The J-M platinum-palladium reef of the Stillwater Complex, Montana; I, Stratigraphy and petrology

Abstract

Exploration of the Stillwater Complex by the Johns-Manville Corporation has led to the discovery and delineation of an approximately stratigraphic zone rich in platinum and palladium comparable to the Merensky Reef. This zone, here called the J-M reef, is generally 1 to 3 m thick and has been traced for 40 km, essentially its maximum possible length within the outcrop area of the intrusion. It is distinguished by small concentrations (0.5-1.0%) of chalcopyrite, pyrrhotite, and pentlanditc in which are contained tiny grains of various platinum-group minerals (mainly moncheite, braggite, cooperite, kotulskite, and Pt-Fe alloy). One segment, 5.5 km long, averages 22.3 g (0.65 troy oz) Pt and Pd per short ton through 2.1 m. The ratio of Pt to Pd typically is about 1:3.5.A new stratigraphic classification of the Stillwater Complex is presented in which three long-standing, major divisions are renamed the basal, ultramafic, and banded series. The first comprises basal norite and basal bronzitite zones totaling 50 to 150 m in thickness; the second, harzburgite and upper bronzitite zones totaling 800 to 1,200 m. The banded series is a 4,500 m succession of plagioclase-rich cumulates with the J-M reef located 400 to 450 m above its base. It is divided into six megacyclic units (I-VI, from bottom to top) in which the principal zones are: (I) norire, gabbro; (II) norite, gabbro; (III) anorthosite, troctolite-gabbro; (IV) anorthosite-troctolite, troctolite-gabbro; (V) anorthosite, troctolite-gabbro; and (VI) gabbro, pigeonite gabbro. Two thin troctolite-anorthosite zones, I and II, occur at the top of units I and II, and troctolite-anorthosite zone I is host to the J-M reef.Where troctolite-anorthosite zone I is best developed in the west-central part of the complex, it is about 100 m thick and is associated with ten relatively continuous olivine-bearing members. These members are generally 1 to 5 m thick and are composed of one or more of peridotite, troctolite, and olivine norite. Members 1 to 4 alternate with varied, discontinuous successions of gabbro, norite, pyroxenite, and anorthosite that are collectively assigned to a gabbro subzone. Members 5 to 10 occur with anorthosite and norite in the anorthosite 1, norite, and anorthosite 2 subzones, and they can each be identified with a cyclic unit. The type rock sequence in five of the cyclic units is anorthosite-peridotite-troctolite; in the sixth, it is anorthosite-olivine norite-norite. The unit associated with olivine-bearing member 5 contains the J-M reef and extends to both ends of the complex; the others have variously been traced for distances of 15 to 36 km.Mineralogical data and 92 whole-rock analyses are presented for troctolite-anorthosite zone 1 and the adjoining zones. Plagioclase becomes gradually less calcic upward through this zone, and normatire anorthite/albite decreases sympathetically, but no cyclic mineralogic or chemical trends are evident. The mafic silicates tend to have higher Fe/Mg within and above the reef than below--a difference that may in part be primary but that is largely a postcumulus effect relating to the larger ratio of trapped (high Fe/Mg) intercumulus liquid to cumulus (low Fe/Mg) mafic minerals in the anorthosite 1 and 2 subzones as compared with the gabbro subzone. Similar variations are shown by MnO and NiO in the mafic minerals. The whole-rock variations of S and Cu indicate that the parental magma(s) of the banded series first reached continuous saturation with immiscible sulfide liquid at the level of the J-M reef, and from the compositions of other sulfide zones at higher levels it appears that the Pt and Pd contents of the magma(s) were effectively exhausted by the formation of the reef.Through the whole of the Stillwater Complex, two principal crystallization orders are indicated for the cumulus silicate minerals. The earlier order, defined in the ultramafic series and in megacyclic units I and II, is olivine, bronzite, plagioclase, augite. The later order, evident in units III through VI, is plagioclase, olivine, augite, bronzite (or hypersthene). The complicated stratigraphy of the troctolite-anorthosite zone I appears to represent an aborted transitional stage. Because both orders are manifest through cumulate successions thousands of meters thick, it would appear that two parental magmas were involved, the first with ultramafic affinities and the second with anorthositic affinities, and indications are that the Pt and Pd in the J-M reef came from the first, and the S from the second. A mechanism for forming the reef by mixing these liquids will be described in the second paper of this series.