Size analysis, visual estimation of phosphate and other minerals, and preliminary estimation of recoverable phosphate in size fractions of sediment samples from drillholes GAT-90, Tybee Island, and GAS-90-2, Skidaway Island, Georgia
Antarctica A Keystone in a Changing World
This report, which continues a series of studies of the phosphatic Neogene sediments from the Georgia coastal plain and continental shelf, documents the estimation of minerals, especially that of phosphate, in bulk and size-separated samples taken from two boreholes drilled in coastal, northeastern Georgia near Savannah. The principal method of investigation is visual microscopic identification of mineral phases in bulk sediment samples and in wet-sieved, size-separated fractions. The reasons
... r releasing this report are to provide: 1) preliminary core analysis information; 2) sediment and mineral identification that can be tied to the offshore seismic stratigrapic framework; 3) preliminary estimates of phosphate resources in the area; and 4) information crucial to the planning of subsequent studies of the coastal and shelf sediments in the region. The results of this report suggest that these coastal sediments contain zones of phosphorite enrichment that approach the presently mined deposits in grade and tonnage of phosphate. Core GAT-90 was drilled on Tybee Island to a depth of 52 m, and Core GAS-90 was drilled on Skidaway Island to a depth of 58 m. Compositionally, the sediments from both cores have varying proportions of detrital quartz and aluminosilicate minerals, carbonate, and phosphate. In both boreholes the most phosphatic zone is the Tybee Phosphorite Member of the middle Miocene Coosawhatchie Formation, which occurs from 24.6 to 27.0 m depth in Borehole GAT-90 and from 33.1 to 41.1 m depth in Borehole GAS-90-2. The phosphate-rich samples of the Tybee Member of the middle Miocene Coosawhatchie Formation average 46 percent of the sample mass as phosphate grains >170 mesh in size. The remainder of the sample is about 25 percent finegrained material (<170 mesh) and about 25 percent >170 mesh quartz. There is little mineral matter other than quartz and phosphate in the >170 mesh fraction of this interval. Clearly, this phosphatic sediment would produce a screened phosphate product comparable to that mined by industry in the southeastern U.S. at present. Size separations usually were >18,18-60,60-170, and <170 mesh, although one of the <170 mesh samples from each borehole was separated into five additional size fractions with the smallest fraction being the <325 mesh. These extended size separations indicate that screening at >170 mesh recovers 93 to 96 percent of the phosphate that is in the >325 mesh fraction. The important results of these studies are: 1) zones of phosphorite enrichment of several meters thickness occur in sediments of these drillholes and, based on several companion studies, extend over the Georgia Shelf; 2) size separation of this sediment to >170 mesh produces enriched phosphate that is about equal to the material presently produced in mines in the southeastern U.S.; 3) in these zones of phosphatic enrichment, phosphate to quartz ratios in the >170 mesh sediment and the amount of fine-grained material (<170 mesh) are similar to presently-mined, onshore phosphate rock deposits in the southeastern U.S.; and 4) resource estimates of recoverable (>170 mesh) phosphate in the most phosphatic horizon in the two boreholes, ranging from 1.5 to 8.5 m in thickness, indicate recoverable phosphate mineral resources of between 1.5 and 3.6 tons/m2.