IOCG STYLE ALTERATION AND MINERALIZATION AT THE SAN FERNANDO PROSPECT, BAJA CALIFORNIA NORTE, MEXICO

The San Fernando mine south of El Rosario, Baja California Norte produced copper during the last two centuries from high grade veins (estimated production 400,000t Cu). The veins cut interbedded dacitic to basaltic flows, lapilli tuffs, and volcaniclastic sediments of the Cretaceous Alisitos Formation. The San Fernando area displays five distinct hydrothermal alteration assemblages. The earliest consists of pervasive albite replacement of host rocks with abundant disseminated to semi-massive magnetite. This early sodic alteration assemblage is cut by a more spatially restricted zone of sodic-calcic alteration defined by an assemblage of scapolite – (locally Cl-rich) actinolite – magnetite. Sodic-calcic altered zones are cut by a replacive potassic alteration assemblage of potassium feldspar and biotite which appears to be largely structurally controlled. The San Fernando mine area contains zones of (Ca- and Fe-rich) tourmaline – quartz alteration along veins which cut earlier alteration assemblages. A late, weakly developed retrograde alteration assemblage of albite-calcite replaces earlier alteration assemblages along the San Fernando vein structure. Sulfide mineralization occurred during both potassic and tourmaline-quartz alteration events. During potassic alteration chalcopyrite replaced magnetite while chalcopyrite-pyrite-magnetite were precipitated during the tourmaline-quartz event. The style and mineralogy of the alteration/mineralization events suggest San Fernando belongs to the iron oxide copper-gold (IOCG) family of deposits.