San Francisco

Jalisco, Mexico

Main commodities: Mn Fe
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The San Francisco manganese deposit is located in Jalisco State, Mexico and is one of a number of Tertiary volcanic-sedimentary hosted Mn oxide lenses spread over an area of around 100 sq. km in the region.

The deposit covers an area of 1.6 x 1 km and is up to 3 m thick.   It is well stratified on a 1 mm to 10 cm scale and is conformable with the enclosing sequence, although individual layers and lenses of ore are discontinuous and can rarely be correlated laterally over more than a few tens of metres.   It is underlain by a conglomerate and a trachytic tuff which are part of a local lacustrine sequence.   The deposit is separated from the underlying trachytic tuff by a 2 m band of silt to shale.   The tuff is also weakly mineralised, with a gradual upward increase in Fe, Mn and Ca in the tuff and shale towards the deposit.   A conglomeratic tuff, indistinguishable from that in the footwall overlies the deposit, with a local intraformational conglomerate at the contact containing Mn oxide clasts.

The Fe and Mn oxides are separated both laterally and vertically, with the Fe oxides to the NW, the Mn oxides to the SE.   The two ores interfinger laterally, but are not mixed in any one band or laminae.   The Fe oxides have a silica gangue, with alternating iron oxide (fine hematite) and cryptocrystalline silica laminae, accompanied by medium grained calcite disseminated throughout.

The Mn oxides are present in a variety of mineralogical assemblages, including:  1). finely laminated, very fine grained braunerite, cryptocrystalline silica and calcite,  2). fine grained braunerite disseminated within coarsely crystalline calcite,  3). medium grained braunerite and medium grained calcite in a salt and pepper mixture, and  4). a small conformable lens about 10 m in diameter of coarsely crystalline pyrolusite.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1981.    
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:
Zantop H  1981 - Trace elements in volcanogenic Manganese Oxides and Iron Oxides: The San Francisco Manganese Deposit, Jalisco, Mexico: in    Econ. Geol.   v76 pp 545-555

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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