Pachuca Real del Monte District - Vizcaina, Santa Ana, Maravillas, Santa Gertrudis,

Hidalgo, Mexico

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The Pachuca Real del Monte district embraces a series of low total sulphide epithermal vein-type silver deposits and is located in Hidalgo state, Mexico, some 90 km north of Mexico City.

The district contains hundreds of productive veins spread out over an area of more than 70 sq. km. These vein systems have been exploited for over 500 years in which time they have produced about 40 000 tonnes of silver from more than 100 Mt of ore at an average total sulphide content of about 1%. Most of the Ag production and all of the bonanzas were in three compact sub-districts: i). western Vizcaina-Santa Ana-Maravillas, ii). Real del Monte, and iii). Santa Gertrudis.

The mineralised veins are hosted by 2700 m of Oligocene-mid Miocene age calc-alkalic series volcanic rocks ofthat total about 2,700 m in thickness. K-Ar dating of the volcanic host rocks and the veins indicates that the veins formed within around 1 m.y. after the termination of volcanism at approximately 21 Ma.

Rocks within the Pachuca field range in composition from basaltic andesite to rhyolite with andesite and dacite flows and flow breccias as the principal components. The volcanic pile has been divisible into a lower succession of mostly intermediate composition flows and breccias and subsidiary volcaniclastic sedimentary rocks of generally uniform thickness comprising 75 percent of the field, and an unconformably overlying upper succession of dacite breccias and rhyolite flows and tuffs of highly variable thickness making up the remainder. Four sets of east-west­trending dykes are cogenetic with the upper succession, the youngest of which are feeders to volcanic domes. The lower succession was deposited under conditions of relative tectonic quiescence, the upper succession indicates episodic volcanism, erosion, warping, intrusion, and faulting. Mineralisation occurred at or near the end of the deposition of the upper succession.

The rocks of the Pachuca volcanic field are cut by east-west trending normal faults with dips of 55 to 70° and vertical offsets of up to 500 m and north-south normal faults with dips >75° and maximum vertical offsets of only 30 m. The widespread east-west faults began their growth during the deposition of the lower succession, and parallel the structural grain of the Trans Mexico volcanic province in contrast with the north-south normal faults which are only found in the Real del Monte area and were active for a relatively brief period of time just prior to and during mineralization.

One example of the veins of the district is the north-south trending Purisima-Colon vein system which is located in a high-angle normal fault system where vertical offset varies systematically from around 30 m in the centre to zero at the northern and southern ends. The uppermost part of the vein describes a concave downward arc, the highest point coinciding with the area of maximum offset. The top of the fault grades upward into a zone of distributed fracturing that extends upward an additional 300 to 350 m.

The zone of distributed fracturing above the fault is characterised by pervasive illite-pyrite-calcite-chlorite alteration, whereas propylitic alteration dominates wall rocks below the top of the fault. Ore-grade silver mineralisation is developed over a 650 m vertical interval extending downward from the top of the vein to a bottom determined by silver grade.

The vein system is vertically zoned, from the top down, as follows: i). an upper zone of 50 m vertical extent in which the vein is <0.5 m wide and has very low total sulphides (<1%) and low Ag grade (<100 g/t), ii). an intermediate zone of about 100 m vertical extent that averages 0.5 to 1 m wide and contains low to moderate sulphides ( 1%) and low to moderate Ag values (150 to 300 g/t), iii). a main ore zone of 350 m vertical extent in which the vein is 2 to 5 m wide and contains moderate to high sulphides (averages 1 to 2 wt %), and high to moderate Ag values (500 to 2000 g/t), and iv). 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 suphides.

The veins are interpretted to have developed in four stages:
i). brecciation of the host rock, which took place prior to the onset of hydrothermal activity when broken wall rocks accumulated in openings produced by separation of the fault walls.
ii). a 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.
iii). an ore stage, which 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. Sulphides were precipitated mainly by fluid-rock reactions in which wall rocks and stage 2 alumino-silicate minerals were altered or totally replaced by sulphides + illite + kaolinite + carbonate + Mg-chlorite.
iv). a postore stage.

The most recent source geological information used to prepare this summary was dated: 2005.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Dreier J E,  2005 - The Environment of Vein Formation and Ore Deposition in the Purisima-Colon Vein System, Pachuca Real del Monte District, Hidalgo, Mexico: in    Econ. Geol.   v100 pp 1325-1347

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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