Summary:
The Autazes Potash Project is located in the Amazon Potash Basin as part of the Central Amazon Basin, a large Paleozoic basin in northern Brazil. Within this basin, a sequence of marine to fluvial-lacustrine sediments of the Tapajos Group has developed, which are of Upper Carboniferous to Permian age. These rocks unconformably overlay rocks of the Upper Devonian to Lower Carboniferous Curua Group and are unconformably overlain by the rocks of the Javari Group of Cretaceous to Palaeogene age and intruded by sill-forming magmatic rocks, geochemically classified as basalts and basaltic andesites rocks related to Penatecaua magmatism.
Within the Autazes area, a sylvinite deposit has developed, which is Lower Permian in age.
The top of the sylvinite deposit (potash-bearing horizon) was determined to be at a depth between 685 m to 863 m. The total thickness in the explored area of the Autazes Potash Project ranges between 0.7 m and 4.0 m, with an average KCI grade of 25.0%.
The exploration results from the Autazes area generally confirm this genetic model. It is assumed that in a first phase of potash deposition, the crystallization of Sylvite together with Halite occurred, which is present as the mostly homogeneous, fine- to medium-grained mixture of Halite and Sylvite and low contents of sulphates (Anhydrite) in the Lower Sylvinite. In a next phase of evaporation, a mixture of Halite, Kainite, Sylvite and Anhydrite has possibly been deposited. Further evaporation was stopped by a next transgressive phase with high water levels, represented by the horizon of Marker 10B (Anhydrite and dark grey clay). During early diagenesis, after sedimentation on top of the evaporite rocks, the Kainite horizon was probably transformed, while a separation of the sulphatic components in the lower part (Middle Sulphates) and a mixture of Sylvite and Halite with small amounts of Anhydrite in the upper part occurred. The upper part of the transformation horizon is characterized by less homogeneous, slightly coarser crystalline formation of the Upper Sylvinite. The Upper Sylvinite is overlain by coarse-grained, secondary Halite that is mixed with Anhydrite and clay.
The Autazes potash deposit is, therefore, subdivided into a lower primary and an upper secondary generated section. Spatially limited differences in the transformation phase led to different stages in the transformation. Due to this fact, not all of the described parts of the transformation sequence are encountered in each part of the deposit.
Mineralization
Except for hole PBAT-15-43A, which was completely destructively drilled, the other 42 holes (including the re-drilling of PBAT-13-22 as PBAT-13-35) drilled by BPC were cored with good recovery in the evaporites.
The potash-bearing horizon can consist of up to three layers. Due to the mostly low thicknesses of the single sylvinite layers as well as the distinction of the sulphate-rich horizon not being possible in each case, the whole potash-bearing section has been summarized as the sylvinite horizon for the current mineral resource estimate. The main potash-bearing mineral for the sylvinite horizon is Sylvite. The sylvinite is considered to be a part of the inferred, indicated and measured mineral resource if the following conditions are fulfilled:
• Thickness of the potash horizon exceeds 1 m;
• Minimum KCl grade of the potash-bearing horizon exceeds 10%.
The thickness ranges from 1.0 to 4.0 m. The isopach map shows that the largest thicknesses occur in the central part of the Autazes area. The maximum thickness of 4.0 m is explored in the north-western center, while the thickness of the sylvinite horizon decreases towards the north, the south-west and south-east. The average thickness for the whole area is 1.89 m.