GEOLOGY AND ALTERATION OF VOLCANICS HOSTING THE JURASSIC SAN NICOLÁS VHMS DEPOSIT (SOUTHERN ZACATECAS, MEXICO)

San Nicolás is a Late Jurassic-Early Cretaceous, stratiform Zn-Cu-Ag-Au-massive sulfide deposit located in central Mexico, with a total resource of 99 Mt of ore grading 1.36 percent Cu, 1.64 percent Zn, 0.41 g/t Au and 24 g/t Ag. The host-rock succession consists of variably altered rhyolite in the hanging wall and a basalt-dacite-dominated volcano-sedimentary sequence in the foot wall. The sulfide deposit and hosting volcanic sequence were metamorphosed under lower greenschist conditions.

A laterally continuous footwall alteration zone extends beneath the entire district (~80 km²) and to a stratigraphic level down least 200 m below the ore lenses. The bulk of this zone is occupied by feldspar-destructive, muscovite-biotite-chlorite-rich, mottled alteration facies with disseminated pyrite. The fault zone with intense quartz-pyrite alteration represents the principal fluid pathway during mineralizing hydrothermal activity. Locally, quartz-K feldspar alteration facies exist on the fringes of the system, and calcareous alteration and chlorite-pyrite alteration facies occur in the upper part of footwall volcanics, next to sulfide lenses. Porphyritic basalts, pillow lavas and volcanic breccias in the hanging wall are unaltered or weakly altered.

Calcareous alteration probably represents the initial phase of hydrothermal activity. This was followed by diffuse upwelling of acidic hydrothermal fluids causing disolution of underling 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, alteration index (AI), chlorite-carbonate-pyrite index (CCPI), Mo, Bi, and As. These geochemical features can be used in exploration for massive sulfide deposits as vectors to ore, at the prospect district scale, and in discrimination of prospective hydrothermal from unprospective diagenetic alteration systems, at the regional scale.