FELDSPAR REPLACEMENT MECHANISMS DURING SODIC-CALCIC ALTERATION

Fluid-rock interaction commonly drives sub-solidus reactions in granitic rocks, and the mechanisms of these reactions can lead to enhanced permeability through fractures and microporosity. Localized sodic-calcic alteration by a dissolution-precipitation mechanism affected Maroon Fm. hornfels and granitic sills along near-vertical joints in the eastern roof rocks of the Tertiary-aged White Rock pluton in the Elk Mountains, Colorado. At the outcrop scale, alteration is strongly controlled by fractures and bedding planes. The alteration envelope is typically zoned from unaltered wall rock to a green outer selvage and a white central zone. In the sills, the green outer selvage is characterized by igneous plagioclase, replacement oligoclase/albite after alkali feldspar, and diopsidic pyroxene or actinolite + titanite ± epidote after biotite, amphibole and Fe-Ti oxides. The white zone contains oligoclase/albite + titanite ± apatite and relict igneous plagioclase.

Cathodoluminescence of replacement feldspar is distinct from that of igneous grains. Red- to red/blue-CL replacement oligoclase pseudomorphs alkali feldspar. Remnant igneous plagioclase persists throughout the alteration but the yellow-CL gradually fades to blue-CL with increasing alteration intensity. In the most altered zones, red-CL oligoclase has preferentially replaced the Ca-rich cores of igneous plagioclase and is cut by late albite along microfractures. Replacement feldspar displays microporosity and sharp interfaces with igneous feldspar, consistent with a dissolution-precipitation mechanism.

Na has been progressively added to the rocks towards the joint surfaces. Green zone samples have gained Ca, but it has been lost in the most-altered white zone. There was loss of Fe, Mg, K, Mn, and trace elements including Rb, Sr, Ba, Cu and Zn. The alteration was likely driven by saline fluids possible sourced from nearby evaporites.