The activation energy values estimated by the Arrhenius equation as a controlling factor of platinum-group mineral formation

In ophiolite complexes and Ural/Alaskan-type intrusions the platinum-group element minerals (PGM) occur as laurite (RuS 2), erlichmanite (OsS 2), irarsite (IrAsS) and alloys (Os–Ir–Ru and Pt–Fe). They are commonly found as small inclusions (normally less than 10 μm, occasionally up to 100 μm) in chromite. The origin of coarse-grained PGM, in the form of 0.5–10 mm nuggets, in placer deposits related with mafic/ultramafic complexes remains still unclear. Literature data on grain size ( r) of platinum-group minerals (PGM) and their formation temperature (range of temperatures between 700 and 1100 °C), revealed an Arrhenius temperature dependence. Correlation of the rate of crystal formation that depends on temperature (T) with the size ( r) of the grain results in a linear relationship between ln( r) and 1/T. From the slope of the line n × ln( r) = −const. + E act/ RT the activation energy for the formation of IPGM (Ir–platinum-group minerals) was estimated, for the first time in the present study, to be approximately 450 ± 45 kJ mol −1. Applying the Arrhenius equation, the corresponding formation temperature for extremely large IPGM grains (up to 1.3 mm) in chromite ores related to ophiolites was found to be approximately 740 °C. It seems to be consistent with a lower formation temperature than with the typical formation temperature of small PGM grains associated with ophiolitic chromitites. This suggests that coarse-grained PGM in mafic/ultramafic complexes, along the permeable shear zones, may have been re-crystallized during plastic deformation at relatively lower temperatures (700–800 °C), under appropriate pressure, temperature, redox conditions and an increased H 2O content. Thus, applying the plot of ln( r) versus 1/T on large Os–Ir–Ru-minerals (sulfides or alloys), characterized by an r value falling into the linear part of the graph and having evidence supporting their formation at relatively high temperatures, then the corresponding formation temperature of those IPGM can be found.