Mertie reported in 1975 in the U.S. Geological Survey Bull. 1390 that placer mining for monazite in North Carolina and South Carolina in the periods 1887-1911 and 1915-17 was done on small deposits in the headwaters of streams that drained monazite-bearing rocks. Exploration was undertaken in 1951-54 by the U.S. Geological Survey and U.S. Bureau of Mines with the objective of finding larger ore bodies of suitable grade farther downstream that would warrant large-scale mining, but such deposits have not been found.
The explanation for the lack of deposits is that a high percentage of the alluvial monazite moves continuously downstream unless it can be held in the interstices of gravel beds. However, the alluvial sediments contain only sparse
gravel deposits, because the weathering of most common crystalline rocks in the Southeastern States generally yields only sand, silt, and clay. Exploration is not yet complete, and workable deposits may still be found in valleys where quartzite, chert, crystalline limestone, or other durable rocks are present to produce the thick beds of gravel.
That part of the Coastal Plain most likely to have received detrital monazite from the belts of monazite-bearing crystalline rocks in the Piedmont is shown to be in Virginia, North Carolina, South Carolina, Georgia, and Florida. The formations of the Coastal Plain range in age from Cretaceous to Holocene. Distinct Pleistocene terraces are present.
The occurrence of monazite in the sedimentary formations of the Coastal Plain is considered, and tables are presented showing the results of panning sedimentary materials from the Pleistocene and some of the Tertiary beds in South Carolina and Georgia. Monazite is generally found in these formations, but the tenors are so low that workable deposits are unlikely. Reconcentration of such deposits by local streams that do not head in the Piedmont province may yield workable placers. A fluvial deposit in Aiken County, S. C., is cited and described as an example of this process.
The Pleistocene terraces are distinguished from one another by their maximum altitudes above sea level. The deposits that form the terraces have well-defined upper limits marked by ancient shorelines, but have no recognizable lower limits. The terraces have been correlated with the several interglaciations of the Pleistocene Epoch, and it is shown that the conditions of weathering, erosion, transportation, and deposition of these sediments were more favorable for the accumulation of heavy minerals during the interglaciations than in Holocene time. Therefore, the largest volume of heavy minerals will be found in the terrace deposits, rather than along the present beaches.
Monazite is generally distributed in all the littoral deposits of Florida, but the content is too low to produce monazite placers. Placers of ilmenite, rutile, and zircon, however, are mined, and at two of the four operating plants, monazite is produced as byproduct. These deposits of heavy minerals, and the methods used in mining them, are briefly described.
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