Land-pebble phosphate, hard-rock phosphate, and river-pebble phosphate are the three types of phosphatic rock found in Florida. This report is concerned primarily with the land-pebble deposits, the only type which contains a significant amount of uranium. The most productive part of the land-pebble district is in Polk and Hillsborough Counties,
in the west-central part of the Florida peninsula.
Phosphate occurs in both the Hawthorn formation of lower
middle Miocene age and the Bone Valley formation of Pliocene age. The
phosphate in the Bone Valley is generally of higher grade, probably as
the result of mechanical reworking and further precipitation during
submergence in Pliocene time. In a few places, leached parts of the
Hawthorn formation are also of minable grade. , Pleistocene terrace
deposits and beach sands unconformably overlie the Bone Valley.
In Polk and Hillsborough Counties, the high-grade part of the
land-pebble phosphate district, uranium occurs principally in the Bone
Valley formation and is concentrated in the upper part of the formation,
which has been leached by ground water. The maximum tenor found
to date is 0.1 percent uranium. The basal member of the Bone Valley
has a maximum grade of .02 percent uranium, and averages about .01
percent uranium. Fresh, unweathered Hawthorn formation has little or
no uranium, but leached Hawthorn, rich in P2O5, contains a maximum of
0.01 percent uranium. The Pleistocene sands have no uranium except
where they contain reworked phosphatic material from the Bone Valley
South of the high- grade district, in Manatee and Hardee
Counties, the Bone Valley, Hawthorn, and Pleistocene formations contain
only very minor amounts of uranium.
The uranium in the land-pebble deposits was probably syngenetic
in origin. Subsequent leaching of soluble material has resulted in
residual enrichment of uranium. The uranium seems to be associated with
the phosphate mineral.