Abstract: Much of west-central Yukon escaped glaciation and is underlain by leached and oxidized, deeply weathered bedrock. Background and anomalous metal values in these soils are presumed to be lower than normal and therefore increase the challenge of interpreting soil geochemical surveys. It is suspected that the geochemical contrast between background and anomalous values can be maximized by sampling a particular soil horizon or analysing a specific size fraction.

One hundred and sixteen samples were collected from soil profiles at ten sites above four porphyry-style mineral deposits - Antoniuk, Revenue, Nucleus and Casino in the Dawson Range of the Yukon-Tanana Terrane. B-horizon soils are locally clay-rich, thin and poorly developed. C-horizon soils are unconsolidated and very coarse grained. Bedrock samples from each site were crushed and analysed for comparison. All samples were analysed using the Au plus 32 (FA-AA and ICP) package which is a popular and economical method currently employed in most exploration programs.

The results are generally consistent within the few samples taken at each deposit, but vary considerably overall, and probably reflect the variable physio-chemical conditions between the deposits. Gold and copper are emphasized in the evaluation of the data, but gold data from the fine-grained fraction are incomplete due to insufficient material from the generally coarse-grained soils. Although statistical accuracy cannot be confirmed with such a small sample set, the following trends are recognized:

- the highest gold values typically occur in the lowest soil horizon (C2) and in the -200 mesh fraction;
- the highest copper values were found in both the -35+80 and -200 mesh fractions (deposit dependent) but were consistently richer in the C2 horizon;
- metal values are commonly depleted in the highest B-horizon (B1), particularly in the +35-80 fraction;
- metal values are most commonly enriched in the B2 and C2 horizons;
- metal depletion is most pronounced in the -35+80 fraction;
- there is a strong positive correlation between gold, copper and molybdenum;
- in addition, As, Bi, P, and to a lesser extent, Sb and Sr have a strong positive correlation with gold, As, Sb, Sr and to a lesser extent Ba and Pb have a strong positive correlation with copper.

Authors: Hart, C.J.R. and Jober, S.A.

Map Scale: 1 : 0

NTS Mapsheet(s): 115I, 115J

Citation: Hart, C.J.R. and Jober, S.A., 1997. Soil Geochemistry above Deeply Weathered Porphyry Deposits in Unglaciated Terrain, Dawson Range, Central Yukon. In: Yukon Exploration and Geology 1996, Roots, C.F. (ed.), Indian Affairs & Northern Development: Exploration & Geological Services Division, p. 107-121.

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