MINERALOGY OF THE EUREKA TUNNEL PEGMATITE OF THE ST PETERS DOME DISTRICT IN EL PASO COUNTY, COLORADO, USA

Eureka tunnel pegmatite is a complex pegmatite with interesting mineralogy that includes two amphiboles. The pegmatite is located off Gold Camp Road, slightly over 2 miles east of Mt. Rosa in El Rosa County, Colorado. The pegmatite is at an elevation of about 9,327 feet above sea level. It is one of many pegmatites in the area known as St. Peters Dome. The samples in this study were collected by geology students from the University of West Georgia on experiential learning trips in 2011 and 2012. The purpose of this research is to understand the evolution of the pegmatite and the complex group of minerals it contains.

The hand samples are very course-grained and contain large amphibole crystals ranging from 1-4 cm in length and 2-3 cm in diameter. Surrounding the large amphiboles is a matrix of quartz, plagioclase, and K-feldspar. Many hand samples contain layers of smaller, rounded amphibole crystals no longer than 1-5 mm. There is reportedly riebeckite and other rare mineral astrophyllite in the pegmatite. Slabs were cut from one of the samples on three different planes. Numerous thin sections were made and observed under a petrographic microscope as well as under a scanning electron microscope (SEM). When observed with a petrographic microscope, the amphibole is dark green under polarized light and pleochroic. There is some overgrowth texture and symplectite. The feldspars show two different forms of twinning. The thin-sections were than carbon-coated and observed under SEM. Spot analyses were conducted on all minerals present. Line scans, multipoint analyses, and elemental maps of silicon, iron sodium, potassium, manganese, titanium, zinc, chlorine, aluminum, magnesium, and oxygen were conducted on the larger amphibole crystals. There is reportedly niobium rutile in the St. Peters Dome pegmatites, and traces of niobium were found within the matrix. Small zircon crystals were found in the matrix and as inclusions inside the amphibole. Fluorite and monazite were also found within the smaller amphibole crystals.