auspices of the Canadian International Development Agency (CIDA). A situation
arose that requir ed a biogeochemical survey in the Tapajo
´
s area of the Amazon, Para
´
State, Brazil (Dunn and Ange
´
lica, 2000). Prior to going into the field, the field party
had virtually no knowledge of the flora to be expected. The staging city for the
project in Brazil was Bele
´
m, where there are the botanical gardens of the Museu
Paraense Emilio Goeldi which, along with advice from their botanists, proved useful
for providing a first assessment of the common flora that might be present in the
survey area, close to the Amazonian village of Creporiza
˜
o. Additional information of
use was obtained from botanists at the University of Bele
´
m, from whom an assess-
ment could be made as to which species were likely to be dominant.
In environments of diverse flora such as the Amazon, Central Africa or Indonesia,
the field knowledge of local guides can be absolutely invaluable. Half a day with local
guides walking through the jungle is usually sufficient to establish the species that are
widely distributed, and which can be the focus for sample collection. Typically, local
guides know the common names of many species and the identification of their
botanical names can be established at a later date by reference to local government,
museum or university publications. In the case of the Amazon survey, of particular
value were two small publications available in Bele
´
m, ‘Guia Botaˆ nico do Museu
Goeldi’ and ‘Frutas Comestı
´
veis da Amazoˆ nia’ (Cavalcante, 1982, 1996).
In northern Saskatchewan, near the eastern edge of the Precambrian (Helikian)
sandstones that comprise the Athabasca Group, there are some of the world’s largest
and richest uranium deposits. A small-scale orientation survey in 1979 identified some
unusually high levels of U in twigs of black spruce (Picea mariana), with >100 ppm U
in twig ash compared to normal background levels of o1 ppm U. Over the succeeding
years, sampling was extended outward from this area (McClean Lake), and it became
apparent the orientation survey happened to be close to the core of what later proved
to be an extraordinarily large biogeochemical U ‘province’. To establish the limits to
this ‘Wollaston Uranium Biogeochemical Anomaly’ (Dunn, 1981), samples were col-
lected at 5 km intervals along the road leading to the core of the anomaly, and, since
there were no other roads in the area at the time, a float plane was used as transport
between lakes to collect additional samples at intervals of 5–10 km across the eastern
side of the Athabasca Group. As a consequence of logistical demands, there was no
consistent grid established, but more than 1000 samples were collected over a large
area on an opportunistic basis, with the highest sample density in the core of the
anomaly. Although not an ideal survey design, for a broad reconnaissance view of an
unknown area, this approach can provide valuable information.
Ultimately, the very low-density sampling of the Athabasca area, with irregularly
spaced sample intervals, sufficed to sketch an approximate limit to the 10 ppm U
contour, and define the limits of the 50 ppm and 100 ppm U contours. Whereas black
spruce twigs usually contain less than 1 ppm U in ash, the regional background of
black spruce twigs from much of northern Saskatchewan is 2 ppm U, no doubt
because of the world-class grade and extent of the many uranium deposits that have
subsequently been discovered in that area.
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Biogeochemistry in Mineral Exploration