| Summary |
The concentration of twenty-one major and trace elements (mainly transition metals) has been determined by inductively coupled argon plasma emission spectroscopy (ICAPES) on 114 samples of phosphate pellets, intraclasts, skeletal material, and microsphorite. Samples are from two major phosphate districts in North Carolina: 1) Aurora Phosphate District (APD) located in the coastal plain and 2) Frying Pan Phosphate District (FPPD) located in Onslow Bay on the continental shelf. Samples from the APD include representatives of the early (?) to early middle Miocene Pungo River Fm. units A and B, the middle Miocene Pungo River Fm. unit C, and the Pliocene Yorktown Fm. unit LY. The early Miocene Pungo River Fm. unit FPF-1 and Pleistocene/Holocene unit FP-H were sampled from the FPPD. Descriptive statistics and common factor analysis (SAS) suggest that the different phosphate grain types are chemically distinctive. 1) Microsphorite grains contain the highest amount of included material (reflected by high A1, Fe, Si, K, and Ti contents) and the smallest amount of carbonate fluorapatite (reflected by the lower P and Ca contents). 2) Skeletal grains contain small amounts of included material and the greatest amount of carbonate fluorapatite. 3) Intraclastic and pelletal grains are intermediate in composition with intraclastic grains more similar to microsphorite and pelletal grains more similar to skeletal grains. 4) Although carbonate fluorapatite is by far the dominant mineral phase, the abundance of many of the major and trace elements including Fe, Al, Si, K, Ti, As, Cr, Mo, Ni, and V are influenced by the clay mineral abundance. 5) Other phases directly affect major and trace element compositions as follows: iron sulfides (Fe), iron oxides (Fe, As, Mn, and Mo), organic matter (Cd, Zn, and Mo), and carbonate fluorapatite (P, Ca, Cd, and Zn). Cluster analysis and descriptive statistics performed on pelletal grains suggest that specific phosphorite units exhibit unique major and trace element signatures. 1) Unit FPF-1 pellets are characterized by moderate concentrations of both major and trace elements. This, in combination with smooth surface textures and associated well preserved foraminifera, suggests that minimal post-depositional alteration has affected these grains. 2) Unit A and lower unit B in the APD exhibit relatively high concentrations of A1, Fe, Cr, and As. Abundances of these elements suggest an increased clay mineral content within the pellets; high major elemental abundances also correspond to higher clay contents within the total sediment. 3) Pellets from upper unit B and unit C in the APD are very similar in composition to those found in unit FPF-1 except that Cd and Zn concentrations are higher in the APD. Elevated Cd and Zn values may reflect enriched paleonutrient conditions at the time of phosphogenesis. 4) Pellets from unit LY in the APD exhibit high values for Mo, relatively high values for P, Ca, Fe, As, Cd, and Zn, and low values for A1 and Si. Low A1 and Si values indicate lower amounts of included material while elevated Fe and As values indicate the presence of iron oxides/hydroxides. Minor oxidation of these grains may be related to past or present subaerial diagenetic events. Elevated Mo, Cd, and Zn value's may reflect unique paleoenvironmental conditions, secondary enrichments (iron oxides), or abundant organic inclusions. 5) Pellets from unit FP-H are characterized by elevated Fe and As and low Cd, Zn, and Mo values. It is suggested that these grains have undergone at least one episode of oxidation leading to gains in Fe and As with corresponding losses of organic matter. |
