Abstract

Thick high-grade high-quality sedimentary marine phosphate deposits have been delineated in the Cretaceous-Tertiary strata of the Cabinda district of the Province of Angola, Portugal. The phosphate-bearing strata, a 1,500-ft sequence of sands, clays, phosphatic beds, and limestone, are underlain by redbeds and greenish-gray shales and overlain by relatively unconsolidated sands and conglomerates. The phosphate is concentrated in 2 units: an upper phosphate zone ranging in thickness from about 45 ft to 75 ft and containing from 15 to 20% P2O5, and a lower phosphate zone ranging in thickness from 80 to 130 ft. These are separated by 65–390 ft of sandstones, shales and conglomerates, with minor phosphate beds. The ore in the lower zone is concentrated in 3 units which are from oldest to youngest, about 10 ft, 40 ft, and 28 ft thick. They contain from about 10 to 20% P2O5 and are separated by 2 waste beds which are about 15–30 ft thick.

The phosphate mineral is carbonate-fluorapatite as inorganic phosphates (pellets, oolites, and nodules), and organic phosphates (fragments of fish teeth, bones, and fish scales). The phosphate in the upper ore zone is predominantly inorganic and that in the lower ore zone is about an equal mixture of organic and inorganic. Some phosphate beds (up to 10 ft in thickness) are primarily apatite and contain as much as 38% P2O5; however, most are mixtures of apatite with quartz sand and silt. Within any ore zone the phosphatic beds are interbedded with sand and silt beds. High-grade and high-quality phosphate concentrates (36–38% P2O5) can be produced by simple sizing and flotation from low-grade ore (10–20% P2O5).

The phosphate was deposited as continuous beds in a marine basin which covered much of the Cabinda district. This basin generally shelved gently westward from near the Congo border on the east, so most lithologic units thicken westward. Phosphate deposition was in part controlled by folding developed before and during the period of deposition. The major known fold which strikes southeast through the middle of the area was probably the predominant structural feature controlling distribution and deposition of the phosphates.

After deposition the folding continued and a strong system of southeast-trending faults developed. The fault system has resulted in the formation of several grabens, which presently form topographic highs where the major reserves of phosphate are preserved.

The major factor bearing on the economic potential of the phosphate is the leaching and oxidation of the phosphate beds by the recent downward movement of meteoric water where the beds are near the surface. Leaching extends to a maximum depth of about 300 ft and increases the grade of the beds by as much as 50%, and the grade of the apatite from 32 to 34% P2O5 to 38% ± P2O5.

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