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: Recognition of Private Property (Reconocimiento de Propiedad Privada, 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. The area is known as the Bagre-Nechí Mining District, located north of the Segovia-RemediosZaragoza Mining District, a host to hydrothermal vein mineralization, which has seen smallscale hard rock mining.
The alluvial mining titles are predominantly underlain by Quaternary alluvial sediments bordered on the east by the Cajamarca Complex rocks and on the west by Sincelejo Group formations. The Quaternary sediments are the main material mined by dredging. The alluvium has been subdivided into four units in which seven types of gold-bearing gravels can be differentiated. The stratigraphy from oldest to youngest is represented by bedrock Cajamarca schists, succeeded in turn by Tertiary clay-rich sediments (false bedrock), alluvial gravel pediment, and terraces.
Pediment gravels cover the Tertiary sedimentary rocks and, in some areas, the metamorphic rock terrain. This unit was developed by broad scale erosion and surface runoff and has smooth slopes that were later downcut by the river or tectonically elevated. The material mainly occurs as coarse sands and gold-bearing quartzite- and amphibolite-rich gravels.
Composed of gold-bearing gravels, the terrace materials generally overlie rock and can be differentiated from those in the riverbed. They are formed from several types of material and their reddish colour is evidence of exposure and oxidation that is not evident in the flood plain alluvium. Elevations of this landform serve to distinguish three generations of terraces.
The highest terraces are the most extensive of the terraces in the area and have been the most exploited by artisanal miners. Their average height above the Nechí River is between 35 m and 40 m. These terraces are found on both margins of the alluvial valley, from El Bagre to Puerto Claver, and from Cuturu to the Caceri Stream, and further downstream to Bijagual, with their sizes diminishing downstream. The highest terraces can be described as a gold-bearing Quaternary gravel lying on Tertiary rocks. Gold is mined to the contact with the Tertiary basement.
Widely distributed on the west flood plain, a second set of terraces are between 18 m and 28 m above the Nechí River. These materials are deposited on Tertiary sedimentary rocks and are approximately 11 m to 12 m thick. The stratigraphic column from the top to the base is: fine sands with fine gravel lenses; a layer of mottled violet, yellow, and white clayey silt of variable thickness; and a layer of gold-bearing medium gravels with three centimetre to eight centimetre coarse clasts.
The lowest generation of terraces is a maximum of eight metres above the river and is interpreted to represent a once higher elevation of the alluvial plain. This unit has less gravel than the other terraces and appears to have lower gold content. These low terraces were dredged near the municipality of El Bagre. Some of these terraces may be buried below the current flood plain surface.
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%.
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 Types
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.
The Nechí deposits appear to have formed at the same time as the gold-bearing gravels of Tertiary age mined in the Sierra Nevada Mountains of Nevada, California, and Oregon in the western USA, however, the US placers formed in higher energy environments, are higher grade and have been subjected to uplift and burial by late Tertiary volcanic rocks and sediments. The Nechí deposits are more akin to the dredging fields of Hammonton and Folsom, California.