The Zn-Pb-Cu (- Ag ± Au) San Martín deposit in northwestern Zacatecas is one of the most economically important and largest skarns in México. Mineral associations in this deposit belong to the sulfide skarn type (with rather "classical" prograde and retrograde zones) and contain peripheral sub-epithermal to epithermal veins.

The San Martín skarn deposit was formed by a hydrothermal system associated with intrusion of the 46-m.y.-old Cerro de la Gloria quartz monzonite stock into the middle Cretaceous Cuesta del Cura limestone. The deposit is exploited by two major mines. The San Martín mine extracts Cu-Zn-Ag ore from veins and replacement bodies hosted by skarn, and the Sabinas mine extracts Zn-Pb-Ag (+ or - Au) ore from veins hosted by skarn and recrystallized limestone.

The vein system represents a series of intrusion-related fractures that roughly parallel the intrusive contact and that served as major conduits for the ore-forming fluids. Formation of both the vein system and sulfide-hosting in skarn probably was aided by volume loss during metasomatism. Other retrograde phases include wollastonite, vesuvianite, epidote, and chlorite; fluorite and calcite are common, and minor quartz is also present.

In the area of San Martín the Cretaceous Cuesta del Cura and Indidura Formations consist of limestones with thin interbeds of flint and shale. It has been observed in the lower levels of the San Martín mine, as well as in deep diamond holes of exploration, that the Cuesta del Cura Formation is approximately 900 m thick, presents changes such as: the presence of carbonaceous shale horizons and almost absence of flint. The Peña formation are intensely folded and faulted by the Laramide orogeny of (40 to 80 Ma), and discordantly covered by tertiary conglomerates, and by Rhyolites and Basalts of the Quaternary across the region.

The mineralization (replacements) inside the mantles/layers is variable and irregular. The length of the mineralization downdip reach 800-850 m and drilling has found that the mineralization is still open at depth, including areas where the mineralization extends for an additional 400 m. The
width of the mantles varies from few meters up to 50-60 m or more locally.

Mineralization is related to the retrograde alteration event overprinting the skarns. This retrograde event is thought to have occurred during the cooling of the porphyry intrusions is related to the retrograde alteration that accompanied the porphyry feldspathic quartz. Fluid inclusions show that sulfide mineralization was associated with high and low salinity fluids (46 %wt Na Cl eq. and 3-8 %wt NaCl eq. Gonzalez Partida, 1997) and temperatures between 250°-300°C, suggesting magmatic and meteoric sources respectively.

Alteration veins associated with porphyries include: early quartz-chalcopyrite-molybdenite, quartzchalcopyrite-arsenopyrite, fluorite-arsenopyrite with sphalerite and quartz, quartz-sericite pyrite veins. In the central part of the deposit there is a horizontal zone with respect to the intrusive contact, presenting Ag, Cu and As enrichment close to the contact. Sphalerite tends to be deposited later and in greater quantity than Fe, Mo, As, Cu, but it is strongly associated with marmatite and chalcopyrite. At depth, there is an increase of pyrrhotite and marmatite, with a horizontal zonation similar to that described. In the NE portion of the deposit structures concentric to the intrusive “Ibarra, Gallo – Gallina”, there is an increase of Pb, Zn, Ag in the Skarn.

Mining is completed using a mechanized Cut and Fill mining method with rockfill and with tailings (hydraulic) and dry tailings. Ramps and levels are developed to provide access to the ore. Attack ramps are then driven to access each cut. The ramps and level development are performed using jumbos. San Martín has been testing the long hole stopping mining methodology after the operation resuming.

Mineral Processing is completed via conventional flotation processes, including crushing, milling, flotation, filtration, thickening and conduction and disposal of tailings. The Minera San Martín unit has two processing plants or concentrators. Three concentrates are produced:
• Zinc Concentrate
• Copper Concentrate
• Lead Concentrate