Summary:
Mineralization in the Nuestra Señora area includes mineralized calc-silicate skarns, carbonate replacement deposits, veins, and breccia-hosted deposits that occur within limestone and granodiorite. Pb-Zn-Cu-Ag mineralization is primarily associated with variably retrograde-altered garnet-pyroxene exoskarn with lesser mineralization within pyroxene-garnet endoskarn in the Nuestra Señora mine. Within the Nuestra Señora Main Zone, post-skarn brecciation and calcite emplacement appear to be contemporaneous with mineralization, with sulfides occurring as fracture-fill and large “clots” within the calcite-filled breccia matrix.
Pb-Zn-Cu-Ag mineralization at Nuestra Señora is primarily associated with variably retrograde-altered garnet-pyroxene exoskarn (bedded limestone protolith) with lesser mineralization within pyroxenegarnet endoskarn. In general, exoskarn mineralization occurs within preferential horizons in the general stratigraphy which strikes northwest and dips to the northeast 30-50o. Thrust faulting sub-parallel to bedding has been proposed to create more favorable fluid pathways and localize mineralization.
Within the Nuestra Señora Main Zone, post-skarn brecciation and calcite emplacement appear to be contemporaneous with mineralization, with sulfides occurring as fracture-fill and large “clots” (up to 10cm across) within the calcite-filled breccia matrix. The breccia texture can be coarse with clasts greater than one meter in width. Though there are some weak disseminated sulfides within the clasts, the majority of sulfides occur within the highly irregular calcite-quartz-chlorite matrix.
In contrast to the skarn-hosted mineralization within the Nuestra Señora mine area, carbonatereplacement mineralization occurs at the Candelaria mine located about 200m to the northeast and 150m higher in elevation than the Nuestra Señora mineralization. Highly irregular, massive-sulfide base-metal bodies, that can be over one meter across, occur within a coarse crystalline, relatively unmineralized marble which formed by thermal metamorphism distal to the skarn alteration. The sulfide/marble contacts can be knife sharp. The sulfide mineralization is highly erratic, although there is evidence that the more significant mineralization is localized along southeast-dipping structures.
Deposition of sulfides occurred during several cycles, with the presence and relative abundance of chalcopyrite with sphalerite and galena indicating fluctuating temperatures during formation. The order of deposition of the sulfides appears to be pyrite, sphalerite, chalcopyrite, galena, and tetrahedrite. The distribution of silver may be related to deposition of copper and not lead. Deposition of silver, copper, and lead probably occurred independently to that of zinc.
The Nuestra Señora, Santo Domingo, and Santa Teresa deposits all have surface expressions. Scorpio found two additional large mineralized zones – Hoag and Sept 9 – adjacent to the main Nuestra Señora zone and between Nuestra Señora and Santo Domingo-Santa Teresa that do not crop out. Scorpio’s initial interpretations postulated that a series of stacked thrust faults provided the main conduit for mineralizing fluids. Subsequent deformation along the thrust faults created dilational zones, which provided structural traps for the emplacement of mineralization. The Sept 9 Zone appears to be a mineralized feeder for emplacement of mineralization into the Hoag and Santa Teresa zones. Recent reinterpretation suggests that the Main, Hoag, and possibly the Santa Teresa deposits appear to have been part of the same, formerly contiguous mineralized zone that has subsequently been offset by a series of northeast-striking, normal and/or strike-slip faults (i.e. Hoag fault). This interpretation is new and requires more investigation and modification.
Mineralization in the Hoag Zone consists predominantly of zinc and lead with over 100 g Ag/t and only minor amounts of copper. The zinc and lead sulfides in the Hoag Zone are generally finer grained, and zinc tends to be more enriched and silver slightly less enriched relative to the Nuestra Señora Main Zone. Mineralization in the Sept 9 Zone consists of coarser-grained sphalerite, galena, and chalcopyrite, similar to that at Nuestra Señora, with higher grades of silver and copper. The mineralization is located at the contact of a skarn and a granodiorite body.
Carbonate replacement mineralization occurs in re-crystallized limestones near or at the faulted contact between granodiorite and limestone at the Candelaria, Santo Domingo, and Santa Teresa deposits, with most of this mineralization occurring at Candelaria. At Candelaria, irregularly shaped massive sulfide pods vary considerably in size, shape, and orientation, which makes it difficult to define them with widely spaced core drilling. The pods are cut and displaced by steep north-northeast-trending faults. There is a spatial relationship between mineralization and a quartz-feldspar porphyry sill or dike that is from 2 to 10m thick and is predominantly sub-parallel to bedding. In addition to the massive sulfides, disseminated mineralization occurs along the interface between endoskarn and exoskarn developed at the contact between the limestones and intrusion. It is associated with retrograde skarn and mylonitic material within the faulted contact.
The Santo Domingo deposit is located on the north side of the Habitas River, 300m northeast of the Nuestra Señora mine portal. Santo Domingo is situated at the intersection of a regional N50°W-trending fault with the northeast-trending Hoag fault. Chimney and manto-type mineralization is locally disrupted and overprinted by intense silica flooding and discrete quartz veining that contains coarse sphalerite, chalcopyrite, minor pyrite, and locally enriched gold.
Santa Teresa is located about 150m northeast of Santo Domingo and about 200m northwest of Candelaria. The surface expression of skarn mineralization extends over a width of 100m along the river. Alteration consists of epidote, actinolite, tremolite, hornfels, grossular garnets, and silicification.