San Martin

Zacatecas, Mexico

Main commodities: Ag Au Pb
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The San Martin deposit is located at Sombrerete in Zacatecas state, Mexico, and is owned and operated by GIMMSA [Grupo Industrial Minera Mexico SA de].(1994) Published reserve and historic production figures include:

Proven+probable reserves, 1993 - 21.3 Mt @ 96 g/t Ag, with associated Pb, Zn, Cu (AME, 1995),
Production 1548 to 1821 - 0.25 Mt @ 450 g/t Ag, 0.5 g/t Au (Ruvalcaba & Thompson, 1988),
Production 1822 to 1900 - 0.05 Mt (Ruvalcaba & Thompson, 1988),
Production 1959 to 1987 - 10.5 Mt (Rubin & Kyle, 1988).

In 1992 the mill treated 1.57 Mt of ore to output 21.7 t Ag, 3923 t Pb, 12 000 t Cu, and 67 300 t Zn (AME, 1995). This would equate with recovered grades of 14 g/t Ag, 4.3% Zn, 0.25% Pb, 0.75% Cu.

Regional Setting

For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to the deposits of that belt, see the Regional Setting section of the Fresnillo record.

Geology & Mineralisation

The San Martin district is located on the western margin of the Mexican Thrust Belt, near the boundary with the Sierra Madre Occidental. Mineralisation occurs within a sequence of middle Cretaceous (Albian to Cenomanian) dark grey limestones with common thin shale and chert bands. It is present as a skarn zone on the margin of a major 4.5 km diameter quartz-monzonite porphyry stock, with associated quartz-rhyolite porphyry represented by apophyses and as an outer shell. In addition to the skarn, mineralisation is also present as sulphide chimneys and as veins. Mineralisation is hosted by favourable carbonate beds between shaly horizons, and has been dated at 46.2 Ma (Megaw, et al., 1988; Rubin & Kyle, 1988).

There is a zonation outwards from the intrusive contact of the stock, commencing with a Cu-Ag contact skarn, to Cu-Zn skarn veins, to Pb-Ag veins, then Au-Sb rich veins, and finally marble. Au is farthest from the contact, occurring within recrystallised limestone. The content of Fe, Cu, Zn and Pb increase with depth within the ore zone. The skarn is characterised by an assemblage comprising chalcopyrite, tetrahedrite-tennantite, molybdenite, sulphosalts, sphalerite, galena, pyrite, pyrrhotite and arsenopyrite, in a gangue of andraditic-grandite garnet, hedenbergite, tremolite, actinolite, wollastonite, vesuvianite, epidote and chlorite. The chimneys contain sphalerite, galena, chalcopyrite, tetrahedrite-tennantite, pyrite, silver and stibnite, with fluorite, calcite and quartz. Vein mineralisation is composed of galena, proustite-pyrargyrite, silver, sphalerite, pyrite and stibnite with fluorite, calcite and quartz (Megaw, et al., 1988; Rubin & Kyle, 1988).

The larger orebodies at San Martin are centred around several major veins. One of these, the San Marcial vein, which is the most economically important, has an average width of 2 m and can be traced for approximately 850 m in a direction of 30°. The dip of the vein is dependent upon the attitude of the intrusive contact. The bulk of the economic mineralisation associated with this vein is located within the skarn replacement body, which is centred on the vein, and varies in width from 2 to 100 m (Rubin & Kyle, 1988).

It has been deduced that early high temperature hydrothermal alteration produced hedenbergite and chalcopyrite bearing skarn. Garnet rich skarns followed, although as the temperature continued to drop, garnet and later clinopyroxene, became unstable relative to the retrograde calc-silicate assemblage that includes tremolite-actinolite, wollastonite, epidote and chlorite. Dissolution of CaCO3 raised the pH, while interaction with meteoric water may have resulted in the release of metals from the hydrothermal solutions. Metals were deposited within inter-crystal pores in the grandite-garnet skarn (Rubin & Kyle, 1988).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1994.    
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:
Gonzalez-Partida, E. and Camprubi, A.,  2006 - Evolution of mineralizing fluids in the Zn-Pb-Cu(-Ag±Au) skarn and epithermal deposits of the world-class San Martin district, Zacatecas, Mexico: in    J. of Geochemical Exploration   v.89, pp, 138-142.
Megaw, P.K.M., Ruiz, J. and Titley, S.R.,  1988 - High-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of Northern Mexico: in    Econ. Geol.   v.83, pp. 1856-1885.
Rubin J N, Kyle J R  1988 - Mineralogy and geochemistry of the San Martin Skarn deposit, Zacatecas, Mexico: in    Econ. Geol.   v83 pp 1782-1801

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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