1981 Volume 31 Issue 167 Pages 169-183
The main Pb-Zn deposits in this district are found to be confined in the specific horizons of Upper Jurassic and Lower Cretaceous sequences, which consist of reef limestone and calcarenite with dolomitic layers. The deposits are also clearly related to dome or anticlinorium structure. The stratigraphic and structural peculiarities of them are thus very similar to those of petroleum and natural gas deposits of Mexican Gulf oil field.
The mineralization is structurally controlled by longitudinal and cross faults and bedding planes of the country rock, resulting in three major forms; manto, chimenea and vein, which are occasionally gradational and mutually connected with each other. Depending on the alteration status of the host carbonate rocks, the deposits can be divided into three groups; no alteration or weak recrystallization group, normal recrystallization group with or without small amounts of skarn minerals and skarn group. The first group that includes such ore deposits as Dos Marias and Plomosas has a simple ore mineral paragenesis; pyrite-sphalerite-galena, and the second, represented by Ojuela, Potosi (Santa Eulalia) and Providencia (Avalos), shows a more complex mineral association; pyrite-sphalerite-galena-chalcopyrite-pyrrhotite-arsenopyrite-tetrahedrite with some Ag-minerals. The third group comprising the deposits such as Naica, San Antonio (Santa Eularia), La Encantada, San Martin, Charcas, La Paz, Fresnillo and Concepcion. del Oro, has various other minerals including magnetite, hematite, scheelite, molybdenite and some Bi-minerals, in addition to the above mentioned minerals. Fluorite, barite and gypsum or anhydrite are commonly present in all three groups of the deposits.
The main mineralization in this district has been dated as Late Eocene or Early Oligocene, correlated to the serorogenic phase of Laramide Orogeny, by both radiometric and stratigraphic analyses. The ore deposits are thus evidently of epigenetic type. While many authors consider the mineralization to be of magmatic origin, some favour a Mississippi Valley-type genesis. Although the detailed features of the deposits show much difference from the typical Mississippi Valley-type mineralization, it seems probable that highly saline brine commonly associated with the oil-bearing sedimentary sequences would have been involved in primary concentration of lead and zinc in this ore field.
