sentences:  Some MVT deposits can, however, be very iron-rich and some sulfide replacement and alteration zones are associated with no lead-zinc at all, resulting in massive accumulations of pyrite-marcasite, which are essentially worthless.There is sometimes an association with quartz veining and colloform silica, however silicate gangue minerals are often rare.The importance and synergies between hydrocarbon source-transport-trap 'fairways' and MVT and Irish Type lead-zinc deposits has been known for several decades.