The Campo Morado group of massive sulfide deposits is situated in the northern part of the State of Guerrero, Mexico. The oxide portions of a number of these deposits had been mined prior to 1939, principally for gold and silver. Between 1973 and 1977, a program of exploration at Reforma, the largest known deposit of the Campo Morado group, delineated several million metric tons of massive pyritic sulfides grading at 3.12 pecent Zn, 1.07 percent Pb, 0.68 percent Cu, 111.8 grams per metric ton Ag, and 1.2 grams per metric ton Au.The Campo Morado bodies belong to a class of mineral deposits characterized as strata-bound, polymetallic, massive sulfides, believed by many to be genetically related to felsic volcanic rocks and to have formed as a result of a process which might be termed exhalative-sedimentary.The Campo Morado massive sulfide deposits are generally lenticular and they are peneconcordant within a shallow-water volcano-sedimentary sequence of Lower Cretaceous age. All of the known deposits are associated with laharic breccias. The sulfide bodies consist of pyrite, sphalerite, chalocpyrite, galena, tetrahedrite, arsenopyrite, marcasite, and pyrrhotite in decreasing order of abundance. The pyrite contains gold and silver and the tetrahedrite carries silver. Compositional zoning is present within the sulfide bodies. The sulfide grains are very fine and they are intergrown. The major gangue mineral with the sulfides is quartz in both cryptocrystalline and crystalline forms.It is suggested in this paper that the Campo Morado group of massive sulfide bodies was deposited in an environment transitional between a true volcanogenic and a true sedimentary environment.