Summary:
Mineralization at San Felipe can be classified as a zinc-lead skarn (Einaudi et al, 1981). These skarn systems commonly occur in continental settings associated with either subduction or rifting. They are sulphide rich with Zn + Pb commonly ranging from 10-20 % and Ag from 30-300 g/t. Zinc-lead skarns are often transitional to massive-sulphide veins and often lack significant calc-silicate alteration. The San Felipe district is characterized by a strong structural control on hydrothermal fluid movement and resulting alteration / mineralization in the northern areas (Ventana, Transversales and San Felipe) and a more disseminated style to the south (Lamas). Calc-silicate alteration at San Felipe is Mn-rich including bustamite-rhodonite, piemontite, garnet and pyroxene.
The San Felipe District contains a series of easterly-trending Pb-Zn-Ag-Mn skarn veins and pipes that cut the Lower Metamorphic sequence and intrusive rocks. The district hosts five principal, westerly-striking, vein systems that include Artemisa-Cornucopia, Las Lamas, San Felipe, Transversales and La Ventana. Primary minerals are sphalerite, galena, pyrite, and magnetite with lesser native silver, chalcopyrite, arsenopyrite, scheelite, and covelite within a gangue of garnet, pyroxene, epidote, quartz, rhodonite, and carbonate (Roldan-Quintana, 1979: Calmus et al., 1996).
Three types of felsic intrusions are spatially associated with the deposits and include the San Felipe rhyolite porphyry, a coarse grained quartz-feldspar porphyry with large amoeboidshaped quartz eyes (up to 1 cm-dia.) in a pink ground mass; the fragmental rhyolite porphyry, a medium-grained rhyolite with fragments of Lower sequence including epidote skarnoid; and granite, a medium-grained, equigranular plutonic rock with rhyolitic composition.
Skarn veins are late, structurally controlled, and crosscut all rock types. Hydrothermal fluid flow paths followed the dike margins, and the same fractures and minor faults that controlled the rhyolite porphyry intrusions.
Skarn-related calc-silicate minerals and sphalerite are useful indictors of system zonation and temperatures. Those minerals with high Fe/Mn ratios formed at higher temperature closer to the hydrothermal source, and minerals with decreased Fe/Mn ratios formed at lower temperatures further from the source. The San Felipe skarns are distal expressions of a larger hydrothermal system, and the skarn mineralogy and Ag-Pb-Zn-bearing sulfides display a metal zonation across the district from high Zn, low Mn in the south to high Pb, high Mn in the north.
Skarn in the south parts of the district at Las Lamas and Artemisa-Cornucopia contains Fe-rich, dark brown garnets (andradite) and dark green pyroxenes (hedenbergite) with epidote, magnetite, quartz, and carbonate associated with low Pb/Zn ratios, low Mn contents, and high Fe contents. This sphalerite is dark brown to deep red with high Fe contents and indicates high formation temperatures >300°C (Meinert, 2007).
In contrast, skarn in the north part of the district at La Ventana and San Felipe Este contain Mn-rich pyroxenoids (pinkish-tan rhodonite and bustamite), Mn-rich epidote, and quartz with increased Mn and Ag contents, increased Pb/Zn ratios, and decreased Fe contents. This sphalerite is honey-colored, an indication of decreased iron content, and the galena is argentiferous, and both are consistent with decreased temperatures of crystallization. Grossular garnet and wollastonite are common in areas with more abundant limestone (ie. Santa Rosa and Las Lamas).