Mineralization

PGE in the Ultramafic Series

The Stillwater Project covers portions of 17 km of strike in the Ultramafic Series and Basal Series rocks.  The Ultramafic Series contains 13 laterally extensive chromitite seams, of which the two lowermost seams, the “A” and “B” chromitites, comprise promising PGE target horizons similar to the UG2 Reef of the Bushveld Complex.

A significant portion of PGE production from the Bushveld Complex now comes from the UG2 Reef, which is about 0.6 m thick and contains 8 to 9 g/t PGE.

The PGE-bearing “A” and “B” chromitite seams at the Stillwater Project have been sampled sporadically by trenching and limited drilling. The seams vary from less than 1 m up to 3 m or more in thickness, with the widths and grades apparently increasing gradually down-dip into an as-yet untested primary magmatic basin developed during emplacement of the Stillwater Complex. In the Stillwater Complex, as in the Bushveld Complex, PGE grades typically increase as the mineralized seams (reefs) thicken into troughs or basins thought to be caused by structural subsidence in the floor of the magma chamber.

Crescent Creek
Approximately 10 km west of the Stillwater River, the Crescent Creek area is underlain by 3.7 km of strike along the “A” and “B” chromitite seams. Limited surface channel sampling conducted by Anaconda Company (“Anaconda”) in 1979 indicates the presence of anomalous PGE over narrow widths in these two seams. (For a complete list of drill hole intercepts please see the Historical Data Section).

Historical drilling at Crescent Creek by Anaconda, in the late 1980s, and by Chrome Corporation of America in the late 1990s, totals about 20 shallow (<150 m depth) small-diameter core holes. All of these historical holes reportedly intersected anomalous PGE over narrow widths (generally 1 m or less). (For a complete list of drill hole intercepts please see the Historical Data Section).

Three of the historical Crescent Creek holes were drilled on a section to examine the “B” Chromitite seam at increasing depth. The results indicate a progressive thickening of the seam down-dip over a 65 m interval accompanied by an increase in Pt, Pd, and rhodium (Rh) values. This may represent thickening into a primary magmatic basin, similar to that observed in the Bushveld Complex. The Rh values in the Crescent Creek area are particularly interesting, with an average Pt:Pd:Rh ratio of approximately 4:10:1, generally similar to the 4:7:1 ratio in the UG2 Reef in the Bushveld Complex. The high Rh content is especially encouraging considering this rare element is currently valued in the $8,500 to $9,500 per ounce range.

Mouat Mine Area
Approximately 2 km west of the Stillwater River, the PGE-bearing “A” and “B” chromitite seams are exposed sporadically along 2.5 km of strike. These represent potentially important PGE exploration targets, but they have not been adequately tested by surface sampling or drilling.

Locally, the “B” chromitite seam exceeds 2.3 m in width and trench samples collected along a 360 m long exposure average 2.49 g/t PGE, with one very narrow interval (0.12 m) containing 2.8 g/t Pt, 6.4 g/t Pd, 0.9 g/t Rh and 0.6 g/t Ru. Only one hole (drilled in 1988) was drilled to test the “B” seam at depth, but this hole failed to intersect the seam due to unexpected fault complications.

Nye Basin Area
Immediately east of the Stillwater River, the PGE-bearing “A” and “B” chromitite seams extend for at least another 4 km along strike. This area has been scarcely tested for PGE. In 1985, Anaconda drilled five shallow, small-diameter core holes in the “A” and “B” chromitite seams for claim assessment requirements, but none of these holes was assayed for PGE and the cores were subsequently misplaced.

Ni-Cu-Co±PGE in the Basal Series

Mouat Mine Area
The Stillwater Project includes more than 12 total km of strike of the Basal Series. In the 1960s and 1970s, Anaconda explored this sequence in the Mouat Mine area, 2 km west of the Stillwater River, with a grid of 127 closely spaced core holes, totaling more than 33,000 m, and 500 m of underground development. This work defined a published historical resource estimate (1987) of 92 million tons grading 0.27% Ni and 0.29% Cu, using a 0.2% Ni cutoff, or 23 million tons grading 0.62% Ni and 0.45% Cu, using a 0.4% Ni cutoff. These are historical resource estimates only and not current resources as defined by NI 43-101. Nickel and copper prices at the time were both less than $1.00 per pound. No systematic sampling was done for PGE.

Nye Basin
Anaconda tested continuity of Ni-Cu mineralization in the Basal Series in the Nye Basin–Benbow areas. They drilled a series of 32 widely spaced drill holes (6,450 m), of which 25 successfully intersected the correct magmatic stratigraphy.

All 25 of these holes were mineralized, averaging 0.6% Ni, 0.5% Cu and 0.06% Co over drill widths of 2 to 30 m, with individual holes ranging from 0.3% to 1.0% Ni, 0.2% to 1.0 % Cu, and 0.04% to 0.09% Co.  Eleven of the historical holes — principally those located close to transverse faults which cut across the stratigraphy — intersected multiple stratiform mineralized zones. (For a complete list of drill hole intercepts please see the Historical Data Section).

Historical estimates of mineralization published by the United States Geological Survey (1987) for the Nye Basin area are in the range of 70 to 90 million tons grading 0.6 to 0.8% Ni and 0.6 to 0.8% Cu, and for the Benbow mine area are 10 to 20 million tons at 0.4 to 0.5% Ni and 0.5 to 0.6% Cu.  

The core from these Ni-Cu±Co zones was subsequently consumed in metallurgical testing and never assayed for PGE. However, Anaconda did electron-probe microanalysis of a single sulfide concentrate sample from hole # NB-17A in their Tucson laboratory on a few grains each of pyrrhotite (iron sulfide), chalcopyrite (iron-copper sulfide), and cobaltiferous pentlandite (iron-nickel-cobalt sulfide). The actual concentration factor is unknown, but because the sample was reportedly taken from massive to semi-massive sulfide interval containing 50% or more sulfide minerals, the sulfide concentrate was presumably about 2 times. The average PGE tenor of the three sulfide phases in the concentrate are as follows:

These results provide encouraging evidence of possible PGE enrichment and suggest that the Ni-Cu±Co zones in the Nye Basin–Benbow areas should be systematically explored for PGE.

Chromite in the Ultramafic Series
The Stillwater project also includes portions of the “G” and “H” chromitite seams that host four massive chromite deposits of potential commercial importance. Two of these deposits, Mountain View and Benbow, are sites of historic mines responsible for a significant portion of the United States’ chrome supply during the 20th century up to the Korean War. These two deposits plus two other drill-defined deposits, Benbow East and Nye Basin, contain substantial amounts of chromite consistently similar in grade and thickness to that historically produced.

Extensive studies, including drilling and underground exploration, done by Chrome Corporation of America and others since the Korean War demonstrate in-place historical resources as follows:

Collectively, these four deposits represent the largest historical chromitite resource known in North America. They have had only limited sampling for PGE. 

In 1986–87, Chrome Corporation of America completed a four-volume feasibility study of the Mountain View deposit as a prelude to opening a 132,000 ton/yr high-carbon ferrochrome operation, but the project was subsequently abandoned due to lack of funding. No work has been done since.

Other Potential PGE Targets
The Stillwater project has other potential, but untested, PGE targets. These include the Volatile Enriched Zone ("VEZ"), at the very top of the Ultramafic Series, and scattered dunite bodies of unknown size, which cross-cut stratigraphy of the two lower lithologic sequences and may represent the feeders to PGE mineralization in the J-M Reef