Undergraduate Honors Theses

Thesis Defended

Spring 2015

Document Type

Thesis

Type of Thesis

Departmental Honors

Department

Geological Sciences

First Advisor

Charles R. Stern

Abstract

Veins and pods of rare earth element (REE)-rich minerals occur northeast of Jamestown, CO, in the Silver Plume-type granites and pegmatites of the Long’s Peak-St. Vrain batholith as described by Goddard and Glass (1940) and Allaz et al. (in press). The REE-rich veins and pods consist of the epidote-member allanite, monazite, fluorite, and fluorbritholite. These veins and pods occur in association with texturally distinctive fine-grained aplites composed of oligoclase, alkali feldspars, quartz, and biotite. In this study I collected samples of the Idaho Springs formation metamorphic country rocks, Silver Plume-type granites and pegmatites from the Long’s Peak-St. Vrain batholith, and the aplites associated with and containing examples of the REE-rich mineralization. I prepared more than 30 thin sections of these samples, described them petrologically, determined their mineral chemistry using an electron microprobe, and obtained their Nd isotopic compositions and whole-rock chemistry. I conclude that the REE-rich veins, pods and associated aplites formed around the same Proterozoic time as their Silver Plume-type host rocks and are likely co-genetic as products of a F and REE-rich late-stage magmatic fluid intruding into the granite. However, I also determine that the felsic aplitic matrix found in association with the REE-rich veins and pods is petrochemically different from the granites and pegmatites of the Long’s Peak-St. Vrain pluton. I propose a model for separation of the F and REE-rich late-stage magmatic fluid into the felsic aplitic matrix and the REE-rich veins and pods by either sequential crystallization (aplite crystallizing first further concentrating the REE’s in the residual fluid) or by liquid immiscibility.

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