CATHODOLUMINESCENCE OF SODIC CALCIC ALTERATION, WHITE ROCK STOCK CONTACT AUREOLE, ELK MOUNTAINS, COLORADO

Sodic calcic alteration is a widespread alteration type in contact aureoles, including some porphyry deposits, and can reflect significant mass transfer in these settings. It is also commonly associated with iron oxide copper gold (IOCG) deposits. This case study of the Tertiary-aged White Rock stock aureole in the Elk Mountains, Colorado, evaluates cathodoluminescence (CL) as a characterization tool for this alteration type.

In the eastern roof rocks of the White Rock stock, limited sodic calcic alteration cuts Maroon Fm. hornfels and granitic sills along near-vertical joints. Alteration envelopes consisting of a series of symmetrical zones range from a few cm to nearly a meter thick. Alteration is strongly fracture controlled and extends along bedding planes in the hornfels. The alteration typically displays white central zones that transition sharply to a green outer margin. In the sills, the green outer zone is characterized by oligoclase/albite after alkali feldspar, and diopsidic pyroxene + titanite ± epidote after biotite, amphibole and magnetite. In some envelopes actinolite replaces diopside towards the joint. Nearest the joint is a white zone of oligoclase/albite + titanite ± apatite. In rare cases a titanite-free oligoclase/albite zone is present at the center of the alteration. Major and trace element chemical changes across the alteration zones correlate with mineralogic changes. Na has been progressively added to the rocks towards the joint surface. There was loss of Fe, Mg, K, Mn, and a variety of trace elements including Rb, Sr, Ba, Cu and Zn. Diopside zone samples have gained Ca, but it has been lost in the most altered zones; Cr and Sc follow similar trends.

Igneous alkali feldspar (blue CL) and plagioclase (yellow-green CL) are readily distinguished from red- to blue-luminescent replacement plagioclase. Red-blue-luminescent feldspar pseudomorphs alkali feldspar phenocrysts, involving an initial step to microperthite. Remnant igneous plagioclase persists throughout the alteration zone but the yellow luminescence gradually fades to blue CL with increasing alteration. In the most altered zones, red-CL albite has preferentially replaced the Ca-rich cores of igneous plagioclase. CL of matrix and phenocryst feldspar documents dissolution/replacement and diffusional reaction mechanisms.