Colluvium is a general term applied to any loose, heterogeneous, and incoherent mass of soil material and/or rock fragments deposited by rainwash, sheetwash, or slow, continuous downslope creep or slumping. It generally includes angular fragments, not sorted according to size. In Rondonia, the Espigao d’Oeste region, we use it to specifically refer to the top-level soil horizon containing manganese mineral bearing clasts. We also refer to this as cobble fields. It is usually found as a cap along a ridge or down slope below a ridge line. It can occur in flat areas as well. The clasts can be angular to sub rounded, platy to spherical, and from sand to boulder sized particles. They can be loose to densely packed within the soil and be mixed with clasts of other rock types. Limited exploration below the colluvium horizons have revealed structural zones containing hydrothermal vein systems of manganese mineralization.
Figure 1 Sidewall of 1m X 1m X 1m hand dug pit showing colluvium.
Mining of the colluvium or cobble fields is done by removing the soil with a bucket loader / excavator into a haul truck and carrying it back to the plant for processing. The colluvium can be from a half metre to greater than a metre thick.
Figure 2 Typical mine site. Colluvium excavated and loaded into truck for transport to processing plant.
Manganese was first explored for by Rio Madeira Mineracao in 2005. While others in the area were prospecting for diamonds and doing bulk sample tests on kimberlites, Rio Madeira recognized an opportunity and created a niche market for high grade, high value manganese cobbles. The cobble fields were known from road cuts and discussions with local farmers. The black cobbles were visible in roadbeds, especially after a rain. Minimal exploration uncovered several occurrences of these “boulder patches”. Three partners underwrote the exploration and capital investments needed to build a processing plant, which became operable during 2008.
Figure 3 Manganese cobbles exposed in road bed.
The Rio Madeira plant was designed in simple fashion after common gravel washing and sorting plants. The raw material from the field is fed through a hopper and jaw crusher if needed, to the first wash station - vibrating screens. The majority of soil is removed with the soil / water slurry being pumped to settling ponds. The remaining clasts are fed through a trommel for a second wash before being sorted by size with vibrating screens. The sorting places the material into 3 piles - a coarse, medium and fine fraction that is now available for sale as the final product. Further beneficiation can be done to separate manganese clasts from gangue by jigging the fine and medium material producing a cleaner, enriched material for the end client, depending on their need.
During 2011, a second company, Eletroligas, optioned the properties known as Jaburi and set up a similar plant based on Rio Madeira’s plant. They produced material for themselves, as they are a ferro-manganese producer and needed a source of high-grade manganese. Their plant was designed to produce product for the ferro-manganese market only, which means they can tolerate higher minor elements in their final product then can material destined for the fertilizer market. Increased silica (quartz) and iron minerals are detriments to fertilizer producers but not ferro-manganese producers.
During 2014, both plants came under the ownership of Brasil Manganese Corporation (BMC), Cancana’s joint venture partnership with Ferrometals. BMC’s first actions after ownership transfer were to bring the safety features into line with modern practices. Both plants needed extensive safety features such as re-wiring of overhead electrical infrastructure, hand rails, cat walks, chain guards, upgraded trommels, water pumps, tailing / settling ponds. Communications were improved by installing a radio network between all vehicles and base stations at each plant capable of reaching Espigao, over 40 km away.
The Eletroligas plant, since renamed the Jaburi Plant, was improved to provide feed for the fertilizer market. The washing cycle and particle separating jigs were improved. The trommel feed and rotation were adjusted. New water pumps were installed.
Both plants follow similar circuits to produce finished product from raw materials. The raw material, which is a mixture of soil and clasts of all sizes, is brought to the plant by 20 tonne capacity trucks and stored near the plant. The clasts within the soil are heterogeneous in nature, made from manganese bearing materials, granitic, mafic and vein quartz bearing clasts.
The role of the plants are two fold; remove the less than 5mm sized material including the soil, and separate the manganese bearing clasts from the gangue material (granitoids, mafic rocks and vein quartz) thus concentrating the manganese cobbles as the final product. This is accomplished by preliminary washing to remove the fines then a series of vibrating screens and jigs to sort the final product into three size fractions. Coarse material is greater than 25mm, medium sized material is 15 to 24mm in size while the fine material is 5mm to 14mm in size. The less than 5mm material is being stored for further processing. Currently studies are being conducted to understand the applicability of using cyclones, and the quantity and quality of the very fine material.
Figure 4 Raw material stockpiled for processing
Figure 5 Hopper with crusher initiates the citcuits
Figure 6 Initial wash to remove soil
Figure 7 Second wash within scrubber / trommel
Figure 8 Hand sorting oversize material
Figure 9 washed and sorted by size
Figure 10 Final product - coarse material
Figure 11 Loading final product - fine material
Numerous steps are involved in the pre production process. Several involve permitting from the various government agencies that over see mining and environmental concerns. Once a showing is identified, usually by surface exposure of clasts, the aerial dimensions must be estimated. Prospecting through hand dug pits or auger drilling will give a first pass estimate of the size of the showing in square metres. If the showing warrants it, follow-up trenching can be performed with a backhoe to get a more thorough cross section through the horizon and identify richer zones of clast accumulations.
Figure 12 Exploration trench used to map the extents of mineralization
While this exploration is being performed, other groups within BMC are active following up with the various permits required. We have a forestry engineer and an environmental engineer on staff to spearhead these permits.
The actual Process or claim must be in good standing and authorized for exploration. DNPM is the federal department of mines that oversees all claims within Brazil. Each State has a branch that is responsible for that state. The Processes or claims are granted for a 3-year term with the possibility of an additional 3-year extension. Annual taxes are paid to DNPM to hold the ground and comprehensive reporting is necessary to get any extensions granted. Two types of mining permits are granted by DNPM. The first, a GUIA, is an exploration bulk sample license. It is intended to allow a company to bulk sample the occurrence, sell the material and use the proceeds to further the exploration. The other or full mining license known as a Lavra is based upon economic studies after extensive exploration and is granted once the environmental, land issues or other concerns are met.
The landowner or farmer must have the proper land use permits in place. If the area of interest is overlain by pasture then the permitting is fairly straightforward. Environmental concerns require reclamation plans with follow-through after the mining.
If the area of interest has forest cover then the permits are much more involved. Removal of any forest needs special permits. Tracts of forest elsewhere, similar in size to the area of interest, has to be put aside and protected against cutting. Reforestation has to be planned from tree farms set up and maintained by BMC. Any commercial logging from the cut down forest has to be arranged for by local companies. Both State and Federal environmental departments have a say in the licensing steps. If water is needed at the mine site, permits stating how much water and where it is coming from need to be granted.
An agreement with the farmer stating how much surface area we want to use needs to be in place. Typically we will fence the area of activities off to keep out the cattle. We will pay the farmer a monthly rental fee based on the area enclosed by the fence and he will also receive royalties from the sale of any manganese material.
Production feed for the plants is scheduled depending on the status of these showings, the weather (wet or dry seasons), distance to the plants and estimated size and contents of each area.
Quality Control begins with grab samples from the field and follows through to the loading of trucks destined for the client. We send samples to SGS Geosol in Belo Horizonte, MG. According to their website, “SGS is the world's leading inspection, verification, testing and certification company” Every batch of samples we send to SGS is accompanied by standard samples of known manganese content as a test of SGS labs quality control. The standard values are unknown to SGS and are used to track performance within their labs. In addition to sampling as part of the exploration process, each truck of final material is weighed and sampled as part of the delivery steps to our warehouse. Each pile is channel sampled from bottom to top. The sample may weigh over 20 kgs. It is sent off to SGS for complete analysis of major and minor elements. As a final step, each truckload going to a client is channel sampled as a reference for comparison against the client’s analysis.