 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; November 2005; v. 33; no. 11; p. 893–896; doi: 10.1130/G21912.1; 4 figures; Data Repository item 2005173. 893 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 Chromitites from layered mafic intrusions are of great eco- nomic importance, yet the origin of these deposits remains enig- matic. We describe multiphase silicate inclusions trapped within chromite grains from the G chromitite seam of the Stillwater Com- plex, Montana, United States. These inclusions are interpreted to represent melt trapped during chromite growth and hence provide information on chromitite formation. Most reheated inclusions have variable quench textures and chemical compositions that are consistent with variable degrees of mixing between a high-Mg ba- saltic parental magma and a Na-rich trondhjemitic melt. The tron- dhjemite is suggested to derive from partial melting of mafic or metasedimentary country rocks. Based on these inclusions, we out- line a model for chromitite formation involving ponding of a new pulse of primitive magma at the roof of the Stillwater magma chamber, followed by localized partial melting and assimilation of the country rock. The newly formed hybrid melts become oversat- urated in chromite, leading to extensive chromite crystallization. Chromitite horizons are proposed to form from dense chromite- rich plumes that periodically sink down from the roof zone to settle out as layers at the basal cumulate mush zone. Numerous radio- genic isotope studies, together with the widespread occurrence of similar multiphase inclusions in chromite from other cumulate complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in many layered intrusions. Keywords: chromitite, melt inclusions, Stillwater Complex, parent magmas, layered intrusions. INTRODUCTION Layered mafic intrusions not only represent natural laboratories for studying processes of magmatic differentiation and assimilation within the crust, but may also contain extensive precious and base metal mineralization. Chromite-rich seams (chromitites) within layered intrusions, such as the Bushveld and Stillwater Complexes, host the majority of the world’s Cr reserves and may contain significant plati- num group element (PGE) mineralization. These chromitite horizons have been subject to extensive prior study, yet their origin and evo- lution remain highly debated. Chromium is relatively immobile during hydrothermal processes, and chromite is only a minor phase produced during closed-system cotectic crystallization of mafic parent magma (e.g., Campbell and Murck, 1993). Therefore, exceptional magmatic processes are required for chromitite formation. Most commonly pro- posed triggers for extensive chromite crystallization include: (1) a pres- sure change in the magma chamber (Cameron, 1977); (2) a change in oxygen fugacity of the magma (Ulmer, 1969); (3) interaction of sea- water or alkaline fluids and primitive magma (Talkington et al., 1984; Whittaker and Watkinson, 1984); (4) mixing of primitive magma with fractionated residual magma (Irvine, 1977; Campbell and Murck, *Corresponding author. E-mail: Carl.Spandler@anu.edu.au. 1993); and (5) assimilation of country rock by primitive magma (Ir- vine, 1975; Kinnaird et al., 2002). The antiquity and slow cooling of most chromitite-bearing layered intrusions have hindered our understanding of the genesis of chromi- tites and layered intrusions in general. The primary petrologic and geo- chemical evidence that are crucial for determining the evolution of cumulate rocks are often removed during subsolidus reequilibration and subsequent hydrothermal alteration or metamorphism. Furthermore, the compositions of the parental magmas of these intrusions—critical in- formation for understanding chromitite formation—remain poorly constrained. Melt inclusion studies have been extensively used for investigat- ing volcanic systems, yet have largely been ignored by researchers of layered intrusions. Trapped within cumulus minerals, melt inclusions may remain unaffected by postcrystallization alteration and hence may be useful for determining parent magma compositions (Spandler et al., 2000) or for unraveling complex processes such as magma mixing or assimilation. In this paper we examine multiphase inclusions within chromite from the G chromitite seam of the Stillwater Complex, Mon- tana. These inclusions are interpreted to be crystallized melt inclusions that were trapped during chromite growth, and hence they provide fun- damental information on the composition of the cumulate-forming magmas and, more significantly, allow us to establish the processes responsible for chromitite formation. GEOLOGICAL SETTING AND SAMPLE DESCRIPTION The 2.7 Ga Stillwater Complex is composed of a 6-km-thick sequence of mafic and ultramafic cumulates emplaced into the Archean metasedimentary rocks of the Beartooth Mountains, Montana, United States. Mineralogical and isotopic variations through the sequence in- dicate that several magma types and multiple magma injections were responsible for cumulate formation (McCallum, 1996). The peridotite zone near the base of the complex includes at least 20 cyclic units consisting of olivine, olivine-orthopyroxene, and orthopyroxene cu- mulates (Raedeke and McCallum, 1984). Chromitite layers that occur near the base of many of the cyclic units are sequentially labeled from A (lowermost) through K (uppermost). The G and H chromitites are the thickest and most economically important seams (Campbell and Murck, 1993). In this study we examine a sample of the main G chromitite seam collected from above the Benbow Mine head frame. The sample con- sists of 1–2 mm cumulus chromite grains set in a matrix of foliated serpentinite. Polished sections of the sample reveal that isolated mul- tiphase inclusions or inclusion clusters occur within the core zones of at least 20% of the chromite grains. Similar inclusions in ultramafic zone chromites were described by Jackson (1961) and Page (1971). It is well known that chromitite layers undergo recrystallization during cooling (e.g., Campbell and Murck, 1993). Nonetheless, high- resolution backscattered-electron imaging of the chromite grains (see GSA Data Repository 1 ) reveals the location of the original magmatic 1 GSA Data Repository item 2005173, analytical techniques, Table DR1, 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 .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.