Exploration of the Geological Formations of the Luquillo Mountain Range of North Eastern Puerto Rico using X-ray Florescence

Nawal C.. Exploration of the Geological Formations of the Luquillo Mountain Range of North Eastern Puerto Rico using X-ray Florescence. Master's Capstone. University of Pennsylvania 2011.

X-ray Florescence (XRF) is a widely used non-destructive method that measures the elemental composition of materials. This technology was applied to investigate the rocks and sediments in the Luquillo Mountains / El Yunque region of Puerto Rico. Initial testing of wet and dry sediments revealed that the machine records higher elemental concentrations in dry compared to wet sediments as it seems that the water molecules interfere with the X-ray beam on wet samples. The XRF method on dried samples produced reliable results and allowed for the chemical separation of the five basic bedrock types found in the Luquillo Mountains. Of the volcanoclastic the Fajardo Formation can be distinguished from the others by its concentration of Barium (Ba) and Rubidium (Rb). The Unnamed formation was distinguished by Copper (Cu) and the Hato Puerto Formation was distinguished by Nickel (Ni) and Strontium (Sr). The Rio Blanco granodiorite is the youngest rock type of the region and was the only formation whose elemental chemistry was not distinguishable from the othersapparently because it was formed directly from the basic magma that also formed the Luquillo Mountains volcanic rocks. Recent studies have found high levels of Mercury (Hg) in Luquillo stream water. Knowing that the Luquillo region was heavily mined for Gold (Ag) and Silver (Au), the Hg used in historic mining is a possible source of the elevated Hg values. The XRF analysis indicated small quantities of Hg in some rocks but no Hg was found in the sediments and soils surrounding the historic mining sites. Therefore if Hg had been used in historic mining operations it is no longer apparent in the sediments and has presumably been removed by erosion of the site.


Cardona,W.A., 1984. El Junque mineral prospects, eastern Puerto Rico. Caribbean
Journal of Science 20, 79-87.

known. To date, no extensive reconnaissance or geochemical work has been carried out. In order to promote future exploration, the author searched among spanish historical documents for clues of previous mining operations. Through the research a total of twenty three (23) sites were relocated which reveal a predominantly gold placer mining operation in the region. Furthermore, the data clearly denotes two zones of mineral occurrence: a northern gold-rich deposit zone and a southern copper dominant area. The extensive hydrothermal processes and contact metamorphism associated with the Rio Blanco Stock and its proximity to the southern hydrothermal alteration belt and San Lorenzo Batholith appear to account for this mineral differentiation.
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