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 San Marcos vein strikes N20°W on average, dips at 60° to the northeast (340°/60°), and has a known strike length of 600 metres. The vein is mineralized for a vertical extent of 280 metres, and its thickness varies from 0.5 to 17 metres. The structure is a fault-vein that marks the eastern contact of the stock with the Indidura Formation and it is concordant with bedding. The structure pinches and swells, reaching its maximum thickness in flexure zones along strike were it generally develops cymoid loops. The mineralogy of the vein consists of hematite, goethite, native silver, cerargyrite, quartz, and calcite.