1981 Volume 31 Issue 165 Pages 27-42
In the northern part of Sierra Madre Oriental, lead-zinc deposits occur in a thick sequence of Mesozoic calcareous sediments, which are folded and faulted by the Laramide orogeny. The ore deposits are divided into two groups, non-alteration(or partly marmorization)-type and skarn-type, depending upon the alteration status of the country rock. The former group, typified by such deposits as Plomosas and Dos Marias, consists of the manto-type orebodies in the Jurassic and Cretaceous calcareous sediments with no distinct alteration, being characterized by very simple ore metal association, Fe, Zn and Pb with low concentration of Ag. On the contrary, the latter group of deposits such as Santa Eulalia, Naica and La Encantada are composed of the chimney, manto and vein-type orebodies with skarn minerals. They have been found in the lower Cretaceous Aurora limestone with dome structure. Their ore metals are mainly Fe, Zn, Pb, Cu and As, being associated with minor amounts of Ag, W, Mo, So and Sb. Both groups of mineralization have calcite, dolomite, fluorite, gypsum, anhydrite, barite and quartz as the gangue minerals. In general, the conspicuous intrusive bodies as the possible source of the mineralization are scarecely recognized in the ore field concerned, although discontinuous exposures of dikes and sills of felsite and small stocks of diorite have been known.
