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Here, we critically examine models for the development of layering in the ultramafic rocks of the Stillwater Complex of Montana, USA. A fine-grained, high-magnesium (high-Mg) dike in the Mountain View area of the complex is a plausible example of the type of magma that was parental to the layered cumulates in the Ultramafic Series. Modeled assimilation and crystallization of a primary komatiitic magma, involving 20% contamination by Archean granodiorite and 1% iron formation in a batch process, produces a liquid and solid assemblage like that in the dike. Equilibrium crystallization during assimilation of the crust produces cumulus olivine (Fo87–85) and chromite (Cr# = 64–55) consistent with the mineral chemistry of cumulus phases in the Peridotite Zone; each internally differentiated ultramafic layer could have formed by injection of a single highly contaminated batch of komatiite magma carrying about 10–20% suspended crystals. Whereas previous models of fractional crystallization to form the limited range of mineral and bulk-rock compositions observed in the Ultramafic Series required that these rocks accumulated from tens to thousands of times greater thicknesses of melt, the present model requires only four times as much melt as cumulates and dramatically diminishes the apparent volume of missing magma. The modeled crystallization of the proposed parental magma also demonstrates that, relative to olivine, only a small amount of cumulus chromite crystallizes in cotectic volume ratios of around 100:1 to 100:2 olivine to chromite. The modal abundance of chromite in the olivine-bearing cumulates from the Mountain View area varies continuously between 0·1 and 80 vol. %, with an average ratio of 100:9·2 olivine to chromite, rarely showing proportions consistent with either chromite-only or chromite + olivine cotectic crystallization. This, in turn, suggests that if olivine and chromite did precipitate in cotectic proportions, the crystals have subsequently been sorted mechanically to variable extents, producing the observed continuous range of modal proportions of chromite in the cumulates, and olivine has been preferentially removed from the system along with considerable amounts of escaped melt. Furthermore, the behavior of crystallizing assemblages at the olivine–orthopyroxene and the chromite–orthopyroxene peritectics forbids the deposition of the commonly observed mineral assemblages olivine + orthopyroxene + chromite or orthopyroxene + chromite by a process of fractional crystallization, but does permit them to form by deposition of mechanically sorted crystals derived from magmas carrying large crystal loads at internal equilibrium. Extremely efficient sorting of crystal-rich magma during lateral transport of crystals may therefore be responsible for the nearly monomineralic chromite seams in the Peridotite Zone. These considerations lead us to question the purported cyclicity of the cumulate rocks in the Peridotite Zone. Markov Chain analysis of the layering in logged drill core at various locations in the complex shows considerable departure from the suggested regular cycles of cumulate layers and provides no support for the classic model that units formed by closed-system equilibrium crystallization from mixed magmas following a succession of discrete magma recharge events. We suggest that the degree of contamination in the supply of crystal-laden contaminated magma transported to the site of deposition may have varied randomly, leading to the formation of layered rocks that do not conform to the previously accepted cyclic unit model of settled or in situ liquidus minerals. The layered rocks observed in the Peridotite Zone may have formed at the base of a magma chamber or they may not have, but their compositions and mineral modes do not convey any information about such a magma body if it was present at the time of their formation, because each layered unit appears to have formed from a completely separate volume of komatiitic magma. [ABSTRACT FROM AUTHOR]

The Cambridge Drill Core provides a continuous sample of the lower part of the floor cumulates of the Skaergaard Intrusion, including ~150m of stratigraphy from the unexposed Hidden Zone. Bulkrock geochemistry together with olivine mineral compositions and augite-plagioclase-plagioclase dihedral angles from the drill core are interpreted as a record of the early history of the Skaergaard magma chamber. A detailed geochemical and microstructural study of the mode and morphology of augite reveals no unambiguous markers that can be used to pinpoint the first appearance of cumulus augite in the stratigraphy sampled by the drill core. The early history of the Skaergaard magma chamber involved the arrival of multiple small batches of magma, each with a variable load of olivine and plagioclase phenocrysts. The region of the core between -120 and -108.8m records at least three separate, but closely spaced, influxes of new magma, with more magma added at -85 and -65 m. The last influx of magma is recorded by the cumulates just below the lowest exposed horizons of the Layered Series, involving a large volume of magma that inflated the chamber to its final size. The Skaergaard magma chamber therefore formed by the progressive inflation of what was likely to have originated as a sill intruded at the discontinuity between the Precambrian gneisses and the overlying plateau lavas. The 'Skaergaard parental magma' should therefore be viewed as the integrated and mixed composition of the various magma influxes that filled the chamber. [ABSTRACT FROM AUTHOR]

• Mafic cumulates in the Lower Banded series of Stillwater Complex are modeled by crystallization of komatiitic liquid contaminated by lower crust. • The olivine-bearing rocks and high tenor sulfides in the J-M Reef Pd-Pt deposit are generated by infiltration into and reaction with mushy gabbronoritic footwall cumulates by incoming contaminated komatiitic melt. The origin and parental magma for layered cumulates in the Lower Banded series (LBS) and the J-M Reef Pd-Pt deposit of the Stillwater Complex remains poorly constrained. We present whole-rock lithogeochemistry and mineral chemistry from LBS rocks collected from drill holes and surface samples from the Mountain View area of the complex that in total span nearly the entirety of the LBS stratigraphy. Excess S, Pt, and Pd in the noritic and gabbronoritic cumulates of the LBS indicate that small amounts of high tenor sulfide liquid generated at very low degrees of sulfide oversaturation were ubiquitous parts of the cumulate assemblage. We show that a simple two-stage thermodynamic model of assimilation-batch crystallization of a komatiitic parental magma in the lower crust, produces a close match to a common suite of fine-grained gabbronorite dikes and sills that intrude both the complex and its footwall. After fractionating ultramafic cumulates in the lower crust, the model contaminated komatiitic liquid produces upper crustal cumulates by batch crystallization en route to or at the level of the intrusion. The modeled rocks have compositions and mineral assemblages closely resembling pyroxenite of the Bronzitite zone and both norite and gabbronorite cumulates in the lower LBS. The trends from the Bronzitite zone through Norite zone I and Gabbronorite zone I can be understood as the result of deposition of crystals from successive batches of the same contaminated parental magma, with an upward trend toward greater amounts of cooling before the separation of crystals from liquid. The olivine-bearing suite of Olivine-bearing zone I, which includes the J-M Reef, can be modeled by partial remelting of the same norite and gabbronorite cumulates due to a temporarily increased flux of hot, moderately less contaminated LBS parental magma that infiltrated partially molten cumulates because its density exceeded that of the interstitial liquid. This model suggests that infiltration of hot Mg-rich parental liquid into moderately PGE-enriched footwall cumulates may be fundamental to the formation of the extremely high tenor sulfide mineralization in the J-M Reef ore zone, and perhaps other reef-type deposits worldwide. The same metal tenors that would require silicate/sulfide mass ratios (i.e., R-factors) of 105 to 106 in a single stage of equilibration would be attained during this second stage of interaction by the incremental infiltration and passage of LBS parental magma through previously sulfide saturated cumulate mush. [ABSTRACT FROM AUTHOR]