The phosphate deposits in the western Sechura Desert, Peru, occur as pelletal phosphate in marine sediments of Miocene age. Most of the pellets are concentrated in beds 1 to 1.5 meters thick that contain about 20 percent P 2 O 5 . Interbedded with the phosphorites are diatomite beds 3 to 20 meters thick, that contain 1 to 7 percent P 2 O 5 . Combinations of these beds form three major phosphatic zones which, in ascending order, are about 38 meters thick with 5.2 percent P 2 O 5 , 6 meters thick with 9.0 percent P 2 O 5 , and 2.5 meters thick with 5.2 percent P 2 O 5 . Within these zones are combinations of beds that are relatively thick and of higher grade, such as the upper part of the lower zone which is about 10 meters thick and contains about 12 percent P 2 O 5 . The composition, general character, thickness, and P 2 O 5 , content of the individual phosphorite and diatomite beds are highly uniform over large areas. Sandstone and tuffs form a minor but characteristic part of this sequence.Ore from the individual beds and zones is easily upgraded by washing and desliming because of the marked differences between the diatomite particles and the phosphate pellets in size, shape, and density.Low-amplitude folds and possibly minor faults that developed during deposition play an important role in the distribution of beds within the phosphatic sequence which range from 135 to 215 meters thick.The Sechura deposits are different from other major marine deposits in that they consist chiefly of phosphate and diatomite and the pellets are composed of a fluorine-deficient carbonate hydroxyl apatite of a type not known in other marine deposits. The apatite, however, is somewhat similar in composition to that in Holocene diatomaceous ooze on the sea floor off the coasts of Chile and Peru.

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