Massive sulfides are not always present but where present are composed of >90% Fe-Ni-Cu sulfides occasionally with exotic enclaves of olivine, metasedimentary or melted material derived from the footwall to the lava flow. The massive sulfide normally sits upon a footwall of basalt or felsic volcanic rock, into which the massive sulfide may locally intrude, forming veins, interpillow sulfides, and interbreccia sulfides. Semi-massive sulfides are more common and are composed of 75-90% Fe-Ni-Cu sulfides with inclusions of olivine and wall rocks.