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In Mexico massive sulphide deposits are widespread and located in a belt parallel to the Pacific border. Two distinctive sub-belts are recognized: (1) a middle Jurassic to early Cretaceous belt, located inland Mexico, including Francisco I. Madero; and (2) an Albian to Cenomanian belt located close the present Pacific border.
Francisco I Madero Zn-Cu deposit is located close the city of Zacatecas (Zacatecas state, Mexico). This deposit is enclosed within a tuffaceous submarine arc to back-arc basin volcano-sedimentary sequence. Both the deposit and enclosing rock are highly deformed and affected by a metamorphic event in the greenschist facies. Deformation is represented by a highly penetrative foliation in relationship with isoclinal folds and shear zones. A late deformation event provokes the formation of kink bands affecting the whole sequence. A Tertiary extensional deformation originated strike slip faults and a conjugate system of normal faults of NW and NE trends.
The lower limit of the ore body is marked by the presence of a decimetric to metric thick segregation quartz blanket that spans all the ore body, probably reflecting a sub-horizontal shear/detachment zone. Lower ore bodies are composed by massive, highly brecciated pyrrothite-pyrite bodies with subordinated sphalerite and chalcopyrite; the middle part is mainly composed by rich sphalerite-galena lenses, from 6 to 65 meter thick and from 2.5 to 3 km long, floating in the chlorite-epidote enclosing rock; the upper part of the system is characterized by Cu-Ag rich lenses composed by chalcopyrite, pyrite and minor sphalerite, enargite and cubanite. A very late, acid dike swarm crosscuts the whole, remobilizes sphalerite and develops very narrow skarn-like assemblages marked by the presence of wollastonite. Local native copper and silver accumulations appear as remobilization products caused by late tertiary faults.
This deposit is characterized by an intense deformation of the ore bodies that transposed the original bedding (S0), originating a schistosity (S1) mainly affecting the incompetent sphalerite-rich lenses and provoking the cataclasis of the more rigid (competent) pyrite-pyrrothite rich bodies. The latter often present rotated pyrite porphyroclasts with "pressure shadows" filled up by segregation quartz. Only in the pyrite-pyrrothite ore bodies the original sedimentary bedding (S0) is preserved suggesting a VMS or SEDEX affiliation.
The Zn and Cu-rich bodies are rooted within the pyrite-pyrrothite bodies, arranged as duplex-like structures. This, coupled with the presence of the basal quartz blanket suggests that metamorphic fluids after dehydration of the volcano-sedimentary sequence, were expelled and focused through detachment structures acting as heat transport vectors that improved the metamorphic remobilization of sulfides, giving rise to the Zn and Cu-rich lenses.
This study has been supported by projects PAPIIT IN100707, IN114106 and Conacyt 49234-F.
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