ASSEMBLAGES OF PLATINUM-GROUP MINERALS AND SULFIDES IN SILICATE LITHOLOGIES AND CHROMITE-RICH ROCKS WITHIN THE SHETLAND OPHIOLITE

A large variety of platinum-group minerals (PGM) have been recorded al ready in chromite-rich lithologies from the Shetland ophiolite complex , in the Shetland Islands, northeast of Scotland. Further work describ ed in this paper documents the nature of the PGM and sulfides from ult ramafic lithologies in the ophiolite sequence, including cumulate duni te, wehrlite and pyroxenite. PGM assemblages from all known platinum-g roup-element-bearing lithologies in the ophiolite are summarized. A pa ragenetic sequence of PGM formation is described. Primary magmatic lau rite is preserved within chromite. Pd- and Pt-rich inclusions isolated in a single crystal of clinopyroxene are considered to represent a re -equilibrated late magmatic phase containing sulfur, base metals and p latinum-group elements (PGE). Pt- and Pd-bearing PGM are clustered aro und the sulfides and enclosed by serpentine or chlorite, suggesting fo rmation after release, during alteration, of PGE from magmatic sulfide assemblages. The PGM enclosed by altered silicates are commonly As-, Sb- and Te-bearing. These elements have been introduced during alterat ion of the primary silicate minerals. Subsequent removal of the arseni c and antimony produces Pt- or Pd-bearing alloys; late-stage weatherin g changes the Pt- and Pd-bearing minerals to ochres and, in one case, to a composite Pt-bearing mineral containing oxygen and carbon. Textur al evidence from dunite, wehrlite and pyroxenite suggests that the maj ority of the secondary PGM formed only a few micrometers from the prim ary source of the PGE. However, longer-range mobility of PGE is indica ted by the presence of Pt and Rh in alloclase - gersdorffite(ss) along the basal contact of the ophiolite at Cliff, a few hundred meters awa y from a primary magmatic source of the PGE.