sulfides is a chemical reaction which occurs as a consequence of concentration of sulfur, often hydrocarbons, and zinc and lead which are absorbed by the hydrocarbons. The hydrocarbons can either leak out of the fault zone or fold hinge, leaving a stockwork of weakly mineralized carbonate-sulfide veins, or can degrade via pyrolysis in place to form bitumens. Once hydrocarbons are converted to bitumen, their ability to chelate metal ions and sulfur is reduced and results in these elements being expelled into the fluid, which becomes saturated in zinc, lead, iron and sulfur. Sulfide minerals such as galena, sphalerite, marcasite and pyrite thus form. Commonly MVT deposits form by the combination of hydrocarbon pyrolysis liberating zinc-lead ions and sulfur to form an acidic solution which dissolves the host carbonate formation and replaces it with massive sulfide accumulations. This may also take the morphology of fault-hosted stockworks, massive tabular replacements and so forth. Porous limestones may form disseminated ores, however most MVT deposits are massive sulfides with knife-like margins between carbonate and sulfide mineralogies. Mineralogy and alteration Ore minerals in carbonate replacement deposits are typically lead sulfide, galena, and zinc sulfide sphalerite. Weathered equivalents form anglesite, cerussite, smithsonite, hydrozincite and secondary galena and sphalerite within the supergene zone. MVT and Irish type deposits are commonly associated with a 'dolomite front' alteration, which manifests as a yellow-cream wash of dolomite (calcium-magnesium carbonate) within calcite-aragonite assemblages of unaltered carbonate formations. Most ore bodies are quite sulfidic, and most are very low-iron, with pyrite-marcasite contents typically below 30% of the mass of sulfides. This makes MVT lead-zinc deposits particularly easy to treat from a metallurgical view. 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. Oil synergies The importance and synergies between hydrocarbon source-transport-trap 'fairways' and MVT and Irish Type lead-zinc deposits has been known for several decades. Often the prospectivity of particular carbonate formations for lead-zinc deposits of this nature is first identified by core drilling by oil explorers. This concept of a cogeneration of hydrocarbons and precursor brines by the same process allows many lead-zinc explorers to use hydrocarbon basin models to predict if a carbonate sequence is likely to host MVT