Mid-Tertiary polymetallic quartz sulfide vein deposits of the San Francisco del Oro-Santa Barbara district are among the largest Pb-Zn-Cu-Ag deposits in Mexico.

Vein formation occurred in at least four distinct stages that define an unusual paragenetic sequence when compared to similar systems. Early massive sulfide veins are cut by veins rich in calc-silicates, quartz, and late sulfides. This group is cut by two stages of postore quartzfluorite-calcite veins with minor sulfides. Early sulfides and late calc-silicates represent a paragenetic sequence which is the reverse of that normally observed in systems containing calcsilicates and sulfides.

Stage 1 veins are characterized by abundant sphalerite and galena, and minor quartz, with weak alteration halos of epidote, axinite, chlorite, quartz, and minor andradite. Stage 1 vein quartz was precipitated from 250 ø to 320øC solutions with salinities ranging from 2 to 14 equiv wt percent NaC1. Vein-related replacement bodies were also formed during this stage of hydrothermal activity. Stage 2 veins are characterized by an abundance of calc-silicate minerals within the veins, sulfides (dominantly chalcopyrite-pyrite assemblages), a small amount of gold mineralization, and a pervasive, calc-silicate-rich alteration assemblage containing manganoan hedenbergite, manganoan ilvaite, quartz, minor andradite, and hematite. Homogenization temperatures of fluid inclusions in late stage 2 quartz range from 180 ø to 300øC and salinities range from 1 to 14 equiv wt percent NaC1. Stage 3 and 4 veins are characterized by quartz, calcite, and fluorite. These veins were formed by 220 ø to 260øC fluids with salinities ranging from 1 to 3 equiv wt percent NaCI.

Veins
Pb-Zn-Ag quartz veins are localized along small displacement faults that cut obliquely across the hinge zone of a broad asymmetric anticline. Single veins are more or less continuous for distances up to 800 m. Veins average about 1.5 m wide but in detail pinch and swell from several centimeters to as much as 3 m in width. Pb-Zn-Ag mineralization is found over a vertical interval of more than 500 m.

The ore was emplaced in several distinct vein stages that display crosscutting relations. Stage 1 ores contain massive sulfides: sphalerite, galena, and very minor chalcopyrite. Stage 2 ores contain abundant calc-silicates, chalcopyrite, and a small amount of gold, but only minor sphalerite and galena. Silver was deposited during both stages, but the bulk of the silver was associated with galena and deposited in stage 1. Stages 3 and 4 contain quartz, calcite, and fluorite and are not important ore-bearing stages.

Replacement bodies
In addition to their occurrence in vein ores, silver, lead, and zinc are also recovered from two large, massive sulfide bodies on levels 11 and 12 of the Granadefia mine. Replacement bodies in the San Francisco del Oro district are large (the Boreal body on level 11 is roughly 200 x 70 x 200 m) but contain only 10 to 50 vol percent total sulfide. The remainder of the ore consists of wall rock more or less completely recrystallized to axinite, andradite, epidote, and chlorite. Relict bedding within these bodies, defined by silicate and sulfide layering, is seen in all but the most completely replaced volumes. In the district as a whole, greater than 95 percent of the ore comes from veins, but within the Granadefia mine, the replacement bodies can account for 20 to 30 percent of production.

The mineral extracted from this site is milled at a plant. It then undergoes a flotation process which culminates in the production of lead, zinc, and copper concentrates with gold, silver, lead, zinc and copper content.