Chromite from the Swartkop chrome mine; an estimate of the effects of subsolidus reequilibration
| Autor: | Gerhard Von Gruenewaldt, Christopher J. Hatton |
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| Rok vydání: | 1985 |
| Předmět: | |
| Zdroj: | Economic Geology. 80:911-924 |
| ISSN: | 1554-0774 0361-0128 |
| DOI: | 10.2113/gsecongeo.80.4.911 |
| Popis: | Chromites from the LG-3, LG-4, and LG-6 chromitite layers at the Zwartkop mine are investigated and subsolidus behavior of chromite in olivine and orthopyroxene-hosted environments is compared. Chromitite layers LG-3 and LG-6 are orthopyroxene hosted, but layer LG-4 is olivine hosted, with abundant orthopyroxene formed by a reaction relation between olivine and liquid. Chromite has reequilibrated with host silicate to a degree dependent on the nature of the host silicate and the modal amounts of chromite present. In an olivine environment, reequilibration between chromite and silicate proceeds to lower temperatures with consequently more iron-rich chromite than in an orthopyroxene-hosted chromite. In a given environment the closure temperature for silicate-chromite reequilibration decreases systematically as the modal amount of chromite decreases. From this systematic relation the solidus temperature can be estimated and some inferences about the original chromite composition can be made. In the LG-6 layer, chromite compositions can be projected back to solidus compositions along a trajectory of constant activity of the MgA1204 component, and at the solidus temperature, chromite composition within and adjacent to the chromitite layer is essentially the same. The activity of MgA1204 remained constant because chromite was in equilibrium with the host orthopyroxene throughout the temperature range in which reequilibration took place. In layer LG-4 the activity of MgA1204 increased during cooling because the solidus chromite was not in equilibrium with orthopyroxene formed by reaction between olivine and liquid. Because the liquid which reacted with olivine was more aluminous than the liquid from which the olivine originally crystallized, subsolidus reequilibration between orthopyroxene and chromite led to an increase in the concentration of AlcOa in chromite. Consequently the decrease in Mg content of chromite during cooling is accompanied by an increase in A1 content, in contrast to the more common decrease in A1 content observed in layer LG-6, which results from the higher stability of |
| Databáze: | OpenAIRE |
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