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 (RuS₂), erlichmanite (OsS₂), 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 H₂O 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.