Mineros S.A., through Mineros Aluvial S.A.S. BIC., its wholly owned subsidiary, holds a 100% interest in the Nechí Alluvial Property under two types of mining titles: RPP and mining concession contracts.
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Summary:
The Nechí Alluvial Property lies within the Central Cordillera, one of the three physiographic subdivisions (Central, Western, and Eastern Cordillera) of the Andes Mountains in northern South America.
Mineralization
Gold in the Nechí alluvial gold deposits consists of free grains, which are predominantly No. 4 or smaller, hosted by flood plain Tertiary fluvial gravels and sands. Eighty percent of the gold grains are 150 µm to 180 µm. No. 4 gold 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, because of the large differential in grain weight, the weight contribution of No. 3 and No. 4 grains is 86%.
Fluvial flow regime is the primary geologic control on alluvial gold deposition. The dominance of fine gold grain sizes indicates a low-gradient system consistent with Mineros’ interpretation of Tertiary paleochannels within flood plain gravels.
As determined from fire assays of bullion, the gold is historically 850 to 900 fine (85% to 90% gold), with approximately 9% silver, 1% iron, and traces of platinum. Mineros currently uses 890 fine for the alluvial gold.
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, 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, ferroactinolite, and minor muscovite, biotite, and pyrite.
Deposit Type
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 gold-bearing gravel plain placers likely grade into river placers.