Bronzite

Abstract: The Lower Critical Zone of the Eastern Bushveld Complex is a layered sequence of bronzitite layers interrupted by layers of chromitite and chromitic bronzitite and by two units in which olivine is an abundant cumulus mineral. Layering involves variations in both number and proportions of cumulus minerals and in mineral compositions.Bulk mineralogical and chemical compositions of successive units of the zone, as determined from modal analyses and electron microprobe analyses, indicate that the pile is the product of fractional crystallization, but the sequence of rock units and the upward changes in the enstatite content of the bronzite indicate a major reversal of normal fractionation, culminating in the appearance of the olivine-bearing units in the middle part of the zone. Of the various possible explanations of the reversal, resorption of olivine sinking from the top of the magma chamber (Macdonald, 1967) and slow decrease in total pressure (Osborn, 1980) appear most attractive, but slow influx and mixing of new parental magma cannot be excluded.The layering of the Lower Critical Zone reflects a system in which the composition of the liquid phase of the magma was on or close to the bronzite-chromite boundary on the liquidus during accumulation of most of the zone but was on or close to the bronzite-olivine boundary, the olivine-chromite boundary, or the bronzite-olivine-chromite triple junction at various stages of the development of the two olivine-bearing units. Slight changes in T, P, or X could therefore result in changes in the phases precipitating. It is suggested that changes in total pressure, causing shifts in phase boundaries on the liquidus, were the prime factor controlling the changes in mineral assemblages which are represented in the layering of the zone. One result was the formation of the remarkably regular and persistent chromitite layers for which the Lower Critical Zone is justly famous.Structural relations, variations in the enstatite content of bronzite, and the bulk mineralogical and chemical compositions of the Lower Critical Zone as a whole and its lowest unit in particular indicate that the zone continues the fractionation represented in the Lower Zone of the Olifants River trough and is not the product of a separate, later magma.

Abstract: Exploration of the Stillwater Complex by the Johns-Manville Corporation has led to the discovery and delineation of an approximately stratigraphic zone rich in platinum and palladium comparable to the Merensky Reef. This zone, here called the J-M reef, is generally 1 to 3 m thick and has been traced for 40 km, essentially its maximum possible length within the outcrop area of the intrusion. It is distinguished by small concentrations (0.5-1.0%) of chalcopyrite, pyrrhotite, and pentlanditc in which are contained tiny grains of various platinum-group minerals (mainly moncheite, braggite, cooperite, kotulskite, and Pt-Fe alloy). One segment, 5.5 km long, averages 22.3 g (0.65 troy oz) Pt and Pd per short ton through 2.1 m. The ratio of Pt to Pd typically is about 1:3.5.A new stratigraphic classification of the Stillwater Complex is presented in which three long-standing, major divisions are renamed the basal, ultramafic, and banded series. The first comprises basal norite and basal bronzitite zones totaling 50 to 150 m in thickness; the second, harzburgite and upper bronzitite zones totaling 800 to 1,200 m. The banded series is a 4,500 m succession of plagioclase-rich cumulates with the J-M reef located 400 to 450 m above its base. It is divided into six megacyclic units (I-VI, from bottom to top) in which the principal zones are: (I) norire, gabbro; (II) norite, gabbro; (III) anorthosite, troctolite-gabbro; (IV) anorthosite-troctolite, troctolite-gabbro; (V) anorthosite, troctolite-gabbro; and (VI) gabbro, pigeonite gabbro. Two thin troctolite-anorthosite zones, I and II, occur at the top of units I and II, and troctolite-anorthosite zone I is host to the J-M reef.Where troctolite-anorthosite zone I is best developed in the west-central part of the complex, it is about 100 m thick and is associated with ten relatively continuous olivine-bearing members. These members are generally 1 to 5 m thick and are composed of one or more of peridotite, troctolite, and olivine norite. Members 1 to 4 alternate with varied, discontinuous successions of gabbro, norite, pyroxenite, and anorthosite that are collectively assigned to a gabbro subzone. Members 5 to 10 occur with anorthosite and norite in the anorthosite 1, norite, and anorthosite 2 subzones, and they can each be identified with a cyclic unit. The type rock sequence in five of the cyclic units is anorthosite-peridotite-troctolite; in the sixth, it is anorthosite-olivine norite-norite. The unit associated with olivine-bearing member 5 contains the J-M reef and extends to both ends of the complex; the others have variously been traced for distances of 15 to 36 km.Mineralogical data and 92 whole-rock analyses are presented for troctolite-anorthosite zone 1 and the adjoining zones. Plagioclase becomes gradually less calcic upward through this zone, and normatire anorthite/albite decreases sympathetically, but no cyclic mineralogic or chemical trends are evident. The mafic silicates tend to have higher Fe/Mg within and above the reef than below--a difference that may in part be primary but that is largely a postcumulus effect relating to the larger ratio of trapped (high Fe/Mg) intercumulus liquid to cumulus (low Fe/Mg) mafic minerals in the anorthosite 1 and 2 subzones as compared with the gabbro subzone. Similar variations are shown by MnO and NiO in the mafic minerals. The whole-rock variations of S and Cu indicate that the parental magma(s) of the banded series first reached continuous saturation with immiscible sulfide liquid at the level of the J-M reef, and from the compositions of other sulfide zones at higher levels it appears that the Pt and Pd contents of the magma(s) were effectively exhausted by the formation of the reef.Through the whole of the Stillwater Complex, two principal crystallization orders are indicated for the cumulus silicate minerals. The earlier order, defined in the ultramafic series and in megacyclic units I and II, is olivine, bronzite, plagioclase, augite. The later order, evident in units III through VI, is plagioclase, olivine, augite, bronzite (or hypersthene). The complicated stratigraphy of the troctolite-anorthosite zone I appears to represent an aborted transitional stage. Because both orders are manifest through cumulate successions thousands of meters thick, it would appear that two parental magmas were involved, the first with ultramafic affinities and the second with anorthositic affinities, and indications are that the Pt and Pd in the J-M reef came from the first, and the S from the second. A mechanism for forming the reef by mixing these liquids will be described in the second paper of this series.

Abstract: Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~≤22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.

Abstract: Platinum mineralization in the Stillwater Complex occurs within a complex sequence of olivine-bearing, plagioclase-rich cumulates about 400 m above the base of the Banded zone. The reef consists of a 2-m-thick layer of disseminated sulfides extending over the entire strike length of the complex. In the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine-plagioclase cumulate ("mixed rock") to a plagioclase-olivine cumulate containing a high proportion of postcumulus bronzite, with olivine preserved only as rare resorbed cores on bronzite grains.Platinum-group elements within the reef correlate strongly with abundances of sulfides, which show convincing textural evidence for having originated as immiscible sulfide liquid. Strong correlations are also observed between all six platinum-group elements, Ni, and Cu. Despite the gradation in host-rock mineralogy, the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length, containing 9.3 percent Ni, 6.9 percent Cu, and approximately 0.7 percent total platinum-group elements (principally Pd). These platinum-group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt.Olivines from the reef show a range of compositions from Fo 71 to Fo 85 , and from 2,400 to 4,500 ppm Ni. A positive correlation is observed between NiO and FeO in olivines from sulfide-bearing samples, indicating Ni-Fe exchange between olivine and sulfides on a local scale, such that Ni contents of olivines were buffered by that of the sulfide phase. A value of 9.70 + or - 1.49 is obtained for the Ni-Fe distribution coefficient between sulfide and olivine, in agreement with values obtained at magmatic temperatures by Boctor (1982).These observations may be interpreted in the framework of the plume model of Campbell et al. (1983). A pulse of replenishing magma enters the Stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt. Sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling. Sulfide liquid attains high platinum-group element concentrations during turbulent ascent and spreading of the plume. Crystallization of olivine from the hybrid layer gives rise to olivine-sulfide "boulders," which settle to the floor of the chamber. Reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich. Subsequent reequilibration between olivine and sulfide grains gives rise to the observed Ni-Fe distribution.