involving 656 kilometres of air travel. A set of 66 test samples was prepared for analysis to determine if the optimum procedure would be the analysis of needles or twigs, and from ash or dry tissue. Results indicated that for a Zn/Pb target, analysis of dry twigs by ICP-MS was the preferred method. The final dataset comprised, including controls, data for 53 elements in 628 milled twig samples for a total of over 33,000 analytical determinations. Not all elements were detectable, and some were too close to the detection limit to generate reliable data. However, data with good to excellent precision was obtained for more than 30 elements. Predictably, given the coarseness of the sampling grid, no unusually high concentrations of elements were encountered, although an area to the west of Mabel Lake yielded anomalous levels of gold which are under further investigation. Subtle multi-element trends and associations were sought in the data. Procedures for plotting the data involved kriging and the production of gradationally coloured contour plots for each element. In addition, a factor analysis was conducted to ascertain relationships among all physical (field) and chemical ii parameters. The age range of the twigs prepared for analysis was found to make a difference to the concentrations of some elements. Whereas the preferred amount of growth was the most recent five years, at some locations trees were young and spindly such that it was only practical to collect 2-3 years of growth. With respect to the complete data set, the weak response in base metals to the Kingfisher deposits may be partly related to the relatively young growth of Douglas-fir in that area. A subset of samples from the Kingfisher area revealed some base-metal trends including Zn and Cd toward the centre of the known deposits, and zones of metal enrichments (e.g., Pb) peripheral to the deposits. Initially, plots of element distributions in all samples were prepared. However, for some elements (notably S) the age range of the twigs proved to be a critical factor and samples of consistent age were required for meaningful plotting and interpretation of the data. The dataset was reduced to the 475 samples for which there was 5 years of growth available and further statistical investigations of the data were made on this reduced dataset. For most elements of interest to this study, plots demonstrated that the element distribution patterns were robust, and that the over all influence of twig age did not significantly change these patterns. A procedure to create a 'Mineralization Score' succeeded in defining two main suites of base-metal-related elements - 1) Pb, Fe, Hg, REE, Al and Ti, and 2) Zn, Cd, Tl and Mn. Scores obtained for these suites were plotted and spatially-related zones of relative enrichment were identified: a) east of Mabel Lake, in the vicinity of the Tsuius River, adjacent to the Tertiary Ladybird granite; b) a zone between Mabel Lake and the Kingfisher deposits; and c) several small zones scattered around the survey area. The areas of Zn enrichment with associated elements tend to show a spatial relationship to areas of Pb enrichment (with Pb-associated elements), especially in the south central area of the Tsuius River. A strong Pb/Zn zonation is typical of Broken Hill-type Sedex mineralization. The geological unit that generates the strongest multi-element responses of the Zn association is comprised of amphibolitic schist, calcsilicate, marble and pelitic marble, mapped as Pcgma, notably in survey Grid B (Fig. 7) of the south central region. In particular, this unit occurs to the south of the Tsuius River, adjacent to the Tertiary Ladybird Granite. The unit that generates the strongest multi-element responses of the Pb association is adjacent the zone of relative Zn enrichment to its northeast, primarily in metasediments dominated by quartz-feldspar-biotite schist, mapped as Pqfh, across the Tsuius River and to its north. From this study, it appears that, when used in conjunction with other geological data and concepts, the careful application of biogeochemical methods may assist in the exploration for base metal deposits in this environment by identifying spatially-related multi-element zones of subtle enrichments. The technique is rapid and cost effective, and particularly appropriate for use in rugged areas such as the area of this survey where access is difficult and other surface sampling media may be inconsistent or difficult to obtain. The method can be used to provide focus for more detailed exploration activities.
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