Chihuahua, Mexico

Main commodities: Zn Pb Cu Ag
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The Bismark skarn related zinc, lead, copper and silver massive sulphide deposit is located within northern Chihuahua in Mexico.

The deposit is classified as a high temperature carbonate replacement deposit, occurring as massive sulphides hosted by altered Cretaceous limestone adjacent to the contact of the 2.5 km diameter, 42 Ma Bismark quartz-monzonite stock and the Bismark fault.   The main phase of the quartz-monzonite is porphyritic

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.

Bismark is within the Chihuahua tectono-stratigraphic terrane, developed over a basement of Mesoproterozoic igneous and sedimentary rocks metamorphosed in Grenvillean time (1.3 to 1.1 Ga), overlain by Palaeozoic and Mesozoic siliciclastics and carbonates.   It is located on the western margin of the Chihuahua Trough, a marine embayment containing a thick Jurassic succession of basal red beds, evaporites and shales followed by lower Cretaceous limestones and capped by thick, upper Cretaceous shales and limestones.

The immediate host to ore is a massive, medium to thick bedded, variably fossiliferous limestone unit belonging to the extensive Aurora Group that hosts all the major deposits of this type in northern Mexico.

Alteration which developed concurrently in both the stock and the intruded limestone comprises:
• K-silicates in the stock (K-feldspar and biotite, overprinted by kaolinite rich veins and pervasive alteration and by subsequent quartz, sericite and pyrite with minor sphalerite and chalcopyrite),
• the limestones have been altered to a prograde exoskarn assemblage of green andradite and diopside with a transitional red-brown andradite, green hedenbergite and minor vesuvianite, calcite, fluorite and quartz.   The main ore stage post dates the calc-silicates and comprises sphalerite and galena with gangue pyrite, pyrrhotite, calcite, fluorite and quartz.

The hydrothermal system was developed synchronously with faulting on the Bismark fault.

The main orebody is hosted by exoskarn and by massive limestone in both the footwall and hangingwall of the Bismark fault within 100 metres of the Bismark stock contact, and has the form of a sheet like body dipping 75 to 80° SW.   It is around 700 m long and persists down dip for 500 m.   Ore occurs as  i). incipient to massive replacement of exoskarn (the bulk of the ore),  ii). massive replacement of recrystallised grey limestone or white marble,  iii). veins cutting all other styles  iv). replacement of the Bismark stock.   The massive sulphides follow the host stratigraphy, grading rapidle out through semi-massive to irregular sulphide veins to peripheral chlorite-calcite-quartz-sulphide veins, extending as far as 150 m into the hangingwall.

Other mineralisation includes sub-economic Pb-Zn-Ag mineralisation away from the main mining area, but adajcent to the stock, as veins and small replacements of pyrite or pyrrhotite massive sulphides.   In addition there are peripheral prospects adjacent to the major structures which lack calc-silicate alteration and are restricted to iron and mangenese oxides within faults.

The deposit contains:   8.5 Mt @ 8% Zn, 0.5% Pb, 0.2% Cu and 50 g/t Ag.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2002.    
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:
Baker T, Lang J R  2003 - Reconciling fluid inclusion types, fluid processes, and fluid sources in skarns: an example from the Bismark Deposit, Mexico: in    Mineralium Deposita   v38 pp 474-495

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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