| Abstract | Studies over the past half century have greatly advanced the understanding of biogeochemical processes. Mapping of multi-element patterns in southern British Columbia suggests that, among other
elements, Tl and Hg may be biogeochemical pathfinder elements for Broken Hill-type Sedex deposits. The halogens in plants, too, may provide additional vectors toward base metal-mineralized areas. Opportunistic sampling during field mapping projects
can add important focus for detailed exploration, and help trace the source of till anomalies. Molybdenum in cedar foliage has outlined the MAX Moly mine area and generated additional targets of similar magnitude of unknown origin. Investigation of
one of these in southern BC has resulted in the discovery of low-grade mineralization in rocks exhibiting similar characteristics to the MAX Moly deposit. Moving forward, more emphasis on the distribution of volatile elements (e.g. Hg; halogens
associated with fluid inclusions) as well as commodity elements, and an improved understanding of the role of bacteria in mobilizing metals should further entrench the role of biogeochemical methods for assisting in the exploration for mineral
deposits.
KEYWORDS: Biogeochemistry, Exploration, Base Metals, Gold, REE, Molybdenum
INTRODUCTION
Over the past half century there have been significant developments in the understanding and application of biogeochemical methods
to the search for buried mineral deposits - both deep and shallow. It is now apparent that depth of root penetration and relationship of plant chemistry to soil composition are not critical to obtaining subtle biogeochemical responses to deeply
buried mineralization. Various processes invoked for explaining partial leach soil anomalies are equally relevant for explaining signatures of elemental anomalies in plant tissues. These include movement by diffusion, electrochemical cells, seismic
pumping, artesian flow, and particularly bacterial movements. These processes are, however, somewhat modified and controlled by plant requirements and tolerances, so that the 'barrier mechanisms' identified by Kovalevsky (1979) come into play and
need to be considered when interpreting data from biogeochemical surveys.
The Geological Survey of Canada has a program, which in south and south-central British Columbia helps stimulate base metal exploration activities. The terrain in much
of the south-central British Columbia 'Targeted Geoscience Initiative' (TGI-3) area is rugged, has variable thickness of till cover and is heavily forested, yet in light of past discoveries it remains highly prospective for various types of metal
deposit. Of five known groups of Zn/Pb deposits within a cover sequence that mantles Palaeoproterozoic core gneiss, mineralization at Kingfisher and adjacent deposits comprises the only group below the tree-line. The widespread regional distribution
of thin calcareous 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.
Mabel Lake Area
As part of
the TGI-3 program several biogeochemical surveys have been undertaken. A helicopter-supported survey was conducted over 700 km2 that included the stratigraphic setting of the Kingfisher deposit and similar stratigraphic units that border Tsuius
Creek, to collect 562 Douglas-fir tree tops at 1 km spacing.
Analysis of dry twigs revealed areas of Zn enrichment with associated elements (Cd, Tl and Mn) that show a spatial relationship to areas of Pb enrichment (with Fe, Hg, REE, Al, and
Ti), especially south of Tsuius Creek. A strong Pb/Zn zonation is typical of Broken Hill-type Sedex mineralization. Of particular note were localized high Tl values.
Subsequent analysis of the ashed needles from these twigs confirmed the
patterns established from the dry twigs. It served to enhance some signatures (notably Ag) and added a layer of data for the halogens, demonstrating a F association. Also from these additional analyses a pattern of Cd and B enrichments at Kingfisher
was evident.
Follow-up ground studies involved the analysis of outer bark from western hemlock collected at 100 m sample spacing over the Tl anomaly south of Tsuius Creek. This permitted more clearly defining the extent of the Tl anomaly (600
m x 600 m) and its relationship to other trace elements. Figure 1 shows the Tl anomaly and the spatial relationship of Hg which appears to define a conjugate set of lineaments interpreted as leakage from structural weakness. The multi-element
association at this locality suggests that Tl and Hg may be pathfinder elements to concealed base-metal mineralization.
Bonaparte Lake Area (Thuya Batholith)
In the Bonaparte Lake area of south-central BC, samples of outer bark from
Engelmann spruce and lodgepole pine were collected on an opportunistic basis during the course of field mapping. Consequently, the sample distribution was uneven, but expedient and sufficient to provide indications of areas with potential concealed
mineralization.
Five hundred samples were collected. The analytical data provided a basis for comparing and contrasting the geochemical signatures of the two types of bark and defining those elements that generated the same or similar
signatures while establishing other elements that generated different distribution patterns because of tolerances to, or requirements for, those elements. Most elements generated similar distribution patterns attesting to the robustness of the
biogeochemical method and reinforcing the significance of the signatures (Dunn & Anderson 2009). Elements that tended to show different patterns (e.g., Ba, Sr) were those that were significantly more concentrated in a particular sample medium. The
pine bark was more enriched than the spruce in Ag, Al, Cd, La, Pb and Sb. Conversely, spruce was more enriched in Ba, Ca, Mn, Rb, Sr, and Zn.
Results from a recent till survey (Plouffe et al., 2009) revealed unusually high concentrations of
thorianite in the heavy mineral concentrates. Plots of thorianite-related elements - Th, U, and REE - in the conifer bark samples indicate an area of subtle enrichment that is located up-ice from the proven direction of ice movement. This may help
to more closely focus on the source of the thorianite. Multi-element signatures in the bark assisted, too in identifying known Cu mineral occurrences, delineating areas of Au enrichment and possibly PGE, as well as defining signatures characteristic
of the principal plutonic and volcanic units. For example, there are significant increases in Ni, Cr and Co over known and concealed mafic to ultramafic units.
MAX Moly
The MAX property lies near the north end of the Kootenay Arc in
tightly folded, strongly sheared Palaeozoic metasedimentary rocks. The reported commodities are Mo, W, Pb, Zn and Cu. The area is heavily forested rugged terrain with outcrop largely obscured by a cover of till. Cedar foliage was collected along
several traverses up the steep hillside toward known mineralization.
The location of the discovery outcrop was clearly outlined by Mo in cedar, and two additional Mo anomalies of unknown source and similar intensity were identified (Fig. 2).
In mid-2008, Roca Mines drilled two drill holes, 100 m apart that targeted the northern biogeochemical anomaly. They reported "Intense silicification, hornfelsing, locally strong quartz veining, and pervasive sericite alteration with trace
molybdenite throughout¿.reminiscent of the MAX resource itself where the extent of a relatively minor molybdenite mineralized zone on surface lies atop a large-scale mineralized deposit currently being mined." Roca Mines press release, 12th August
2008 (www.rocamines.com).
CONCLUSIONS
(1) Examination of data from past biogeochemical surveys in context of subsequent discoveries has established their relationship to mineralization.
(2) Pathfinder and commodity element
distribution patterns from recent surveys have delineated new targets for base metals and other commodities, and resulted in new discoveries.
(3) It is anticipated that focus on volatile elements (especially halogens and Hg) and improved
understanding of processes (notably bacteria) in mobilizing metals from depth will provide further successes for biogeochemical surveys. |