Field and petrological characteristics of PGE-rich podiform chromitites: Implications for efficient exploration

Abstract

Two distinct types of podiform chromitites have been extensively described within the mantle sections of Neoproterozoic ophiolites in Saudi Arabia and Phanerozoic ophiolites in Oman. The first type is concordant with the foliation plane of the harzburgite host, generally lensoidal in shape and is characterized by its low platinum-group elements (PGE) contents. The second type occurs as discordant or dike-like bodies and predominantly rich in PGE. In addition to their mode of occurrence, several lines of evidence can be employed to differentiate these two types of chromitites, both in the field and during laboratory investigations. Concordant PGE-poor chromitites are typically homogeneous in appearance (with regards to chromite grains distribution) and massive, featuring chromian spinel (Cr-spinel) with olivine as the dominant silicate inclusions. In contrast, discordant PGE-rich chromitites are often heterogeneous in appearance, exhibiting a semi-massive to disseminated, and are rich in both hydrous (amphiboles) and anhydrous (clinopyroxene) silicate inclusions. Discordant dunite veins, sharing similar petrological characteristics, are consistently associated with PGE-rich chromitites, with Cr-spinel containing a notable abundance of clinopyroxene inclusions. Compared to podiform chromitites found worldwide, concordant chromitites have lower to intermediate PGE contents (typically < 500 ppb), display a gently sloping negative trend in PGE patterns, contain very few platinum-group mineral (PGM) grains, and exhibit a wide compositional range of Cr-spinel (Cr# = Cr/(Cr + Al) atomic ratio, varying from 0.5 to 0.9). In contrast, discordant chromitites have higher PGE contents (commonly > 500 ppb), show a strong decoupling between the IPGE (Os, Ir, Ru) and PPGE (Rh, Pt, Pd) subgroups of the PGE distribution patterns, contain numerous PGM grains, and consist of high-Cr varieties (Cr# mostly > 0.6). In-situ analysis of clinopyroxene inclusions within Cr-spinel of discordant chromitites and associated dunite veins revealed characteristic U-shaped rare-earth elements (REE) patterns. This suggests the involvement of a hydrous SiO2-rich melt in the formation of PGE-rich chromitites and their associated discordant dunites. The wide range of Os-isotope composition of the Os-rich PGM, spanning from sub-chondritic to supra-chondritic 187Os/188Os values in the PGE-rich chromitites serves as further evidence for the heterogeneous hydrous melting process that led to the formation of these chromitites. These features, encompassing field and petrological characteristics, can be attributed to variations in the tectonic setting during the formation of podiform chromitites and can serve as valuable guidelines for PGE exploration.