The nickel in the Oregon deposit occurs in both the peridotite and the overlying soil. A deep lateritic-like soil is developed locally on the peridotite and serpentine and constitutes the ore. The average nickel content of the lateritie soil is less than one per cent, whereas the nickel content of the peridetite is about 1 to 1.5 per cent. The nickel occurs as garnierite (Mg, Fe, Ni, Mn)3(OH)4 (SiAl)2O5) in the soil, and as lattice replacements in the olivine and pyroxene
in the peridotite.
Although the areas are different geologically, the approach in prospecting for them is fundamentally the same. The procedure is twofold:
- A rapid reconnaissance survey to outline an area of high nickel content.
- A more detailed survey to outline zones of ore grade within the area of high nickel.
Chemical analyses of nickel were made by a dimethylglyoxime colorimetric test, similar to standard test for nickel utilizing a concentrated sulfuric acid for extraction.
The lower limit of detection for nickel in soil by the method used was about 10 ppm., and for nickel in plants about 5 ppm. The relative deviation was about twenty-five per cent.
The soil samples were taken in a zone from one inch to one foot below the humus layer, within a 15 foot radius, around a station. Approximately 100 grams of soil were taken in each sample,
The nickel content in the soil over the Oregon locality is considerably greater than the background nickel content, and at some stations, is 500 times the background metal concentrations.
In general, two zones of high nickel content are noted, one at about 6 inches, and the other at about 18 inches. Thus, a composite sample taken from 6 inches to 18 inches would include the zones of strong nickel enrichment.
The nickel content of the soil over the norite rocks at the base of the Stillwater complex is about 250 ppm., and is more than the nickel content of soil over underlying gravels, glacial debris, and granite, which is about 10 ppm. nickel.
Samples of first, second, and third year growth twigs were collected from four or five individuals of each species, within a 15-25 foot radius about the station. Trees of about the same height were sampled, and samples were taken at eye level around the tree. Enough samples were taken to fill a paper bag measuring 3 x 6 x 11 inches.
The nickel that can be absorbed by plants is the nickel readily available and readily soluble in the soil. Although much more nickel is present in the soil at the Oregon locality than at the Montana locality, the latter soil has sufficient nickel that might be available to the plants.
A way to estimate the amount of nickel in the soil available to plants was desired. One way might be to use a weak chemical extraction that would give a nickel content comparable to the soluble nickel available to plants. All the samples which contain more than 5 ppm. nickel by an acetate buffer extraction were collected at the Oregon locality, although the samples from Montana contain a high nickel content.