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 intrusionrelated 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.
Rosarios Vein
The Rosarios vein strikes north 70 degrees (°) west on average, dips at 64° to the northeast (290°/64°) and has a known strike length of 2,000 metres (m). The mineralization extends vertically for 900 m, and its thickness varies from 0.2 to 14 m. The vein sits roughly at the northern contact of the granodiorite stock and the Indidura limestone. The vein pinches and swells; economic grades can occur either at the footwall or at the hangingwall of the main controlling structure. Stockwork zones are developed either at the footwall or hangingwall of the vein; vein splays and replacements are typically developed at the hangingwall. The mineralogy of the vein consists of galena, sphalerite, acanthite, native silver, pyrite, pyrrhotite, arsenopyrite, anglesite, willemite, quartz, calcite, and fluorite.
C1100 (Intermedia)
Vein The C1100 vein strikes north 50° west on average, dips at 75° to the northeast (310°/75°) and has a known strike length of 500 m. The vein is mineralized for a vertical extent of 430 m, and its thickness varies from 0.5 to 1.5 m. The structure is a fault-vein that pinches and swells and is hosted by the Indidura Formation and the granodiorite stock. The vein is oxidized in the upper 150 m and it usually develops mineralized stockwork. The mineralogy of the vein consists of mainly of galena, sphalerite, acanthite, native silver, pyrite, pyrrhotite, quartz, calcite, and fluorite.