Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# < 0.6) chromite deposits are relatively rare, occurring as interspersed orebodies in regions of Types 2 and 3. Sulfide-bearing Type 1 high-Cr deposits are PGE-enriched; the high-Al deposits have lowest Ir and Pd. Among the PGEs, Ir ranges from <20 to 550 ppb, Rh from <100 to 1100 ppb, Rh from 0.7 to 760 ppb, Pt from <1.0 to 5960 ppb and Pd from <0.5 to 8350 ppb. Chondrite-normalized (CN) PGE patterns are diverse, and include those with both negative slope (Ir > Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits).
Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides.
