In their analysis of whole rock data from the ultramafic section of the Mirabela Layered Intrusion, Barnes and Williams suggest that the rocks formed as a closed system of crystals + trapped liquid and that compaction played essentially no role in the formation of the rocks. Here it is shown that their stratigraphic trends are fully consistent with a system undergoing concurrent crystallization and compaction (CC&C). Specifically, phosphorus decreases upward owing to more time for compaction prior to apatite saturation away from the cooler margins. In contrast, sodium and strontium (mainly incorporated in interstitial plagioclase) increases upward owing to the initial interstitial liquid becoming more enriched in the plagioclase components over the crystallization of the ultramafic section, causing interstitial plagioclase to crystallize earlier in the compaction history up-section. Local coincident peaks in P, Na and Sr, typically related to changes in rock type, can be ascribed to porosity variations that can develop during compaction of a layered, heterogenous mush. These and other observations lead to a conclusion contrary to that of Barnes and Williams: the Mirabela Intrusion is instead an excellent case study for the consequence of CC&C in layered intrusions and potential role of non-conventional fractionation mechanism of the parent magma.

The chromatographic model for chromitite formation previously presented is updated to include sulfide as a possible participating phase. In the model, chromite is precipitated at an upward-moving reaction front as a Cr-bearing (gabbro)norite protolith reacts with a Cl-rich fluid that becomes progressively undersaturated in pyroxene as it rises into hotter parts of the crystal pile, leaving an anorthositic residue. The liberation of Fe during the dissolution of orthopyroxene can drive concurrent sulfide saturation. If the reaction front encounters an orthopyroxenite the model produces a first-order approximation for the Merensky Reef section of anorthosite–chromite–orthopyroxenite. In addition, MELTS modelling shows that the isothermal addition of Cr 2 O 3 alone to a nearly solid (gabbro)norite assemblage increases the amount of both chromite and liquid at the expense of other silicate minerals owing to the liberation of Ca, Na and Si, the latter two acting as fluxing agents. A generalized constitutional zone refining model is presented in which a variety of spinel–silicate layering types (e.g. Bushveld magnetite layers and Skaergaard trough layering) can develop over time.

Abstract The presence of pegmatoid bodies in the Stillwater Complex is poorly understood, but they have been suggested to have resulted from the presence of fluids in the complex. To better understand the origin of the pegmatoids and to trace the possible influence of country-rock-derived fluid in the Stillwater Complex, bulk rock Rb-Sr, Sm-Nd, and Pb-Pb isotopes for samples from the Archean Stillwater Complex and its metamorphic aureole are reported. Pegmatoid bodies are compared to spatially associated host rock and the underlying hornfels facies country rocks. Evidence of resetting of radiogenic isotopes during regional metamorphism at 1700 Ma is not observed, and the initial radiogenic isotopic ratios in Stillwater Complex rocks overlap those of the underlying hornfels. Despite the isotopic similarity of the country rock to the Stillwater Complex, the intrusion is modestly isotopically heterogeneous. In Stillwater samples, the average εNd,2710Ma = −1.1 ± 6.9, 206Pb/204Pb2710 Ma = 15.24 ± 2.26, and 87Sr/86Sr2710Ma = 0.703043 ± 0.002747 (1σ). The similarity between country rock and intrusive rock isotopic compositions at Stillwater contrasts with the data reported for the Bushveld Complex, South Africa, where the country rock is isotopically distinct from the intrusion. The variability in radiogenic isotope signatures in Stillwater rocks show a noisy but decreasing influence of country rock up through the Lower Banded series interpreted to reflect variable crustal contamination, in part from <1.0 wt % country rock fluids released during intrusion of the Stillwater Complex. The influence of crustal fluid contamination as compared to more traditional crustal assimilation models or simple magmatic heterogeneity suggests that hydrothermal fluids modified the isotopic compositions of more fluid-mobile elements and can explain aspects of isotopic heterogeneity in layered intrusions.

Several lines of evidence, including hydrous melt inclusions and unusually Cl-rich apatite, have been used to suggest that the reappearance of olivine and PGE-sulfide of the J-M Reef in the Stillwater Complex, Montana, is due to fluid infiltration and hydration melting. This study builds upon the hydration melting model using the programs MELTS and PELE with Stillwater bulk rock compositions for the original protolith. Cl-bearing phases are not modeled by MELTS and thus simple oxide mixtures of either a pure H2O or a H2O + Na2O “faux brine” are added to norite, gabbronorite, and melanorite protoliths at 1050 °C at 2 kbar pressure, conditions for which the nominally “dry” protolith is > 95

Layered intrusions present a number of problems in understanding how they crystallized, including identification of parental magma and the effect of trapped liquid on modifying the original mineral assemblage. This work explores how simulated remelting of rocks from layered intrusions using the MELTS program can potentially allow one to retrieve the original liquid composition (both major and trace elements) as well as the original mineral compositions. Although it can ideally distinguish between conventional factional crystallization and in situ crystallization models, model calculations demonstrate that equilibrium crystallization of interstitial liquids trend normal to parent liquid fractionation trends on AFM plots and are thus sensitive to errors in estimates of initial liquid fraction. Although major elements of retrieved interstitial liquids are only modestly affected by the late loss of a small but incompatible-element-rich liquid late in the crystallization of the rock, it is the concentration of the incompatible element in this late liquid fraction that are commonly used to estimate the original liquid fraction. This technique is applied to the Stillwater Complex, Montana, where modeled major-element interstitial liquid trends are broadly normal to possible major-element Rayleigh or in situ fractionation trends of the parent liquid. Although modeled incompatible trace-element ratios show more consistent trends, relying on incompatible elements to estimate the proportion of residual liquid can underestimate the trapped liquid shift effect on mafic minerals. A potentially better estimate can be found by simulated melting to reproduce primocryst plagioclase core compositions, which do not re-equilibrate easily in the absence of wholesale recrystallization. It is concluded that modeled Stillwater major-element liquid compositions are strongly affected by open-system behavior, particularly late-stage liquid migration and loss during solidification.

Abstract The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, U.S.A.), which have been linked to platinum and palladium mineralization in the economically important Johns-Manville (J-M) Reef ore body. We present the first data set for heavy halogens (Cl, Br, and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13 500 ppm for Cl, 26 ppb to 360 ppm for Br, and <1 ppb to 9 ppm I over the whole sample set. Cl, Br, and I correlate well with each other implying a shared process and/or distribution in mineral species. Br/Cl and I/Cl ratios span a range from 0.3 to 35 × 10−3 and 5 to 900 × 10−6 by weight, respectively, encompassing MORB-like to more enriched compositions, particularly for Br/Cl. High-Br/Cl ratios compared to MORB in some Stillwater samples suggest fractionation of halogens during the exsolution of a volatile-rich fluid to explain the most Br-enriched samples. More generally, the presence of minerals such as scapolite, hornblende, and apatite in the most halogen-enriched samples suggests that the halogen-bearing fluids were derived from the cooling of the intrusion rather than late-stage (low-temperature) metamorphism. The combined halogen abundance and noble gas isotope data set imply that crustal contamination may have played a limited role in the crystallization of pegmatoids and the G Chromitite but is not required to account for the halogen budget of the J-M Reef. High-halogen contents in the sulfide-bearing J-M Reef and associated lithologies are consistent with the influence of fluid-related activity during platinum-group element (PGE)-Reef formation, lending weight to the hydromagmatic model for mineralization in the Stillwater intrusion. Our new data also imply chalcophile tendencies of Br and I over Cl in sulfides in natural systems, hinting at the importance of sulfide liquid interaction with halogen-rich fluids in the formation of sulfide-hosted precious metal ore deposits.

Drilling related to development of the platinum-group element deposit of the J-M Reef of the Stillwater Complex returned samples of a rare chromitite seam between anorthosite and norite in a discordant anorthositic body. Plagioclase core An concentrations are marginally higher and modestly reversely zoned on the norite side (average Ancore = 83·8; average Ancore – Anrim = –1·1) as compared with the anorthosite side (average Ancore 82·5; average Ancore – Anrim = +1·0). The anorthosites are also characterized by a slightly smaller average plagioclase grain size than plagioclase in the norite (1·41 mm and 1·54 mm, respectively). The chromite can contain single and polyphase inclusions of orthopyroxene, plagioclase, amphibole, biotite and Cl-rich apatite. These and other compositional and textural features, as well as inference from other discordant anorthositic bodies in the Banded series, are all consistent with a chromatographic model of chromite precipitation at a reaction front as a norite protolith reacts with a Cl-rich aqueous fluid saturated in plagioclase alone. Chromitite seam formation is modeled using an infiltration metasomatic model, in which a fluid becomes progressively undersaturated in pyroxene as it rises into the hotter part of the crystal pile. As this pyroxene-undersaturated fluid moves through a noritic protolith, it dissolves the Cr-bearing orthopyroxene to produce an anorthosite. Chromite precipitates at the reaction front between the anorthosite and the norite owing to liberation of Mg and Cr from pyroxene. Continuous redissolution and reprecipitation of chromite occurs as the pyroxene dissolution front moves in the direction of fluid flow, collecting the Cr lost from the anorthosite. Owing to Cr dissolved mainly as a neutral divalent cation complex, CrCl(OH)0, in the solution, but incorporated as a trivalent cation in chromite, the required redox reaction can involve concurrent precipitation of sulfide with chromite. This mechanism differs from some recent models in that the anorthosites are themselves replacement bodies and are not original precipitates from a magma nor formed by loss of mafic material by partial melting. The results show the need for experimental mineral solubility data at T and P conditions appropriate to upper crustal mafic–ultramafic intrusions.

In their discussion of our recent publication, Scoon and Mitchell (2020) put forward a number of arguments against the hydromagmatic model of Bushveld Complex formation that we present. Their criticisms of our model focus primarily on the formation mechanisms of the discordant bodies present at Bushveld, namely the iron-rich ultramafic pegmatites and the dunite pipes. While this was a minor portion of our paper, we here review evidence in favor of a fluid-related origin for these discordant bodies, in contrast to the primarily magmatic origin that Scoon and Mitchell present.

The role of hydrothermal fluids during the crystallization of layered intrusions and the ore deposits they contain has long been debated. This book summarizes the evidence for fluid-crystal-liquid (hydromagmatic) interactions and their importance for the understanding of the formation of platinum-group deposits in layered intrusions. It discusses the composition of igneous fluids in mafic magmatic systems, the generation and movement of these fluids in layered intrusions, their impact in altering the mineralogy and composition of the originally precipitated assemblages, and their role in the transport of the platinum-group elements (PGE). Using examples from the Bushveld complex of South Africa and other intrusions, this book provides a comprehensive overview of the hydromagmatic model for the origin of various features of layered intrusions. It is a useful reference for academic researchers and professional geologists working on economic mineral exploration, layered igneous intrusions, and hydrothermal metallogenesis.

Abstract Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as crossbedding, trough structures or layer termination. It is now accepted that basaltic Chapter 2 Igneous Layering in Basaltic Magma Chambers

Sulfide assemblages, precious metals, transition metal alloys, and associated accessory phases were characterized throughout the Skaergaard intrusion to better constrain the sulfide saturation history of the intrusion and the role of late magmatic volatiles in modifying the Skaergaard metal budget and distribution. Sulfides in and below the Middle Zone of the Layered Series of the intrusion are readily replaced by low-Ti magnetite. The ratio Σ(low-Ti magnetite mode)/Σ(sulfide mode), indicating oxidation of sulfides, reaches maximum values in the Lower Zone of the Layered Series. Sulfide assemblages below the Middle Zone are typically accompanied by minor biotite, apatite, and rare calcite as well as trace compositionally distinctive clinopyroxene and orthopyroxene. The occurrence of Ag, Au, Pt, Cu, and metal alloys outside of the Middle Zone is further evidence of the Skaergaard intrusion parental magma being S-poor. Native Ag, commonly accompanied by trace amounts of Cl, occurs both in and below the Middle Zone. Evidence of coexisting precious metal + brine assemblages exists where native metals are accompanied by sylvite ± halite and Ag is accompanied by Ag halides. Ag occurrences in the Middle Zone are of irregular morphology with trace Cl ± S ± calcite. Further evidence supportive of a metal + brine assemblage is observed where Ag + quartz is found in an apparent open clinopyroxene-hosted fluid inclusion consisting of Na, Si, Cl, Ca, K, and S. Ag is used to model the behavior of precious and transition metals in the presence of an exsolving fluid phase. Numerical modeling suggests that, in a sulfide-bearing system, residual Ag concentrations and concentrations in the exsolved fluid are most affected at the point where sulfide is lost to a separating volatile fluid phase. It is suggested that, owing to the low-S nature of the Skaergaard system, fractional crystallization and early fluid saturation produced enrichment of Ag, with other precious and transition metals, in the interstitial silicate liquid much higher than normal due to delayed sulfide saturation. As this interstitial liquid evolved, Ag was lost to an exsolved volatile phase of high salinity and migrated upward. A similar process likely occurred for Au and other elements with high affinities for Cl.

The Banded Series of the Archean-aged Stillwater Complex contains three thick anorthosites overlain by olivine-bearing rocks with apparently different crystallization sequence in that orthopyroxene appears late, if at all, as compared with that of the Ultramafic and Lower Banded series where orthopyroxene appears before plagioclase. Conventional models suggest that the rocks above the anorthosites represent injection of a magma with a different liquid line of descent than the rest of the complex. Alternatively, it has been suggested that the plagioclase + pyroxene mush protolith above the anorthosites was hydrated by the degassing of the underlying thick anorthosite units to produce olivine rather than pyroxene by either expansion of the olivine phase field by addition of H2O, incongruent melting of pyroxene, and/or silica loss to the vapor. To test for hydration melting and a late silica loss, electron microprobe core/rim analyses were done on plagioclase grains from troctolite of Olivine-bearing zone V (OB-V). Approximately a dozen grains were analyzed for each of six thin sections. In total, 69 pairs were reversely zoned, and 22 were normal or unzoned. The most negative An difference was − 8.25, the most positive was 4.32, but the majority of pairs were clustered from − 5 to 1. The average was − 2.1. Core grains had an average An number of 77.8, typical for other plagioclase from the Middle Banded series. A green, Na-rich hornblende is commonly associated with plagioclase where the latter is enclosed in olivine. The prevalence of reversely zoned grains is consistent with silica and sodium loss from pre-existing plagioclase. This evidence, as well as the lack of plagioclase core compositions being reset to more primitive, An-rich compositions, the amoeboidal, poikilitic habit of the olivine, and the presence of amphibole are all consistent with the troctolite and olivine gabbro of the Middle Banded series and lower parts of the Upper Banded series having formed as the result of a combination of processes including hydration, incongruent melting, and element leaching during degassing of the underlying anorthosites.

We documented occurrences of native copper (Cu), silver (Ag), and gold (Au) in a pāhoehoe flow from Kīlauea volcano (Hawaii, USA), an a‘ā flow from Mauna Loa volcano (Hawaii), and a mid-oceanic-ridge basalt (MORB) from the Chile Ridge (southeastern Pacific Ocean). Native Ag in Kīlauea and MORB samples consistently contained minor Cl (<1 wt%). Native Ag in Hawaiian basalts can occur at the center of nearly circular patches of relatively evolved minerals, which presumably formed after late-stage silicate liquid infilled pipe vesicles. Sulfur loss and oxidation of a Cu-sulfide phase can explain the native Cu, but not Au and Ag deposition. The rare occurrence of native Cu-Au-Ag alloys and the large native Au and Ag grain size suggest separate metal precipitation mechanisms. A fractional crystallization and degassing model envisions initial Au and Ag enrichment in crystallizing interstitial liquid and further enrichment in a separating vapor phase. From the flow interior, the metals ascend through ephemeral pipe vesicles as bisulfide (Au) or chloride (Ag) vapor complexes and precipitate in the transition zone below the upper vesicular zone, owing to temperature and oxidation state changes. Our results support igneous vapor transport of ore elements in mafic plutonic systems and imply preconcentration of gold during lava solidification before later hydrothermal remobilization.