The main orebody presently mined in Philex Mines (Sto. Tomas II orebody) is localized at the contact between andesites on the north and a complex suite of dioritic rocks on the south. The andesites are microporphyritic with relic trachytic groundmass. Minor amounts of plagioclase feldspars occur as phenocrysts in altered microlites. The rock is predominantly altered to sericite. Biotite, quartz and apatite are sparsely distributed. Zeolites as fine neddles are noted.