Hydrothermal alteration of volcanic rocks hosting the Late Jurassic- Early Cretaceous San Nicolás VMS deposit, southern Zacatecas, Mexico
San Nicolás is a Late Jurassic-Early Cretaceous, stratiform Zn- Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit located in central Mexico, with a total resource of 99 Mt of ore grading 1.36 % Cu, 1.64 % Zn, 0.41 g/t Au and 24 g/t Ag. At San Nicolás mining district the ore deposits are related with submarine graben-horst structures produced by the opening of the Gulf of México. San Nicolás VMS deposit is at the right side of a graben structure, fed through a normal growing fault called "La Panza". The host-rock succession consists of a variably altered rhyolite lava dome in the hanging wall and a basalt- dacite-dominated volcano-sedimentary sequence in the footwall. The sulfide deposit and hosting volcanic sequence were metamorphosed under lower greenschist facies. A laterally continuous footwall alteration zone extends beneath the entire district (~80 km2) and to a stratigraphic level at least 200 m below the several ore lenses. The bulk of this zone is occupied by feldspar-destructive, muscovite-biotite-chlorite-rich, mottled alteration facies with disseminated pyrite. The ore mineralization was controlled by a fault zone with intense quartz-pyrite alteration and represents the principal fluid pathway during mineralizing hydrothermal activity. Locally, quartz-K feldspar alteration facies occur on the edges of the system, whereas calcareous alteration and chlorite- pyrite alteration facies occur in the upper part of footwall volcanics, next to sulfide lenses. Porphyritic basalts, plus basaltic pillow lavas and volcanic breccias in the hanging wall are unaltered or weakly altered. Carbonate alteration (formation of dolomite and/or ankerite) probably represents the initial phase of hydrothermal activity. This was followed by diffuse upwelling of acidic hydrothermal fluids causing dissolution of underlying limestones and destruction of primary feldspars, precipitation of pyrite, and formation of sericite, chlorite, and clay minerals. Subsequently, intense quartz- pyrite alteration was directly associated with mineralization. The San Nicolás footwall alteration zone shows systematic geochemical changes with increasing proximity to the ore bodies. These include Na depletion and elevated Mg, S, AI (AI=100*[MgO+K2O]/ [MgO+K2O+Na2O+CaO]), CCPI(CCPI=100*[MgO+FeO]/ [MgO+FeO+Na2O+K2O]),as well as Mo, Bi, and As. These geochemical features can be used in exploration for massive sulfide deposits as guides to ore, at the district scale, and in discrimination of prospective hydrothermal from unprospective diagenetic alteration systems at the regional scale.