Mexican epithermal deposits can be broadly grouped into two
types based on their contained metals, distribution, associated igneous rocks
and fluid inclusion compositions. Silver-gold deposits (eg Tayoltita) mostly
occur in the western part of northernMexico
where host rocks consist of Eocene, intermediate composition volcanic and
intrusive rocks; fluid inclusion studies indicate that mineralising fluids were
<300c and dilute (3 eq. wt. % nacl). in contrast, silver-lead-zinc deposits>
(eg Fresnillo) mostly occur in the central part of northern Mexico where they
are temporally associated with Oligocene-Miocene volcanic rocks and intrusions
of felsic composition, but commonly hosted within underlying Paleozoic-Mesozoic
sedimentary rocks; fluid inclusion studies indicate that mineralizing fluids
ranged from 200 to 400 C and were saline (10 eq. wt. % NaCl). The
distribution of these two deposit types, crosscutting major crustal sutures, and
the close spatial-temporal relationships between mineralisation and magmatism,
together suggest that magmatic processes were more important in the formation of
mineralising solutions, as a source of both ligands and metals, than the
segments of crust hosting deposits.