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.

The Vacas replacement vein strikes N17°W on average, dips at 58° to the northeast (343°/58°), and has a known strike length of 200 metres. The zone is mineralized for a vertical extent of 400 metres, and its thickness varies from 0.2 to 18.0 metres. The replacement body is hosted by the Indidura Formation and is concordant with a fault zone running along bedding planes. This is thought to be a favourable structural setting for replacement mineralization. This fault structure is also locally occupied by later andesite dikes. The mineralogy of the replacement vein consists of galena, sphalerite, acanthite, freibergite, jamesonite, native silver, pyrite, pyrrhotite, arsenopyrite, quartz, calcite, and fluorite.

Production in 2016 was from five underground sources and from the Quebradillas open pit. Oxide ores where supplied mainly by the Quebradillas open pit, and Quebradillas and San Marcos underground mines, while sulphide ores were supplied from Rosarios, Intermedia, Vacas and Quebradillas underground mines.

The Vacas mine began production in the second half of 2013 at a rate of 300 tpd. The mine produced sulphide ore with high silver grades ranging from 200 to 240 g/t. The known mineral reserves were exhausted by December 2016.

La Parrilla operates two parallel processing circuits that recover metals from the two types of ore. The two ore types are oxide ores and sulphide ores; both types are polymetallic, containing silver, lead, zinc, and gold.

The flotation plant receives sulphide ores and produces a silver-rich lead concentrate and a zinc concentrate as saleable products. Precious metals are preferably deported to the lead concentrate with both concentrates showing payable values of silver.

The leaching plant receives oxide ores and produces doré bars as a saleable product.

The processing plants were originally designed to process 1,000 tonnes per day (tpd) per circuit for a combined throughput of 2,000 tpd. Total throughput was generally maintained at 2,000 tpd until mid-2015 when production from the Quebradillas open pit neared completion. After this time, the mining operations have been unable to provide 1,000 tpd of oxide ore from the remaining underground sources. FMS has partially offset this by working to increase the throughput of sulphide ores through the flotation plant. The 2016 average throughput for oxide ores was 520 tpd and 1,146 tpd for sulphide ores.

La Parrilla operates a conventional concentration plant consisting of a single multi-stage crushing plant, a flotation circuit, and a leaching circuit.

The single crushing plant batches sulphide ore and oxide ore to provide ore to the two parallel circuits. The crushing plant is a conventional three-stage crushing operation. The primary crusher operates in open circuit, the secondary crusher operates in open circuit with preclassification using a vibrating screen, and the tertiary crushing stage operates in closed circuit with a vibrating screen.

The flotation plant consists of a conventional ball mill, a multi-stage flotation plant that floats the lead concentrate first, then the zinc concentrate. After dewatering, the final concentrates are ready for trucking off site.

The leaching plant uses conventional agitated leaching, followed by the Merrill-Crowe process to recovery precious metals from the pregnant solution. Precious metals are smelted into a doré bar that is trucked off-site.

Tailing from both circuits are filtered separately before being dry-stacked in the tailings storage facility. Water for each circuit is managed independently.