Extended Abstract

Layered intrusions and ophiolite complexes host platinum-group minerals (PGM). Although layered intrusions are the dominant host of economically-viable PGM deposits, recent works on ophiolite and ophiolitic complexes showed that they can be targets for PGM mineralization (e.g. Prichard and Lord, 1993; Economou-Eliopoulos, 1996). The PGM, occurring in native, alloy or solid solution forms, are dominantly hosted by either oxide or sulfide minerals. Geochemical evidence show that PGM formation and accumulation are dominantly affected by partial melting, crystal fractionation and alteration (e.g. Leblanc, 1992; Boudreau and Meurer, 1999). Interestingly, several geological characteristics are recognized to be common among crust-mantle sequences that host PGM. These sequences are island arc-related, associated with boninitic lavas that are second/ third stage melts and are products of multi-stage/ high degrees of partial melting (e.g. Yumul and Balce, 1994; Proenza et al., 1999). These are usually the characteristics of supra-subduction zone (SSZ) ophiolites as opposed to mid-ocean ridge (MOR) ophiolites (e.g. Pearce et al., 1984). The Zambales Ophiolite Complex (ZOC), for that matter, is classified as a SSZ ophiolite (Hawkins and Evans, 1983; Yumul 1996).

The ZOC is made up of the MOR-related Coto block and island arc-related Acoje block. This crust-mantle sequence hosts PGM and platinum group-elements (PGE) in the Acoje block (e.g. Bacuta et al. 1990). The melts responsible for the PGE-bearing sulfide and chromitite deposits are of orthomagmatic origin characterized by high-MgO basalt/ boninitic composition (e.g. Yumul, 1992; Zhou et al., 2000). The orthomagmatic origin and boninitic geochemical characteristics of the Acoje block melt suggest, as discussed above, that these are products of multiple stages and high degrees of partial melting. Being second or third stage melts allow them to scavenge and to accumulate PGEs from sources until they are PGE-saturated (Hamlyn et al., 1985). Any change in the physical or chemical parameters of the melt destroy the solubility of the PGE and lead to the eventual crystallization of the PGE. Metal ratio diagrams, utilizing base and precious metals, reveal that the distribution and deposition of the PGE in the Acoje block are affected by olivine, chromite and sulfide crystallization (Barnes et al., 1988; Barnes, 1990; Manjoorsa and Yumul, 1996). The generation, accumulation and segregation of the PGE, oxide and sulfide minerals from the melts are governed by the combined factors of high degrees of partial melting, multiple melt replenishment with concomitant magma mixing and fractional crystallization. Although previous sulfide segregation events could have occurred below the prolific PGE-bearing nickel sulfide horizon as shown by the Ni/Cu (>1), the Cu/Pd (Fig. 1) and Ni/Pd strongly suggest that the main platiniferous zone is confined within the Acoje block transition zone dunite. Any effort geared to look for PGM in the ZOC must prioritize the transition zone dunite suite as an exploration target.