Summary:
The Farim phosphate deposit is a flat-lying sedimentary phosphatic bed, which underlies an area greater than 60 km2. The geological sequence at Farim displays the following lithological units from top to bottom:
• sandy-argillaceous overburden with soft, alternating sandy, clayey and sandy-clayey layers
• phosphatic interval (FPO)
• upper dolomitic limestone
• decarbonized phosphate unit (FPA) corresponding to the Saliquinhé phosphate deposit
• calcareous phosphate member (FPB)
• limestone at the footwall of the phosphate sequence, white, soft and porous.
Two main types of phosphate, differentiated by their petrography and chemical composition, have been identified on the Farim property:
• FPA layer – A de-carbonated phosphate matrix with very high P2O5 content of about 30% P2O5, formed exclusively in the shallow water of the Saliquinhé basin
• Lower grade FPB layer – Highly carbonated phosphate, generally containing 5 to 15% P2O5 (mean 13% P2O5) with some values up to 20%.
Deposit Geology and Mineralization
The three phosphate-bearing horizons referred to as FPO, FPB and FPA are located below a variable thickness of overburden.
Overburden
The overburden waste at Farim typically consists of a layer of reddish-brown laterite gravel, followed by cream-colored clay with occasional cobbles and boulders of cemented orange sand and brown clay. This is followed by a layer of stiff brown to orange sandy clay and a layer of firm light grey, moist, high-plasticity clay of a similar thickness. No laboratory test results are currently available for these materials.
The thickness of the overburden layers ranges from 26 m to 70 m with a mean of 41 m in the mining areas, whereas the phosphate matrix layer which is also a sedimentary deposit ranges from 1.0 m to 6.2 m with a mean of 2.7 m in thickness to over 5 m thick in places. Below these two layers is a soft rock limestone layer which increases quickly with depth to medium and hard bedrock.
FPO
The FPO is a clayey dolomitic limestone that is weakly phosphatic and has limited economic potential and is sparsely distributed across the deposit. It comprises laminated green clays and aluminophosphate and is 0.5 m to 1 m thick. At the surface in the higher zones, laterite with a ferruginous cover in places may be found. The FPO is not included in the mineral resource estimate at Farim.
FPA
The FPA phosphate matrix is homogenous and has a grainstone texture, with grains less than 800 µm in size. It is a soft, poorly cemented unit of phosphatic sand, which includes phosphatized shell and bone material, teeth, fecal pellets and crustacean coprolites. There is no calcareous cement, and it contains little silica and clay. It is mildly indurated and includes siliceous or pyritized layers 5 cm to 20 cm thick which comprise an average of 6% of the unit. The FPA layer has a P2O5 content of approximately 30% (consistently higher than 25%). The FPA unit is currently considered the potentially economic phosphate horizon. Grades of sedimentary phosphate deposits of worldwide distribution as compiled by IMC (2011) are in the range of 15 to 32%. The Farim deposit is at the higher end of that range (Champion, 2000).
The FPA is localized within the Saliquinhé bay sub-basin and is the potentially economic phosphate bed. The sub-basin is bounded to the south and east by carbonate platform rocks against which the FPA wedges out. The northwestern limit of the FPA has not yet been defined. To the north, the Tambato submarine bar, which formed a barrier between the Saliquinhé bay and the deeper Casamance basin, will likely form the northern limits of the FPA unit but this has not been demonstrated by drilling.
The limits of the FPA unit, the hanging wall and footwall, are clearly defined. A mixture of saprolitic fine sand and clays, which are generally unconsolidated, overlies the FPA. The immediate hanging wall to the FPA is 20 cm to 60 cm thick unconsolidated sand. The hanging wall rocks are oxidized reddish brown to an elevation of about 10 m below sea level. The FPA is grey to beige and brown and lies in a generally reducing environment below the oxidized interval. This is important because iron oxide, which is soluble in sulfuric acid, is a contaminant in phosphate deposits whereas iron sulfide, which is insoluble in sulfuric acid, is not (Champion, 2000).
The FPA is very regular, sub-horizontal and continuous. The FPA unit has an average width of about 3 m (in the resource area) and underlies an area of about 60 km². In the northern part of the basin, north of the village of Saliquinhé, a northeasterly trending area about 5.5 km long and 1.5 km wide has FPA thickness typically greater than 3.0 m and up to 6.0 m. A smaller area to the south of Saliquinhé, near the River Cacheu, also exceeds 3.0 m in thickness.
FPB
The FPB is a calcareous phosphate unit consisting of alternating soft phosphate strata with carbonaceous gangue and thinner, hard strata of slightly phosphatic bioclastic limestone. The lower grade FPB layer consists of highly carbonated phosphate, generally containing 5% to 20% P2O5 with an average of 13% P2O5. The FPB phosphatic limestone is indurated and much harder than FPA.
FPB is located immediately below FPA but exists under only 50% of the area of FPA. FPB also has a large extent outside of FPA. This horizon is known to extend 20 km north to south and 50 km east to west with thickness variable from 1 to 15 m with an average thickness of approximately 5.3 m.