The Pachuca Real del Monte district is a leading example of a low total sulfide epithermal vein-type silver district. In its 500-year history, Pachuca has produced about 40 × 106 kg of silver from more than 100 million metric tons (Mt) of ore at an average total sulfide content of about 1 wt percent. Hosting the more than 100 veins of the district are calc-alkalic series volcanic rocks of Oligocene-mid Miocene age that total about 2,700 m in thickness. K-Ar dating of the volcanic rocks and the veins indicates that the veins formed within ~1 m.y. of the termination of volcanism at about 21 Ma.

The present study is of the north-south–trending Purisima-Colon vein system of the Pachuca Real del Monte district where field, petrographic, and fluid inclusion studies provide a three-dimensional and time transgressive view of vein formation and important clues to the genesis of the general class of low total sulfide silver veins. The Purisima-Colon vein deposits are located in a high-angle normal fault system where vertical offset varies systematically from ~30 m in the center to nil at the north and south ends. The fault system is blind and its top describes a concave downward arc, the highest point coinciding with the area of maximum offset and the lowest points, at the distal north and south ends, with the least offset. The top of the fault transitions upward into a zone of distributed fracturing that extends upward an additional 300 to 350 m. Illite-pyrite-calcite-chlorite alteration pervades the zone of distributed fracturing above the fault, whereas propylitic alteration dominates wall rocks below the top of the fault. Ore-grade silver mineralization is within a 650-m vertical interval that extends downward from the top of the vein to a bottom determined by silver grade. The vein system is vertically zoned, from the top down, with respect to thickness, sulfide content, and silver grade as follows: (1) an upper zone of 50-m vertical extent in which the vein is <0.5 m wide and has low total sulfides (<<1 wt %) and low Ag grade (<100 g/t), (2) an intermediate zone of about 100-m vertical extent that averages 0.5 to 1 m wide and contains low to moderate sulfides (≥ 1 wt %) and low to moderate Ag values (150–300 g/t), (3) a main ore zone of 350-m vertical extent in which the vein is 2 to 5 m wide and contains moderate to high sulfides (averages 1–2 wt %), and high to moderate Ag values (500–2,000 g/t), and (4) a lower zone of unknown vertical extent (extending >500 m below the mine workings) that is similar to the main ore zone but contains only base metal sulfides.

The veins developed in four stages: (1) brecciation of the host rock, (2) a silicate stage, (3) an ore stage, and (4) a postore stage. Stage 1, brecciation of the host rock, took place prior to the onset of hydrothermal activity when broken wall rocks accumulated in openings produced by separation of the fault walls. Stage 2, the silicate stage, is dominated by various forms of fine-grained to microcrystalline quartz and chalcedony plus coarse-grained crystalline quartz, johansennite, K-feldspar, albite, clinozoisite, epidote, and hematite. Petrographic and fluid inclusion data indicate that much of the fine-grained to microcrystalline quartz and chalcedony of stage 2 was precipitated rapidly from <180°C fluids that were supersaturated to highly supersaturated with silica, whereas the coarse-grained crystalline quartz and associated johansennite, K-feldspar, and clinozoisite precipitated from fluids at near-quartz saturation and at temperatures of 230° to 310°C. The coarse crystalline quartz was deposited by fluids that ascended the veins along the boiling curve, as evidenced by all-vapor fluid inclusion assemblages and by fluid homogenization temperatures plotted versus depth; PCO2 for the silicate-stage fluids was <10–2 bar and PO2 was in the hematite field. Fluid inclusion ice-melting temperatures from quartz, adularia, epidote, and johansennite average ~1 wt percent NaCl equiv (range = 0–4 wt % NaCl equiv). The silicate-stage mineral assemblage, quartz + adularia + clinozoisite, in conjunction with fluid homogenization and freezing data, indicates that the silicate stage was deposited from low-salinity, oxidizing, neutral pH hydrothermal fluids similar to the equilibrated, deeply circulated meteoric fluids of modern dilute, neutral pH geothermal systems.

The ore stage was deposited at 270° to 300°C by fluids that were acid, of moderate salinity, and which did not deposit silica or quartz except at the very beginning of the ore stage or as very local alteration products of johansennite. Sulfides were precipitated mainly by fluid-rock reactions in which wall rocks and stage 2 alumino-silicate minerals were altered or totally replaced by sulfides + illite + kaolinite + carbonate + Mg-chlorite. Ore-stage fluids, with salinity in the range of 5 to 7 wt percent NaCl equiv, resemble the “magmatic” fluids of Giggenbach (1997) diluted by one to two parts of equilibrated meteoric fluids. Whereas most sulfides precipitated from water-rock reactions, sulfides lining vugs probably precipitated due to cooling and/or dilution.

The separate quartz and sulfide deposition at Pachuca is believed to be a common feature of silver-dominant epithermal veins based on descriptions of other mineral deposits of this type. This observation suggests that the absence of quartz at Pachuca may be a defining feature of this class of deposits and might provide important clues to the ultimate source of the metals. The separate genesis of quartz and ore-stage minerals also requires caution when applying the results of fluid inclusion studies in quartz to problems of ore transport and deposition in such systems.

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