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The Acoje Block is one of two major units comprising the Zambales Ophiolite . The bottom to top sequence of the Acoje block consists of metamorphic harzburgite with associated lherzolite and dunite, well developed ultramafic and mafic cumulates and a high level plutonic-volcanic suite of gabbro-diorite-dolerite and basalt. The massive and intensely fractured residual harzburgites are associated with pockets of lherzolite, and are much fresher compared to their counterparts in the Coto Block. Like in the Coto Block, the residual-cumulate transition is marked by a dunite layer. This dunite in Acoje, called ‘black’ dunite in the mine, has a very dark appearance probably due to abundant iron oxide dust inclusions. In addition, these dunites host chromite and nickel mineralization. Several gabbro dikes intrude the dunite. The ultramafic cumulates consist of rhythmically layered clinopyroxenites, dunites, wehrlites and harzburgites. Olivine, spinel and pyroxene are the main minerals, although completely altered plagioclases are very sparsely present. Talc, serpentine and iron oxide stains are very common. The mafic cumulates - gabbro, eucrite, gabbronorite and anorthosite - exhibit normal and reverse graded bedding and other structures that include chanelling, scour and fill, slumping and flame structures luminescent of turbilites. The rocks are mostly medium- to coarse-grained, consisting chiefly of plagioclase, pyroxene and olivine. Unlike in the Coto Block, orthopyroxene is an important cumulus phase in the Acoje Block. The diorite-diabase sill/dike complex associated with basalts is best exposed in the Barlo-Sual area. The basalt-dolerite units are usually aphanitic and greenish gray and slightly chloritized and argillized. The diorites are fine- to medium-grained and grayish-white in color. The basaltic flows and pillow basalts in Barlo are host to Cyprus-type massive sulfide deposit.

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