Geochemical Variations Within Podiform Chromitite Deposits in the Shetland Ophiolite: Implications for Petrogenesis and PGE Concentration

Abstract

The Shetland ophiolite, located to the northeast of the Scottish mainland, contains podiform chromitites. Two of these are known to host extremely anomalous PGE concentrations. The chromitite at Cliff is enriched in Pt, Pd, and Rh PGE (PPGE) and the Harold's Grave chromitite is enriched in Ir, Ru, and Os PGE (IPGE). Analysis of multiple samples of chromitite from chromite quarry spoil tips and surrounding chromite-bearing dunite and chromite-bearing harzburgite has revealed that there are links between the chromite geochemistry and the PGE. Both Cliff and Harold's Grave have a characteristic chromite geochemistry, which distinguishes them from the other PGE-poor chromitites.The IPGE-rich Harold's Grave chromite contains elevated trace elements (TiO2, V2O5, and Zn), lower Mg#, and a lower Fe 3+ /Fe 2+ ratio than in the other chromitite deposits. This suggests formation from a more reduced magma that favors the concentration of IPGE in chromitite. It is proposed that the IPGE have been concentrated to such enriched and consistent concentrations due to large throughput of magma, leaving an unusually large dunite envelope that surrounds a rather small concentration of chromitite. The geochemistry of the Harold's Grave deposit suggests formation from a mid-ocean ridge basalt (MORB)/island-arc tholeiite-type magma. There is a range of chromite compositions at Cliff that suggests formation of the chromitite from a variety of mantle melts from mantle of variable composition. PPGE-rich Cliff chromite has a lower Cr# combined with a higher TiO2 than the other chromite in chromitites at Cliff. It is suggested that a more island-arc tholeiite-type magma formed these PPGE-rich chromitites compared to a more boninitic magma that formed the other chromitites at Cliff. This island-arc tholeiite-type magma was closer to sulfur saturation than the boninitic magma and reached sulfur saturation, producing the very PPGE-rich chromite at Cliff.

Factors Controlling the Gallium Preference in High-Al Chromitites

“…The highest Co, Mn, Zn, Fe and lowest Ga were mainly recorded in strongly altered small chromite occurrences from the Rhodope massif in Bulgaria. They are in a good agreement with other chromitites [21][22][23][24][25] and are independent of the age of the associated ophiolites. Platinum-group elements (PGE) show total contents ranging from 30 to 6120 ppb and Pd/Ir ratios from 0.06 to 34, which are independent of the major element composition of chromitites ( Table 2).…”

“…A tectonic discrimination of peridotites, using the oxygen fugacity (f O 2 )-Cr#[Cr/(Cr + Al)] diagram, and the Ga-Ti-Fe 3+ # [Fe 3+ /(Fe 3+ + Cr + Al)] systematics in chrome-spinels, has been proposed [16]. A negative correlation between Ga and Cr in chromitites has been established, that may be related to the composition of parental magmas [17][18][19][20][21][22][23][24] or to the outer electronic structure of Ga that is similar to that of Al [25]. Also, the investigation of spinels in lithospheric mantle xenoliths from distinct tectonic settings has demonstrated that trace elements contribute in discriminating between spinels hosted in peridotites and those crystallized from magmas [11].…”

Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

“…1), have particularly anomalously high ug/g values of PGEs: The Cliff locality is extremely enriched in Pt, Pd and Rh compared with ophiolitic chromitites worldwide; whereas, in contrast, he Harold's Grave local y s IPGE e r ched (Table 1), both containing total PGE concentrations in excess of 60 ug/g (Prichard and Lord, 1993). The PGE concentrations in the ophiolite and the history of their discovery are summarised in Brough et al (2015). Detailed petrogenetic studies of the various chromitite pods within the Baltasound area highlight significant short-range variability in chemical and Os isotopic characteristics, implying derivation for a source with short-range heterogeneity feeding magmas through a series of distinct co d s (O'Dr scoll e al , 2012; Derbysh re e al , 2013).…”

Peridotite enclaves hosted by Mesoarchaean TTG-suite orthogneisses in the Fiskefjord region of southern West Greenland

“…Thus, it appears that at least some of the PGEs may have been distributed according to local fractional crystallisation processes, which led to the differentiation of peridotites, norites and orthopyroxenites in the Fiskefjord region. Unfortunately, currently no PGE data is available for the chromitite layers hosted by the Seqi dunites, but it is not unlikely that these could also host PGE mineralisation (e.g., [30,65,69,20]. Interestingly, the dunites all have Pd contents below the detection limit, which may thus be concentrated elsewhere in the magmatic system perhaps as a result of sulfide saturation (cf., [74].…”