The Nechí Alluvial Project is situated in the alluvial valley and flood plain of the Nechí River lower basin that lies within the foothills of the Central Cordillera of the Andes Mountains.

The Nechí alluvial deposits may be classed as a Tertiary gravel plain gold placer with bordering bench or terrace deposits. The gravel plain style of placer is Mineros’ primary exploration target for dredging. Gravel plain deposits are generally characterized by well-rounded gravels, few boulders, and fine gold distributed vertically and laterally in the pay formations. Such placers are formed in low-velocity shifting stream channels in low-gradient valleys. Gold grades in the Nechí River flood plain are elevated at the mouths of creeks draining the high ground (Segovia batholith) to the east and these areas likely represented the merging of creek or river placers with the main gravel plain. In the upper reaches of the Nechí River, the goldbearing gravel plain placers likely grade into river placers.

Gold in the Nechí alluvial gold deposits consists of free grains that are predominantly No. 4 or smaller. No. 4 grains are very fine (flour or powder gold), with individual grains weighing approximately 0.02 mg. No. 2 and No. 3 gold grains are also present. In terms of grain counting, No. 4, and smaller grains account for 96% of the grains logged in drill hole sampling, however, the weight contribution of No. 3 and No. 4 grains is 86%. As determined from fire assays of bullion, the gold grains are 850 to 900 fine (85% to 90% gold), with approximately 9% silver, 1% iron, and traces of platinum. Mineros currently uses an assumption of 890 fineness for resource and reserve estimation.

Resistate heavy minerals separated from various process streams on the dredges have been studied for potential economic interest as industrial minerals (Lamus et al., 2006). Those present in Nechí River black sands have been identified as magnetite, ilmenite, titanomagnetite, zircon, monazite, and minor hematite, chromite, and rutile. The monazite is sometimes intergrown with xenotime and thorite. Ilmenite averages 49% TiO2 and has elevated Mn. Silicate gangue and other minerals are principally quartz plagioclase, chlorite, clinozoizite, hornblende, ferro-actinolite, and minor muscovite, biotite, and pyrite.

There are three types of mining carried out at the Nechí Alluvial Project. The first, and predominant method, is alluvial mining using a combination of suction and bucket line dredges. The second method is suction plain mining using new suction dredge equipment that was purchased by Mineros in 2020. The third method is terrace mining using an amphibious excavator.

Dredging operations are carried out 24 hours per day, 365 days per year, with 80% availability due to equipment maintenance and clean-up. The nominal production capacity of each of five bucket line dredges, used to mine pay gravel, is 500 m3/h, The nominal capacity of each of seven suction dredges, used to mine overburden, ranges from 300 m3/h to 500 m3/h. Planned annual production of 27.5 Mm3 results in an average hourly production rate of 3,950 m3 per hour.

The alluvial mining operation consists of overburden removal and excavation of the goldbearing gravel using large floating dredges. The deposits extend for more than 50 km along the Nechí River, with widths up to 3.5 km. The current mining process overall consists of the following four basic phases:
1. Overburden removal and deposit with suction dredges.
2. Gravel removal with bucket line dredges.
3. Trommel screen and sluice gravity gold extraction.
4. Final metallurgical processing of doré at the metallurgical plant and laboratory at Mineros’ El Bagre complex.

The first three operations are carried out by a production unit, consisting of a wheel cuttersuction dredge, a bucket line dredge, and support equipment including a bulldozer, a trackmounted crane, an amphibious backhoe, a boat, and other minor equipment. Each unit is a self-contained independent operation. Five production units are currently in operation.

The suction dredge then proceeds to remove the first four metres of surficial material. The removal of material continues downwards, suctioning volumes in two metre by four metre cuts, with a variable dredge operation width between 60 m and 300 m. The clay removal proceeds in front of the dredge for 16 m, then the dredge returns to the starting point and excavates another four metre deep volume. The excavation continues to the average dredge depth of 12 m (with a maximum of 18 m for Dredge No. 19). The removed material is pumped through a discharge pipe behind the bucket line dredge and discharged between two bucket line dredge
backfill dumps.

The bucket line dredge is specifically designed to exploit low grade, deep alluvial deposits at a large scale. The dredge combines the extraction of alluvial material, processing, and recovery of the valuable material, and the discharge of tailings under one roof. The removal of gold-bearing gravel comprises several operations: dredging, advance forward, lateral displacement, tailings disposal, and water supply.

The operations support equipment includes track excavators, amphibious backhoes, track bulldozers, wheel backhoes, and mobile cranes.

There are three main mining methods – bucket line dredges, suction dredges, and terrace mining – each with a different gold processing plant associated with it. Bucket line dredge mining utilizes the processing plants located on the five bucket line dredges. The suction dredge mining uses a floating processing plant that is fed from Dredge No. 21. Terrace mining utilizes a backhoe to feed a floating processing plant, which uses gravimetric recovery units to obtain a concentrate.

Gold concentrates from the areas are treated in a gold recovery and smelting process in a facility and laboratory in the El Bagre complex.

BUCKET LINE DREDGE ALLUVIAL PROCESSING
Mineral beneficiation from the bucket line dredge consists of gravity concentration and gravity gold treatment on board the dredge.

Screening
The bucket line arm excavates gravel at a rate of approximately 500 m3/h and delivers it to a main hopper feeding a rotating trommel with screen at 9.5 ........