Summary:
Mineral deposits at Del Toro occur in veins, chimneys, breccias and mantos. The deposits are associated with a quartz monzonite–granodiorite intrusion and are hosted by Cretaceous limestone and shale that has been altered to marble, hornfels, skarnoid and skarn.
Because of their spatial relation with intrusions and metamorphic/motasomatic rocks, the deposits are proposed to be of the intrusion-related hydrothermal type. Potassic alteration observed at depth in the San Juan mine suggests high-temperature alteration at depth. Although fluid inclusion- microthermometry studies have not been carried out for the Del Toro deposits, the association intrusions and skarn suggests they could be of the mesothermal to epithermal type. The occurrence of distal quartz– calcite veins containing fluorite in the Dolores mine is suggestive of an epithermal environment.
Mineralization to the south at San Nicolas and San Juan seems to be more mesothermal whereas the mineralization to the north in Dolores seems to be epithermal. No attempt was made to fit Del Toro deposits to the settings of the well-studied porphyries or epithermal deposits described by Sillitoe or Hedenquist elsewhere, since the geologic features observed suggest that Del Toro sits in between these end-member environments.
Mineral deposits at Del Toro occur in veins, chimneys, breccias and mantos. It is interpreted that some of the mineralization is skarn-related mesothermal in style, similar to that of San Martin Sombrerete. Some epithermal features, such as quartz–calcite veins containing fluorite, occur at Dolores and the Navidad claims north of Dolores. Veins at Del Toro can be of two types, open space filling or fault vein.
The open space filling type can be massive sulphide veins containing galena, sphalerite and pyrite, quartz– carbonate veins containing pyrite, sphalerite and galena, and massive carbonate veins consisting of calcite, siderite and manganiferous calcite.
The second type of vein, fault-vein, consists of breccia or gouge with disseminated sulphides and oxides. Open space filling veins can transition into fault veins, and vice versa, along a structure. Most veins were likely open or partially open faults that were flooded with hydrothermal fluids carrying metals, and some of these were reactivated by later faulting events. Good examples of massive open space filling veins that transition into fault veins are Lupita, Santa Teresa and San Nicolas. Cuerpo 1 and Cuerpo 2 in the San Juan mine are good examples of vein-faults. Veins typically range in width from a few centimetres to up to five metres.
Breccia pipes and chimneys also occur in the district. Sulphide-rich (galena and sphalerite) chimneys were mined in the Perseverancia mine (Perseverancia and San Nicolas chimneys) where they occur at the intersection of northwest trending structures and north-trending structures. Cuerpo 3 is a sub-vertical pipe like structure located on the eastern footwall of the Cuerpo 1 vein-fault system. It extends from 2,200 masl to at least 1,955 masl, varies in width from 40 to 80 m and forms where the Cuerpo 1 and Cuerpo 2 bodies intersect with one another. It contains sulphide and oxide mineralization in cross-cutting structures and disseminations.
Manto-type structures are more common in the Cotorras and Magistral areas, although some manto development has also been observed in the San Juan mine associated with the Lupita vein. The mantos may vary in thickness from 20 cm at Las Cotorras to 1.5 m at Magistral.