Re--Os and Sm--Nd Isotope Geochemistry of the Stillwater Complex, Montana: Implications for the Petrogenesis of the J-M Reef

Lambert, David D; Walker, Richard J.; Morgan, John; Shirey, Steven B.; Carlson, Richard W.; Zientek, Michael L.; Lipin, Bruce R.; Koski, Michael S.; Cooper, R. L.

doi:10.1093/petrology/35.6.1717

Cited by 104 publications

(30 citation statements)

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“…Ford and Himmelberg, 1991); and it is difficult not to see the great similarity between Dufek and Stillwater. Detailed isotopic studies of Stillwater cumulates suggest, for example, an involvement of geochemically distinct magmas throughout the history of the body (Lambert et al, 1994). Textural studies of the Kap Edvard Holm layered gabbro complex in eastern Greenland reveal a series of quench zones reflecting sequential injections along the chamber floor (Tegner et al, 1993).…”

Section: Solidification Front Instabilitymentioning

confidence: 99%

Solidification fronts and magmatic evolution

1996

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From (3. F. Becker's and L. V. Pirsson's early enunciations linking the dynamics of magma chambers to the rock records of sills and plutons to this day, two features stand at the centre of nearly every magmatic process: solidification fronts and phenocrysts. The structure and behaviour of the envisioned solidification front, however, has been mostly that akin to non-silicate, non-multiply-saturated systems, which has led to confusion in appreciating its role in magmatic evolution. The common habit of intruding magmas to carry significant amounts of phenocrysts, which can lead to efficient ffactionation, layering, and interstitial melt flow within extensive mush piles, when coupled with solidification fronts, allows a broad understanding of the processes leading to the rock records of sills and lava lakes. These same processes are fundamental to understanding all magmas.The spatial manifestation of the liquidus and solidus is the Solidification Front (SF); all magmas, stationary or in transit, are encased by SFs. In the ideal case of an initially crystal-free, cooling magma, crystallinity increases from nucleation on the leading liquidus edge to a holocrystalline rock at the trailing solidus. The package of SF isotherms advances inward, thickening with time and, depending on location --roof, floor, or walls --and the initial crystallinity of the magma, is instrumental in controlling magmatic evolution. Bimodal volcanism as well as much of the structure of the oceanic crust may arise from the behaviour of SFs.In mafic magmas, somewhere near a crystallinity (N) of 55% (vol), depending on the phase assemblage, the SF changes from a viscous fluid (suspension (0

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Section: Solidification Front Instabilitymentioning

confidence: 99%

Solidification fronts and magmatic evolution

1996

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“…Typically, contact-style PGE-Ni-Cu mineralization similar to that present within the GNPA member and Platreef is often attributed to the development of an immiscible sulfide liquid through in situ contamination by assimilation of crustal S (e.g., Duluth Complex; Mainwaring and Naldrett 1977;Ripley 1981;Ripley et al 1986 and the Basal Series of the Stillwater Complex; Lambert et al 1994;Lee 1996;McCallum 1996). Within the Platreef, it is now accepted that early contamination at depth induced S saturation, with localized contamination acting only as an ore-modifying process McDonald and Holwell 2007;PennistonDorland et al 2008;Ihlenfeld and Keays 2011).…”

Section: Introductionmentioning

confidence: 99%

Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

2014

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The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma-mass spectrometry. These data, coupled with wholerock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt-As and Pd-Bi-Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400-800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE teno...

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“…Luanga is comparable to chromitites from layered mineralized complexes, like Stillwater and UG2, in terms of its Pd/Ir ratio and PGE content, whereas Niquelândia values are similar to chromitites genetically related to depleted mantle portions and their 'trapped' melting products, thus characterized by low Pd/Ir ratios and PGE contents. Lambert et al (1994) emphasize the association between high 187 Os/Os 188 ratios and important PGE deposits. The origin of high contents of both components is a matter of controversy, even in Bushveld and Stillwater, two of the best studied maficultramafic complexes in the world.…”

Section: Chromititesmentioning

confidence: 80%

“…Several authors advocate the assimilation of crustal rocks by the parental magma (Lambert et al, 1989(Lambert et al, , 1994Horan et al, 2001). Boudreau et al (1986) attribute the increase of 187 Os to hydrothermal fluids in massive chromite layers, contemporaneous to the last stages of crystallization or during postmagmatic reequilibration.…”

Section: Chromititesmentioning

confidence: 99%