"Rock of the Month # 286, posted for April 2024" ---

Platinum-palladium ore from the Stillwater complex, Montana, U.S.A.

The MINLIB bibliography contains some 400 references to the Stillwater complex, a testament to its size, long history of research, and economic importance. Here we will draw a brief history of the evolving understanding and mining ventures on the Stillwater, citing just 34 of those references, and see how the Stillwater inspired scientists and miners to look at layered intrusions in new and more sophisticated ways. The period 1979-1991 saw intense activity, with parallel developments in research and mineral exploration.

Layered intrusions

Here we will look at the host intrusion, as much as at the one representative yet essentially random rock sample! So we will look at the development of scientific research, exploration and mining over some eight decades. The Stillwater complex is composed of a vast volume of plutonic igneous rocks, a large layered intrusion, one of the most famous and best-studied in the world: amongst the best-known examples are the Bushveld complex (South Africa); Skaergaard (east Greenland); Rhum (Inner Hebrides of Scotland); the Great Dyke (Zimbabwe); and Stillwater (Montana). Each formed in the Earth's crust, from one or more parent magmas in magma chambers of widely varying size and geometry. Well-preserved examples are found across some 3 billion years of Earth history, Stillwater and Rhum being old and young examples, respectively. These and others are described, often in great detail, in the classic textbook by Wager and Brown (1968, e.g., Stillwater, pp.298-342). The more refractory minerals such as olivine and chromite crystallize first, at the highest temperatures and, being denser than the host magma, sink to the bottom and form deposits of one or more minerals with distinctive textures, known as cumulates (e.g., Jackson, 1967).

The Stillwater layered intrusion

Though reports of PGE [platinum group elements] appeared in the 1920s, the economic potential of the Stillwater complex was first exploited in terms of chromite deposits (for steel production) in the lower, ultrabasic zone (e.g., Peoples and Howland, 1940). Postwar, in the 1970s, a different style of mineralization was found in the feldspathic, banded zone that overlies the ultrabasic zone. This became known as the Johns-Manville prospect (the J-M Reef or Howland Reef), a thin layer containing sulphides and values of Pt and Pd (Conn, 1979). Page et al. (1985) reviewed the early decades of exploration at the Stillwater complex.

The mineralogy of the Stillwater has also been of great interest, for in terms of igneous petrology (pyroxenes: Hess, 1941) and later for ore minerals. The banded zone was soon investigated (e.g., McCallum et al., 1980). The principal sulphides are pyrrhotite, pentlandite and chalcopyrite (Fe-Ni-Cu-S), while prominent amongst many PGM species are the braggite-vysotskite solid solution series (PtS-PdS), typically in zoned base-metal sulphide globules (Wolfgram and Evans, 1980; Bow et al., 1982). A wealth of PGM (sulphides, tellurides, arsenides, etc) were soon described (e.g., Volborth et al., 1986). Zientek and Oscarson (1987) conducted an SEM study on a small high-grade sample, and located some 160 PGM grains of at least 14 species, in a 5 cm² area. Notably, 80% of 37 ppm assayed Pd is hosted in pentlandite, whereas all the 55 ppm assayed Pt is accounted for by observed PGM species. Pentlandite in veinlets carries an astounding 3.5 to 8.7 wt.% Pd. Various authors have undertaken detailed studies on the textures and speciation of the PGM in these rich ores (e.g., Godel and Barnes, 2008).

The PGE-enriched base metal sulphides occur in the Lower Banded Zone, some 400 m above the top of the Ultrabasic Zone (Barnes, 1981). This early work by Steve Barnes found that S and PGE are strongly correlated, and sulphide liquid immiscibility within the silicate magma led to extreme PGE concentrations, the sulphide liquid containing about 6500 ppm Pd and 2000 ppm Pt. Barnes, Ian Campbell and Tony Naldrett found that the J-M Reef occurs "at an interval marked by temporary reappearance of olivine and chromite as cumulus phases, implying an origin from an influx of primitive magma into the chamber" (Barnes et al., 1982; also Campbell et al., 1983). Immiscible sulphide liquid precipitated on the magma mixing of ultramafic and anorthositic parent liquids in the magma chamber (Irvine et al., 1983). A later review (Barnes and Naldrett, 1985, 1986) described the PGE mineralization in olivine-bearing, plagioclase-rich cumulates: "the reef consists of a 2-m-thick layer of disseminated sulfides extending over the entire strike length of the complex". At the Minneapolis Adit, the metal content in the sulphide fraction was both extraordinary and constant over a 700-metre strike length: 100% sulphide contained an estimated 9.3% Ni, 6.9% Cu and 0.7% total PGE (mostly Pd). Given that the clarke, the estimated average crustal concentration, of each PGE is on the order of 5 parts per billion and less, the elementral abundances in the sulphide phase are quite extraordinary.

Keays et al. prepared a geochemical profile through the western part of the complex, in the Chrome Mountain and Contact Mountain areas, where the J-M Reef is located about 1460 m above the intrusion floor. Given the high grade, it is perhaps no surprise that the J-M Reef in the section is readily pinpointed by plotting Pt, Pd, Pd/S, Pd/Se and (Pt+Pd) as a function of height in the intrusion.

Meanwhile, geophysical work (gravity surveys) indicated that the Stillwater intrusive complex was likely far more extensive than its surface area of 194 km², the total area perhaps exceeding 4400 km², extending far to the east of the rugged outcrop, as a flat sheet under sedimentary cover (Bonini, 1981). The geochronologists were also busy. U-Pb zircon and baddeleyite age dates for the Lower Banded Series and the JM Reef indicate an age of 2702 ± 2 Ma (Premo et al., 1989).

Mining palladium, platinum and associated metals

Resource estimation, engineering designs and permitting led to the onset of mining and then mine expansion along the J-M Reef in the 1980s and 1990s. The Stillwater became a major source of palladium, and for some time the only primary palladium mine in North America. The first two decades of work on the deposit are summarized below.

Rapid exploration and delineation of ore-grade rock along the J-M Reef, which in idealised form is a conceptually and geometrically simple target, rapidly expanded the estimated resources and reserves of Pd-rich ore. New data from adit and drillcore provided a more complex detailed view (Raedeke and Vian, 1986). Van der Veen (1987) summarised the rapidly-evolving picture of the deposit: the host intrusion crops out over a strike length of 48 km; overall, the J-M Reef is almost 2 m thick and carries 0.5 to 2% sulphides in noritic rocks; the bulk of PGE occur as PGM of the braggite-vysotskite series; the Ni / Cu ratio is relatively high at 1.3; and the combined (Pt+Pd) grade is extremely high at an estimated average of 32 ppm.

Mining was slated for a start in 1987, with an estimated Pt:Pd ratio of 1:3, and a modest initial mining rate of 500 tons/day (Dingwall, 1986; Vian, 1987). The initial plan called for a series of 13 adits to be driven over a 20 year period, employing cut-and-fill mining. The scale of mining increased rapidly in the 1990s. The Stillwater mine was projected to produce 500,000 oz combined Pt plus Pd in 1998 (Anon, 1998). The total proven and probable resource on the 45-km strike length of known mineralization, at the end of 1997, was estimated at 29.5 MT grading 0.79 oz/T PGE (23.4 million oz PGE), and further expansion of annual output to 1.2 million oz combined Pt + Pd was planned for 2001 (see Danielson, 1998, who also summarised the history of exploration and the mine site details of the project; see also Anon, 1999: the actual 1998 Pt+Pd production was an impressive 444,000 oz). An expansion of the Stillwater mine to 3,000 T/day was scheduled for completion in 2001, and a second mine was planned at East Boulder, 21 km west of the original mine, with projected production of 2,000 T/day (Worthington, 2000). By the end of 2003, the total proven and probable resources were quoted as 40.4 MT grading 0.58 oz/T PGE, with a Pd : Pt metal ratio of 3.6:1 (Danielson, 2004). Jumping to the present, a recent statement of remaining identified resources at the J-M Reef, in the 2023 annual report of operator Sibanye-Stillwater, identified 15.7 million oz Pt+Pd at the Stillwater mine, and 10.6 million oz Pt+Pd at the East Boulder mine.

The J-M Reef appears to have remarkable continuity along strike, that is, forming a consistently located marker layer within the intrusion. There are additional features, such as graphite-sulphide-PGE mineralization in pegmatoids stratigraphically below the main J-M Reef (Volborth et al., 1985, 1986). These can be compared to the well-known "pothole" features associated with the Merensky Reef of the Bushveld complex. Also noted along the J-M Reef are rich mineralized masses at the base of the Reef, known as "ballrooms" (e.g., Danielson, 1998). These ballrooms (analogous to pothole structures described previously) comprise tens of metres of olivine-bearing rocks and/or PGE-rich sulphide mineralization forming structures that cut down into the footwall rocks. The ballroom olivine has a composition of Fo_79-76, and sulphides leaking down into the mushy footwall can produce high grades (Jenkins et al., 2021). These may also occur in other intrusions, perhaps where the mineralized intrusive intersects a discontinuity, such as faulting in the footwall, offering a zone of weakness into which sulphide-charged melt may sink (e.g., the Current Lake chonolith in northwest Ontario: Clean Air Metals, 2025).

Fig. 2: A close-up of one sample, showing a clot of granular sulphides, largely pyrrhotite and chalcopyrite. Sample 529B.