Origin of chromitites in the Ring of Fire Part II: trace element fingerprinting of contaminants
Authors: Brenan J M , Woods K , Mungall J E , Weston R 
Published: August 2019

To expand on the existing database documenting the trace element composition of ROFIS chromites, and to constrain models of their origin, a total of 45 chromite-bearing samples from the Black Thor, Big Daddy, Blackbird, Black Label chromite deposits have been
analysed for major and trace elements. The samples represent three textural groups, as defined by the relative abundance of cumulate silicate phases and chromite. A search of the element partitioning literature has shown that of the elements that are readily detectable in the chromites, Ga, Zn and V
show contrasting behaviour in their olivine- and chromite-melt partitioning. Specifically, these elements are ambivalent (D ~ 1, Zn) to moderately incompatible in olivine (V, Ga), but moderately compatible in chromite (D~3-6), and hence their behaviour in magmas will depend on the relative
proportions of the two phases that are crystallizing. Simple fractional crystallization models are developed that monitor the change in element behaviour based on the relative proportions of olivine to chromite in the crystallizing assemblage; from 'normal' cotectic proportions involving
predominantly olivine, to chromite-only crystallization. Comparison of models to the natural chromite V-Ga array suggests that the overall positive correlation between these two elements is consistent with chromite formed from komatiite magma crystallizing olivine and chromite in normal cotectic
proportion, and no evidence of the strong depletion in these elements expected for chromite-only crystallization. The spread in the Ga-V data can be explained if the magma responsible for chromite formation has assimilated up to ~10% of wall-rock banded iron formation, or granodiorite, or
up to 50% of metasediment. The Zn-Ga variation amongst the RoFIS chromitites shows large variation, well outside that expected for the extent of crystallization or assimilation that can reproduce the V-Ga array. This is interpreted to be the consequence of both the availability and mobility of
zinc at subsolidus conditions. A filter applied to the data to exclude the most silicate-rich, and hence 'zinc exchangeable' samples, as well as those with visible alteration, results in a small subset of samples whose zinc contents are consistent with BIF assimilation, excluding granodiorite or
metasediment as important contaminants, at least for the samples considered. Despite the evidence for contamination, results suggest that the RoFIS chromitites crystallized from normal cotectic proportions of olivine to chromite, and therefore no specific causative link between contamination and
chromitite formation. The specific fluid dynamic regime during magma emplacement may therefore be responsible for crystal sorting and chromite accumulation. Ongoing work will be to conduct laboratory experiments to better constrain the partitioning of Ga, V and Zn between olivine, chromite and
komatiite at conditions relevant to genesis of the RoFIS to further refine the crystallization models developed thus far.