SEDIMENTARY PHOSPHATE DEPOSITS- DISTRIBUTION AND ISSUES OF CLASSIFICATION

Sedimentary phosphate deposits are the most common type of phosphate deposits with known occurrences in over 80 countries. For this study, we have compiled a global data set of more than 1000 economic, subeconomic, and non-economic sedimentary phosphate deposits that range in age from Early Proterozoic through Holocene. Data for the deposits include location, basin, age of mineralization, host rock and formation, size and (or) grade, and a variety of other geologic and economic data. Unique classification of these deposits has remained elusive despite the voluminous descriptive literature; no one classification scheme has received widespread acceptance. The large sedimentary phosphate deposit data set offers an opportunity to test past classification criteria, identify previously unrecognized common geologic factors or spatial patterns, compare spatial/temporal associations with other sedimentary deposits, and investigate more thoroughly the relationship of the deposits with differing paleogeographic conditions through geologic time.

For this study, deposits are grouped by age of mineralization and plotted on paleogeographic reconstructions with other selected paleogeographic indicators, including organic-rich sedimentary rocks, diatomite, and chert. Many deposits plot in areas, such as shallow seas, where phosphate deposits might be expected to form based on existing classification schemes. Other deposits appear to have been incorrectly dated as they plot on land or, alternatively, suggest by their presence that shallow seas were more extensive than indicated by the paleogeographic reconstructions. For some time periods, there is a clear relationship between phosphate deposits and organic-rich sedimentary rocks; for others, the two sediments occur in very different locations. The commonly cited tendency of phosphate deposits to form on continental shelves and slopes primarily off the west coast of continents seems to hold for more recent deposits (Eocene and Miocene), but does not consistently apply to older deposits. The data set provides the opportunity to refine our understanding of the genesis and occurrence of sedimentary phosphate and may offer the potential to identify new exploration targets in areas without currently known sedimentary phosphate deposits.