PGE-rich Podiform Chromitites in the Al 'Ays Ophiolite Complex, Saudi Arabia: An Example of Critical Mantle Melting to Extract and Concentrate PGE

摘要

The Al 'Ays ophiolite complex in Saudi Arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements (PGE) in podiform chromitite with maximum values of 2,570 ppb Pt, 6,870 ppb Pd, 840 ppb Rh, 5,800 ppb Ru, 6,200 ppb Ir, and 3,300 ppb Os. Smooth chondrite-normal-ized PGE profiles indicate igneous PGE ratios. These suggest that in situ alteration of the PGM caused only minor mobility of PGE during secondary modification of the mineralogy. Thus the geochemistry of the igneous con-centration processes can be examined despite the mineralogical changes caused by subsequent alteration. Three main types of PGE mineralization are observed that are defined by their relative abundances of individual PGE. Type 1 has Ru > both Pt and Pd, with negative-slope chondrite-normalized profiles. Type 2 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 1 to 5, and convex upward chondrite-normalized profiles. Type 3 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 5 to 60, positive-slope chondrite-normalized profiles and is associated with elevated Cu and Ni concentrations. The unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition; for example, Cr_2O_3 varies from 39 to 69 wt percent. PGE mineralization types 1, 2, and 3 are related to the composition of the chromite. Type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent Cr_2O_3, type 2 occurs in chromitite having a range of 53 to 61 wt percent Cr_2O_3, and type 3 occurs in chromitite having a range of 39 to 51 wt percent Cr_2O_3. The PGE form a great variety of platinum group minerals (PGM) and they differ among the three types of PGE mineralization. Type 1 is characterized by euhedral Os, Ir, and Ru (IPGE) alloys and laurite, both commonly enclosed in chromite, as well as members of the irarsite hollingworthite solid-solution series and Pt-IPGE-bearing PGM, both commonly interstitial to the chromite grains. Type 2 PGE enrichment is characterized by IPGE-, Pt- and Rh-bearing PGM. Type 3 PGE enrichment hosts predominantly Pd- and Pt-bearing PGM associated with Ni- and Cu-bearing minerals. Where exposed to the serpentinization process, the PGM are altered to alloys, arsenides, antimonides, and oxides that form irregular shapes or may form pseudomorphs of former PGM. They are commonly associated with Ni- and Cu-bearing minerals, including ruthanian pent-landite, millerite, arsenides, and PGE-bearing awaruite. Mantle melting and subsequent crystallization were at an optimum to concentrate PGE in the Al 'Ays ophiolite complex. Crystallization of IPGE, commonly prior to chromite crystallization and latterly with some Pt and Rh, occurred across the range of chromitite composition. Crystallization of Pd with some remaining Pt occurred during sulfur saturation in chromitite formed from a more evolved magma. We propose that this crystallization was from a magma that was enriched in PGE because the degree of mantle melting was just sufficient to extract the PGE, but not dilute them in a melt that includes further mantle melting. This feature is likely to be common to other PGE-rich ophiolite complexes such as in Shetland in the United Kingdom, Leka in Norway, Thet-ford in Canada, Pindos in Greece, Tropoja in Albania and in New Caledonia. If equilibrium partial melting had continued in Al 'Ays, then the magma would have been diluted by subsequent PGE-poor melt. This would have prevented sulfur saturation until much higher in the sequence, producing Pt- and Pd-bearing base metal sulfides in the crustal wehrlite and gabbro, as has occurred in the Cyprus and Oman ophiolites.