Platinum‐Group Elements Geochemistry and Chromian Spinel Composition in Podiform Chromitites and Associated Peridotites from the Cheshmeh‐Bid Deposit, Neyriz, Southern Iran: Implications for Geotectonic Setting
Authors: TAGHIPOUR Batoul , AHMADNEJAD Farhad 
Published: February 2018

Dunite and serpentinized harzburgite in the Cheshmeh‐Bid area, northwest of the Neyriz ophiolite in Iran, host podiform chromitite that occur as schlieren‐type, tabular and aligned massive lenses of various sizes. The most important chromitite ore textures in the Cheshmeh‐Bid deposit are massive, nodular and disseminated. Massive chromitite, dunite, and harzburgite host rocks were analyzed for trace and platinum‐group elements geochemistry. Chromian spinel in chromitite is characterized by high Cr#(0.72–0.78), high Mg#(0.62–0.68) and low TiO2 (0.12 wt%–0.2 wt%) content. These data are similar to those of chromitites deposited from high degrees of mantle partial melting. The Cr# of chromian spinel ranges from 0.73 to 0.8 in dunite, similar to the high‐Cr chromitite, whereas it ranges from 0.56 to 0.65 in harzburgite. The calculated melt composition of the high‐Cr chromitites of the Cheshmeh‐Bid is 11.53 wt%–12.94 wt% Al2O3, 0.21 wt%–0.33 wt% TiO2 with FeO/MgO ratios of 0.69–0.97, which are interpreted as more refractory melts akin to boninitic compositions. The total PGE content of the Cheshmeh‐Bid chromitite, dunite and harzburgite are very low (average of 220.4, 34.5 and 47.3 ppb, respectively). The Pd/Ir ratio, which is an indicator of PGE fractionation, is very low (0.05–0.18) in the Cheshmeh‐Bid chromitites and show that these rocks derived from a depleted mantle. The chromitites are characterized by high‐Cr#, low Pd + Pt (4–14 ppb) and high IPGE/PPGE ratios (8.2–22.25), resulting in a general negatively patterns, suggesting a high‐degree of partial melting is responsible for the formation of the Cheshmeh‐Bid chromitites. Therefore parent magma probably experiences a very low fractionation and was derived by an increasing partial melting. These geochemical characteristics show that the Cheshmeh‐Bid chromitites have been probably derived from a boninitic melts in a supra‐subduction setting that reacted with depleted peridotites. The high‐Cr chromitite has relatively uniform mantle‐normalized PGE patterns, with a steep slope, positive Ru and negative Pt, Pd anomalies, and enrichment of PGE relative to the chondrite. The dunite (total PGE = 47.25 ppb) and harzburgite (total PGE = 3 4.5 ppb) are highly depleted in PGE and show slightly positive slopes PGE spidergrams, accompanied by a small positive Ru, Pt and Pd anomalies and their Pdn/Irn ratio ranges between 1.55–1.7 and 1.36–1.94, respectively. Trace element contents of the Cheshmeh‐Bid chromitites, such as Ga, V, Zn, Co, Ni, and Mn, are low and vary between 13–26, 466–842, 22–84, 115–179, 826—‐1210, and 697–1136 ppm, respectively. These contents are compatible with other boninitic chromitites worldwide. The chromian spinel and bulk PGE geochemistry for the Cheshmeh‐Bid chromitites suggest that high‐Cr chromitites were generated from Cr‐rich and, Ti‐ and Al‐poor boninitic melts, most probably in a fore‐arc tectonic setting related with a supra‐subduction zone, similarly to other ophiolites in the outer Zagros ophiolitic belt.