The geological model for the Los Helados deposit consists of a Miocene porphyry/breccia system emplaced at the contact between a Permo-Triassic dacite porphyry intrusion and late Paleozoic granite.

Core drilling indicates that the magmatic- hydrothermal breccia system and dacite porphyries host the largest part of the Los Helados Cu and Au mineralization. Both breccias and porphyries intrude a coarse-grained granite pluton that is locally cut by a swarm of mafic dykes. Copper and Au mineralization extends into the granite country rock.

An intrusive complex of tonalitic composition pre-dates the Los Helados breccia body; clasts of the intrusive complex are entrained within the breccia.

The main mass of the intrusive complex is situated in the north of the Los Helados area. Intrusive breccia textures with coarse-grained tonalite are common. Dikes mapped in the area adjacent to the breccia are black and fine- grained to aphanitic.

At least three generations of dacite porphyry that accompanied the Miocene mineralization event are now recognised at Los Helados.

The earliest of the porphyries is mainly encountered as clasts at depths of >1,000 m in the central parts of the polymict breccia. The porphyry is clearly early in timing because it is cut by early dark micaceous (EDM) and A-type quartz veinlets that contain medium Cu grades.

Mineralization.
The copper–gold mineralization at Los Helados is primarily hosted by the Miocene magmatic– hydrothermal breccia which forms a roughly circular, pipe-like body with minimum dimensions of 1,100 m east–west, 1,200 m north– south and at least 1,500 m vertically. The breccia body is surrounded by a broad halo of moderate to low grade Cu–Au mineralization which diminishes in grade with increasing distance from the breccia contact.

The breccia limits have been established by drilling to the west, east and south; however, the northern limit of the breccia body has not yet been identified. The system also remains open at depth, and the lateral extent of the breccia at depth is also poorly constrained by the current drilling. The eastern contact appears to be subvertical, whereas the western contact dips outwards at roughly 70°, hence the width of the breccia body increases progressively downwards.

Copper grade increases downwards, either in the lower parts of the sericitic zone or in the underlying chlorite–sericite alteration zone, and elevated grades are maintained into the potassic alteration zone.

Gold grades generally correlate well with Cu; however, within the sericitic alteration zones, where pyrite content exceeds chalcopyrite, high Au grades can be locally independent from Cu values and are hosted by narrow veins.

Consistently high Cu and Au grades are present in the potassic plus chlorite–sericite and potassic zones where chalcopyrite is more abundant than pyrite.

The main conclusions and recommendations from the block cave geomechanics study were:

- Based on empirical methods, caving of the Los Helados rock mass can be achieved with a minimum hydraulic radius (HR) of 39. This equates to a cave initiation footprint of approximately 135 x 185 m. It is concluded that cave initiation can be successfully achieved at Los Helados given the rock mass conditions and proposed mining footprints.

- The rock mass characteristics suggest that unassisted cave propagation may be an issue at Los Helados, with a planned lift height of 1,000 m. The proposed height/width ratios of the cave are 0.56 to 2.75, depending on which side of the cave is considered. This indicates that there could be a risk of cave-stalling, depending on the initiation strategy. The proposed single lift of more than 800 m over the full footprint area is greater than what has currently been done in other existing traditional block cave operations. In order to ensure cave propagation of the proposed 1,000 m column, the design will incorporate a HF preconditioning lift located at 3,840 mRL. HQ diameter preconditioning holes will be drilled on a 70 m x 82 m grid spacing, using 300 m upholes and 300 m down-holes. To complete the full column HF, two design options are possible. Both options will have the same lower HF level at 3,840 masl. The difference between the two options is that one has a second HF lift at 4,120 masl and the second option assumes 500 m down-holes are drilled from surface within the central footprint zone.

- The results of the primary fragmentation analysis indicate that in-situ block sizes alone are not viable for efficient cave mining, and that the effects of secondary fragmentation will need to be considered. Draw heights of around 100 m are required before adequate/productive fragment size distributions are produced through secondary fragmentation. The use of confined blasting (CB) preconditioning of the first 100 m of columns is recommended for efficient productivity using 6 yd3 LHDs. The current design assumes 100 m of CB, using up-holes from the undercut on a 12 m x 17 m spacing.

- Based on empirical analyses, the likely cave angle at Los Helados will be around 75°. The predicted mean caving angle of 75° from the empirical analysis is comparable to data from benchmarking studies from similar block cave mines in porphyry copper systems. A maximum subsidence angle of 60° has been estimated for Los Helados.

The deposit is massive, approximately cylindrical, and has a nearvertical dip. Based on a combination of cost and the deposit geometry, block caving was selected as the preferred mining method.

The block cave mine design is based on a 1,022,000 m2 footprint area. The footprint area contains two production lifts. Lift 1, the upper and smaller of the two lifts, and Lift 2, the lower and larger of the two lifts. Designs include:

- Lift 1 will have its undercutting level (UCL- 1) at elevation 3,630 masl, 90 m above the UCL of Lift 2. It will have a 194,000 m2 footprint, and was designed with a rectangular shape (200 m wide north–south, 970 m long east–west). Mining assumes block caving with load-haul-dump (LHD) equipment for extraction. Upper Lift 1 has the LHDs dumping into 120 m ore passes connected to the haulage level, from where the mill feed material will be trucked to one of three underground gyratory primary crushers.

- Lift 2, has its undercutting level (UCL-2) at elevation 3,540 masl. The lift will have an 828,000 m2 footprint, and be mined by block caving using LHD equipment. Lift 2 has LHDs dumping into 30 m ore passes connected to the haulage level, from where the mill feed will be trucked to the three primary crushers.

There are two common levels serving Lifts 1 and 2:

- The truck haulage level, 120 m below the production level in Lift 1 and 30 m below the production level in Lift 2, receives the ore dumped in the production level into near vertical ore passes connecting to the truck haulage level. The receiving end of the ore pass is equipped with chutes to load the trucks that haul the ore to the three underground primary crushers.

- The conveyor level is located 40 m below the truck haulage level. Intensive pre-conditioning of the whole rock mass was incorporated in the design and will use both hydraulic fracturing (HF) and fracturing by confined blasting (CB) methods. To achieve HF, two hydrofracturing levels were included, the upper hydrofracturing level at 4,120 masl. (HFL-1), 280 m above the lower hydrofracturing level at 3,840 masl. (HFL-2), which in turn is 210 m above Lift 1 and 300 m above Lift 2.

The entire footprint area and the complete height of the columns will be hydrofractured using a 70 m x 82 m drill hole grid with hydrofractures every 1.5 m in each hole. To achieve CB, 100 m long upward vertical blast holes drilled in a 20 m x 17 m grid from the undercutting levels will be used.

On the production level, an El Teniente-type design was adopted on a 17 m x 20 m grid, adequate for the expected fragmentation of the ore. The design pattern considers production drifts (4.0 m x 4.0 m) every 34 m and straight draw bell drifts (4.0 m x 4.0 m) at 60° angle to the production drifts, spaced every 20 m. The draw bell drives are used to construct the trenches (draw bells) that connect to the undercut level. These draw bells are created by drilling and blasting. Once they are opened, two draw points per draw bell are constructed, with heavy support in the brows in order to withstand the erosion of the mineralized material that must flow through the draw points.

Overall, the mine plan assumes that to produce from Lifts 1 and 2, there will be 12 different levels and about 249 km of drifts and raises.

The metallurgical testwork programs reviewed two alternative comminution technologies:
- Conventional semi-autogenous grind (SAG) milling
- High pressure grinding rolls (HPGR).

While both methods appeared to be effective, based on the testwork completed, a decision was made to proceed with HPGR technology due to the estimated significant LOM operating cost savings.

During the first seven years of operation, the plant will only treat material from Josemaría. The Josemaría open pit will be located approximately 4 km from the proposed process plant site. In year 8, mill feed material from the underground operation at Los Helados will be introduced to the plant, and blended with Josemaría open pit material. The blended feed will continue for a six-year period while Los Helados ramps-up to full production. In year 14 of operations, the open pit mining at Josemaría ceases, and for the remaining mine life, only underground feed from Los Helados will be process ........