BIMODAL-SILICICLASTIC MASSIVE SULFIDE DEPOSITS OF THE LEON-GUANAJUATO DISTRICT, CENTRAL MEXICO

In central Mexico, the Esperanza Formation of the Fresnillo-Guanjauato subterrane is host to thirteen massive sulphide occurrences. This volcano-sedimentary assemblage appears to have an oceanic affinity, with the massive sulphide mineralization indirectly related to rifting. An Upper Triassic age has previously been assigned to the Esperanza Formation based upon regional correlations with the Aperos, Zacatecas, El Maguey and Taray Formations. However; a recent U-Pb zircon determination gives a late Jurassic age of 146.1 Ma for a rhyolites that hosts the El Gordo Deposit.

General stratigraphic relationships consist of a basal limestone that is overlain by argillaceous sediments. These are overlain by a 100-200 metre thick sequence of mafic to felsic flows and pyroclastics that host the massive sulfide mineralization, and a second interval of argillaceous sediments.

The massive sulphides are stratiform and well bedded consisting of pyrite, with lesser amounts of sphalerite, chalcopyrite, pyrrhotite, chalcocite, digenite, bornite, galena, and barite. Quartz-sericite is dominant form of alteration. Silicification is particularly intense immediately below the mineralized intervals, and is commonly associated with strong chloritization. Siliceous exhalite consisting of fine-grained quartz with fine laminations of pyrite and sphalerite is also present. Lateral sulphide zoning consisting of distal pyrrhotite-sphalerite mineralization with a proximal pyrite-chalcopyrite core is a feature of the Los Gavilanes prospect. At the El Gordo prospect a siliceous iron formation consisting of magnetite and hematite occurs adjacent to the massive sulphides. Tourmalization is also a common feature.

Analogous deposits in terms of tectonic setting and stratigraphic relations to the Leon-Guanajuato District and Francisco I Madero deposit belong to a class known as “Bimodal-Siliciclastic”. These occur in the Iberian Pyrite Belt, Ural Mountains of Europe and the Bathurst District of eastern Canada. This class of deposit hosts a large proportion of the world’s largest volcanogenic massive sulphide deposits and is characterized by mafic to felsic volcanism that is hosted by a thick sequence of sedimentary rocks. The dominantly sedimentary setting of these deposits, along with their large average size suggests a genetic link to Sedex deposits.