THE GEOCHEMISTRY OF GADOLINITE-BEARING PEGMATITES IN THE SOUTH PLATTE DISTRICT

Pegmatites are intrusive igneous rocks that contain interlocking crystals that are two centimeters or larger that form in the final stages of solidification of a magma chamber. Pegmatites are important for their economic value as industrial and gem resources as well as for their rare earth elements. Therefore, the geochemistry of the minerals and fluid inclusions trapped in those minerals offer valuable data for understanding the formational conditions in which the pegmatite crystallized. There are two primary types of pegmatites that are categorized by their trace element composition. The first type of pegmatites is the Lithium-Cesium-Tantalum (LCT) type and the second type of pegmatites are niobium-yttrium-fluorine (NYF) type. This project aimed at investigating the geochemical nature of NYF-type pegmatites that are less commonly utilized for gem-minerals, but are potentially more economic for their Y-, Nb-, and heavy rare earth element (HREE)-minerals.

The focus of this study is to investigate the petrogenesis and geochemistry of the White Cloud pegmatites located in Jefferson, C.O. During exploration, the major minerals found included quartz, perthitic microcline, plagioclase, and fluorite. In the Lesser White Cloud pegmatite the Y-Ce silicate mineral gadolinite was found in small amounts. The White Cloud pegmatite and Lesser White Cloud Pegmatite are both located in the South Platte Pegmatite District approximately 500 meters from one another. Fluid inclusions (FI) were examined to determine the history of crystallizing fluids as well as the mechanisms of formation. The focus of our research is to examine primary fluid inclusions in growth zone assemblages as they represent the fluid from which the minerals crystallized. Petrographic and microthermometric analyses were conducted to determine the geochemical signatures of these fluid inclusions.