Paper No. 9-4
Presentation Time: 2:20 PM
GEOCHEMICAL FINGERPRINTING OF BEACH PLACER DEPOSITS, MONTAUK POINT, LONG ISLAND, NEW YORK
Garnet and magnetite-rich sand, also enriched in monazite and zircon, has been observed near Montauk Point, Long Island. The sediment is derived from the till and stratified drift of the Ronkonkoma Moraine (deposited 90,000 to 70,000 BP), by mechanical weathering and erosion due to wave action at Montauk Point, the headland on the eastern tip of Long Island. Although the proximal source of the sediment is obviously the Ronkonkoma Moraine, the ultimate source remains to be determined. In order to determine the ultimate source, the chemical composition of selected minerals in the placer deposit needs to be compared with respect to same minerals and rocks of the till (mostly composed of granite, gneiss, schist, and quartzite derived from the New England Highlands region) and stratified drift. The possibility of a mixed provenance for some of the beach sand minerals exists. For example, stratified drift in the Montauk Point area may, in part, be derived from the Harbor Hill Moraine (about 20,000 BP); as well as from the Ronkonkoma Moraine and ultimately, bedrock over which the glaciers moved during the Pleistocene. Incorporation of the results of XRF, XRD, SEM and microscopic analysis allowed greater insight and promised to lend a greater resolution to the general conclusions made in previous studies. The distribution of representative bulk oxides and trace elements within the heavies was presumably controlled by the original magma composition and enrichment of certain trace elements such as Zr, Cr, Ti, and Nb indicated their immobile nature during alteration. The similarities among beach placer deposits, stratified drift and till lithology were explored more fully using multi-element plots. Overall bulk oxides and elemental abundances and patterns of individual element enrichment or depletion can be directly compared; however the levels of most mobile elements such as Rb and Ba tend to be distinctly variable considering the effect of chemical and mechanical weathering of the source materials. Previous studies did not adequately incorporate detailed geochemical investigations and could not provide a clear understanding of the provenance scenario with respect to the segregation and concentration of the garnet-magnetite beach placer deposits at or near Montauk Point.