Summary:
The mineralization at La Parrilla occurs in veins, breccias, stockworks and replacements that are hosted by the Cretaceous limestones and shales of the Indidura Formation and by the granodiorite–quartz monzonite intrusion. Contact metamorphism and metasomatism resulted in the development of marble, hornfels, skarnoid, and skarn at the intrusive contact. Because the mineralization is related to the intrusive contact and skarn development, the deposits are proposed to be of the intrusion related hydrothermal type, and may represent mesothermal to epithermal environments.
Veins at La Parrilla are of two types: open space filling veins and fault-veins. The open space filling veins can consist of massive sulphides veins; quartz-calcite veins containing pyrite, sphalerite, and galena; and breccia veins cemented by quartz-calcite. Fault-veins consists of matrix-supported breccias or gouge containing disseminated sulphides and oxides. Open space filling veins can transition along strike into fault-veins and vice versa, and the presence of stockwork is common at the contacts of the vein with the host rock. Thus, it is interpreted that most veins were open or partially open faults and fractures, that they were flooded with hydrothermal fluids, and that some of these were reactivated by later faulting. Replacement deposits, on the other hand, occur as oblique or perpendicular splays to veins and faults, and as larger replacement deposits concordant with sedimentary bedding. Replacement deposits generally have limited strike extent and have irregular shape and thickness.
The La Parrilla deposits contain primary sulphides such as galena, sphalerite, pyrite, pyrrhotite, arsenopyrite, chalcopyrite, covellite, acanthite, native silver, and silver sulphosalts (tetrahedrite–freibergite solid solution). Due to supergene oxidation, the primary sulphides in the upper parts of some deposits have been altered to cerussite, anglesite, hemimorphite, hydrozincite, jarosite, goethite, hematite, cervantite, malachite, chrysocolla, chalcanthite, and native silver. The main nonmetallic gangue minerals present in the deposits are calcite, quartz, fluorite, and siderite. The main clay minerals associated with the deposits and alteration halos are smectite, illite-smectite, and kaolinite.
The major mineralized zones are:
• Quebradillas N-S Vein: strikes north 45° west on average, dips at 71° to the northeast (315°/71°), and has a known strike length of 125 m. The vein is mineralized for a vertical extent of 465 m, and its thickness varies from 0.25 to 5.0 m.
• The Quebradillas Vein: strikes north 88° east on average, dips at 86° to the south (88°/86°) and has a known strike length of 280 m. The vein is mineralized for a vertical extent of 175 m and ranges in thickness from 1 to 2.5 m.
• Quebradillas Q-38: strikes north 7° west on average, dips at 68° to the northeast (353°/68°) and has a known strike length of 150 m. The zone is mineralized for a vertical extent of 180 m, and has a variable thickness of 0.3 to 8.7 m.
• Quebradillas 460 Replacement vein: strikes north 16° west on average, dips at 63° to the northeast (344°/63°), and has a known strike length of 425 m. The zone is mineralized for a vertical extent of 570 m, and ranges in thickness from 0.1 to 8.5 m.
• Quebradillas 550 Vein: strikes north 80° east on average, dips at 83° to the south (80°/83°) and has a known strike length of 315 m. The vein is mineralized for a vertical extent of 185 m and varies in thickness from 0.5 to 5.5 m.
• Quebradillas Tiro Vein: strikes north 60° west on average, dips at 56° to the northeast (300°/56°) and has a known strike length of 70 m. The vein is mineralized for a vertical extent of 120 m, and its thickness ranges from 0.50 to 4.0 m.
• Quebradillas Viboras Vein: strikes north 80° west on average, dips at 80° to the south (100°/80°) and has a known strike length of 700 m. The vein is mineralized for a vertical extent of 250 m, and its thickness varies from 0.3 to 3.5 m.
• Quebradillas El Recuerdo Vein: strikes north 80° west on average, dips at 70° to the northeast (280°/70°) and has a known strike length of 550 m. The vein is mineralized for a vertical extent of 150 m, and its thickness varies from 1.3 to 9.0 m.