Subsolidus Diffusion Research Articles

A contribution on the platinum-group mineralogy of the Merensky Reef at the Rustenburg platinum mine

The different modes of occurrence, compositional variations, and possible paragenetic positions of the platinum-group minerals (PGM) in samples of Merensky Reef pyroxenite from exposures of unoxidized and undisturbed normal reef facies in the central section of the Rustenburg mine are described and discussed. The study indicates that the main platinum-group minerals are braggite (38 vol %), cooperite (17 vol %), moncheite (13 vol %), laurite (8 vol %), kotulskite (8 vol %), merenskyite (4 vol %), Pt-Fe alloy (3 vol %), palladian electrum (1 vol %), and sperrylite (1 vol %). The remainder (less than 1 vol %) include atokite, paolovite, rustenburgite, mertieite II, a (PdHg)(BiTe) phase, and a palladium arsenide. Electron microprobe analyses of these platinum-group minerals are tabulated and graphically presented to display compositional ranges and substitutional trends. The reflected light properties of cooperite and braggite, about which a confusion still exists in the literature, are described, and reflectance and hardness data are presented. The platinum-group minerals are an intimate part of the crystallization history of the base metal sulfides, and their paragenetic sequence, as determined from a textural interpretation, conforms to the known geochemical character of the platinum-group elements, especially with regard to the fractionation of palladium and platinum. Thus, the earliest Pt-Pd minerals to crystallize from the sulfide liquid are platinum rich, with the replacement of cooperite by braggite, and moncheite and merenskyite by kotulskite as evidence of a later palladium metasomatism. Subsolidus diffusion of siderophilic platinum in the monosulfide solid solution during decomposition is favored for the formation of myrmekitic Pt-Fe alloy in pyrrhotite, with the relatively more chalcophilic palladium initially concentrating in intermediate solid solution and finally forming tellurides and stannides in chalcopyrite. Some palladium would be retained in solid solution in pentlandite. Fractionation of platinum earlier than palladium during chalcopyrite genesis resulted in the overgrowth and replacement of early formed platinum-rich tellurides and stannides from intermediate solid solution by palladium-rich ones.

Grain boundary concentration of rare earth elements in a hornblende cumulate.

The distribution of REE (La and Ce) in a hornblende cumulate was investigated with an electron probe microanalyser. The cumulate consists of cumulus kaersutite, augite, titanomagnetite and apatite and such intercumulus minerals as green hornblende, titanomagnetite, plagioclase, apatite and sphene. La and Ce are concentrated in grain boundaries between minerals as well as in apatite and sphene. Grains of cumulus apatite are homogeneous in REE contents, except for rims in contact with augite where REE contents conspicuously increase towards augite. Intercumulus apatite and sphene exhibit decreasing REE distributions from the core to the rim, which are followed by increases when the rims have contact with augite, kaersutite and green hornblende. The constant and the outward decreasing REE distributions may be best explained in terms of surface equilibrium crystallization from infinite main liquid and finite pore liquid, respectively. The formation of REE-rich rims of apatite and sphene and the grain boundary concentration of REE are hardly explained by the model, but may be attributed to a subsolidus diffusion process.

The geochemical role of primary copper-sulphur mineralization in the crystallization of the Freetown (Sierra Leone) layered gabbro

SummaryThe separation of iron-titanium oxide droplets and the formation of olivine, in the Freetown layered gabbro, depleted the magma in iron and enriched the magma in copper. In magma injections in which the sulphur fugacity was sufficiently high, this resulted in the separation of immiscible copper sulphide droplets closely associated with the iron-titanium oxide droplets, and both were subsequently enclosed by olivine. A lower sulphur fugacity in other injections of magma suppressed the separation of the copper, which later appeared as native copper associated with euhedral titanomagnetite, and was enclosed by feldspar. Crystallization of the iron-titanium oxide droplets and subsolidus diffusion has resulted in granular exsolution of ilmenite from the droplets and lamellar exsolution of ilmenite in the euhedral titanomagnetite.