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Origin of Chromitites in Layered Intrusions: Evidence from Chromite-Hosted Melt Inclusions from the Stillwater Complex

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Origin of Chromitites in Layered Intrusions: Evidence from Chromite-Hosted Melt Inclusions from the Stillwater Complex Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex Carl Spandler* Research School of Earth Sciences, Australian National University, Canberra 0200, Australia John Mavrogenes Research School of Earth Sciences, and Department of Earth and Marine Sciences, Australian National University, Canberra 0200, Australia Richard Arculus Department of Earth and Marine Sciences, Australian National University, Canberra 0200, Australia ABSTRACT 1993); and (5) assimilation of country rock by primitive magma (Ir- Chromitites from layered ma®c intrusions are of great eco- vine, 1975; Kinnaird et al., 2002). nomic importance, yet the origin of these deposits remains enig- The antiquity and slow cooling of most chromitite-bearing layered matic. We describe multiphase silicate inclusions trapped within intrusions have hindered our understanding of the genesis of chromi- chromite grains from the G chromitite seam of the Stillwater Com- tites and layered intrusions in general. The primary petrologic and geo- plex, Montana, United States. These inclusions are interpreted to chemical evidence that are crucial for determining the evolution of represent melt trapped during chromite growth and hence provide cumulate rocks are often removed during subsolidus reequilibration and information on chromitite formation. Most reheated inclusions subsequent hydrothermal alteration or metamorphism. Furthermore, the have variable quench textures and chemical compositions that are compositions of the parental magmas of these intrusionsÐcritical in- consistent with variable degrees of mixing between a high-Mg ba- formation for understanding chromitite formationÐremain poorly saltic parental magma and a Na-rich trondhjemitic melt. The tron- constrained. dhjemite is suggested to derive from partial melting of ma®c or Melt inclusion studies have been extensively used for investigat- ing volcanic systems, yet have largely been ignored by researchers of metasedimentary country rocks. Based on these inclusions, we out- layered intrusions. Trapped within cumulus minerals, melt inclusions line a model for chromitite formation involving ponding of a new may remain unaffected by postcrystallization alteration and hence may pulse of primitive magma at the roof of the Stillwater magma be useful for determining parent magma compositions (Spandler et al., chamber, followed by localized partial melting and assimilation of 2000) or for unraveling complex processes such as magma mixing or the country rock. The newly formed hybrid melts become oversat- assimilation. In this paper we examine multiphase inclusions within urated in chromite, leading to extensive chromite crystallization. chromite from the G chromitite seam of the Stillwater Complex, Mon- Chromitite horizons are proposed to form from dense chromite- tana. These inclusions are interpreted to be crystallized melt inclusions rich plumes that periodically sink down from the roof zone to settle that were trapped during chromite growth, and hence they provide fun- out as layers at the basal cumulate mush zone. Numerous radio- damental information on the composition of the cumulate-forming genic isotope studies, together with the widespread occurrence of magmas and, more signi®cantly, allow us to establish the processes similar multiphase inclusions in chromite from other cumulate responsible for chromitite formation. complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in GEOLOGICAL SETTING AND SAMPLE DESCRIPTION many layered intrusions. The 2.7 Ga Stillwater Complex is composed of a .6-km-thick sequence of ma®c and ultrama®c cumulates emplaced into the Archean metasedimentary rocks of the Beartooth Mountains, Montana, United Keywords: chromitite, melt inclusions, Stillwater Complex, parent States. Mineralogical and isotopic variations through the sequence in- magmas, layered intrusions. dicate that several magma types and multiple magma injections were responsible for cumulate formation (McCallum, 1996). The peridotite INTRODUCTION zone near the base of the complex includes at least 20 cyclic units Layered ma®c intrusions not only represent natural laboratories consisting of olivine, olivine-orthopyroxene, and orthopyroxene cu- for studying processes of magmatic differentiation and assimilation mulates (Raedeke and McCallum, 1984). Chromitite layers that occur within the crust, but may also contain extensive precious and base near the base of many of the cyclic units are sequentially labeled from metal mineralization. Chromite-rich seams (chromitites) within layered A (lowermost) through K (uppermost). The G and H chromitites are intrusions, such as the Bushveld and Stillwater Complexes, host the the thickest and most economically important seams (Campbell and majority of the world's Cr reserves and may contain signi®cant plati- Murck, 1993). num group element (PGE) mineralization. These chromitite horizons In this study we examine a sample of the main G chromitite seam have been subject to extensive prior study, yet their origin and evo- collected from above the Benbow Mine head frame. The sample con- lution remain highly debated. Chromium is relatively immobile during sists of 1±2 mm cumulus chromite grains set in a matrix of foliated hydrothermal processes, and chromite is only a minor phase produced serpentinite. Polished sections of the sample reveal that isolated mul- during closed-system cotectic crystallization of ma®c parent magma tiphase inclusions or inclusion clusters occur within the core zones of at least 20% of the chromite grains. Similar inclusions in ultrama®c (e.g., Campbell and Murck, 1993). Therefore, exceptional magmatic zone chromites were described by Jackson (1961) and Page (1971). It processes are required for chromitite formation. Most commonly pro- is well known that chromitite layers undergo recrystallization during posed triggers for extensive chromite crystallization include: (1) a pres- cooling (e.g., Campbell and Murck, 1993). Nonetheless, high- sure change in the magma chamber (Cameron, 1977); (2) a change in resolution backscattered-electron imaging of the chromite grains (see oxygen fugacity of the magma (Ulmer, 1969); (3) interaction of sea- GSA Data Repository1) reveals the location of the original magmatic water or alkaline ¯uids and primitive magma (Talkington et al., 1984; Whittaker and Watkinson, 1984); (4) mixing of primitive magma with 1GSA Data Repository item 2005173, analytical techniques, Table DR1, fractionated residual magma (Irvine, 1977; Campbell and Murck, representative compositions of chromite and inclusion phases, and Table DR2, representative homogenized melt inclusion compositions, is available online at www.geosociety.org/pubs/ft2005.htm, or on request from editing@geosociety *Corresponding author. E-mail: [email protected]. .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; November 2005; v. 33; no. 11; p. 893±896; doi: 10.1130/G21912.1; 4 ®gures; Data Repository item 2005173. 893 Figure 1. A±B: High-resolution back- scattered-electron (BSE) images of chro- mite grains with original magmatic grain boundaries and melt inclusions evident. Melt inclusions are clearly contained within original grain boundaries. Bright halos around melt inclusions are Fe-rich chromite zones formed by Fe-Mg-Mn ex- change between melt inclusions and host spinel (see text). C: BSE image of melt in- clusion prior to rehomogenization with numerous daughter minerals exposed. grain boundaries, and in almost all cases, the inclusions are within the remelting. Complete remelting and rehomogenization of all inclusions original chromite crystals (Figs 1A, 1B). was only achieved at 1450 8C, so we only present data for inclusions We examined more than 100 individual inclusions ranging in size quenched at 1450 8C. Three types of homogenized inclusions were from 10 to 100 mm. All inclusions have rounded or negative crystal observed by SEM. Roughly half of the inclusions were quenched to a shapes and consist of two or more phases. Enstatite and aspidolite (Na- homogeneous glass (type 1; Fig. 2A), whereas most other inclusions phlogopite) are always present, and in all cases compose the majority quenched to glass intergrown with spinifex-textured olivine (type 2; of the inclusions (Fig. 1C). Other common phases include magnesio- Fig. 2B). Both type 1 and 2 inclusions may have contained one or katophorite (Na-Ca amphibole), albite, and diopside. Tiny chalcopyrite more vapor bubbles, but the vapor phase composed ,5% of these crystals were identi®ed in only two of the inclusions. Between inclu- inclusions in all cases. Type 3 inclusions composed ,5% of the total sions there is little variation in mineral compositions, but mineral pro- inclusion population and consisted of a large vapor bubble with a small portions may be highly variable. Representative compositions of these rim of melt (Fig. 2C). minerals and the host chromite are presented in Table DR1 (see foot- Representative compositions of the homogenized inclusion are note 1). The ferromagnesian inclusion minerals are close to the pure presented in Table DR2 (see footnote 1). All inclusions have signi®- 1 Mg end-member compositions, and mica and amphibole in the inclu- cantly higher FeO MgO and lower Al2O3 compared to typical ba- sions are signi®cantly
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