The Santander Property hosts intrusion-related, carbonate-hosted, distal ‘passive’ replacement deposits, or carbon replacement deposits (“CRDs”). Controls on mineralization vary, however, with the majority of mineralization displaying very strong structural and lithological controls. The Santander CRD, in common with those in other districts, displays a strong mineralogic/metallic zonation: a ‘classic’ lateral or vertical zonation (from distal to proximal) would be Mn » Ag » Pb+Ag » Zn » Cu+Au. The 44-square-kilometre Santander Property is underlain by an approximately 2,600 m thick package of Cretaceous clastic and carbonate sedimentary rocks known as the Goyllarisquisga Group, within which an approximately 1,000 m thick sequence of massive limestones and limestoneshales of the Jumasha and Chulec Formations are the principal hosts identified to date. Pre-mineralization diabase dykes and sills are locally present within the section. The entire sequence is tightly folded into a series of orogenparallel, northwest-trending anticlines and synclines. The lower, predominantly clastic part of the section has been thrust over the mainly carbonate-rich upper portion (the favourable host rocks) along the regional northwest-trending Santander Fault Zone. Unconformably overlying the tightly folded Cretaceous sediments are moderately folded and faulted andesitic Tertiary volcanics of the Calipuy Formation. Syn-mineralization intrusive activity, considered to be the heat and fluid sources that produced base metal mineralization, has not been recognized on the Property to date. This absence is empirically considered indicative of a relatively large CRD system, and such bodies are inferred to be present at depth (>800 m) on the Property.

As part of the year-end tasks, Trevali has conducted aresource estimate for its Santander Property. The Santander Property includes four discrete deposit areas: the Magistral deposits, the Santander Pipe, the Puajanca Pipe, and the Tailings area.

The Magistral deposits consist of three main bodies: Magistral North, Magistral Central, and Magistral South; and six minor bodies: Rosa (depleted 2017), Bono, Fatima North, Fatima South, Magistral Central-North, and Oyon. Magistral mineralization is hosted in limestone of the Chulec formation, and the upper limits (or hanging wall) broadly correspond to a siliciclastic facies section of the Farat formation, often in fault contact. The lower limit (or footwall) of the mineralization is defined as the base of the last significant sulphide horizon and is occasionally gradational. Mineralized, narrow (generally less than 50 centimetres), but very high grade veins occur perpendicular to the main Magistral bodies and occasionally present massive sulphide replacement in between the veins, as in the cases of Rosa, Fatima South, and Fatima North. At depth, Oyon splits into two parallel (stacked) mantos: Oyon 1 and Oyon 2.

The Santander Pipe is located approximately 350 metres east of the Santander Mine’s processing plant. Sulphide mineralization is hosted within the Santander anticline and is associated with skarn and/or associated gangue (silicification, dolomitization, and calcic alteration) in various proportions largely dependent on the original character of the host rock and postulated distance from heat/mineral source or pathways. In detail, the skarn mineralization forms a circular, massive, plug-like body in the Jumasha and Pariatambo limestone formation to depths of approximately 250 metres below surface prior to forming more discrete skarn hosted replacements in the underlying interbedded Chulec limestone formation to 480-metre vertical depth, which is the vertical limit of historic mining operations. The average diameter of the skarn/sulphide body is approximately 120 metres. The mineralization remains open for expansion as underground exploration drilling indicates that the Santander Pipe extends a minimum of an additional 250 metres below the lowermost development levels for an approximate total vertical extent of 730 metres.

The Puajanca prospect is located approximately one km east-northeast of the Magistral North deposit. Mineralization is hosted within the Santander anticline, which also hosts the Santander Pipe approximately three km along strike and at a 700 metre lower elevation to the south-southeast. Mineralization occurs within the Pariatambo and uppermost portions of the underlying Chulec Formations. Within the preferred Chulec limestone, host replacement and vein mineralization is associated with weak skarn alteration. At Puajanca South, mineralization is exposed over an area approximately 120 metres long by 20 metres wide with variable thickness. Exploration drilling indicates that mineralization extends from surface to vertical depths of approximately 225 metres and remains open.

Underground mining commenced in 2013 and commercial production was declared effective January 31, 2014. All mining and mineral processing activities are performed by contractors.

The underground mine is accessed via three operational portals at Magistral North, Magistral Central, and Magistral South. Each portal has an associated ramp system, and the Magistral Central and Magistral South ramps are connected at the 4,510-metre and 4370-metre levels, with one ramp servicing both Magistral Central and Magistral South for the remainder of the depths of the currently defined mineralized lenses. By-passes connect the Magistral North ramp system to the Magistral Central and Magistral South system on the main levels on elevations of 4,510 metres, 4,580 metres, 4,370 metres, 4,300 metres and newly planned 4,230-metre level.

Avoca is the main mining method utilized at the Santander underground operations. It is supplemented by up-hole retreat for partial sill pillar recovery and by modified Avoca in some extremity stopes along strike of the mineralization. Underground mining is divided into 70-metre vertical height mining blocks, in which each mining block consists of three 18 metre high sublevel production stopes and a 10 to 12 metre thick sill pillar to be left in place for overall stability reasons.

Stope sequencing is retreated along strike from lens extremities. Ore is hauled to surface by 20-tonne capacity trucks loaded by Load-Haul-Dump vehicles (LHDs). Waste rock broken underground is hauled by LHDs to empty stopes as backfill, to underground temporary storage (remuck bays), or to surface temporary waste storage. Ore mucked from stopes is either loaded directly into trucks or stored in remuck bays along the ramps prior to truck haulage. LHDs fill empty stopes with waste rock from development, supplemented with waste rock back-hauled from existing surface waste rock stockpiles.

The Santander processing plant is a conventional sulphide milling and flotation plant, comprised of three stages of crushing with two stages of grinding and differential flotation to produce zinc and lead concentrates. Concentrates are dewatered on-site and temporarily stored in concentrate sheds before being trucked to the Callo Glencore port facility near Lima as part of off-take agreements with Glencore. Process plant tailings are delivered to an on-site tailings management facility.

The Santander mill and flotation plant was designed and built by Trevali, Los Quenuales, and Holland & Holland. Los Quenuales’ closed 1,250 tonne-per-day Rosaura mill was purchased, relocated, refurbished, and expanded to process Santander material at a nameplate capacity of 2,000 tonnes per day. The mill receives ore mainly from the Magistral underground zones, which is supplemented by excavated historical tailings from a nearby tailings pond.

Since commissioning, the mill has been operating at better than predicted metallurgical results, in addition to a higher than predicted plant availability. The operating performance of the Santander mill in 2017 averaged approximately 2,300 tonnes per day (versus nameplate capacity of 2,000 tpd) with utilization of above 91.8%.