THE FIRST OCCURRENCE OF PLATINUM-GROUP MINERALS (PGM) IN A CHROMITE DEPOSIT IN THE EASTERN DESERT, EGYPT

The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated. The most abundant b ase metal sulfides (EMS) associated with the Lawi chromite are pentlan dite, millerite and heazlewoodite. The major platinum-group minerals i dentified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS), tellurian arsenopalladinite (PdTeSbAs), potar ite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgT e phases. Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian gold shares bound ar ies with chromite. Potarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium is the most abundant PGE in the Gebel Lawi chromite. A paragenetic sequence of PGM formation is descr ibed. Textural evidence indicates that Os-, Ir- and Ru-bearing PGM for med early and were followed by Rh- and Pd-bearing PGM. The concentrati on of all five PGE could be magmatic, but much of the PGE mineralogy e xcept for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes. At later stages, the envir onment became Te-, Sb-, As- and Hg-rich, which finally led to the form ation of low-temperature alteration minerals.